Abstract:
A knee prosthesis including a tibial component and a meniscal component adapted to be engaged through the tibial component through an asymmetrical engagement.

Description:
RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/075,813, filed on May 12, 1998, now U.S. Pat. No. 6,090,144. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a knee prosthesis. 
     2. Description of Related Art 
     The knee joint or articulation may be considered two condyloid joints, lateral and medial, between femur and tibia, and one arthrodial joint between the patella and the femur. The chief movements at the knee are flexion (decrease in the angle between two bones) and extension (increase in the angle between two bones) and rotation. These movements can be referred to as asymmetrical in that the movement of the left knee joint differs from the movement of the right knee joint. The individual displacement of the right and left knee joint during flexion and extension is also asymmetrical. 
     The knee joint combines a wide range of movement in one direction with a great weight-bearing capacity and considerable stability. The superior end of the tibia is the largest weight-bearing surface of the human skeleton. Its two articulating condyles or menisci are thickened and convex on their peripheral borders, and thin, concave, and free on their opposite borders. They are connected anteriorly and peripherally by transverse ligaments, and by part of the capsule of the knee joint, to the head of the tibia. These menisci lend some stability to the joint. Additional stability is given by the strong anterior and posterior cruciate ligaments which connect the tibia and femur inside of the joint and cross each other like the letter “X”. The anterior cruciate ligament extends from the front of the intercondylar eminence of the tibia, upward and backward to the medial side of the lateral condyle of the femur. The posterior cruciate ligament extends from the posterior intercondylar fossa of the tibia, upward and forward to the lateral side of the medial condyle of the femur. The stability of the knee is secured by the muscles of the thigh, the joint capsule (system of tendons and ligaments that pass over the knee joint) and four ligaments—the two lateral ligaments and two cruciate ligaments. 
     Injuries to the knee are very common. The injuries often result to the menisci or the ligaments that hold them. Significant research and development in recent years has been directed to the development of knee prostheses that are reliable, i.e., prostheses that are not subject to unacceptable dislocation, not subject to bearing failure, not subject to loosening from the bones, and which provide a substantial duplication of the motion of the natural joint. In general, knee replacement prostheses are indicated for bi-cruciate retention application, unicondylar applications and for posterior cruciate retention applications. Other prostheses are indicated where neither posterior nor anterior cruciate ligaments are retained. The types of knee prostheses available can generally be classified as fixed prostheses and mobile prostheses. 
     Generally, either a fixed or mobile knee prosthesis involves a femoral component, a meniscal component, and a tibial component. The meniscal component generally is seated between the femoral component and the tibial component, each mated with the femur and tibia, respectively. The reference to either fixed or mobile prostheses generally concerns the meniscal component. In the fixed system, the meniscal component is fixedly attached to the femur or tibia. In the mobile system, prior art knee prostheses offer some limited range of symmetrical motion for each of the right and left knee joint prosthesis. 
     The fixed prosthesis is generally used on patients where there is severe damage to the femur and/or tibia around the knee joint or where neither the posterior or anterior cruciate ligaments of the knee joint cannot be retained. The fixed prosthesis generally does not allow any movement of the motion of the femur on the tibia, e.g., the “sliding-rolling” motion of the femur on the tibia. Instead, the meniscal component is fixed to the tibial component and/or the femoral component. This fixation generally includes screw and bands. The fixed prosthesis also does not allow correction for a misplacement in rotation of the tibia component. Finally, the fixed prosthesis contributes to accelerated wear of the generally polyethylene meniscal component. 
     Mobile, i.e., sliding or moving, knee prostheses generally accommodate some movement by the meniscal component or the tibial component during knee joint movement. As noted above, the individual biomechanical displacement of the right and left knee joint during flexion and extension is asymmetrical. The natural meniscal displacement of a knee joint during extension, for example, is approximately 15 millimeter (mm) for the external (lateral) meniscus and 5 mm for the internal (medial) meniscus. 
     The general interest in the mobile prosthesis is to obtain a dimunition of the constraint on the meniscal component by delivering a proper positioning of the meniscal component on the tibial component during and after movement. In most instances, the motions of prior art prostheses are limited to a simple rotation (flexion/extension) which is in some instances combined with anterior-posterior clearance. These protheses generally offer no lateral translation or anterior-posterior translation of the components, e.g., the meniscal component. The range of motion of the components for the displacement is limited generally because the guidance is accomplished on rails or the motion around a fixed axis. The existing mobile motions are also symmetrical and non-conforming to human biomechanical movements. For example, most mobile knee prostheses have an axis of rotation about which movements of flexion and extension take place[lace]. In these systems, the displacement of the meniscal component about the axis of rotation is symmetrical. For example, displacement of the external (lateral) portion of the meniscal component is equivalent to the displacement of the internal (medial) portion for extension and flexion. During flexion, this type of symmetrical displacement will cause the femoral component to strike and erode the internal meniscal component and reduce flexion. 
     Prior art mobile prostheses also offer no rotational misalignment correction, such as, for example, where the meniscal component is misaligned between the femoral and tibial components. This is especially true in those systems that provide guide rails in the seat of the tibial component for placement of the meniscal component. The mobile prostheses further provide a lack of simple transformation toward a fixed tibial plate in cases of lateral instability, risk of incorrect positioning, luxation of the meniscal component, and rupture of the posterior cruciate ligament. In this instance, additional surgery is necessary to place a fixed knee prosthesis. 
     The invention seeks to address the limitations inherent in prior art knee prostheses. 
     SUMMARY OF THE INVENTION 
     A fixed knee prosthesis and a mobile knee prosthesis are disclosed. The knee prosthesis includes a tibial component and a meniscal component adapted to be engaged to the tibial component in an asymmetrical manner. The mobile knee prosthesis of the invention is adapted for and addresses the biomechanical movements of a right and a left knee joint or articulation separately. In one embodiment, the tibial component of the knee prosthesis of the invention includes a tibial seat including a Y-shaped cavity having a first arm and a second arm intersecting at a base. The meniscal component includes a meniscal plate selectively configured about a sagittal plane for either a right or left knee and a protuberance extending from a bottom surface of the meniscal plate. 
     The protuberance of the meniscal plate has a shape adapted to conform in some measure with the base and one of the first arm and the second arm of the cavity of the tibial seat, according to whether the prosthesis is adapted for the right or left knee joint of the patient. In one embodiment, the engagement of the protuberance of the meniscal component with the cavity of the tibial component is such that the protuberance is free to move within a portion of the cavity in conformance with the biomechanical movements of a natural knee joint, e.g., larger displacement of exterior (lateral) meniscal component than the interior (medial) portion of the meniscal component. In this manner, the invention provides a knee prosthesis or system with asymmetrical movements that emulate the asymmetrical movements of natural biomechanics. 
     An alternative embodiment of the invention describes a meniscal component including a meniscal plate including a Y-shaped cavity having a first arm and a second arm intersecting at a base. In this embodiment, the tibial component includes a tibial seat and a protuberance extending from a top surface of the tibial seat. The protuberance of the tibial component has a shape adapted to conform in some measure with the base and either the first arm or the second arm of the cavity of the meniscal plate according to whether the replacement is for the right or left knee joint of the patient. Accordingly, in one embodiment, the protuberance of the tibial component and the shape of the meniscal component, particularly about a sagittal plane, is specific for a left or a right knee joint prosthesis. In one embodiment, the cavity of the meniscal component is adapted to move about the protuberance in the tibial component in accordance with the asymmetrical movements of natural biomechanics. 
     Additional features and benefits of the invention will become apparent from the detailed description, figures, and claims set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of a knee prosthesis in accordance with an embodiment of the invention. 
     FIG. 2 is an exploded side sectional view of a knee prosthesis in accordance with an embodiment of the invention. 
     FIG. 3 is a top perspective view of the tibial component of the knee prosthesis with a Y-shaped cavity in accordance with an embodiment of the invention. 
     FIG. 4 is a top or superior side view of the left meniscal component of a knee prosthesis in accordance with an embodiment of the invention. 
     FIG. 5 is a bottom or inferior side view of the left meniscal component of a knee prosthesis with a protuberance adapted to conform with a portion of the Y-shaped cavity of the tibial component in accordance with an embodiment of the invention. 
     FIG. 6 is a top or superior side view of the right meniscal component of a knee prosthesis in accordance with an embodiment of the invention. 
     FIG. 7 is a bottom or inferior side view of the right meniscal component of a knee prosthesis with a protuberance adapted to conform with a portion of the Y-shaped cavity of the tibial component in accordance with an embodiment of the invention. 
     FIG. 8 is a top cross-sectional view of the right meniscal component of a knee prosthesis inserted into the Y-shaped cavity of the tibial component in a first position in accordance with an embodiment of the invention. 
     FIG. 9 is a top cross-sectional view of the right meniscal component of a knee prosthesis inserted into the Y-shaped cavity of the tibial component in a second position in accordance with an embodiment of the invention. 
     FIG. 10 is a top cross-sectional view of the right meniscal component of a knee prosthesis inserted into the Y-shaped cavity of the tibial component in a third position in accordance with an embodiment of the invention. 
     FIG. 11 is a top or superior side view of a meniscal component for a fixed knee prosthesis with a Y-shaped protuberance adapted to conform to the Y-shaped cavity of the tibial component in accordance with an embodiment of the invention. 
     FIG. 12 is an exploded side sectional view of a knee prosthesis in accordance with a second embodiment of the invention. 
     FIG. 13 is a bottom or inferior view of the meniscal component of a knee prosthesis with a Y-shaped cavity in accordance with a second embodiment of the invention. 
     FIG. 14 is a top perspective view of the tibial component of a left knee prosthesis with a protuberance adapted to conform with a portion of the Y-shaped cavity of the meniscal component in accordance with a second embodiment of the invention. 
     FIG. 15 is a top perspective view of the tibial component of a right knee prosthesis with a protuberance adapted to conform with a portion of the Y-shaped cavity of the meniscal component in accordance with a second embodiment of the invention. 
     FIG. 16 is a top perspective view of the tibial component of a knee prosthesis with a Y-shaped protuberance adapted to conform with the Y-shaped cavity of the meniscal component in accordance with a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to a knee prosthesis. The knee prosthesis provides three degrees of liberty in accordance with biomechanical movement data. An anterior-posterior translation is obtained as well as a median lateral motion. The knee prosthesis achieves these goals through the use of asymmetrical components specific for either the left or right knee. The invention is also designed to allow simple transformation, for example, from a mobile knee system to a fixed knee system. 
     FIG. 1 shows a side sectional view of a knee prosthesis according to an embodiment of the invention. FIG. 2 is an exploded view of the knee prosthesis of FIG.  1 . Knee joint prosthesis  10  is functionally secured to a tibia and a femur of a human leg. Knee joint prosthesis  10  includes femoral component  15  that is rigidly connected to the superior end of a femur, generally after the femur has been resected in a manner that is well known in the art. Femoral component  15  includes a condylar portion  17  that contacts or engages meniscal component  20 , which is more fully described below. Superiorly adjacent to condylar portion  17  on femoral component  15  is femoral stem  18  that acts as a fixing device to fix femoral component  15  to a femur. 
     In one embodiment, femoral component  15  is made of a biocompatible metal, such as, for example, titanium, titanium alloy, or cobalt-chromium alloy, or made of a biocompatible ceramic, such as, for example, alumina ceramic or zirconia ceramic. Femoral component  15  is fixed to a femur, for example, by cement or a hydroxyaptite coating on femoral stem  18 . The hydroxyaptite coating is used in the instance to induce bone growth. It is to be appreciated that the femoral component is not required in the knee prosthesis of the invention. Instead, the meniscal component can be adjusted to conform and be compatible with the femur of the patient. However, to increase the longevity of the knee prosthesis and to avoid damage to the femur, femoral component  15  is generally recommended. 
     Meniscal component  20  is located between femoral component  15  and tibial component  25 . The overall shape of meniscal component or plate  20  will be described in detail below, but here it is notable that meniscal component  20  has a generally planar inferior surface with protuberance (here labeled reference numeral  60 ) selectively chosen for the left or right knee, respectively, of a patient. In one embodiment, a superior surface of meniscal component or plate  20  has a generally condylar (concave) shape to match the opposing condylar (convex) surface of a femur or femoral component  15 . In this manner, meniscal component  20  is able to articulate with condylar portion  17  of femoral component  15 . The top surface of meniscal component  20  may also be configured to conform to prior art femoral components. In one embodiment, meniscal component  20  is made from biocompatible ultra-high molecular weight polyethylene (UHMWP) It is to be appreciated, however, that other suitable materials may be used consistent with the properties of biocompatibility and durability. 
     Meniscal component  20  is connected to tibial component  25  by inferiorly extending protuberance  60  that fits in a receiving cavity (not shown) of tibial component  25  depending upon whether knee prosthesis  10  is to be assembled in the left or right leg of a patient, respectively. Tibial component  25  is described in detail below, but here it is notable that tibial component  25  contains tibial seat  30  having a generally planar superior surface  27  to support the generally planar inferior surface  22  of meniscal component  20 . Inferior surface  28  of tibial seat  30  contains inferiorly extending tibial keel  45  which is secured to the tibia of a patient. 
     In one embodiment, tibial component  25  is made of a biocompatible high-strength metal such as, for example, titanium, titanium alloy, or cobalt-chromium alloy or a biocompatible ceramic such as, for example, alumina ceramic or zirconia ceramic. Tibial component  25  is fixed to a tibia of a patient by, for example, making a hole in the tibia to support tibial keel  45  and cementing keel  45  to the tibia. In another embodiment, tibial component  25  is secured to the tibia of a patient by applying a hydroxyaptite coating on keel  45  to induce bone growth onto tibial component  25 . 
     FIG. 3 shows an embodiment of tibial component  25  in accordance with the invention. Tibial component  25  includes tibial seat  30  having a generally planar superior surface  27 . In one embodiment, the shape of tibial seat  30  resembles a painter&#39;s-pallet with an elliptic configuration incurvated or indented at one side. Indentation  36  defines tibial seat  30  with medial condylar portion  32  and lateral condylar portion  34 . In this configuration, one skilled in the art will realize that the knee prosthesis of the invention can be affixed to a patient without the destruction of a viable posterior cruciate ligament. Indentation  36  between medial condylar portion  32  and lateral condylar portion  34  allows for posterior cruciate ligament retention. 
     The width of tibial seat  30  may be made to be specific for a patient. In one instance, for example, tibial seat  30  will have a standard thickness of, for example, approximately 5 millimeters (mm). In another instance, where more of a patient&#39;s tibia requires resection for placement of the knee prosthesis or system of the invention, tibial seat  30  may have a thickness of 10 mm or more. In the embodiment where the meniscal component is made of UHMWP, it is appreciated that conforming the meniscal components to the specifics of the patient&#39;s knee is much more cost effective than machining or casting a specific tibial component. 
     Tibial seat  30  has a substantially Y-shaped cavity  35  with a first arm and a second arm intersecting at a base. The base is proximally adjacent indentation  36  between medial condylar portion  32  and lateral condylar portion  34 . In one embodiment, Y-shaped cavity  35  is formed with substantially arcuate surfaces and arcuate or softened edges throughout. In this embodiment, central axis  37  bisects cavity  35  between medial condylar portion  32  and lateral condylar portion  37 . 
     Extending from inferior surface  28  of tibial seat  30  of tibial component  25  is keel  45 . An upper portion  40  of keel  45  includes a cavity extending about and having the same shape as Y-shaped cavity  35  of tibial seat  30 . In this manner, the opening through cavity  35  extends into upper portion  40  of keel  45 . This extension of the Y-shaped cavity allows the stability and range of motion of the meniscal component to be adjusted, for example, by modifying the thickness or depth of protuberance  60 —a deep or thick protuberance will be more stable and allow less meniscal component  20  motion, while a shallow or thin protuberance will be less stable but allow greater meniscal component  20  motion. In one embodiment, keel  45  is a fixed length such as, for example, approximately 12 mm. In another embodiment, keel  45  is modular and can be made of varying lengths. 
     FIGS. 4-7 show different views of meniscal components for a left and right knee prosthesis, respectively. FIG. 4 shows superior surface  55  of meniscal component  20  for a left knee prosthesis. Left meniscal component  20  has an asymmetrical shape similar to a painter&#39;s-pallet with an elliptic configuration incurvated or indented at one side. It is noted that the shape of left meniscal component  20 , in this embodiment is not identical to the shape of tibial seat  30 . The symmetrical shape of tibial seat  30  is presented in outline form beneath meniscal component  20  to demonstrate this difference. Similar to tibial seat  30 , meniscal component has two condylar portions, medial condylar portion  56  and lateral condylar portion  57 , to preserve a posterior collateral ligament. In one embodiment, superior surface  55  has concave condylar shapes to accommodate opposing convex condylar portions of femoral component  15 . The shape of meniscal component  20  is chosen, in this embodiment to provide the closest duplication of human biomechanics by the movement of meniscal component  20  about cavity  35 . 
     As shown in FIG. 6, right meniscal component  21  has an asymmetrical painter&#39;s-pallet (elliptical) shape to preserve a posterior collateral ligament. The shape is compared in the figure with the symmetrical shape of tibial seat  30 . Meniscal component  21  includes medial condylar portion  66  and lateral condylar portion  67 . In one embodiment, superior surface  65  of meniscal component  21  has concave condylar shapes to accommodate opposing convex condylar portions of femoral component  15 . 
     FIG. 5 shows an inferior view of left meniscal component  20 . Inferior surface  58  is substantially planar to match substantially planar superior surface  27  of tibial seat  30 . Inferior surface  58  includes protuberance  60  having a shape adapted to conform in part with the base and one arm of cavity  35  of tibial seat  30  but to allow some movement or play in this portion of the cavity. As shown in FIG. 7, inferior surface  68  of right meniscal component  21  similarly has a substantially planar surface and protuberance  70  adapted to conform in part with the base and the other arm of cavity  35  of tibial seat  30 . In this manner, meniscal components  20  and  21  are specific for a left and a right knee of a patient, respectively. 
     Protuberances  60  and  70  have an asymmetrical shape with a mirror symmetry for the left and right meniscal components  20  and  21 , respectively. The shape of protuberances  60  and  70  is of an asymmetrical bean form with a larger internal portion to mate with the base of Y-shaped cavity  35  of tibial component  25  and a smaller external portion to mate with an arm portion of Y-shaped cavity  35 . The shape of protuberances  60  and  70  and the shape of meniscal components  20  and  21  shift the axis of displacement of meniscal component  20  or  21  on tibial seat  30  to a more medial or more lateral position, respectively. As noted, in this embodiment, protuberances  60  and  70  do not fit snugly in Y-shaped cavity  35  of tibial component  25 , but instead are slightly smaller, particularly at their external ends, to allow movement of meniscal component  20  and  21  once the knee prosthesis is placed in the left or right leg of a patient, respectively. In one embodiment, protuberances  60  and  70  also have rounded edges to facilitate the movement of the protuberance in tibial cavity  35 . 
     The asymmetrical shape of meniscal components  20  and  21  and protuberances  60  and  70 , respectively, creates a controlled complex motion that includes a rotation combined with an anterior-posterior and a lateral translation. The range of motion duplicates the biomechanics of the human knee by privileging a larger displacement on the external portion of meniscal component  20  and  21  while limiting the displacement of the internal portion of meniscal component  20  and  21 . 
     FIGS. 8-10 illustrate the controlled complex motion of right meniscal component  21  in tibial seat  30 . Each of these figures present top or superior cross-sectional views of right meniscal component  21  seated in tibial seat  30 . FIG. 8 illustrates a neutral position, FIG. 9 a flexion, and FIG. 10 an extension. Referring to FIG. 8, there is presented protuberance  70  mated with an arm portion and the base of cavity  35  of tibial seat  30 . Bean-shaped protuberance  70  does not conform precisely to the dimensions of an arm and the base of cavity  35 . Instead, protuberance  70  is able to move about a portion of cavity  35 . The neutral position of FIG. 8 presents some point between flexion and extension. 
     As noted, FIG. 9 demonstrates the movement of meniscal component  21  after a flexion. The displacement of meniscal component  21  may be characterized as follows. Medial condylar portion  66  moves anteriorly along medial condylar portion  32  of tibial seat  30 . Lateral condylar portion  67  of meniscal component  21  moves posteriorly about lateral condylar portion  34  of tibial seat  30 . At the same time, meniscal component  21  rotates in a clockwise direction as illustrated in FIG.  9 . There is also a medial translation. 
     FIG. 10 is directly opposite FIG.  9 . FIG. 10 shows an extension having the following movements. Medial condylar portion  66  moves posteriorly along medial condylar portion  32  of tibial seat  30 . Lateral condylar portion  67  of meniscal component  21  moves anteriorly about lateral condylar portion  34  of tibial seat  30 . At the same time, meniscal component  21  rotates in a counter-clockwise direction as illustrated in FIG.  10 . There is also a lateral translation. 
     The range of motion and the asymmetry of the motion of the knee prosthesis of the invention is created by the asymmetrical shape of meniscal component and  21  and the relationship of the symmetrical shape of cavity  35  of tibial seat  30  of tibial component  25  with the bean shape of protuberance  60  and  70  of left meniscal component  20  and right meniscal component  21 , respectively. In the embodiment described, the asymmetrical shape of meniscal component  20  and  21  allows the external (lateral) portion (e.g., lateral condylar portion  66 ) to be displaced a greater distance than the internal (medial) portion (e.g., medial condylar portion  67 ). In one embodiment, for example, condylar portion  66  of meniscal component  21  may be displaced  14  mm while medial condylar portion  67  may be displaced 4 mm in the same flexion/extension displacement. 
     By placing either left meniscal component  20  or right meniscal component  21  on tibial component  25 , protuberance  60  or  70 , respectively, will be positioned in cavity  35  and have a range of controlled motion that will privilege an asymmetrical displacement of, for example, meniscal component  21  (lateral condylar portion  66  and medial condylar portion  67 ) on tibial component  25 . It is to be appreciated that, in certain instances, the asymmetry described above with greater displacement privilege on, for example, lateral condylar portion  66  than medial condylar portion  67 , can be reversed to obtain a larger displacement on the medial condylar portion and positioning the axis of displacement in a more lateral position on tibial plate  30 . 
     In other instances, it may be necessary to limit the motion of the meniscal component. A surgeon may desire, for example, to place a fixed knee prosthesis or the patient&#39;s diagnois may require additional posterior stabilization, such as, for example, where the posterior cruciate ligament may not be retained. FIG. 11 shows an embodiment of the invention wherein meniscal component  23  is configured as a fixed insert. FIG. 11 shows the inferior side view of meniscal component  23 . In this embodiment, meniscal component  23  has an elliptical shape similar to the shape of tibial seat  30  of tibial component  25 , i.e., symmetrical. Extending from inferior surface  78  of meniscal component  23  is Y-shaped protuberance  75 . Y-shaped protuberance  75  is symmetrical and complementary with Y-shaped cavity  35  of tibial seat  30  of tibial component  25 . In one embodiment, Y-shaped protuberance  75  fits snugly in a conformal tibial cavity, such as tibial cavity  35  of tibial component  25  of FIG.  3 . 
     The embodiment described with reference to FIG. 11 demonstrates an advantage of the configuration of the invention: the conversion, for example, from a mobile knee prosthesis or system to a fixed knee prosthesis or system does not require a new tibial component. Instead, the conversion is accomplished by replacing, for example, meniscal component  20  with meniscal component  23 . 
     FIGS. 12-15 show a knee prosthesis according to another embodiment of the invention. FIG. 12 is an exploded side sectional view of a knee prosthesis according to this embodiment. Knee prosthesis  100  includes femoral component  15  that is rigidly connected to the superior end of a femur as described above with respect to FIGS. 1 and 2 and the accompanying text. Meniscal component  120  is located between femoral component  15  and tibial component  125 . Meniscal component  120  has a generally planar lower surface and, in one embodiment, a superior surface with a generally condylar (concave) shape to match the opposing condylar (convex) surface of a femur or femoral component  15 . In one embodiment, meniscal component  20  is made from biocompatible UHMWP. 
     Meniscal component  120  is connected to tibial component  125  by a superiorly extending protuberance from tibial seat  130 . The protuberance (labeled here as reference numeral  160 ) fits in a portion of a receiving cavity on the inferior side of meniscal component  120  depending upon whether knee prosthesis  100  is to be assembled in the left or right leg of the patient, respectively. Similar to the first embodiment, tibial component  125  includes tibial seat  130  having a generally planar superior surface  127 . Tibial seat  130  resembles a painter&#39;s-pallet with an elliptic configuration incurvated or indented at one side. The indentation defines tibial seat  130  with two condylar portions. Extending from superior surface  127  of tibial seat  130  is protuberance  160 , the orientation of which depends upon whether the knee prosthesis is for the left or right leg of a patient, respectively. It is to be noted here, unlike the first embodiment where meniscal component  20  was configured to be placed in one of a left or right knee prosthesis, respectively, in this embodiment, tibial component  125  is configured to be placed in either the left or right knee prosthesis, respectively. Alternatively, protuberance  160  may be modular and, thus, exchangeable allowing a single tibial component  125  for each of a right leg and a left leg and an individual protuberance selective for each articulation. Extending from inferior surface  128  of tibial seat  130  of tibial component  125  is keel  145  to affix tibial component  125  to a tibia. 
     FIG. 13 shows a view of inferior surface  138  of meniscal component  120 . Meniscal component  120  resembles a painter&#39;s pallet with an elliptic configuration incurvated or indented at one side. The indentation defines two condylar portions: lateral condylar portion  166  and medial condylar portion  167 . The shape of lateral condylar portion  166  and medial condylar portion  167  are not symmetrical in this embodiment. FIG. 13 is an embodiment of meniscal component  120  for a right knee prosthesis. In this instance, when viewed in a sagittal plane, lateral condylar portion  166  is slightly smaller or narrower than medial condylar portion  167  to allow more movement of lateral condylar portion  166 . 
     In this embodiment shown in FIG. 13, inferior surface  138  of meniscal component  120  is generally planar with a substantially Y-shaped cavity  135  with a first arm portion and a second arm portion intersecting at a base. The base is proximally adjacent indentation  136  between the condylar portions of meniscal component  120 . In this embodiment, axis of displacement  137  does not bisect cavity  135 , but is slightly offset toward lateral condylar portion  166 . It is to be appreciated that in some instances, it may be desirable to reverse the range of motion such that medial condylar portion  167  has a larger displacement. This may be accomplished by mirroring meniscal component  120  and shifting the axis of displacement for a right knee prosthesis. 
     FIGS. 14 and 15 shows top perspective views of embodiments of tibial components for a left and right knee prosthesis, respectively. Superior surface  127  of tibial seat  130  is substantially planar to match substantially planar superior surface  138  of meniscal component  120 . In FIG. 14, superior surface  127  includes protuberance  160  having a shape adapted to conform in part with the base and an arm of cavity  135  of meniscal component  120 . FIG. 14 is, for example, for a left knee prosthesis. FIG. 15 shows a similar tibial component having protuberance  170  extending from a superior surface of the tibial seat for a right knee prosthesis. In this manner, tibial components  125  and  126  are specific for a left and a right knee of a patient, respectively. Protuberances  160  and  170  have an asymmetrical shape with a mirror symmetry for the left and right tibial components, respectively. The shape of protuberances  160  and  170  is of an asymmetrical bean form with a larger internal portion to mate in part with the base of Y-shaped cavity  135  of meniscal component  120  and a smaller external portion. In this embodiment, protuberances  160  and  170  do not fit snugly in Y-shaped cavity  135  of meniscal component  120 , but instead are slightly smaller, particularly at their external ends, to allow movement of meniscal component  120  once the knee prosthesis is placed in the left or right leg of a patient, respectively. In one embodiment, protuberances  160  and  170  have rounded edges to facilitate the movement of the protuberance in meniscal cavity  135 . 
     The asymmetrical shape of meniscal component  120  and of the protuberances of tibial components  125  and  126  create a controlled complex motion that includes the rotation combined with an anterior-posterior and a lateral translation similar to that described above with reference to FIGS. 8-10 and the accompanying text. The significant difference is in the location of Y-shaped cavity (meniscal component) and protuberance (tibial seat). Once again, however, the range of motion duplicates the biomechanics of, for example, the human knee by privileging a larger displacement on the external (lateral) portion of meniscal component  120  while limiting the displacement of the internal (medial) portion of meniscal component  120 . 
     FIG. 16 shows an embodiment of tibial component  122  that may be used in a fixed knee prosthesis configuration. Extending from the superior surface of tibial seat  146  of tibial component  122  is Y-shaped protuberance  165 . Y-shaped protuberance  165  is symmetrical and complementary with a Y-shaped cavity of symmetrical meniscal component  123 . In one embodiment, Y-shaped protuberance  165  fits snugly in the meniscal cavity. 
     It is to be appreciated that in the second embodiment, the height of the protuberance extending from the tibial seat is such that it sits within meniscal cavity  135  so that inferior surface  138  of meniscal component  120  contacts superior surface  127  of tibial seat  130 . It is to be appreciated that meniscal component  120  may be made of different thicknesses to accommodate the proper placement and positioning of a knee prosthesis in a patient. 
     The use of the knee prosthesis of the invention can be used as a primary or revision knee system, for example, where a prior knee prosthesis failed. Further, the knee prosthesis of the invention allows a surgeon to make a decision during surgery whether to put a moving meniscal component or a fixed meniscal component into a knee prosthesis simply by choosing an appropriate meniscal component. Thus, the invention offers an asymmetrically designed knee prosthesis or system that in one sense mimics the biomechanical movements of a natural knee and in another is relatively easy to configure to an individual patient&#39;s needs. 
     The above embodiments described a Y-shaped cavity, protuberances that mate with a portion of the Y-shaped cavity, and asymmetric meniscal components. The invention is not to be interpreted as limited to any particular shape of meniscal component or cavity/protuberance. Instead, the invention recognizes the importance of an asymmetrical design, particularly in a mobile knee prosthesis or system (i.e., the asymmetrical design of the meniscal component relative to the tibial component or vice versa) commensurate with natural biomechanics. FIGS. 1-10 and  12 - 15  and the accompanying text presented various embodiments for a mobile knee prosthesis. The invention, however, recognizes that other designs based on the principles described herein are conceivable that capture this asymmetry and are therefore within the scope of the invention. Similarly, FIGS. 11 and 16 and the accompanying text presented various embodiments of a fixed knee system. These embodiments are particularly suitable for transformation from one of the mobile knee prostheses described herein. The invention recognizes that other designs based on the principles described herein are similarly conceivable and within the scope of the invention. 
     In summary, the preceding detailed description, described the invention with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.