Abstract:
An artificial knee joint that includes a femoral component with a specially shaped bearing surface and a tibial component, whose surface interacts with the femoral surfaces. The interaction provides for the required motion and stability characteristics. The interaction between the femoral and tibial surfaces is such that as the knee is flexed to maximum, the femoral component moves posteriorly on the tibial surface, by an amount similar to that in the anatomic knee. This is accomplished primarily by the depth and width of the femoral trochlea diminishing as the femoral component is flexed from zero to maximum, together with a ramp on the center of the tibial surface. The opposite motion, roll forward of the femur from a fully flexed to a more extended position, is accomplished by varying the outward radii of the lateral and medial femoral bearing surfaces, together with a ramp on the postero-lateral and postero-medial regions of the tibial surfaces. A variation of this is to generate a tibial surface which provides for a progressive internal rotation of the tibia as flexion proceeds.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to knee prostheses and more specifically to surface guided total knee replacement prostheses.  
       BACKGROUND OF THE INVENTION  
       [0002]     One of the major objectives of an artificial knee joint is to restore the mechanics of the anatomic knee. However the anatomic knee is complex, consisting of structures such as the articular surfaces, ligaments, and muscles, which cannot all be replicated by artificial parts at this time. This problem is mitigated by the fact that arthritis involves destruction of the condylar bearing surfaces and the menisci, while the remaining parts of the knee are largely intact and can still function. Hence, if an artificial knee joint provides a replacement for the bearing surfaces using metal and plastic materials, reasonably normal joint function can theoretically be achieved. This is the fundamental principal behind the large majority of the resurfacing types of knee joint that have been designed since about 1970.  
         [0003]     There have been a number of design features introduced to optimize the configurations of artificial knees. Dishing of the tibial bearing surfaces has increased the inherent stability of the artificial knee and has also reduced the contact stresses on the plastic, to increase durability. This dishing has been considered especially important in many designs, because of the requirement to resect the anterior cruciate ligament. Even when it has been present, the difficulty of the surgical technique in retaining the anterior cruciate ligament has led to a preference for resection. Surgical considerations have also led to the further step of resecting the posterior cruciate. When both the anterior and posterior cruciate ligaments are resected, the dishing of the tibial surfaces becomes even more important to provide anterior-posterior, as well as rotational, stability to the knee. When there is only a compressive force across the knee, and no shear force, the femur will locate at a “dwell point” or “bottom-of-the-dish” in the plastic tibial surface. This dwell is constant throughout the range of flexion in a standard total knee design.  
         [0004]     A feature of normal knee motions that was seen as an advantage was posterior displacement of the femur on the tibia as flexion proceeded. This was thought to increase the lever arm of the quadriceps and to allow for a higher range of flexion. In configurations which retained the posterior cruciate ligament, this motion was achieved so long as the placement of the components allowed for the natural tightening of the posterior cruciate with flexion. But in configurations where both cruciates were resected, some other mechanism was required to obtain the posterior displacement.  
         [0005]     The most widely used mechanism was an intercondylar cam, where a plastic post in the center of the tibial component, projected upwards into a metal housing between the metallic medial and lateral femoral condyles. In one of the first artificial knees using an intercondylar cam (Walker et al, U.S. Pat. No. 4,209,861), the housing articulated against the post throughout the entire flexion range, producing continuous rollback. In subsequent designs, the cam came into contact in the mid-range of flexion. This type of artificial knee configuration has been termed posteriorly stabilized, or PS, knees.  
         [0006]     The large majority of the knees implanted today are of the posterior cruciate retaining (CR) or PS types. These have functioned well, but a number of disadvantages remain. Due to variations in surgical techniques, it has been difficult to obtain an optimal tightness with the CR of the posterior cruciate throughout flexion, resulting in variable rollback patterns. For the PS, considerably extra bone needs to be resected from the center of the femur to accommodate the intercondylar housing of the femoral component, there is often long-term damage to the plastic post, and there is a tendency for overconstraint in rotation due to the dishing of the tibial bearing surfaces.  
         [0007]     A disadvantage common to both CR and PS types, although more pronounced with CR designs due to their shallower tibial bearing surfaces, is a paradoxical motion in which the femur slides forwards on the tibial surface during flexion, rather than backwards which is the required motion. Also, uneven or jerky motion occurs in many cases. These abnormalities occur because in flexion, the smaller distal-posterior sagittal radius of the femoral component comes into contact with the tibial bearing surface resulting in less constraint to anterior sliding of the femur. This dilemma was partially addressed for mobile bearing condylar knees, when femoral-tibial conformity was a particular goal, as in U.S. Pat. Nos. 5,906,643 and 6,264,679B1 to Walker wherein a solution was disclosed in which notches at the sides of the lateral and medial condyles, continuations of the distal-posterior radius, articulated with matching surfaces on the plastic tibial component. This arrangement was claimed to extend the degrees of flexion over which the condyles were in close contact. One of the main rationales for this arrangement was to provide a single anterior contact in early flexion and a progressively posterior contact in late flexion.  
         [0008]     Another feature of anatomical knee motion, as well as a general posterior translation of the femur on the tibia, is rotation of the tibia about its long axis. There are two ways in which this rotation can occur. Firstly, as the knee flexes, there is usually a continuous internal rotation of the tibia. The axis has been found to locate on the medial side of the knee and hence the lateral femoral condyle displaces posteriorly, but the medial femoral condyle displaces only a small amount in comparison. While artificial knees have included configurations that accommodate medial pivotal rotation, these configurations lack an integral mechanism that promotes the anatomical motions of rollback in flexion, medial pivotal rotation and roll forward in extension.  
         [0009]     The second type of rotation, termed laxity, is that at any angle of flexion, the tibia can be rotated internally and externally with respect to the neutral position. The amount of rotation towards extension is about 10 degrees on either side of neutral, but this increases to about 20 degrees on either side of neutral by about 30 degrees flexion. In high flexion the laxity apparently reduces again, but there is little quantitative data available. In condylar replacement artificial knees, laxity occurs due to the partial conformity between the femoral and tibial bearing surfaces. The shallower are the tibial surfaces, the greater the laxity. Towards extension, there is closer conformity and less laxity, compared with the laxity in flexion. In a general way, this laxity reproduces the situation in an anatomic knee joint. While partial conformity in an artificial knee is desirable in providing laxity, the contact point tends to be located at the ‘bottom of the dish’ such that it does not displace anteriorly and posteriorly in extension and flexion, nor does it rotate internally with flexion.  
         [0010]     An ideal artificial knee configuration would be one where the neutral path of motion, and the laxity about that neutral path, was similar to that of the anatomic knee. In this way, the motion would be compatible with the surrounding soft tissue envelope and the musculature of the knee. In our configuration, the femoral and tibial bearing surfaces are constructed to replace the mechanical function of the anatomic cartilage surfaces, the menisci, and the cruciate ligaments. A surface design with smooth transitions of curvatures has the advantages that large contact areas can be maintained, the motion will be smooth, and there is no possibility of damage to cams or other projections. In fact, in extreme motion conditions or if the artificial joint has been installed so that it is too loose in flexion, actual dislocation over a cam post can occur. For a surface configuration with no cam or projections, this could not occur.  
         [0011]     The subject of this invention is a surface design of an artificial knee with continuous curvatures whereby posterior displacement occurs automatically with flexion, with the effect that the dwell point moves progressively posterior with flexion, as in the anatomic knee. Conversely, as the knee is extended from a flexed position, the dwell point moves anteriorly. To more closely replicate the neutral path of motion of the anatomic knee. The surfaces can be designed or configured so that the neutral path in flexion involves internal rotation of the tibia, and conversely as the knee is extended again. Finally the surfaces can also be made to minimize paradoxical motion in early flexion. In all these schemes, the surfaces are such that rotational laxity is possible about the neutral path as in the anatomic knee.  
       SUMMARY OF THE INVENTION  
       [0012]     A number of characteristics which are ideally included in the configuration are as follows. The posterior displacement is small from approximately 0-45 degrees flexion, which has the advantage that a large area of contact can be maintained in that major functional range. The total posterior translation should be approximately 10-15 mm, but where the posterior limit is not too close to the posterior edge of the tibial component. The reason is that some rotational laxity is still needed in high flexion and it is undesirable if the femoral condyles ride on or over the very posterior edge of the tibial component. The patella mechanism is an important consideration in an artificial knee.  
         [0013]     In the anatomic knee, past about 110 degrees flexion, the contact between the patella and the digital end of the femur splits into two separate areas on the lateral and medial condyles. Most artificial knees cause the same scheme, especially when intercondylar cams are used. However, it would be an advantage is contact in high flexion was over the entire width of the patella, which would considerably reduce the stresses on the artificial patella and the remaining bone. Continuous contact is possible with a surface guided design or configuration.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Preferred embodiments of the invention are described below with reference to the drawings, wherein like numerals are used to refer to the same or similar elements.  
         [0015]      FIG. 1A  is an anterior-medial isometric view of a typical femoral component and tibial component showing the anterior position of the patella flange of the femoral component;  
         [0016]      FIG. 1B  is a side cross-sectional sagittal plane view of an outer surface of a femoral component that has sections from −30 to 150 degrees constructed in accordance with the present disclosure;  
         [0017]      FIG. 1C  is a series of partial cross-sectional frontal views of the surface of each of the sections of the femoral surface of  FIG. 1A  at different flexion angles from −30 to 150 degrees;  
         [0018]      FIG. 2A  is the generating femoral block of  FIG. 1A  seated inside a solid rectangular tibial block at zero degrees flexion;  
         [0019]      FIG. 2B  is the tibial block of  FIG. 2A  after a Boolean subtraction has been performed for the imprint of the generating femoral block at a position of approximately zero degrees flexion;  
         [0020]      FIG. 2C  is one embodiment of the tibial block of  FIG. 2A  showing facets after the generating femoral block has been placed in a limited number of successive positions through the flexion range and an overall tibial surface is produced;  
         [0021]      FIG. 3A  is an antero-medial perspective exploded view of a surface guided knee replacement that has a femoral component and a tibial component constructed in accordance with the present disclosure;  
         [0022]      FIG. 3B  is a postero-medial perspective exploded view of the knee replacement of  FIG. 3A ;  
         [0023]      FIG. 3C  is a postero-medial perspective exploded view of the knee replacement of  FIG. 3A  showing an anterior to posterior descending ramp in the center of the tibia;  
         [0024]      FIG. 3D  is a bottom perspective exploded view of the knee replacement of  FIG. 3A  showing the patella groove;  
         [0025]      FIG. 4A  is a side view of the knee replacement of  FIG. 3A  with the tibial component positioned at a five degrees posterior slope and the femoral component at a position of approximately zero degrees flexion;  
         [0026]      FIG. 4B  is a side view of the knee replacement of  FIG. 4A  at a position of approximately 60 degrees flexion;  
         [0027]      FIG. 4C  is a side view of the knee replacement of  FIG. 4A  at a position of approximately 120 degrees flexion;  
         [0028]      FIGS. 5A-5L  show the movement of the contact areas of the femoral component on the tibial bearing surface of the knee replacement of  FIG. 4A  from the approximate zero degrees flexion through the approximate maximum flexion;  
         [0029]      FIG. 6A-6C  are mid-sagittal cross-sectional views of the knee replacement of  FIG. 3A  at 60, 90 and 120 degrees flexion showing the lowest contour of the femoral intercondylar region for each respective angle of flexion that contacts the ramp of the tibial component;  
         [0030]      FIG. 7A  is a mid-condylar sagittal plane cross-sectional view of a simplified replacement knee at approximately zero degrees flexion showing the closely matching interface of the femoral anterior curvature with the tibial anterior curvature and their respective radii;  
         [0031]      FIG. 7B  is a simplified mid-condylar sagittal plane cross-sectional view of the replacement knee of  FIG. 7A  at approximately 60 degrees flexion with the contact point in the central region of the tibial curvature;  
         [0032]      FIG. 7C  is a simplified mid-condylar sagittal plane cross-sectional view of the replacement knee of  FIG. 7A  at approximately 60 degrees flexion that shows anterior sliding of the femoral component on the tibial component;  
         [0033]      FIG. 8A  is an antero-medial plan perspective exploded view of the surface guided knee replacement of  FIG. 4A  that shows notches positioned on the side of the femoral component and corresponding pads on the tibial component;  
         [0034]      FIG. 8B  is a postero-medial plan perspective exploded view of the surface guided knee replacement of  FIG. 8A ;  
         [0035]      FIG. 8C  is an antero-medial plan perspective view of the surface guided knee replacement of  FIG. 8A  with the femur component at the zero degree flexion position on the tibial component;  
         [0036]      FIG. 8D  is a side view of the surface guided knee replacement of  FIG. 8A  with the femur component at the 30 degree flexion position on the tibial component; and  
         [0037]      FIG. 8E  is a side view of the surface guided knee replacement of  FIG. 8A  with the femur component at the 60 degree flexion position on the tibial component;  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0038]     Referring to  FIG. 1A , a surface guided knee replacement  10  is shown in a position of approximately zero degrees flexion and includes a femoral component  20  and a tibial component  40 . Femoral component  20  connects to a shaped distal end of a femur. In the embodiment shown, the tibial component  40  is one-piece, but can also be a bearing component fixed into a tibial tray or baseplate. The tibial component  40  connects to a shaped proximal end of a tibia and includes a bearing surface  62  that receives and guides the flexion of femoral component  20 .  
         [0039]     Femoral component  20  has a proximal-anterior portion  22  and an opposed proximal-posterior portion  24  that define an arcuate curve. Component  20  includes an intercondylar surface  25  that connects a medial condyle  26  and a lateral condyle  28 . Intercondylar surface or patella groove  25  defines a recess or notch that has a depth that progressively diminishes from the proximal-anterior portion  22  to the proximal-posterior portion  24 . Condyles  26  and  28  have convex outer surfaces. The portions of condyles  26  and  28  that interface with tibial bearing surface  62  have radii that decrease along the arcuate curve in the direction from the proximal-anterior portion  22  to the proximal-posterior portion  24 .  
         [0040]     As shown in  FIGS. 1B-1C , the outermost section of femoral component  20  has a sagittal profile  21  that is composed of five separate arcs joined tangentially. Sagittal profile  21  includes arc AB, with center OAB, arc BC with center OBC, arc CD with center OCD, arc DE with center ODE and arc EF with center OEF. Arcs AB, BC and CD extend from greater than 150 degrees to −30 degrees. Proximal-anterior portion  22  of profile  21  includes point A and proximal-posterior portion  24  includes point F. In profile  21 , arc AB or the posterior-distal portion of femoral condyles  26  and  28 , retains contact with bearing surface  62  of tibia  40  in high flexion to avoid bony impingement.  
         [0041]     In flexion from about zero degrees at point C through approximately 90 degrees flexion at point B, a constant femoral radius of arc BC contacts receiving surface  62  and advantageously assists in maintaining the same arcuate profile in contact with receiving surface  62 . From 60 to 90 degrees flexion the constant radius of femoral component  20  is maintained to achieve the desired height requirement that arc AC is taller than that in the anatomic knee. The radii from points C to F are chosen to be a close match to the anatomic knee and give the desired shape of the patella trochlea and the average anterior-posterior dimension of femoral component  20 . Condyles  26  and  28  can have the same radii and arc profiles  21 , but different shapes can also be used. In an anatomic femur, the profiles of the lateral and medial condyles are different.  
         [0042]     Continuing with  FIGS. 1B-1C , the location of the frontal plane sections of profile  21  are shown in  FIG. 1C , designated by their angular position along their respective arcs. From 0-150 degrees, there is a correspondence with the angle of flexion of the knee.  FIG. 1C  shows the shape of these sections seen in frontal views of the femoral component  20  at the corresponding flexion angle. From −30 to 0 degrees flexion, the sections are approximately anatomic including the central height and shape of the patella groove  25 , bearing spacing (distance between the high points on the lateral condyle and medial condyle) and frontal radii which is reflected in a side height  44 . From 0-90 degrees flexion there is a progressive reduction in the central height of the patella groove that reaches zero at approximately 90 degrees flexion.  
         [0043]     In the anatomic knee, the patella contacts the distal region of the femur on the 0-30 degrees section when the knee is flexed maximally at about 150 degrees. Hence the shape and depth of the anatomic knee are essentially maintained for this purpose. It will later be shown that the reducing height of the grooved intercondylar surface  25  with flexion will create a ramp  64  on tibial receiving surface  62  of similarly diminishing height and width.  
         [0044]     After 30 degrees flexion, the side height  44  of the sections of the tibial surface  62  steadily increases. The increased side height  44  supports the posterior displacement of femoral component  20  during flexion, preserves bearing surface  62  for stability in early flexion and creates ramps  66  at the postero-lateral and postero-medial corners of the tibial component that promotes roll forward as the knee is extended from maximum flexion. In the sections from 90-150 degrees, the side curvature begins closer to the center than the bearing spacing. This increases the thickness of the ramp mentioned above and further promotes roll forward.  
         [0045]     Femoral component  20  and tibial component  40  are preferably made of metal and a polymer material, respectively. The interface between femoral component  20  and receiving surface  62  is intended to provide the extended wear characteristics that are desirable in a knee replacement.  
         [0046]     As shown in  FIGS. 2A and 2B , the generating solid femoral block  12  is three-dimensionally shaped into the sections of  FIG. 1C  along the longitudinal axis between the approximately −30 degrees to approximately 150 degrees of flexion as shown in  FIG. 1B . Generating femoral block  12  can be exactly the same or have an approximately similar shape and dimension as that of femoral component  20 . Femoral block  12  is seated into a tibial block  16 , to a depth of seating of approximately 10 mm that provides sufficient dishing of the final tibial component. Femoral block  12  is at zero degrees flexion. Using a Boolean subtraction process that is common to present 3-D modeling software packages, the generating femoral block  12  is subtracted from the tibial block  16 . This initial process produces a smooth tibial receiving surface  17 .  
         [0047]     The generating femoral block  12  is then restored and adjusted in position. This adjustment of position includes a flexion of the generating block about a defined axis, a posterior translation, and a small downwards translation that simulates the movements of the actual knee joint. A table is made of these movements. In our example, the defined axis of rotation was OBC for 0-90 degrees flexion and OAB for 90-150 degrees flexion. The posterior translation was approximately zero from a range of approximately 0 degrees to as much as approximately 50 to 60 degrees flexion. From as early in flexion as 50 degrees, the translation is preferably increased in steps to 12 mm until approximately 150 degrees with a range of translation from approximately 10-15 mm being acceptable. The downward translation paralleled the five degrees posterior slope of the tibia. Other patterns of motion than the above could be used so long as the essential posterior translation was included.  
         [0048]     Referring now  FIG. 2C , generating femoral block  12  is placed on the tibial block  16  in increments of flexion that correspond to angles of flexion and Boolean subtractions are then performed at each angle. This produces a tibial surface  62  that includes two laterally opposed condyle receiving areas  68  separated by an anterior to posterior descending ramp  64 . The individual facets reflect the interface between the femoral block  12  and tibial block  16  for each of the increments of flexion. To produce a smooth surface, steps at small angular increments, and/or smoothing in software, can be carried out.  
         [0049]     When the generating femoral block  12  is placed on the tibial bearing surface  17 , from zero to 50 degrees, there is a large area of contact between the full width of femoral block  12  and tibial bearing surface  62 . This provides stability in the anterior-posterior direction but also prevents more than a few degrees of internal-external rotation. In order to allow for more rotational laxity in early flexion, the generating femoral block  12  can be expanded by a small percentage in the anterior-posterior direction, which opens out the condyle receiving areas  68  positioned on the opposing lateral sides of ramp  64  on tibial surface  62 . After 50 degrees flexion, it is found that there is adequate internal-external rotation possible.  
         [0050]     Referring now to FIGS.  2 A and  3 A- 3 D, starting from generating femoral block  12  and tibial block  16 , femoral component  20  and the tibial component  40  are generated. In this one preferred embodiment, the distal end of the femur is cut with one standard method of shaping that includes five facets that mate with the five planar inwardly facing sides  27  of femoral component  20 . It is understood that the inwardly facing sides  27  that oppose the outwardly positioned convex condyles  26  and  28  can have any shape. For added fixation to the femur, femoral component  20  can also include two or more short pegs  29 , although many other fixation means are possible.  
         [0051]     Towards the proximal-anterior portion or end portion  22 , component  20  converges inwardly  31 , especially on the medial side. At the proximal-anterior portion  22  of femur component  20 , the edges  31  of the patella flange are rounded. The proximal-posterior portion  24  also has rounded edges  33  to conform with the shape of the femur. Rounded edges  31  and  33  advantageously provide smooth sliding of soft tissues including muscle, tendon and capsular tissue, over the edges of the femoral component  20 .  
         [0052]     Overall, the contour of femoral component  20  is shaped and dimensioned to be a close match to that of the anatomic femur, except that the height of the posterior portion of condyles  26  and  28  is preferably approximately 2 mm higher to avoid impingement of the posterior femoral condyles with the posterior edge of the tibial component  40 , in high flexion. On the anterior of femoral component  20 , the patella groove  25  is anatomic in profile. This profile carries round to the proximal-anterior portion  22  where the patella contacts in high flexion. From the proximal-anterior portion  22  to region  27  or at approximately 110 degrees flexion, the depth of patella groove  25  resembles the anatomic. Posterior to the region  27 , the depth of groove  25  steadily diminishes until it reaches zero in proximity to region  35  in the vicinity of proximal-posterior portion  24 . In the posterior region of the femoral component  20 , the central region of the surfaces can be close to cylindrical.  
         [0053]     The bottom of tibial component  40  is shaped to fit the resected surface of the upper tibia. Tibial component  40  preferably uses three fixation pegs  43 , although many other fixation means are possible. In another embodiment, tibial component  40  can be mounted on a metal baseplate which is fixed to the upper tibia, such that the tibial component  40  can rotate on the surface of the baseplate.  
         [0054]     As shown in FIGS.  1 C and  4 A- 4 C, femoral component  20  is shown in contact with receiving surface  62  of tibial component  40  through a range of motion from approximately zero degrees flexion to approximately 120 degrees of flexion. The area of contact between femoral component  20  and tibial component  40  moves posteriorly from the approximately zero degree position of flexion to the approximately 60 degree and approximately 120 degree positions during flexion.  
         [0055]     Side height  34  is preferably approximately 2-3 mm between 0 degrees to about 50 degrees flexion. Thereafter, the side height increases at a preferably steady rate to approximately 6-8 mm in the higher ranges of flexion from approximately 50 to 150 degrees in the proximal-posterior portion or end portion  24  of femoral component  20 .  
         [0056]     This arrangement produces elevated ramps  66  ( FIGS. 3A and 3B ) in the postero-lateral and postero-medial corners of the tibial surface  62 . When the knee is extended from a fully flexed position, these ramps facilitate rolling of the femoral component  20  in an anterior direction. When the femoral component  20  is nearing extension, it now seats in the lower regions of condyle receiving surfaces  68  on tibial surface  62 . At approximately 120 degrees flexion and beyond, the contact is approximately 10 mm from the posterior of tibial component  40 .  
         [0057]     As shown in FIGS.  4 A and  5 A- 5 B contact area  72 , shown as the darkened area of the anterior of bearing surface  62  defines the interface between femoral component  20  and tibial component  40  at zero degrees flexion. The large contact area  72  shows the close conformity between components  20  and  40  at approximately zero degrees flexion. Contact area  72  covers from the anterior to just behind the center of tibial receiving surface  62 . The center of pressure will depend on the prevailing joint force. If there is an extending moment on the femur, the center of pressure will tend to lie at the anterior, acting as a brake to further extension.  
         [0058]     Referring now to FIGS.  4 B,  5 B- 5 D as flexion increases, contact area  72  moves progressively posteriorly, as indicated by the arcuate lines which pass across the component in a medial-lateral direction. At 30 degrees flexion the contact moves to the posterior part of the anterior contact region, due to the arc BC ( FIG. 1B ) being in contact. In particular, patella groove  25  receives central ramp  64  on bearing surface  62 . Ramp  64  has a height that decreases from the anterior to the posterior of tibial component  40  and sides that are concave slopes. The height and slope of the sides of ramp  64  guide femoral component  20  to displace posteriorly during flexion and displace anteriorly during extension.  
         [0059]     Up to approximately 50 degrees flexion, contact area  72  does not move further posterior than shown contact regions  72 . At approximately 60 degrees flexion, contact area  72  transitions to a band across the tibial surface just to the posterior to the center of tibial receiving surface  62 .  
         [0060]     As shown in FIGS.  4 C and  5 E- 5 L, from approximately 60 degrees flexion to 90, 120, and 150 degrees flexion, contact area  72  remains as a lateral band across the tibial surface that moves progressively more posterior with flexion. At each angular position of flexion, there is both anterior and posterior stability of femoral component  20  on the tibial receiving surface  62  due to the different discrete slopes across the tibial surface.  
         [0061]     Referring now to  FIGS. 6A-6C , are sagittal sections through the center of femoral component  20  and tibial component  40  and show the interaction of patella groove  25  between the base of patella grove  25  at approximately 60 degrees flexion and the posterior extension of patella groove  25  with ramp  64  in the center of the tibial receiving surface  62  at approximately 120 degrees flexion. In this one preferred embodiment, groove  25  initially makes contact with ramp  64  between approximately 30-60 degrees flexion. As femoral component  20  flexes to 90 degrees ( FIG. 6B ), contact has moved down ramp  64  and ramp  64  has effectively displaced the femoral component  20  posteriorly. On further flexion to 120 degrees ( FIG. 6C ), the extension of the patella groove  25  has reached the bottom of the ramp  64 .  
         [0062]     At 120 degrees flexion and beyond, the contact area of f the femoral component  20  on the tibial receiving surface  62  is preferably at least 8 mm and as much as approximately 10-12 mm from the posterior of the tibial component  40  and receiving surface  62  is shaped to maintain femoral component  20  at this position up to maximum flexion. As noted above, this is an advantage because internal-external rotation of the femur on the tibia is possible without the femur contacting the extreme posterior edge of tibial component  40 .  
         [0063]     As shown in  FIG. 7A , a simplified replacement knee is positioned at zero degrees flexion where there is a close conformity between the anterior curves of the femoral component  20  and tibial receiving surface  62 . If there is a shear force applied that would move the femoral component  20  anterior relative to the tibial component  40 , there will only be a small relative displacement because of the close conformity.  
         [0064]     Referring now to  FIGS. 7B and 7C , when the replacement knee is flexed, the reduction in the radius of condyles  26  and  28  can cause a loss of contact area and conformity between femoral component  20  and tibial receiving surfaces  62 . If an anteriorly directed shear force is applied as shown by arrow A in  FIG. 7C , there will be an anterior displacement of femoral component  20  on tibial receiving surface  62 . This has been termed paradoxical motion and does not resemble normal anatomic motion in which the femur displaces posteriorly with flexion. Such paradoxical motion is not desirable in an artificial knee because it reduces the lever arm of the quadriceps muscle, and can impair the ability to climb stairs and reduce the range of flexion.  
         [0065]     One of the methods for avoiding paradoxical motion is shown in  FIG. 8A-8E . Notches  37  are cut in the sides of medial condyle  26  and lateral condyle  28  of femoral component  20 . Notches  37  articulate against corresponding pads  67  in proximity to the anterior outside edges of tibial component  40 . As best seen in  FIG. 8C , notch  37  in femoral component  20  has the same radius R as the distal-posterior radius of the lateral  28  and medial  26  femoral condyles. When femoral component  20  is flexed to approximately 30 degrees ( FIG. 8D ) or 60 degrees ( FIG. 8E ), the notches  37  mate with the pads  67 , and prevents anterior translation of femoral component  20  on tibial component  40 . Or in other words, the interaction of the pads and notches prevents the femoral component from skidding forward on the tibial component during early flexion. Therefore, when the knee is close to extension, as shown in  FIG. 8C , for example, there is a small gap or space between the upper shelf of notch  37  on the femoral component  20  and the top of pad  67  on tibial component  40  in order to allow a few degrees of hyperextension.  
         [0066]     It is an advantage if the edge of the notches  37  and pads  67  are rounded to avoid catching if the alignment of the knee is not exactly central as the knee is extended. The width of the notches  37  on the femoral component  20  are such that they do not interfere with the region that the patella traverses. In this regard, the medial notch can be wider than the lateral, because the patella bearing area is more extensive on the lateral side.  
         [0067]     In the preceding specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident, however, that various modifications, combinations and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow. It is understood that the present invention can combine one or more novel features of the different embodiments. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.