Prosthetic head, neck and stem components for use in hip arthroplasty

Abstract

A dual mobility prosthetic bearing comprises: an outer femoral head component (9020) having a part-spherical outer bearing surface (9022) and a head recess (9024) comprising a proximal portion (9026) defined by a part-spherical inner bearing surface and a distal portion (9028) defined by a generally tubular inner bearing surface, the proximal portion having a maximum diameter that is greater than a maximum diameter of the distal portion; an inner femoral head component (9030) having a neck recess (9032) for receipt of a femoral neck component (9004), and a part-spherical outer bearing surface (9034) configured to be received within the proximal portion of the head recess; wherein the part-spherical outer bearing surface of the outer femoral head component and the part-spherical outer bearing surface of the inner femoral head component have a coincident centre of rotation; and wherein the inner femoral head component comprises a generally tubular skirt (9036) configured for receipt within the distal portion of the head recess.

Description

[0001] Prosthetic Head, Neck and Stem Components for Use in Hip Arthroplasty

[0002] Technical Field of the Disclosure

[0003] The disclosure relates to prosthetic head, neck and stem components for use in hip arthroplasty and to a femoral component including any or all of the prosthetic head, neck and stem components.

[0004] Background of the Disclosure

[0005] The present disclosure relates to prosthetic head, neck and stem components for use in hip arthroplasty (also known as hip replacement). Hip arthroplasty includes total hip arthroplasty (THA) whereby an artificial femoral component including a head (i.e. ball) component is replaced along with an artificial acetabular cup (i.e. socket), and hemiarthroplasty whereby only the femoral component is replaced and the natural acetabulum is retained.

[0006] In some cases, hip resurfacing may be performed instead of total hip arthroplasty or hemi-arthroplasty. In which case, only a damaged bone surface (e.g. of the head of the femur and the acetabular surface) is removed and replaced by prosthetic (head and cup) resurfacing components.

[0007] History of bearing development for hip arthroplasty

[0008] The first hip arthroplasty bearings to give some success in the 1950's were Metal on Metal bearings comprising metal femoral heads configured to articulate in metal acetabular cups. The early developers were Mr Ken McKee in Norwich UK and Mr Peter Ring from Redhill UK. When the bearing was well-manufactured, 30 year successes were observed. Despite gaining some traction in the market as there was no viable alternative at that time, the number of implants inserted was not very high. When Metal on Polyethylene bearings (comprising metal femoral heads and polyethylene acetabular cups) were developed by Sir John Charnley (Charnley), surgeons abandoned Metal on Metal THA's.

[0009] 55677900-1 The inventor led the development and reintroduction of modern Metal on Metal bearings in a technique known as Birmingham Hip Resurfacing (BHR). This comprised using a metal cap on a prepared femoral head for articulation in a metal acetabular cup.

[0010] Some hip bearings comprise a polymer forming one part of the bearing, often the acetabular cup, with the other part usually consisting of a metal or ceramic head. However, sometimes the head can be formed of a polymer (such as polyethylene) for articulation in a metal or ceramic cup.

[0011] Charnley was first to identify polytetraflouroethylene (PTFE) as a potential bearing material. Charnley was searching for a polymer which exhibited very low frictional torque and he was pointed in this direction by a Dr Bowden from Cambridge University. Charnley tried PTFE on PTFE resurfacing as his first implant. Unfortunately, these implants failed after a few months as the PTFE head and cup seized in use causing the rear side of the cup to articulate in the acetabulum with high PTFE wear and therefore this design was abandoned. Charnley then tried large metal heads on PTFE cups but the cups soon loosened. He gradually reduced the femoral head diameter and eventually Charnley found success using much smaller heads with a 7 / 8 inch (22.225mm) diameter head. Charnley implanted these small metal heads on PTFE cup designs in over 300 patients in 1958, 1959, 1960 and part of 1961. The patients were delighted with their new hip joints and early patient satisfaction lured Charnley into operating on many more patients. Unfortunately wear, osteolysis and cup loosening became apparent. By Christmas 1961 , it was clear that the Charnley design had a huge problem. The patients needed revision surgery but it was not clear what polymer could be used for the revision operations.

[0012] In early 1962, Charnley was due to travel to Zurich for a meeting of hip surgeons, hosted by Prof Maurice Muller, and he was expected to be abroad for some 6 weeks. Before departing, Charnley called into the laboratory to speak to his engineer Harry Craven (Craven). Craven had starting wear testing on a new polymer. Charnley enquired what the polymer was and Craven replied, high density polyethylene. At a later meeting of the British Hip Society, Craven related this story and claimed that Charnley had been unimpressed and had told Craven to throw the polyethylene away.

[0013] 55677900-1 Fortunately, for those who develop hip and other joint prosthesis and those who need joint replacement surgery, Craven ignored Charnley’s advice and carried on testing polyethylene and it became apparent that the wear of polyethylene was dramatically lower than PTFE.

[0014] When Charnley returned home he had the very pleasant surprise that Craven had found very low wear of polyethylene and it looked like a viable replacement for PTFE. Charnley had a piece of polyethylene inserted under the skin of his arm and when no reaction was seen over time, Craven made some polyethylene cups for Charnley to insert into patients. Thus modern joint replacement was born with Sir John Charnley getting the full credit.

[0015] Charnley had to revise many PTFE cup hip replacements in his patients and he had his engineers measure both linear and volumetric wear on every PTFE cup retrieval. This work is illustrated in the graph 100 of Figure 1 , which shows that the bigger the femoral head (increasing diameter metal sphere), the lower the linear wear. However, for volumetric wear the opposite was observed. The smaller the femoral head, the lower the volumetric wear. Lower volumetric wear causes less osteolysis. This relationship between head size and linear and volumetric wear has also been found to hold true for non-cross-linked polyethylene cups.

[0016] When such non cross-linked polyethylene cups became the norm for total hip arthroplasty, many surgeons were nervous about using Charnley’s 22 mm diameter metal head as they were concerned about dislocation as a complication. As a result, many surgeons used 28mm or 32mm or larger diameter heads to try and reduce the dislocation rate. However, as noted above, higher volumetric wear was predicted from the use of such larger heads and osteolysis became a major medium term complication which required revision surgery.

[0017] This lead to the French development of so-called dual mobility hip bearings 200, illustrated in Figures 2A and 2B. The argument was that using a small diameter inner metal head 202 on an inner polyethylene mobile bearing would keep volumetric wear to a minimum whilst a larger outer polyethylene head 204 surface on a metal cup 206 bearing would not contribute significantly to volumetric wear for the following reason: lower frictional torque of the small inner head 202 against the polyethylene would mean

[0018] 55677900-1 that the majority of articular motion would occur at the inner bearing and because only a minority of articular motion occurred at the outer bearing, this would not contribute significantly to volumetric wear of polyethylene or resulting osteolysis.

[0019] As illustrated in Figures 2A and 2B, the small diameter inner metal head 202 was mounted on a conically tapered neck 210 of a femoral stem component 208.

[0020] Initially the French surgeon developers used concentric centres of rotation C for both the small diameter inner metal head 202 and the outer polyethylene head 204. However, use of this bearing in patients showed that the mobile polyethylene head 204 tilted into varus, as shown in Figure 2B, with impingement of an edge 104a of the polyethylene head 204 against the medial femoral neck 110 and this adverse position of the polyethylene head 204 never recovered back to the normal state shown in Figure 2A. The impingement of the polyethylene head 204 on the medial femoral neck 210 therefore became constant resulting in wear of the polyethylene head 204. In addition, the superior portion of the inner metal head 202 was no longer covered by the outer polyethylene head 204 resulting in a risk of dislocation of the inner head 202 from the polyethylene head 204.

[0021] A design change rectified this problem, but it also created a different problem. For the design change, it was necessary to know the direction of load in the hip joint. Theory had predicted that the joint reaction force acts on the hip in a supero-medial direction. However, Charnley had already discovered the load direction in practice, by observing the wear direction in high wearing PTFE cups. Figure 3 illustrates a sketch 300 from Charnley’s book “Low Friction Arthroplasty of the Hip, Theory and Practice”, 1979, p7. The sketch 300 shows a 22mm diameter stainless steel head 302 mounted on a neck 304 of a femoral stem 306, for articulation in a Teflon acetabular cup 308, after 3 years. The sketch 300 clearly shows a vertical wear track created by the head 302 in the cup 308, indicating a vertical load direction L.

[0022] The design change is shown in the dual mobility hip bearing 400 illustrated in Figures 4A and 4B. In this case, the centre Ci of rotation of the small diameter inner metal head 402 is offset from the centre Coof rotation of the larger outer polyethylene head 404. As before, the small diameter inner metal head 402 is mounted on a conically tapered neck 410 of a femoral stem component 408 and the outer polyethylene head 404 surface is

[0023] 55677900-1 arranged to articulate in a metal acetabular cup 406. More specifically, the centre Ci of the small diameter inner metal head 402 is offset from the centre Coof the larger outer polyethylene head 404 in a proximal direction along with the axis of the neck 410. In some cases the small diameter inner head 402 may be formed from ceramic as opposed to metal.

[0024] Offsetting the centre Ci of the small diameter inner metal head 402 from the centre Coof the larger outer polyethylene head 404 causes the mobile polyethylene head 404 to rotate into a valgus position, as shown in Figure 4B, during the stance phase of gait. Referring to the arrangement in Figure 4A, the vertical load direction L applies a torque force T to the polyethylene head 404 to rotate the polyethylene head 404 around the inner metal head 402 centre Ci until the two centres of rotation Ci and Coalign with the vertical load direction L as shown in Figure 4B.

[0025] In this position, it is common for the edge 404a of the polyethylene head 404 to impinge on the prosthetic femoral neck 410. Designers and manufacturers try to reduce the diameter of the femoral neck 410 to minimise the incidence of impingement but there is a limit, as strength of the prosthetic femoral neck 410 cannot be compromised. Impingement of the edge 404a of the polyethylene head 404 is a risk factor for intra- prosthetic dissociation.

[0026] Radiation cross-linking of polyethylene (XLPE) was introduced into surgical practice circa 1990 and XLPE has now largely replaced conventional polyethylene in hip arthroplasty. There is no doubt that XLPE massively reduces wear in hip arthroplasty.

[0027] In the XLPE era, dual mobility bearings have become ever more popular as surgeons seek to reduce the risk of dislocation in their patients.

[0028] A comprehensive study has recently been published on experience with the use of modern dual mobility THA's from the Mayo clinic. This was published in The Bone & Joint Journal (Bone Joint J 2024;106-B(5 Supple B):98-104) of the International Hip Society by K.E. Mallett et al. and was entitled “Incidence, treatment, and outcomes of modern dual-mobility intraprosthetic dissociations”. This article explained that although dual mobility articulations provide increased stability, they can fail as either a large head dislocation or as an intraprosthetic dissociation (IPD), defined as the large head

[0029] 55677900-1 separating from the smaller, inner head. Of the 1 ,453 dual mobility implants in the study period, 49 patients sustained a dislocation of the large polyethylene head from the acetabular component, representing a total dislocation incidence of 3.4%.

[0030] A dislocation rate of 3.4% for a prestigious institution using modern dual mobility cups is disappointing and raises the question of whether progress in reducing dislocation risk in THA has really been made with dual mobility bearings.

[0031] The study also found an intraprosthetic dissociation (IPD) rate of 0.76% with a mean follow-up of 4.2 years, which may be even more worrying. In addition, recurrent impingement contact of the polyethylene mobile bearing edge 404a on the femoral neck 410 plus free rotation of the polyethylene head 404 on the inner metal head 402, does not bode well. It seems that the ingredients exist for worsening plastic deformation of the polyethylene mobile bearing edge 404a. Such plastic deformation might remove the approximate 1.25mm extent of snap fit which retains the polyethylene head 404 on the inner metal head 402 and this might lead to an even greater IPD risk with increasing follow-up. It is good that the Mayo clinic has reported on this issue now and hopefully they will report regularly going forward to keep alert surgeons informed.

[0032] No longer is there a constraint on prosthetic femoral head size due to XLPE wear resistance. Now the problem is, how to fix native sized large femoral heads to traditional 12 / 14 femoral neck taper cones without causing severe wear and corrosion of the taper junction.

[0033] The inventor has led the development of a hip resurfacing with a cemented metal resurfacing cap and a titanium porous ingrowth XLPE cup. Simulator wear to 10 million cycles shows extremely low wear of the XLPE. This so-called Polymotion® Hip Resurfacing (PHR) has already began an Investigation Device Exemption (IDE) study in the USA soon.

[0034] Fixation of large diameter resurfacing femoral components are known to be successful with the BHR after 27 years in use.

[0035] Neck taper wear and corrosion are already a problem with moderate size femoral heads. The inventor predicts that if natural size femoral heads start to be used on 12 / 14 taper

[0036] 55677900-1 cones, wear and corrosion of the neck taper cone will become a much bigger problem. The inventor has had extremely positive patient follow-up with 27 years of the BHR. However the inventor also has a smaller series of BHR cups with BHR sized modular metal heads, from the same manufacturer as the large diameter resurfacing components, fixed to 12 / 14 taper cones of THA stems (again, from the same manufacturer) and taper wear and corrosion has caused an unacceptably high revision rate.

[0037] Some surgeons claim that modular ceramic heads will fix this problem. The inventor has performed and presented metal ion studies on ceramic on ceramic total hip replacement patients and compared these to pre-operative control patients. Chromium and Strontium, both constituent oxides of Delta ceramics did not have increased ion levels in serum compared to as the controls. However, Titanium and Aluminium (Aluminum), both constituents of titanium alloy, had significantly raised ion levels in serum, suggesting femoral neck taper damage. Figure 5 illustrates the results from these metal ion studies with both the Titanium and Aluminium (Aluminum) showing significantly increased values for delta compared to the controls (with p<0.005).

[0038] Aluminum is a known neurotoxin. High levels of this metal in patients presents serious ongoing risks to patient health.

[0039] Whilst initially it appeared that large metal heads were responsible for causing metal reactions and pseudotumours due to wear and corrosion at the taper interface junction. More recently, use of large ceramic heads are also implicated in causing unwanted metal taper damage and wear as noted in hip simulator studies, such as those described by Rohan M Bhalekar et al. in “Wear at the taper-trunnion junction of contemporary ceramic-on-ceramic hips shown in a multistation hip simulator”, Journal of Biomedical Materials Research Part B, 2018:00:00:1-11. For example, Figure 6 shows a vertical load L on a ceramic head 602, which is fitted via a conical tapered recess 604 on a tapered 12 / 14 trunnion cone 606. The simulation showed material loss from the cone 606 at the proximal-superior end 610 and the distal-inferior end 612, with no damage to the ceramic head 602.

[0040] The best scientific work on the cause of dislocation following total hip arthroplasty is described in a paper by Richard J. van Arkle et al. entitled “Capsular ligament function

[0041] 55677900-1 after total hip arthroplasty”, Journal of Bone Joint Surgery Am. 2018;100;e94(1-10). This shows that performing a standard total hip arthroplasty, as practised today, massively increases the range of movement of the replaced hip compared to the natural hip prior to surgery. This increased range of movement increases the risk of prosthetic femoral neck to cup edge impingement and femoral bone to pelvic bone impingement. Both of these unwanted mechanisms tend to lever out the prosthetic femoral head from the prosthetic acetabular cup resulting in dislocation.

[0042] In the above paper, the authors measured the maximum ranges of internal and external rotation of native hip joints in 8 cadavers (n=8) for five different positions of flexion (including extension and abduction (Ext-Abd), standing, heel strike, sitting and flexion and abduction (Fix-Abd) and the results are shown in the graph 700 of Figure 7.

[0043] The authors then carried out THA using three different diameter head sizes (28mm, 32mm and 36mm) through a medial acetabular wall trap door approach without interfering with the intact hip capsule, but still removing the femoral head and the upper half of the femoral neck, and measured again the maximum range of internal and external rotation for the same five positions of flexion. The results for each of these head sizes are shown in the graph 800 of Figure 8, along with the results for the native hip reproduced from Figure 7. This clearly shows an increased range of internal rotation for each of the three head sizes when compared to the native hip, which leads to a risk of posterior dislocation. There is also an increased range of external rotation, particularly in extension, leading to a risk of anterior dislocation.

[0044] This has enormous implications for hip arthroplasty. Surgeons think of a 36mm head as large, but these results show that a 36mm head is little better than a traditional 28mm head.

[0045] The last thing that is wanted following hip arthroplasty is to massively increase the range of movement beyond normal. This increased range of - particularly internal rotation movement - results in impingement of the femoral neck of a THA stem against the edge of the acetabular cup, resulting in lever-out posterior dislocation. Increased range of movement also makes femoral bone on pelvic bone contact possible, increasing dislocation risk.

[0046] 55677900-1 These results show that the modern trend in using a direct anterior approach will do nothing to stop posterior dislocation.

[0047] The same group of investigators found that only a hip resurfacing had a near comparable range of movement to the native hip joint thus reducing the risk of posterior dislocation. The main differences between a THA and hip resurfacing are: a) Hip resurfacing has a femoral head diameter close to the patients’ native femoral head size; and b) Unlike THA, where the femoral head and the head-neck junction and approximately half the femoral neck are resected, hip resurfacing retains the tapered head-neck junction and the large prosthetic head plus the retained head-neck junction more completely fills the hip capsule volume, which allows the hip capsule to perform its most important function of stabilising the hip joint against dislocation.

[0048] The inventor has been a proponent of hip resurfacing for 35 years. There are, however, downsides of hip resurfacing. The surgery is difficult. Retention of the femoral head, head-neck junction and femoral neck requires considerable training, an extensile exposure and increased operative time to perform hip resurfacing. Most surgeons will not consider performing a hip resurfacing. There are contraindications, particularly poor quality bone which is seen in many elderly women. Avascular necrosis (AVN) is a relative contraindication. The inventor had a 10% failure rate in his AVN patients at 10 years. However, there was a zero % failure rate over the next 10 years making the decision to resurface or not really difficult for patients and surgeons. Developmental dysplasia is little problem with a Crowe grade 1 hip classification but difficult or very difficult with a Crowe grade 3 hip classification. AVN as a complication of hip resurfacing in the inventor’s series occurs in 0.5% of cases.

[0049] All of the above means that hip resurfacing is not for every patient and not for every surgeon.

[0050] However, it is the inventor’s desire to make THA as good as hip resurfacing with respect to dislocation risk. This is a big ask as the dislocation rate for hip resurfacing is 1 per 1 ,000 resurfacing cases. Currently, THA has a dislocation risk over 10 times higher than hip resurfacing.

[0051] 55677900-1 One procedure to consider is an Ultra Short Stem (USS) hip arthroplasty. This implant anchors in the tapered head-neck junction and the femoral neck only, without entering the shaft of the femur. This avoids stress shielding of the femur, a completely unsolved problem with THA. The inventor has over 20 years of follow-up with USS hip arthroplasty. Fixation in the head-neck junction and femoral neck is not problematic. The problem with this implant was fixation of a large head to a 12 / 14 taper resulting in taper wear and corrosion requiring revision surgery.

[0052] If the conventional THA patient is young with good bone then an uncemented femoral component may be selected. There is much less scope with an uncemented stem for the surgeon to adjust the effective neck length by adjustment of the uncemented stem position in the femur. In the past, modular femoral necks were provided by every large orthopaedic company.

[0053] Figure 9 shows an example of an uncemented modular prosthetic femoral component 900 comprising a stem component 902, a head component 904 and a separate neck component 906. The neck component 906 may be provided in different lengths and angles so anteversion / retroversion and leg length could be optimised. Each neck component 906 comprises a conical proximal taper 908 for insertion into a conical recess in the head component 904 and a flat distal taper 910 for insertion into a recess in the stem component 902. However, it was found that the junction 912 of the modular neck component 906 in the recess in the femoral stem component 902 had massive wear and corrosion problems and all of these modular neck designs have been withdrawn.

[0054] It is therefore an aim of the present disclosure to provide prosthetic heads, neck and stems for use in hip arthroplasty that address one or more of the problems above or at least provides a useful alternative.

[0055] In general, this disclosure proposes to overcome the above problems by providing improved head, neck and stem components for use in hemi-arthroplasty and total hip arthroplasty (THA).

[0056] In one aspect of the disclosure there is provided a dual mobility prosthetic bearing for use in hip arthroplasty comprising:

[0057] 55677900-1 an outer femoral head component having a part-spherical outer bearing surface and a head recess comprising a proximal portion defined by a part-spherical inner bearing surface and a distal portion defined by a generally tubular inner bearing surface, the proximal portion having a maximum diameter that is greater than a maximum diameter of the distal portion; an inner femoral head component having a neck recess for receipt of a femoral neck component; and a part-spherical outer bearing surface configured to be received within the proximal portion of the head recess such that the part-spherical outer bearing surface of the inner femoral head component is in bearing engagement with the part- spherical inner bearing surface of the outer femoral head component; wherein the part-spherical outer bearing surface of the outer femoral head component and the part-spherical outer bearing surface of the inner femoral head component have a coincident centre of rotation; and wherein the inner femoral head component further comprises a generally tubular skirt configured for receipt within the distal portion of the head recess to retain the outer femoral head component on the inner femoral head component during use.

[0058] Thus, embodiments of this disclosure provide a dual mobility prosthetic bearing that may be used in any hemi-arthroplasty or total hip arthroplasty (THA) in order to reduce a risk of post-operative dislocation.

[0059] The bearing may be configured to prevent the outer femoral head component from non- axial (e.g. off-axis I transverse) rotation with respect to a longitudinal axis of the inner femoral head component. More specifically, the smaller diameter generally tubular (e.g. parallel-sided) distal portion of the head recess may be configured to prevent the outer femoral head component from sliding or rotating into a varus or valgus or anteversion / retroversion position where it may be more likely to dislocate out of the acetabulum.

[0060] Moreover, the generally tubular (parallel) skirt of the inner femoral head component is prevented from making contact with an edge of an acetabular cup component because it is fully contained within the outer femoral head component (e.g. within a radius of the outer femoral head component).

[0061] 55677900-1 The head recess of the outer femoral head component may comprise a recessed shoulder portion between the proximal portion and the distal portion and / or the inner femoral head component may comprise a recessed shoulder portion between the part- spherical outer bearing surface of the inner femoral head component and the generally tubular skirt.

[0062] The outer femoral head component and the inner femoral head component may be configured for snap-fit engagement.

[0063] The snap-fit engagement may serve as a locking engagement against relative longitudinal movement and / or relative off-axis movement.

[0064] The outer femoral head component may be configured for rotation about a longitudinal axis of the inner femoral head component.

[0065] The outer femoral head component and the inner femoral head component may comprise co-operative inter-engaging features.

[0066] The neck recess of the inner femoral head component may be tapered and configured for accommodating a tapered neck projection from a femoral stem component.

[0067] The tapered neck recess and / or the tapered neck projection may comprise one or more anti-rotation features to prevent relative rotation there-between.

[0068] The tapered neck recess may be provided centrally within the inner femoral head component or the tapered neck recess may be off-set from a longitudinal axis of the inner femoral head component.

[0069] The outer femoral head component may be formed of one or more of: polyethylene, LIHMWPE, cross-linked polyethylene (XLPE), polyether ether ketone (PEEK), polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material.

[0070] The inner femoral head component may be formed of one or more of: metal, ceramic.

[0071] 55677900-1 In another aspect, the inventor has found that the difference in the post-operative dislocation rates between hip resurfacing procedures and total hip arthroplasty (THA) may be due to the fact that, with hip resurfacing, the available hip capsular volume is filled whereas that is not the case with total hip arthroplasty (THA) or hemi-arthroplasty. The femoral head size in current hemi-arthroplasty is returned to normal, but the prosthetic head-neck junction and the femoral neck diameter nowhere near fill the capsule volume.

[0072] The main mechanism of action of the hip capsule is to close down like an iris and force the femoral head into the acetabulum thus reducing the chance of dislocation. If the internal volume of the hip capsule is replaced accurately on the femoral head and femoral neck as is the case in hip resurfacing, like the normal (native) hip prior to surgical interference, then the risk of dislocation is low.

[0073] The inventor therefore proposes an implant in which the femoral head and upper femoral neck have outer dimensions that more faithfully reproduce the space (i.e. volume) occupied by the hip capsule prior to surgery.

[0074] Typically, a range of femoral head and neck components are provided to match the outer dimensions of the natural femoral head and upper femoral neck for each patient. A range of head-neck outer (e.g. polymer) components ranging in diameter from 36mm to 64mm are provided with increasing neck diameters to also match the outer dimensions of the natural femoral necks for each patient size. Typically, for THA, femoral head components are provided in 2mm increment sizes to be able to match each patient’s native anatomy. For hemi-arthroplasty, typically femoral head components are provided in 1mm increments to more accurately match the acetabular articular cartilage. However, if the sizing of the prosthetic components does not take into account the internal volume of the hip capsule then high dislocation rates will occur.

[0075] The inventor has also done some work looking at ion production in the blood serum of patients who have had ceramic on ceramic THA's compared to normal pre-operative controls (see Fig 5). Through this work, there is clear evidence of damage and wear to the titanium alloy femoral components of the neck taper, with significant elevation of titanium and aluminium ions being detected in the THA recipient’s serum.

[0076] 55677900-1 It is therefore an aim of the of the present disclosure to provide a prosthetic head, neck and stem for use in hip arthroplasty that may reduce the post-operative dislocation rate and / or reduce the risk of damage or wear to the femoral components. As far as possible, 12 / 14 head / neck taper cones have been designed out.

[0077] In relation to this aspect, the applicant presents two different designs of hip arthroplasty with a dual mobility bearing using (e.g. cross-linked polyethylene (XLPE)) outer head articulating on a (e.g. metal) inner femoral head. Both these designs have coincident centres of rotation like the original French design as in Fig 2A. However, two different approaches are taken to prevent tipping of the XLPE head into varus like Fig 2B. In the Ultra Short Stem (USS) hip replacement the native neck and the trumpet shape headneck junction are preserved and only the femoral head is removed. As will be seen later, the USS has a collar for bone fixation on the under surface of the collar and the XLPE head articulates with the polished top surface of this collar allowing free rotation but preventing tipping over into varus. The femoral head diameter is close to native size and of course the head-neck junction and the femoral neck are native. The hip capsule volume will be filled giving a stable hip joint. The problem taper junction is eliminated and instead a small integral inner metal femoral head has an outer XLPE head component snap fitted onto the inner metal femoral head. This will provide an ultra-conservative hip arthroplasty for the very young and active patients.

[0078] In the THA design and hemi arthroplasty design presented later, the femoral head and half the femoral neck are resected as is common in conventional THA. However, the difference is that the prosthetic (metal) femoral neck diameter and the inner (metal) femoral head diameter are the same, typically 22.225mm. Here the inner (metal) head and outer (XLPE) head centres are coincident. A different mechanism is used with this implant to prevent tipping of the outer (XLPE) head. In order to fill the hip capsule volume, having removed the native femoral head, the head-neck junction and half the femoral neck, not only is the femoral head replaced with the outer (XLPE) head, so too is the head-neck junction and the upper part of the femoral neck is also replaced with an integral downward skirt from the outer (XLPE) head component. The longitudinal cavity in the centre of the XLPE articulates with free rotation on the metal inner head and neck but tipping over of the head-neck XLPE component is prevented. This design of THA implant having filled the hip capsule volume should have a dislocation rate close to hip resurfacing of 1 :1 ,000.

[0079] 55677900-1 According to one aspect of the present disclosure, there is provided a dual mobility prosthetic bearing for use in total hip arthroplasty or hemi arthroplasty with an inner (e.g. metal) femoral head diameter typically 22.225mm (7 / 8”) and inner (e.g. metal) femoral neck of the same diameter with a recess at the inner head / neck junction for snap fitting on an outer (e.g. polymer) head and neck replacement. An outer (e.g. polymer) head and neck replacement is proposed that comes in typically 1mm increments with the aim of reproducing the native femoral head and neck dimensions so that each patients capsule volume is filled by the replacement thus providing hip stability to reduce dislocation. The outer (e.g. polymer) head and neck replacement rotates freely on the inner (e.g. metal) head and neck part.

[0080] According to an aspect of the present disclosure, there is provided a (outer) femoral head component having a part-spherical outer bearing surface and a recess forming a part- spherical inner bearing surface; a (inner) femoral neck component having a proximal portion configured to be received within the recess; the proximal portion having a part-spherical outer bearing surface (e.g. inner head) configured for bearing engagement with the part-spherical inner bearing surface of the (outer) femoral head component; wherein the part-spherical outer bearing surface of the (outer) femoral head component and the part-spherical outer bearing surface of the proximal portion (e.g. inner head) have a coincident centre of rotation; and wherein an extension is provided on the (outer) femoral head component and / or the femoral neck component to retain the femoral head component on the femoral neck component during use.

[0081] Thus, embodiments of this disclosure provide a dual mobility prosthetic bearing that may be used in any hemi-arthroplasty or total hip arthroplasty (THA) in order to reduce a risk of post-operative dislocation.

[0082] The extension may be configured to prevent the femoral head component from non-axial (e.g. off-axis I transverse) rotation with respect to a longitudinal axis of the femoral neck component. Thus, the extension may be configured to prevent the femoral head component from sliding or rotating into a varus or valgus or anteversion / retroversion position where it may be more likely to dislocate out of the acetabulum.

[0083] 55677900-1 The extension may be provided substantially around a circumference of the femoral head component and / or the femoral neck component.

[0084] The extension may comprise a skirt depending from the femoral head component. The skirt may be configured to extend along a distal portion of the femoral neck component. The skirt may have a maximum diameter of less than a maximum diameter of the part- spherical outer bearing surface so as reduce a risk of impingement of the skirt portion on the acetabulum or acetabular cup edge. In use, the skirt may be configured to extend over substantially an entire length of the femoral neck component to substantially fill a hip capsular volume.

[0085] The skirt may have a maximum diameter that is at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% or at least 95% of the maximum diameter of the outer bearing surface of the femoral head component in order to adequately fill the hip capsular volume. The aim is to replace the femoral head diameter to match the native head diameter and replace the trumpet shaped head - neck junction and neck diameter to match native in that patient size.

[0086] A distance from a centre of the coincident centre of rotation to the edge of the skirt may be greater than a radius of the outer bearing surface of the femoral head component. For example, the edge of the skirt may be at least 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50% greater than the radius of the outer bearing surface of the femoral head component.

[0087] The polyethylene skirt portion may be configured to extend over the distal portion of the inner metal femoral neck component to effectively increase the diameter of the femoral neck.

[0088] The femoral head component may comprise a recessed shoulder portion between the part-spherical outer bearing surface and the skirt to match the native head neck junction. The recessed shoulder portion may serve to reduce a risk of impingement of the prosthetic on the acetabular cup edge. The recessed shoulder portion may serve to

[0089] 55677900-1 provide a gentle transition between the part-spherical outer bearing surface and the skirt forming the neck.

[0090] In some cases, e.g. for USS hip replacement, the extension may comprise a collar extending transversely with respect to a longitudinal axis of the femoral neck component. The collar may form a proximal end of a flared trumpet portion of the femoral neck component.

[0091] Both the USS and the design of THA presented both only allow rotation at the inner bearing. This is very advantageous as the motion at the inner bearing is termed noncrossing path motion. Non-crossing path motion has been described in rotating platform knee replacements from Leeds UK and has the lowest wear of any articulation involving metal on polyethylene or crosslinked polyethylene (XLPE). The background is that however perfect manufacturing of metal or ceramic articulating partners are, there are always asperities and imperfections. In non-crossing path motion, if an asperity causes a wear track of the polyethylene countersurface, with non-crossing path motion, the asperity keeps articulating in the same wear path without causing any extra wear. The applicants believe that these two designs of hip arthroplasty will have minimal wear at the inner bearing. The inner bearing is smaller than the outer bearing and by definition will have lower frictional torque, so preferentially all rotation in the hip joint will occur at the inner bearing. This minimises the movement required at the outer bearing. In stark contrast is conventional dual mobility joints. The movements at both the inner and outer bearings are random with crossing path motions, with higher overall polyethylene wear resulting.

[0092] In some cases, e.g. for USS hip replacement, the femoral component may comprise a stepped stem including a plurality of discrete steps located between a tip and a base of the stepped stem. The steps may be concentrated more towards the base than towards the tip. The steps may be distributed along the body such that the distance between each adjacent step gradually decreases towards the base. The steps may comprise the same widths or may have widths that increase towards the base. A surface of the steps may be configured for porous ingrowth to aid bone attachment after press-fitting in the femoral neck.

[0093] 55677900-1 The effect of this arrangement of steps is that the most distal parallel-sided section guides the placement of the step above and so on as the stem is inserted. This creates a self-aligning stem. This is necessary because the density of cancellous bone is rarely homogenous all the way around the stem. Without the self-aligning effect of the steps, lateral, medial, anterior or posterior drift of the stem could occur with stem impaction.

[0094] The inner (metal) femoral head and neck component and the outer (e.g. polymer) femoral neck component may be configured for snap-fit engagement. The snap-fit engagement may serve as a locking engagement against relative longitudinal movement (thereby ensuring the centres of rotation of the part-spherical outer bearing surfaces remain coincident).

[0095] The outer (polymer) femoral head and neck component may be configured for rotation only with respect to the inner (metal) femoral head and neck component. The outer femoral head and neck component may be configured for rotation about a longitudinal axis of the inner femoral head and neck component.

[0096] The polymer femoral head and neck component and the metal femoral head and neck component may comprise co-operative inter-engaging features. For example, an annular groove at the metal head -neck junction is provided for snap fitting of the polymer head and neck component such that rotation of the polymer on the metal is permitted, but longitudinal distraction of the polymer on the metal head- neck part is prevented.

[0097] The femoral neck component may be provided in the form of an ultra short stem (USS) femoral component where the femoral head is removed but the head-neck junction and the native neck are retained.

[0098] In other examples, the inner femoral head and neck component may be provided integral with the femoral stem component for use in cemented stem THA. A monoblock femoral metal inner head, neck and stem component machined and polished from forgings of high nitrogen stainless steel is viable in cemented THA which is particularly applicable in elderly people with femoral neck fracture and poor bone quality. Each size of femoral stem comes with a size specific length of femoral head and neck. If at trial reduction, the leg length is too long, the surgeon has choices. The surgeon may elect to site the femoral stem at a lower position in the femoral canal to cement the femoral component if room

[0099] 55677900-1 in the femoral canal is available. If room is not available, the surgeon may select a one size smaller stem to trial. This one size smaller stem will have a corresponding shorter head and neck portion. The surgeon can then identify at trial reduction exactly where this smaller stem will be sited in the femoral canal to give exactly the correct leg length. In addition minor abnormalities of neck version can corrected with cemented stems by rotating the position of the stem in the femoral canal. In summary, cemented stems can be carefully selected to give correct leg length and version so no modularity is required.

[0100] This is totally different with uncemented stems where a tight press-fit of the stem in the femoral bone is required to get osseo-integration. The ideal position of an uncemented stem in the femur to get stable fixation and osseo-integration may be wrong from a leg length, offset or version viewpoint. There is a need for modularity therefore to optimise leg length, offset and version with uncemented hip stems. Lessons must be learned from history to prevent continuing to make the same mistakes going forward.

[0101] The most successful taper junction in the history of THA is the S-ROM™ stem taper junction from DePuy™. The 30 year results of this stem taper junction are superb with taper junction failure almost unheard of. The characteristics of this stem taper are noteworthy. The material of the stem and the sleeve are both titanium alloy. The length of the taper junction is 38mm in all stem sizes. Offset between the stem and sleeve is present always so the long Morse taper junction is capable of withstanding long term torsional loading. The maximum diameter of the Morse taper junction varies from 12mm in small sizes to 18 mm in large sizes. The applicants view is that the length of the Morse taper junction is decisive in its success in the S-ROM™ stem.

[0102] In complete contrast is the history of the almost ubiquitous 12 / 14 modular head / neck taper junction. If enough detail is examined this short <12 mm long taper junction has been a failure. The applicants view is that modular 12 / 14 head / neck tapers will soon disappear from the THA armentarium.

[0103] In order to design a successful taper junction for uncemented THA, the applicant has increased the length of the taper junction as much as anatomy constraints will allow and in excess of 30mm long.

[0104] 55677900-1 An uncemented stem would ideally be manufactured from titanium alloy with a suitable bone ingrowth surface. Ideally, the modular head and neck components would also be titanium alloy from a taper wear and corrosion viewpoint. However, polished titanium alloy on the outer head and neck is not a good wear partner against polymers including XLPE. History shows that Titanium Nitrite coating from two particular vendors have successful 20+ year outcomes. Titanium Nitride coating from other vendors have a less good history.

[0105] The other option for a long modular THA neck junction is to manufacture the outer modular head and neck part from Zirconium or Zirconium alloy. History shows that this would be successful at the taper junction. In order to make the outer modular head and neck a good bearing material against a head of XLPE, the outer surface would need to be hardened by oxidation.

[0106] The option of including a rounded corner hexagon shape to resist rotational forces is available. The history of the long S-ROM™ taper shows that this is not necessary. Also, particularly the inner long taper with a hexagon shape is difficult to accurately manufacture.

[0107] The femoral neck component may be generally longitudinal comprising the proximal portion at one end and a distal portion at an opposite end.

[0108] In a modular example, the femoral head-neck component may comprise a generally tubular component having the proximal portion at one end thereof and a tapered recess at an opposite end thereof, for accommodating a tapered neck projection from a femoral stem component. Different lengths of the femoral head-neck component may be provided for use with the same or different femoral stem components to suit different sizes of patients and different leg lengths.

[0109] The tapered recess and / or the tapered neck projection may comprise one or more antirotation features to prevent relative rotation there-between.

[0110] The tapered recess may be provided centrally within the femoral neck component or may be off-set from a longitudinal axis of the femoral neck component. As such, a femoral

[0111] 55677900-1 neck component may be selected to alter one or more of: varus / valgus and a nteve rs i o n / re trove rs i o n .

[0112] The part-spherical outer XLPE bearing surface of the outer femoral head and neck component may be configured for articulation in a patient’s acetabulum or in a prosthetic acetabular cup.

[0113] The femoral head component may comprise a polymer. In particular, the part-spherical inner bearing surface may comprise a polymer. In some cases, the part-spherical outer bearing surface of the femoral head component may comprise a polymer for articulation on a ceramic or metal acetabular surface but, in other cases, the femoral head component may comprise a metal or ceramic cap forming the part-spherical outer bearing surface for articulation on acetabular cartilage in hemi arthroplasty. .

[0114] The part-spherical outer bearing surface of the femoral head component may be hemispherical or slightly more than hemi-spherical.

[0115] The prosthetic head component may be formed of a polymer material. The part-spherical outer bearing surface of the femoral head component and the skirt (or at least a majority thereof) may be formed as an integral component, for example by compression moulding or the femoral head component may be machined from a compression molded polymer.

[0116] In the case of total hip arthroplasty (THA), the part-spherical outer bearing surface of the femoral head component may have a polymer articulating surface. This may be suitable for articulation with a metal or ceramic surface of an acetabular cup component.

[0117] The femoral head component may be formed of one or more of: polyethylene, LIHMWPE, cross-linked polyethylene, polyether ether ketone (PEEK), and polyimide (or more correctly, a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material as described in US6,686,437).

[0118] Notably, grinding wheel tests have shown that polyimide material (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) exhibits little to no wear. Thus, instead of having to mould polyethylene into a ceramic head of 1mm increment heads and machining and crosslinking to XLPE, which is a very

[0119] 55677900-1 expensive process - it is proposed to machine the femoral head component and / or the femoral neck component out of polyether ether ketone (PEEK) or polyimide bar stock.

[0120] From a health economics viewpoint, no country would find the cost of ceramic top hemiarthroplasty acceptable as 40% of the patient’s requiring this procedure have died by the one year post-operative period. However, polyether ether ketone (PEEK) or polyimide materials may present a more cost-effective and still wear resistant option.

[0121] Polyether ether ketone (PEEK) or polyimide (or more correctly, a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) wears well as a head against polyethylene and XLPE and so it acts like a ceramic or a metal head. Polyether ether ketone (PEEK) and polyimide also wears well against metal and ceramic and so it acts like XLPE. Polyether ether ketone (PEEK) and polyimide performs well in an abrasion test and so it behaves like a ceramic. Polyether ether ketone (PEEK) and polyimide has much better mechanical properties that polyethylene. That gives a wide range of unique use cases even just considering the hip joint.

[0122] The BHR™ implant manufactured by Smith and Nephew Inc. works well as a conservative joint replacement implant giving high function and a very low dislocation rate 1 :1 ,000. This is better that 10 times lower than any current THA design. The BHR™ implant generally comprises a part-spherical hollow head for implantation over a partially resected femoral head. It comprises a narrow longitudinal stem for fixing the head in the femoral neck of a patient.

[0123] However, the BHR™ implant is designed for metal on metal articulation and it produces metal ions causing an elevation of blood levels of Cobalt and Chromium ions. Metal ion production has become unpopular among the surgical community. However a use case for polyether ether ketone (PEEK) or polyimide (or more correctly, a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) would be to substitute the cemented Cobalt Chrome femoral head shell of the BHR with a cemented polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) femoral head shell and metal ion production would no longer be problem.

[0124] According to an aspect of the disclosure there is provided a hip resurfacing implant (e.g. of the form of the BHR™ implant) comprising an articular head comprising polyether

[0125] 55677900-1 ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material.

[0126] As described in this document with reference to Figures 2A-2B, there is widespread and increasing use of traditional dual mobility XLPE heads against predominantly a metal cup articular surface but, rarely, also against a ceramic articular surface or an oxidised zirconium alloy articular surface. As described in this document, a very worrying complication arising is Intra Prosthetic Dissociation (IPD) at relatively short term followup. Since impingement of the traditional dual mobility XLPE head edge on the femoral neck is a designed in feature of this implant type, it is the applicants opinion that the rate of IPD will inevitably increase with increased duration of follow-up.

[0127] Replacement of XLPE femoral head parts in the traditional design of dual mobility articulations with polyether ether ketone (PEEK) or Polyimide (or more correctly, a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) would reduce the risk of IPD as the much tougher polyether ether ketone (PEEK) or Polyimide (or more correctly, a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) will be much more resistant to polymer edge deformation and wear and loss of snap fit from recurrent edge impingement.

[0128] According to an aspect of the disclosure there is provided a dual mobility bearing comprising an articular head comprising polyether ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material.

[0129] Dual Mobility Metal or Ceramic femoral heads or fixed Metal or Ceramic heads as a Hemi Arthroplasty in Fractured neck of Femur Population.

[0130] With excellent wear and exceptional abrasion resistance polyether ether ketone (PEEK) or polyimide (or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) articulating on acetabular cartilage or in the event of wear through of the acetabular cartilage, polyether ether ketone (PEEK) or polyimide (or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) would likely offer a low cost and durable articulation on acetabular bone. Low

[0131] 55677900-1 cost is particularly important in the elderly fractured neck of femur population where 40% of hemi-arthroplasty patients are deceased at one year follow-up.

[0132] The easiest way for US manufacturers to incorporate polyether ether ketone (PEEK) or polyimide (or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) into their hemi arthroplasty range for the fractured neck of femur population would be to mount a polyether ether ketone (PEEK) or polyimide (or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) articulating surface onto either an existing ceramic or metal head of a dual mobility hemi arthroplasty. The low cost will be apparent as currently a dual mobility bearing does not exist for hemi arthroplasty as neither conventional polyethylene nor XLPE can be used for articulation against cartilage or bone. Instead, currently a bipolar hemi arthroplasty has to be manufactured and sold in 1 mm increments. This bipolar hemi arthroplasty has a small modular metal or ceramic head cone fixed to the neck of a THA femoral stem. The bipolar head implant has a polished metal or ceramic head shell with either conventional polyethylene or XLPE either snap-fitted or moulded into the metal head shell.

[0133] A polyether ether ketone (PEEK) or polyimide (or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) articulating head may be machined from bar stock keeping cost low. In which case, no metal or ceramic head shell has to be manufactured from metal or ceramic in 1 mm increments thus saving on expensive manufacturing operations.

[0134] According to an aspect of the disclosure there is provided a dual mobility bearing for hemi-arthroplasty comprising an articular head comprising polyether ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non- halogenated, aromatic polyimide material.

[0135] According to an aspect of the disclosure there is provided an outer femoral head for hemi-arthroplasty comprising an articulating surface comprising polyether ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non- halogenated, aromatic polyimide material.

[0136] 55677900-1 In this disclosure we have described an all XLPE femoral head component to be articulated with rotation only on a metal femoral neck component. On the acetabulum the femoral head has to be articulated with a metal or ceramic acetabular cup.

[0137] According to an aspect of the disclosure there is provided a prosthesis for THA comprising a metal or ceramic acetabular cup and a dual mobility bearing according to the first aspect of the disclosure, wherein the femoral head component comprises XLPE and the femoral neck component comprises metal.

[0138] In this disclosure we have described a hemi arthroplasty femoral head with an XLPE body moulded into either a ceramic or metal cap / shell. This would work well as a hemi arthroplasty in the fractured neck of femur population.

[0139] Rotation only of the inner bearing reduces wear at the outer bearing.

[0140] However this implant may be superseded by a machined polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) femoral head component on the femoral neck component, mainly on account of cost.

[0141] According to an aspect of the disclosure there is provided a prosthesis for hemi arthroplasty comprising a dual mobility bearing according to the first aspect of the disclosure, wherein the outer femoral head component comprises either a XLPE body and a metal or ceramic articulating cap; or a polyether ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material. In this case, the inner femoral head and neck component will be made from metal.

[0142] In this disclosure we have described an USS implant with a XLPE femoral head articulating on a metal or ceramic articular surface.

[0143] According to an aspect of the disclosure there is provided an USS implant for THA comprising a metal or ceramic acetabular cup and a dual mobility bearing according to the first aspect of the disclosure, wherein the femoral head component comprises a XLPE body.

[0144] 55677900-1 According to an aspect of the disclosure there is provided an USS implant for THA comprising a stepped stem portion and a fixed articular head portion wherein the fixed articular head portion terminates in a flat annular base extending radially from a proximal end of the stepped stem.

[0145] The USS implant may be formed of polymer (e.g. PEEK), metal or ceramic. For example, the USS implant could comprise PEEK injection moulded into a stem shell (which may be stepped) of metal such as trabecular titanium for bone ingrowth. The fixed articular head portion may be solid or hollow.

[0146] The stepped stem may include a plurality of discrete steps located between a tip and a base of the stepped stem. The steps may be concentrated more towards the base than towards the tip. The steps may be distributed along the body such that the distance between each adjacent step gradually decreases towards the base. The steps may comprise the same widths or may have widths that increase towards the base. A surface of the steps may be configured for porous ingrowth to aid bone attachment after pressfitting in the femoral neck.

[0147] Spacer implants

[0148] Spacer implants have particular use in medial compartment knee arthritis. The McKeever metal spacer was used in the US in the 1950's and the Unispacer metal implant was used in the early 2,000's. Occasional spectacular successes have been observed giving hope for this conservative type of treatment in young patients. Both the McKeever and Unispacer implants were off the shelf implants and poor fit was a recurring problem.

[0149] Today it is easy to define the size and shape a spacer implant should be from MRI scans or CT scans. But it is a problem getting a patient specific defect size and shape manufactured, particularly in ceramic but also metal. Polyether ether ketone (PEEK) or Polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) may come into its own when the patient specific spacer implant can be machined from this wear and abrasion resistant material without any need for polishing.

[0150] 55677900-1 Medial compartment arthritis of the knee is given as an example for a candidate joint for a custom patient specific spacer implant. It is likely that polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) has a strong use case to assist repair of articular surface damage in many different synovial joints.

[0151] According to an aspect of the disclosure there is provided a spacer implant comprising polyether ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material.

[0152] The part-spherical outer bearing surface of the femoral head component may comprise a metal or ceramic articulating surface. This may be useful in the case of hemiarthroplasty, where the original acetabulum is retained, or total hip arthroplasty (THA).

[0153] The part-spherical outer bearing surface of the femoral head component may comprise a metal or ceramic cap including the part-spherical outer bearing surface and wherein the metal or ceramic cap is mounted on a polymer base.

[0154] The metal or ceramic cap may comprise one or more recesses and / or undercuts for attachment to the polymer base. The attachment may be by injection moulding, compression moulding or melt-fusing the polymer base to the metal or ceramic cap or by press-fitting the metal or ceramic cap onto the polymer base.

[0155] The inner femoral head and neck component may be formed of a metal.

[0156] The proximal portion may comprise an annular groove disposed at the inner (e.g. metal) head-neck junction.

[0157] The prosthetic outer head and neck component may be configured for use in hemiarthroplasty or total hip arthroplasty (THA).

[0158] According to a second aspect of this disclosure, there is provided an outer femoral head and neck component for use in hip arthroplasty comprising a dual mobility prosthetic bearing as described above.

[0159] 55677900-1 The femoral component may comprise a stem portion at a distal end thereof and the femoral neck component described above, at a proximal end thereof. In some cases, the femoral neck component may be integrated with the stem portion. In other cases, the femoral neck component described above may form a separate component having a tapered recess at a distal end thereof, for accommodating a tapered neck projection projecting from the stem portion of the femoral component.

[0160] The femoral component may be configured for use in hemi-arthroplasty or total hip arthroplasty (THA).

[0161] Cemented Stems For Distal Migration

[0162] According to a third aspect of this disclosure, there is provided a prosthetic (e.g. metal) femoral stem for cemented fixation, comprising: a head and neck portion and a stem portion; the stem portion having a distal tip and a concavely curved medial surface having a single radius of curvature extending from a junction between the neck portion and the stem portion to a region a third of the distance from the distal tip.

[0163] Advantageously, the curved medial surface may ensure that distal migration of the stem portion does not result in medial translation of the stem portion. Instead, lateral translation of the stem portion may be preferentially induced.

[0164] The neck portion of the prosthetic femoral stem may comprise the femoral neck component of the dual mobility prosthetic bearing described above. In some cases, the neck portion may form a tapered neck projection for engagement in a tapered recess at a distal end of a modular femoral neck component, particularly useful with uncemented stems as described above.

[0165] Any features described above may also apply to the third aspect of the disclosure.

[0166] In some cases, PEEK could be injection moulded into an all in one stem, neck and head PEEK hemi-arthroplasty for elderly patients (who have a 40% mortality at 1 year). The implant may include a curved stem shape as described herein and the risk of stem fracture would be low as the activity level in these elderly patients would be very low. Such a stem may be cemented as described herein.

[0167] 55677900-1 If the clinical experience of a PEEK cemented one piece hemiarthroplasty in the fractured neck of femur population worked out well and there were no fractures of stems or no excess wear of heads against acetabular cartilage, a curved cemented modular stem as described herein could be formed of PEEK with a modular snap on XLPE head and outer neck and a PEEK modular cup injection moulded into a metal (e.g. trabecular titanium) thin shell to give a porous coating on the acetabular reamed bone.

[0168] Another option may comprise using Porcine or Bovine skin collagen as an interposition material between an arthritic femoral head and an arthritic acetabulum. This may be particularly useful in young patients to stop arthritic bone surfaces from rubbing and causing pain.

[0169] Furthermore, a one piece hemi-arthroplasty in PEEK could be inserted in an arthritic hip. Instead of reaming and inserting a traditional acetabular component - one, two or three sheets of Porcine or Bovine skin collagen could be inserted into the acetabulum and sutured to the surrounding acetabular soft tissues to stop the PEEK head rubbing on painful arthritic acetabular surfaces.

[0170] Advantages

[0171] In summary, aspects of the disclosure aim to provide a modified dual mobility prosthetic bearing for use in hip arthroplasty that may help to: a) reduce dislocation risk of hemi-arthroplasties and THA's; b) reduce intraprosthetic dissociation risk in dual mobility bearings; c) reduce the use of metal or ceramic heads connected to 12 / 14 neck taper cones of hemi-arthroplasties and THA's, which in turn will reduce patient exposure to Aluminium (Aluminum), thus reducing the risk of dementia in patient recipients; and d) reduce wear of the femoral head component of the modified dual mobility prosthetic.

[0172] Brief Description of the Drawings

[0173] Some embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which:

[0174] 55677900-1 Figure 10 shows a cross-sectional view of an Ultra Short Stem (USS) hip prosthesis comprising a dual mobility bearing according to the disclosure, when implanted in a femoral neck of a patient.

[0175] Figure 11 shows cross-sectional views of a range of differently sized USS hip prostheses of the same design as that shown in Figure 10.

[0176] Figure 12 shows an enlarged cross-sectional view of a USS hip prosthesis of Figure 11 , having a 58mm polymer outer head diameter and a 32mm inner metal head diameter.

[0177] Figure 13 shows a cross-sectional view of a cemented stemmed THA with dual mobility bearing according to the disclosure.

[0178] Figure 14 shows a view similar to that of Figure 13 but including an acetabular cup for articulation with the femoral head component.

[0179] Figure 15 shows a cross-sectional view of another design of a dual mobility bearing according to the disclosure, which could be configured for articulation on a polymer cup articular surface or could be used as a hemi- arthroplasty articulating on acetabular cartilage when conventionally it is termed bi polar hemi arthroplasty.

[0180] Figure 16 shows an enlarged view of a portion of a metal or ceramic cap from the femoral head component of Figure 15.

[0181] Figure 17 shows a cross-sectional view of a further design of a modular neck dual mobility prosthetic bearing according to the disclosure.

[0182] Figure 18A shows a cross-sectional view of an offset neck design of a dual mobility bearing according to the disclosure.

[0183] Figure 18B shows a cross-sectional view of a neck component with a hexagonal form.

[0184] Figure 19 shows forced distal migration of a known prosthetic femoral stem component, during use.

[0185] Figure 20 shows forced distal migration of a prosthetic femoral stem component, according to the disclosure, during use.

[0186] Figures 21A, B and C show, respectively, the prosthetic femoral stem component of Figure 20, the differently curved side portions of the prosthetic femoral stem component and the prosthetic femoral stem component when implanted in a resected femur.

[0187] Figure 22 shows an exploded view of a dual mobility prosthetic bearing for use in hip arthroplasty, including an acetabular cup component and a femoral neck portion of a femoral stem prosthesis.

[0188] Figure 23A shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component is 26mm,

[0189] 55677900-1 the maximum diameter of the outer femoral head component is 32mm, and the neck recess is configured for a minus 3mm leg length reduction.

[0190] Figure 23B shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component is 26mm, the maximum diameter of the outer femoral head component is 32mm, and the neck recess is configured for a plus 6mm leg length extension.

[0191] Figure 24A shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component is 26mm, the maximum diameter of the outer femoral head component is 40mm, and the neck recess is configured for a minus 3mm leg length reduction.

[0192] Figure 24B shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component is 26mm, the maximum diameter of the outer femoral head component is 40mm, and the neck recess is configured for a plus 12mm leg length extension.

[0193] Figure 25A shows a longitudinal cross-sectional view of the outer femoral head component of Figure 22.

[0194] Figure 25B shows a longitudinal cross-sectional view of the inner femoral head component of Figure 22.

[0195] Figure 26A illustrates initial insertion of the inner femoral head component of Figure 25B into the outer femoral head component of Figure 25A.

[0196] Figure 26B shows the completed insertion of the inner femoral head component of Figure 25B in the outer femoral head component of Figure 25A.

[0197] Figure 27 shows a longitudinal cross-sectional view of another Ultra Short Stem (USS) hip prosthesis comprising a dual mobility bearing according to the disclosure, when implanted in a femoral neck of a patient.

[0198] Figure 28 shows a longitudinal cross-sectional view of a further Ultra Short Stem (USS) hip prosthesis, similar to that of Figure 27 but with a solid head, when implanted in a femoral neck of a patient.

[0199] Detailed Description of Embodiments

[0200] Generally speaking, the disclosure provides various examples of femoral heads, necks and stems, which have been designed to reduce a risk of post-operative dislocation following a hemi-arthroplasty or a total hip arthroplasty (THA) procedure or revision surgery.

[0201] 55677900-1 Figure 10 shows a cross-sectional view of an Ultra Short Stem (USS) hip prosthesis 1000 comprising a dual mobility bearing 1100 according to the disclosure, when implanted in a femoral neck 1200 of a patient. Figure 10 is based on a computed tomography (CT) scan of a proximal end of a femur 1202 and includes a head resection line R. A cortical bone 1204 outline of the femur 1202 is indicated from a coronal slice on the CT scan, while the cancellous bone in the femur 1202 and femoral neck 1200 has been removed for clarity.

[0202] The now popular technique of preoperative CT scan and use of a surgical robot makes accurate resection of the femoral head possible. Early resection of the femoral head makes this USS procedure possible for surgeons who are not used to an extensile exposure. For limited access exposures, like a direct anterior approach, removal of the femoral head allows access to ream the acetabulum and insertion of the acetabular cup (not shown).

[0203] As best shown in Figure 12, the USS stem with a dual mobility bearing 1100 comprises a polymer femoral head component 1102 having a part-spherical outer bearing surface 1104 and a recess forming a part-spherical metal inner bearing surface 1106 and a femoral component 1110 having a proximal portion 1112 configured to be received within the recess. The metal proximal portion 1112 has a part-spherical outer bearing surface 1114 configured for bearing engagement with the part-spherical inner bearing surface 1106 of the polymer femoral head component 1102. The part-spherical outer bearing surface 1104 of the femoral head component 1102 and the part-spherical outer bearing surface 1114 of the proximal portion 1112 have a coincident centre of rotation C. In this example, a collar 1116 (which forms an extension) is provided on the femoral component 1110 to support the femoral component 1102 on the bony femoral neck component 1110 during use. The extension 1116 comprises a collar 1118 extending transversely with respect to a longitudinal axis of the femoral neck component 1110. The collar 1118 forms a proximal end of a flared trumpet portion 1120 of the femoral neck component 1110. A surface of the trumpet portion 1120 may be configured for porous ingrowth to aid bone attachment after press-fitting in the femoral neck 1200.

[0204] Like hip resurfacing, a near normal size femoral head component 1102 is used and the tapered head-neck junction of the patient’s femoral neck 1200 are retained as shown in

[0205] 55677900-1 Figure 10. Like with hip resurfacing, the hip capsule is filled so a very low dislocation rate is expected.

[0206] Figure 11 shows cross-sectional views of a range of differently sized USS hip prosthetics 1000 of the same design as that shown in Figure 10. Each USS hip prosthetics 1000 comprises a dual mobility prosthetic bearing 1100 as described above including a femoral head component 1102 and a femoral neck component 1112. The part-spherical outer bearing surface of the femoral head component 1102 and the part-spherical outer bearing surface of the proximal portion of the femoral neck component 1112 have a coincident centre of rotation C. In Figure 11 , the diameter of each of the proximal portions and the corresponding femoral head components 1102 are indicated in millimeters (mm), from a 22mm proximal portion to a 32mm proximal portion and from a 40mm femoral head component 1102 to a 58mm femoral head component 1102. From experience, it is believed that this range of sizes will cover over 99% of all patients.

[0207] Notably, as shown in Figure 11 , larger implants 1000 have larger sized femoral heads 1102 to maintain the co-incident centres of rotation C. In addition, the gradually increasing head sizes help to position the collars precisely on the resected femoral head.

[0208] Figure 12 shows an enlarged cross-sectional view of a USS hip prosthesis 1000 of Figure 11 , having a 58mm outer head diameter and a 32mm inner head diameter. In this case, the inner head is hemi-spherical and the outer head is greater than hemi-spherical for secure attachment to the femoral neck component 1110.

[0209] The femoral head component 1102 is formed of polymer, which may comprise polyethylene, XLPE, polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material). The femoral neck component 1110 may be formed of metal. In use, the femoral head component 1102 may articulate on a thin metal or ceramic acetabular cup surface (not shown).

[0210] The implant 1000 comprises a short stem 1130 configured to extend through the femoral neck 1200 but not along the length of the femur 1202. A distal end of the stem 1130 includes anti-rotational splines 1132 in the form of parallel-sided longitudinal channels 1134 terminating at a proximal end in a transverse step 1136.

[0211] 55677900-1 The femoral head component 1102 and the femoral neck component 1110 comprise cooperative inter-engaging features. In this case, the femoral neck component 1110 comprises an annular groove 1122 between the extension 1116 and the part-spherical outer bearing surface 1114 of the proximal portion 1112 and the femoral head component 1102 comprises an annular projection 1124 received in the annular groove 1122. Thus, the outer (e.g. polymer) femoral head component 1102 and the femoral neck component 1110 are configured for relative rotation of the projection 1124 in the groove 1122. As such, the femoral head component 1102 is configured for rotation about a longitudinal axis of the femoral neck component 1110.

[0212] The femoral head component 1102 and the femoral neck component 1110 are configured for snap-fit engagement of the projection 1124 in the groove 1122. The snap- fit engagement serves as a locking engagement against relative longitudinal movement (thereby ensuring the centres of rotation C of the part-spherical outer bearing surfaces 1104, 1114 remain coincident).

[0213] A selected size of a known dual mobility polyethylene bearing to be used for an operation is snap-fitted by the surgical team onto the femoral head in the operating room. This is possible because the deformation required for the snap fit is only approximately 1 ,25mm. A greater snap fit deformation would likely reduce IPD but would make it very difficult or impossible to assemble the bearing in the operating room.

[0214] The USS implants 1000 of Figures 10-12 are designed to be factory-fitted with the polyethylene femoral head component 1102 already mounted on the metal femoral neck component 1110. This allows a 2.5mm snap fit to be used as the surgical team do not have to assemble the device.

[0215] Unlike current dual mobility devices, as illustrated in Figure 2B, impingement contact of the polyethylene edge of the femoral head component 1102 on metal of the femoral neck component 1110 does not occur with the present design. Rotation only movement is guided by the large area contact between the polyethylene femoral head component 1102 and the polished top surface of the collar 1118. Offset centres of rotation are not required or desirable between the metal femoral neck component 1110 and the polyethylene femoral head component 1102.

[0216] 55677900-1 The coincident centres of rotation C and the upper surface of the collar 1118 stops the polyethylene femoral head component 1102 from tipping into varus. So, with the implant 1000, the head centre is maintained, leg length is unaltered and the hip capsule is filled by the large femoral head component 1102 and the patient’s native femoral neck 1200.

[0217] For the massive numbers of generally elderly people who fall and fracture their femoral necks, retaining the head-neck junction or upper femoral neck 1200 is not possible as this typically breaks off with the proximal fragment of bone. However, it is imperative that following hemi-arthroplasty surgery, the hip must not dislocate as a dislocated hip in elderly people often results in death soon after. The present disclosure therefore provides for new design of hemi-arthroplasty that is configured to fill the hip capsule and give the greatest resistance possible to dislocation.

[0218] For elderly people with a fractured neck of femur, generally a cemented stem is best and allows early full weight bearing without the hazard of an uncemented stem splitting the weak bone of the femoral shaft. An early peri-prosthetic fracture in an elderly frail person is also a death sentence. Such patients must get out of bed soon after surgery and walk to sustain life.

[0219] Getting the correct leg length is important. Each femoral stem has a neck length that is on average the correct length for that size of stem. However, a cemented stem gives the surgeon freedom to either lengthen or shorten the leg. If the leg is too long on trial reduction, two options are available. It is usually possible in these patients with a capacious femoral canal to site the position of the femoral stem lower down in the cement mantle.

[0220] However, if the trial stem is too tight in the prepared femoral canal, then a size smaller femoral stem trial is selected. This stem can be sited lower in the cement mantle, thus shortening the leg. However, this smaller stem will have a femoral neck length which is shorter than with the larger sized stem, again giving the surgeon freedom to get the leg length correct for that patient. This is discussed in detail previously.

[0221] 55677900-1 Some surgeons might object that a cemented stem design leads to massive embolisation into the right heart and lungs resulting in 'fat embolism syndrome'. This author has taught a method for suction venting of the femur below a hip resurfacing femoral component which has been shown to completely eliminate embolisation into the right side of the heart. A method for suction venting of the femoral canal in THA surgery is the subject of another US patent application.

[0222] Figure 13 shows a cross-sectional view of a cemented stem THA with a monoblock inner femoral head and neck. A dual mobility polymer bearing 2000 has been fitted according to the disclosure. The dual mobility prosthetic bearing 2000 comprises a femoral head component 2200 having a part-spherical outer bearing surface 2202 and a recess forming a part-spherical inner bearing surface 2204; and a femoral neck component 2300 having a proximal portion 2302 configured to be received within the recess. The proximal portion 2302 has a part-spherical outer bearing surface 2304 configured for bearing engagement with the part-spherical inner bearing surface 2204 of the femoral head component 2200. The part-spherical outer bearing surface 2202 of the femoral head component 2200 and the part-spherical outer bearing surface 2304 of the proximal portion 2302 have a coincident centre of rotation C.

[0223] In this example, an extension 2400 is provided on the femoral head component 2200 to retain the femoral head component 2200 on the femoral neck component 2300 during use. The extension 2400 is in the form of a skirt depending from the femoral head component 2200 and including a recess 210 configured to receive a tubular distal portion 2306 of the femoral neck component 2300. The skirt 2400 has a maximum diameter of less than a maximum diameter of the part-spherical outer bearing surface 2202. As shown, the skirt 2400 is configured to extend over substantially an entire distal portion 2306 of the femoral neck component 2300 to substantially fill a hip capsular volume and designed to approximate to the outer dimensions of that size of bony head neck junction and femoral neck (not shown).

[0224] The recess of the femoral head component 2200 is configured to receive substantially the entire femoral neck component 2300, during use. As shown, the recess extends beyond a center C of the femoral head component 2200. The recess comprises a tubular side surface extending from the edge of the skirt 2400 into the articulating femoral head component 2200. The tubular side surface shown is substantially cylindrical.

[0225] 55677900-1 The skirt 2400 may have a maximum diameter that is at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% or at least 95% of the maximum diameter of the part-spherical outer bearing surface 2202 in order to adequately fill the hip capsular volume.

[0226] A distance L from the centre of rotation C of the part-spherical outer bearing surface 2202 (e.g. determined by an origin of the radius R of curvature of the articulating surface) to the edge of the skirt 2400 is greater than the radius R of the articulating portion. For example, the edge of the skirt 2400 in Figure 13 is at least 35% greater than the radius R of the articulating portion,

[0227] The femoral head component 2200 in this example is formed entirely of polyethylene, although in some embodiments, the femoral head component 2200 may be formed entirely of cross-linked polyethylene, polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) or the femoral head component 2200 may comprise two or more of these materials.

[0228] The skirt 2400 is integrally formed with the articulating portion of the femoral head component 2200 and serves to provide a neck extension portion thereto. A recessed shoulder portion 2402 is provided between the articulating portion and the skirt 2400. The recessed shoulder portion 2402 serves to reduce a risk of impingement of the femoral head component 2200 on the acetabulum or acetabular cup edge, during use. The recessed shoulder portion 2402 also serves to provide a gentle transition between the articulating portion and the skirt 2400 to reduce the chances of soft tissue like synovium catching on a prominent edge.

[0229] The femoral neck component 2300 of the dual mobility prosthetic bearing 2000 is integrated into a femoral stem component 2500, which comprises a stem portion 2502 at a distal end thereof and the femoral neck component 2300 at a proximal end thereof.

[0230] 55677900-1 The proximal portion 2302 of the femoral neck component 2300 is advantageous in evenly distributing forces to or from the femoral head component 2200, through the distal portion 2306 of the femoral neck component 2300.

[0231] The femoral neck component 2300 further comprises an annular groove 2310 disposed at an extremity of the proximal portion 2302, for snap-fit engagement with an internal surface projection of the femoral head component 2200.

[0232] The stem portion 2502 is connected to the femoral neck component 2300 by an intermediate portion 2504. The intermediate portion 2504 may provide a transition between the stem portion 2502, which extends substantially in a first direction and the femoral neck component 2300, which extends substantially in a second direction intersecting the first direction. The intermediate portion 2504 may be disposed between a neck resection line 2506 at a proximal end of the stem portion 2502 and the distal end of the femoral neck component 2300.

[0233] Notably, the stem portion 2502 shown in Figure 13 is merely illustrative and any design of stem portion 2502 may be used in embodiments of the invention. For example, the femoral neck component 2300 may be provided at a proximal end of any stem portion 2502. As such, implant manufacturers could relatively easily and cheaply modify their forging tools or casting tools to reproduce the femoral neck component 2300 of Figure 13 on any of their desired stem portions 2502. Consequently, expensive bone preparation tools like rasps and broaches, which are made to match each femoral stem size and design may still be used with embodiments of the disclosure.

[0234] In some embodiments, the femoral neck component 2300 may be manufactured integrally with the stem portion 2502 using a metal such as cobalt chromium alloy or stainless steel. These two alloys are typically used for cemented stem fixation in a femur in both hemi-arthroplasty and THA. Polished surfaces of these two alloys are good metals for neck articulation against polymers such as polyethylene or cross-linked polyethylene, which may be used for the femoral head component 2200. Titanium or titanium alloys are typically used for uncemented stem fixation, but titanium or titanium alloys are not good materials for articulation on polyethylene or cross-linked polyethylene. Consequently, if titanium or titanium alloys are used for the femoral neck component 2300, they may need to be surface hardened for satisfactory articulation with

[0235] 55677900-1 a polyethylene or cross-linked polyethylene femoral head component 2200. A common method for surface hardening titanium or titanium alloys is to coat the metal surface with polished titanium nitrite giving the surface a gold colour. However, other methods of surface hardening may be applied.

[0236] The proximal portion 2302 may form a hemispherical or near hemispherical inner head of the femoral neck component 2300. This is advantageous as a direction of load on this inner head varies as the patient’s activity and gait changes during the activities of daily living. A part-spherical proximal portion 2302 means that whatever the load direction, the part-spherical proximal portion 2302 can transfer the load evenly to or from the polyethylene or cross-linked polyethylene femoral head component 2200. By contrast, if a flat top design was employed, varying load direction could cause massive load concentration and wear at an edge of the flat top (as discovered in the 1960's and 1970's where flat top designs plus poor wearing polymer materials caused accelerated wear and failure in use).

[0237] Optionally, the annular groove 2310 is provided immediately below the part-spherical proximal portion 2302. This allows for a snap fitting protrusion of polyethylene or crosslinked polyethylene to rotate within the annular groove 2310. Although the present implant is specifically designed to markedly reduce dislocation episodes, strange clinical situations can occur. For example, if a patient has a fall in the early post-operative days, the capsular repair sutures can be torn out or broken and the polyethylene or crosslinked polyethylene femoral head component 2200 could prolapse through the capsular suture line with hip dislocation. Without a snap fitting of the polyethylene or cross-linked polyethylene into a recess in the metallic femoral neck component 2300, attempts at a closed reduction could result in the femoral head component 2200 being knocked off the femoral neck component 2300. This would therefore require an open reduction of the dislocation and repair of the hip capsule. However, provision of a snap fitting for the femoral head component 2200 on the femoral neck component 2300 does not in any way inhibit free rotation of the femoral head component 2200 on the femoral neck component 2300.

[0238] In the event that the acetabular cartilage is discovered at surgery to already be badly worn, then an all XLPE femoral head component 2200 may be matched with a smooth surface thin metal or ceramic acetabular articular surface.

[0239] 55677900-1 Figure 14 shows a view similar to that of Figure 13 but including an acetabular cup 3000 for articulation with the femoral head component 2200.

[0240] The acetabular cup 3000 comprises an articulation surface 3002 for articulation with the femoral neck component 2300.

[0241] The acetabular cup 3000 may take the form of a cobalt chromium cup including a porous surface for bone ingrowth, a titanium cup with a titanium nitride polished articular surface, a ceramic cup with a porous ingrowth surface, a titanium uncemented cup with a ceramic liner or a titanium uncemented cup with an oxidised zirconium alloy liner.

[0242] In the implant of Figure 13 and 14, the diameter of the proximal portion 2302 is 22.225mm. This diameter of femoral head has been used by Sir John Charnley since early 1962 with success against polyethylene cups. Just below the hemi-circular articulating surface of the proximal portion 2302 is the groove 2310 where a ridge of polymer femoral head component 2200 rests. Here the femoral head component 2200 is snap-fitted with apparatus provided with the surgical instruments to easily achieve this. The femoral head component 2200 may be provided in 1mm increments and each head size comes with a factory fitted polymer skirt 2400 that mimics the head neck junction diameter and femoral neck diameter that is on average the correct size for each diameter of femoral head component 2200. The femoral head component 2200 freely rotates on the metal femoral neck component 2300. The groove and snap fit polymer ridge ensures that in the event of a post-operative dislocation, a closed reduction can be performed without the femoral head component 2200 falling off the metal femoral neck component 2300. Dislocation is not always the fault of the designer or the surgeon. Elderly people can fall in the early post-operative period and wrench their femoral head out of the socket before the hip capsule has had time to heal. If this highly undesirable event occurs, a closed reduction can be performed.

[0243] The outer femoral head component 2200 can only rotate on the metal femoral neck component 2300. Thus, the edge of the femoral head component 2200 does not slip around the proximal portion 2302 to impinge on the metal femoral neck with every step the patient takes.

[0244] 55677900-1 Sir John Charnley has demonstrated clearly that a 22.225mm diameter head has a lower frictional torque against polymers than any larger femoral head. This means that in this design of hip arthroplasty rotational movement will preferentially occur at the inner metal and polymer interface rather than at the outer femoral head and acetabular cartilage interface. This protects the acetabular cartilage from future wear and pain and requirement for revision surgery.

[0245] Another reason for an inner rotation only movement is to minimise wear of the polymer against the metal femoral neck component 2300. Wear of a polymer surface occurs by crossing path motion against a metal component which even having a highly polished surface does have micro-spikes on its surface. This is well known in design of total knee arthroplasty. A rotating platform total knee arthroplasty advantageously has rotation only at the polyethylene-polished tibial baseplate interface giving non-crossing path motion. The rationale is that scratching of the polymer surface by asperities on the polished metal with a rotation only movement causes the asperities to move in the same polymer wear track that has already been created, thus minimising wear.

[0246] For hemi-arthroplasty use, a polished ceramic or metal surface is provided for articulation with the acetabular articular surface. Articulation of polyethylene on the normal acetabulum in hemi-arthroplasty has been tried in the past with disastrous wear consequences. The inventor has had XLPE wear tested against an eburnated bone model in Newcastle University, UK and XLPE wear is only marginally better than non- cross-linked polyethylene and is definitely not good enough for clinical use. It is believed that a form of polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material) may prove to have less wear against eburnated bone in view of its impressive abrasion resistance.

[0247] Figure 15 shows a cross-sectional view of another femoral head component 4000 in accordance with the present disclosure. This femoral head component 4000 comprises a polymer body 4010 compression moulded into an inner head porous surface of a ceramic femoral head cap 4002. The other features of the femoral head component 4000 and the dual mobility prosthetic bearing of figure 15 are identical to those described above and not repeated here.

[0248] 55677900-1 The femoral head cap 4002 may be formed of metal or ceramic and includes an articulating surface 4004. As shown in the enlarged portion in Figure 16, the cap 4002 is provided with a plurality of projections 4006 on the inner surface, which form recesses with undercuts there-between, for locking attachment to the polymer body 4010 provided by compression moulding.

[0249] The femoral head component 4000 is suitable for hemi-arthroplasty or THA.

[0250] The cap 4002 may be formed of titanium nitride coated titanium, cobalt chromium, stainless steel or oxidised zirconium alloy.

[0251] Figure 17 shows a cross-sectional view of a further design of a dual mobility prosthetic bearing 5000 according to the disclosure. The dual mobility prosthetic bearing 5000 includes the same femoral head component 4000 as described above and a modular femoral neck component 5100. The modular femoral neck component 5100 has an outer form identical to the femoral neck component 2300 of Figure 13 and comprises a proximal portion 5102 configured to be received in the femoral head component 4000. The proximal portion 5102 has a part-spherical outer bearing surface 5104 configured for bearing engagement with the part-spherical inner bearing surface of the femoral head component 4000. The part-spherical outer bearing surface of the femoral head component 4000 and the part-spherical outer bearing surface 5104 of the proximal portion 5102 have a coincident centre of rotation C. An annular groove 5106 is provided below the proximal portion 5102 at a proximal end of the distal tubular portion 5108.

[0252] In this design, the distal tubular portion 5108 includes a central (non-offset) frusto-concial recess 5110 for receipt of a projecting tapered neck portion 5200 of a femoral stem component 5300.

[0253] This modular neck design is proposed for uncemented stems, typically titanium alloy stems. The design aims to optimise design and materials. The femoral neck component 5100 may be provided in a range of different neck lengths (i.e. having lengths of the tubular portion 5108) to enable the surgeon to adjust the patient’s leg length intra- operatively by selecting the appropriate neck length. The 22.225 mm diameter of the tubular portion 5108 offers the possibility to insert a long 12 / 14 taper on the inside of the femoral neck component 5100 with mechanical advantage.

[0254] 55677900-1 Figure 18A shows a cross-sectional view of a yet further design of a dual mobility prosthetic bearing 6000 according to the disclosure. This design is similar to that described above in relation to Figure 15 but includes an offset (angled) frusto-concial recess 6110 for receipt of the projecting tapered neck portion 5200 of a femoral stem component 5300.

[0255] In order to adjust offset, either an increase or a decrease in femoral component offset, or adjust for an increase or a decrease in femoral component anteversion, an offset recess 6110 in the femoral neck component 5100 is proposed. With an offset design, a torque is created which must be mechanically resisted. Every 12 / 14 titanium alloy neck cone has machined small circular grooves and ridges. In this new design it is proposed that the titanium alloy neck taper cone 5200 has machined in longitudinal grooves and ridges. As discussed in relation to the excellent history of the S-ROM™ modular stem connection, a long length taper is preferable. This may be achievable in this design having a long inner head and neck with an outer profile of 22.225 mm diameter. A further advantage of this modular neck is that length and offset can be optimised without altering the concentric centers of the inner and outer heads.

[0256] One material for this modular femoral neck component 5100 is zirconium or zirconium alloy. On the inside of the neck taper recess 5110, 6110 the material is non-oxidised zirconium or zirconium alloy. This is relatively soft and corrosion resistant. The longitudinal ridges on the male taper titanium alloy surface will bite into the non-oxidised zirconium surface giving resistance to rotational motion. The outer surface of the zirconium or zirconium alloy is optimised for articulation against XLPE or other suitable polymer by hardening the surface through oxidation and polishing said surface.

[0257] There will be situations where a conventional ceramic head needs to be fitted to an existing stem of the above design. Clearly a modular femoral neck component 5100 needs to be available to fit onto the existing stem neck junction and needs to provide suitable fixation to femoral head components. Options that could be provided would be that all of the femoral neck component 5100 could be manufactured from zirconium or zirconium alloy and the same tapered neck design shown in Figure 2A could be designed and manufactured for attachment to the femoral neck component 5100. For attachment to a ceramic head several options are available. A 12 / 14 neck taper could be fitted to a

[0258] 55677900-1 standard ceramic inner head taper. However, the applicant regards this as sub-optimal and a salvage situation only.

[0259] There is also the option that the whole modular femoral neck component 5100 as shown in Figures 17 and 18A is manufactured from titanium alloy. The outer surface would be coated with Titanium Nitride and polished for a good articulation on XLPE. The inner male taper of the femoral stem component could be coated in Titanium Nitride. This would also harden the machined in ridges and grooves on the male taper encouraging the ridges to penetrate into the inner taper female recess 5110 thus providing resistance to rotation. Titanium Nitride at this interface would discourage wear and corrosion.

[0260] Similarly, a male taper of the femoral stem component could be manufactured with an upper end formed into a 12 / 14 cone and this could be coated with titanium nitride to discourage wear and corrosion of the male taper.

[0261] As an alternative to longitudinal ridges and grooves to give rotational stability, a hexagonal section, as illustrated in Figure 18B, may be provided on the lower approximately 8 mm of both the male and female neck tapers 5200. The hexagonal sections continue to have an extension of the same Morse tapers that are on the upper round section parts of the male and female tapers. The corners of the hexagonal section are rounded off.

[0262] The material for the neck could either be titanium alloy or zirconium I zirconium alloy as above. Both materials could be formed to receive the polyethylene head parts as already described.

[0263] Also both materials could be formed into a 12 / 14 taper cone to receive either a metal or ceramic head. As the S-ROM™ taper shows, and as the neck proposed has a length to accommodate a taper length close to 40 mm, this should be ideal to resist torsional forces without any special features.

[0264] All of these options, with shortened or lengthened tubular parts to give a range of different neck lengths or offsets are proposed and all maintaining concentric inner and outer head centers.

[0265] Cemented Stems For Distal Migration

[0266] 55677900-1 An example of a known Exeter stem 6000 for cement fixation is shown in Figure 19. This stem 6000 comprises a neck portion 6010 and a stem portion 6012. The stem portion 6012 has a front to back dimension that is less than the anterior to posterior dimension of the femur. The stem portion 6010 is also wider medio-laterally (side to side) than in an anterior to posterior (front to back) direction and therefore it is known as a blade stem. The Exeter stem 6000 is flat on the anterior and posterior surfaces of the stem portion 6012 throughout its length. The Exeter stem 6000 has a small radius curve 6014 at the upper lateral aspect of the stem portion 6012.

[0267] The inventor has observed that for such Exeter stems 6000, follow up x-rays 5 years post-operatively, reveal that nearly all patients have experienced migration of the stem 6000, which can be seen by a gap in the cement mantle at the top lateral side. As shown in Figure 19, when the Exeter stem 6000 distally migrates (i.e. when the distal tip migrates distally from location 6004a to 6004b), the small radius curve 6014 of the stem 6000 parts from the surrounding cement mantle resulting in a tapering curved gap, illustrated between points 6002a and 60002b, with no fixation to bone in that area. Unloaded bone undergoes osteopenia with weakening of this region of the femur bone.

[0268] There is accompanying medial translation of the upper portion of the stem 6000, as shown in Figure 19, but that is not normally noticeable by surgeons. For example, point 6006a on the upper medial aspect is translated to point 6006b. In the case of the Exeter stem 6000, the lower medial aspect is migrated along the straight taper of the blade stem portion 6012 from point 6008a to 6008b.

[0269] In the translated position, the patients have no pain and have good function. Instead of the bone at the upper medial side fracturing, the bone remodels to accommodate the medial translation of the stem 6000 from point 6006a to point 6006b. However, a problem can arise, particularly in elderly patients with weak bone - if they fall at any postoperative time point, then their femur can fracture. In which case, an acute forced distal migration (e.g., of 5mm) of the stem 6000 in the cement is accompanied by a similar acute medial translation of the stem 6000, which can fracture the upper medial femur in the region 6006b. The medial aspect of the femur is already weakened by resection of the femoral head and at least half of the femoral neck during THA. In addition, there is little soft cancellous bone to provide a buffer between the medial side of the Exeter stem 6000 and the hard cortical bone of the femur. This risky situation can lead to peri-prosthetic femoral fracture particularly in elderly people if they have a fall. However, this fracture is

[0270] 55677900-1 generally not confined to the upper medial femur. The spreading of the fracture may depend on the loading regimen at the fall. If there is a twisting element accompanying the fall, then the fracture can propagate far down the femur, in some circumstances well below the tip of the stem 6000. The variety of different types of peri-prosthetic fractures are documented in the Vancouver classification. The different types of fractures need different operative approaches, different length and types of internal fixation, with some, in addition, needing revision of the original stemmed implant.

[0271] Unfortunately, such peri-prosthetic fractures are not rare. The Australian joint replacement register has reported peri-prosthetic fractures after the un-cemented Corail stem. In small and medium sized stems, out of 51 ,760 recipients, 3.1% had a peri- prosthetic fracture. Out of 7,758 recipients of a large Corail stem, 3.9% had a peri- prosthetic fracture. Having a peri-prosthetic fracture as a result of a fall is very bad news for elderly patients and surgery and resulting slow mobilisation from such a fall carries a high mortality rate.

[0272] The inventor has therefore designed a prosthetic femoral stem component 7000, as shown in Figure 20, which does not result in the same medial translation as described above.

[0273] The prosthetic femoral stem component 7000 is specifically designed to reduce dislocation and peri-prosthetic femoral fracture. The stem 7000 comprises a neck portion 7010 and a stem portion 7012. The stem portion 7012 has a round cross-section distally and gradually transitions into an oval cross-section about one third of the distance from the tip 7004. The oval cross-section is wider medio-laterally than in an anterior to posterior direction. The anterior to posterior dimension of the stem portion 7012 is designed to be similar to the anterior to posterior dimension of the femur in which it is implanted.

[0274] As best illustrated in Figure 20B, on the medial side of the stem portion 7012 there is a single outwardly concave radius medial curve 7500 that generally follows the curvature of the inner medial cortical wall of the femur. As such, the length and / or curvature of the medial curve 7500 will alter with different sizes of stem 7000. The medial curve 7500 should have a single radius of curvature from the junction 7006 of the neck 7010 to the distal third of the stem 7000. Approximately two thirds of the way down the stem portion

[0275] 55677900-1 7012 towards the distal tip 7004, the medial curve 7500 transitions into a round cross- sectional cone portion 7014 of the stem portion 7012, which extends to the tip 7004. The cone portion 7014 is angled perfectly in harmony with the direction of migration of the stem 7000 which is induced by the medial curve 7500. Referring back to Figure 20, the medial profile of the stem 7000 ensures that distal migration of the tip from point 7004a to point 7004b is not accompanied by medial translation of the stem 7000. Instead as the junction 7006 between the neck portion 7010 and the stem portion 7012 migrates from point 7006a to point 7006b, the medial curve 7500 ensures that the stem 7000 migrates along the radius of the medial curve 7500, for example, point 7008a is migrated to point 7008b without any medial translation.

[0276] Referring back to Figure 20B, on the lateral side of the stem 7000, there are three parts. The distal third is formed by the cone portion 7014. A central curved lateral portion 7504 of the stem portion 7012 comprises a large outwardly convex radius of curvature (much larger than that of the medial curve 7500) forming a lateral border which does not follow the inner lateral cortical wall of the femur. The central curved lateral portion 7504 laterally cannot continue to the top proximal end of the stem portion 7012, otherwise the lateral top of the stem portion 7012 would impinge on the inner aspect of the greater trochanter and would fracture the greater trochanter. Accordingly, a top curved lateral portion 7506 is provided at the lateral top of the stem portion 7012, having a much smaller radius of curvature than the medial curve 7500 and the central curved lateral portion 7504. This means that the lateral top of the stem portion 7012 will not impinge on the greater trochanter. The top curved lateral portion 7506 extends to a generally transverse proximal surface 7508 of the stem portion 7012. Referring back to Figure 20, distal migration of the stem 7000, results in the junction 7002 between the top curved lateral portion 7506 and the transverse proximal surface 7508 moving both distally and laterally from point 7002a to point 7002b. Similarly, the central curved lateral portion 7504 is laterally translated along its length.

[0277] As can be seen in Figure 21 C, distal migration of the stem 7000, as shown in Figure 20, will not result in the stem 7000 parting company with the cement mantle 8000, thus maintaining fixation and bone loading. Thus, the stem 7000 is specifically designed to migrate without translation around the medial side and instead specifically translates solely on the lateral (safer) side.

[0278] 55677900-1 Notably, the stem 7000 may be cemented in the femur using any available techniques. In addition, the 7000 may or may not include a dual mobility prosthetic bearing as disclosed herein.

[0279] Peri-prosthetic fractures of traditional stems are a major problem with both uncemented stems and straight polished collarless stems like the Exeter stem described above. The fact is that with a polished tapered stem migrating into the bone cement, a translation has to occur somewhere.

[0280] The inventor considers that all of the translation would preferentially be in a lateral direction where a) the femoral bone is not weakened so much by resection of the femoral head and neck, and b) there is much more cancellous bone on the lateral side of the femur to serve as a buffer, particularly in the greater trochanter region between the lateral side of the metal stem and the cortical bone of the femur. The inventor has therefore designed the stem 7000, for cement fixation, with an aim of minimising femoral fracture and considers it likely that with this curved stem design the risk of peri-prosthetic fracture would be reduced.

[0281] The stem 7000 may be machined and polished from forgings of high nitrogen stainless steel. The risk area for fracture of such femoral stem components is at the junction of the proximal two thirds of the stem and the distal one third of the stem. At this junction, the cross-section of the stem is preferably circular.

[0282] Figure 22 shows an exploded view of a dual mobility prosthetic bearing 9000 for use in hip arthroplasty, including an acetabular cup component 9002 and a femoral neck component 9004 of a femoral stem prosthesis.

[0283] The acetabular cup component 9002 comprises an outer part-spherical metal shell 9006 and an inner part-spherical polymer liner 9008.

[0284] The femoral neck component 9004 comprises a tapered conical tip 9010 and a neck shaft 9012. The neck shaft 9012 comprises anti-rotation features in the form of planar longitudinal cut-outs 9014 to prevent relative rotation of the femoral neck component 9004 with respect to a correspondingly profiled recess (not shown).

[0285] 55677900-1 In practice, the femoral neck component 9004 may be provided on or as a part of any suitable femoral stem prosthesis. For example, the femoral neck component 9004 may be provided as part of any femoral stem prosthesis described herein.

[0286] The dual mobility prosthetic bearing 9000 comprises an outer femoral head component 9020 having a part-spherical outer bearing surface 9022 and a head recess 9024 comprising a proximal portion 9026 (as best shown in Figure 25A) defined by a part- spherical inner bearing surface and a distal portion 9028 (as best shown in Figure 25A) defined by a generally tubular inner bearing surface, the proximal portion 9026 having a maximum diameter that is greater than a maximum diameter of the distal portion 9028.

[0287] The dual mobility prosthetic bearing 9000 also comprises an inner femoral head component 9030 having a neck recess 9032 for receipt of the femoral neck component 9004 and a part-spherical outer bearing surface 9034 configured to be received within the proximal portion 9026 of the head recess 9024 such that the part-spherical outer bearing surface 9034 of the inner femoral head component 9030 is in bearing engagement with the part-spherical inner bearing surface of the outer femoral head component 9020 (as shown in Figure 23A).

[0288] The inner femoral head component 9030 further comprises a generally tubular skirt 9036 configured for receipt within the distal portion 9028 of the head recess 9024 to retain the outer femoral head component 9020 on the inner femoral head component 9030 during use.

[0289] Notably, the part-spherical outer bearing surface 9022 of the outer femoral head component 9020 and the part-spherical outer bearing surface 9034 of the inner femoral head component 9030 have a coincident centre of rotation. This ensures that applied forces are evenly distributed within the dual mobility prosthetic bearing 9000.

[0290] As previously explained, such a dual mobility prosthetic bearing 9000 may be used in any hemi-arthroplasty or total hip arthroplasty (THA) in order to reduce a risk of postoperative dislocation by preventing the outer femoral head component 9020 from sliding into a varus or valgus or anteversion / retroversion position where it may be more likely to dislocate out of the acetabular cup component 9002.

[0291] 55677900-1 Figures 23A and 23B show the components of Figure 22 after they have been assembled and implanted into a patient,

[0292] More specifically, Figure 23A shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component 9020 is 26mm, the maximum diameter of the outer femoral head component 9030 is 32mm, and the neck recess 9032 is configured for a minus 3mm leg length reduction.

[0293] Thus, it can be seen that the part-spherical outer bearing surface 9022 of the outer femoral head component 9020 fits snuggly in the polymer liner 9008 of the acetabular cup component 9002 for articular bearing therein and the part-spherical outer bearing surface 9034 of the inner femoral head component 9030 fits snuggly in the proximal portion 9026 of the head recess 9024 for articular bearing therein.

[0294] The outer femoral head component 9020 and the inner femoral head component 9030 comprise co-operative inter-engaging features ensuring a snap-fit of the inner femoral head component 9030 within the head recess 9024 of the outer femoral head component 9020. The co-operative inter-engaging features in this case are constituted by the shape of the head recess 9024 and the corresponding outer profile of the inner femoral head component 9030, which has a wider proximal portion and a narrower distal portion to retain the inner femoral head component 9030 in the head recess 9024.

[0295] Notably, the outer femoral head component 9020 is capable of rotation about a longitudinal axis A of the inner femoral head component 9030. However, relative longitudinal movement is prevented by the press-fit engagement of the inner femoral head component 9030 in the head recess 9024.

[0296] As shown in Figure 23A, the tapered conical tip 9010 of the femoral neck component 9004 is pressOfitted into the neck recess 9032. In this case, the neck recess 9032 extends through almost the entire length of the inner femoral head component 9030 such that the dual mobility prosthetic bearing 9000 sits as far down the femoral neck component 9004 as possible. This provides the shortest leg length adjustment, which, in the present case, is minus 3mm. In other cases, the neck recess 9032 and / or the tapered conical tip 9010 may be configured to provide a different leg length (e.g. minus 2mm,

[0297] 55677900-1 minus 1mm, 0mm, plus 1mm, plus 2mm, plus 3mm, plus 4mm, plus 5mm, plus 6mm, plus 7mm, plus 8mm, plus 10mm, plus 11mm, plus 12mm). It will be understood that a 0mm adjustment will correspond to a neutral position in which the leg is neither shortened or lengthened, whereas minus dimensions will result in a corresponding shortening of the leg and plus dimensions will result in a corresponding lengthening of the leg. As such, a set of parts with different dimensions may be provided for selective use during surgery.

[0298] As shown in Figure 23A, the generally tubular (parallel) skirt 9036 of the inner femoral head component 9030 is prevented from making contact with an edge of the acetabular cup component 9002 (even if rotated) because it is fully contained within the outer femoral head component 9020 (e.g. within a radius of the outer femoral head component 9020).

[0299] Figure 23B is similar to Figure 23A and shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component 9030 is 26mm, the maximum diameter of the outer femoral head component 9020 is 32mm, and the neck recess 9032 is configured for a plus 6mm leg length extension.

[0300] Figure 24A is similar to Figure 23A shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head component 9030 is 26mm, the maximum diameter of the outer femoral head component 9020 is 40mm, and the neck recess 9032 is configured for a minus 3mm leg length reduction.

[0301] Figure 24B is similar to Figure 24A shows a longitudinal cross-sectional view of the assembled components of Figure 22, where the maximum diameter of the inner femoral head 9030 component is 26mm, the maximum diameter of the outer femoral head component 9020 is 40mm, and the neck recess 9032 is configured for a plus 12mm leg length extension (i.e. the neck recess 9032 extends into the inner femoral head 9030 by 6mm less than the neck recess 9032 of Figure 24A).

[0302] Figure 25A shows a longitudinal cross-sectional view of the outer femoral head component 9020 of Figure 22 in isolation. This clearly shows the part-spherical outer bearing surface 9022 and the head recess 9024 which comprises the proximal portion

[0303] 55677900-1 9026 defined by a part-spherical inner bearing surface and the distal portion 9028 defined by a generally tubular inner bearing surface, the proximal portion 9026 having a maximum diameter that is greater than a maximum diameter of the distal portion 9028.

[0304] The head recess 9024 of the outer femoral head component 9020 comprises a recessed (sloped) shoulder portion 9040 between the proximal portion 9026 and the distal portion 9028.

[0305] By way of example, the dimensions of the outer femoral head component 9020 of Figure 25A comprise the outer bearing surface 9022 having a diameter of OD of 40mm, the inner bearing surface of the proximal portion 9026 having an inner diameter ID of 26.1mm, a head recess 9024 height HH of 24mm, a width of the distal portion 2028 DW of 22.1 mm, a thickness T of the outer bearing surface 9022 of 6.95mm and an outer radius of curvature R of 13.5mm in the region adjacent the distal portion 2028.

[0306] Figure 25B shows a longitudinal cross-sectional view of the inner femoral head component 9030 of Figure 22 in isolation. This clearly shows the neck recess 9032 for receipt of the femoral neck component 9004 and the part-spherical outer bearing surface 9034 configured to be received within the proximal portion 9026 of the head recess 9024. The inner femoral head component 9030 further comprises the generally tubular skirt 9036 configured for receipt within the distal portion 9028 of the head recess 9024 to retain the outer femoral head component 9020 on the inner femoral head component 9030 during use.

[0307] The inner femoral head component 9030 comprises a recessed (sloped) shoulder portion 9042 between the part-spherical outer bearing surface 9034 of the inner femoral head component 9030 and the generally tubular skirt 9036.

[0308] By way of example, the dimensions of the inner femoral head component 9030 of Figure 25B comprise a head height HH of 24mm, a head bearing diameter BD of 26mm, a skirt diameter SD of 22mm, a snap-in distance SIN of 1.6mm, an insertion angle In of 32 degrees and an extraction angle Ex of 45 degrees.

[0309] Figure 26A illustrates initial insertion of the inner femoral head component 9030 of Figure 25B into the outer femoral head component 9020 of Figure 25A. Although the diameter

[0310] 55677900-1 of the part-spherical outer bearing surface 9034 is greater than the width of the distal portion 2028, the flexible material of the outer femoral head component 9020 is configured to flex around the inner femoral head component 9030 in a snap-fit engagement to lock the components against relative longitudinal movement and / or relative off-axis movement.

[0311] Figure 26B shows the completed insertion of the inner femoral head component 9030 of Figure 25B in the outer femoral head component 9020 of Figure 25A.

[0312] The outer femoral head component 9020 may be formed of one or more of: polyethylene, LIHMWPE, cross-linked polyethylene (XLPE), polyether ether ketone (PEEK) or polyimide or a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material, whilst the inner femoral head component 9030 may be formed of metal or ceramic.

[0313] The neck recess 9032 of the inner femoral head component may be tapered and configured for accommodating a tapered neck projection 9010 from a femoral stem component.

[0314] The tapered neck recess 9032 and / or the tapered neck projection 9010 may comprise one or more anti-rotation features (e.g. planes 9014) to prevent relative rotation therebetween.

[0315] The tapered neck recess 9032 may be provided centrally within the inner femoral head component 9030, as illustrated, or the tapered neck recess 9032 may be off-set from the longitudinal axis A of the inner femoral head component 9030.

[0316] Figure 27 shows a longitudinal cross-sectional view of another Ultra Short Stem (USS) hip prosthesis 10000 comprising a dual mobility bearing 11100 according to the disclosure, when implanted in a femoral neck 1200 of a patient. The USS hip prosthesis 10000 is similar to that shown in Figure 12 but with a stepped stem 11130.

[0317] Previous hip prosthesis included a metal two-piece stem and head design, in which a screw-in handle was attached to the proximal end of the stem for insertion into the femur and then removed before attachment of the metal head on the stem. When inserting a

[0318] 55677900-1 press-fit tapered device, the density of cancellous bone around the stem is never perfectly even. As the stem is tapped into position, there is inevitably some sideways drift. With the surgeon being alert, the attached handle could be used to correct any tendency to drift sideways. However, this device suffered a handful of gross head / neck taper wear failures.

[0319] A fixed metal inner head on the proposed USS hip prosthesis 1000, such as that shown in Figure 12, has the advantage of no head / neck taper wear problems, however, it does have the problem of there being no option to attach a handle for insertion. The USS hip prosthesis 10000 of Figure 27 is therefore proposed to include a self-aligning stepped stem, similar to those disclosed in US8926707B2.

[0320] Figure 27 is based on a computed tomography (CT) scan of a proximal end of a femur 1202. A cortical bone 1204 outline of the femur 1202 is indicated from a coronal slice on the CT scan, while the cancellous bone in the femur 1202 and femoral neck 1200 has been removed for clarity.

[0321] The USS hip prosthesis 10000 comprises a polymer femoral head component 11102 having a part-spherical outer bearing surface 11104 and a recess forming a part- spherical inner bearing surface 1106 and a metal femoral component 11110 having a proximal head portion 11112 configured to be received within the recess. The proximal head portion 11112 has a part-spherical outer bearing surface 11114 configured for bearing engagement with the part-spherical inner bearing surface 11106 of the polymer femoral head component 11102. The part-spherical outer bearing surface 11104 of the femoral head component 11102 and the part-spherical outer bearing surface 11114 of the proximal head portion 11112 have a coincident centre of rotation C. In this example, a collar 11116 (which forms an extension) is provided on the femoral component 11110 to support the polymer femoral head component 11102 on the metal femoral component 11110 during use.

[0322] The stepped stem 11130 is configured to extend through the femoral neck 1200 but not along the length of the femur 1202. The stepped stem 11130 comprises a rounded tip 11140 for insertion into the femoral neck 1200, in use, and a body of generally tapering form extending in a distal direction from the collar 11116 to said tip 11140. The body of the stepped stem 11130 includes a plurality of discrete steps 11150 located between

[0323] 55677900-1 said tip 11140 and said collar 11116, wherein the steps 11150 are concentrated more towards the collar 11116 than towards the tip 11140 and wherein the steps 11150 are distributed along the body such that the distance between each adjacent step 11150 gradually decreases towards the collar 11116. The steps 11150 may comprise the same widths or may have widths that increase towards the collar 11116. A surface of the steps 11150 may be configured for porous ingrowth to aid bone attachment after press-fitting in the femoral neck 1200.

[0324] For completeness, the femoral head component 11102 may be formed of polymer, which may comprise polyethylene, XLPE, polyether ether ketone (PEEK) or polyimide (e.g. a formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide material). The femoral component 11110 may be formed of metal or ceramic. In use, the femoral head component 11102 may articulate on a thin metal or ceramic acetabular cup surface (not shown).

[0325] Like hip resurfacing, a near normal size femoral head component 11102 is used and the tapered head-neck junction of the patient’s femoral neck 1200 are retained as shown in Figure 27. Like with hip resurfacing, the hip capsule is filled so a very low dislocation rate is expected.

[0326] As before, the femoral head component 11102 and the femoral component 11110 comprise co-operative inter-engaging features. In this case, the femoral component 11110 comprises an annular groove 11122 between the collar 11116 and the part- spherical outer bearing surface 11114 of the proximal inner head portion 11112 and the femoral head component 11102 comprises an annular projection 11124 received in the annular groove 11122. Thus, the outer (e.g. polymer) femoral head component 11102 and the proximal inner head portion 11112 are configured for relative rotation of the projection 11124 in the groove 11122. As such, the femoral head component 11102 is configured for rotation about a longitudinal axis of the femoral component 11110.

[0327] The femoral head component 11102 and the femoral component 11110 are configured for snap-fit engagement of the projection 11124 in the groove 11122. The snap-fit engagement serves as a locking engagement against relative longitudinal movement (thereby ensuring the centres of rotation C of the part-spherical outer bearing surfaces 11104, 11114 remain coincident).

[0328] 55677900-1 Unlike current dual mobility devices, as illustrated in Figure 2B, impingement contact of the polyethylene edge of the femoral head component 11102 on metal of the femoral component 11110 does not occur with the present design. Rotation only movement is guided by the large area contact between the polyethylene femoral head component 11102 and the polished top surface of the collar 11116.

[0329] The coincident centres of rotation C and the upper surface of the collar 1118 stops the polyethylene femoral head component 1102 from tipping into varus. So, with the implant 1000, the head centre is maintained, leg length is unaltered and the hip capsule is filled by the large femoral head component 1102 and the patient’s native femoral neck 1200.

[0330] The common thread with the above described prosthesis is that they all have a coincident centre between either the centre of the metal inner head (in the case of the USS) or the centre of the articular portion of the metal head (in the case of THA and Dual Mobility implants) and the Polyethylene, preferably XLPE outer head component (or in the case of hemi-arthroplasty), ceramic or metal covered XLPE implant.

[0331] As we have already explained, the early dual mobility implants had coincident centres of rotation between the inner metal head and the outer polyethylene head, but in use, the outer polyethylene heads rotated into a varus position. The French developers solved this problem by designing in offset centres of rotation of the inner and outer heads and this certainly solved the problem of the outer polyethylene heads rotating into a varus position.

[0332] We have also explained that after a surgical implantation procedure, the biomechanics of the hip is significantly adversely affected. As noted before, Charnley and Halley published evidence that the direction of polyethylene wear was not as predicted by biomechanical theory. Biomechanical theory would mean that the wear direction would be upper and medial (only a small percentage was in practice). The predominant wear direction was upper and lateral.

[0333] Had the wear direction been upper and medial, the French solution of offset centres would have been good. With the direction of wear and joint reaction force direction being predominantly upper and lateral, this creates a major problem for all current designs of dual mobility implants.

[0334] 55677900-1 The edge of the outer polyethylene head impinges with each step on the metal femoral neck. The outer polyethylene head freely rotates on the inner metal or ceramic head and recurrent impingement causes symmetrical deformation of the polyethylene edge. Plastic deformation of the outer polyethylene edge eventually increases, leading to decreasing 'snap fit' with increasing time. The end result is Intra Prosthetic Dissociation (IPD), a problem that always needs an open revision procedure.

[0335] The problem currently is to decide what type of revision implant to use. Using the same implants again can be predicted to end with the same IPD again.

[0336] The current disclosure details an alternative strategy. In all of the above proposed implants, the inner and outer centres of rotation are coincident.

[0337] With the USS, the polished top of the collar, approximately 90 degrees to the long axis of the implant provides support for the polymer head component above it and prevents the polymer head component tipping into varus.

[0338] For implant resistance to post operative dislocation, the near normal diameter of the polymer outer head plus the retained outer diameter of the natural femoral neck both serve to fill the volume of the hip capsule, thus providing resistance to unwanted dislocation.

[0339] With the total hip arthroplasty (THA) design, the polished outer surface of the metal neck articulates with the adjacent rotating inner surface of the polymer outer neck component to prevent tipping of the component into varus.

[0340] The near normal prosthetic head diameter together with the near normal external diameter of the polymer neck both serve to fill the hip capsule, thus providing resistance to unwanted dislocation.

[0341] With the dual mobility component configured to fit onto an existing and typically used femoral neck, when used to revise a dual mobility component that has failed with IPD, the new components have coincident head centres. The polymer head articulates with a

[0342] 55677900-1 short length of the outer oxidised zirconium alloy femoral neck to prevent the polymer tipping into varus.

[0343] Instead of using an oxidised zirconium alloy revision component to fix onto a used and damaged existing femoral neck, the possibility exists of using a metal head component e.g. CoCr or stainless steel or indeed a ceramic head with super glue to fill in any irregularities between the new head / neck component and the existing damaged femoral neck.

[0344] Figure 28 shows a longitudinal cross-sectional view of a further Ultra Short Stem (USS) hip prosthesis 20000, similar to that of Figure 27 but with a fixed solid head 20002, when implanted in a femoral neck 1200 of a patient.

[0345] In this case, the USS hip prosthesis 20000 comprises a one-piece solid head 20002 fixed at the proximal end of a stepped stem 11130 which is configured to extend through the femoral neck 1200 but not along the length of the femur 1202. As above, the stepped stem 11130 comprises a rounded tip 11140 for insertion into the femoral neck 1200, in use, and a body of generally tapering form extending in a distal direction from the head 20002 to said tip 11140. The body of the stepped stem 11130 includes a plurality of discrete steps 11150 located between said tip 11140 and said head 20002, wherein the steps 11150 are concentrated more towards the head 20002 than towards the tip 11140 and wherein the steps 11150 are distributed along the body such that the distance between each adjacent step 11150 gradually decreases towards the head 20002. The steps 11150 may comprise the same widths or may have widths that increase towards the collar 11116. A surface of the steps 11150 may be configured for porous ingrowth to aid bone attachment after press-fitting in the femoral neck 1200.

[0346] Notably, as shown in Figure 28, the solid head 20002 terminates in a flat annular base 11160 extending radially from a proximal end of the stepped stem 11130.

[0347] In US8926707B2, the hollow heads included an annular skirt around retained cancellous bone (also known as trabecular bone or woven bone) in the base of the head. Experience has shown this is not ideal as cancellous bone which is unsupported by the normal cortical subchondral bone outside it is weak and can fail when supporting an implant on top. To accommodate the devices of US8926707B2, a peripheral cut in the axis of the

[0348] 55677900-1 neck has to be made to accommodate the outside of the implant thus removing the strong subchondral bone support. In addition, certain diseases (e.g. avascular necrosis of the femoral head) can affect and weaken further the already weak cancellous bone by necrosis, on occasion extending down to the head / neck junction.

[0349] However, the proposed USS hip prosthesis 20000 of Figure 28 removes all the cancellous bone in the base of the femoral head by having the resection level precisely at the annular base 11160 of the head / neck junction.

[0350] The USS hip prosthesis 20000 may be formed of metal or ceramic. In the case of an all metal component, the fixed solid head 20002 may be hollowed out to remove weight and then welded on the stepped stem 11130, to provide a fixed hollow head.

[0351] Furthermore, it may be possible to 3D print the USS hip prosthesis 20000 from metal or ceramic materials. In which case, the fixed head 20002 may be solid or hollow.

[0352] Embodiments of the present disclosure can be employed in hemi-arthroplasty or total hip arthroplasty (THA), including in revision surgery, to reduce a risk of dislocation.

[0353] The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘below’, ‘along’, ‘side’, etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

[0354] Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure, which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiments, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

[0355] 55677900-1

Claims

CLAIMS:1 . A dual mobility prosthetic bearing for use in hip arthroplasty comprising: an outer femoral head component having a part-spherical outer bearing surface and a head recess comprising a proximal portion defined by a part-spherical inner bearing surface and a distal portion defined by a generally tubular inner bearing surface, the proximal portion having a maximum diameter that is greater than a maximum diameter of the distal portion; an inner femoral head component having a neck recess for receipt of a femoral neck component; and a part-spherical outer bearing surface configured to be received within the proximal portion of the head recess such that the part-spherical outer bearing surface of the inner femoral head component is in bearing engagement with the part-spherical inner bearing surface of the outer femoral head component; wherein the part-spherical outer bearing surface of the outer femoral head component and the part-spherical outer bearing surface of the inner femoral head component have a coincident centre of rotation; and wherein the inner femoral head component further comprises a generally tubular skirt configured for receipt within the distal portion of the head recess to retain the outer femoral head component on the inner femoral head component during use.

2. The dual mobility prosthetic bearing of claim 1 wherein the head recess of the outer femoral head component comprises a recessed shoulder portion between the proximal portion and the distal portion and / or the inner femoral head component comprises a recessed shoulder portion between the part- spherical outer bearing surface of the inner femoral head component and the generally tubular skirt.

3. The dual mobility prosthetic bearing of any preceding claim wherein the outer femoral head component and the inner femoral head component are configured for snap-fit engagement.55677900-14. The dual mobility prosthetic bearing of claim 3 wherein the snap-fit engagement serves as a locking engagement against relative longitudinal movement and / or relative off-axis movement.

5. The dual mobility prosthetic bearing of any preceding claim wherein the outer femoral head component is configured for rotation about a longitudinal axis of the inner femoral head component.

6. The dual mobility prosthetic bearing of any preceding claim wherein the outer femoral head component and the inner femoral head component comprise co-operative inter-engaging features.

7. A dual mobility prosthetic bearing for use in hip arthroplasty comprising: a femoral head component having a part-spherical outer bearing surface and a recess forming a part-spherical inner bearing surface; a femoral neck component having a proximal portion configured to be received within the recess; the proximal portion having a part-spherical outer bearing surface configured for bearing engagement with the part-spherical inner bearing surface of the femoral head component; wherein the part-spherical outer bearing surface of the femoral head component and the part-spherical outer bearing surface of the proximal portion have a coincident centre of rotation; and wherein an extension is provided on the femoral head component and / or the femoral neck component to retain the femoral head component on the femoral neck component during use.

8. The dual mobility prosthetic bearing of claim 7 wherein the extension is provided substantially around a circumference of the femoral head component and / or the femoral neck component.

9. The dual mobility prosthetic bearing of claim 7 or 8 wherein the extension comprises a skirt depending from the femoral head component and configured to extend along a distal portion of the femoral neck component.55677900-110. The dual mobility prosthetic bearing of claim 9 wherein skirt has a maximum diameter of less than a maximum diameter of the part-spherical outer bearing surface.11 . The dual mobility prosthetic bearing of claim 9 wherein the femoral head component comprises a recessed shoulder portion between the part- spherical outer bearing surface and the skirt.

12. The dual mobility prosthetic bearing of any of claims 7 to 11 wherein the extension comprises a collar extending transversely with respect to a longitudinal axis of the femoral neck component.

13. The dual mobility prosthetic bearing of claim 12 wherein the collar forms a proximal end of a flared trumpet portion of the femoral neck component.

14. The dual mobility prosthetic bearing of any of claims 7 to 13 wherein one of the femoral head component and the femoral neck component comprises an annular groove and the other of the femoral head component and the femoral neck component comprises an annular projection for receipt in the annular groove.

15. The dual mobility prosthetic bearing of any of claims 7 to 14 wherein the femoral neck component is provided in the form of an ultra-short stem (USS) femoral component.

16. The dual mobility prosthetic bearing of any of claims 7 to 15 wherein the femoral neck component comprises a generally tubular component having the proximal portion at one end thereof and a tapered recess at an opposite end thereof, for accommodating a tapered neck projection from a femoral stem component.

17. The dual mobility prosthetic bearing of claim 16 wherein the tapered recess and / or the tapered neck projection comprise one or more anti-rotation features to prevent relative rotation there-between; and, optionally, wherein55677900-1the tapered recess is provided centrally within the femoral neck component or wherein the tapered recess is off-set from a longitudinal axis of the femoral neck component.

18. The dual mobility prosthetic bearing of any preceding claim wherein the femoral head component is formed of one or more of: polyethylene, LIHMWPE, cross-linked polyethylene (XLPE), polyether ether ketone (PEEK), polyimide or a formable, pyromellitic, dianhydride (PMDA)- free, non-halogenated, aromatic polyimide material.

19. A femoral component for use in hip arthroplasty comprising a dual mobility prosthetic bearing of any of claims 7 to 18 and a femoral stem component; wherein the femoral neck component of the dual mobility prosthetic bearing is integrated with the femoral stem component.

20. A prosthetic femoral stem comprising: a neck portion and a stem portion; the stem portion having a distal tip and a concavely curved medial surface having a single radius of curvature extending from a junction between the neck portion and the stem portion to a region a third of the distance from the distal tip.55677900-1

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