Abstract:
A recess-ramp knee joint prosthesis comprising a femoral and a tibial component is configured to reproduce normal kinematics and function. Asymmetric condular surfaces and a cupola of the femoral component interact with corresponding dished surfaces and a ramp of the tibia thereby duplicating the behavior of the anatomical knee.

Description:
CROSS-REFERENCE TO RELATED DOCUMENTS 
     This U.S. Utility Patent Application claims priority to U.S. Provisional Patent Application 61/075,158, filed Jun. 24, 2008, and U.S. Provisional Patent Application 61/100,488, filed Sep. 26, 2008. This reference and all additional references cited in this specification, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features, and/or technical background. 
    
    
     BACKGROUND 
     The term Guided Motion Knees was formulated in the mid-1990&#39;s in an effort to conceptualize features in the femoral and tibial components of a total knee replacement (TKR) which would guide the motion of the knee during flexion and extension. The particular motion characteristics of interest were those on a natural anatomic knee itself; as the knee is flexed, posterior displacement (or rollback) of the femur on the tibia, external rotation of the femur on the tibia, and rotational and anterior-posterior laxity at all angles of flexion. Guided Motion in a basic form has already been addressed in many previous designs dating back to the early 1970&#39;s, using the geometry of the lateral and medial bearing surfaces, as well as central cam-post mechanisms. (Raymond P. Robinson,  The Early Innovators of Today&#39;s Resurfacing Condylar Knees , Journal of Arthroplasty, Vol. 20, Suppl 1, 2005). Almost all of the total knee replacements on the market today have similar shapes for the lateral and medial sides, such that there is little lateral or medial bias to the motion. However, in recent years, designs have emerged which have attempted to produce asymmetric motion. One of the first was the Medial Pivot Knee (based on early concepts by Freeman et al., Wright Manufacturing) and the Journey Knee (Smith &amp; Nephew). The Medial Pivot Knee is based on a completely stable medial side and a rotatable lateral side. The Journey Knee has more conformity medially than laterally with a slightly convex lateral tibial surface, together with a cam-post mechanism to produce femoral rollback with flexion. There is evidence that these designs do bring knee kinematics and function closer to natural anatomic than symmetric designs. However, there continues to be a need for a total knee replacement which, more perfectly, reproduces normal kinematics and function, and feels like a natural knee. 
     BRIEF SUMMARY 
     In one embodiment of the present invention there is provided a prosthetic knee joint comprising: a tibial component comprising an asymmetric lateral dished surface and an anteriorly elevated medial dished surface, and a protrusion located between the lateral dished surface and the anteriorly elevated medial dished surface, anterior portion of the protrusion defining an anterior ramp and posterior portion of the protrusion defining a posterior ramp, the medial dished surface further comprising an external rotation axis; a femoral component comprising asymmetric lateral and medial condylar shaped surfaces, an anterior femoral groove, and a cupola located between the lateral and medial condylar surfaces, the cupola being a continuation of the femoral groove; the lateral condylar shaped surface is in sliding contact with the lateral dished surface, the medial condylar shaped surface is in sliding contact with the medial dished surface, and, for angles of flexion within a specified range, a surface of the cupola is in contact, and conformal with the posterior ramp; the lateral and the medial condylar surfaces and the posterior ramp and the surface of cupola are respectively configured to maintain contact as the lateral condylar surface displaces posteriorly, with respect to the external rotation axis, in concert with flexure. 
     In embodiments is additionally disclosed a prosthetic knee joint wherein the medial condylar surface and the medial dished surface, at zero degree flexure, is substantially conformal. Embodiments also comprise the lateral condylar surface and the lateral dished surface, at zero degree flexure, is substantially conformal anteriorly and non-conformal posteriorly; that the medial and the lateral condylar surfaces have arcuate sagittal profiles comprising a connected sequence of substantially circular arcs; and that the medial condylar surfaces have arcuate sagittal profiles comprising a connected sequence of substantially circular arcs, having a diminished radius relative to the arc immediately anterior; and that the cupola surfaces and the ramp surfaces are mathematically continuous and have substantially non-constant derivatives; and that the cupola surface transitions to the adjoining surface with a rounded edge having a radius of greater than 3 millimeters. Additional embodiments include that the specified range of angles of flexion extends from 30 or 60 degrees to maximum flexion. Maximum flexion is typically approximately 155 degrees. 
     Embodiments further include that the medial dished surface, the lateral dished surface, and the ramp are defined as surfaces conformal to an envelope of maxima of the distal femoral surfaces resulting from a succession of incremental placements of the femoral component along the desired motion track corresponding to joint flexure; and that the tibial component further comprises a ligament clearance depression in the posterior vertical wall and the femoral component further comprises a ligament clearance notch located between the condylar surfaces; and that the tibial component and femoral component comprise separable portions contained by the ligament clearance depression and the ligament clearance notch respectively. 
     Embodiments also include an artificial knee comprising: a first component having an outer generally J-shaped surface and an inner generally J-shaped surface: the outer generally J-shaped surface comprising a first asymmetric bilateral lobular profile comprising a first lobe and a second lobe, the first lobe having first radius, and the second lobe having a larger second radius, the first asymmetric bilateral lobular profile being adjacent to a second accurate asymmetric bilateral profile, the bilateral profiles defined by an intermediate off-center depression of non-uniform depth traversing along the generally J-shaped outer surface, the depression including, along its traverse, a pit; the inner generally J-shaped surface comprising a first lateral surface, a bottom surface and a second lateral surface, the second lateral surface being taller in height than the first lateral surface, and the bottom surface comprising a ridge extending between the first lateral surface and the second lateral surface; a second component having an upper surface, a bottom surface and a circumscribing transverse surface between the upper surface and the bottom surface: the upper surface defining a first and second dished section, asymmetric to one another, the first dished section being anteriorly elevated with respect to the second dished section and the second dished section having a shallower profile than the first dished section, and the upper surface further defining an intervening elevated section between the two dished sections, the intervening elevated section having a discrete mound component emanating therefrom, the mount having a first slope and a second opposing slope, the first slope being angled steeper than the second opposing slope; the bottom surface defining one or more protrusions from such surface, wherein the mound of the intervening elevated section of the second component is configured to fit within the pit of the first component, the first lobe of the first component is configured to fit within the first dished section of the second component and rest on a surface thereof when the mound is fit within the pit, and the second lobe of the first component is configured to fit within the second dished section of the second component and rest on a surface the reof when the mound is fit within the pit. 
     Definitions 
     Condylar shaped surface is a surface located on the distal portion of the femoral component and having the shape of an anatomic condyle. 
     Conformity between two curves means that the radii at the contact are nominally the same. 
     Cupola is a cavity or depression in the distal surface of the femur located between the medial and lateral condyles. The posterior of the cupola serves as a surface which contacts the ramp or post, protruding from the tibia, during flexion. 
     Cupola height is the distance between the base of the cupola and the profile of the lateral and medial femoral condyles as seen in the sagittal view. 
     Drape is a free-form surface which overlays a composite of surfaces without penetrating any of the surfaces. 
     External rotation of the femur is the rotation of the femur about an axis, located on the medial condylar surface of the tibia, which is parallel to the long axis of the tibia. 
     External rotation axis is the axis on the medial side of the tibia about which the external femoral rotation takes place. 
     Frontal plane is mutually perpendicular to the sagittal and transverse planes. 
     Laxity is the amount of displacement or rotation that can occur due to lack of conformity between two adjacent surfaces. 
     Post is a ramp with a posterior surface having a slope of greater than approximately 45 degrees. 
     Protrusion is a mound-like structure, projecting upward from the tibial component. The surface of the protrusion defines an anterior ramp having an average slope rising from the anterior of the protrusion toward the posterior, and a posterior ramp having an average slope rising from the posterior of the protrusion toward the anterior. 
     Ram is the surface of a protrusion from the proximal surface located between the medial and lateral tibial bearing surfaces. The posterior of the ramp serves as a contact surface which contacts the cupola contained portion of the femur. 
     Sagittal plane is a plane which divides the femur and tibia into left and right halves 
     Transverse plane is a horizontal plane perpendicular to the long axis of the tibia. 
     Transverse plane projection is the geometric projection of a specified line segment onto the transverse plane. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The following detailed description, given by way of example, will be best understood in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of the femoral component. 
         FIG. 2  is an overhead view of the femoral component on the transverse plane, with anterior above, posterior below, lateral left, and medial right. 
         FIG. 3  is a frontal view of the femoral component on the frontal plane, with superior above and inferior below. 
         FIG. 4  is a side view of the femoral component on the sagittal plane, with anterior to the right, posterior to the left. 
         FIG. 5  shows sections S 1  thru S 9  from the sagittal view, showing the profiles of the femoral bearing surfaces. 
         FIG. 6  shows sagittal sections FL, FC, and FM, from the frontal view, showing the lateral profile, the cupola profile and the medial profile. 
         FIG. 7  shows the construction of a typical medial profile of the femoral component using circular arcs. 
         FIG. 8  shows the construction of a typical lateral profile of the femoral component using circular arcs. 
         FIG. 9  shows the medial profile located on a section of the tibial bearing surface at zero degrees flexion. 
         FIG. 10  shows the medial profile located on a section of the tibial bearing surface at 30 degrees flexion. 
         FIG. 11  is a perspective view of the tibial component. 
         FIG. 12  is a view of the tibial component on the transverse plane. 
         FIG. 13  is a view of the tibial component on the frontal plane. 
         FIG. 14  is a view of the tibial component on the sagittal plane. 
         FIG. 15  shows sections F 1  thru F 5  from the sagittal view, showing the profiles of the tibial bearing surfaces. 
         FIG. 16  shows sagittal sections TL, TC, and TM, from the frontal view, showing the lateral profile, the ramp profile and the medial profile. 
         FIGS. 17 ,  18 ,  19  show the sagittal sections through the lateral condyle, the ramp-post, and the medial condyles, at zero degrees flexion. 
         FIGS. 20 ,  21 ,  22  show the sagittal sections through the lateral condyle, the ramp-post, and the medial condyles, at 60 degrees flexion. 
         FIGS. 23 ,  24 ,  25  the sagittal sections through the lateral condyle, the ramp-post, and the medial condyles, at 120 degrees flexion. 
         FIG. 26  shows a section in the transverse plane through the contact area of the cupola and ramp at 60 degrees flexion. 
         FIG. 27  shows a section in the transverse plane through the contact area of the cupola and ramp at 90 degrees flexion. 
         FIG. 28  shows a section in the transverse plane through the contact area of the cupola and ramp at 120 degrees flexion. 
         FIG. 29  is a composite of femoral components at multiple positions in the prescribed motion path throughout the full range of flexion. 
         FIG. 30  is a sagittal plane view of the medial sections, corrected so that the lowest points lie on an arc RM. 
         FIG. 31  is a sagittal plane view of the lateral sections, corrected so that the lowest points lie on an arc RL. 
         FIG. 32  is a drape of the lower surface of the composite of corrected femoral components, which defines the tibial surface. 
         FIG. 33  is a tibial component where the surface including the bearing surfaces and ramp, is the aforementioned drape surface. 
         FIG. 34  shows two femoral components, the left being intended for resection of the cruciate ligaments, and the right being intended for retention of the posterior cruciate, together with a tibial component below which can be used with either femoral component. 
     
    
    
     DETAILED DESCRIPTION 
     During everyday activities, the knee joint experiences a variety of forces, including axial compressive and anterior-posterior shear, and moments, including varus-valgus and axial torque. The knee can achieve flexion angles of up to approximately 155 degrees, while the relative motions between the femur and the tibia include numerous combinations of femoral-tibial positional relationships at the bearing surfaces. Stability is essential, which is provided by a combination of bearing surface interaction, muscle forces and the soft tissues in and around the joint. There is now considerable evidence that the major anterior-posterior stability, of the femur with respect to the tibia, is derived from the medial side, which allows only a few millimeters of anterior-posterior displacement. In the anatomic knee, this stability is provided by the cruciate ligaments, together with the medial collateral ligament. The higher the compressive load, the more the stability is provided by the medial meniscus in combination with the dishing and anterior upsweep of the tibial plateau. In contrast, the lateral side of the knee is extremely mobile. During the full range of flexion, the lateral femoral condyle displaces posteriorly by about 20 mm. while the medial femoral displaces posteriorly only a few millimeters, and that only at the higher flexion angles. Hence the concept of knee mechanics is that the stability is provided by the medial side while the mobility is provided by the lateral side. This mode of function is necessary for the patient to feel that their artificial knee feels like their natural anatomic knee. 
     The Ramp Knee, a type of Guided Motion Knee, reproduces these mechanical properties, due to the design of the femoral and tibial bearing surfaces and the interaction of a central ramp or post on the tibial component which locates within a housing or cupola in the center of the femoral component. The cupola is blended to the surrounding bearing surfaces laterally, medially, anteriorly and posteriorly. The sagittal profiles in the centers of the lateral and medial condyles preferably resemble natural anatomic shapes. The radius of curvature of the distal sagittal profile of the medial side condyle is constant up to about 30 degrees flexion while the lateral condylar surface has a radius of curvature, at the point of tibial contact, which reduces with flexion. The depth of the femoral cupola reduces steadily from the distal end of the femur, where it can preferably be 10-15 mm in depth, to the posterior, where it becomes less than 7 mm in depth. 
     The respective medial and lateral tibial surfaces may be generated by mathematically superimposing multiple femoral surfaces, each of which corresponds to the correct orientation of the femur, with respect to the tibia, for a full range of flexion angles. The correct orientation of the femur may be determined to be a predefined function of the external femoral rotation and posterior displacement of the femur as a function of flexion angle based on empirical data of the neutral path of motion. The neutral path of motion is the trajectory followed by the femur, without the influence of superimposed shear or torque forces. Therefore, characterization of the orientation includes, in part, axial rotation of the femur about an external rotational axis in the tibia, together with corresponding posterior displacements of the femur on the tibia. The external rotation axis can change in position with flexion, but is within approximately 10 mm of the medial femoral-tibial contact point. The point of contact of the medial condyle and the associated external rotational axis undergoes a small displacement over the full range of flexion. A resulting surface of femoral contact is created by incrementing the flexion angle of the femur in small increments (i.e. 5 to 15 degrees) and generating a drape or envelope of the lower surfaces of the composite femoral positions. 
     Typically the medial side of the femur displaces 2-4 mm while the axial rotation is about 15-20 degrees, resulting in a lateral side posterior displacement of about 15-20 mm. In order to accommodate such a large lateral displacement, the transverse axis of the femur at zero degrees flexion, is rotated internally on the tibia, so that the lateral contact location is anterior to the center of the tibial plateau, resembling the screw-home mechanism of the femur on the tibia, as the femur comes into terminal extension. The lower surface of the composite envelope of the femoral surfaces, will be conformal with the tibial surface and is consistent with the required neutral path of motion. However it will be understood that for purposes of tolerances and to allow some laxity to occur, the tibial surface will be relieved slightly to avoid a tight femoral-tibial fit. In any case, laxity is inherent in this tibial surface except at the extremes of the flexion range. To produce this behavior, extra femoral surfaces can be added to provide the required laxity to the composite at the extremes. 
     After generating the composite femoral positions, a modification in the sagittal plane is carried out whereby the profiles are placed on arcs. On the medial side, the arc is of small radius, for example 40-50 mm, while on the lateral side, the arc is of large radius, for example 70-100 mm. The anterior parts of the arcs will preferably be of smaller radius than the posterior, to allow for a high flexion range and posterior displacement of the lateral femoral condyle in flexion. The final step is to mathematically smooth the composite of the corrected femoral surfaces, using a drape function. This resulting smoothed surface defines that portion of the tibia which is contacted by the condyles. The tibial surface also includes the central ramp or post surface which is similarly generated by the envelope of successive positions of the femoral cupola. This process results in a central ramp or post which is not as steep as a typical central post on typical PS total knees. However it will be appreciated that the steepness of the ramp will be determined by the pattern of cupola heights from the distal end of the femur to the posterior. An important feature of both the femoral and tibial surfaces is that all of the curves are continuous without corners or edges, for the purpose of avoiding stress concentrations and providing large areas of contact. 
       FIG. 1  is a perspective view of the femoral component  10 , where the general peripheral shape matches an average anatomical knee shape, Two short posts  20 ,  25  are typically used for fixation. The upper surface of a shallow cupola  30  is seen centrally.  FIG. 2  shows at the superior the typical anatomic shape of the patella groove  40  or trochlea. At the inferior, the lateral femoral condyle  50  is more prominent than the medial  60 .  FIG. 3  shows the frontal view, where the anterior view of the lateral and medial femoral condyles are shown. The radii are 23 mm, which blends well with the patella groove  40 , and is typical of an anatomic shape. For a femoral component this radius can be increased, particularly towards the outsides of the component.  FIGS. 2 and 3  show the planes of sagittal sections through the lateral femoral condyles (FL)  70 , center of the cupola (FC)  90 , and the medial femoral condyles (FM)  80 .  FIG. 4  shows the sagittal view, with anterior to the right. The two fixation posts  20 ,  25  can be seen. This view also shows the sections of the profiles of the condylar bearing surfaces  100 , which are shown in  FIG. 5 . 
       FIG. 5  shows the condylar profiles around the femoral component, S 1 -S 6  being the profiles which contact the tibial bearing surfaces, S 7 -S 9  being on the patella trochlea. The height of the cupola H  110  is maximum in the region of profile  6 , and then the height reduces around the bearing surface until it reaches a minimum at about profile S 2 . This can be seen more clearly in  FIG. 6 . The depth P 130  of section S 1  can be zero resulting in a cylindrical section of bearing surface running from lateral to medial. The difference between the maximum D  120  and minimum P  130  represents the height of the ramp or post on the center of the tibial component. The rate of change of heights also control the slope of the ramp or post. The angle of the posterior surface of the ramp to the horizontal will usefully be in the range of 30-90 degrees. The height at section S 1  will be less than or equal to the depth of the patella groove A  45  which is typically 7mm. However this may not have sufficient medial-lateral stability and hence a minimum depth of approximately 3 mm is preferable. As shown in  FIGS. 6 ,  7 , and  8 , the shapes of the lateral and medial profiles are different. 
       FIG. 7  shows a preferred profile of the medial bearing surface. Arc FE  140 , center S  150 , is the upper trochlea. From E to C  160 , center R  170 , is a constant radius, or close to constant. The arc CB  180 , center Q  190 , is reduced; and the arc BA  200 , center P  210 , is further reduced to facilitate a high range of flexion.  FIG. 8  shows the equivalent profile of the lateral bearing surface. In this case, arc D′C′  220 , center Q′  230 , is much larger than arc C′B′  240 , center P′  250 . These profiles resemble anatomic, and many ways of describing these profiles by arcs or spirals can be accomplished while retaining the general shapes. For the medial profile, the advantage is described by  FIGS. 9 and 10 . At zero flexion, the femoral and tibial surfaces are close to conformity (arrow), such that anterior sliding of the femur on the tibia is restricted. When the knee flexes to 30 degrees, the sliding is still restricted  260 . From 30-60 degrees, there is less restriction. However the ramp-cupola will start to act between 30-60 degrees, preventing the femur from displacing anteriorly on the tibia. 
       FIG. 11  shows a perspective view of the tibial component  270 , with the posterior to the lower left. In the center of the face is a curved notch  280 , both to fit the anatomic contour of the upper tibia, and for passage of the posterior cruciate if this is retained. The notch  280  can also be seen at the bottom of  FIG. 12 .  FIG. 13  shows a posterior view where dished surfaces  290 , 300  that receive the corresponding condyles  50 , 60  are separated by a central protrusion. The anterior medial side  320  to the right of  FIG. 13  is higher than lateral side  310  to the left. This again illustrates the differences between the more conforming medial side  300  and less conforming lateral side  290 . The dished medial tibial surface  300  will restrict anterior femoral sliding. The sections TL  340 , TC  350 , and TM  360 , are the locations of sagittal sections on the lateral side  370 , ramp  380 , and medial side  390 , shown in  FIG. 15 . In  FIG. 14 , the slope of the posterior ramp  400  and the slope of the anterior ramp  405  is seen. In this case the slope of the posterior ramp is 45 degrees, but various slopes are possible. A shallower slope will provide less definitive motion guidance, while a steeper slope will generally require a higher cupola, a disadvantage in regard to removal of bone when fitting to the femur. F 1 -F 5  are the locations of frontal plane sections, shown in  FIG. 16 . 
     In  FIG. 15 , the frontal radii of the lateral and medial bearing surfaces are shown. Except for the extreme anterior section F 5 , the frontal radii are constant from anterior to posterior, shown with the dashed arcs  410 . However, towards the posterior, F 1 , the arc radius is the same but the arc length is reduced because of the reduced central height. The advantage of the constant radius is that there can be close conformity of the tibial bearing surface with the femoral bearing surface throughout the entire flexion range, minimizing contact stresses. In this figure, it can be seen that the posterior ramp  400  is disposed approximately 2 mm to the lateral side, to match the cupola  110  seen in  FIG. 5 , this feature of a lateral shift being anatomic. 
       FIG. 16  shows a comparison between the sagittal profiles of the lateral TL  370  and medial TM  390  bearing surfaces. The medial is more dished both anteriorly and posteriorly to provide anterior-posterior stability, although the femoral radius is larger in order to allow 2-4 mm of anterior-posterior laxity, especially in high flexion. The lateral profile is shallow anteriorly to allow internal femoral rotation in extension, the so-called screw-home mechanism, and posteriorly to allow posterior displacement of the lateral femoral condyle in flexion. 
       FIGS. 17-20 , respectively show the lateral, central and medial sections at zero degrees flexion. The low lateral conformity and high medial conformity have already been described. The anterior ramp  405  now acts to limit extension, although rocking is possible to allow up to 5 degrees of hyperextension.  FIGS. 20-22  show the sections at 60 degrees flexion. Here, the posterior ramp  400  and cupola  110  is seen to be in contact. The ideal initial contact is in the range of 30-60 degrees flexion. Finally,  FIGS. 23-25  show the sections at 120 degrees flexion. The lateral femoral condyle  50  is posterior on the tibial surface  330 , while the medial femoral condyle  60  has displaced 2-4 mm, these actions due to the posterior ramp  400  and cupola  10  and the relative dishing of the lateral  290  and medial  300  sides. Due to the differential displacements, the femoral component has rotated approximately 20 degrees externally about an axis  420  on the medial side of the medial tibial bearing surface. During rotation, the location of axis  420  may minimally displace within the medial tibial bearing surface. Such minimal displacement may, for example, be limited to less than approximately 5 millimeters. 
       FIG. 26  shows a section in the transverse plane of the cupola  110  and the posterior ramp R  400 , at 60 degrees flexion. The interior of the cupola is rounded, and so is the posterior surface of the ramp, such that there is close conformity, which maximizes the contact area and minimizes the contact stresses. The same situation occurs at 90 degrees flexion ( FIG. 27 ) and 120 degrees flexion ( FIG. 28 ). This conformal contact is a major advantage in protecting the ramp, or post, from edge damage. 
       FIGS. 29-33  show one method for generating surface of the tibial component  270 . A composite is made of the femoral components at increments of flexion ( FIG. 29 ). The motion path is described by simple empirical equations which describe the axial rotation and the posterior displacement of the femoral component  10  on the tibial component  270 . In our case we use 20 degrees of external rotation and 4 mm of posterior displacement. However there are many values which will produce similar tibial component shapes and which would function satisfactorily.  FIGS. 30 and 31  show that the sagittal sections are aligned on arcs RM  440  medially and RL  450  laterally. The radii of the arcs have been determined in previous studies of knee replacements to provide the correct combinations of stability and laxity. A drape function over the lower part of the composite of femoral components produces a surface which will replicate the combined motions of the femur, as shown in  FIG. 32 . It will be appreciated that the tibial surface is modified to avoid an exact fit with the femoral component. This can be done by building in small side-to-side laxity movements in the femoral composites. The final tibial component  270  is thus generated ( FIG. 33 ) and completed by making a posterior notch  280 , and relieving the posterior of the medial side with a 45 degree chamfer to avoid impingement with the posterior femoral cortex in high flexion. 
       FIG. 34  shows a convenient combination of components which can be made. The standard femoral component  10  and tibial component  270  as described thus far are shown on the left. However there are many surgical cases where it is preferred to retain the posterior cruciate ligament. To accommodate this, a slot or ligament clearance notch  460  is made in the femoral component  470 . The posterior notch or ligament clearance depression  280  on the tibial component  270  allows passage of the posterior cruciate as stated already. The ramp does not interfere with motion of the cruciate retaining femoral component. However there is now no cupola to interact with the ramp and provide the posterior displacement. This function is now carried out by the posterior cruciate ligament. In an embodiment, the ligament clearance depression and ligament clearance notch may be formed by removal of separable portions of the respective tibial and femoral components. 
     Statement Regarding Preferred Embodiments 
     While the invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications can be made to the invention without departing from the spirit or scope of the invention, in particular the embodiments of the invention defined by the appended claims. All documents cited herein are incorporated in their entirety herein.