Abstract:
A distal component for a wrist prosthesis that utilizes fixation within the carpus complex, incorporates features that improve the probability of intercarpal fusion and allows modularity of articulating components. In one form, the subject invention includes an ovoid fixation plate having three holes and a boss. The three holes allow attachment of stems and/or screws distally for fixation in the carpus. The boss projects proximally from the plate, and serves as an attachment point for the articulating head. Modularity of the fixation and articulating components allow a surgeon many options for optimizing fixation distally, and articulation proximally. The proximal head of the distal component is designed to articulate with existing radial components. Since the proximal head of the distal component is modular, however, the head can be redesigned to accommodate later design changes in proximal components.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/368,740, filed Mar. 29, 2002. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to wrist prostheses particularly for total wrist arthroplasty. 
   BACKGROUND OF THE INVENTION 
   A normal human wrist may be considered as comprising three sets of bones: the distal forearm, constituting the distal portion of the radius and the ulna; the carpals, constituting eight bones divided into two rows, i.e. the proximal bones (scaphoid, lunate, triquetrum, and pisiform) and the distal bones (trapezium, trapezoid, capitate, and hamate), that are most closely associated with the motion of the wrist; and the metacarpals, constituting the distal segments (i.e. thumb and four fingers). 
   The wrist is commonly considered a biaxial joint, meaning that there are two principle movements of the wrist, namely an extension-flexion movement and a radial/ulnar movement. Although the wrist has no intrinsic mechanism for active supination/pronation deviation movement, it is currently thought that there is likely some degree of passive motion associated with a torsional force transmitted across the radial-carpal joint. While various wrist prosthetics have been developed and patented, they all suffer from loosening of one of the two components of the wrist prosthetic. The torsional loads cannot be passed onto soft tissue due to the constrained design of prostheses. The torsional loads combined with media ulnar and radial deviation causing off center loads can lead to a “window-wiper” action of the central stem of the metacarpal component against the dorsal aspect of the middle metacarpal. 
   Recognition of such passive torsional forces has led to various wrist prosthetic designs that attempt to compensate for such passive torsional forces. These designs attempt to provide a more stable fixation. One type of stable fixation design that attempts to compensate for passive torsional forces adds rotational control pegs to a distal component of the wrist prosthetic. Another type of stable fixation design relies on screw-type fixation of a metacarpal component. Such designs have not been well received due to the inherently weak bone stock available for the metacarpal component in typical wrist implant patients. Also, some designs fail because there is an effort to obtain greater fixation, when motion is still present. 
   Another manner of attempting to compensate for such torsional forces is mismatching of wrist components. Particularly, a surgeon may match small metacarpal components with larger radial components. This, however, provides a less conforming articulating surface, thus allowing for greater contact stresses and greater potential for dislocation. 
   Another problem with wrist prosthetics is loosening of the distal implant component after implantation. In an attempt to solve this problem, various solutions have been proposed. These solutions, however, typically involve the creation of more and/or longer stems which are intended to penetrate deeper into the second, third, and fourth metacarpal canals. While at a first glance this solution seems probable to solve the problem. This solution, however, has not produced superior results to the primary implant outcome. 
   It would thus be advantageous to provide a distal component for a wrist prosthesis that overcomes one or more of the disadvantages of the prior art. 
   SUMMARY OF THE INVENTION 
   The subject invention is a distal component for a wrist prosthesis. Particularly, the subject invention is a distal component for a wrist prosthesis that utilizes fixation within the carpus complex, incorporates features that improve the probability of intercarpal fusion, and allows modularity of articulating components. 
   In one form, the subject invention provides a distal wrist component for a wrist prosthesis. The distal wrist component includes a fixation plate having a middle bore, two side bores, and a boss, a bushing disposed in the two side bores, and a head received on the boss. Each bushing is configured to receive a bone screw such that an end of the bone screw is flush with a surface of the fixation plate. 
   In another form, the subject invention provides a distal wrist component for a wrist prosthesis. The distal wrist prosthesis includes a fixation plate having first and second side bores and a middle bore with the first and second side bores having a spheric inner surface, a boss extending from a surface of the fixation plate and about the middle bore, a bushing disposed in the first and second side bores with each bushing having a spheric outer diameter matching the spheric inner surface of the first and second side bores such that the bushings are articulatable about the center of the spherical bore radius within the first and second side bores, and a head received on the boss. 
   In yet another form, the subject invention provides a distal wrist component for a wrist prosthesis. The distal wrist component includes a fixation plate having first and second peripheral bores and a middle bore, a boss extending from a surface of the fixation plate and about the middle bore, a bushing disposed in the first and second peripheral bores with each bushing having internal tapered threads adapted to receive a bone screw having an end with matching tapered thread such that when the bone screw is engaged in the bushing, the bushing is radially loaded to lock a trajectory of the bone screw at moments consistent with an amount of friction between the bushing and the fixation plate, and a head received on the boss. 
   In a particular form, the subject invention includes an ovoid fixation plate having three holes and a boss. The three holes allow attachment of stems and/or screws distally for fixation in the carpus. The boss projects proximally from the plate, and serves as an attachment point for the articulating head. Modularity of the fixation and articulating components allow a surgeon many options for optimizing fixation distally, and articulation proximally. The proximal head of the distal component is designed to articulate with existing radial components. Since the proximal head of the distal component is modular, however, the head can be redesigned to accommodate later design changes in radial components. 
   Particularly, the subject invention includes an ovoid-shaped base or fixation plate designed to accept attachment of two screws and a modular central stem. The two screws are peripherally attached to the ovoid-shaped fixation plate through the use of particularly designed bushings. The bushings allow the trajectory of the screws to be locked relative to the ovoid-shaped plate. The plate has two peripheral holes that are machined with an internal diameter that is preferably spherical in shape. The bushings have a matching spherical outer diameter is press fit into the plate such that the bushings may articulate within the hole of the plate. The bushing also contains internal threads that are tapered in diameter from largest at the proximal end of the bushing and smallest at the distal end. These threads accept a screw that has matching tapered threads such that when the screw is firmly engaged into the bushing, the bushing is radially loaded. This radial load(ing) locks the trajectory of the screw at moments consistent with the amount of friction between the bushing and the plate. 
   The distal side of the plate has a central hole with a precisely machined Morse taper. This allows a central stem to be received in the taper, rigidly attached by means of a set screw positioned through the proximal side of the plate. The plate also includes a proximal boss positioned centrally with respect to the medial/lateral direction and offset slightly dorsally in the anterior/posterior direction. The outer diameter of this boss has a precisely machined Morse taper designed to firmly engage an articulating surface proximally, and a flat surface distally, that roughly simulates a football sliced along its long axis. The articulating surface is designed such that is mates with certain existing radial component articulating surfaces. 
   The attachment of the articulating head to the fixation plate can exist in two embodiments. In one embodiment, the hole and boss are precisely machined so that the mating surfaces of the head and plate firmly engage, and provide a solid construct in vivo. In a second embodiment, the hole and boss are precisely machine such that the distal surface of the head bottoms out on the proximal surface of the plate. In the condition where these surfaces mate, the hole and boss do not engage, allowing rotation about the axis of the hole of the articulating head. Further, the modularity of the articulation heads allows options for offset rotation centers, varying head heights, varying degrees of articular laxity, materials, and/or motion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partially exploded side view of an exemplary wrist prosthesis; 
       FIG. 2  is an exploded perspective view of an exemplary distal wrist component of the wrist prosthesis of  FIG. 1 ; 
       FIG. 3  is a bottom perspective view of an exemplary head of the exemplary distal wrist component of  FIG. 2 ; 
       FIG. 4  is a bottom plan view of the exemplary head of  FIG. 2 ; 
       FIG. 5  is a sectional view of the exemplary head taken along line  5 — 5  of  FIG. 4 ; 
       FIG. 6  is an enlarged perspective view of one side of an exemplary carpal fixation plate of the exemplary distal wrist component of  FIG. 2 ; 
       FIG. 7  is an enlarged perspective view of another side of an exemplary carpal fixation plate of the exemplary distal wrist component of  FIG. 2 ; 
       FIG. 8  is a sectional view of the exemplary carpal fixation plate taken along line  8 — 8  of  FIG. 7 ; 
       FIG. 9  is an enlarged plan view of an exemplary body screw of the exemplary distal wrist component of  FIG. 2 ; 
       FIG. 10  is an enlarged plan view of an exemplary bushing of the exemplary distal wrist component of FIG. 
       FIG. 11  is a sectional view of the exemplary bushing taken along line  11 — 11  of  FIG. 10 ; 
       FIG. 12  is an enlarged plan view of an exemplary stem of the exemplary distal wrist component of  FIG. 2 ; 
       FIG. 13  is a sectional view of the exemplary stem taken along line  13 — 13  of  FIG. 12 ; 
       FIG. 14  is an enlarged plan view of an exemplary set screw of the exemplary distal wrist component of  FIG. 2 ; 
       FIG. 15  is a sectional view of the exemplary set screw taken along line  15 — 15  of  FIG. 14 ; 
       FIG. 16  is a sectional view of an assembled exemplary distal wrist component in accordance with the principles of the subject invention; and 
       FIG. 17  is a front view of an alternative embodiment of a wrist prosthesis particularly having an alternative embodiment of a distal wrist component. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views. 
   DETAILED DESCRIPTION OF THE INVENTION 
   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein by described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   Referring now to  FIG. 1  there is shown an exemplary embodiment of a wrist prosthesis in accordance with the principles of the subject invention, the wrist prosthesis generally designated  20 . The wrist prosthesis  20  includes a radial or proximal component generally designated  22  and a carpal or distal component, generally designated  24 . The proximal component  22  is configured, adapted and/or operative to be implanted into a radius of a patient. The distal component  24  is configured, adapted and/or operative to be implanted into carpals of the patient. 
   The proximal component  22  includes a stem  26  that is implanted into the radius of the patient. Situated on the stem  26  is a platform  28  that includes a curved or arcuate surface  30 . The curved surface  30  provides a bearing surface for the distal component  24 . Particularly, the curved surface  30  provides an articulation surface for the distal component  24  to articulate thereon. The proximal component  22  is manufactured from a material such as metal suitable for implantation, or various materials suitable for implantation with a plastic (e.g. polyethylene) insert (not shown) within the platform  28  to provide an articulating surface. 
   The distal component  24  includes a head, head portion, head component or the like generally designated  32 , a fixation plate generally designated  38 , and an anchor or stem generally designated  42 . As explained below, the anchor  42  may constitute a stem as shown in  FIG. 1  or may be a bone screw, or the other type of anchoring device. Shown in FIG.  1  and described hereinbelow, are two bone screws  80  that extend from the fixation plate  38 . The bone screws  80  are configured to be anchored into the carpus of the patient. The head  32  may or may not be rotatable with respect to the fixation plate  38 . 
   Referring to  FIGS. 3-5 , the head  32  is shown in greater detail. The head  32  is preferably made from a metal such as cobalt-chrome but other materials suitable for implantation and providing a bearing or articulation surface are acceptable. The head  32  is defined by a body  34  that is generally egg-shaped (ovoid or the like) cut along its longitudinal axis. As such, the body  34  defines an outer surface  36  that provides an arcuate or curved bearing surface. Preferably, the arcuate shape of the outer surface  36  matches the arcuate shape of the bearing surface  30  of the platform  28  of the proximal component  22 . In this manner, the head  32  can essentially congruently bear against and articulate with respect to the proximal component  22 . 
   The head  32  also has a bottom surface  46  that is preferably essentially flat. Since the head is ovoid-shaped, the bottom surface  46  essentially defines an ellipse. Disposed at the center of the bottom surface  46  is a recess, bore, hole, or the like  48 . As discussed further below, the recess  48  is configured to receive or be received on a boss, protuberance, or the like. 
   Referring now to  FIGS. 6-8 , the exemplary fixation or mounting plate  38  is shown in greater detail. The fixation plate  38  is defined by a body  40  that is generally shaped as an ovoid, ellipse, or the like. The shape of the body  40  generally corresponds to the shape of the bottom  46  of the head  32 . The body  40  has a bottom surface  41 , a first hole  54  at one oblong end of the body  40  and a second hole  56  on another oblong end of the body  40 . It should be appreciated that the terms first and second are arbitrary and thus may be interchanged without consequence. The body further includes a third hole  62  that may be slightly off center from a center point of the body  40 . A small height rim  51  extends about the hole  62  from the bottom surface  41 . 
   As seen in  FIG. 7 , the body  40  has a top surface  43  through which the holes  54  and  56  extend. Extending from the top surface  43  around hole  52  is a large height rim  50 . The rim  50  defines a boss that is configured, adapted, and/or operative to receive the recess  48  of the head  32 . The rim  50  may be configured to allow the surface  46  of the head  32  to touch the surface  43  of the fixation plate  38  or may be configured to allow the surface  46  of the head  32  to be immediately adjacent but not touching the surface  43  of the fixation plate. Typically, when the surfaces  46  and  43  touch, the head  32  is rotatable with respect to the fixation plate  38 , and when the surfaces  46  and  43  do not touch, the head  32  is not rotatable with respect to the fixation plate  38 . This, however, may not be the case. 
   The holes  54  and  56  are adapted to receive bushings therein, the bushings of which are described below. The bore  52  is adapted to receive the stem  42  and the set screw  96 . Particularly, the bore  52  is defined by three diameter bores. More particularly, the bore  52  has a first diameter bore  61  opening onto the surface  43 , a second diameter bore  62  opening onto the surface  41 , and a third diameter bore  60  connecting the first and second diameter bores  61  and  62 . The third diameter bore  60  is smaller than the first and second diameter bores  61  and  62 . In one form, the second diameter bore  62  is slightly tapered. This is to receive the tapered end  90  of the stem body  44  when a stem is used as the center anchor. The first diameter is shown as a constant diameter bore. The bore  61  is sized to receive the head  100  of the set screw  96  or other like fastener if one is used (see FIGS.  14 - 15 ). The bore  60  is sized to allow the shank  98  of the set screw  96  to extend therethrough but not the head  100 . The stem end  90  is received in the bore  62  while the shank  98  is threadedly received in the threaded bore  92  of the end  90  of the stem  42 . It should be appreciated that other bore configurations may be utilized such as oppositely tapered bores that connect, a single one diameter bore, all depending on the main or center anchor and the manner in which the center anchor is affixed to the fixation plate  38 . 
   The boss of the plate is positioned centrally with respect to the medial/lateral direction and offset slightly dorsally in the anterior/posterior direction. The outer diameter of this boss has a precisely machined Morse taper designed to firmly engage an articulating surface proximally, and a flat surface distally, that roughly simulates a football sliced along its long axis. The articulating surface is designed such that is mates with certain existing radial component articulating surfaces. 
   Referring to  FIG. 9  the exemplary bone screw generally designated  80  is shown. The exemplary bone screw is utilized to fix the fixation plate  38  to the carpus of a hand. The bone screw  80  has a body  82  that has a plurality of radially expanded threads extending from a rounded tip  84  to an end  86 . The end  86  is threaded with typical, smaller threads. The threads  86  may be straight or may be tapered depending on the configuration of the internal threads of the bushing (see below). An exemplary bone screw that may be used is described in further detail in U.S. Pat. No. 5,954,722 issued on Sep. 21, 1999 to Bono, entitled “Polyaxial Locking Plate” the specification of which is hereby incorporated by reference in its entirety. 
   Referring to  FIGS. 10 and 11  the exemplary bushing generally designated  70  is shown. The bushing  70  is defined by a generally annular or ring-shaped body  72  that includes a gap  74 . The gap  74  allows the bushing to radially compress when inserted into the bores  54  and  56 . This provides a press fit to retain the bushing  70  within the respective bore. The bushing  70  includes internal threads  76  that correspond to the threads of the end  86  of the bone screw  80 . In one form, the threads  76  of the bushing  70  taper with respect to one end to the other end. This is illustrated by the distances d and D in FIG.  11 . When d is smaller than D (i.e. d&lt;D) there is a taper from the axial ends. When d equals D (i.e. d=D) there is no taper. It should be appreciated that the bone screw(s)  80 , and particularly the threaded end  86  of the bone screw(s), interact with the bushing(s)  70 . Various combinations of tapers and no tapers with respect to the bushing threads  76  and the bone screw end threads  86 . The interaction of the bushing(s), the bone screw(s), and the fixation plate  38  will be described. 
   Particularly and with reference to  FIGS. 2 and 16 , the two bone screws  80  are peripherally attached to the fixation plate  38  through the use of the particularly designed bushings  70 . The bushings  70  allow the trajectory of the bone screws  80  to be locked relative to the fixation plate  38 . The two peripheral bores  54  and  56  of the fixation plate  38  are machined with an internal diameter that is preferably at least partially spherical in shape. The bushings  70  have a substantially mating spherical outer diameter (e.g. different radii) that is press fit into the bores  54  and  56  of the fixation plate  38  such that the bushings  70  may articulate within the bores  54  and  56  of the fixation plate  38 . As indicated above, each bushing  70  also contains internal threads  76  that are preferably tapered in diameter from largest at a proximal end (“d”) of the bushing and smallest at a distal end (“D”). These threads  76  accept the bone screw end  86  that has mating tapered threads such that when the bone screw  80  is firmly engaged into the bushing  70 , the bushing  70  is radially loaded. This radial load(ing) locks the trajectory of the bone screw at moments consistent with the amount of friction between the bushing  70  and the fixation plate  38 . 
   As described above, the bone screws  80  mate with the bushings  70 . It should be appreciated that various combinations of threads, tapers, no tapers, and the like with respect to both the bone screws and the bushings can be made. Therefore, such combinations are contemplated and within the scope of the subject invention. 
   Referring to  FIGS. 12 and 13 , the exemplary stem  42  is shown in greater detail. The stem  42  when used, functions as an anchor into the carpus, metacarpal and or bone of a finger. The stem is defined by a fluted and tapered body  44  that essentially forms a spike. The stem  44  has a neck  90  that may be tapered opposite to the taper of the body  44 . A bore  92  that may be threaded as shown, is disposed at the end of the neck  90 . The bore  92  is adapted to receive a pin (threaded or not depending on whether or not the bore  92  is threaded. As shown in  FIG. 13 , the tapered portion of the body  44  includes a hollow  94 . It should be appreciated that more or less flutes than that shown may be provided on the stem. The stem flutes aid in anchoring the stem  44  against torsion. 
   Referring now to  FIGS. 14 and 15 , the exemplary set screw  96  is shown in greater detail. The set screw  96  has a shank  98  that is threaded and a head  100 . The head  100  includes a bore  102  that is configured to accept a driver for setting the set screw  96 . The bore  102  may be configured as a hex, octagon, star, or the like. The threads of the shank  98  are complementary with the threads of the threaded bore  92  of the stem  42 . While the set screw  96  is shown threaded, the set screw may not be threaded, depending on the type of fit or joining of the stem  42  and the fixation plate  38 . 
     FIG. 2  depicts the distal component  24  in an exploded view to better illustrate the various constituent components thereof described above. It can be seen in  FIG. 2  that the fixation plate  38  provides a base upon which the various components are or may be assembled. In particular, and during implantation, the stem  42  (central anchor) is inserted into the bore  52  of the fixation plate  38  from one side of thereof, particularly from the surface  41 . The stem  42  may be affixed to the fixation plate  38  via the set screw  96  (or other fastener) that is inserted into the bore  52  from the other side (from surface  43 ) of the fixation plate  38 . 
   A bushing  70  is press fit into each bore  54  and  56 . Thereafter, a bone screw  80  is inserted through each bushing  70 . As the tip  84  of each bone screw, then the shank  82  is received in a carpal (bone), the threaded end  86  is threadedly received by the threads  76  of the bushing  70 . The bone screws are installed until the end of the end  86  is flush or below the surface  43 . During this time, the fixation plate  38  is preferably held in place. Thereafter, the head  32  is placed onto the fixation plate  38 . Particularly, the recess  48  of the head  32  is received onto the boss  50  either loosely or by a press fit. The entire construct constituting an exemplary distal component  24  in accordance with the principles of the subject invention is shown in cross-section in FIG.  16 . 
   Referring now to  FIG. 17 , there is shown an alternative embodiment of a wrist prosthesis generally designated  20   a . The wrist prosthesis includes a radial or proximal component generally designated  22  and a distal wrist component generally designated  24   a . It should be appreciated that the components of the wrist prosthesis  20   a  and particularly the distal wrist component  24   a  that are different from the distal wrist component  24  are designated with an “a”. 
   The distal wrist component  24   a  is generally the same as that described above with the exception of the central anchor. In this embodiment, the central anchor is a bone screw  150  here shown as larger than the bone screws  80 . It should be appreciated, though, that the bone screw  150  may be the same size as the bone screws  80  or may be smaller. In the case of a central bone screw  150 , the bore  52  (not seen in  FIG. 17 ) of the fixation plate  38  would be adapted to receive the bone screw  150 . 
   The above paragraph illustrates that the central anchor may be different depending on circumstances. In another example that is not shown herein, the distal wrist component does not have a central anchor. Of course, various combinations are contemplated. 
   The subject invention provides various features and/or advantages. For example, the subject distal wrist component provides a screw and bushing locking mechanism that provides a firmly positioned trajectory. This may lead to firmer carpal complex in the short term, thereby leading to higher probability of carpal fusion. As another example, the modular central stem attachment allows for varying shapes, lengths and/or sizes of central stems. As yet another example, the modular articulating head allows for varying shapes, heights, and/or forms of attachments. These can vary with patient indications and/or mating radial components. As still another example, the modular head allows the application of a mobile bearing concept in an embodiment where the hole in the head does not lock onto a boss on the plate. As a final example, the offset nature of the hole pattern in the plate more accurately matches anatomy, allows better placement of screw centers, and provides the potential of superior fusion performance. 
   There is thus a plurality of advantages of the subject invention arising from the various features of the wrist prosthesis described herein. It will be noted that alternative embodiments of the wrist prosthesis of the subject invention may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of a wrist prosthesis that incorporate one or more of the features of the subject invention and fall within the sprit and scope of the subject invention.