Apparatus for fixing a prosthesis to a bone

Abstract

The invention relates to a prosthesis retaining plug (200) for securing a prosthesis (302) comprising polymer material to a bone. The prosthesis retaining plug comprises a body (202) having a first end (208) and a second end (210), the first end insertable into a cavity in the bone, and the second end having an opening (204) adapted to receive at least a portion of the prosthesis. The body comprises a polymeric material having a repeat unit of formula (1) where t1 and w1 independently represent 0 or 1 and v1 represents 0, 1 or 2, and at least a portion of the body is resiliently deformable to receive said portion of the prosthesis.

Description

APPARATUS FOR FIXING A PROSTHESIS TO A BONETECHNICAL FIELD

[0001] The present invention relates to an apparatus for fixing a prosthesis to a bone, for example an apparatus for fixing a knee implant (e.g. a femoral implant and / or a tibial implant) to a bone.BACKGROUND

[0002] Prosthetic implants can be used to partially or completely replace diseased and / or damaged joint tissue. Such implants may include articulating surfaces that replace the bone’s natural articulating surfaces. For example, an implant for a knee replacement may include a femoral implant and / or a tibial implant. A femoral implant may be implanted on the distal end of the femur and may replace the articulating surfaces of the femur. A tibial implant may be implanted on the proximal end of the tibia and may replace the articulating surfaces of the tibia. In operation, the articulating surfaces of the femoral implant articulate against the articulating surfaces of the tibial implant.

[0003] Various materials have been used for the femoral and tibial implants. For example, the implants may be made from metal, for instance, cobalt-chrome. Metal implants may have a significantly higher stiffness and tensile strength than bone. As a result, metal implants can have an increased tendency to shield the underlying bone from stresses that would normally be applied to the joint during use. According to Wolff’s law, bone remodels in response to applied loads. If a bone is shielded from these loads, the bone will not be exposed to the stimulus required to maintain bone mass. This can lead to a loss of bone mass that may e.g. increase the chances of the implant coming loose. Knee implants formed from polymeric compositions may be formulated to have mechanical properties that are more compatible with the mechanical properties of bone.

[0004] To implant a femoral implant onto the distal femoral bone, the bone may need to be resected. The implant may be held in position using a bone cement, e.g. polymethylmethacrylate (PMMA). Preparation and application of the bone cement in an operating theatre may add time and complexity to the surgical procedure. A bone cement fixation requires a mechanical bond to be formed between the cement and the bone, as well as a mechanical bond between the cement and the implant. In some instances, large loads transferred through the implant can cause one, or both, of these mechanical bonds to degrade, particularly over time. Furthermore, some patients may have adverse reactions to the bone cement.

[0005] Cementless implants may be used. Primary fixation of cementless implants may be obtained by the formation of compressive forces and subsequent shearing forces atthe bone-implant interface during implantation. These compressive and shearing forces can be influenced by, for example, the interference fit and the frictional properties of the implant surface. The interference fit refers to the dimensional difference between the prepared bone cuts and the implant. The femoral bone may be cut slightly larger than the internal implant dimensions, resulting in a press fixation after implantation. The coefficient of friction may be altered by the roughness of the implant surface.

[0006] It is an object of the invention to ameliorate the aforementioned disadvantages associated with implantation using bone cement by providing an improved prosthesis fixing kit and method for cementless implantation.BRIEF SUMMARY

[0007] According to a first aspect of the invention, there is provided a prosthesis retaining plug for securing a prosthesis comprising a polymer material to a bone, the prosthesis retaining plug comprising a body having a first end and a second end, the first end insertable into a cavity in the bone, the second end having an opening adapted to receive at least a portion of the prosthesis, wherein the body comprises a polymeric material having a repeat unit of formula (I)where t1 and w1 independently represent 0 or 1 and v1 represents 0, 1 or 2, and at least a portion of the body is resiliently deformable to receive said portion of the prosthesis.

[0008] Preferably, the body is dimensioned to receive a fixing pin of the prosthesis.

[0009] Preferably, the body is substantially tubular in shape.

[0010] Advantageously, the body is configured so as to resiliently deform or expand when the portion, preferably the fixing pin, of the prosthesis is inserted into the prosthesis retaining plug. In so doing, the retaining plug can be secured into the bone cavity and the prosthesis secured into the plug so itself also to locate in said cavity.

[0011] Preferably, the prosthesis retaining plug comprises one or more slots, preferably the or each slot extending longitudinally to the body, preferably parallel to a longitudinal axis of the body.

[0012] Preferably, the first end is a distal end of the body. Preferably, the second end is a proximal end.

[0013] Preferably, the or each slot may extend to the distal end, preferably adjacent said end.

[0014] The one or more slots may consist of a plurality of slots evenly distributed around a circumference of the prosthesis retaining plug (e.g., four, five or 6 slots).

[0015] The prosthesis retaining plug may also include a flange.

[0016] The prosthesis retaining plug may also include a non-return feature arranged to engage the prosthesis, preferably the fixing pin thereof, when the fixing pin is inserted into the prosthesis retaining plug and prevent removal of the fixing pin from the prosthesis retaining plug.

[0017] The non-return feature may comprise one of a groove and a protrusion. The fixing pin may include the other of the groove and the protrusion. The protrusion may be arranged to be received in the groove when the fixing pin is received in the prosthesis retaining plug.

[0018] The fixing pin may be retained in the prosthesis retaining plug by the non-return feature when it has been inserted into the passage of the prosthesis retaining plug to its full extent.

[0019] The prosthesis retaining plug may include the protrusion. The protrusion may be inwardly protruding at or near an end of the prosthesis retaining plug. The protrusion may be inwardly protruding at or near the distal end of the prosthesis retaining plug.

[0020] In another example, an end (e.g., the free end) of the prosthesis retaining plug may form a non-return feature. In particular, the end of the prosthesis retaining plug may be configured to abut an overhanging part of the fixing pin when the fixing pin is received in the prosthesis retaining plug. The overhanging part of the fixing pin may include an overhanging abutment surface. For example, the fixing pin may include a step that abuts the free end of the prosthesis retaining plug when the fixing pin is received in the prosthesis retaining plug so as to retain the fixing pin in the prosthesis retaining plug.

[0021] Preferably, the body may also include an external surface shaped to grip an internal surface of the cavity in the bone. The external surface of the prosthesis retaining plug may include a plurality of projections. The projections may form ribs or ridges.

[0022] The plurality of projections may extend in a circumferential direction around the prosthesis retaining plug. The plurality of projections may extend around a circumference of the prosthesis retaining plug.

[0023] Each of the plurality of projections may include a distal surface and an opposing surface. The distal surface may be angled relative to the longitudinal axis of the prosthesis retaining plug to aid insertion of the prosthesis retaining plug into the cavity in the bone.

[0024] The opposing surface may be substantially perpendicular to the longitudinal axis of the prosthesis retaining plug. Alternately, the opposing surface may be angled relative to the longitudinal axis of the prosthesis retaining plug. In this wat the opposing surfaces act against removal of the prosthesis retaining plug from the hole in the bone.

[0025] A longitudinal length of the prosthesis retaining plug may be less than a longitudinal length of the fixing pin. A longitudinal length of an element may be taken to be a straight end to end distance of the element measured substantially parallel to the longitudinal axis of the element. In this way, an end of the fixing pin protrudes from the end of the prosthesis retaining plug when fully inserted.

[0026] The fixing pin may include a basal end at the prosthesis and a free end opposite the basal end.

[0027] The body may be tapered inwards towards the first end.

[0028] Said polymeric material and / or the polymer material preferably consists essentially of a repeat unit of formula I. Preferred polymeric and / or polymer materials comprise (especially consist essentially of) a said repeat unit wherein t1 = 1 , v1=0 and w1=0; t1 =0, v1=0 and w1=0; t1 =0, w1 = 1 , v1=2; or t1 =0, v1 = 1 and w1=0. More preferred comprise (especially consist essentially of) a said repeat unit wherein t1 = 1 , v1=0 and w1=0; or t1=0, v1=0 and w1=0. The most preferred comprises (especially consists essentially of) a said repeat unit wherein t1 = 1 , v1=0 and w1=0.

[0029] In preferred embodiments, said polymeric material and / or the polymer material is selected from polyetheretherketone, polyetherketone, polyetherketoneetherketoneketone and polyetherketoneketone. In a more preferred embodiment, said polymeric material and / or the polymer material is selected from polyetherketone and polyetheretherketone. In an especially preferred embodiment, said polymeric material and / or the polymer material is polyetheretherketone.

[0030] The prosthesis retaining plug may consist of a polyaryletherketone (PAEK). For example, the prosthesis retaining plug may consist of polyetheretherketone (PEEK). In a most preferred arrangement, both the prosthesis and the retaining plug each consist of PAEK, preferably both PEEK. In so doing, the retaining plug resiliently deforms to be received on or over a portion of the prosthesis. For example, the or each retaining plug is received by a stem or fixing pin on a femoral implant in use.

[0031] The PAEK may comprise a filler material. Such filler materials may be selected from inorganic fibrous materials and non-melting and high-melting organic fibrous materials, such as aramid fibres, carbon fibre and the like. For instance, the fibres may be selected from glass fibre, carbon fibre, silica fibre, zirconia fibre, silicon nitride fibre, boron fibre, fluorocarbon resin fibre and potassium titanate fibre. Most preferred fibres are carbon fibres. Nanofibres may be employed.

[0032] Suitably, at least 30 vol%, more preferably at least 40 vol%, especially at least 50 vol% of the PAEK material is present. Preferably, up to 70 vol%, up to 65 vol%, up to 50 vol% of carbon fibre filler is present in the PAEK.

[0033] The PAEK may comprise barium sulphate, preferably in an amount at least 1 wt%, suitably at least 2 wt%, preferably at least 3 wt% barium sulphate, for example, 5 or 6wt%. Said barium sulphate is preferably intimately mixed with the PAEK polymer, suitably so the barium sulphate and said polymer define a substantially homogenous mixture.

[0034] Preferably, the PAEK comprises a bioactive material, preferably selected from the group consisting of apatites, calcium phosphates and calcium sulfates.

[0035] Suitably, the bioactive material comprises an apatite. Suitably, the bioactive material comprises hydroxyapatite (HA). The bioactive material may comprise calcium phosphate. The bioactive material may comprise tri calcium phosphate. The bioactive material may comprise alpha and / or beta tri calcium phosphate.

[0036] In one arrangement, the retaining plug comprises PAEK including bioactive material, and the prosthesis comprises or consists of PEEK.

[0037] The polyaryletherketone of the prosthesis and / or the retaining plug may have a tensile strength, measured in accordance with IS0527 (specimen type 1 b) tested at 23°C at a rate of 50mm / minute of at least 20 MPa, preferably at least 60 MPa, more preferably at least 80 MPa. The tensile strength is preferably in the range 80-1 10 MPa, more preferably in the range 80- 100 MPa.

[0038] The polyaryletherketone of the prosthesis and / or the retaining plug may have a flexural strength, measured in accordance with IS0178 (80mm x 10mm x 4mm specimen, tested in three-point-bend at 23°C at a rate of 2mm / minute) of at least 50 MPa, preferably at least 100 MPa, more preferably at least 145 MPa. The flexural strength is preferably in the range 145- 180MPa, more preferably in the range 145-164 MPa. The polyaryletherketone may have a flexural modulus, measured in accordance with IS0178 (80mm x 10mm x 4mm specimen, tested in three-point-bend at 23°C at a rate of 2mm / minute) of at least 1 GPa, suitably at least2 GPa, preferably at least 3 GPa, more preferably at least 3.5 GPa. The flexural modulus is preferably in the range 3.5-4.5 GPa, more preferably in the range 3.5-4.1 GPa.

[0039] The polyaryletherketone of the prosthesis and / or the retaining plug may be amorphous or semi-crystalline. The polyaryletherketone is preferably crystallisable. The polyaryletherketone may be semi-crystalline. The level and extent of crystallinity in a polymer may be measured by wide angle X-ray diffraction (also referred to as Wide Angle X-ray Scattering or WAXS), for example as described by Blundell and Osborn (Polymer 24, 953, 1983). Alternatively, crystallinity may be assessed by Differential Scanning Calorimetry (DSC). The level of crystallinity of said polyaryletherketone may be at least 1 %, suitably at least 3%, preferably at least 5% and more preferably at least 10%. In especially preferred embodiments, the crystallinity may be greater than 25%. It may be less than 50% or less than 40%. The main peak of the melting endotherm (Tm) of said polyaryletherketone (if crystalline) may be at least 300°C.

[0040] The retaining plug may have a length smaller than the portion of a prosthesis, for example, the fixing pin such that after insertion of the fixing pin into the prosthesis retaining plug, the free end of the fixing pin protrudes beyond the prosthesis retaining plug in the hole in the bone.

[0041] The fixing pin may also include a non-return feature arranged to engage the prosthesis retaining plug, in particular a corresponding non-return feature of the prosthesis retaining plug or an end of the prosthesis retaining plug, to prevent removal of the fixing pin from the prosthesis retaining plug after insertion of the fixing pin into the prosthesis retaining plug.

[0042] The non-return feature may include one of a groove, or a protrusion, or an overhanging abutment surface (e.g., a step). The non-return feature may be positioned at or near a free end of the fixing pin.

[0043] The present disclosure further provides a prosthesis fixing kit for fixing a prosthesis in a cavity in a bone, the prosthesis fixing kit including a fixing pin integrally formed with, or configured to be secured to, the prosthesis, and a prosthesis retaining plug for insertion into the cavity in the bone, where the prosthesis retaining plug has a passage configured to receive the fixing pin and where the prosthesis retaining plug is configured to expand upon receiving the fixing pin to fix the prosthesis to the bone.

[0044] The fixing pin and the prosthesis retaining plug may include any of the features as set out above.

[0045] The present disclosure further provides a prosthesis kit including a prosthesis and a prosthesis fixing kit.

[0046] The prosthesis may be a prosthetic joint.

[0047] The prosthetic joint may comprise or consist of a femoral implant, a tibial implant or a patellar implant.

[0048] The prosthesis and the prosthesis fixing kit may include any of the features as set out above.

[0049] The prosthesis kit may include a plurality of prosthesis fixing kits, for example two, three, four or six prosthesis fixing kits.

[0050] The fixing pins of each of the prosthesis fixing kits may be integrally formed with the prosthesis.

[0051] The present disclosure further provides a method of fixing a prosthesis to a bone, including the steps of: providing a prosthesis fixing kit according to an aspect; inserting the prosthesis retaining plug into a hole in the bone; inserting the fixing pin into the prosthesis retaining plug such that the prosthesis retaining plug expands upon receiving the fixing pin thereby fixing the prosthesis to the bone.

[0052] The method may include an additional step of securing the fixing pin to the prosthesis.

[0053] Surgical preparation steps may include resecting the bone to shape the bone to receive the prosthesis, and drilling one or more holes in the bone to receive the prosthesis retaining plug(s).

[0054] Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0055] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

[0056] FIG. 1 illustrates a prosthetic assembly.

[0057] FIG. 2 illustrates a prosthesis retaining plug of the prosthetic assembly of FIG. 1.

[0058] FIG. 3 illustrates a fixing pin of the prosthetic assembly of FIG. 1.

[0059] FIG. 4 provides an enlarged view of the fixing pin of FIG. 3.

[0060] FIG. 5 illustrates a method of fixing a prosthesis to a bone.DETAILED DESCRIPTION

[0061] FIG. 1 illustrates a prosthetic assembly 100. The prosthetic assembly 100 includes a fixing pin 300 and a prosthesis retaining plug 200. The fixing pin 300 and the prosthesis retaining plug 200 are used to fix a prosthesis 302 to a bone (not illustrated) by first inserting the prosthesis retaining plug 200 into a hole in the bone, then inserting the fixing pin 300 into the prosthesis retaining plug 200. The prosthesis retaining plug 200 is shown in more detail in FIG. 2, and the prosthesis 302, including the fixing pin 300, is shown in more detail in FIG. 3 and FIG. 4.

[0062] In this example, the prosthetic assembly 100 includes two retaining plugs 200, 201 , and two fixing pins 300, 301. Specifically, the prosthetic assembly 100 includes the prosthesis retaining plug 200 and a second prosthesis retaining plug 201 , and the fixing pin 300 and a second fixing pin 301. Except where mutually exclusive: features described in conjunction with the prosthesis retaining plug 200 are to be understood to be applicable to the second prosthesis retaining plug 201 ; and features described in conjunction with the fixing pin 300 are to be understood to be applicable to the second fixing pin 301. In some examples the prosthetic assembly 100 may include only a single prosthesis retaining plug 200 and a single fixing pin 300, or more than two prosthesis retaining plugs 200 and more than two fixing pins 300, for example three prosthesis retaining plugs 200 and three fixing pins 300.

[0063] In the illustrated example each of the fixing pin 300 and the second fixing pin 301 are integrally formed with the prosthesis 302 such that these parts are formed as a single molded element. However, in other examples the fixing pin 300 and the second fixing pin 301 (where present) may be attachable to the prosthesis 302, for example by a threaded connection or similar.

[0064] As shown most clearly in FIG. 3, the fixing pin 300 comprises a basal end 304 at the prosthesis 302 and a free end 306 opposite the basal end. The fixing pin 300 has a length greater than a length of the prosthesis retaining plug 200 such that, after insertion of the fixing pin into the prosthesis retaining plug, the free end 306 protrudes beyond the prosthesis retaining plug.

[0065] As shown most clearly in FIG. 2, the prosthesis retaining plug 200 comprises a substantially cylindrical body 202 which is insertable into a hole formed in the bone. In this example, the bone (not shown) into which the prosthesis retaining plug 200 is inserted is a femur. The substantially cylindrical body 202 has a proximal end 210 and a distal end 208. In use, the distal end 208 is inserted into the hole in the bone. An external surface 212 of the substantially cylindrical body 202 is shaped to grip an internal surface of thehole in the bone. The external surface 212 of the prosthesis retaining plug 200 is preferably generally tapered, being narrower at the distal end 208 than at the proximal end 210.

[0066] The substantially cylindrical body 202 comprises a passage 204 for receiving the fixing pin 300. The prosthesis retaining plug 200 is configured to expand (in this example, resiliently expand), when the fixing pin 300 is inserted into the prosthesis retaining plug 200. The expansion of the prosthesis retaining plug 200 creates an interference fit with the inside of the hole formed in the bone, which fixes the fixing pin 300 and the prosthesis 302 to the bone. Following this, bone growth in and around the prosthesis retaining plug 200 further secures the prosthesis retaining plug 200 to the bone.

[0067] As shown in FIG. 2, the external surface 212 of the substantially cylindrical body 202 is shaped to grip the surface of the hole in the bone. In particular, the external surface 212 of the substantially cylindrical body 202 includes a plurality of projections 214. Each of the plurality of projections 214 extend in a circumferential direction around the prosthesis retaining plug. The plurality of projections 214 form a plurality of ridges or ribs.

[0068] In the illustrated example, each of the plurality of projections 214 comprises a distal surface 216a and an opposing surface 216b. In this example, the distal surface 216a is angled relative to a longitudinal axis 222 of the prosthesis retaining plug 200 to aid insertion of the prosthesis retaining plug 200 into the hole in the bone. In the illustrated example the distal surface 216a is curved, but it will be appreciated that it may be a flat, angled surface or similar. As shown in FIG. 2, the opposing surface 216b is arranged substantially perpendicular to the longitudinal axis 222 of the prosthesis retaining plug 200. In this way, the opposing surface 216b acts against removal of the prosthesis retaining plug 200 from the hole in the bone. In other examples, the opposing surfaces 216b are angled back towards the substantially cylindrical body 202 to resist removal of the prosthesis retaining plug 200 from the hole in the bone. Such plurality of projections 214 may be termed fir-tree projections.

[0069] The passage 204 of the prosthesis retaining plug 200 is shaped complementarily to the fixing pin 300. In this example, the passage 204 tapers inwards from the proximal end 210 towards the distal end 208.

[0070] The passage 204 of the prosthesis retaining plug 200 is sized to receive the free end 306 of the fixing pin 300. Before the fixing pin 300 is inserted into the prosthesis retaining plug 200, the passage 204 has a slightly smaller internal diameter than the external diameter of the fixing pin 300. The internal diameter of the passage 204 may be less than the external diameter of the fixing pin 300 along at least part of the length of theprosthesis retaining plug 200. This configuration ensures the prosthesis retaining plug 200 expands when the fixing pin 300 is inserted into the passage 204.

[0071] The prosthesis retaining plug 200 consists of a resiliently deformable material, being polyaryletherketone (PAEK), particularly polyetheretherketone [PEEK],

[0072] Said deformable material allows the prosthesis retaining plug 200 to deform to accommodate the fixing pin 300 when it is inserted into the passage 204.

[0073] In the illustrated examples the prosthesis retaining plug 200 has a plurality of slots 206 which extend longitudinally along a length of the substantially cylindrical body 202. The plurality of slots 206 are evenly distributed around a circumference of the prosthesis retaining plug 200. Each of the plurality of slots 206 extends to the distal end 208 so as to each form a slit in the substantially cylindrical body 202. In this way, the plurality of slots 206 divide the substantially cylindrical body 202 into segments. The plurality of slots 206 divide the plurality of projections 214 into segments.

[0074] The plurality of slots 206 aid in the expansion of the prosthesis retaining plug 200 when the fixing pin 300 is inserted into the passage 204, as the segments of the substantially cylindrical body 202 will splay outwards on insertion of the fixing pin 300. However, it will be appreciated that the plurality of slots 206 are not essential, and the expansion of the prosthesis retaining plug 200 may instead be by deformation of the substantially cylindrical body 202. In addition, in some examples the plurality of slots 206 do not extend all the way to the distal end 208 of the substantially cylindrical body 202. In such examples, the parts of the substantially cylindrical body 202 separated by the plurality of slots 206 would bulge outwards on insertion of the fixing pin 300.

[0075] The expansion of the prosthesis retaining plug 200, when the prosthesis retaining plug 200 is within the hole in the bone, ensures that the external surface 212 of the prosthesis retaining plug 200 (including the plurality of projections 214 if present) are pushed outwards away from the longitudinal axis 222 of the prosthesis retaining plug 200. This creates an interference fit between the external surface 212 and the bone to secure the fixing pin 300 and prosthesis 302 to the bone. Following this, bone growth into and around the prosthesis retaining plug 200 further secures the prosthesis retaining plug 200 to the bone.

[0076] When the plurality of projections 214 are present the opposing surfaces 216b of the plurality of projections 214 secure the prosthesis retaining plug 200 with respect to the bone, preventing the prosthesis retaining plug 200 from being withdrawn from the hole. In particular, the opposing surfaces 216b of the plurality of projections 214 resist movement of the prosthesis retaining plug 200 out of the hole. Advantageously, bone growth mayoccur between and around the plurality of projections 214 to increase fixation of the prosthesis retaining plug 200 to the bone after implantation. The plurality of projections 214 thereby provide short term fixation of the prosthesis retaining plug 200 (and the prosthetic assembly 100) to the bone, and longer term the fixation is improved by bone growth between and around the plurality of projections 214. Such bone growth typically occurs within 6 weeks of implantation.

[0077] Securing the prosthesis retaining plug 200 with respect to the bone also secures the fixing pin 300 and the prosthesis 302 to the bone. This is because the prosthesis retaining plug 200 is secured to the fixing pin 300 after the fixing pin 300 has been inserted into the prosthesis retaining plug 200. In the present example, this is achieved by way of a non-return feature provided on each of the prosthesis retaining plug 200 and the fixing pin 300. The non-return feature connects the prosthesis retaining plug 200 to the fixing pin 300 on full insertion of the fixing pin 300 into the prosthesis retaining plug 200.

[0078] As shown in FIG. 2, the non-return feature on the prosthesis retaining plug 200 is a protrusion 218 arranged to engage the fixing pin 300 when the fixing pin 300 is inserted into the prosthesis retaining plug 200. In this example, the protrusion 218 extends around an internal circumference of the passage 204 at the distal end 208 of the prosthesis retaining plug 200. The protrusion 218 is configured to engage the non-return feature of the fixing pin 300 when the fixing pin 300 is received within the prosthesis retaining plug 200. Interaction between the non-return features of the fixing pin 300 and prosthesis retaining plug 200 inhibits removal of the fixing pin 300 from the prosthesis retaining plug 200.

[0079] The protrusion 218 projects towards the longitudinal axis 222 of the prosthesis retaining plug 200. The protrusion 218 has a first surface 224a, which faces away from the proximal end 210, and a second surface 224b which faces towards the proximal end 210. In this example, the first surface 224a forms part of an end face of the distal end 208.

[0080] As shown in FIG. 3 and FIG. 4, the fixing pin 300 includes a non-return feature positioned at the free end 306 of the fixing pin 300. In this example, the non-return feature consists of a groove 318 arranged to engage the protrusion 218 of the prosthesis retaining plug 200 to prevent removal of the fixing pin 300 from the prosthesis retaining plug 200 after insertion of the fixing pin 300 into the prosthesis retaining plug 200.

[0081] The first surface 224a is configured to prevent the fixing pin 300 from being withdrawn from the passage 204 once non-return features of the prosthesis retaining plug 200 and the fixing pin 300 have engaged. In this example, the first surface 224a is substantially perpendicular to the longitudinal axis 222. In contrast, the second surface224b is angled with respect to the longitudinal axis 222 preventing the protrusion 218 from limiting the extent to which the free end 306 of the fixing pin 300 can be inserted into the passage 204.

[0082] It will be appreciated that the non-return feature may be a groove on the prosthesis retaining plug 200 and a protrusion on the fixing pin 300. In an alternative example the fixing pin 300 may include a step facing the basal end 304, and the step may abut against the distal end 208 of the prosthesis retaining plug 200 after insertion of the fixing pin 300 into the prosthesis retaining plug 200 to prevent the fixing pin 300 being removed from the prosthesis retaining plug 200. The step may be formed by an overhanging abutment surface.

[0083] Referring to FIG. 2, the prosthesis retaining plug 200 comprises a flange 220 disposed at the proximal end 210 of the substantially cylindrical body 202. The flange 220 has a larger external diameter than the remainder of the prosthesis retaining plug 200. The flange 220 also has a larger external diameter than the internal diameter of the hole in the bone into which the prosthesis retaining plug 200 is to be inserted. In this way, the flange 220 is configured to remain outside of the hole when the prosthesis retaining plug 200 is inserted into the hole in the bone. In use, the flange 220 limits the extent to which the prosthesis retaining plug 200 can be inserted into to the hole. The flange 220 prevents the prosthesis retaining plug 200 from being pushed completely inside the hole when the fixing pin 300 is inserted into the passage.

[0084] An outer diameter of proximal end 210 is substantially same as a maximum outer diameter of the plurality of projections 214. The proximal end 210 and the flange 220 are configured to prevent fluid ingress between hole and prosthesis retaining plug 200.

[0085] As shown in FIG. 3, the fixing pin 300 (and the second fixing pin 301 where present) are integrally formed with the prosthesis 302 so as to form a single molded element. In this example, the fixing pin 300 and the prosthesis 302 consists of a polyaryletherketone such as polyetheretherketone.

[0086] The prosthesis 302 shown in FIG. 3 is a femoral implant having a bone-facing interface 308 and an articulating interface 310 that, during use, articulates against the bearing interface of a tibial implant (not shown). As shown in FIG. 3, the articulating external surface 212 comprises a first condyle 312 and a second condyle 314 corresponding to lateral and medial condyles of the femoral tissue of the original knee joint respectively. An intercondylar notch 316 is provided between the first condyle 312 and second condyle 314 and arranged to receive a stem of the tibial implant.

[0087] The basal end 304 of the fixing pin 300 is contiguous with the prosthesis 302. The fixing pin 300 is shaped complementarily to the passage 204 of the prosthesis retaining plug 200. In this example, the fixing pin 300 tapers inwards from the basal end 304 towards the free end 306.

[0088] FIG. 4 shows a cross-section of the fixing pin 300 of FIG. 3. As shown by comparison of FIG. 3 and FIG. 4, the groove 318 extends partially, in this case half-way, around the circumference of the fixing pin 300. The groove 318 is positioned to align with the protrusion 218 on the prosthesis retaining plug 200 as described above. In this example, the groove 318 is positioned on a side of the fixing pin 300 which faces away from the intercondylar notch 316. In other examples the groove 318 extends fully around the fixing pin 300.

[0089] The groove 318 is arranged to receive a width of the protrusion 218 of the prosthesis retaining plug 200 when the fixing pin 300 is received within the prosthesis retaining plug 200. As best seen is FIG. 4, the groove 318 is inwardly protruding and arranged to engage the protrusion 218 of the prosthesis retaining plug 200 to prevent removal of the fixing pin 300 from the prosthesis retaining plug 200 after insertion of the fixing pin 300 into the prosthesis retaining plug 200. The groove 318 comprises a first groove surface 320a, which faces towards the basal end 304 of the fixing pin 300, and a second groove surface 320b, which faces towards the free end 306. The first groove surface 320a is configured to align and cooperate with the first surface 224a of the prosthesis retaining plug 200, when the fixing pin 300 is received within the prosthesis retaining plug 200, to prevent the fixing pin 300 from being withdrawn from the prosthesis retaining plug 200. In this example, the first groove surface 320a is substantially perpendicular to a long axis 322 of the fixing pin 300. In contrast, the second groove surface 320b is angled with respect to the long axis 322 such that when the fixing pin 300 is received within the prosthesis retaining plug 200, the second groove surface 320b is aligned with and engages the second surface 224b of the prosthesis retaining plug 200.

[0090] The length of the fixing pin 300 is greater than the length of the prosthesis retaining plug 200. As such, after insertion of the fixing pin 300 into the prosthesis retaining plug 200, the free end 306 protrudes beyond the prosthesis retaining plug 200.

[0091] As shown in FIG. 4, the free end 306 of the fixing pin 300 has a convex curved surface. This shape enables the free end 306 to slidingly interact with the second surface 224b of the protrusion 218, thereby allowing the protrusion 218 to pass over the free end 306 and engage the groove 318 when the fixing pin 300 is inserted into the passage 204.

[0092] FIG. 5 is a schematic illustration of a method 500 of fixing a prosthesis to a bone.

[0093] Preparatory surgical steps include resection of the bone to shape the bone to receive the prosthetic assembly 100. In the example of a femoral knee implant the bone resection includes cutting the bone to form a flat surface on each of the medial and lateral condyles, and, if appropriate, further shaping the medial and lateral condyles to receive the prosthetic assembly 100. The preparatory surgical steps also include drilling or otherwise forming a hole in the bone. In the example of a femoral knee implant, one hole is formed in each of the medial and lateral condyle.

[0094] After surgical preparation, a first step 501 of the method 500 includes providing a prosthesis kit that contains each of the parts discussed in relation to the prosthetic assembly 100, such as the prosthesis retaining plug 200, the fixing pin 300 and the prosthesis 302.

[0095] A second step 502 includes inserting the prosthesis retaining plug 200 into the hole in the bone.

[0096] A third step 503 includes inserting the fixing pin 300 into the prosthesis retaining plug 200 such that the prosthesis retaining plug 200 resiliently expands upon receiving the fixing pin 300, thereby fixing the prosthesis 302 to the bone.

[0097] In all figures hereinbefore described, both the femoral knee implant together with the retaining plug comprise PEEK.

[0098] Features described in conjunction with a particular aspect, embodiment or example of the disclosure are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of the method so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of the method so disclosed.

Claims

CLAIMS1. A prosthesis retaining plug for securing a prosthesis comprising a polymer material to a bone, the prosthesis retaining plug comprising a body having a first end and a second end, the first end insertable into a cavity in the bone, the second end having an opening adapted to receive at least a portion of the prosthesis, wherein the body comprises a polymeric material having a repeat unit of formula (I)where t1 and w1 independently represent 0 or 1 and v1 represents 0, 1 or 2, and at least a portion of the body is resiliently deformable to receive said portion of the prosthesis.

2. A prosthesis retaining plug as claimed in claim 1 , wherein the body is substantially cylindrical in shape, and the second end comprises a flange.

3. A prosthesis retaining plug as claimed in any one of the preceding claims, wherein one or more slots are provided, the or each slot extending longitudinally along the body.

4. A prosthesis retaining plug as claimed in claim 3, wherein the one or more slots consists of a plurality of slots evenly distributed around a circumference of the body.

5. A prosthesis retaining plug as claimed in any one of the preceding claims, wherein said plug comprises a non-return feature arranged to engage a fixing pin of a prosthesis when the fixing pin is inserted into said plug so as to prevent removal of the fixing pin.

6. A prosthesis retaining plug as claimed in claim 5, wherein the non-return feature comprises one of a groove and a protrusion.

7. A prosthesis retaining plug as claimed in claim 6, wherein said plug comprises the protrusion, and wherein the protrusion is inwardly protruding at or near an end of the prosthesis retaining plug.

8. A prosthesis retaining plug as claimed in any one of the preceding claims wherein the body is tapered.

9. A prosthesis retaining plug as claimed in any one of the preceding claims, wherein the body comprises an external surface shaped to grip an internal surface of the hole in the bone, and wherein the external surface comprises a plurality of projections, optionally wherein the plurality of projections extend in a circumferential direction around the prosthesis retaining plug.

10. A prosthesis retaining plug as claimed in claim 9, wherein each of the plurality of projections comprises a distal surface that is angled relative to a longitudinal axis of said plug to aid insertion thereof into the bone cavity, and an opposing surface substantially perpendicular to the longitudinal axis of the prosthesis retaining plug to act against removal of said plug from the bone.

11. A prosthesis fixing kit for fixing a prosthesis in a cavity in a bone, the comprising at least one prosthesis retaining plug as claimed in any one of claims 1 to 10 and a prosthesis comprising a polymer material, the prosthesis having a fixing pin, wherein said plug has a passage configured to receive the fixing pin and wherein the prosthesis retaining plug is configured to resiliently expand upon receiving the fixing pin to fix the prosthesis to the bone.

12. A prosthesis fixing kit as claimed in claim 11 , wherein both the polymer material and the polymeric material comprise polyetheretherketone.

13. A prosthesis fixing kit as claimed in claim 12, wherein both the prosthesis and the retaining plug comprise polyetheretherketone having a tensile strength, measured in accordance with IS0527 (specimen type 1 b) tested at 23°C at a rate of 50mm / minute of at least 20 MPa.

14. A prosthesis fixing kit as claimed in claim 12 or 13, wherein both the polymer material and the polymeric material consist of polyetheretherketone.

15. A prosthesis fixing kit as claimed in any one of 11 to 14, wherein the prosthesis is a femoral implant, a tibial implant or a patellar implant.

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