Abstract:
A hinged knee prosthesis includes a femoral component with a tibiofemoral articular surface that is distinct from the patellofemoral articulating surface. Fully congruent tibiofemoral articulation is provided for virtually all flexion angles. Additionally, the bearing is capable of at least limited axial rotation relative to the tibial component but is restrained against dislocation. Accordingly, dislocation is much less likely even n those situations where collateral ligaments are insufficient.

Description:
[0001]     This application claims priority on U.S. Provisional Patent Appl. No. 60/566,214, filed Apr. 28, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a knee joint prosthesis, and particularly a hinged knee joint prosthesis.  
         [0004]     2. Description of the Related Art  
         [0005]     A natural knee joint includes the distal end of the femur with articular cartilage, the proximal end of the tibia with articular cartilage and a meniscus between the femur and tibia. The femur and the tibia are held in a proper relationship to the meniscus by ligaments. These stabilizing ligaments include the posterior cruciate ligament, the anterior cruciate ligament and collateral ligaments.  
         [0006]     The condyles at the distal end of the femur define compound curves. Hence, the degree of congruency between the condyles of the natural femur and the superior surface of the meniscus varies at different degrees of flexion. Flexion of the knee causes the tibia to rotate relative to the femur about an axis that extends generally in a medial-to-lateral direction. The non-congruent shapes of the femoral condyles and the superior face of the bearing causes the contact area of the femur to roll back relative to the tibia during certain ranges of flexion. Flexion also generates rotation of the tibia about its own axis. The amount of rotation of the tibia during flexion of the knee is controlled and limited by the ligaments.  
         [0007]     The natural knee joint can become damaged or diseased. For example, damage or disease to the knee can deteriorate the articular surfaces of the femur or tibia and can damage the articular cartilage between the bones. The prior art includes prosthetic knee joints to replace a damaged or diseased natural knee. A prosthetic knee joint typically includes a femoral component that is mounted to the distal end of a resected femur, a tibial component mounted to the proximal end of a resected tibia and a bearing between the femoral and tibial components. The inferior face of the femoral component of a prosthetic knee joint typically defines a pair of arcuate convex condyles. The superior face of the bearing has regions for articular bearing engagement with the condyles of the femoral component. The superior face of the tibial component may be substantially planar and is engaged with the inferior face of the bearing.  
         [0008]     Prior art prosthetic knee joints have taken many different forms, depending upon the preferences of the orthopedic surgeon, the condition of the natural knee and the health, age and mobility of the patient. Some prior art knee joint prostheses fixedly secure the inferior surface of the bearing to the superior surface of the tibial component. Other prior art knee joint prostheses provide somewhat greater mobility and permit rotational movement between the bearing and the tibial component. Still other prior art knee joint prostheses allow even greater mobility and permit a controlled amount of anterior-posterior sliding movement between the bearing and the tibial component in addition to the rotational movement.  
         [0009]     Prior art prosthetic knee joints for patients with viable ligaments and good stability have no hinges and rely upon retained ligaments to hold the femoral component in an acceptable range of positions relative to the bearing and the tibial component. However, patients without viable collateral ligaments require a knee joint prosthesis where the femoral component is hinged to substantially prevent both anterior-posterior movement of the femoral component relative to the tibial component and to prevent medial-lateral movement.  
         [0010]     U.S. Pat. No. 5,824,096 issued to the inventors herein and discloses a hinged knee prosthesis where the condyles of the femoral component are defined by compound curves. Hence, congruency will exist for certain ranges of flexion, but a non-congruent line contact will exist between the femoral component and the bearing during other ranges of flexion. More particularly the femoral component of the prosthesis shown in U.S. Pat. No. 5,824,096 is configured to provide a substantially congruent bearing in the critical peak loading phase of the normal walking cycle. However, a reduced posterior femoral radii of the condyles produces a more normal knee motion during flexion with an adequate line contact in deeper flexion phases of non-critical activity. The hinged knee prosthesis of U.S. Pat. No. 5,824,096 also permits a controlled range of movement of the bearing relative to the tibial and femoral components along an anterior-posterior axis. Additionally, the hinged connection disclosed in U.S. Pat. No. 5,824,096 permits a controlled movement of the femoral component away from the tibial component. The compound curves of the femoral component combined with the ability of the bearing to move in anterior and posterior directions causes the femoral component to climb and roll back on the bearing during certain ranges of flexion. The hinge of the prosthesis shown in U.S. Pat. No. 5,824,096 is used primarily for stabilization and performs only a minimal load bearing function. Thus, the hinge can be much smaller than knee prostheses where the hinge performs a primary load bearing function. The smaller hinge minimizes bone removal and hence helps to achieve an improved implant fixation. The smaller hinge also is lighter, and hence minimizes the effect of prosthesis weight on the gait of the patient.  
         [0011]     The hinged knee prosthesis of U.S. Pat. No. 5,824,096 has performed very well since its first introduction and use. However, wear could be reduced further if femoral congruency existed for a greater range of knee motion, rather than only during peak loading phases during walking. For example, congruent contact during many high load activities, such as stair climbing and decent, arising from a chair and other moderate to deep flexion activities would be helpful for improving wear of the prosthesis. Additionally, a prosthesis with a tibial component that extended a smaller distance into the tibia would require less bone removal for implantation, and hence would be received well.  
       SUMMARY OF THE INVENTION  
       [0012]     The invention relates to a hinged knee prosthesis with a condylar bearing. The prosthesis includes a femoral component for mounting to the resected distal end of the femur, a tibial component for mounting to the resected proximal end of the tibia, a bearing for disposition between the femoral and tibial components and a hinge assembly for providing stability during articulation between the femoral and tibial components. The prosthesis may further include a patellar component.  
         [0013]     The hinged knee prosthesis of the subject invention differs from prior art hinged knee prostheses by providing patellofemoral articulating surfaces that are distinct from the tibiofemoral articulating surfaces. The patellofemoral articulation may be similar or identical to that of prior art knee prostheses and may include a compound curve as the articulating surface on the femoral component. However, the tibiofemoral articulating surface is configured for congruent contact over a larger range of motion than is available for knee prosthesis without distinct tibiofemoral and patellofemoral articulating surface. Preferably the congruent contact for the tibiofemoral articulation extends for substantially the entire range of motion. This provides a significant advantage of reduced wear, as compared to prior art prosthetic knees, including prior art hinged knee prostheses.  
         [0014]     Prosthetic knees with distinct patellofemoral and tibiofemoral articulating surfaces have been contemplated in non-hinged prosthetic knee designs. However, such a design would not accommodate valgus-varus motion, which is motion in a generally medial to lateral direction. However, a hinged knee provides adequate resistance to valgus-varus motion, and hence is well suited to distinct patellofemoral and tibiofemoral surfaces.  
         [0015]     The tibial component of the prosthetic knee includes an axial support, such as a stabilizing rod that can be mounted in a prepared cavity in the resected proximal end of the tibia. The tibial component further includes a tibial plate that extends transverse to the axial support and that can be mounted on the resected proximal end of the tibia. The tibial plate includes a superior tibial surface that may be substantially planar. The tibial component further includes a conical hole extending through the tibial plate and towards the axial support and stabilization rod. A groove may be formed near the distal end of the conical hole.  
         [0016]     The bearing of the prosthetic knee includes a plastic cone that can be mounted in the conical hole of the tibial component so that the bearing can rotate relative to the tibial component. The cone of the bearing preferably includes axial separation means for preventing axial separation or dislocation of the bearing from the tibial component. Thus, the prosthetic knee can provide dislocation resistance at least as good as prior art hinged prostheses, but with a shorter cone and hence less bone removal. The axial separation means may include a groove in the cone of the bearing that will align with the groove in the conical hole of the tibial component when the cone of the bearing is mounted in the conical hole of the tibial component. A snap ring may be engageable in the aligned grooves of the conical hole in the tibial component and in the cone of the bearing. The snap ring prevents the bearing from moving proximally and out of the conical hole in the tibial component. However, the snap ring permits the cone of the bearing to rotate relative to the tibial component. Hence, rotational mobility for the bearing is permitted, but dislocation of the bearing is prevented. Other connections that permit rotation but not separation can be provided  
         [0017]     The bearing further includes an inferior bearing surface for bearing engagement on the superior tibial surface of the tibial component. Means may be provided for limiting rotational movement of the bearing on the superior tibial surface. For example, the inferior surface of the bearing may be formed with an arcuate groove, and a pin may project from the superior surface of the tibial component for engagement in the groove of the bearing. Thus, the angular extend of the groove in the bearing will limit the range of pivotal movement of the bearing relative to the tibial component. The bearing further includes a superior condylar bearing region for articular bearing engagement with the femoral component. The bearing further include a hole extending from the proximal or superior end to or towards the distal end. Lower portions of the hole in the bearing may be conically tapered.  
         [0018]     The hinge assembly includes a carriage with a shaft configured for insertion into the hole of the bearing. The carriage further includes a head with smoothly polished side surfaces that may be substantially parallel to one another. A hinge support hole extends through the head and transverse to the axis of the shaft. The hinge assembly further includes a hinge pin that can be inserted into the pin support hole in the head of the carriage and a set screw for mounting in a threaded hole in the head to hold the hinge pin in position in the head.  
         [0019]     The femoral component includes a superior surface configured for mounting on the resected distal end of the femur. A stabilizing rod may project from the superior surface for mounting in a cavity prepared in the resected distal end of the femur. The femoral component further include hinge support walls that project proximally from the superior surface of the femoral component. The walls are substantially parallel to one another and are spaced apart sufficiently for receiving the head of the carriage. The hinge support walls further include holes that align with one another for receiving opposite ends of the hinge pin. The inferior region of the femoral component include a tibiofemoral articular surface and a distinct patellofemoral articulating surface. The patellofemoral articulating surface may define a curved shape similar to the shape of the articular surface on the femoral component disclosed in the above-referenced U.S. Pat. No. 5,824,096. The tibiofemoral articulating surface is configured for congruent articular bearing engagement with the condylar bearing region of the bearing.  
         [0020]     The congruent articular bearing engagement of the tibiofemoral articular surface of the femoral component with the condylar bearing region on the superior surface of the bearing provides congruency through virtually all ranges of motion from a slightly hyperextended condition to deep flexion. Thus, the prosthetic knee of the subject invention provides congruent contact during many high load activities, such as stair climbing and descent and arising from a chair. Additionally, the prosthetic knee of the subject invention provides very good resistance to valgus-varus moments. Furthermore, the engagement of the cone of the bearing in the conical hole of the tibial component resists dislocation and the engagement of the stop pin with the groove in the inferior bearing surface of the bearing controls and limits the range of permissible rotation of the tibia relative to the femur. Thus, the prosthetic knee of the subject invention is well suited for those situations where the ligaments are not present or are ineffective. 
     
    
     BRIEF DESCRIPTON OF THE DRAWINGS  
       [0021]      FIG. 1  is a longitudinal cross-sectional view of the assembled and implanted hinged knee prosthesis of the subject invention.  
         [0022]      FIG. 2  is a top plan view of the tibial component.  
         [0023]      FIG. 3  is a cross-sectional view taken along line  3 - 3  in  FIG. 2 .  
         [0024]      FIG. 4  is a side elevational view of the bearing.  
         [0025]      FIG. 5  is a top plan view of the bearing.  
         [0026]      FIG. 6  is a cross-sectional view taken along line  6 - 6  in  FIG. 5 .  
         [0027]      FIG. 7  is a front elevational view of the carriage of the hinge assembly.  
         [0028]      FIG. 8  is a side elevational view of the carriage.  
         [0029]      FIG. 9  is a front elevational view of the hinge pin of the hinge assembly.  
         [0030]      FIG. 10  is a side elevational view of the hinge pin.  
         [0031]      FIG. 11  is a side elevational view of the set screw.  
         [0032]      FIG. 12  is a rear elevational view of the femoral component.  
         [0033]      FIG. 13  is a side elevational view of the femoral component.  
         [0034]      FIG. 14  is a front elevational view of the hinge bearing.  
         [0035]      FIG. 15  is a side elevational view of the hinge pin bearing.  
         [0036]      FIG. 16  is a longitudinal cross-sectional view of the tibial component, bearing and carriage in their assembled condition.  
         [0037]      FIG. 17  is a cross-sectional view similar to  FIG. 16 , but showing movement required for dislocation of the assembled bearing and carriage.  
         [0038]      FIG. 18  is a cross-sectional view similar to  FIGS. 16 and 17 , but showing the movement required for dislocation of the carriage relative to the assembled tibial component and bearing.  
         [0039]      FIG. 19  is a rear elevational view of the assembled knee prosthesis.  
         [0040]      FIG. 20  is a front elevational view thereof.  
         [0041]      FIG. 21  is a side elevational view of the assembled prosthesis showing the knee in a 5° hyperextension.  
         [0042]      FIG. 22  is a cross-sectional view showing the assembled knee at approximately 150° flexion, and showing the set screw in an exploded orientation.  
         [0043]      FIG. 23  is a top plan view showing relative positions of the patella and femoral component during flexion. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]     A hinged knee prosthesis in accordance with the invention is identified generally by the numeral  100  in  FIGS. 1 and 19 - 23 . The knee prosthesis  100  includes a femoral component identified generally by the numeral  200  in  FIG. 1 . The femoral component  200  is configured for mounting to the resected distal end of the natural femur  600 . The femoral component  200  is configured for articulation relative to an assembly that includes a hinge subassembly  300 , a plastic bearing  400  and a tibial component  500 . The tibial component  500  is configured for mounting in the resected proximal end of the natural tibia  700 . The prosthesis  100  may further include a patellar component  800  that may be implanted in the natural patella  900 .  
         [0045]     The tibial component  500  includes a body  501  and a stabilizing rod  502  as shown in  FIG. 1 . Both the body  501  and the stabilizing rod  502  are formed from a metallic material that will provide appropriate strength and biocompatibility. A preferred tibial component  500  is made from a titanium alloy with a TiN coating. However, the tibial component  500  may also be formed from a CoCr alloy. Other metallic materials appropriate for use in the manufacture of the tibial component  500  will be known to those skilled in the art.  
         [0046]     The body  501  of the tibial component  500  is illustrated most clearly in  FIGS. 2 and 3 . The body  501  includes a conical hole  503  that extends distally from a superior tibial surface  504 , and that tapers to smaller dimensions at locations further from the superior tibial surface  504 . The conical hole  503  communicates with a cavity  505  that receives the proximal end of the stabilizing rod  502 .  
         [0047]     A tibial plate  506  extends transverse to the axis defined by the conical hole  503  and includes the superior tibial surface  504 . The inferior face of the tibial plate  506  is mountable on the resected proximal end of the natural tibia  700  as shown in  FIG. 1 . Additionally, portions of the tibial component  500  below the tibial plate  506  are implanted into a cavity prepared in the resected proximal end of the tibia  700 . The tibial component  500  may be used with a bone cement to achieve secure anchoring of the tibial component  500  in the tibia  700 . Alternatively, external surface regions of the body  501  near the plate  506  may have a bone ingrowth surface region that will encourage growth of the natural bone for secure anchoring of the tibial component  500 .  
         [0048]     The body  501  of the tibial component further includes an annular groove  507  formed near the distal end of the conical hole  503 . Additionally, the tibial component  500  includes a stop pin  508  mounted to an anterior portion of the tibial plate  506 . As explained further herein, the stop pin  508  cooperates with the bearing  400  to limit relative rotation between the bearing  400  and the tibial component  500 .  
         [0049]     The bearing preferably is formed unitarily from a non-metallic material and most preferably from UHMWPe. The plastic of the bearing performs well under loads, exhibits good biocompatibility and does not interact with the metallic materials of the prosthesis  100  that are adjacent the bearing  400 . The bearing  400  includes superior condylar bearing surfaces  401  at medial and lateral positions on the bearing  400 . A hole  402  extends in a proximal to distal direction through the bearing  400 . As shown most clearly in  FIG. 6 , the hole  402  includes a generally cylindrical proximal portion and a conically generated distal portion. Proximal portions of the hole  402  extend through a portion of the bearing  400  that is formed with opposite substantially planar side surfaces  403 . The side surfaces  403  extend generally in anterior-to-posterior directions and are approximately parallel to one another. The bearing further includes an inferior bearing surface  406  configured for congruent bearing engagement with the superior tibial surface  504 . In the illustrated embodiment, both the superior tibial surface  504  and the inferior bearing surface  406  of the plastic bearing  400  are substantially planar.  
         [0050]     A cone  407  extends distally from the inferior bearing surface  406  and has an outer surface configured for substantially congruent engagement in the conical hole  503  of the tibial component  500 . Thus, the bearing  400  can rotate relative to the tibial component about the central axis of the conical hole  503  in the tibial component  500 . This rotation will cause the inferior bearing surface  406  of the bearing  400  to rotate in engagement with the superior tibial surface  504 . An anterior and superior position of the bearing  400  includes stop surfaces  408  that limit rotation of the femoral component  200  relative to the bearing in a hyperextension direction, as explained below. An annular groove  409  is formed in the cone  407  of the bearing  400  near the distal end of the cone  407 . The groove  409  is disposed to align with the groove  507  of the tibial component  500 . A snap ring then can be engaged simultaneously in the groove  409  and  507  to retain the cone  407  of the bearing in the conical hole  503  in the tibial component  500 . This engagement will permit rotation of the bearing  400  relative to the tibial component  500 , but will prevent dislocation of the bearing  400  from the tibial component  500 .  
         [0051]     The bearing  400  further includes an arcuate slot  410  formed in an anterior portion of the inferior bearing surface  406 . The slot  410  is configured to engage the stop pin  508  of the tibial component  500  and extends through an arc of preferably about 30°. The engagement of the stop pin  508  in the slot  410  limits the range of rotational motion of the bearing  400  relative to the tibial component. The size of the slot  508  and hence the range of rotational movement of the bearing  400  relative to the tibial component  500  will be selected in accordance with the mobility of the patient. In some instances, the slot  410  can be replaced by a cylindrical opening to prevent all rotation between the bearing  400  and the tibial component  500 .  
         [0052]     The hinge subassembly  300  includes a metal carriage  310  formed unitarily from a sufficiently strong biocompatible material, such as the material used to form the body  501  of the tibial component  500 . The metal carriage  310  includes a head  311  with opposite planar highly polished surfaces  312 . A shaft  313  extends distally from the head  311 . The shaft  313  of the preferred embodiment includes a substantially cylindrical proximal portion and a conically tapered distal portion. The shaft  313  of the carriage  310  is configured for rotational engagement in the hole  402  of the bearing  400 . A threaded hole  314  extends through the head  311  from an outer surface region substantially adjacent the shaft  313  to a pin support hole  315  formed in the head  311 .  
         [0053]     The hinge subassembly  300  further includes a hinge pin  320  configured for engagement in the pin support hole  315 . The hinge pin  320  include cylindrical bearing surfaces  321  adjacent opposite longitudinal ends and an engagement groove  322  between the cylindrical bearing surfaces  321 ..  
         [0054]     The hinge subassembly  300  further includes a set screw  330  with a conical leading end  331 . The set screw  330  can be threadedly engaged in the threaded hole  314  of the carriage  310  so that the leading end  331  of the set screw  330  engages in the groove  322  of the hinge pin  320 . Thus, the hinge pin  320  can be retained fixedly in the pin support hole  315  of the carriage  310 .  
         [0055]     The femoral component  200  is formed from a metallic material that exhibits sufficient strength and biocompatibility. For example, the femoral component  200  may be formed from the same material described above for the tibial component  500 . The femoral component includes a femoral body  201  with a superior surface and a stabilizing rod  202  that extends proximally from the superior surface of the femoral body  201 . The femoral body  201  can be mounted to the resected distal end of the natural femur  600  so that the stabilizing rod  202  can be mounted in a cavity prepared in the resected proximal end of the femur  600 . The femoral component  200  can be affixed in the femur by bone cement or by natural bone ingrowth that may be promoted by an appropriate external surface configuration on portions of the femoral component  200 .  
         [0056]     Inferior regions of the femoral body  201  define tibiofemoral articular surfaces  203  that are configured for congruent articular bearing engagement with the condylar bearing surfaces  401  of the bearing  400 . More particularly, the tibiofemoral articular surfaces  203  are configured for congruent bearing articular engagement with the condylar bearing surface  401  of the bearing through a broad range of flexion extending at least from full extension to most ranges of flexion that are likely to be generated during high load conditions, such as stair climbing or standing from a sitting position. In a preferred embodiment, congruency will exist from approximately 5° hyperextension to approximately 150° flexion. This congruency results in reduced stress as compared to a line contact or point contact that might be achieved with non-congruent articulating surfaces. As a result, the load is distributed over a wider area and failure during high load activities is much less likely.  
         [0057]     Superior regions of the femoral body  201  include spaced apart hinge support walls  204  that are distanced from one another appropriate amounts for receiving the outer side surfaces  312  of the head  311 . The superior face of the femoral body  201  also includes a rod support  205  that is engagement with the stabilizing rod  202  of the femoral component  200 .  
         [0058]     Holes  206  extend through the hinge supports  204  and substantially align with one another. Plastic bushings  210  are engageable in the holes  206  and define internal diameters appropriate for rotatably bearing engaging the hinge pin  320 .  
         [0059]     The femoral body  201  includes a patellofemoral articulating surface  207  with a superior region  208  and an inferior region  309  as shown in  FIGS. 13 and 20 . The superior region  208  of the patellofemoral articulating surface  207  is wider than the inferior region  209  since at lower flexion angles the patella  900  is in a relatively superior position, and may be displaced medially, as shown in  FIG. 23 . At moderate to large flexion, the patella  900  is central and the inferior patellofemoral articulating surface  209  need not be wider than the patella  900 .  
         [0060]     The prosthesis  100  is implanted by assembling an appropriate femoral stabilizing rod  202  to the femoral body  201  as described for example, in U.S. Pat. No. 5,074,879. The plastic bushings  210  also are mounted in the holes  206  of the femoral body  201 . As a result, the thrust flanges  211  of the plastic bushings  210  limit the insertion of the plastic bushings  210  into the holes  206 . Additionally, the substantially cylindrical hinge bearing surfaces  212  are located centrally in the holes  206 . This subassembly of the femoral body  201 , the femoral stabilizing rod  202  and the plastic bushings  210  define the femoral component  200 .  
         [0061]     The bearing  400  then is assembled with the carriage  310  of the hinge subassembly  300 . More particularly, the shaft  313  of the carriage  310  is inserted into the hole  402  of the bearing  400 . The snap ring  409  engages in the groove  507  to prevent unintended separation of the carriage  310  from the bearing  400 . The pin support hole  315  of the carriage  310  then is aligned with the hinge bearing surface  212  in the bushing  210  of the femoral component  200 . The hinge pin  320  then is passed into a first hinge bearing surface  212 , through the support hole  315  of the carriage  310  and into the second hinge bearing surface  212 . The set screw  330  then is introduced into the threaded hole  314 . As the set screw  330  is tightened, the conical end  331  thereof engages in the groove  322  in the hinge pin  320 , thereby clamping the pin  320  in place. This clamping is important to avoid metal-to-metal micro-motion that could generate harmful metallic wear debris. The tibial body  501  and the tibial stabilizing rod  502  then are assembled to form the component  500 .  
         [0062]     The tibia and the femur are prepared in a known manner, including forming channels to receive the stabilizing rods  202  and  502  respectively. A box-like cavity is prepared in the central, distal and posterior aspect of the femur. The cavity is dimensioned to define an envelope surrounding the two support walls  204 . The tibial component  500  then is implanted into the tibia and the femoral component  200  and hinge subassembly  300  are implanted in the femur  600 . The joint then is distracted and the tapered end of the cone  407  of the bearing  400  is inserted into the conical hole  503  of the tibial body  501 . The joint then is closed so that the implanted prosthesis  100  is in the disposition shown in  FIGS. 1 and 19 - 23 . The assembled prosthesis permits rotation about the axis A and the axis B in  FIG. 19 .  
         [0063]     The prosthesis  100  provides several advantages. First, the prosthesis  100  provides the valgus-varus stability with full congruency through the complete anticipated range of tibiofemoral articulation. Additionally, the superior patellofemoral articulation still maintains patellar tilt at low to moderate flexion angles. A patient who requires a hinged prosthetic joint is likely to have collateral ligaments that are deficient or absent. Hence, the remaining natural components of the knee joint may not be sufficient to resist dislocation. However, the snap ring or other such retention mechanism between the bearing  400  and the tibial component  500  prevents the relatively small amount of distraction shown in  FIG. 17  that could lead to dislocation. Rather, the much larger distraction shown in  FIG. 18  would be required to achieve by completely separating the much longer shaft  313  of the carriage  310  from the bearing  400 . Accordingly, the knee prosthesis  100  provides very good dislocation resistance with a relatively short cone on the tibial component  500 .  
         [0064]     While the invention has been described with respect to certain embodiments, it is apparent that various changes can be made without departing from the scope of the invention as defined by the appended claims.