Abstract:
The invention concerns an arthroplasty implant for providing a joint between first and second members of the body, such as a metatarsophalangeal (MTP) joint, a metacarpophalangeal (MCP) joint or a proximal interphalangeal (PIP) joint. The implant of the invention includes a fist component ( 10 ) defining a concave surface ( 14 ), a second component ( 20 ) defining a convex surface ( 24 ) and an intermediate component ( 32 ) which is located between the first and second components. The intermediate component has a convex surface ( 36 ) which is slidable on the concave surface of the first component to allow articulation between the first component and the intermediate component and a concave surface slidable on the convex surface of the second component to allow articulation between the second component and the intermediate component. A means ( 16, 38; 50, 60 ) is provided to prevent the intermediate component from separating laterally from at least one of the first and second components.

Description:
BACKGROUND TO THE INVENTION  
       [0001]     THIS invention relates to an arthroplasty implant.  
         [0002]     The invention is particularly concerned with arthroplasty implants of the wrist and small bones of the hand and foot, such as metatarsophalangeal (MTP) joint implants, metacarpophalangeal (MCP) joint implants and proximal interphalangeal (PIP) joint implants.  
         [0003]     Various types of implants for such joints have been proposed and are in use. It is however believed that the known implants, most of which are of two part construction, suffer from one disadvantage or other that either limits their flexibility, load-transmitting ability or life expectancy.  
       SUMMARY OF THE INVENTION  
       [0004]     According to the present invention there is provided an arthroplasty implant for providing a joint between first and second members of the body, the implant comprising: 
        a first component defining a concave surface and having first connection means for connecting it to the first body member;     a second component defining a convex surface and having second connection means for connecting it to the second body member;     an intermediate component for location between the first and second components and defining a convex surface which is slidable on the concave surface of the first component to allow articulation between the first component and the intermediate component and a concave surface slidable on the convex surface of the second component to allow articulation between the second component and the intermediate component, and     means for preventing the intermediate component from separating laterally from at least one of the first and second components.        
 
         [0009]     The concave surface of the first component and the convex surface of the intermediate component are preferably complementally, spherically curved. In the preferred embodiments, the convex surface of the second component and the concave surface of the intermediate component are defined by radii of curvature which differ in mutually orthogonal directions. The length of the convex surface in a direction defined by a relatively large radius of curvature is preferably greater than the length of that surface in a direction defined by a relatively small radius of curvature.  
         [0010]     In all cases, the first and second components should be capable of translation and articulation relative to the intermediate component.  
         [0011]     One embodiment of the invention comprises a central projection on the concave surface of the first component and a central opening in the convex surface of the intermediate component, the projection in use locating loosely in the opening to prevent lateral separation of the intermediate and first components.  
         [0012]     In another embodiment of the invention one of the first component and the intermediate component includes a laterally outwardly facing projection and the other of the first component and the intermediate component includes a laterally inwardly facing recess, the projection in use interacting with the recess to prevent lateral separation of the intermediate and first components. Typically in this embodiment, the first component includes an annular wall bounding the concave surface of that component, the peripheral wall being formed with an annular undercut defining the laterally inwardly facing recess, and the intermediate component includes an annular rib defining the laterally outwardly facing projection, interaction in use between the rib and the undercut preventing lateral separation of the intermediate and first components.  
         [0013]     Other features of the invention are defined in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:  
         [0015]      FIG. 1  shows a perspective view of a first component of an arthroplasty implant according to a first embodiment of the invention;  
         [0016]      FIG. 2  shows a side view of the component seen in  FIG. 1 ;  
         [0017]      FIG. 3  shows a perspective view of a second component of an arthroplasty implant according to first embodiment of the invention;  
         [0018]      FIG. 4  shows a side view of the component seen in  FIG. 3 ;  
         [0019]      FIG. 5  shows a plan view of the component seen in  FIG. 3 ;  
         [0020]      FIG. 6  shows a perspective view of an intermediate component of an arthroplasty implant according to the first embodiment of the invention;  
         [0021]      FIG. 7  shows a plan view of the intermediate component seen in  FIG. 6 ;  
         [0022]      FIG. 8  shows a cross-section at the line  8 - 8  in  FIG. 7 ;  
         [0023]      FIG. 9  shows a cross-section at the line  9 - 9  in  FIG. 7 ;  
         [0024]      FIG. 10  shows a side view of an assembled arthroplasty implant according to the first embodiment of the invention with the first and intermediate components in a neutral position before articulation between them;  
         [0025]      FIG. 11  shows a similar side view of the assembled arthroplasty implant seen in  FIG. 10  after maximum articulation between the first and intermediate components;  
         [0026]      FIG. 12  shows a plan view of the assembled arthroplasty implant seen in  FIG. 10  after maximum articulation between the first and intermediate components.  
         [0027]      FIG. 13  shows a perspective view of the first component of an arthroplasty implant according to a second embodiment of the invention;  
         [0028]      FIG. 14  shows a side view of the first component seen in  FIG. 13 ;  
         [0029]      FIG. 15  shows a perspective view of the intermediate component of an arthroplasty implant according to the second embodiment of the invention;  
         [0030]      FIG. 16  shows a side view in the direction of the arrow  16  of the intermediate component seen in  FIG. 15 ;  
         [0031]      FIG. 17  shows a side view in the direction of the arrow  17  of the intermediate component seen in  FIG. 15 ;  
         [0032]      FIG. 18  shows a side view of an assembled arthroplasty implant according to the second embodiment of the invention with the first and intermediate components in a neutral position before articulation between them; and  
         [0033]      FIG. 19  shows a similar side view of the assembled arthroplasty implant seen in  FIG. 18  after maximum articulation between the first and intermediate components. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]     The drawings illustrate individual components of preferred metatarsophalangeal (MTP) joint implants, and the assembled MTP implants. In each case the implant consists of three individual components.  
         [0035]     A first embodiment of the invention is illustrated in FIGS.  1  to  12  of the drawings. In this embodiment,  FIGS. 1 and 2  illustrate a first, phalangeal component  10  which is connected in use to a phalanx. It includes a body  12  formed with a spherically curved, concave surface  14 . Projecting centrally from the surface  14  is a conical peg  16  and projecting rearwardly from the body  12  is a tapered post  18  of square cross-section. In use, the post  18  is placed and anchored in a predrilled hole in the phalanx.  
         [0036]     FIGS.  3  to  5  illustrate a second, tarsal component  20  which is connected in use to the associated tarsus. It includes a body  22  with a convexly curved surface  24  and curved skirts  26 ,  28 . The radius of curvature of the surface  24  in the view of  FIG. 4  is less than the radius of curvature in the view of  FIG. 5 . Projecting rearwardly from the body  22  is a tapered post  30  of square cross-section. In use the post  30  is placed and anchored in a predrilled hole in the tarsus.  
         [0037]     Both the phalangeal component  10  and the tarsal component  20  are made in one piece of grade 5 titanium, their curved surfaces  14  and  24  being provided with a titanium nitride finish.  
         [0038]     FIGS.  6  to  9  illustrate an intermediate component in the form of a meniscus  32  which is located in the assembled MTP implant between the phalangeal and tarsal components  10  and  20 . The meniscus  32  is made of a low friction plastic material, in this case an ultra high molecular weight polyethylene (UHMWPE) available under the name ORTHOSOL™. One side of the meniscus is formed with a concave surface  34  and the opposite side with a convex surface  36 . The convex surface is spherically curved and is formed centrally with a conical recess or socket  38 . The concave surface  34  is not spherical. The radius of curvature of the surface  34  in  FIG. 8 , which matches radius of curvature of the surface  24  in  FIG. 4 , is less than the radius of curvature of the surface  34  in  FIG. 9 , which matches the radius of curvature of the surface  24  in  FIG. 5 . It will accordingly be understood that the concave surface  34  of the meniscus is complemental to the convex surface  24  of the tarsal component  20 , and that the convex surface  36  of the meniscus is complemental to the concave surface  14  of the phalangeal component  10 .  
         [0039]     FIGS.  10  to  12  illustrate an assembled MTP arthroplasty implant  40 , according to the first embodiment of the invention, and consisting of the three components  10 ,  20  and  32 . The meniscus  32  is located between the phalangeal and tarsal components  10  and  20  with the various concave and convex surfaces in cooperating relationship with one another. The peg  16  of the phalangeal component is located in the socket  38  of the meniscus  32 . In this regard it will be noted that the transverse dimension of the peg is somewhat less than the transverse dimension of the socket at any given point along the length of the peg and socket.  
         [0040]     In  FIG. 10 , the phalangeal component  10  and the meniscus  32  are at a neutral orientation with one another, i.e. they are axially aligned and no articulation or translation has taken place between them.  FIG. 11  illustrates the situation after maximum permitted articulation and translation has taken place between these components. It will be noted that in  FIG. 11 , edge regions of the phalangeal component  10  and meniscus  32  come into contact with one another, as indicated by the arrow  42 . Further articulation in the same sense past this condition is impossible. The fact that the socket  38  is oversize with respect to the peg  16  permits translation and maximum articulation to take place, but it will be noted that in  FIG. 11  the peg  16  also abuts the side of the socket  38  to prevent further articulation or translation.  
         [0041]     Throughout the permitted range of movement between the phalangeal component and the meniscus, the peg  16  remains located in the socket  38 . This prevents the meniscus from separating laterally from the phalangeal component, i.e. holds the meniscus captive relative to the phalangeal component at all times.  
         [0042]     In  FIGS. 10 and 11 , there is no change in the positional relationship of the meniscus and the tarsal component  20 . Given the complemental curvature of their respective convex and concave surfaces, it will however be understood that these components are free to slide over one another and, in doing so, to articulate relative to one another. This is illustrated by  FIG. 12 , which shows the meniscus, and with it the phalangeal component, after relative sliding, i.e. translation, and articulation has taken place. The convex surface  24  of the tarsal component  20  is substantially larger than the complemental concave surface  34  of the meniscus, allowing translation and articulation to take place over a wide range of positions and angles.  
         [0043]     A second embodiment of the invention is illustrated in FIGS.  13  to  19  of the drawings. In these Figures features corresponding to features seen in FIGS.  1  to  12  are indicated by the same reference numerals.  
         [0044]     Notable differences between the first or phalangeal component  10  illustrated in  FIGS. 13 and 14  and the first component  10  seen in  FIGS. 1 and 2  are the absence of the central peg  16  and the inclusion of an annular, peripheral wall  52  which bounds the concave surface  14  and which is formed with an undercut  50  defining a laterally inwardly facing recess. The wall  52  and concave surface in combination define a cup-shaped receptacle  53 .  
         [0045]     Another difference between the component  10  of  FIGS. 13 and 14  and that of  FIGS. 1 and 2  is the face that the post  18  has a round cross-section and is provided at its end with barb formations  54 .  
         [0046]     The second or tarsal component  20  of the second embodiment is seen in  FIGS. 18 and 19 . This component differs from the second component illustrated in FIGS.  3  to  5  in that the post  30  is of round cross-section and carries barb formations  56  at its end. The structure defining the convex surface  24  is also slightly different, as illustrated.  
         [0047]     It is believed that the barb formations  54  and  56  will be able to provide better anchorage of the posts  18  and  30  in their respective predrilled holes in the phalanx and tarsus respectively.  
         [0048]     The intermediate component or meniscus  32  of the second embodiment is illustrated in FIGS.  15  to  17 . As will be apparent from  FIGS. 16 and 17 , the concave surface  34  of this meniscus is similar to that of the first embodiment. The convex surface  36  of the second embodiment is however defined by a somewhat greater radius of curvature than the corresponding surface in the first embodiment. Anther difference between the convex surface  36  of the second embodiment and that of the first embodiment is the absence of the central recess or socket  38 . The meniscus also cinludes an annular groove  58 , with a portion  59  of the meniscus beneath this groove presenting an annular, outwardly facing rib  60 .  
         [0049]      FIGS. 18 and 19  illustrated the second embodiment in an assembled condition. The portion  59  of the meniscus is received in the cup-shaped receptacle  53 . Typically the outer diameter of the rib  60  will be such that the portion  59  is either be a very close fit or a press fit through the opening defined by the inner rim  63  of the undercut side wall  52 . A comparison of  FIGS. 18 and 19  shows how the first component and meniscus can both articulate and translate laterally relative to one another. It will however be understood that throughout the range of permitted translation and articulation, the lower portion  59  of the meniscus is held captive relative to the first component  10  by the undercut side wall  52 . Thus in this embodiment the interaction of the rib  60  and the undercut recess  50  prevents the components from separating laterally from one another.  
         [0050]     A comparison of  FIGS. 18 and 19  also illustrates the ability of the implant to accommodate a wide range of translation and articulation between the second component  20  and the meniscus.  
         [0051]     Referring again to the first embodiment, a comparison of  FIGS. 10 and 11  on one hand and  FIG. 12  on the other hand indicates that articulation and translation between the components can take place in mutually orthogonal directions. The range of translation and articulated movement between the tarsal component  20  and the meniscus in one direction, illustrated by  FIGS. 10 and 11 , is greater than the corresponding range of movement in the orthogonal direction, illustrated by  FIG. 12 . This feature is attributable to the shape of the structure defining the convex surface  24  and is provided for the reason that most small joints of the hand or foot are designed to flex primarily in one direction. Considering, for instance, a toe joint, the primary flexural movement of the toe is towards or away from the foot rather than at right angles thereto, although at least a small degree of movement in the latter sense must also be accommodated.  
         [0052]      FIGS. 18 and 19  only illustrate the primary flexural movement, but it will be understood that a similar range of movement in the orthogonal direction is also possible in this embodiment.  
         [0053]     In both embodiments the concave surface  34  of the meniscus and the convex surface of the tarsal component  20  is defined by radii of curvature which differ from one another in mutually orthogonal direction. This feature also contributes to movement in the primary direction. In the case of the first embodiment compare, for instance,  FIGS. 4 and 5  illustrating the tarsal component and  FIGS. 8 and 9  illustrating the meniscus. In the case of the second component, a similar comparison may be made between  FIGS. 16 and 17  illustrating the meniscus. In each case, it will be understood that sliding movement is easier in the direction defined by the larger radius of curvature, i.e. the more gentle curvature.  
         [0054]     It is believed that the three component implants described above and illustrated in the drawings will provide for substantial flexibility in the implanted arthroplasty. Also, the relatively large bearing areas between the respective components will, it is believed, provide the arthroplasty with substantial longevity. Referring in particular to the phalangeal components  10  and the menisci  32 , the fact that these components are retained in their cooperating relationship either by the interaction of the peg  16  and socket  38  or by the interaction of the portion  60  and the undercut  50  means that there still remains a large bearing area between the components to transmit generally axial loading.  
         [0055]     Many modifications are possible within the scope of the invention. For instance, although it is considered beneficial in the first embodiment for the peg to abut the side of the socket, as illustrated in  FIG. 11 , this is not critical to the performance of the implant. In the second embodiment described above, it is the interaction of an outwardly facing projection, i.e. the rib  60  and the inwardly facing recess, i.e, the undercut  50 , which prevents lateral separation of the phalangeal component and meniscus. The rib and recess are included in the phalangeal component and meniscus respectively. It will however be understood that the situation could equally well be reversed, with a projection on the phalangeal component interacting with a recess on the intermediate member. In similar fashion the peg and recess arrangement of the first embodiment could be reversed with the peg on the intermediate component and the recess in the first or phalangeal component. Still further the concave and convex surfaces could be reversed so that, for instance, the palangeal component has the convex surface and the tarsal component has the concave surface, such surfaces interacting with a concave and convex surfaces respectively on the intermediate member. It is also feasible for means to be provided, eg in the form of the described peg and recess combination or the described rib and recess combination, to prevent separation of the intermediate member from both of the first and second components. It is the intention that all such variations are included with the scope of the invention.  
         [0056]     Further, while specific mention has been made of MTP arthroplasty implants, it will be understood that the principles of the invention are equally applicable to other arthroplasty implants, including those mentioned at the outset, typically for the wrist or small bones of the hand or foot.