Patent Publication Number: US-2005143833-A1

Title: Patello-femoral joint arthroplasty

Description:
REFERENCE TO RELATED APPLICATION  
      This application is a continuation of co-pending application Ser. No. 10/212,853, filed on Aug. 6, 2002, which in turn claims priority to U.S. Provisional Application Ser. No. 60/310,527, entitled “Femoral Implant for Patello-Femoral Joint Arthroplasty and Associated Surgical Method”, filed on Aug. 7, 2001. The disclosures of each of the above-identified provisional and utility patent applications are hereby totally incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to prosthetic patello-femoral joint assemblies, and more particularly, to individual components of such prosthetic assemblies and associated surgical methods of implantation.  
     BACKGROUND OF THE INVENTION  
      The knee joint is a frequent place for joint damage, and the loss of normal (i.e. relatively pain-free) ambulatory function is a frequent result of such damage. Damage to the knee joint can occur as a result of any one of a plurality of causes, or a combination of causes. For example, a modest overextension of a knee weakened by osteoporosis can result in damage. Moreover, the extent of the damage to the knee joint can vary greatly depending on cause, age of the patient, pre-existing conditions and other factors.  
      The knee is a common source of problems because the joint has an unusually large range of motion and bears nearly half of the weight of the entire body. A primary knee movement, known as flexion-extension movement, includes the bending (flexion) and straightening (extension) of the leg in which a lower part of the leg (tibia and fibula bones) flex in relation to an upper part of the leg (femur bone). Ideally, the knee joint is capable of almost 180 degrees of flexion motion. The knee joint can also accommodate a certain amount of rotational motion in which the lower leg rotates a few degrees in relation to the upper leg.  
      This wide range of motion requires extensive contact surface between the femur and the tibia. The knee joint is rather loosely held together by tendons and ligaments to permit such a wide range of motion. The front or anterior side of the knee joint is protected by the knee cap or patella. The patella is held in place by ligaments and slides over a femoral joint surface during flexion movement. The patella and its ligaments are mechanically involved in joint extension. If any of the joint surfaces (femoral surface, patellar surface, or tibial surface) becomes damaged or roughened, the knee joint will not operate properly.  
      A common problem is damage to the patello-femoral joint that causes free motion of the patella to be inhibited and painful. Such damage is sometimes referred to as “runner&#39;s knee”. Patello-femoral joint (PFJ) damage can make normal joint movement almost impossible.  
      A variety of prosthetic replacements have been developed for different joint surfaces of the knee joint. In extreme cases, the entire joint can be replaced with a prosthetic device. Such a prosthetic replacement is referred to as a total knee replacement. However, total knee replacement requires a considerable time for recovery. In less extreme cases it may be advantageous to replace only the damaged part of the joint.  
      In some cases, PFJ damage may be adequately addressed with a PFJ arthroplasty, as opposed to a total knee replacement system. This type of knee surgery is less drastic than total knee replacement. It is designed for patients whose main problems involve only the patello-femoral part of the knee and is directed to providing a smooth sliding relationship between the femur and the patella. The surface of the femur on which the patella slides is referred to as the trochlear groove. The trochlear groove is the indentation or groove located between the medial and lateral condylar surfaces at the inferior end of the femur.  
      In prior art PFJ prosthetic systems, a prosthetic patellar bearing surface is introduced. The prosthetic bearing surface typically includes an anchoring portion for receiving natural patellar remnants. As a result, the final patellar structure includes a posterior prosthetic bearing surface and an anterior natural patella surface. The anterior natural patella surface typically retains the connective tissue that connects the patella to the quadriceps and tibia.  
      In order to achieve adequate translational movement of the prosthetic patellar bearing surface, particularly in the presence of damage to the trochlear groove, a cooperating prosthetic femur implant is typically affixed onto the end of the femur. The prosthetic femur implant in most cases includes a bearing surface that is specially adapted to receive the prosthetic patellar bearing surface to ensure reliable travel during flexion movement.  
      Such prior art systems, however, are typically highly artificial systems that employ unnatural patello-femoral tracking. One drawback of such systems is that they are not compatible with total knee replacement systems. In many cases, the PFJ system requires so significant an amount of bone removal as to render subsequent total knee replacement almost impossible.  
      More natural patellar devices employ a saddle-shaped design. The saddle-shaped design may be used with or without a femoral implant and is intended to track the within the natural trochlear groove. While the current saddle-shaped designs track within the natural trochlear groove and/or implants that closely approximate the natural trochlear groove, it has been observed that designs of this nature can be prone to a phenomenon referred to as sudden posterior rotation.  
      Sudden posterior rotation sometimes occurs after a deep flexion movement in patients that have a weakened tendon condition known as patella infera. In particular, as the knee is flexed farther and farther into acute flexion, it reaches a point where the patella suddenly rocks back over the sharp superior edges of the patella bearing. The patella bearing rotates around the transverse axis of the patella with the superior pole moving posteriorly and the inferior pole going anteriorly. Sudden posterior rotation often results in significant patient discomfort. Even without discomfort, the sudden posterior rotation can be annoying to the patient.  
      Another drawback of the prior art saddle-shaped patellar devices is that many require a femoral implant relatively deep trochlear groove to receive the peak edge of the saddle. Deep trochlear grooves also require relatively significant bone removal and thus render subsequent knee replacement difficult.  
      There is a need, therefore, for a patella prosthesis having the advantages of more naturally tracking designs but which is less prone to sudden posterior rotation. There is a further need for a femoral implant that requires less bone removal for implantation.  
     SUMMARY OF THE INVENTION  
      The present invention address the above cited need, as well as others, by providing a prosthetic patellar bearing surface that includes first and second femoral engaging surfaces disposed between first and second edge surfaces, the first and second edge surfaces being rounded, or otherwise having a gradual transition from a nearly backward (or posterior) facing portion to a nearly vertical upward or downward (superior or inferior) facing portion. Moreover, the height (or inferior-superior) dimension is at least approximately 90% of the width (or medial-lateral) dimension. The additional relative height, as well as the rounded or otherwise gradually transitioning edges, significantly reduces the likelihood of sudden posterior rotation during deep flexion movement.  
      A first embodiment of the invention is a prosthetic patellar component that includes a base and a bearing element. The base is operable to be affixed to an outer patellar surface. The bearing element comprises first and second femoral engaging surfaces that are separated by a convex peak. The first engaging surface extends medially from the peak and the second engaging surface extends laterally from the peak, the bearing element having a medial-lateral length and a largest inferior-superior length, wherein a ratio of the largest inferior-superior length is at least about 90% of the medial-lateral length. The first and second engaging surfaces are disposed between first and second edge surfaces, the first edge surface extending from a substantially posterior facing portion proximate to the first and second femoral engaging surfaces and a substantially vertical facing portion proximate the base. Adjacent medial-laterally extending surface portions of the first edge surface have an angular displacement less than about 30 degrees in the anterior-posterior direction.  
      Because the edge surfaces include adjacent surface portions having an angular displacement of less than about 30 degrees, no abrupt corners at the edge are present. The lack of abrupt corners reduces the likelihood of sudden posterior rotation and its associated discomfort. In a preferred embodiment, the edge surfaces are rounded, such that the adjacent surface portions are continuous tangential portions of the rounded edge surface. However, alternative embodiments may include discrete polygonal edge portions that simulate a rounded edge surface by employing less than 30 degree displacement between adjacent portions. The present invention may be employed in a PFJ system that engages a natural trochlear groove or a prosthetic femur implant that includes a trochlear groove.  
      Another aspect of the present invention is a femoral implant device for use with a prosthetic patella arrangement. The femoral implant device preferably requires a reduced amount of bone removal. In one embodiment the femoral implant device for use in patello-femoral joint arthroplasty includes a medial bearing surface, a lateral bearing surface and a channel disposed between the medial bearing surface and the lateral bearing surface. The channel extends generally transverse the medial-lateral direction. The lateral bearing surface, the medial bearing surface and the channel form an anterior surface of the implant device. The femoral implant further includes a posterior surface, the posterior surface having a maximum slope in medial-lateral cross-section of less than about 42 degrees.  
      The slope limitation helps ensure that the implantation process will require relatively less bone removal. The medial and lateral bearing surfaces are preferably convex and of differing sizes, both of which provide for better tracking of the patellar device.  
      In a further feature, the posterior face of the femoral implant includes outwardly projecting anchors that are configured for fixation within prepared bores in the femur. The anchors are substantially mutually parallel and aligned along the impaction direction for driving the femoral implant into the femur.  
      The above-described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a side fragmentary view of a knee joint in which an exemplary prosthesis arrangement according to the invention has been implanted, the knee joint in approximately 45 degrees of flexion;  
       FIG. 2  shows a side fragmentary view of a knee joint in which an exemplary prosthesis arrangement according to the invention has been implanted, the knee joint in approximately 120 degrees of flexion;  
       FIG. 3  shows a top plan view of an exemplary patella bearing prosthesis according to the present invention;  
       FIG. 4  shows a bottom plan view of the bearing element of the patella bearing prosthesis of  FIG. 3 , the bearing element separated from the base;  
       FIG. 5   a  shows a top plan view of the base of the patella bearing element of  FIG. 3 , the base separated from the bearing element;  
       FIG. 5   b  shows a side plan view of the base of the patella bearing element of  FIG. 3 ;  
       FIG. 6  shows a cutaway view of the bearing element of  FIG. 4  taken along line VI—VI of  FIG. 4 ;  
       FIG. 7  shows a cutaway view of the bearing element of  FIG. 4  taken along line VII—VII of  FIG. 4 ;  
       FIG. 8  shows a front plan view of a femoral implant for use in connection with the patella bearing element of  FIG. 3 ;  
       FIG. 9  shows a top plan view of the femoral implant of  FIG. 8 ;  
       FIG. 10  shows a side plan view of the femoral implant of  FIG. 8 ;  
       FIG. 11  shows a cutaway view of the femoral implant of  FIG. 8  taken along line XI—XI of  FIG. 9 ;  
       FIG. 12  shows a perspective view of a femoral implant template disposed on a femur in accordance with a surgical method according to the present invention;  
       FIG. 13  shows a side plan view of patellar tissue resection of a surgical method according to the present invention;  
       FIG. 14  shows a patella bearing template for use in connection with a surgical method according to the present invention;  
       FIG. 15  shows a plan view of the use of the patella bearing template of  FIG. 14  in preparing the patellar tissue for receiving the patella bearing prosthesis of  FIG. 3 ; and  
       FIG. 16  shows a plan view of the patellar tissue being affixed to the patella bearing prosthesis of  FIG. 3  in a surgical method according to the present invention.  
    
    
     DETAILED DESCRIPTION  
       FIGS. 1 and 2  show side fragmentary views of a knee joint  10  in which an exemplary prosthesis arrangement  12  according to the invention has been implanted.  FIG. 1  shows the knee joint  10  in approximately 45 degrees of flexion while  FIG. 2  shows the knee joint  10  in approximately 120 degrees of flexion.  
      In addition to the prosthesis arrangement  12 , the knee joint  10  shown in  FIGS. 1 and 2  includes a portion of a femur  14 , a portion of a tibia  16 , quadricep connective tissue  18  and a patellar ligament  20 . The prosthesis arrangement  12  further includes a bearing element  22 , a base  24  and natural patellar bone tissue  26 . The bearing element  22  is secured to the base  24  such that partial rotation between the bearing element  22  and the base  24  may occur. The base  24  is securely affixed to the patellar bone tissue  26 . The patellar bone tissue  26  is naturally affixed between the quadricep connective tissue  18  and the patellar ligament  20 . In accordance with one aspect of the present invention, the bearing element  22  includes edge surfaces  28  and  30 . At least the superior edge surface  28  has a gradual transition, for example, a rounded edge. As will be discussed further below, the superior-inferior dimension of the bearing element  22  is relatively large compare to prior art devices of like construct.  
      The prosthesis arrangement  12  moves or slides substantially in the inferior-superior direction during flexion motion of the knee.  FIG. 2  illustrates a condition that may occur in patients having patella infera (weakened connective tissue). In particular, as the knee  10  moves to deep flexion as shown in  FIG. 2  the weakened patellar ligament  20  allows the prosthetic arrangement to rotate slightly in the posterior direction. However, because of the relatively large inferior-superior dimension and the gradual transition of the superior edge surface  30 , the prosthetic arrangement  12  may rotate smoothly back into position as the knee joint  10  moves out of deep flexion.  
      Further detail regarding an exemplary embodiment of the prosthesis arrangement  12  is provided in connection with  FIGS. 3-7 .  FIG. 3  shows a bearing prosthesis  32  that includes the bearing element  22  and the base  24 .  FIGS. 4, 6  and  7  show different views of the bearing element  22  apart from the base  24 , while  FIGS. 5   a  and  5   b  show different views of the base  24  apart from the bearing element  22 .  
      With reference to  FIGS. 3, 4 ,  6  and  7 , the bearing element  22  includes a posterior side  34  and an anterior side  36 . The posterior side  34  includes a bearing surface  38  defined by first and second femoral engaging surfaces  40  and  42 . The first and second femoral engaging surfaces  40  and  42  are separated by a peak surface  44 . The surfaces  40 ,  42  and  44  preferably cooperate to form an asymmetric saddle-type surface. To this end, the first femoral engaging surface  40  extends medially away from the peak surface  44 , also sloping in the anterior direction as it extends medially away from the peak surface  44 . Analogously, the second femoral engaging surface  42  extends laterally from the peak surface  44 . The second femoral engaging surface  42  also slopes in the anterior direction as it extends laterally away from the peak surface  44 .  
      In a preferred embodiment discussed herein, the sagittal cross-section (e.g.  FIG. 6 ) of the peak surface  44  is concave, forming a slightly U-shaped channel. Likewise, the first and second femoral engaging surfaces  40  and  42  have similarly shaped sagittal cross-sections.  
      The first and second engaging surfaces  40  and  42  are thus disposed end to end (i.e. serially) in the medial-lateral direction, with the peak surface  44  forming an intersection. The first and second engaging surfaces  40  and  42  further co-extend width-wise along the inferior-superior dimension. Also extending medial-laterally and bordering the inferior edges of the first engaging surface  40 , the second engaging surface  42  and the peak surface  44  is the superior edge surface  28 . Extending medial-laterally and bordering the superior edges of the first engaging surface  40 , the second engaging surface  42  and the peak surface  44  is the edge surface  30 .  
      With particular reference to  FIGS. 3 and 6 , the superior edge surface  28  extends from a substantially posterior facing portion  46  (located proximate to the first and second femoral engaging surfaces  40  and  42 ) to a substantially vertical (superior) facing portion  48  proximate to the anterior side  36 . Between the substantially posterior facing portion  46  and the substantially superior facing portion  48  is a gradually transitioning surface that may be considered to be divided into a plurality of adjacent medial-laterally extending surface portions. To ensure a gradual transition, it is preferable that the angle displacement between any two adjacent surface portions be less than about 30 degrees as measured in the anterior-posterior direction (i.e. measured in the view shown in  FIG. 6 ).  
      In the exemplary embodiment described herein, the first edge surface  28  includes a curved portion  50 , thereby guaranteeing throughout such portion that the angle displacement between adjacent surface portions is always less than about 30 degrees. The curved portion  50  extends downward until it encounters the substantially posterior facing portion  46 . In the exemplary embodiment described herein, the substantially posterior facing portion  46  extends substantially straight in the posterior direction from the anterior side  36  to a portion of the arc of the curved portion  50  that is approximately 20-25 degrees from the inferior-superior line that intersects its radius. Accordingly, the angle displacement between the tangent at the end of the curved portion  50  and the substantially posterior facing portion  46  is also 20-25 degrees, consistent with the overall 30 degree limitation discussed above.  
      In some embodiments, it may not be practical to limit the angle displacement between adjacent portions of the edge surface to about 30 degrees throughout the entire edge surface  28 . In such cases, it has been found that by at least providing a curved portion such as the curved portion  50  can assist is reducing the likelihood of sudden posterior rotation, even if the angle displacement between the end of the curved portion and the substantially superior facing portion exceeds about 30 degrees. In particular, as long as the curved portion  50  extends sufficiently outward in the superior direction with an appropriate radius of curvature, the effect shown in  FIG. 2  may typically be achieved. For example, if the curved portion  50  extends in the superior direction such that it covers at least about 20 percent of the largest inferior-superior dimension of the bearing surface  38 , and if the curved portion  50  has a radius of curvature that is less than one-half of the largest inferior-superior dimension of the bearing surface  38 , then enough of a gradual transition surface is provided by the edge surface  28 .  
      If the radius of curvature is too large in such an embodiment, then the resulting edge surface would have too sharp of a cutoff and would not represent a gradual transition surface sufficient to effectively eliminate sudden posterior rotation. Likewise, if the curved portion  50  does not extend sufficiently far in the superior direction before terminating in the substantially superior facing portion  48 , then the resulting edge surface would not exhibit enough of a transition area to effectively reduce sudden posterior rotation.  
      In an acceptable alternative, the angle of transition between the end of the curved surface  50  and the substantially superior facing portion may be about 45 degrees or less if the curved portion  50  extends in the superior direction such that it covers at least about 20 percent of the largest inferior-superior dimension of the bearing surface  38 . While 45 degrees of angular displacement on the edge is somewhat abrupt, the length and curvature of the curved portion  50  will generally provide an adequate transition surface.  
      In other embodiments, the gradual transition surface may be accomplished by individual, non-curved (in the posterior-anterior direction) portions that form a polygonal pseudocurve that extends from the substantially posterior facing portion  46  to the substantially superior facing portion  48 , as long as the angle between the adjacent portion is less than about 30 degrees. In still other embodiments, the pseudocurve may have an angle of up to about 45 degrees with respect to the substantially superior portion if the pseudocurve extends to at least until about 20 percent of the largest inferior-superior dimension.  
      All of the above limitations stress the idea of a gradual, convex transition surface to reduce the likelihood of sudden posterior rotation of the prosthetic arrangement  12 . Prior art devices typically employed abrupt corners, such as an 80-90 degree transition with an insignificantly rounded corner. Such abrupt corners could result in sudden posterior rotation because the superior surface of the corner surface could “catch” on the femur when the knee joint comes out of deep flexion.  
      Another aspect of the present invention that assists in the inhibiting sudden posterior rotation problems is the relatively large inferior-superior length as compared to the medial-lateral length. In particular, the medial-lateral length is typically dictated in part by the medial-lateral length of the natural patella. The medial-lateral length is preferably as large as is practical to ensure optimal tracking, while not exceeding the approximate medial-lateral length of the natural patella. By using a superior-inferior size that is, at its longest point, at least approximately 90%, and preferably at least approximately 92% of the medial-lateral length, a transition edge surface (i.e. the edge surface  28 ) of significant length may be provided without sacrificing the inferior-superior dimensions of the femoral engaging surfaces  40  and  42 .  
      The combination of the gradual transition surfaces and increased inferior-superior dimension thus provide good tracking, adequate contact surface, and inhibition of sudden posterior rotation during deep flexion of the knee. Such advantages of the prosthetic arrangement  12  are further enhanced because the arrangement is configured to allow for partial rotation of the natural patella tissue  26  with respect to the bearing element  22 . To this end, the base  24  is configured to be attached to the bearing element in such a manner as to allow for partial relative rotation. As a result, when the natural patella tissue  26  is affixed to the base  24 , the natural patella tissue  26  may rotate in a limited way with respect to the bearing element  22 , which more closely mimics the natural range of motion of a healthy knee joint.  
      Referring to  FIGS. 4, 5   a ,  5   b  and  6 , the anterior side  36  of the bearing element  22  includes a recess  48  which is configured to receive a corresponding bearing  52  of the base  24 . The corresponding bearing  52  may rotate within the recess. The recess  48  in the exemplary embodiment described herein has the shape of an elevated and inverted cone. Accordingly, the bearing  52  has the shape of an elevated cone such that the bearing fits into the recess  48 . The bearing  52  includes an annular lip  54  that cooperates with a corresponding annular lip  56  of the recess to retain the bearing  52  within the recess after being press fit.  
      The anterior side  36  of the bearing element  22  further includes a rotation limiting channel  60  that is configured to receive a small protrusion  58  that is disposed on the base  24 . The rotation limiting channel  60  is preferable arc-shaped to allow the protrusion  58  to move in an arc, thereby allowing rotation of the bearing element  22  with respect to the base  24 . However, the limits of the arc are chosen such that they correspond to the desired limitation of rotational freedom.  
      In general, the base  24  has a size and shape roughly correlated to the size and shape of a human patella. The base  24  includes a posterior side  62  on which the bearing  52  and the protrusion are located and an opposing anterior side  64 . The anterior side  64  includes a relatively flat patella receiving surface  66  and a plurality of anchors  68 . As will be discussed below the anchors  68  are received into drilled bores in the natural patella bone tissue  26  to assist in securing the base  24  to the bone tissue  26 .  
      The base  24  and the bearing element  22  are press fit together such that the bearing  52  is received into the recess  48  and the small protrusion  58  is received in to the rotation limiting channel  60 . The annular lips  54  and  56  retain the base  24  and the bearing element together. The rotation limiting channel  60  limits the relative rotational movement of the base  24  and the bearing element  22  by only allowing limited travel of the small protrusion  58  within the channel  60 .  
      When the assembled bearing prosthesis  32  is secured to the natural patella bone tissue  26 , the resulting prosthetic arrangement  12  is capable of relatively natural movement within the body. In particular, the first and second femoral engaging surfaces  40  and  42  are advantageously configured to engage relatively normal femoral condyles to allow sliding movement of the arrangement  12  within the condyles. In a preferred embodiment, the femur is further prepared with a femoral insert or implant that is configured to receive the bearing prosthesis  32 .  
       FIGS. 8, 9 ,  10  and  11  show an exemplary embodiment of a femoral implant  70  according to the present invention. Features of the femoral implant  70  include and asymmetrical wing shape that allows for better tracking of the asymmetrical bearing prosthesis  32 . Another feature is the relatively shallow trochlear groove, which requires less bone removal prior to implantation. Requiring less bone removal provides the advantage of allowing subsequent procedures to be performed on the knee joint. In particular, patients who have PFJ replacement are more likely to require a total knee replacement at some point in their lives. Accordingly, it is advantageous to limit the amount of bone removed during PFJ replacement in order to ensure that adequate femur bone tissue is intact for later implementation of the total knee prosthesis.  
      Referring now to  FIGS. 8, 9   10  and  11 , the femoral implant  70  includes a first (medial) condylar wing  72 , a second (lateral) condylar wing  74 , and a trochlear channel  76  that forms the intersection of the wings  72  and  74 . The first condylar wing  72 , the second condylar wing  74  and the trochlear channel  76  all include anterior bearing surfaces that, as a group, define the anterior bearing surface  82  of the femoral implant  70 .  
      The first condylar wing  72  is roughly triangular shaped and is configured to mimic the curvature of a condyle of a human femur. To this end, the anterior surface of the first condylar wing  72  forms a convex crescent arc shape in inferior-superior dimension, thereby curving somewhat in the posterior direction at both the inferior end  78  and the superior end  80 , as shown in  FIG. 10 . The posterior surface of the first condylar wing  72  is substantially complementary, and thus concave. In addition, the anterior surface of the first condylar wing  72  has a convex arc shaped defined through its medial-lateral dimension, as shown in  FIG. 11 .  
      The second condylar wing  74  has a similar shape as the first condylar wing  72 , although the second condylar wing  74  is generally wider in the medial-lateral dimension than the first condylar wing  72 . The trochlear channel  76  runs generally from the inferior end  80  to the superior end  78  and forms the intersection of the convex condylar wings  72  and  74 .  
      In general, the femoral implant  70  is installed at the inferior end of the femur  16  such that the trochlear channel  76  aligns with the natural trochlear groove of the femur. As will be discussed below, the femoral bone tissue must be prepared to receive the femoral implant  70 . In particular, the femoral bone tissue is shaped such that it conforms substantially to the posterior surface  84  of the femoral implant  70 .  
      In accordance with the exemplary embodiment described herein, the depth of the groove defined by the trochlear channel  76  is advantageously configured to balance the need for reducing the amount of femoral bone tissue that must be removed and need for sufficient tracking of the bearing element  22  of the patella prosthetic arrangement  12 . To this end, the posterior surface  84  of the femoral implant  70  has a maximum slope of less than approximately 40 to 42 degrees, taken in any medial-lateral cross-section, such as is shown in  FIG. 11 . As a result, less femoral bone tissue need be removed from the vicinity of the trochlear groove than in prior art implants having a deeper (more severely sloped) channel. Preferably, the anterior bearing surface  82  has a complementary slope limitation.  
      The posterior surface  82  further includes a plurality of anchors  86  for securing the femoral implant to the femoral bone tissue. Each anchor  86  may suitably be a posteriorly extending member. As depicted in  FIGS. 8 and 10 , the anchors  86  are substantially parallel to each other. The anchors  86  are also generally perpendicular to a plane tangent to the femoral bone surface as prepared in accordance with the steps outlined below using the implant template  88 .  
      A process for performing a PFJ replacement employing the prosthetic patellar arrangement  12  and the femoral implant  70  is discussed with reference to  FIGS. 12 through 16 . Initially, it is advisable to review x-rays of the knee joint to determine which of a plurality of sizes should be employed. In general, the bearing element  22  is preferably available in four or five sizes ranging from 1.015 inches (inferior-posterior) by 1.126 inches (medial-lateral) to 1.520 inches (inferior-posterior) by 1.615 inches (medial-lateral). The femoral implant  70  is preferably available in four or five corresponding sizes ranging from 1.51 inches (inferior-posterior) by 1.18 inches (medial-lateral) to 2.4 inches (inferior-posterior) by 1.7 inches (medial-lateral).  
      Routine total joint arthroplasty protocols should be followed. The incision should be a midline skin incision, unless previous surgical scars indicate otherwise. A lateral retinacular release is performed up to but not including the superior lateral geniculate artery. If a more extensive release is necessary, it should be dissected and preserved for patellar blood supply. The patella should be dislocated and everted laterally.  
      Once the patella has been laterally dislocated, the trochlear groove and surrounding femoral surfaces must be prepared to receive the femoral implant  70 . To this end, an implant template  88  is employed.  FIG. 12  shows the implant template  88  fitted to the trochlear groove  90  of the femur  14 . The implant template  88  has a shape that is substantially similar to that of the femoral implant  70 , except that the implant template includes drill guides or drill bosses  94  instead of, and in the same position as, the anchors  86 .  
      The implant template  88  is first aligned within the trochlear groove  90  as shown in  FIG. 12  (however, alignment occurs without the drill bit  96  shown in  FIG. 12 ). Once the template  88  is properly aligned, the outline of the template is marked on the cartilage and bone using a marking pen, knife or the like. It is noted that the inferior end should not protrude into the intercondylar notch, but instead should be just proximal to the notch as shown in  FIG. 12 .  
      The cartilage within the outline should be sharply resected. High-speed burrs having small sharp osteotomes at the edges should be used to cut away a small portion of the subchondral bone within the outline. The implant template  88  is then placed into the groove again. An outline is drawn again, and further cuts may be made if the implant template  88  is not yet flush with the articular cartilage surface. The outline and cut steps may be repeated until the implant template  88  lays flush. Care should be taken to remove only small layers at a time to avoid the possibility of significant over-removal.  
      When the implant template  88  is flush, the components of the inferior end  78  of the femoral implant  70  will be flush, thereby reducing the possibility of overhang in which the prosthetic patellar arrangement  12  could get caught during deep flexion. By contrast, the portion of the wings  72  and  74  proximal the superior end  80  may protrude anteriorly from the bone without substantial ill effect.  
      After the trochlear cavity is created as discussed above and the implant template  88  fits properly, the implant template  88  may be used to drill holes in the femur  14  in which the anchors  86  will be received. This process is illustrated in part by  FIG. 12 . Once the holes have been drilled the femoral implant  70  is implanted. To this end, the anchors  86  are aligned with the drilled holes and an impacting device is used to drive the anchors  86  into the holes and the implant  70  into the cavity of the femur  14 . Since the anchors are mutually parallel and generally perpendicular to the tangent plane to the prepared femur, the anchors can be readily driven along the impaction direction directly into the bone.  
      After the femoral implant  70  is in position, the patellar prosthetic arrangement  12  is prepared. To this end, the synovial tissue must be freed from the periphery of the patella down to the plane of the quadriceps and patellar tendon reflections. As shown in  FIG. 13 , the patellar articular surface  100  is resected parallel to and on the level of the quadriceps tendon connective tissue  18 , thereby leaving the natural patella anterior bone tissue  26  connected to both the tissue  18  and the tibial ligament  20 . The resection may suitably be performed using a patellar resection guide and an oscillating saw, not shown. Suitable devices are commercially available.  
      Once the patella articular surface  100  is removed, a template  102  is used to drill the holes in the remaining bone tissue  26  for receiving the anchors  68  of the base  24  of the bearing prosthesis  32 . (See  FIGS. 1 and 5   b ). As shown in  FIG. 14 , the patellar template  102  includes three drill bosses  104  that are in the same configuration and alignment as the anchors  68  of the bearing prosthesis  32 . The patellar template  102  otherwise has a shape and size similar to that the remaining bone tissue  26 .  FIG. 15  illustrates use of the patellar template  102  to drill the holes.  
      Thereafter, the bearing prosthesis  32  is pressed onto the remaining bone tissue  26  such that the anchors  86  are received into the drilled holes. The resulting prosthetic arrangement  12  then includes the base  24 , the bearing element  22  and the natural patellar bone tissue  26 . However, the prosthetic arrangement  12  and the femoral implant  70  have only been prepared for trial reduction. To perform the trial reduction, the knee joint  10  is put through a full range of motion.  
      During the full range of motion, patellar excursion should be checked. If the patellar prosthetic arrangement  12  must be held in place with a thumb, then the alignment is not proper. Proper alignment of the extensor mechanism is important because the femoral implant  70  has a relatively deep anatomic sulcus. As a guideline, if the Q-angle is less than about 20 degrees, then a slightly larger lateral release will usually suffice. If the Q-angle is over 20 degrees, then a medial tibial tubercle transfer to a Q-angle of about 10 degrees should be considered. The Q-angle is measured intraoperatively with the knee extended and the limb rotated to that the patella is straight up and reduced into the trochlear channel  76 .  
      The travel of the arrangement  12  should be checked to ensure that the bearing element  22  engages the trochlear channel  76  smoothly going from extension to flexion as well as going from flexion to extension. The travel of the arrangement  12  should also be checked to ensure that it does not catch at the inferior end  78  or superior end  80 .  
      If the trial reduction is successful, the prosthetic arrangement  12  may be finally assembled. To this end, the bearing prosthesis  32  is removed from the patellar bone tissue  26  and the femoral implant  70  is removed from the femur  14 .  
      The trochlear area of the femur  14  is prepared using pulse lavage. After the femur dries, bone cement is applied to the posterior surface  84  of the femoral implant  70 . The femoral implant  70  is then reinserted into the trochlear area of the femur  14  using an impact device, as discussed above. Excess cement should be removed. The bearing prosthesis  32  is implanted onto the patellar bone tissue  26  using either a porous-coated implant or a cement technique. A patellar clamp  106  as shown in  FIG. 16  may suitably be used to implant the bearing prosthesis  32 . The resulting prosthetic arrangement should again be tested for proper excursion.  
      A number 0 braided polyester or a similar non-absorbable suture should be used for capsular closure, to allow for expedited range of motion for post-operative exercise.  
      It will be appreciated that the above described embodiments are merely exemplary, and that those of ordinary skill in the art may readily devise their own implementations of the present invention that incorporate the principles of the present invention and fall within the spirit and scope thereof.  
      It will further be appreciated that the shape of the bearing element is compatible with the LCS Total Knee system available from Depuy Orthopedics of Warsaw, Ind. Thus, if the patient subsequently (many years later) requires a total knee replacement, then the femoral implant  70  may be removed, and replace by the total knee system. The patellar prosthetic arrangement  12 , however, need not be removed and may be used in conjunction with the total knee system.