WO2009073924A1

WO2009073924A1 - Endoprosthesis for a metatarsophalangeal joint

Info

Publication number: WO2009073924A1
Application number: PCT/AU2008/001827
Authority: WO
Inventors: Timothy Edward Schneider
Original assignee: Timothy Edward Schneider
Priority date: 2007-12-12
Filing date: 2008-12-12
Publication date: 2009-06-18

Abstract

An endoprosthesis for a metatarsophalangeal (MTP) joint having at least one sesamoid bone proximate thereto. The endoprosthesis includes at least one prosthesis-half defining a prosthetic sliding surface, and being configured to be anchored in a metatarsal head of the MTP joint. The prosthetic sliding surface is designed with one or more contours configured to accommodate the one or more sesamoid bones during articulation of the MTP joint. The invention is particularly applicable as an endoprosthesis for the basal joint of a large toe.

Description

ENDOPROSTHESIS FOR A METATARSOPHALANGEAL JOINT

Field of the Invention

The present invention generally relates to an endoprosthesis for a metatarsophalangeal joint. The invention is particularly applicable as an endoprosthesis for the basal joint of a large toe and it will be convenient to hereinafter disclose the invention in relation to that exemplary application. However, it is to be appreciated that the invention is not limited to that application and could be used for other similar joints, for example a finger joint.

Background of the Invention

The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

A number of endoprosthesis are available for joint replacement of a metatarsophalangeal (MTP) joints such as a basal joint of a large toe. Each of these endoprosthesis includes one or more prosthesis halves that can be implanted in one or both of the proximal phalanx and the metatarsal to replace the joint surface of these bones.

United States Patent Publication No. 2006/0074492 discloses one prior endoprosthesis for a MTP joint. The total joint replacement form of this endoprosthesis comprises a mushroom shaped proximal prosthesis-half having a convex prosthetic sliding surface and an elongate anchor pin configured to be inserted into the metatarsal head. The prosthetic sliding surface of the proximal prosthesis-half is configured for optimal cooperation with a cooperating concave prosthetic sliding surface of a corresponding distal prosthetic-half. However, some recipients of this type of endoprosthesis have found that articulation of a MTP joint that includes the endoprosthesis can develop pain, particularly in the region of one or more of the sesamoid bones of the joint. Some recipients have also experienced some stiffness when articulating a MTP joint that includes this type of endoprosthesis. In this respect, it is thought that the shape of the one or both of the proximal prosthesis-half or distal prosthesis- half may not be optimal to accommodate all the features and adjoining structures of the MTP joints for which these types of endoprosthesis are designed to replace.

It would therefore be desirable to provide an endoprosthesis that can improve articulation of a MTP joint including the endoprosthesis.

Summary of the Invention

According to a first aspect of the present invention, there is provided an endoprosthesis for a metatarsophalangeal (MTP) joint having at least one sesamoid bone proximate thereto, the endoprosthesis including at least one prosthesis-half defining a prosthetic sliding surface, the prosthesis-half configured to be anchored in a metatarsal head of the MTP joint, the prosthetic sliding surface having one or more contours configured to accommodate the one or more sesamoid bones during articulation of the MTP joint.

In this regard, it has been found that the design of the prosthetic sliding surface of existing MTP endoprosthesis do not take into account the natural shape of the MTP joint and in particular the shape of the metatarsal head (MTH) of that joint. For example, the MTH of the basal joint of a big toe includes one or more sesamoid grooves which accommodate movement of the proximate sesamoid bones during articulation of the joint. During articulation of the basal joint of a big toe, the sesamoid bones tend to follow these sesamoid grooves. Essentially, each sesamoid bone uses the sesamoid grooves as a track along which the sesamoid bones can slide. However, most existing MTP endoprosthesis do not account for this groove, and therefore provide a shape where the natural sesamoid groove comes to an abrupt end at the start of the endoprosthesis. When this type of endoprosthesis is implanted into a MTP joint which includes proximate sesamoid bones, each sesamoid bone can only slide in the remaining natural sections of the sesamoid grooves until each sesamoid bone engages the prosthetic sliding surface of the implanted prothesis half. At this point each sesamoid bone is forced in an unnatural path over the prosthetic sliding surface of the prothesis half. As can be appreciated, this unnatural path can place strain on each sesamoid bone, causing the user discomfort. The present invention addresses this problem by including a contour to accommodate the movement of each sesamoid bone during articulation of a MTP joint that includes the endoprosthesis.

The MTP joint in which the endoprosthesis of the present invention can be used can be a basal joint of a large toe, a toe joint, or a finger joint. However, the invention is preferably directed to the basal joint of a large toe, and more preferably the big toe. It should also be appreciated, that each individual will have a different toe size. Accordingly, it is preferable for the endoprosthesis to be provided in various sizes to fit with various sized MTP joints. More preferably, the endoprosthesis will be provided in at least three different sizes to accommodate varying toe anatomy.

The prosthetic sliding surface is designed to slidingly cooperate with a corresponding surface associated with the proximal phalanx of the MTP joint. Accordingly, the prosthetic sliding surface can have various configurations ranging from planar to curved. In a preferred embodiment, the prosthetic sliding surface is selected from a concave surface, or a convex surface. More preferably, the prosthetic sliding surface is convex.

The contours of the sliding surface are configured to accommodate movement of the sesamoid bones during articulation of the MTP joint. It is therefore preferred for the contours to include one or more grooves shaped to accommodate an upper surface of each sesamoid bone proximate to the MTP joint. Preferably, each groove has a complementary configuration to the upper surface of each sesamoid bone. In order to provide for this complementary configuration, it is also preferred for the shape of the groove to substantially correspond with the configuration of the natural groove of MTH of the basal joint that accommodates each sesamoid bone.

The contour of the prosthetic sliding surface is also preferably configured to accommodate an optimal range of movement between the MTH and proximal phalanx of the MTP joint. It is therefore preferable for the shape of the contour to be minimally intrusive on the convex shape of the prosthetic sliding surface as possible. Accordingly, in embodiments where the prosthetic sliding surface is a convex surface, it is preferable for the prosthetic sliding surface to have an upper end and a lower end, the lower end thereof including at least two grooves spaced apart about a centreline of the convex surface to accommodate each sesamoid bone. Preferably, each groove is a mirror image of the other groove about the centreline of the convex surface. In this way, at least half of the prosthetic sliding surface is specifically shaped for accommodating movement between the MTH and proximal phalanx of the MTP joint. More preferably, more than 2/3 of the prosthetic sliding surface is specifically shaped for accommodating movement between the MTH and proximal phalanx of the MTP joint.

In some embodiments, the optimal shape of the prosthetic sliding surface includes a contoured section that includes dimples or recesses set into the overall convex shape of the prosthetic sliding surface. In one particular embodiment in which the prosthetic sliding surface has a convex surface, the prosthetic sliding surface has an upper end and a lower end, the lower end thereof including a substantially saddle shaped contour to accommodate each sesamoid bone.

As can be appreciated, the contours of the prosthetic sliding surface have been included to accommodate the sesamoid bones with a cooperating section in the prosthetic sliding surface over which the sesamoid bones can easily slide during articulation of the MTP joint. This can result in the prosthetic sliding surface having a first section configured with a first radius of curvature for cooperating with a corresponding surface associated with the proximal phalanx to facilitate relative movement therebetween, and a second section configured with a second radius of curvature associated with the one or more contours of the prosthetic sliding surface configured to accommodate the one or more sesamoid bones during articulation of the MTP joint. It is preferable for the second radius to substantially correspond with the radius of curvature of the sesamoid grooves in the MTH. Of course, each of the first and second radius of curvature can vary in different anatomies. Preferably, the second radius of curvature is less than the first radius of curvature.

The prosthesis-half of the endoprosthesis according to the present invention can include an anchor designed to be anchored in a MTH of the MTP joint. The anchor can be any suitable member. In preferred embodiments, the anchor comprises a pin or a screw.

The anchor can be orientated in various angles relative to the prosthetic sliding surface. In some arrangements, the anchor is orientated parallel to an axis about which the prosthetic sliding surface is arranged. In other embodiments, the anchor is orientated at an acute angle to an axis about which the prosthetic sliding surface is arranged. In one preferred embodiment, the prosthetic sliding surface is arranged about a first axis; and further comprises an anchor connected to the prosthesis-half and extending about a second axis, the anchor configured to anchor the prosthesis-half in a bone of a joint, the first axis and second axis being offset by a predetermined amount. Preferably, the predetermined amount is between 0.5 mm and 5.0 mm.

In some embodiments, the endoprosthesis can be formed of a single integral piece of material. However, it is more preferable for the endoprosthesis to have a modular construction. In a modular form, the endoprosthesis preferably further includes a connection element configured to be implanted in the bone, the connection element having a bore configured to couplingly receive the anchor therein. In some embodiments, the prosthesis-half is releasably fixed to the connection element by a self-limiting plug connection formed by a conical anchoring pin and a complementahly conical pin receiving means. In other embodiments, the prosthesis-half is fixed to the connection element by a screw fixing means, and the connection element has a plurality of threaded bores spaced apart from one another to accommodate a fixation screw associated with the prosthesis-half so that the prosthesis-half can be fixed to the connection element.

According to a second aspect of the present invention there is provided a prosthesis for a MTP joint having at least one sesamoid bone proximate thereto, the prosthesis comprising: a first prosthesis-half comprising an endoprosthesis according to the first aspect of the present invention, the first prosthesis-half defining a first prosthetic sliding surface, and including a first anchor configured to anchor the first prosthesis-half in a MTH of the MTP joint; and a second prosthesis-half defining a second prosthetic sliding surface, and including a second anchor configured to anchor the second prosthesis-half in the adjoining proximal phalanx bone of the joint, wherein the first and second prosthetic sliding surfaces are configured to slidingly cooperate each other.

The first and second prosthetic sliding surfaces can have substantially complementary configurations in some forms to facilitate good sliding engagement between the respective first and second prosthetic sliding surfaces.

Again, while the prosthetic sliding surface can have various configurations ranging from planar to curved, it is preferred that the first prosthetic sliding surface is convex and the second prosthetic sliding surface is concave.

The anchor of each prosthesis-half can be orientated in various angles relative to the prosthetic sliding surface. In a preferred arrangement, the first prosthetic sliding surface is configured about a first axis and the first anchor extending about a second axis, the anchor configured to anchor the first prosthesis-half in a bone of a joint; and the second prosthetic sliding surface is configured about a third axis and the second anchor extends about a fourth axis, and wherein at least one of the first and third axes is offset relative to the corresponding second or fourth axis by a predetermined amount. Preferably, the predetermined amount is between 0.3 mm and 5.0 mm.

According to a third aspect of the present invention, there is provided a prosthetic articulation for a metatarsophalangeal joint, the prosthetic articulation including a distal concave surface that articulates with a proximal component, the distal concave surface having a radius of curvature, wherein the proximal component includes a central convexity having a first radius that substantially matches the radius of curvature of the distal surface, and one or more lateral convexities having a second radius of curvature to accommodate a flexor mechanism associated with articulation of the for a metatarsophalangeal joint, the second radius being smaller than the first radius.

This form of prosthetic articulation allows the flexor mechanism of a metatarsophalangeal joint to pass through a shorter course than the central weight bearing articulation during passive extension. In the case of articulation of a great toe, the flexor mechanism includes the two sesamoid bones. The two regions of shorter radius in the great toe give the appearance of two lateral grooves with a central ridge. The lateral convexities are therefore preferably configured to accommodate the one or more sesamoid bones during articulation of the metatarsophalangeal joint. In some forms, the shape of the lateral convexities substantially corresponds with the configuration of a natural groove of the metatarsal head of the basal joint that accommodates a sesamoid bone.

It should be appreciated that this form of prosthetic articulation could be used in similar metatarsophalangeal type joints for example in similar robotic type joints.

According to another aspect of the present invention, there is provided a method of implanting an endoprosthesis in a bone of a MTP joint having at least one sesamoid bones proximate thereto, comprising: providing a prosthesis-half comprising an endoprosthesis according to the present invention; and implanting the prosthesis-half into a bone of the MTP joint. In those embodiments where endoprosthesis includes a contour that can be aligned with the natural groove of MTH of the basal joint that accommodates each sesamoid bone, the method can further include the steps of: locating the prosthesis-half into the MTH of the basal joint; and orientating and aligning the contours of the prosthetic sliding surface with the natural groove of the MTH of the basal joint that accommodates each sesamoid bone.

The endoprosthesis can be made of metal, cobalt chromium alloys or titanium alloys or pure titanium. However, it should be appreciated that ceramic material, polyethylene or other biocompatible composite or synthetic materials could also be used. Preferably, the endoprosthesis is manufactured from stainless steel, and more preferably medical grade stainless steel (s316).

Brief Description of the Drawings

The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

Figure 1 is an exploded perspective view of a prior art MTP endoprosthesis produced by Plus Orthopaedics.

Figure 2 is a representation of a normal basal joint of a big toe.

Figure 3 is a photograph showing one form of a prior art MTP endoprosthesis implanted in the metatarsal head of the basal joint of a big toe.

Figure 4 is a positive view of an X-ray showing one form of the MTP endoprosthesis shown in Figure 1 implanted in the basal joint of a big toe.

Figure 5 is a front view of one form of the improved MTP endoprosthesis according to the present invention. Figure 6 is a side view of the improved MTP endoprosthesis shown in Figure 5.

Figure 7 is a front perspective view of the improved MTP endoprosthesis shown in Figure 5.

Figure 8 is a rare perspective view of the improved MTP endoprosthesis shown in Figure 5.

Figure 9 is an exploded perspective view of another embodiment of an improved MTP endoprosthesis according to the present invention.

Detailed Description

Figure 1 shows a prior art MTP endoprosthesis 10 produced by Plus Orthopaedics similar to the endoprosthesis 10 described in for example United

States Patent Publication No. 2006/0074492. The MTP endoprosthesis 50 according to the present invention has a generally similar construction to this endoprosthesis 10 and accordingly the disclosure of US2006/0074492 should be taken as being incorporated into this specification by this reference. The prior art MTP prothesis 10 comprises a proximal prosthesis-half 12 and a complementary distal prosthesis-half 14. As is shown by the exploded view shown in Figure 1 , the prosthesis-halves 12, 14 have a modular construction.

As best shown in Figure 1 , the proximal prosthesis-half 12 has a mushroom shape which includes a convex prosthetic sliding surface 16 with a conically shaped anchoring pin 18. The prosthetic sliding surface 16 is arranged about a central axis 23. Similarly, the anchoring pin 18 is arranged about a central axis 24. Central axis 24 is arranged eccentrically with respect to a central axis 23 of the prosthetic sliding surface 16. In the illustrated embodiment, the central axes 23, 24 are offset (e1 ) from one another by about 0.5 mm. The proximal prosthesis-half 10 also defines a rear side or surface 17, which is located opposite the prosthetic sliding surface 16. This offset provides a degree of annular adjustment to the position of the prosthetic sliding surface 16 once implanted in the MTH. The distal prosthesis-half 14 has a complementary form to the proximal prosthesis-half 12. In the illustrated embodiment, the distal prosthesis-half 26 comprises a concave prosthetic sliding surface 20 having a central axis 22, and a conically shaped anchoring pin 25 having a central axis 26. The distal prosthesis-half 14 also defines a rear surface 28, which is located opposite the prosthetic sliding surface 20. Again, the two central axes 22 and 26 are offset

(e2) from one another by about 0.5 mm. This offset provides a degree of annular adjustment to the position of the concave prosthetic sliding surface 20 once implanted in the distal end of a proximal phalanx bone.

Each of the proximal prosthesis-half 12 and the distal prosthesis-half 14 is anchored into bone of the MTP joint using a threaded connection element 27, 29 respectively. Each threaded connection element 27, 29 include a receiving bore 27A and 29A that is co-axial with the connection element 27, 29, so that the receiving bore connection element 27A, 29A and the connection element connection element 27, 29 extend about the same axis. Each receiving bore 27A, 29A is centrally located in the respective connection element 27, 29. Receiving bore 27A is configured to receive anchoring pin 18 of the proximal prosthesis-half 12. Receiving bore 27A is configured to receive anchoring pin 25 of the distal prosthesis-half 14. The proximal prosthesis-half 12 is designed to be implanted in the metatarsal head (32 in Figure 2) of a metatarsal bone (31 in Figure 2). The distal prosthesis-half 14 is designed to be implanted at the distal end (33 in Figure 2) of a proximal phalanx (34 in Figure 2). Each prosthesis-half 12, 14 can be anchored in the respective connection element 27, 29 using a self-limiting plug connection between the respective anchoring pins 18, 24 and a complementahly conical pin receiving bore 27A and 29A. Each connection element 27, 29 is anchored within the respective bone 32, 34 using a cementless means.

Referring now to Figure 2, there is shown a schematic representation of the MTP joint 30 for the big toe (the first MTP joint). The first MTP joint 30 is essentially a ball formation at a metatarsal head (MTH) 32 of a metatarsal bone 34 rotating in a corresponding socket at the distal end 33 of a proximal phalanx bone 34. The joint 30 also includes a plantar plate ligament (not shown), and two sesamoid bones 35, 36 (medial and lateral). As shown in Figure 3, the MTH 32 includes two grooves 38 or tracks in the end of the bone 34 that are configured to accommodate each sesamoid bone 35, 36 during articulation of the joint 30.

During articulation of the MTP joint 30 in the normal gait cycle, the forefoot of a foot (not shown in Figure 2) is planted on the ground and the MTP joint 30 rotates in the socket of the proximal phalanx 34. This is a passive event driven by the user's forward momentum. During articulation of the MTP joint 30, the sesamoid bones 35, 36 are drawn along the grooves 38 in the MTH 32 following the curve radius defined by arc S in Figure 2, and ultimately move to a position where the sesamoid bones 35, 36 engage the distal end 40 of the MTH 32. The sesamoid bones 35, 36 move distally until the MTP joint 30 has and an angle α of about 70 degrees between the proximal phalanx 34 and metatarsal 31. Thereafter, the various ligaments and restraints (not shown) are tensioned at the end of range. A flexor mechanism inherent in the joint 30 then contracts the joint 30 and pulls the proximal phalanx 34 back to its starting point. This second phase requires active muscle contraction and acts through a chain of structures including the sesamoid bones 35, 36.

In a normal foot, the sesamoid bones 35, 36 track in a pair of grooves 38 having a different radius of curvature to the central/distal end 40 of the MTH 32 which articulates with the socket formed at the distal end 33 of the proximal phalanx 34.

Figure 3 and 4 show one embodiment of a total prosthesis shown in Figure 1 implanted in a first MTP Joint (big toe). Figure 3 shows a surgical view of the exposed MTH 32 of the joint with the proximal prosthesis-half 12 implanted. Figure 4 shows an X-ray-derived side view of the MTP endoprosthesis 10 after surgery. In the illustrated embodiment, the prosthesis 10, such as is shown in Figure 1 , is disposed in a phalanx bone 34 and a metatarsal bone 31 , proximal a sesamoid bone 22, of a large toe 31. Fixed to the distal end 40 of the metatarsal bone 31 is the spherical cap-like proximal prosthesis-half 12 with the corresponding prosthetic sliding surface 16, which is in a sliding engagement with the complementary concave prosthetic sliding surface 23 of the distal prosthesis-half 14, which is implanted at a proximal end of the phalanx 34.

As is best shown in Figure 3, the prosthetic sliding surface 16 of the prior art endoprosthesis 10 does not include a contour following the sesamoid grooves 38 of the MTH 32. Accordingly, during articulation of the joint 30, the sesamoid bones 35, 36 track in the natural sesamoid grooves 38 of the MTH 32 until they abut the prosthetic sliding surface 16 of the proximal prosthesis-half 12. Thereafter, the sesamoid bones 35, 36 are forced to continue to over the prosthetic sliding surface 16. It should be appreciated that the prosthetic sliding surface 16 of the proximal prosthesis-half 12 has a different radius of curvature to the sesamoid grooves 38. Accordingly, the sesamoid bones 35, 36 are forced to move in an unnatural curvature during articulation of the joint 30. In some cases, it is possible for the sesamoid bones 35, 36 to become tethered under the MTH 32, creating stiffness in articulation of the joint 30. Without wishing to be limited to any one theory, it is thought that this is due to the sesamoid bones 35, 36 being forced out onto a different radius of curvature path over the prosthetic sliding surface 16 of the proximal prosthesis-half 12 than is the case in a normal MTH 32.

Referring now to Figures 5, 6 7, and 8 there is shown one embodiment of a proximal prosthetic-half 52 of an endoprosthesis 50 (Figure 1 ) according to the present invention. With reference to Figure 1 , it should be appreciated that an endoprosthesis 50 according to the present invention comprises a variation of the proximal prosthesis-half 12 shown in Figures 1 , 3 and 4, and is configured to fit with the other described and illustrated modular components 14, 27, 29 comprising this endoprosthesis 10. Accordingly, the preceding description should be taken as being applicable to this form of the proximal prosthetic-half 52 with respect to the modular components 14, 27, 29.

As shown, the proximal prosthesis-half 52 according to an embodiment of the present invention also has a generally mushroom shape which includes a convex prosthetic sliding surface 56 with a conically shaped anchoring pin 58. The prosthetic sliding surface 56 is arranged about a central axis 63. Similarly, the anchoring pin 58 is arranged about a central axis 64. In the illustrated embodiment, the central axes 63, 64 are co-axial. However, it should be appreciated that in other embodiments, the central axes 63, 64 can also be offset (e3) by about 0.5mm similar to the central axes 23 and 24 of the proximal prosthesis-half 12 shown in Figures 1 , 3 and 4. The proximal prosthesis-half 52 also defines a rear side or surface 57, which is located opposite the prosthetic sliding surface 56.

A similar distal prosthesis-half 14 as shown in Figures 1 , 3 and 4 can be used with this form of the proximal prosthesis-half 52. Similarly, the proximal prosthesis-half 52 can be anchored into bone of the MTP joint using a threaded connection element (not shown) similar to the threaded connection element 27 shown in Figure 1. Again, this proximal prosthesis-half 52 is designed to be implanted at the MTH (32 in Figure 2).

The illustrated proximal prosthetic-half 52 includes sesamoid grooves 59 within the prosthetic sliding surface 56 that conform with and substantially continue the sesamoid grooves 38 of the MTH 32. The shape of the grooves is shown by contour lines on each of Figures 5 to 8. Essentially, the sesamoid grooves 59 form part of the contour of the lower half 60 of the prosthetic sliding surface 56. Each sesamoid groove 59 is configured with a radius that substantially corresponds with the radius of curvature C1 (Figure 2) of the sesamoid grooves 38 of the MTH 32 to allow the sesamoid bones 35, 36 to move normally within the MTP joint 30 during articulation of that joint 30.

In the illustrated embodiment, the contours of the lower half 60 of the prosthetic sliding surface 56 has been shaped to accommodate an upper surface (37 in Figure 2) of each sesamoid bone 35, 36 proximate to the MTP joint 30. In this respect, the upper surface 37 of each sesamoid bone 35, 36 moves within the sesamoid grooves 38 of the MTH 32 during articulation of the MTP joint 30. Similarly, this upper surface 37 of each sesamoid bone 35, 36 will be seated in the sesamoid grooves 59 in the prosthetic sliding surface 56 of the prosthetic- half 52. Each sesamoid groove 59 therefore has a complementary configuration to the upper surface 37 of each sesamoid bone 35 and 36. Similarly, the shape of each sesamoid groove 59 substantially corresponds to the shape of the natural sesamoid grooves 38 of MTH 32.

As is best shown in Figures 5 and 7, the lower half 60 of the prosthetic sliding surface 56 has a saddle shape. This saddle shape is formed from the two grooves sesamoid grooves 59 being recessed into the convex dome shape of the prosthetic sliding surface 56. Each of the sesamoid grooves 59 are spaced apart about a centreline G of the prosthetic sliding surface 56 which forms a saddle point or ridge at the centreline G. Each sesamoid grooves 59 is a mirror image of the other sesamoid grooves 59 about the centreline G. The contoured surface of the illustrated prosthetic sliding surface 56 occupies less than Vi the surface area of the prosthetic sliding surface 56, thereby leaving a substantial surface area specifically shaped for accommodating movement between the prosthetic sliding surface 56 of this proximal prosthetic-half 52 and a corresponding distal prosthesis-half 14.

The overall shape of the prosthetic sliding surface 56 therefore provides a first radius of curvature R (on Figure 6) to cooperate with a corresponding surface associated with the proximal phalanx to facilitate relative movement therebetween, and a second radius of curvature C (Figure 5 and 6) associated with the radius of curvature of the sesamoid grooves 59 in the prosthetic sliding surface 56. This radius of curvature C is substantially the same as the radius of curvature C1 (Figure 2) of the arc S that the sesamoid bones 35, 36 travel along during articulation of the MTP joint 30. Furthermore, this radius of curvature C substantially corresponds with the radius of curvature of the sesamoid grooves 38 in the MTH 32. Of course, this radius of curvature C can vary in different anatomies. Accordingly, the contoured prosthetic sliding surface 56 allows the passively flexing sesamoid bones 35, 36 to glide up a shorter track provided by the grooves 59 than provided by the non-contoured sections of the prosthetic sliding surface 56 which has a wider curve. This allows the sesamoid bones 35, 36 to operate under less load as the MTP joint 30 flexes. The MTH 32 stabilizes the joint centrally as it rolls into dorsiflexion with normal walking. This arrangement 50 therefore allows a MTP joint 30 under load to enjoy the central stability of the MTP joint 30 between the end MTH 32 and the proximal phalanx 34. The sesamoid bones 35, 36 which are associated with the muscles for active flexion will easily and passively track along their shorter curve to allow a large range of motion at the MTP joint 30 than is possible with a non- contoured prosthetic sliding surface 16 (Figure 1 ). Accordingly, there will be minimal stress in the MTP joint 30 flexor mechanism during this phase. The sesamoid's 35, 36 muscle group (Flexor Hallucis Brevis (not illustrated)) can then contract with maximum power when required.

While not wishing to be limited to any one theory, it is thought that this mechanism also introduces a certain amount of elasticity into the system to smooth the transition from passive dorsiflexion to active plantar flexion during the gait cycle.

The endoprosthesis 50 according to the present invention can be fitted using a method similar to that set out in United States Patent Publication No. 2006/0074492. In the illustrated embodiment, the connection element 27 is implanted (in this case screwed therein using cementless means) in the MTH 32 and then the conical pin 58 of the proximal prosthesis-half 52 is anchored in the receiving bore 27A in the connection element 27 using a self-limiting plug connection. However, due to the unique configuration of the sliding surface 56 of the proximal prosthesis-half 52 according to the present invention, the prosthesis-half 52 is orientated with the sesamoid grooves 59 of the lower half 60 of the sliding surface 56 aligned with the sesamoid grooves 58 of the MTH 32. The complementary distal prosthesis-half 14 can also be anchored in the phalanx bone 34 in the same manner, with the connection element 29 implanted in the phalanx bone 34 (in this case screw therein using cementless means). The distal prosthesis-half 14 can be coupled to the connection element 29, using a self limiting plug connection between the anchoring pin 25 and the pin receiving bore 29A of the connection element 29.

It should be appreciated however that in other embodiments, the connection element 27, 29 could alternatively have a plurality of threaded bores (not illustrated) spaced apart from one another for accommodation of a fixation screw (not illustrated) associated with the prosthesis-half 52, 14. The threaded bores would be arranged so that the prosthesis-half 52, 14 can be fixed to the connection element 27, 29 with the respective prosthetic sliding surface 56, 20 of the former either centrally or, if so required, eccentrically arranged with respect to the central axis of the latter.

Figure9 shows another embodiment of a MTP endoprosthesis 150 according to the present invention. Like the previously described embodiment, this MTP prothesis 150 comprises a proximal prosthesis-half 152 and a complementary distal prosthesis-half 154. As is shown by the exploded view shown in Figure 8, the prosthesis-halves 152, 154 have a modular construction.

As shown, the proximal prosthesis-half 152 according to this embodiment of the present invention also has generally the same configuration as the proximal prosthesis-half 52 shown in Figures 5 to 7. Like parts in Figure 8 have therefore been given the same reference numerals as those in Figures 1 , 5, 6 and 7 plus 100.

Again, the proximal prosthesis-half 152 has a generally mushroom shape which includes a convex prosthetic sliding surface 156 with a conically shaped anchoring pin 158. The prosthetic sliding surface 156 is arranged about a central axis 163. Similarly, the anchoring pin 158 is arranged about a central axis 164. In this embodiment, central axis 163 of the sliding surface 156 is centrally aligned with the central axis 164 of the anchoring pin 158. Again, the illustrated proximal prosthetic-half 152 includes sesamoid grooves 159 within the prosthetic sliding surface 156 that conform with and substantially continue the sesamoid grooves 38 of the MTH 32. The sesamoid grooves 159 form part of the contour of the lower half 60 of the prosthetic sliding surface 156. Each sesamoid groove 159 is configured with a radius that substantially corresponds with the radius of curvature C1 (Figure 2) of the sesamoid grooves 38 of the MTH 32 to allow the sesamoid bones 35, 36 to move normally within the MTP joint 30 during articulation of that joint 30. The distal prosthesis-half 154 shown in Figure 8 has the same configuration as the distal prothesis half 54 shown and described in relation to Figure 1.

The endoprosthesis 150 according to the present invention can be fitted using a method similar to that set out in United States Patent Publication No. 2006/0074492 and similar to the method described above for the embodiment of the invention shown in Figures 5 to 7.

It should be appreciated that the described arrangement can also be implemented at other toe joints or finger joints. Use of the described invention is not to be limited in that respect. It should be appreciated that this form of prosthetic articulation could be used in similar metatarsophalangeal type joints for example in similar robotic type joints.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Throughout the description and claims of the specification the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

Claims

Claims:

1. An endoprosthesis for a metatarsophalangeal joint having at least one sesamoid bone proximate thereto, the endoprosthesis including at least one prosthesis-half defining a prosthetic sliding surface, the prosthesis-half configured to be anchored in a metatarsal head of the metatarsophalangeal joint, the prosthetic sliding surface having one or more contours configured to accommodate the one or more sesamoid bones during articulation of the metatarsophalangeal joint.

2. An endoprosthesis according to claim 1 , wherein the prosthetic sliding surface is selected from a concave surface, or a convex surface.

3. An endoprosthesis according to claim 1 or 2, wherein the contours include one or more grooves shaped to accommodate an upper surface of each sesamoid bone proximate to the metatarsophalangeal joint.

4. An endoprosthesis according to claim 3, wherein each groove has a complementary configuration to the upper surface of each sesamoid bone.

5. An endoprosthesis according to claim 3 or 4, wherein the shape of the groove substantially corresponds with the configuration of a natural groove of the metatarsal head of the basal joint that accommodates a sesamoid bone.

6. An endoprosthesis according to any one of claims 1 to 5, wherein the prosthetic sliding surface is a convex surface, the prosthetic sliding surface having an upper end and a lower end, the lower end thereof including at least two grooves spaced apart about a centreline of the convex surface to accommodate each sesamoid bone.

7. An endoprosthesis according to claim 6, wherein each groove is a mirror image of the other groove about the centreline of the convex surface.

8. An endoprosthesis according to any one of claims 1 to 7, wherein the prosthetic sliding surface is a convex surface, the prosthetic sliding surface having an upper end and a lower end, the lower end thereof including a substantially saddle shaped contour to accommodate each sesamoid bone.

9. An endoprosthesis according to any one of claims 1 to 8, wherein the prosthetic sliding surface has a first section configured with a first radius of curvature for cooperating with a corresponding surface associated with the proximal phalanx to facilitate relative movement therebetween, and a second section configured with a second radius of curvature associated with the one or more contours of the prosthetic sliding surface configured to accommodate the one or more sesamoid bones during articulation of the metatarsophalangeal joint.

10. An endoprosthesis according to claim 9, wherein the second radius of curvature is less than the first radius of curvature.

11. An endoprosthesis according to any one of claims 1 to 10, wherein the prosthetic sliding surface is arranged about a first axis; and further comprising an anchor connected to the prosthesis-half and extending about a second axis, the anchor configured to anchor the prosthesis-half in a bone of a joint, the first axis and second axis being offset by a predetermined amount.

12. An endoprosthesis according to claim 1 1 , wherein the predetermined amount the first axis and second axis are offset is between 0.5 mm and 5.0 mm.

13. An endoprosthesis according to any one of claims 1 to 12, wherein the endoprosthesis has a modular construction and further includes a connection element configured to be implanted in the bone, the connection element having a bore configured to couplingly receive the anchor therein.

14. An endoprosthesis according to claim 13, wherein the prosthesis-half is releasably fixed to the connection element by a self-limiting plug connection formed by a conical anchoring pin and a complementarity conical pin receiving means.

15. An endoprosthesis according to claim 14, wherein the prosthesis-half is fixed to the connection element by a screw fixing means, and the connection element has a plurality of threaded bores spaced apart from one another to accommodate a fixation screw associated with the prosthesis-half so that the prosthesis-half can be fixed to the connection element.

16. An endoprosthesis according to any one of claims 1 to 15, wherein the metatarsophalangeal joint is a basal joint of a large toe.

17. A prosthesis for a metatarsophalangeal joint having at least one sesamoid bone proximate thereto, the prosthesis comprising: a first prosthesis-half comprising an endoprosthesis according to any one of the preceding claims, the first prosthesis-half defining a first prosthetic sliding surface, and including a first anchor configured to anchor the first prosthesis- half in a metatarsal head of the metatarsophalangeal joint; and a second prosthesis-half defining a second prosthetic sliding surface, and including a second anchor configured to anchor the second prosthesis-half in the proximal phalanx bone of the joint, wherein the first and second prosthetic sliding surfaces are configured to slidingly cooperate each other.

18. A prosthesis according to claim 17, wherein the first and second prosthetic sliding surfaces have substantially complementary configurations.

19. A prosthesis according to claim 17 or 18, wherein the first prosthetic sliding surface is convex and the second prosthetic sliding surface is concave.

20. A prosthesis according to claim 17, 18 or 19, wherein the first prosthetic sliding surface is configured about a first axis and the first anchor extending about a second axis, the anchor configured to anchor the first prosthesis-half in a bone of a joint; and the second prosthetic sliding surface is configured about a third axis and the second anchor extends about a fourth axis, and wherein at least one of the first and third axes is offset relative to the corresponding second or fourth axis by a predetermined amount.

21. A prosthesis according to claim 20, wherein the predetermined amount that at least one of the first and third axes is offset relative to the corresponding second or fourth axis is between 0.3 mm and 5.0 mm.

22. A prosthetic for articulation of a metatarsophalangeal joint, the prosthetic including a distal concave surface that articulates with a proximal component, the distal concave surface having a radius of curvature, wherein the proximal component includes a central convexity having a first radius that substantially matches the radius of curvature of the distal surface, and one or more lateral convexities having a second radius of curvature to accommodate a flexor mechanism associated with articulation of the metatarsophalangeal joint, the second radius being smaller than the first radius.

23. A prosthetic according to claim 22, wherein the lateral convexities are configured to accommodate the one or more sesamoid bones during articulation of the metatarsophalangeal joint.

24. A method of implanting an endoprosthesis in a bone of a metatarsophalangeal joint having at least one sesamoid bones proximate thereto, comprising: providing a prosthesis-half comprising an endoprosthesis according to any one of claims 1 to 16; and implanting the prosthesis-half into a bone of the MTP joint.

25. A method of implanting an endoprosthesis according to claim 24, wherein the prosthesis-half is an endoprosthesis according to claim 5, and further including the steps of: locating the prosthesis-half into the metatarsal head of the basal joint; and orientating and aligning the contours of the prosthetic sliding surface with the natural groove of the metatarsal head of the basal joint that accommodates each sesamoid bone.