Abstract:
A prosthesis assembly configured for use with a scapula in one embodiment includes a spherical humeral component, an elongated glenoid bearing including a first bearing surface with (i) a first spherical end portion with a first radius of curvature, (ii) a second spherical end portion with a second radius of curvature, and (iii) a central portion located between the first spherical end portion and the second spherical end portion, and a first coupling portion extending from a second bearing surface opposite the bearing surface, and a base configured to rotatably support the elongated glenoid bearing, wherein the first radius of curvature is substantially equal to the second radius of curvature and the central portion does not have a radius of curvature that is substantially equal to the second radius of curvature.

Description:
This application is a utility application claiming priority to U.S. provisional application No. 61/331,458, filed May 5, 2010, entitled “Mobile Bearing Glenoid Prosthesis,” the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to shoulder prostheses, and more particularly to shoulder prostheses configured for use in shoulders having glenoid vault erosion or defects. 
     A typical shoulder or glenohumeral joint is formed in a human body where the humerus  10  movably contacts the scapula  12  as shown in  FIG. 1 . The scapula  12  includes a glenoid fossa  14  that forms a socket against which the head of the humerus  10  articulates. At this socket, the scapula  12  includes cartilage  16  that facilitates such articulation. Beneath the cartilage is subchondral bone  18  that forms a wall of a glenoid vault  20  that defines a cavity which contains cancellous bone  22 . The subchondral bone  18  that forms the glenoid vault  20  defines a glenoid rim  21  at a periphery of the glenoid vault that is attached to the cartilage  16  (see  FIG. 1 ). During the lifetime of a patient, the glenoid fossa  14  may become worn, especially at its posterior and/or superior portions thereby causing severe shoulder pain and limiting the range of motion of the patient&#39;s shoulder joint. To alleviate such pain and increase the patient&#39;s range of motion, a shoulder arthroplasty may be performed. 
     Shoulder arthroplasty often involves surgical replacement of the glenoid fossa with a conventional glenoid prosthesis such as the one disclosed in U.S. Pat. No. 6,911,047, the disclosure of which is herein incorporated by reference. The glenoid prosthesis, when implanted, provides a new laterally-facing bearing surface, which may be concave or convex, for articulation with a complementary bearing surface of a natural or prosthetic humeral head. Such conventional glenoid prosthesis is typically formed from UHMW polyethylene, titanium, or cobalt chrome and includes bone anchor(s) such as peg(s), screw(s), post(s), or a keel extending from a back side of the device opposite its bearing surface. So configured, the back side of the prosthesis is typically secured against subchondral bone of the glenoid vault while the bone anchor(s) may extend into the cavity of the glenoid vault whereby it may become anchored to cancellous bone located within the glenoid vault. 
     Shoulder prostheses such as those described above are very effective. During typical movement of the shoulder joint after arthroplasty, however, the humeral head rotates and slides against the glenoid surface. Glenoid components, however, are typically spherical in shape. Accordingly, the translation of the humeral head in the shoulder joint after arthroplasty can result in edge loading of the prosthesis assembly. Edge loading can result in limited motion, instability, and accelerated wear. 
     Some attempts have been made to reduce the problems associated with edge loading. Some prosthesis assemblies introduce a mismatch in the diameter of the head and the diameter of the glenoid component. This approach accommodates translation of the humeral head but does not eliminate edge loading. Additionally, the resulting loss of optimal coverage of the humeral head results in increased instability. Another approach that has been used is to use varying diameters in the glenoid component. This approach also fails to eliminate edge loading and further results in reduced stability. 
     What is needed therefore is an improved prosthesis assembly for use in patients requiring shoulder arthroplasty. An improved prosthesis assembly which reduces edge loading without increasing instability of the shoulder joint is also needed. 
     SUMMARY 
     In accordance with one embodiment of the present disclosure, there is provided a prosthesis assembly configured for use with a scapula which includes a spherical humeral component, an elongated glenoid bearing including a first bearing surface with (i) a first spherical end portion with a first radius of curvature, (ii) a second spherical end portion with a second radius of curvature, and (iii) a central portion located between the first spherical end portion and the second spherical end portion, and a first coupling portion extending from a second bearing surface opposite the bearing surface, and a base configured to rotatably support the elongated glenoid bearing, wherein the first radius of curvature is substantially equal to the second radius of curvature and the central portion does not have a radius of curvature that is substantially equal to the second radius of curvature. 
     Pursuant to another embodiment of the present disclosure, a prosthesis assembly configured for use with a scapula includes a spherical humeral component, a stretched glenoid bearing including a first bearing surface with (i) a first spherical outer portion with a first radius of curvature, (ii) a second spherical outer portion with a second radius of curvature, and (iii) a central portion located between the first spherical outer portion and the second spherical outer portion, and a first mating portion extending from a second bearing surface opposite the first bearing surface, and a base including a third bearing surface and a second mating portion extending from the third bearing surface, the base configured to rotatably support the stretched glenoid bearing, wherein the first radius of curvature is substantially equal to the second radius of curvature and the central portion does not have a radius of curvature that is substantially equal to the second radius of curvature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a cross-sectional view of an anatomically normal glenohumeral joint of a human patient; 
         FIG. 2  depicts a perspective view of a shoulder prosthesis assembly of the present disclosure; 
         FIG. 3  depicts an end cross-sectional view of the glenoid bearing base of  FIG. 2 ; 
         FIG. 4  depicts a side cross-sectional view of the glenoid bearing base of  FIG. 2 ; 
         FIG. 5  depicts a top plan view of the base of  FIG. 2 ; 
         FIG. 6  depicts a perspective view of the glenoid bearing of  FIG. 2 ; 
         FIG. 6A  depicts a cross sectional view of a spherical humeral component that may be used with the glenoid bearing of  FIG. 6 ; 
         FIG. 7  depicts an end plan view of the glenoid bearing of  FIG. 6 ; 
         FIG. 8  depicts a side cross-sectional view of the glenoid bearing of  FIG. 6 ; 
         FIG. 9  depicts a top perspective view of a shoulder assembly prosthesis with a stretched bearing component that does not include a rim replacement vault; 
         FIG. 10  depicts a bottom perspective view of the shoulder assembly prosthesis of  FIG. 9 ; 
         FIG. 11  depicts a top plan view of the shoulder assembly prosthesis of  FIG. 9 ; 
         FIG. 12  depicts a cross-sectional view of the shoulder assembly prosthesis; 
         FIG. 13  depicts a cross-sectional view of a shoulder assembly prosthesis with a stretched bearing component which is snap-fit to a base component; 
         FIG. 14  depicts a partial cross-sectional view of the mating recess of the base component of  FIG. 13 ; and 
         FIG. 15  depicts a partial plan view of the mating portion of the stretched bearing component of  FIG. 13 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the shoulder prosthesis assembly described herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the shoulder prosthesis assembly 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. 2 , there is shown a shoulder prosthesis assembly  100  that is configured to be implanted in a human scapula. The prosthesis assembly  100  includes a glenoid base component  102  and a glenoid bearing  104 . The glenoid base component  102  in this embodiment is made entirely of a metallic material, while the glenoid bearing  104  is made entirely of a polymeric material. Preferably, the glenoid base component  102  is made of a biological grade stainless steel or titanium material. Also, the glenoid bearing support may include a porous-coating on its entire outer surface to facilitate biological ingrowth of a patient&#39;s bone. The glenoid bearing  104  is preferably made entirely of a polymer such as polyethylene. One particular polyethylene that is well suited for use as the bearing component is a high molecular weight polyethylene, for example, ultra-high molecular weight polyethylene (UHMWPE). 
     The glenoid base component  102  is described with further reference to  FIGS. 3 and 4 . In particular, the glenoid base component  102  includes a glenoid vault-occupying portion  106  and a glenoid rim replacement portion  108 . The glenoid rim replacement portion  108  is attached to the glenoid vault-occupying portion  106  as shown in  FIGS. 3-4 . If desired, the glenoid rim replacement portion  108  and the glenoid vault-occupying portion  106  may be attached to each other by being integrally formed together as a single part. 
     Alternatively, the glenoid rim replacement portion  108  and the glenoid vault-occupying portion  106  may be separately formed. In such embodiments, a cavity may be formed in the glenoid vault-occupying portion  106  which receives a complementary shaped cavity occupying portion of the glenoid rim replacement portion  108 . If desired, the separately formed glenoid rim replacement portion  108  and glenoid vault-occupying portion  106  may be attached to each other by snap-fit or friction-fit features or the like. One such friction fit feature is a ball taper connection which allows for version correction of the glenoid rim replacement portion  108  independent of the version of the glenoid vault-occupying portion  106 . Alternatively, soft tissue may be relied upon to maintain the glenoid rim replacement portion  108  mated with the glenoid vault-occupying portion  106 . 
     The glenoid vault-occupying portion  106  is configured to occupy at least a portion of the glenoid vault of a scapula, such as the glenoid vault  20  shown in  FIG. 1 . If desired, the glenoid vault-occupying portion  106  may configured to substantially completely fill the glenoid vault of a scapula, such as glenoid vault  20  shown in  FIG. 1 . The glenoid vault-occupying portion  106  has an exterior wall  110  and an exterior wall  112  as best shown in  FIG. 3 . When the glenoid vault-occupying portion  106  is viewed in cross-section (see  FIG. 3 ) the exterior wall  110  and the exterior wall  112  are positioned with respect to each other to define a generally V-shaped wedge  114 . 
     The glenoid vault-occupying portion  106  has a mating portion  120  which extends inwardly from a bearing surface  122 . The bearing surface  122  provides support for the glenoid rim replacement portion  108  and may be polished to reduce generation of wear products. The mating portion  120  includes a wall portion  124  which defines a generally cone shaped inner periphery of the base  102 . A ridge  126  and a ridge  128  extend into the recess formed by the wall portion  124 . The ridges  126  and  128  extend partially along the inner periphery defined by the wall portion  124 . 
     A rim  130  extends from the bearing surface  122 . The rim  130  and wedge  114  define a pocket  132  which extends completely about the wedge  114 . If desired, bone graft material may be placed into the pocket  132 . 
     The glenoid vault-occupying portion  106  further includes fastener channels  134  and  136 . Fasteners may be inserted through the mating recess  120  and through the channels  134  and  136  to affix the glenoid base component  102  to a glenoid. 
     The glenoid bearing  104  is shown in more detail in  FIGS. 6-8 . In particular, the bearing  104  includes a body  140  and a mating member  142 . The body  140  includes a bearing surface  144  configured to articulate with a spherical humerus component, such as the spherical humeral component  30  shown in  FIG. 6A  with a glenoid component  32 , and a bearing surface  145  configured to articulate with the bearing surface  122 . 
     The glenoid bearing  104  is a stretched bearing. A “stretched bearing” is a bearing that includes a bearing surface with at least three distinct geometries. With reference to  FIG. 8 , the bearing surface  144  includes an outer bearing portion  146 , a central bearing portion  148 , and an outer bearing portion  150 . The outer bearing portion  146  is spherically shaped with a radius of curvature  152  and the outer bearing portion  150  is spherically shaped with a radius of curvature  154 . The radius of curvature  152  is preferably the same length as the radius of curvature  154 . The central portion  148 , however, has a radius of curvature  156  that is much larger than the radius of curvature  152  and the radius of curvature  154 . In some embodiments, the central portion  148  includes a substantially planar portion. A rim  158  extends completely around the bearing surface  144 . 
     The mating member  142  includes a wall  160  that defines a conical outer periphery and slots  162  and  164 . The slots  162  and  164  extend inwardly from the outer periphery defined by the wall  160  and are configured to receive the ridges  126  and  128 , respectively. The slot  162  includes two end portions  166  and the slot  164  includes two end portions  168 . The length of the slots  162  and  164  about the outer periphery defined by the wall  160  is greater than the length of the ridges  126  and  128  about the inner periphery defined by the wall portion  124 . The conical shape defined by the wall  160  is complementary to the conical shape defined by the wall portion  124 . 
     The shoulder prosthesis assembly  100  is assembled by implanting the glenoid base component  102  in the glenoid vault  20  of a patient. If desired, fasteners may be inserted through the mating recess  120  and the channels  134  and  136  to affix the base component  102  to the glenoid vault. A glenoid bearing  104  is then selected. The glenoid bearing  104  is selected such that the outer bearing portions  146  and  150  have radii of curvatures  152  and  154  which provide the desired coverage for the diameter of the spherical humeral head that is used. Accordingly, a number of different glenoid bearings  104  may be provided in a kit with a number of different radii of curvature  152  and  154 . 
     The selected glenoid bearing  104  is then coupled with the implanted base component  102  by axially aligning the slots  162  and  164  with the ridges  126  and  128  and inserting the mating member  142  into the mating recess  120 . As the mating member  142  is inserted, the mating member  142  contacts the ridges  126  and  128  and the mating member  142  is slightly compressed until the slots  162  and  164  are vertically aligned with the ridges  126  and  128  at which point the mating member  142  decompresses thereby locking the bearing  104  within the mating recess  120 . In devices with different radii of curvature  152  and  154 , the vertical height of the ridges  126  and  128  may be offset, with a similar offset in the vertical height of the slots  162  and  164  to ensure a desired orientation of the bearing  104  on the base component  102 . 
     Once the bearing  104  is locked with the base  102 , the bearing surface  145  is rotatably supported on the bearing surface  122 . Rotation of the bearing  104  on the base  102  is provided since the length of the slots  162  and  164  about the outer periphery defined by the wall  160  is greater than the length of the ridges  126  and  128  about the inner periphery defined by the wall portion  124 . Rotation is limited by contact of the end portions  166  with the ridge  126  and contact of the end portions  168  with the ridge  128 . The extent of rotation may be adjusted by providing slots of differing lengths. By joining the slots, 360 degree rotation may be allowed. 
     Rotation is effected when the shoulder prosthesis assembly  100  is implanted as the spherical humeral head contacts the ridge  158 . At the central portion  148 , the curvature of the rim does not complement the curvature of the spherical head. Accordingly, a torque is generated on the bearing  104 . The torque causes the bearing  104  to rotate. As the bearing  104  rotates, the spherical head continues to contact the rim  158  until the spherical head moves into one of the outer bearing portions  146  or  150 . The radius of curvature of the rim  158  in the outer bearing portions  146  and  150  matches the radii of curvature  152  and  154  in the outer portions  146  and  150 . Thus, since the radii of curvature  152  and  154  complement the radius of curvature of the spherical head, the spherical head is captured at the outer portions  146  or  150 . 
     In the embodiment of  FIG. 2 , rotation of the bearing  104  on the base  102  is limited to less than 360 degrees. Accordingly, the bearing surface  122  need not be circular. Rather, an hour-glass shaped bearing surface may be used to provide rotational support throughout rotation of the bearing  104 . Use of a circular bearing surface  122 , however, allows a surgeon to use either limited rotation bearings  104  or bearings  104  that can rotate 360 degrees with a single base  102 , reducing the number of bases needed in inventory. 
     Another benefit of a circular bearing surface on a base component can be realized with bases using a conical stem in place of the wedge  114 . Specifically, site preparation is simplified for base components incorporating stems and circular bearing surfaces. A guide wire may be used to guide a rotating bone cutting device with multiple cutting edges such that the glenoid vault is shaped to receive the stem and circular bearing surface in a single operation. If desired, fins may be provided on the stem in such base components to prevent rotation of the base component. 
     In embodiments of the shoulder prosthesis assembly  100  allowing 360 degree rotation, the ridges  126  and  128  and the slots  162  and  164  may be omitted. The vertical height of the mating member  142  and the pressure provided by soft tissue around the shoulder joint are sufficient to maintain the mating member  142  within the mating recess  120 . In embodiments incorporating the ridges  126  and  128  and the slots  162  and  164 , the vertical height of the mating member  142  may be reduced to provide a truncated cone since the slot/ridge locking mechanism and the pressure provided by soft tissue around the shoulder joint are sufficient to maintain the mating member  142  within the mating recess  120 . 
     While the embodiment of  FIGS. 2-8  incorporates a rim replacement vault, various modifications may be made within the scope of the invention. By way of example,  FIGS. 9-12  depict a shoulder prosthesis assembly  200  that is configured to be implanted in a vault of a human scapula. The prosthesis assembly  200  includes a glenoid base component  202  and a glenoid bearing  204 . The glenoid base component  202  in this embodiment is made entirely of a metallic material, while the glenoid bearing  204  is made entirely of a polymeric material. Preferably, the glenoid base component  202  is made of a biological grade stainless steel or titanium material. Also, the glenoid bearing support may include a porous-coating on its entire outer surface to facilitate biological ingrowth of a patient&#39;s bone. The glenoid bearing  204  is preferably made entirely of a polymer such as polyethylene. One particular polyethylene that is well suited for use as the bearing component is a high molecular weight polyethylene, for example, ultra-high molecular weight polyethylene (UHMWPE). 
     The glenoid base component  202  includes a stem  206  and a base plate portion  208 . If desired, the stem  206  and the base plate portion  208  may be attached to each other rather than being integrally formed together as a single part. The stem  206  is configured to occupy at least a portion of the glenoid vault of a scapula, such as the glenoid vault  20  shown in  FIG. 1 . 
     The base component  202  has a mating recess  220  which extends inwardly from a bearing surface  222 . The bearing surface  222  provides support for the bearing  204  and may be polished to reduce generation of wear products. The base component  202  further includes a number of fins  224  which prevent rotation of the base component  202  once the base component  202  is implanted. 
     The glenoid bearing  204  includes a body  240  and a mating member  242 . The mating member  242  is shaped complementary to the conical shape of the coupling member  220 . The body  240  includes a bearing surface  244  configured to articulate with a spherical humerus component (not shown) and a bearing surface  245  configured to articulate with the bearing surface  222 . 
     The glenoid bearing  204  is a stretched bearing which includes an outer bearing portion  246 , a central bearing portion  248 , and an outer bearing portion  250 . The outer bearing portions  246  and  250  are spherically shaped, preferably with a similar radius of curvature. The central portion  248 , however, has a radius of curvature that is much larger than the radius of curvature of the bearing portions  246  and  250 . In some embodiments, the central portion  248  includes a substantially planar portion. A ridge  258  extends completely about the periphery of the central bearing portion  248 . 
     The shoulder prosthesis assembly  200  is assembled and operated in substantially the same manner as the shoulder prosthesis assembly  200 . One difference is that the glenoid bearing  204  is not rotatably locked to the base component  202 . Rather, the pressure provided by soft tissue around the shoulder joint is sufficient to maintain the mating member  242  within the mating recess  220 . 
     Rather than relying solely upon pressure provided by soft tissue, vault versions may incorporate friction fit or snap-fit features to maintain the bearing component mated with the base component. By way of example,  FIGS. 13-15  depict a shoulder prosthesis assembly  300  that includes a glenoid base component  302  and a stretched glenoid bearing  304 . 
     The base component  302  includes a mating recess  306  with a conical portion  308 , a neck  310  and a bulbous void  312 . The stretched glenoid bearing  304  includes a mating portion  314  with a conical portion  316 , a neck  318  and a bulbous portion  320 . The conical portion  316 , the neck  318  and the bulbous portion  320  are sized complementary to the conical portion  308 , the neck  310  and the bulbous void  312 , respectively. The bulbous portion  320 , however, has a diameter in a plane orthogonal to the longitudinal axis  322  of the mating portion  306  that is smaller than the diameter of the neck  310  in the plane in which the narrowest portion of the neck  310  lies. Accordingly, the bulbous portion  320  must be compressed somewhat in order to slide the bulbous portion  320  past the neck  310  along the longitudinal axis  324  of the mating portion  306  and into the bulbous void  312 . In some embodiments, a void may be formed within the bulbous portion  320  to facilitate compression of the bulbous portion  320 . 
     There is a plurality of advantages arising from the various features of each of the embodiments of the shoulder prosthesis assembly described herein. It will be noted that alternative embodiments of the shoulder prosthesis assembly 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 the shoulder prosthesis assembly that incorporates one or more of the features and fall within the spirit and scope of the present invention as defined by the appended claims.