Patent Publication Number: US-2007100460-A1

Title: Orthopaedic implant systems with anti-abrasion studs

Description:
FIELD OF THE INVENTION  
      This invention relates generally to prostheses for human body joints, and more particularly, to prostheses for human knees.  
     BACKGROUND OF THE INVENTION  
      When a human skeletal joint is damaged, whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire joint is replaced by means of a surgical procedure that involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-joint arthroplasty.  
      For a damaged human knee, the total knee is commonly replaced with prosthetic components shaped to replace portions of the distal femur, proximal tibia and patella. Prosthetic components for use in replacing the distal femur are shaped to replace the articulating surfaces (shown at  21 ,  23  in  FIG. 1 ) of the medial condyle (shown at  20  in  FIG. 1 ), lateral condyle (shown at  22  in  FIG. 1 ) and trochlea, and prosthetic components for use in replacing the proximal tibia are shaped to replace the tibial plateau. Commonly, the tibial component is two piece: one piece is affixed to the bone and the other piece is a bearing with concave surfaces receiving the femoral condyles. Frequently, a portion of the patella is also replaced with a prosthetic component as part of the total knee replacement.  
      In some patients, only a portion of the knee is damaged or injured. For such patients, individual compartments of the knee may be replaced. For example, the medial or lateral compartment of the knee may be replaced with uni-condylar components that replace the articulating surface of one condyle of the distal femur and one side of the tibial plateau. The patellofemoral compartment may be replaced with a femoral component that replaces a portion of the trochlea and a patellar component that replaces part of the patella. In some instances, two or three unicompartmental components are implanted together in one joint; for example, two sets of uni-condylar components could be implanted together to replace the articulating surfaces of both the medial and lateral sides of the tibio-femoral joint, a trochlear component (and patellar component) and a set of uni-condylar femoral and tibial components could be implanted together, or two sets of uni-condylar components and a trochlear component (and patellar component) could be implanted together. The following journal articles report, among other things: use of patellofemoral components (trochlear component and patellar component) and one or two sets of uni-condylar components, Arciero, Major and Toomey, “Patellofemoral Arthroplasty: A Three-to-Nine Year Follow-Up Study,” 236 Clinical Orthopaedics and Related Research, Vol. 236, Nov. 1, 1988, pages 60-71; and two sets of uni-condylar components, Bourne, Rorabeck, Finlay and Nott, “Kinematic I and Oxford Knee Arthroplasty: A 5-8-year Follow-up Study,” The Journal of Arthroplasty, Vol. 2, No. 4, Dec., 1987, pages 285-291, and Shoji, D&#39;Ambrosia and Lipscomb, “Failed Polycentric Total Knee Prostheses,” The Journal of Bone and Joint Surgery, Vol. 58-A, No. 6, Sep. 1976, pages 773-777, and Stockley, Douglas and Elson, “Bicondylar St. Georg Sledge Knee Arthroplasty,” Clinical Orthopaedics and Related Research, No. 255, June, 1990, pages 228-234.  
      Patents and published applications related to uni-condylar knee implant components or patellofemoral implant components include the following: U.S. Pat. No. 3,852,830; U.S. Pat. No. 3,953,889; U.S. Pat. No. 4,034,418; U.S. Pat. No. 4,340,978; U.S. Pat. No. 4,838,891; U.S. Pat. No. 5,871,541; U.S. Pat. No. 6,616,696; and U.S. Pat. No. 6,709,460.  
      Commercial uni-condylar knee implant components or patellofemoral implant components include the LCS® UNI Unicompartmental Knee System (DePuy Orthopaedics, Warsaw, Ind.), the Preservation™ Uni-Compartmental Knee (DePuy Orthopaedics, Warsaw, Ind.), the LCS® PFJ Prosthesis (DePuy Orthopaedics, Warsaw, Ind.), the Patella MOD III and Patella II (Smith &amp; Nephew/Richards) and the Oxford (Biomet).  
      When knees are replaced with common total joint prostheses, substantially all of the potential articulating surface of the distal femur is replaced and covered with metal; no native articular cartilage remains exposed in the potential area of articulation. In contrast, when one or more compartments of a knee are replaced with unicompartmental components, substantial areas of native cartilage are not covered by metal, and remain exposed.  FIG. 1  illustrates an example of a human femur  10  with an implanted trochlear implant component  11 .  FIG. 2  illustrates an example of a human femur  10  with an implanted trochlear implant component  11  replacing the articulating surface of the trochlea together with a uni-condylar femoral component  13  replacing the articulating surface of one of the femoral condyles. In  FIG. 2 , the areas of exposed native tissue include the intercondylar notch  16 , and areas  18 ,  19  of the distal femoral condyles  20 ,  22  adjacent to the intercondylar notch  16  and an area  24  of the distal femoral condyles  20 ,  22  lying between the distal portion  27  of the trochlear component  11  and the anterior portion  29  of the uni-condylar femoral component  13 . As shown in  FIGS. 1-2 , the distal portion  27  of the trochlear component  11  generally tapers toward its distal end which is positioned near or within the intercondylar notch  16 .  
       FIG. 3  illustrates the femur  10  of  FIG. 2 , shown with a patellar implant component  31  engaging the trochlear component  11 . The patellar component  31  includes a bearing surface  33  that bears against a bearing surface  35  of the patellar component  11 . The exposed bearing surface  35  of the illustrated trochlear implant component  11  has two convex surfaces  39 ,  41  meeting along a groove  43 .  FIG. 4  illustrates the femur of  FIG. 3  with the patellar component  31  positioned with respect to the trochlear component  11  as it would be with the knee in deep flexion. When the knee is in deep flexion, a portion of the patellar component  31  may extend beyond the edges of the distal portion  27  of the trochlear component  11 . Such an overhanging portion (shown at  37  in  FIG. 4 ) of the patellar component  31  may contact and rub against the patient&#39;s native tissue (such as native tissue indicated at  18 ,  19  and  24  in  FIG. 4 ) as the knee flexes and extends. This contact may result in painful irritation of the native tissue. This painful irritation could be prevented through use of a total knee prosthesis; however, use of a total knee prosthesis could result in an unnecessary loss of healthy bone tissue. The pain resulting from this irritation could be treated by revising the surgery, replacing the uni-compartmental components  11 ,  13  with a total knee prosthesis, again resulting in the loss of healthy bone tissue. A need exists for a means for preventing or treating the patient&#39;s native tissue near the intercondylar notch without requiring the removal and replacement of healthy tissue.  
      U.S. Pat. Publication No. 2005/0177242 A 1 , entitled “Prosthesis,”discloses a trochlear component with an intercondylar notch portion with tapered wings extending distally and curved posteriorly. The wings also curve away from each other in the posterior direction. Although the wings provide additional bearing surfaces for the patellar implant component, they may not cover the portions of the femur that potentially contact the patellar prosthesis bearing surface. In addition, individual patient anatomies may prevent use of such a trochlear implant in all patients.  
     SUMMARY OF THE INVENTION  
      The present invention provides an implant system and surgical technique that protects a patient&#39;s native tissue when the patient has been treated with uni-compartmental or multi-compartmental arthroplasty. The protection offered by the present invention can be provided in a wide range of patient anatomies.  
      In one aspect, the present invention provides this protection and wide range of use by providing a knee implant system that includes a trochlear implant, a uni-condylar implant component, a patellar implant component and an implantable stud. The trochlear component is sized and shaped to replace a portion of the femur without covering the distal surfaces of the medial and lateral condyles. The trochlear component has a bearing surface and a bone-facing surface. The uni-condylar implant component is sized and shaped to replace a portion of one of the condyles of the distal femur. The uni-condylar implant component has a bearing surface and a bone-facing surface. The patellar implant component is sized and shaped to replace a portion of the patella. The patellar implant component has a bearing surface and a bone-facing surface. The implantable stud includes a head and a fixation post. The head has a bearing or articulation surface and a bone-facing surface. The fixation post extends outward from the bone-facing surface of the head. The head of the stud is sized and shaped to fit between a portion of the trochlear component and a portion of the uni-condylar implant component without facing either the trochlear component or the uni-condylar implant component when all three components are implanted on the distal femur. The bearing surface of the head has a different shape than the bearing surfaces of the trochlear implant component and the uni-condylar implant component. The bearing surface of the head is sized and shaped to limit contact between the patellar implant component and native tissue during flexion and extension of the knee joint.  
      In another aspect, the present invention provides an orthopaedic implant system comprising a trochlear component and an implantable stud. The trochlear component is sized and shaped to replace a portion of the femur between the medial and lateral condyles without covering the distal surfaces of the medial and lateral condyles. The trochlear component has a bearing surface and a bone-facing surface. The implantable stud has a head with a bearing surface and a bone-facing surface. A fixation post extends outward from the bone-facing surface of the head. The implantable stud is made of a non-bioresorbable material and is implantable independent of the trochlear component. The area of the bearing surface of the head of the stud is less than 900 mm 2 . The bearing surface of the head of the implantable stud is contoured and substantially smooth. The bearing surface and bone-facing surface of the head of the implantable stud meet along a curved edge.  
      In another aspect, the present invention provides an orthopaedic implant system comprising a trochlear implant component, a patellar implant component, a uni-condylar implant component, a first implantable stud and a second implantable stud. The trochlear implant component is sized and shaped to replace a portion of the femur between the medial and lateral condyles without covering the distal surfaces of the medial and lateral condyles. The trochlear component has a bearing surface and a bone-facing surface. The patellar implant component is sized and shaped to replace a portion of the patella. The patellar implant component has a bearing surface and a bone-facing surface. The uni-condylar implant component is sized and shaped to replace a portion of one of the condyles of the distal femur. The uni-condylar implant component has a bearing surface and a bone-facing surface. The first implantable stud has a head with a bearing surface, a bone-facing surface and a fixation post extending outward from the bone-facing surface of the head. The second implantable stud has a head with a bearing surface, a bone-facing surface and a fixation post extending outward from the bone-facing surface of the head. The bearing surface of the first implantable stud differs from the bearing surface of the second implantable stud in at least one of the characteristics of size and shape. The bearing surfaces of the first and second implantable studs have maximum transverse dimensions in the range of 10-40 mm and thicknesses in the range of 2-6 mm. The bearing surfaces of first and second implantable studs have shapes that are different from the shapes of the bearing surfaces of the trochlear implant component and the uni-condylar implant component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a distal femur with an implanted prior art trochlear implant component;  
       FIG. 2  is a perspective view similar to  FIG. 1 , showing the distal femur with both a prior art trochlear implant component and a prior art uni-condylar femoral implant component;  
       FIG. 3  is a perspective view of a distal femur with both a prior art trochlear implant component and a prior art uni-condylar femoral implant component, showing a prior art patellar implant component bearing against the bearing surface of the trochlear implant component;  
       FIG. 4  is an end view of a distal femur, illustrating a possible position of the patella and prior art patellar implant with respect to a prior art trochlear component and prior art uni-condylar femoral component, and further illustrating the potential for the patellar implant component to contact native tissue during flexion and extension of the knee;  
       FIG. 5  is a perspective view of a distal femur, similar to  FIG. 2 , but with a first embodiment of a stud implanted in the space between a trochlear component and uni-condylar implant component;  
       FIG. 6  is a perspective view of a distal femur, similar to  FIGS. 2 and 5 , but with two studs of a second embodiment implanted in areas adjacent to the intercondylar notch of the femur;  
       FIG. 7  is perspective view of a first embodiment of an anti-abrasion stud;  
       FIG. 8  is an elevation of the anti-abrasion stud of  FIG. 7 ;  
       FIG. 9  is a second elevation of the anti-abrasion stud of  FIGS. 7-8 ;  
       FIG. 10  is a cross-section of the anti-abrasion stud of  FIG. 9 , taken along line  10 - 10  of  FIG. 9 ;  
       FIG. 11  is a top plan view of the anti-abrasion stud of  FIGS. 7-10 ;  
       FIG. 12  is a bottom plan view of the anti-abrasion stud of  FIGS. 7-11 ;  
       FIG. 13  is perspective view of a second embodiment of an anti-abrasion stud;  
       FIG. 14  is an elevation of the anti-abrasion stud of  FIG. 13 ;  
       FIG. 15  is a cross-section of the anti-abrasion stud of  FIG. 14 , taken along line  15 - 15  of  FIG. 14 ;  
       FIG. 16  is a top plan view of the anti-abrasion stud of  FIGS. 13-15 ;  
       FIG. 17  is a bottom plan view of the anti-abrasion stud of  FIGS. 13-16 ;  
       FIG. 18  is perspective view of a third embodiment of an anti-abrasion stud;  
       FIG. 19  is a top plan view of the anti-abrasion stud of  FIG. 18 ;  
       FIG. 20  is an elevation of the anti-abrasion stud of  FIGS. 18-19 ;  
       FIG. 21  is a cross-section of the anti-abrasion stud of  FIGS. 18-20 , taken along line  21 - 21  of  FIG. 20 ;  
       FIG. 22  is a bottom plan view of the anti-abrasion stud of  FIGS. 18-21 ;  
       FIG. 23  is perspective view of a fourth embodiment of an anti-abrasion stud;  
       FIG. 24  is an elevation of the anti-abrasion stud of  FIG. 23 ;  
       FIG. 25  is a second elevation of the anti-abrasion stud of  FIGS. 23-24 ;  
       FIG. 26  is a cross-section of the anti-abrasion stud of  FIGS. 23-25 , taken along line  26 - 26  of  FIG. 25 ;  
       FIG. 27  is a top plan view of the anti-abrasion stud of  FIGS. 23-26 ;  
       FIG. 28  is a bottom plan view of the anti-abrasion stud of  FIGS. 23-27 ;  
       FIG. 29  is perspective view of a fifth embodiment of an anti-abrasion stud;  
       FIG. 30  is an elevation of the anti-abrasion stud of  FIG. 29 ;  
       FIG. 31  is a second elevation of the anti-abrasion stud of  FIGS. 29-30 ;  
       FIG. 32  is a top plan view of the anti-abrasion stud of  FIGS. 29-31 ;  
       FIG. 33  is a bottom plan view of the anti-abrasion stud of  FIGS. 29-32 ; and  
       FIG. 34  is a cross-section of a portion of a femur and trochlear implant component, illustrating the position of the first embodiment of the anti-abrasion stud with respect to articular cartilage of the femur. 
    
    
     DETAILED DESCRIPTION  
      The present invention provides an orthopaedic implant system that includes, in addition to uni-compartmental implant components, one or more anti-abrasion studs  50  that extend the bearing areas of other implant components to protect native tissue from damage resulting from engaging a patellar implant component during flexion and extension. In addition to the anti-abrasion studs  50 , the orthopaedic implant system of the present invention may include a trochlear implant component, a patellar implant component, one or more uni-condylar femoral implant components, and one or more uni-condylar tibial implant components against which the uni-condylar femoral components articulate.  
       FIG. 5  illustrates the distal end of a human femur  10 , shown with two compartments of the distal femur  10  replaced by a trochlear implant component and a uni-condylar femoral implant component. The illustrated trochlear and uni-condylar implant components of  FIG. 5  are similar to those disclosed in U.S. Pat. App. Publication No. 2005/0154471 A1, entitled “Systems and Methods for Compartmental Replacement in a Knee,” which is incorporated by reference herein in its entirety. However, it should be understood that the present invention is not limited to the structures disclosed in that patent application; the principles of the present invention, and the addition of anti-abrasion studs  50 , can be broadly applied to other implant systems wherein a portion of native tissue is exposed to potential contact with the articulating surface.  
      The illustrated trochlear implant component  12  is sized and shaped to replace a portion of the patellofemoral compartment of the distal femur without covering the distal articulating surfaces  21 ,  23  of the medial and lateral condyles  20 ,  22 . The trochlear component  12  has an exposed bearing surface  34  and a bone-facing surface underlying the bearing surface. The exposed bearing surface  34  of the illustrated trochlear implant component  12  has two convex surfaces  38 ,  40  meeting along a groove  42 . The illustrated trochlear implant component  12  is sized and shaped to provide an articulating surface for the patellar component  30 , so that the patellar component  30  engages the trochlear component  12  when the leg is in extension as well as through a normal range of flexion.  
      The illustrated uni-condylar implant component  14  is sized and shaped to replace the femoral condyle surface  21  that articulates with the proximal tibia. The uni-condylar femoral implant component  14  has an exposed arcuate articulating or bearing surface  44  and an underlying bone-facing surface. The bone-facing surface can be porous to promote bone ingrowth, or can be adapted for cemented fixation. Overall, the illustrated uni-condylar femoral implant component  14  is sized and shaped to cover the distal and posterior articulating surfaces of one femoral condyle.  
      As shown in  FIGS. 4-6 , the illustrated trochlear component  12  has a distal portion  26  that tapers distally and posteriorly; the illustrated uni-condylar femoral component  14  has an anterior portion  28  that tapers proximally and anteriorly. One end of the illustrated trochlear component  12  is implanted adjacent to the intercondylar notch  16 . The intercondylar notch  16  remains in its native state, as does a portion  24  of the femoral condyle between the tapering edges of the trochlear component  12  and the uni-condylar femoral component  14  and as do portions  18 ,  19  of the distal femur adjacent to the intercondylar notch  16 . These portions  18 ,  19 ,  24  of the femur in their native state include native tissue, such as articular cartilage.  
      Although not illustrated in the accompanying drawings, it should be understood that the illustrated uni-condylar femoral implant component  14  would be used in conjunction with a uni-condylar tibial implant component. Such a uni-condylar tibial implant component would typically be two-piece, with a metal base and a polymer bearing made of a material such as ultra-high molecular weight polyethylene (UHMWPE), but could be a single integral implant component made out of a material such as UHMWPE.  
      When a trochlear component is implanted, the implant system would also typically include a patellar implant component, such as that shown at  30  in  FIGS. 3-4 . The patellar implant component  30  is sized and shaped to replace a posterior portion of the patella. The patellar implant component has a bearing surface  32  and a bone-facing surface. The illustrated patellar implant component  30  is a two-piece component, with a bearing made out of a smooth material such as (UHMWPE), although the patellar component could be a single integral implant component made out of a material such as UHMWPE.  
      To protect the area  24  of native tissue between the opposed tapered edges of the distal portion  26  of the trochlear component  12  and anterior portion  28  of the uni-condylar femoral component  14 , the orthopaedic implant system of  FIG. 5  includes a first embodiment of an anti-abrasion stud  50 A implanted at this area  24  of native tissue. To protect the areas  18 ,  19  of native tissue adjacent the intercondylar notch  16 , the orthopaedic implant system of  FIG. 6  includes a medial anti-abrasion stud  50 B and a lateral anti-abrasion stud  50 C implanted at these areas  18 ,  19 . As described in more detail below, other embodiments  50 D,  50 E of anti-abrasion studs may also be employed to extend the patellar tracking surface and thereby protect native tissue.  
      All of the illustrated anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E include common features. As shown in  FIGS. 7-33 , they each include a head  52 A,  52 B,  52 C,  52 D,  52 E and a fixation post  54 A,  54 B,  54 C,  54 D,  54 E. Each head  52 A,  52 B,  52 C,  52 D,  52 E has a bear surface  56 A,  56 B,  56 C,  56 D,  56 E and an opposite bone-facing surface  58 A,  58 B,  58 C,  58 D,  58 E. The fixation posts  54 A,  54 B,  54 C,  54 D,  54 E extend outward from the bone-facing surface  58 A,  58 B,  58 C,  58 D,  58 E of the head  52 A,  52 B,  52 C,  52 D,  52 E.  
      The head  52 A,  52 B,  52 C,  52 D,  52 E of each of the illustrated anti-abrasion stud  50 A,  50 B,  50 C,  50 D,  50 E is sized and shaped to fit between a portion of the trochlear component  12  and a portion of one uni-condylar femoral implant component  14  without contacting either the trochlear component or the uni-condylar femoral implant component when all of the components are implanted on the distal femur, as illustrated in  FIGS. 5-6 . As can also be seen from  FIGS. 5-33 , the head  52 A,  52 B,  52 C,  52 D,  52 E of each anti-abrasion stud  50 A,  50 B,  50 C,  50 D, shape that is different from the shape of the bearing surfaces of the trochlear implant component  12  and the uni-condylar femoral implant component  14 . Two of the illustrated anti-abrasion studs  50 A,  50 D have heads that are elliptical in top plan view (see  FIGS. 11 and 27 ); two of the illustrated anti-abrasion studs  50 B,  50 C have heads that are circular in top plan view (see  FIGS. 16 and 19 ); and one of the illustrated anti-abrasion studs  50 E has a head that is kidney-shaped in top plan view (see  FIG. 32 ).  
      It should be appreciated that the three illustrated shapes for the heads of the anti-abrasion studs are provided as examples only. Alternative shapes may be used and are within the scope of the invention. For example, for anti-abrasion studs that are intended for use to extend the patellar tracking surface further toward the intercondylar notch, the heads of the anti-abrasions studs can have an edge that is shaped to complement the shape of a portion of the edge of the trochlear implant component.  
      The head  52 A,  52 B,  52 C,  52 D,  52 E of each of the illustrated anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E has a height between the lowest portion of the bone-facing surface  58 A,  58 B,  58 C,  58 D,  58 E and the highest point on the bearing surface  56 A,  56 B,  56 C,  56 D,  56 E. These heights are indicated at “h 1 ” in  FIGS. 8, 10 ,  15 ,  20 ,  25 ,  26  and  31 . Generally, head heights hi in the range of about 2-6 mm should be adequate to raise most of the bearing surface  56  of the head  52  above the exterior surface of the articular cartilage on the bone; in other words, the head heights are generally greater than the thickness of the articular cartilage where the anti-abrasion stud is implanted.  FIG. 34  illustrates the lowermost point of the bone-facing surface  54 A of one of the anti-abrasion studs  50 A positioned against the bone surface  51 , with a substantial part of the bearing surface  56 A of the head  52 A above the top level of the articular cartilage  53  surrounding the anti-abrasion stud  50 A. A portion of another implant component, such as trochlear component  12 , is shown in cross-section in  FIG. 34 . Examples of numerical values for h 1  for the illustrated embodiments are provided in Table 1, below.  
      The head  52 A,  52 B,  52 C,  52 D,  52 E of each of the illustrated anti-abrasion studs  50 A,  50 B,  50 C,  50 D and  50 E has a maximum length and width. These lengths and widths are indicated at “L” and “w” in  FIGS. 12, 16 ,  19 ,  27  and  33 . Examples of numerical values for  1  and w for the illustrated embodiments are provided in Table 1, below. Examples of numerical values for the perimeters of the illustrated heads are also provided in Table  1  below.  
      All of the bearing surfaces  56 A,  56 B,  56 C,  56 D,  56 E of the illustrated anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E are contoured and substantially smooth, to provide a low friction path for the patellar component during flexion and extension. The illustrated bearing surfaces are convex. The radii of curvature for the bearing surfaces are indicated at “r 1 ” in  FIGS. 8, 15 ,  21 ,  24 ,  26  and  30 . Examples of numerical values for r 1  for the illustrated embodiments are provided in Table 1, below. Examples of surface areas for the bearing surfaces of the illustrated heads are also provided in Table 1 below.  
      It should be appreciated that the profiles of the bearing surfaces  56 A,  56 B,  56 C,  56 D,  56 E of the illustrated embodiments are provided as examples only. Various profiles for the bearing surfaces could be used; the most appropriate profile for a bearing surface may relate to the shape of the bearing surface of the implant that the anti-abrasion stud is augmenting or extending. A particular profile or groups of profiles for the bearing surfaces of the anti-abrasion studs can be selected to best augment a wide variety of main implant shapes and sizes. For example, it may be desirable to include a concave portion to form a track. Accordingly, the present invention is not limited to any particular profile for the bearing surfaces of the anti-abrasion studs unless expressly called for in the claims.  
      The head  52 A,  52 B,  52 C,  52 D,  52 E of each of the illustrated anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E has a curved edge  60 A,  60 B,  60 C,  60 D,  60 E around the perimeter of the bearing surface. The curved edges  60 A,  60 B,  60 C,  60 D,  60 E extend toward the bone-facing surfaces  58 A,  58 B,  58 C,  58 D,  58 E. In the illustrated embodiments the curved edges have radii of curvature of about 0.5-5 mm. These radii are indicated at “r 2 ” in  FIGS. 8, 10 ,  14 ,  15 ,  20 ,  21 ,  24 ,  26  and  31 . Examples of numerical values for r 2  for the illustrated embodiments are provided in Table 1, below.  
                       TABLE 1                          Anti-Abrasion               Stud   Dimension (mm)   Surface                                                 Embodiment   “L”   “w”   “h 1 ”   “h 2 ”   “r 1 ”   “r 2 ”   Perimeter   Area (mm 2 )                                                         50A   18   12   2.34   12.66   30   0.5   142.4394   183.6562       50B   10   10   2.34   12.66   10   1   87.2664   98.9987       50C   10   10   2.34   12.66   10   1   87.2664   98.9987       50D   20   14   3.75   11.257   30   2   165.8505   288.3211       50E   41   22   5   13.5   30   5   365.1022   857.9848                  
 
      The fixation posts  54 A,  54 B,  54 C,  54 D,  54 E of each of the illustrated embodiments of anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E are provided for affixation of the studs to the patient&#39;s bone. The illustrated fixation posts are intended to be placed in a prepared bore in the patient&#39;s bone, such as the substantially cylindrical bore shown at  57  in  FIG. 34 , and include raised surface features to aid in affixation of the posts to the walls of the bore  57  in the bone.  
      Each of the illustrated fixation posts  54 A,  54 B,  54 C,  54 D,  54 E has a flat, circular end  70 A,  70 B,  70 C,  70 D,  70 E opposite the head  52 A,  52 B,  52 C,  52 D,  52 E. The illustrated posts include a plurality of spaced cylindrical portions  72 A,  72 B,  72 C,  72 D,  72 E having a first diameter and spaced raised cylindrical portions  74 A,  74 B,  74 C,  74 D,  74 E having a second larger diameter. The cylindrical portions  72 A,  72 B,  72 C,  72 D,  72 E and raised cylindrical portions  74 A,  74 B,  74 C,  74 D,  74 E are concentric about the longitudinal axes  75 A,  75 B,  75 C,  75 D,  75 E of the fixation posts. In the anti-abrasion studs  50 A,  50 B,  50 C,  50 D illustrated in  FIGS. 7-23 , the fixation posts further include conical beveled portions  76 A,  76 B,  76 C,  76 D connecting the raised cylindrical portions  74 A,  74 B,  74 C, and  74 D to the cylindrical portions  72 A,  72 B,  72 C, and  72 D. The conical beveled portions  76 A,  76 B,  76 C,  76 D are concentric about the longitudinal axes  75 A,  75 B,  75 C,  75 D of the fixation posts and taper toward the flat circular ends  70 A,  70 B,  70 C, and  70 D of the fixation posts.  
      The number of fixation posts and the positions of the fixation posts relative to the heads may vary depending on the size and shape of the head. For example, in the first four illustrated anti-abrasion studs  50 A,  50 B,  50 C,  50 D, the longitudinal axes of the fixation posts  54 A,  54 B,  54 C,  54 D are aligned with the centers of the heads  52 A,  52 B,  52 C,  52 D. In the last illustrated anti-abrasion stud  50 E, there are three spaced fixation posts  54 E positioned to support the head  52 E.  
      Examples of dimensions for the fixation posts  54 A,  54 B,  54 C,  54 D,  54 E and their surface features  70 ,  72 ,  74  are provided in Table 2.  
                       TABLE 2                                      Diameter (mm)                             Anti-Abrasion       Smaller diameter   Larger Diameter       Stud   Circular   cylindrical   cylindrical       Embodiment   end 70   portion 72   portion 74               50A   2   5   6       50B   2   5   6       50C   2   5   6       50D   2   5   6       50E   2   4   5                  
 
      It should be understood that the surface features  70 ,  72 ,  74  described above are provided as examples only. Other surface features to aid in fixation of the anti-abrasion studs in the bone could be used in addition to or in place of the surface features illustrated and described above. For example, longitudinal surface features could be employed; grooves, ridges or fins could also be used to enhance fixation and retard rotation of the anti-abrasion studs.  
      It should also be understood that all of the dimensions, areas and radii disclosed herein (including all those set forth in Tables 1 and 2) are provided as examples only. The present invention is not limited to any particular dimension, area or radii unless expressly set forth in the claims.  
      In four of the illustrated anti-abrasion studs  50 A,  50 B,  50 D,  50 E, the entire head  52 A,  52 B,  52 D and  52 E and fixation post  54 A,  54 B,  54 D,  54 E are integrally-formed. However, the anti-abrasion studs could be made as multi-piece implants that can be assembled in the operating room. The anti-abrasion stud  50 C of  FIGS. 18-22  is an example of a two-piece anti-abrasion stud, wherein the fixation post  54 C includes a flange  80  to which an independent bearing  82  is affixed. Together, the flange  80  and bearing  82  form the head  52 C of the stud  50 C. The bearing  82  can be affixed to the flange  80  in any standard manner, such as through an interference fir or frictional lock. With such a two-piece stud, a surgical kit could be modular, including a plurality of bearings  82  of different sizes and shapes from which the surgeon may select the most appropriate size and shape for the particular patient.  
      The anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E of the present invention may be made of any standard bio-compatible material, although it is preferred that the material be one that is not biodegradable and not bioresorbable. Common metal alloys, such as standard medical implant grade cobalt-chrome alloys and titanium alloys, may be used for the entire implant. In the case of a two-piece anti-abrasion stud  50 C of  FIGS. 18-22 , the fixation post  54 C and flange  80  may be made of such a standard material, and the bearing  82  may be made of a different material, such as a ceramic or polymer (for example, ultra-high molecular weight polyethylene), if desired. The entire anti-abrasion stud could also be made of such a ceramic or polymer.  
      If all or part of the anti-abrasion stud is made of a metal alloy, it may be desirable for the surfaces that will contact bone to be treated to be conducive to bone ingrowth. For example, standard industry can be employed to make the bone-contacting surfaces porous. Coatings may also be employed to induce bone ingrowth into the appropriate portions of the stud or to deliver drugs to the site.  
      The bearing surfaces  56 A,  56 B,  56 C,  56 D,  56 E of the anti-abrasion studs preferably provide a low-friction surface for the patellar bearing to move across during the flexion and extension. If the heads  52 A,  52 B,  52 C,  52 D,  52 E are made of metal, the bearing surfaces may be highly polished to maximize smooth movement of the patellar bearing across the stud bearing surface.  
      The anti-abrasion studs of the present invention may be provided in the form of implant system, sets or kits. For example, a knee implant system, set or kit could include a set of trochlear components, patellar components and anti-abrasion studs. The system, set or kit could also include uni-condylar femoral implant components and uni-condylar tibial implant components. All of the implant components could be provided in a variety of sizes to accommodate a wide range of patient anatomies. The anti-abrasion studs included in the system, set or kit could include a variety of sizes of a single head shape or a variety of head shapes, profiles and sizes.  
      To use the anti-abrasion studs  50 A,  50 B,  50 C,  50 D,  50 E and implant systems of the present invention, the orthopaedic surgeon would prepare the patient&#39;s bones in the most appropriate fashion for implantation of the first or major implant components. For example, for a patellofemoral joint arthroplasty, the trochlea of the distal femur would be resected or otherwise shaped or prepared to receive the trochlear implant component and the patella would be resected or otherwise shaped or prepared to receive the patellar implant component (if a patellar implant component is to be used). The trochlear component would then be implanted in a standard manner, as would the patellar implant component, if used. For a tibiofemoral joint arthroplasty, one or both of the femoral condyles would be resected or otherwise shaped or prepared to receive an appropriate uni-condylar femoral implant component and the corresponding side of the tibial plateau would be resected or otherwise shaped or prepared to receive an appropriate tibial implant component (or assembly of components). The femoral uni-condylar implant component or components and the uni-condylar tibial component or components would then be implanted in a standard manner.  
      If the surgeon determines at the time of the original surgery that the patient would benefit from providing an enhanced or augmented patellar track extending further toward the intercondylar notch, or that the transition between the bearing surfaces of the trochlear component and the uni-condylar femoral component or components is uneven or overly extended, the surgeon may chose to use one of the anti-abrasion studs of the system to extend the bearing surfaces of the other implant components.  
      The orthopaedic surgeon may select the most appropriate size and shape of anti-abrasion stud to extend the bearing surface or surfaces. Preferably, the head of the anti-abrasion stud is sized and shaped so that it will not contact any part of the trochlear implant component or uni-condylar femoral implant component. A drill or reamer is then used to prepare a bore in the bone; preferably, the outer diameter of the drill or reamer is slightly less than the outer diameter of the fixation feature (such as larger diameter portion  74 ) of the fixation post. The fixation post is then introduced into the bore and pushed into the bore until the lowermost part of the head (such as the bone-facing portion) contacts the surface of the bone. At least a substantial part of the bearing surface of the head will be above the level of the articular cartilage. This procedure may be repeated with additional anti-abrasion studs as deemed necessary by the surgeon.  
      If the orthopaedic surgeon initially elects to avoid using the anti-abrasion studs, the studs may be implanted in a separate procedure on a later date. For example, if the patient has received a trochlear implant or a uni-condylar femoral implant and complains of pain or of a patellar component catching or making a noise during flexion or extension, the surgeon may opt to implant an anti-abrasion stud at that time. Due to the small size of the anti-abrasion studs, this subsequent procedure can be a minimally invasive one.  
      Thus, the system of the present invention provides the surgeon with the opportunity to enhance and extend the bearing surfaces of standard uni-compartmental implant components to fit the needs of individual patients. The anti-abrasion stud will provide an additional bearing surface that substantially bridges a portion of the gap between the other implant components to provide an augmented bearing surface and covers and protects the native articular cartilage from abrasion.  
      While only specific embodiments of the invention have been described and shown, it is apparent that various alternatives and modifications can be made thereto. Moreover, those skilled in the art will also recognize that certain additions can be made to these embodiments. It is, therefore, the intention in the appended claims to cover all such alternatives, modifications and additions as may fall within the true scope of the invention.