A total arthroplasty implant

Abstract

A total arthroplasty joint implant (1) has a cup (30) for anchoring in a proximal bone, and a head (10) with a ball (11) on a neck (12) on a base (14), the ball being located distally of the cup and retainer and being for engaging a socket of an intramedullary stem (150) for anchoring in a distal bone. The cup and the head are configured to allow a degree of lateral freedom of movement of a base (14) of the head up to 3 mm in some examples, thereby providing orbital freedom of movement of the head. The base of the head is held by a retainer (20) which presses the base against a planar lateral surface (34) of the cup, and the retainer is in turn held by an overhang rim (36) of the cup, and the retainer overhangs (22) the cup in a counter-balancing manner.

Description

[0001] “A Total Arthroplasty Implant”

[0002] Introduction

[0003] The present invention relates to total arthroplasty implants for joints such as joints in the hand or the elbow with multiple axes of rotation.

[0004] An example of an implant with multiple axes of rotation is one for a first carpometacarpal joint for spacing a trapezium bone from a first metacarpal bone (the CMC joint). In this case there is translational motion of a saddle-shaped surface of a proximal implant part over the trapezium and three-dimensional rotational movement of the distal part due to an articulated coupling such as a ball-and-socket j oint.

[0005] The thumb carpometacarpal (CMC) joint is often affected by osteoarthritis. In the CMC joint the locus for abduction and adduction is located inside the base of the first metacarpal and the locus for flexion and extension is located on or abouts the surface of the trapezium. These two loci are approximately 9 mm apart. Historically, CMC implants have not addressed the fact that two loci exist, however the hemi-arthroplasty implants described in WO2017 / 137607 (NUIG) and W02020 / 193078 (Loci Orthopaedics Ltd.) address this issue. The term “hemi-arthroplasty” is used here because the proximal part is not fixed to the proximal bone (the trapezium), rather it slides over it.

[0006] US9,707,084 (Hueber) describes a radial head prosthesis with a floating articular member. CN107049562 describes a double-movable joint vertebra prosthesis. JP4331524 describes a shoulder or hip joint prosthesis.

[0007] Despite the benefits of the hemi-arthroplasties described in the above two documents there are still circumstances where a total arthroplasty is required, a “total arthroplasty” being one in which one element of the implant may be a stem which is fixed in the distal bone and another element is fixed in the proximal bone and may resemble a cup. More specifically, in the case of a total arthroplasty of the CMC joint, either a ball or the socket is fixed in the metacarpal and the corresponding element of the ball and socket is fixed in the trapezium. Due to the complex movements and forces acting within the CMC j oint, there is a tendency for the fixed implant element on the trapezial side to loosen and become dislodged, leading to implant failure. In the case of a hemi-arthroplasty, as the mating element is not fixed in the trapezium, then no such disparate physiological forces are at work on the trapezial element.

[0008] In certain clinical circumstances, it may be desirable for a total arthroplasty implant to be employed, and the present invention addresses the problem of loosening of the proximal element in a total arthroplasty in the CMC joint.

[0009] There may be large forces acting on the proximal fixed cup element of the existing total arthroplasty implants. Post implantation, it is common for joint forces acting on the cup to loosen and dislodge the cup over time.

[0010] The invention addresses these problems.

[0011] Summary of the Invention

[0012] We describe a total arthroplasty joint implant comprising: a cup for anchoring in a proximal bone, a head with a ball on a neck on a base, the neck extending distally in a longitudinal proximal -distal direction, for the ball to engage a socket of a component for anchoring in a distal bone, and a retainer configured to be held within a volume of the cup, and being configured to hold the base of the head, wherein the implant is configured such that the head base has lateral freedom of movement across said longitudinal direction.

[0013] In some preferred examples, said freedom of lateral movement is in the range of 0.5 mm to 3.0 mm. In some preferred examples, said lateral freedom of movement is in the range of 1.0 mm to 2.0 mm.

[0014] In some preferred examples, the retainer has an opening and the head neck (extends through said opening. In some preferred examples, the retainer opening has a diameter D2 which is larger than a diameter DI of the ball.

[0015] In some preferred examples, the head base is disc shaped. In some preferred examples, the head base and the cup have engaging surfaces which extend laterally across the longitudinal axis, the head base being retained by the retainer in contact with said cup surface.

[0016] In some preferred examples, the said head and cup surfaces are planar.

[0017] In some preferred examples, the retainer comprises an overhang rim extending radially inwardly to retain the base of the head within the volume of the cup.

[0018] In some preferred examples, the cup comprises an inwardly directed overhang rim to hold the retainer, and the retainer comprises an inwardly directed overhang rim to hold the head base and press it against a proximal surface of the cup in a lateral plane.

[0019] In some preferred examples, the retainer has a distal rim which overlies a portion of the cup.

[0020] In some preferred examples, the cup comprises proximal anchors for anchoring in a proximal bone.

[0021] In some preferred examples, the retainer extends proximally to surround at least part of the head base.

[0022] In some preferred examples, the retainer extends proximally to surround all of the head base the retainer having a radially inward facing surface (26) for abutment with the head base (13) upon extreme lateral movement of the head base in use.

[0023] In some preferred examples, the retainer comprises a radially outward facing proximal rim for engaging a corresponding groove of the cup and said rim overlaps in the longitudinal dimension with the chamber surface which is for lateral abutment with the head base.

[0024] In some preferred examples, the retainer has a radially outward facing distal rim and said retainer proximal rim and distal rim form together a radially outward facing groove for engagement with a ridge of the cup.

[0025] In some preferred examples, head extends distally from a distal-most surface of the retainer and the cup by an extension distance D3 in the range of 7 mm to 9 mm. In some preferred examples, the head extends distally from a distal-most surface of the retainer and the cup by an extension distance D3, and the extension distance D3 is at least 30% of a total longitudinal dimension of the assembled cup, retainer and head.

[0026] In some preferred examples, the retainer has a Shore hardness in the range of 62 D to 89 D.

[0027] In some preferred examples, the implant further comprises a distal stem having a socket engaging the ball and being configured for intramedullary engagement in a distal bone.

[0028] Detailed Description of the Invention

[0029] The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:

[0030] Fig. 1 is a cross sectional view of an implant of the invention, showing a cup within which there is a retainer, and this retains a head which in this case is a ball of a ball joint,

[0031] Fig. 2 is a front view of the head,

[0032] Fig. 3 is a more detailed cross-sectional view of the retainer, and Fig. 4 is an enlarged view of an edge portion of the retainer showing some dimensions and angles,

[0033] Fig. 5 is a cross sectional view of the cup,

[0034] Figs. 6 to 9 are a set of views of an alternative cup, in which:

[0035] Fig. 6 is a side view,

[0036] Fig. 7 is a cross sectional view,

[0037] Fig. 8 is an underneath plan view, and

[0038] Fig. 9 is an enlarged view of an anchor,

[0039] Fig. 10 is a perspective view of an implant of another embodiment, and

[0040] Fig. 11 is a perspective view showing the implant of Fig. 10 including a stem for insertion in a distal bone. Overall Configuration, Figs. 1-5

[0041] Referring to Figs. 1 to 5 an implant 1 comprises a head 10, a retainer 20, and a cup 30. The cup 30 anchors in the trapezium, the head 10 engages the distal bone via a ball-and-socket connection, and the retainer 20 retains the head 10 with a degree of freedom of movement relative to the cup, as described in more detail below.

[0042] The head 10 comprises a ball 11, a neck 12, and a disc-shaped base 13 with a circumferential edge surface 14 and a base surface 15.

[0043] The retainer 20 comprises a main body of generally annular configuration, having a distal annular part 21(a) and a proximal annular part 21(b). The distal annular part 21(a) has a distal rim 22 extending radially outwardly, and a radially inwardly facing rim 25 having a radially inwardly facing surface 24 defining a distal opening of the retainer. The main body of the retainer 20 has a circumferential groove or socket 23 facing radially outwardly, and this is bordered on the proximal side by a rim or ridge 27 extending radially outwardly. The proximal part 21(b) has a radially inwardly facing surface 26 which, together with the ledge 25, defines a chamber for the base 13 of the head 10.

[0044] As shown in Fig. 4 the distal outer rim 22 has a dimension in the longitudinal (proximal-distal direction) of 1.0 mm, the groove 23 has a longitudinal dimension of 1.3 mm, the concave radius of curvature of the proximal surface of the groove 23 is 0.2 mm, and the proximal rim 27 forms a convex curve around the edge of its proximal-most surface and this has a radius of curvature of 0.5 mm.

[0045] The cup 30 comprises a body 31 having a proximal convex curved surface 32 and three anchors 33 in the form of spikes spaced at 120° spacings around the proximal surface 32. The body 31 also forms a circumferential ledge 36 which decreases in diameter proximally, to form a snap-fitting circumferential groove 37 configured to receive the proximal rim 27 of the retainer 20. The cup 30 also has a base surface in contact with the surface 15 of the head 10.

[0046] The ball 11 of the head 10 is configured to form the ball part of a ball joint completed by a socket of a stem of conventional construction. The complete total arthroplasty implant comprises the components 10, 20, and 30, which are illustrated, and a stem (not shown in Fig. 1 to 4) of known construction for intramedullary insertion in the distal bone and with a socket for engagement with the ball 11. A full implant is shown in Fig. 11.

[0047] Materials

[0048] The materials of the components 10, 20, and 30 are chosen for suitability in use as follows.

[0049] Head 10, for performing articulation as the ball part of a ball joint with the stem. This component does not interface directly with bone.

[0050] Retainer 20, for allowing snap fitting of the head into position in the cup 30 and resisting distal migration of the head 10.

[0051] Cup 30, for interfacing with, and anchoring into, the proximal bone such as the trapezium, an also allowing snap-fitting of the retainer 20 and retention of the retainer in position.

[0052] The cup material is preferably Cobalt Chrome alloy, and both the head and retainer are preferably CFR-PEEK (carbon fibre reinforced polyether ether ketone) such that bearing surfaces are provided for the head at all contact surfaces. Both Cobalt Chrome alloy, PEEK or CFR-PEEK may be coated with commercially pure Titanium or Hydroxy Apatite for securement in bone. The combination of (CoCr) Cobalt Chrome alloy for the cup 30 and CFR-PEEK for the retainer 20 also permits a secure snap-fit fastening between these two components. The internal annular snap-fit of the retainer 20 would also function well if both retainer and cup were made from a form of PEEK (polyether ether ketone) material. The following table lists some preferred combinations of component materials, in which TPS is Titanium Plasma Spray and HA is hydroxyapatite.

[0053] The modular nature of the implant allows choice of an optimum material for each of the three components.

[0054] The CoCr material listed could alternatively be Ti or 316 Stainless Steel among other suitable metals, and the PEEK material could alternatively be UHMWPE (ultra high molecular weight polyethylene) or another suitable polymeric material in respect of the retainer or head. UHMWPE is not preferred for the cup. Where a component is of a metal the interfacing components are preferably a polymer.

[0055] Relationship Between, and Functionality of. Components

[0056] Assembly between the retainer 20 and the cup 30 is by means of an annular snap-fit between the retainer rim 27 and the cup groove 37. Essentially, the retainer 20 is held in position in the cup 30 by the centrally extending overhang rim 36 of the cup, and the head 10 is held in position by the centrally directed overhang rim 25 of the retainer 20. As shown in Fig. 2 the diameter of the ball 11 is DI and the diameter of the distal retainer opening 24 is designated as D2, and D2 is preferably larger than DI so that the ball 11 can easily fit through the opening 24 before or during implantation.

[0057] Also, the dimension D3 denotes the longitudinal distance to which the head extends distally from the distal-most surface of the retainer and the cup. This is preferably in the range of 7 mm to 9 mm. It is preferred that the dimension D3 is at least 30% of the total longitudinal dimension of the cup / retainer / head when assembled, and more preferably at least 40% of this overall dimension. This extent of protrusion of the head allows the distal part of the implant such as an intramedullary part to engage with the head and allows it to articulate with a large degree of freedom of motion about the head. This is very advantageous for a total arthroplasty implant.

[0058] When assembled, the base 13 is retained within the proximal chamber formed by the proximal retainer part 21(b) and the inner rim 25 of the distal part 21(a). The retainer 20 extends fully to the proximal internal surface 34 of the cup, the proximal surface 21(c) of the retainer being against the base surface 34 of the cup. Hence, the retainer 20 provides the radial boundaries and the distal boundaries of the chamber confining the base 13. This allows the material of the retainer 20 to be chosen for optimum snap fitting into the retainer and also for providing optimum resilience for limiting radial movement of the head’s base 13. The distal opening of the retainer, formed by the surface 24, does not contact the neck 12 of the head due to the geometry of this example, its main purpose being to provide longitudinal retention of the base by virtue of the rim 25 and to allow the head ball 11 to pass through during assembly.

[0059] The distal outer rim 22 of the retainer provides a counter balance to the snap-fit engagement of the retainer 20 in the cup 30. The cup 30 has the spikes 33, or in other examples any other suitable fixation anchors protruding proximally, to assist embedment in the trapezium once a suitable aperture is formed by an orthopaedic burr.

[0060] Use

[0061] In use, the cup 30 is fixed in a proximal bone such as the trapezium, but the head 10 is not fixed. Instead, it is free to move laterally by a small amount within the confines of the retainer 20, set by the difference in diameter of the base 13 / 14 of the head 10 and the diameter of the chamber 26, indicated as a gap radially around the base radial surface 14 in Fig. 1. The amount of lateral translation is in the order of 0.5 mm to 3.0 mm and more preferably in the range of 1 mm to 2 mm, this lateral freedom being all around the periphery of the circular base 14 of the head. The amount of allowed longitudinal translation is negligible. The freedom of movement laterally of the base provides a degree of orbital freedom of movement of the head 10. The movement of the head 10 is constrained by the internal diameter of the retainer chamber 26 and by the overhang rim 25.

[0062] It will be appreciated that the invention provides security of cup placement within the trapezium for a total arthroplasty by providing controlled movement of the extended head and ball element in a lateral and orbital fashion within the cup, thus reducing or eliminating disparate forces that act on the total arthroplasty cup 30. The retainer integral outer lip 22 aids insertion of the robust snap- fit features 27 / 37 and 26 / 23 directly into the cup 30 and provides a very rigid integrated structure in which the base 13 of the head 10 is free to translate and orbit while constrained from vertical movement, while at the same time the cup is securely fastened to the trapezium by one or several spikes and is not subject to the disparate forces acting on conventional total arthroplasty cups.

[0063] The implant may be referred to as a Mobile Total Arthroplasty (MTA) for use as a semiconstrained internal distraction implant for the treatment of moderate to severe osteo-arthritis. The head engages a stem in some examples, and the stem may be of any known type and may include a proximal liner forming the socket to receive the head ball 11.

[0064] The implant provides two axes of articulation that can act independently but concurrently, thereby recreating the natural axes of the joint. An Extension-Flexion axis is recreated by the head 10 sliding laterally relative to the cup 30, the interfacing sliding surfaces being the surfaces 15 and 34. An abduction - adduction axis is provided by the ball 11 of the head 10 in its interaction with a stem or stem / liner functioning as a socket in the metacarpal side of the joint. These can be separated in space by preferably 9 mm (the distraction distance) to recreate the natural axes of the joint. Other distraction distances may be obtained by varying the dimensions of the head. In general, it is preferred that the distraction distance is in the range of 8 mm to 10 mm.

[0065] The device is modular, allowing exact matching to the patient’s anatomy, especially to the patient’s biomechanics.

[0066] The invention benefits from our realisation that total arthroplasty implants often fail because almost all joint forces are transmitted through the cup. Even if there is a dual axis, internal articulation (where one liner sits inside another), as the cup and liner construct cannot move laterally, all forces are still ultimately transmitted through the shell of the cup. The implant of the invention addresses this problem by allowing some degree of translation while also providing rotation and two axes of movement. These have been achieved by providing a retainer to reside within the cup and the head to be retained by the retainer, such multiple components providing a degree of freedom of movement of the base of the head to spread the forces across the retainer in addition to the cup. It is particularly beneficial that the retainer extends fully proximally within the cup, its proximal-most surface 21(c) abutting the base internal surface 34 of the cup. The retainer therefore is substantial and hence robust and also beneficially providing resilient boundaries to radial or lateral motion of the base. The functions of surrounding the base and of snap-fitting into the cup both require a resilient material and so these functions are complementary. In some examples the retainer has a Shore hardness in the range of 62 D to 89.

[0067] A major benefit of the invention is that, while it is still important to have the cup in approximately the correct position, the implant allows a slide so the "biomechanical landing zone" is widened to the footprint of the base of the head. The implant provides two axes or articulation that can act independently but concurrently thereby recreating the natural axes of the joint. The Ex-Fl axis is recreated by the head 10 sliding relative to the cup 30. The Ab-Ad axis is recreated by the ball- and-socket in the stem in the metacarpal. These can be separated by 9 mm to recreate the natural axes of the joint or separated by varying amounts by changing the dimensions of the head.

[0068] The device is modular, allowing exact matching to the patient's anatomy, and especially biomechanics.

[0069] It will be appreciated that the invention solves the problem of total arthroplasty implants being intolerant of translation, only allowing rotation, in which any translation causes disruptive forces to be transmitted through the cup, leading to cup failure. The implants of the invention on the other hand allow some degree of translation while not losing two axes.

[0070] Alternative Cup Configuration: Three Spikes

[0071] Figs. 6 to 9 show a cup 50 of an implant of another embodiment. The cup 50 comprises a rim 51 defining a chamber 57 to receive a retainer. On the proximal side the cup 50 has circumferential grooves 53 for improved anchoring in a proximal bone such as the trapezium and three equally spaced spikes 54 having tapered side edges 59 and tips 60. The retainer is held in the cup for the above embodiments.

[0072] Alternative Cup Configuration: Single Spikes

[0073] Referring to Figs. 10 and I l a total arthroplasty implant 100 has the head 10, the retainer 20, and a cup 130 with a single anchor spike 132 which is on -axis. The implant also has an intramedullary stem 150 with a socket to receive the ball 11 and is in this case suitable for CMC joint replacement. The stem 150 may be used with any of the implants described above to complete the total arthroplasty implant.

[0074] Components of embodiments can be employed in other embodiments in a manner as would be understood by a person of ordinary skill in the art. The invention is not limited to the embodiments described but may be varied in construction and detail. Different configurations may be provided to achieve the advantageous features of a cup which can anchor in a proximal bone such as the trapezium and has mechanical lock features to hold a retainer to prevent it moving distally, and a retainer with locking features to retain the head from moving distally. It is preferred that these locking features are overhang ledges such as the ledges 36 of the cup and the ledge 25 of the retainer.

Claims

Claims1. A total arthroplasty joint implant comprising: a cup (30) for anchoring in a proximal bone, a head (10) with a ball (11) on a neck (12) and said next is on a base (14), wherein the neck extends distally in a longitudinal proximal-distal direction for the ball (11) to engage a socket of a component (150) for anchoring in a distal bone, and a retainer (20) configured to be held within a volume (37) of the cup, and being configured to hold said base (13), wherein the implant is configured such that said base (13) has lateral freedom of movement across said longitudinal direction.

2. An implant as claimed in claim 1, further comprising a distal stem (150) having a socket engaging the ball and being configured for intramedullary engagement in a distal bone.

3. An implant as claimed in claim 1 or claim 2, wherein said freedom of lateral movement is in the range of 0.5 mm to 3.0 mm.

4. An implant as claimed in claim 3, wherein said lateral freedom of movement is in the range of 1.0 mm to 2.0 mm.

5. An implant as claimed in any preceding claim, wherein the retainer has an opening (24) and the neck (12) extends through said opening.

6. An implant as claimed in claim 5, wherein the retainer opening (24) has a diameter (D2) which is larger than a diameter (DI) of the ball (11).

7. An implant as claimed in any preceding claim, wherein the base (13) is disc shaped.

8. An implant as claimed in any preceding claim, wherein the base (13) and the cup (30) have engaging surfaces (15, 34) which extend laterally across the longitudinal axis, the base (14) being retained (25) by the retainer (20) in contact with said cup surface (34).

9. An implant as claimed in claim 8, wherein the said base and cup surfaces (15, 24) are planar.

10. An implant as claimed in any preceding claim, wherein the retainer (20) comprises an overhang rim (25) extending radially inwardly to retain the base (13) of the head within the volume of the cup.

11. An implant as claimed in any preceding claim, wherein the cup (30) comprises an inwardly directed overhang rim (36) to hold the retainer (20), and the retainer comprises an inwardly directed overhang rim (25) to hold the base (13) and press it against a proximal surface (34) of the cup in a lateral plane.

12. An implant as claimed in any preceding claim, wherein the retainer (20) has a distal rim (22) which overlies a portion of the cup.

13. An implant as claimed in any preceding claim, wherein the cup comprises proximal anchors (33, 132) for anchoring in a proximal bone.

14. An implant as claimed in any preceding claim, wherein the retainer (20) extends proximally to surround at least part of the base (13).

15. An implant as claimed in claim 14, wherein the retainer (20) extends proximally to surround all of the head base (13), the retainer having a radially inward facing surface (26) for abutment with the base (13) upon extreme lateral movement of the base in use.

16. An implant as claimed in claim 15, wherein the retainer comprises a radially outward facing proximal rim (27) for engaging a corresponding groove (37) of the cup and said rim (27) overlaps in the longitudinal dimension with the chamber surface (26) which is for lateral abutment with the base (13).

17. An implant as claimed in claim 16, wherein the retainer (20) has a radially outward facing distal rim (22) and said retainer proximal rim (27) and said distal rim (22) form together a radially outward facing groove for engagement with a ridge (36) of the cup.

18. An implant as claimed in any preceding claim, wherein the head (11) extends distally from a distal-most surface of the retainer and the cup by an extension distance (D3) in the range of 7 mm to 9 mm.

19. An implant as claimed in any preceding claim, wherein, wherein the head extends distally from a distal-most surface of the retainer and the cup by an extension distance (D3), and the extension distance (D3) is at least 30% of a total longitudinal dimension of the assembled cup, retainer and head.

20. An implant as claimed in any preceding claim, wherein the retainer has a Shore hardness in the range of 62 D to 89 D.

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