Patent Publication Number: US-7708782-B2

Title: Mobile bearing knee prosthesis

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of U.S. application Ser. No. 10/894,146, filed Jul. 19, 2004, now U.S. Pat. No. 7,422,605, which claims the benefit of U.S. Provisional Application Ser. No. 60/487,907, filed Jul. 17, 2003 and U.S. Provisional Application Ser. No. 60/551,369, filed Mar. 9, 2004. This application also claims the benefit of U.S. Provisional Application Ser. No. 60/653,872, filed Feb. 17, 2005. Each of the aforementioned applications is incorporated herein by reference in its entirely. 
    
    
     FIELD OF THE INVENTION 
     In one embodiment of the present invention a mobile bearing knee prosthesis may include an interface (e.g., a spherical radius interface) comprised of a concave superior surface on a tibial tray and a convex inferior surface on a tibial insert. 
     In another embodiment of the present invention a mobile bearing knee prosthesis may include an interface (e.g., a spherical radius interface) comprised of a convex superior surface on a tibial tray and a concave inferior surface on a tibial insert. 
     In another embodiment of the present invention a mobile bearing knee prosthesis may include a bi-concave interface (e.g., having a “wave” like surface geometry). This “wave” like surface geometry may be at the interface between a tibial insert and a tibial tray in the mobile bearing knee (as opposed to the interface between the tibial insert and a femoral component). Further, this “wave” like surface geometry may allow a “virtual” or “artificial” axis of rotation to be provided by the interface between the tibial insert and the tibial tray in the mobile bearing knee. 
     In one example (which example is intended to be illustrative and not restrictive) the tibial insert may include a polyethylene articulating surface. In another example (which example is intended to be illustrative and not restrictive) the tibial tray may include a metal articulating surface (e.g., a highly polished metal articulating surface). In another example (which example is intended to be illustrative and not restrictive) one or both of the articulating surfaces may include diamond (e.g., to improve wear characteristics on one or more mating surfaces). 
     For the purposes of describing and claiming the present invention the term “rotational constraint” is intended to refer to essentially stopping rotation of an object at a given point. 
     Further, for the purposes of describing and claiming the present invention the term “rotational control” is intended to refer to exercising control over the amount of force required to rotate an object. 
     Further still, for the purposes of describing and claiming the present invention the term “superior surface” is intended to be synonymous with the term “top surface”. 
     Further still, for the purposes of describing and claiming the present invention the term “inferior surface” is intended to be synonymous with the term “bottom surface”. 
     Further still, for the purposes of describing and claiming the present invention the term “first bearing” is intended to refer to the articulation between the bottom surface of the femoral component and the top surface of the tibial insert. 
     Further still, for the purposes of describing and claiming the present invention the term “second bearing” is intended to refer to the articulation between the bottom surface of the tibial insert and the top surface of the tibial tray. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 6,319,283 relates to a tibial knee component with a mobile bearing. More particularly, this patent relates to an orthopaedic knee component for implanting within a proximal tibia. The orthopaedic knee component includes a tibial tray with a proximal tibial plateau and a projection extending generally orthogonal to the tibial plateau. The tibial tray also includes a distally extending stem. A bearing is coupled with the tibial plateau and has an articular bearing surface for engagement with a femoral component. The bearing is rotationally movable between a first rotational limit and a second rotational limit about an axis extending generally orthogonal to the tibial plateau. The bearing has a backing surface engaging the tibial plateau which is sized and shaped such that the backing surface is substantially entirely supported by the tibial plateau at any position during rotational movement between the first rotation limit and the second rotational limit. 
     U.S. Pat. No. 5,683,468 relates to a mobile bearing total joint replacement. More particularly, this patent relates to a prosthetic component provided for a condylar joint. The prosthetic component includes a platform having a bearing surface and a pair of side walls. The side walls include a pair of concave surfaces which face one another and define arcs of the same right circular cylinder. The prosthetic component also includes a plastic bearing having a bearing surface slidably engaged with the bearing surface of the platform. The bearing also includes thrust surfaces defining arcs of two right circular cylinders having radii less than the radius of the side wall surfaces of the platform. The thrust surfaces are spaced from one another to permit only limited sliding movement of the bearing in medial to lateral directions, but greater sliding movement in anterior to posterior directions. 
     U.S. Pat. No. 5,556,432 relates to an artificial joint. More particularly, this patent relates to an endoprothesis for the human knee joint, consisting of at least two joint parts moving with respect to each other, a joint head and a joint base, with toroidal joint surfaces, that have function surfaces with differing circular intersection contours in mutually perpendicular planes—a longitudinal plane and a transverse plane—whereby the curve ratios of the function surfaces are defined in each of the planes as either convex-convex, convex-concave, or concave-concave, and the joint geometry of the function areas to each other in each of the two planes is determined by a link chain with two link axes (dimeric link chain), which proceed through the rotation centers of the function areas with the radii of the attendant intersection contours, respectively. 
     U.S. Pat. No. 5,358,530 relates to a mobile bearing knee. More particularly, this patent relates to a prosthetic mobile bearing knee including a femoral implant having condyle sections attached to a femur and a tibial tray implant having a plateau attached to a tibia. The tibial tray implant has a pair of spaced apart, concavely curved plateau bearing surfaces for cooperation and sliding with convexly curved surfaces on a tibial bearing. The tibial tray plateau bearing surfaces are shaped to create a gradually increasing resistance to sliding and rotational movement of the tibial bearing. The tibial bearing that interfits between the femoral and tibial tray implants is constructed in one or two portions. 
     U.S. Pat. No. 4,224,696 relates to a prosthetic knee. More particularly, this patent relates to a prosthetic knee having as its component parts a femoral implant, a tibial implant, and a meniscal plate disposed between the implants. Knee flexion and extension is permitted by compoundly curved condyle surfaces of the femoral implant, which resemble corresponding surfaces of a natural knee, and correspondingly shaped convex bearing surfaces in the meniscal plate. All other motions of the prosthetic knee take place at the interface between the meniscal plate and tibial implant. This interface is defined by a continuous, concave, spherically shaped surface in the upwardly facing plateau of the tibial implant and a corresponding, continuous, convex spherical surface of the meniscal plate. The components are biased into mutual engagement along the cooperating concave and convex surfaces by the natural ligaments which surround the prosthetic knee. The continuous biased engagement of the cooperating convex and concave surfaces of the prosthetic knee assure its stability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a perspective view of a mobile bearing knee prosthesis according to an embodiment of the present invention; 
         FIG. 1B  shows a top plan view of the mobile bearing knee prosthesis of  FIG. 1A ; 
         FIG. 1C  shows a side view of the mobile bearing knee prosthesis of  FIG. 1A ; 
         FIG. 1D  shows another top plan view of the mobile bearing knee prosthesis of  FIG. 1A ; 
         FIG. 1E  shows a cross section taken along line A-A of  FIG. 1D ; 
         FIG. 2A  shows a top plan view of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 2B  shows a cross section taken along line H-H of  FIG. 2A ; 
         FIG. 2C  shows a perspective view of the mobile bearing knee prosthesis of  FIG. 2A ; 
         FIG. 3  shows a perspective view of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 4A  shows a plan view of a tibial tray component of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 4B  shows a side view of the tibial tray component of  FIG. 4A ; 
         FIG. 4C  shows a plan view of a tibial insert component of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 4D  shows a side view of the tibial insert component of  FIG. 4C ; 
         FIG. 5A  shows a plan view of a tibial tray component of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 5B  shows a side view of the tibial tray component of  FIG. 5A ; 
         FIG. 6A  shows a plan view of a tibial tray component of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 6B  shows a side view of the tibial tray component of  FIG. 6A ; 
         FIG. 7A  shows a plan view of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 7B  shows a cross section of the mobile bearing knee prosthesis of  FIG. 7A ; 
         FIG. 7C  shows a side view of the tibial tray of the mobile bearing knee prosthesis of  FIG. 7A ; 
         FIG. 8A  shows a perspective view of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 8B  shows a perspective view (partially cut-away) of the mobile bearing knee prosthesis of  FIG. 8A ; 
         FIG. 8C  shows a perspective view (partially cut-away) of the mobile bearing knee prosthesis of  FIG. 8A ; 
         FIG. 9A  shows a top plan view of a mobile bearing knee prosthesis according to another embodiment of the present invention (wherein a tibial insert is shown rotated and locked in place on a tibial tray); 
         FIG. 9B  shows a cross section of the mobile bearing knee prosthesis of  FIG. 9A ; 
         FIG. 9C  shows certain detail associated with the mobile bearing knee prosthesis of  FIG. 9A ; 
         FIG. 9D  shows certain detail associated with the mobile bearing knee prosthesis of  FIG. 9A ; 
         FIG. 10A  shows a plan view of a tibial tray according to another embodiment of the present invention; 
         FIG. 10B  shows a cross section of the tibial tray of  FIG. 10A ; 
         FIG. 10C  shows a plan view of a tibial insert for use with the tibial tray of  FIG. 10A ; 
         FIG. 10D  shows a plan view of the tibial tray and tibial insert of  FIGS. 10A-10C ; 
         FIG. 10E  shows certain detail taken along line B-B of  FIG. 10D ; 
         FIG. 10F  shows certain detail taken along line B-B of  FIG. 10D ; 
         FIG. 11  shows a side view of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 12A  shows a plan view of a mobile bearing knee prosthesis according to another embodiment of the present invention; 
         FIG. 12B  shows a side view of the mobile bearing knee prosthesis of  FIG. 12A ; 
         FIG. 13A  shows an elevation view of a mobile bearing knee according to another embodiment of the present invention (in this view a tibial insert and a tibial tray are engaged and a uniform curvature between mating parts is seen); 
         FIG. 13B  shows an elevation view of a mobile bearing knee according to the embodiment of  FIG. 13A  (in this view a tibial insert and a tibial tray are partially engaged and a uniform curvature between mating parts is seen); 
         FIG. 13C  shows a side elevation view of a mobile bearing knee according to the embodiment of  FIG. 13A  (in this view a tibial insert and a tibial tray are engaged and a uniform curvature between mating parts is seen); 
         FIG. 13D  shows a side elevation view of a mobile bearing knee according to the embodiment of  FIG. 13A  (in this view a tibial insert and a tibial tray are partially engaged and a uniform curvature between mating parts is seen); 
         FIG. 13E  shows another elevation view of a mobile bearing knee according to the embodiment of  FIG. 13A  (in this view a tibial insert and a tibial tray are engaged, the tibial insert is rotated 10° relative to the tibial tray, and a uniform curvature between mating parts is seen); 
         FIG. 13F  shows a perspective view of a mobile bearing knee according to the embodiment of  FIG. 13A  (wherein a tibial insert and a tibial tray are engaged, and a uniform curvature between mating parts is seen); 
         FIG. 14A  shows another elevation view of a mobile bearing knee according to the embodiment of  FIG. 13A  (wherein the Fig. includes a cross-sectional line indicator through the center of the mobile bearing knee); 
         FIG. 14B  shows a cross-section along the cross-sectional line indicator of  FIG. 14A  (wherein the relationship of the “wave” geometry to an axial post on the tibial insert is seen); 
         FIG. 14C  shows another elevation view of a mobile bearing knee according to the embodiment of  FIG. 13A  (wherein the Fig. includes a cross-sectional line indicator through the center of the mobile bearing knee and the tibial insert is rotated 10° relative to the tibial tray); 
         FIG. 14D  shows a cross-section along the cross-sectional line indicator of  FIG. 14C  (wherein the relationship of the “wave” geometry to an axial post on the tibial insert is seen and the tibial insert is rotated 10° relative to the tibial tray); 
         FIGS. 15A-15C  show perspective views (at various angles) of the inferior surface of a tibial insert of a mobile bearing knee prosthesis according to an embodiment of the present invention; 
         FIGS. 16A and 16B  show perspective views (at various angles) of the superior surface of a tibial tray of a mobile bearing knee prosthesis according to an embodiment of the present invention; 
         FIGS. 17A-17C  show schematic cross-sectional views of a mobile bearing knee according to an embodiment of the present invention; 
         FIGS. 18A and 18B  show schematic cross-sectional views of a mobile bearing knee according to an embodiment of the present invention; 
         FIGS. 19A and 19B  show schematic plan views of a centered pivoting feature ( FIG. 19A ) and an eccentered (or offset) pivoting feature ( FIG. 19B ) according to embodiments of the present invention; 
         FIGS. 20A-20E  show schematic plan views of offset pivot mechanisms according to embodiments of the present invention; 
         FIG. 21  shows an example of a surface having an infinite number of axes of revolution; 
         FIGS. 22A-22D  show examples of surfaces which may be obtained by rotating any desired geometric form around a desired axis of revolution; 
         FIG. 23  shows an example of a conventional MBK associated with a flat second bearing in which the tibial tray post engages in a cylindrical cavity specially designed on the bottom surface of the tibial insert; 
         FIG. 24 , shows a “wave” shape according to an embodiment of the present invention that may be obtained by the revolution of a spherical tool around an axis which is essentially collinear to the neutral axis of a tibial tray hole designed to receive a central peg; 
         FIG. 25  shows that by using a conventional tight gap between a central peg and a hole, excess friction can cause binding over time; 
         FIG. 26  shows that in comparison, for example, with the prosthesis of  FIG. 25 , a prosthesis according to an embodiment of the present invention may provide a larger gap between a central peg and a hole to help avoid any tibial insert-tray binding; 
         FIGS. 27A-27C  show the location of the contact between the tibial components for a conventional MBK associated with a flat second bearing; 
         FIG. 28  shows that in various embodiments of the present invention the location of the contact between the tibial components occurs along the deepest line of the second bearing; 
         FIGS. 29A and 29B  show that (for a conventional MBK associated with a flat second bearing) the shear component of eccentric load is transmitted to the central peg ( FIG. 29A ), which creates high stress on the peg due to the low contact area between the central peg and the hole ( FIG. 29B ). 
         FIG. 30  shows that in comparison, for example, with  FIGS. 29A and 29B , until about 30° the shear component of the joint load (in one example of the present invention) is borne by the internal curvature of the wave; 
         FIGS. 31 and 32  show a test specimen of a prosthesis according to an embodiment of the present invention after the completion (of about 5 million cycles) of a wear test; 
         FIG. 33  shows that for a conventional MBK associated with a flat second bearing, the thickness of the tibial insert is typically thinnest at the site of the maximum load; and 
         FIG. 34  shows that in contrast (for example, with  FIG. 33 ) the thickness of the tibial insert according to an embodiment of the present invention is essentially constant along the medio-lateral axis. 
     
    
    
     Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying figures. The figures constitute a part of this specification and include illustrative embodiments of the present invention and illustrate various objects and features thereof. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely illustrative of the invention that may be embodied in various forms. In addition, each of the examples given in connection with the various embodiments of the invention are intended to be illustrative, and not restrictive. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     In one embodiment a mobile bearing knee prosthesis may include an interface (e.g., a spherical radius interface) comprised of a concave superior surface on a tibial tray and a convex inferior surface on a tibial insert. In another embodiment a mobile bearing knee prosthesis may include an interface (e.g., a spherical radius interface) comprised of a convex superior surface on a tibial tray and a concave inferior surface on a tibial insert. Of note, such a spherical radius may have an inherent tendency to self-align. 
     In one example (which example is intended to be illustrative and not restrictive) the tibial insert may include a polyethylene articulating surface. In another example (which example is intended to be illustrative and not restrictive) the tibial tray may include a metal articulating surface (e.g., a highly polished metal articulating surface). In another example (which example is intended to be illustrative and not restrictive) one or both of the articulating surfaces may include diamond (e.g., to improve wear characteristics on one or more mating surfaces). 
     In another embodiment of a mobile bearing knee prosthesis the interface may have a pivoting location. In one example (which example is intended to be illustrative and not restrictive) the pivoting location may be defined by a female feature (e.g., cylinder, cone or combination) that mates with a male feature (e.g., a post). The pivoting location may be in the center of the interface or the pivoting location may be offset from the center of the interface in one or more of a medial, lateral, anterior and/or posterior directions. 
     In another embodiment of a mobile bearing knee prosthesis a locking feature may be provided to help prevent lift-off of the tibial insert. In one example (which example is intended to be illustrative and not restrictive) the locking feature may be provided by a male feature (e.g., a post) working in combination with a female feature (e.g., cylinder, cone or combination) to help prevent lift-off of an articulating surface (e.g., a polyethylene articulating surface). 
     In another embodiment of a mobile bearing knee prosthesis anterior/posterior translation and/or medial/lateral translation may be provided by utilizing a female feature (e.g., cylinder, cone or combination) which is enlarged to allow for additional movement in one or more desired planes. 
     In another embodiment of a mobile bearing knee prosthesis rotational constraint and/or control may be provided by medial and/or lateral rails that interfere and/or wedge with a tibial insert as the tibial insert rotates to a specific angular displacement (the interference and/or wedging may occur at one or both rails). Further, to aid in containment of the tibial insert, a groove may be provided in one or both rails and a mating feature may be provided on the tibial insert. 
     In another embodiment of a mobile bearing knee prosthesis rotational constraint and/or control may be provided by using a male feature (e.g., a post) as a spring (e.g., a torsion spring) such that a constraining member (e.g., a cross-pin) can be inserted into a receiving member (e.g., a V-groove) in the male feature. 
     In another embodiment of a mobile bearing knee prosthesis rotational constraint and/or control may be provided by using an ellipsoid surface at the rotational interface. 
     Referring now to  FIGS. 1A-1E , Mobile Bearing Knee Prosthesis  100  may include Tibial Tray  102 , Tibial Insert  104  and Femoral Component (not shown) which interfaces with Tibial Insert  104 . 
     In one example (which example is intended to be illustrative and not restrictive) Mobile Bearing Knee Prosthesis  100  may include an interface (e.g., a spherical radius interface) comprised of a concave superior surface on the Tibial Tray  102  and a convex inferior surface on the Tibial Insert  104  (of note, such a spherical radius may have an inherent tendency to self-align). 
     In another example (which example is intended to be illustrative and not restrictive) the Tibial Insert  104  may include a polyethylene articulating surface. In another example (which example is intended to be illustrative and not restrictive) the Tibial Insert  102  may include a metal articulating surface (e.g., a highly polished metal articulating surface). 
     Mobile Bearing Knee Prosthesis  100  may have a pivoting location. In one example (which example is intended to be illustrative and not restrictive) the pivoting location may be defined by Cavity  106  that mates with Post  108 . Post  108  may stabilize Mobile Bearing Knee Prosthesis  100  against shear forces (e.g., medial/lateral forces in the transverse plane) as well as serve as a rotational axis. 
     Mobile Bearing Knee Prosthesis  100  may include locking feature(s) to help prevent lift-off of the Tibial Insert  104 . In one example (which example is intended to be illustrative and not restrictive) the locking feature may be provided by Indentation  110  (disposed within Cavity  106 ) working in conjunction with Raised Portion  112  (disposed on Post  108 ) (see, for example,  FIG. 1E ). 
     Further, Mobile Bearing Knee Prosthesis  100  may provide anterior/posterior translation and/or medial/lateral translation (e.g., by utilizing Cavity  106  which is enlarged to allow for additional movement in one or more desired planes). In one example (which example is intended to be illustrative and not restrictive) A/P translation may be about 4.5 mm. 
     Referring now to  FIGS. 2A-2C , it is seen that the pivoting location may be placed where desired. For example (which example is intended to be illustrative and not restrictive) the pivoting location may be in the center (denoted by the dashed circle “A”), anterior (denoted by the dashed circle “B”), or posterior (denoted by the dashed circle “C”) of Mobile Bearing Knee Prosthesis  200  (which may include Tibial Tray  202  and Tibial Insert  204 ). Of note,  FIGS. 2A-2C  show a mobile bearing knee prosthesis similar to that shown in  FIGS. 1A-1E  but without the Indentation/Raised Portion lift-off prevention mechanism. Of further note, it is believed that moving the pivoting location towards the posterior will tend to minimize moments on Post  208 . In one example (which example is intended to be illustrative and not restrictive) rotational limits may be between about 50-53 degrees. 
     Referring now to  FIG. 3 , Mobile Bearing Knee Prosthesis  300  may include Tibial Tray  302 , Tibial Insert  304  and Femoral Component (not shown) which interfaces with Tibial Insert  304 . Mobile Bearing Knee Prosthesis  300  may include one or more diamond bearing surfaces  310  on an articulating surface of Tibial Tray  302 , on an articulating surface of Tibial Insert  304 , in Cavity  306  and/or on Post  308 . In this regard, isolation of the articulating surface of Tibial Insert  304  (e.g., the polyethylene surface) from the articulating surface of Tibial Tray  302  with the highly wear-resistant diamond bearing surface(s)  310  helps avoid the problem of backside wear typically inherent in conventional mobile bearing knee prostheses. 
     Referring now to  FIGS. 4A-4D , a mobile bearing knee prosthesis may include Tibial Tray  402 , Tibial Insert  404  and Femoral Component (not shown) which interfaces with Tibial Insert  404 . The mobile bearing knee prosthesis may include diamond bearing surface(s)  410  on an articulating surface of Tibial Tray  402 , on an articulating surface of Tibial Insert  404 , in Cavity  406  and/or on Post  408 . In one example (which example is intended to be illustrative and not restrictive) 3-point contact associated with the diamond bearing surface(s)  410  may establish a plane. In another example (which example is intended to be illustrative and not restrictive) one or more of the diamond bearing surface(s)  410  (e.g., the diamond bearing surface(s)  410  on Tibial Insert  404 ) may be spherical or hemi-spherical in shape (e.g., to avoid or attenuate edge loading). In another example (which example is intended to be illustrative and not restrictive) one or more of the diamond bearing surface(s)  410  may be press-fit. 
     In another example a rotary stop mechanism may be provided to help ensure that the diamond bearing surface(s) (e.g., the posterior, medial and lateral diamond bearing surface(s)) remain engaged at all times. In another example (which example is intended to be illustrative and not restrictive) this rotary stop mechanism may be diamond against diamond. In this regard, see  FIGS. 5A and 5B , where the most anterior diamond bearing of Tibial Tray  502 , for example, is elevated (to cause the medial and lateral diamond bearings on the underside of the Tibial Insert (not shown) to abut and constrain rotary motion). 
     Referring now to  FIGS. 6A and 6B , a Tibial Tray  602  for a mobile bearing knee prosthesis may include a large diameter surface, such as a spherical surface (a Tibial Insert (not shown) may have a mating large diameter surface, such as a spherical surface, on a backside thereof). The Tibial Tray  602  may include Plane Surface  602   a  (which Plane Surface  602   a  is essentially flat). In one example (which example is intended to be illustrative and not restrictive) Plane Surface  602   a  may be a polyethylene component (e.g., a molded “puck”). In a further example (which example is intended to be illustrative and not restrictive) the areas designated “A” in  FIG. 6A  may maintain a high contact area. 
     Referring now to  FIGS. 7A-7C , Mobile Bearing Knee Prosthesis  700  may include Tibial Tray  702 , Tibial Insert  704  and Femoral Component (not shown) which interfaces with Tibial Insert  704 . In one example (which example is intended to be illustrative and not restrictive) Tibial Tray  702  may have a concave articulating surface and Tibial Insert  704  may have a convex articulating surface. The aforementioned articulating surfaces may comprise a large radius sphere (e.g., for backside articulation of a rotating/mobile prosthesis). 
     In another example (which example is intended to be illustrative and not restrictive) there may be a tighter clearance at the area designated “A” in  FIG. 7B  then there is at the area designated “B” in  FIG. 7B . 
     Referring now to  FIGS. 8A-8C  an embodiment adapted to aid in rotational constraint and/or control is shown. 
     More particularly, Mobile Bearing Knee Prosthesis  800  may include Tibial Tray  802 , Tibial Insert  804  and Femoral Component (not shown) which interfaces with Tibial Insert  804 . Mobile Bearing Knee Prosthesis  800  may have a pivoting location defined by Cavity  806  that mates with Post  808 . Further, Cross-pin  806 A may mate with Groove  808   a  such that during rotation of Tibial Insert  804  the Cross-pin  806   a  acts as a rotational stop and Post  808  acts as a spring (i.e., a torsion spring to give resistance to rotation). 
     In one example (which example is intended to be illustrative and not restrictive) the diameter of Cross-pin  806   a  and/or the size of Groove  808   a  may be varied to provide different levels of rotational constraint and/or control. 
     In another example (which example is intended to be illustrative and not restrictive) the Cross-pin  806   a  may be installed prior to implantation of Tibial Tray  802  (whereby Groove  808   a  allows Tibial Insert  804  to be installed with Tibial Tray  802  in place in the body (e.g., cemented in place). 
     Referring now to  FIGS. 9A-9D  and  10 A- 10 E various additional embodiments adapted to aid in rotational constraint and/or control and/or to help prevent tibial insert lift-off are shown. 
     More particularly, as seen in  FIGS. 9A-9C , when Tibial Insert  904  is rotated it contacts Rotation Limiting Tabs  902   a  of Tibial Tray  902  (to thereby wedge Tibial Insert  904  in place and inhibit further rotation). 
     Further, each Rotation Limiting Tab  902   a  may include Undercut  902   b  to help prevent lift-off when extremes of rotation have been reached (In this regard, Tibial Insert  904  may include one or more Lips  904   a  for engaging Rotational Limiting Tabs  902   a  and/or Undercuts  902   b ). 
     Further still, Tibial Insert  904  may include Post  908  which resides in Cavity  910  in Tibial Tray  902 , whereby Cavity  910  includes Indentation  912  for receiving Raised Portion  914  of Post  908 . Indentation  912  and Raised Portion  914  may thus cooperate to help prevent lift-off of Tibial Insert  904 . In one example (which example is intended to be illustrative and not restrictive) the running clearance between Post  908  and Cavity  910  may be between about 0.005 and 0.010 inches. 
     Further still,  FIGS. 10A and 10B  show a distance “A” inside an outer wall section of Tibial Tray  1002 ;  FIG. 10C  shows distances B 1 , B 2  and C associated with Tibial Insert  1004  (wherein distance B 1  and B 2  are greater than distance A and distance C is less than distance A); and  FIG. 10D  shows contact points between Tibial Tray  1002  and Tibial Insert  1004  when Tibial Insert  1004  is rotated (in the clockwise direction in this example). 
     Further still,  FIG. 10E  shows detail of the interference between Tibial Tray  1002  and Tibial Insert  1004  at a contact point of  FIG. 10D  and  FIG. 10F  shows that there is no interference at the contact point of  FIG. 10E  when the Tibial Insert  1004  is not rotated past a certain point (e.g., at a “neutral position”). Of note,  FIGS. 10E and 10F  also show Recess  1002   a , which may be used for example for poly flow and/or to aid in preventing lift-off. 
     Referring now to  FIG. 11 , it is seen that Mobile Bearing Knee  1100  according to an embodiment of the present invention may include Tibial Tray  1102  and Tibial Insert  1104 , wherein the rotational axis A of Mobile Bearing Knee  1100  may be placed in-line with the natural axis A′ of the knee. 
     Referring now to  FIGS. 12A and 12B  it is seen that Mobile Bearing Knee  1200  according to an embodiment of the present invention may include Tibial Tray  1202  and Tibial Insert  1204 . Of note, the design of these  FIGS. 12A and 12B  allows retention of the posterior cruciate ligament (PCL) via clearance for the PCL (which does not require posterior stabilization offered with the PS spine (e.g., as may be required on certain other embodiments)). 
     In another embodiment the tibial insert may be made of Ultra High Molecular Weight Polyethylene (“UHMWPE”). In one example (which example is intended to be illustrative and not restrictive) the UHMWPE may be molded UHMWPE (which, it is believed, wears at a lower rate than machined UHMWPE). 
     Referring now to  FIGS. 13A-13F ,  14 A- 14 D,  15 A- 15 C,  16 A,  16 B and  17 A- 17 C, it is noted that under these embodiments of the present invention a mobile bearing knee prosthesis may include a bi-concave interface. 
     In this regard, it is noted that such a bi-concave interface may aid in providing an optimal anatomic configuration of the knee while at the same time providing a sufficiently thick (e.g., in terms of wear resistance) tibial insert articulation structure (e.g., polyethylene articulation structure). 
     In one example (which example is intended to be illustrative and not restrictive), such articulation structure may be about 6.5 mm thick. 
     Further, referring in particular to  FIG. 17A , it is seen that the tibial insert articulation structure may have a homogeneous, or constant, thickness (i.e., thickness “X” in this  FIG. 17A ) and referring in particular to  FIG. 17B , it is seen that the tibial insert articulation structure may have a non-homogeneous, or non-constant, thickness (e.g., thicker by “Y” at the area marked “A” and “B” in this  FIG. 17B ). 
     Further still, such a bi-concave interface may aid in coping with the potential shear stress provided by lift-off during movement by the patient (see  FIG. 17C , showing an aspect of the invention directed to self-centering against lift-off and reduction or elimination of shear stress to the pivot feature (e.g., axial post)). 
     Of still further note, the embodiments of these  FIGS. 13A-13F ,  14 A- 14 D,  15 A- 15 C,  16 A,  16 B and  17 A- 17 C may, of course, include some or all of the various pivoting, translating, locking, rotational constraint and/or control features described above. 
     Referring now to  FIGS. 18A and 18B , a mobile bearing knee prosthesis according to an embodiment of the present invention may include an interface (e.g., a spherical radius interface) comprised of a convex superior surface on the tibial tray and a concave inferior surface on the tibial insert (of note, such a spherical radius may have an inherent tendency to self-align). 
     Of note, the aforementioned configuration may help reduce wear at the interface between the tibial insert and the tibial tray by ejecting abrasive material (e.g., polyethylene particles created by relative movement at the interface) out from the interface (see  FIG. 18A ). 
     Of further note, as seen in  FIG. 18B , the thickness of the material forming the tibial insert may vary as required (e.g., for optimum wear resistance vs. ease of movement). In one example (which example is intended to be illustrative and not restrictive), the areas marked “A” and “B” may be thicker than the area marked “C”. 
     Of still further note, the embodiments of these  FIGS. 18A and 18B  may, of course, include some or all of the various pivoting, translating, locking, rotational constraint and/or control features described above. 
     Referring now to  FIGS. 19A and 19B , it is again noted that a mobile bearing knee prosthesis according to the present invention may incorporate an eccentered (or offset) pivoting feature (e.g., axial post). More particularly, in one embodiment such an eccentered pivoting feature may serve (e.g., during movement by the patient) to decrease the anterior translation associated with the medial condolyte and increase the roll back (posterior translation) associated with the lateral condolyte. In this regard, see, for example,  FIGS. 19A and 19B , where  FIG. 19A  shows the large anterior translation associated with a central pivot (e.g., at 70° of external rotation of the femur in relation to the tibia) and where  FIG. 19B  shows the smaller anterior translation associated with a medially offset pivot (e.g., at 70° of external rotation of the femur in relation to the tibia). More particularly, these  FIGS. 19A and 19B  show that the offset pivot results in a relatively smaller anterior translation associated with the medial condolyte and a relatively larger posterior translation associated with the lateral condolyte. 
     Further, it is noted that the embodiments of these  FIGS. 19A and 19B  may, of course, include some or all of the various pivoting, translating, locking, rotational constraint and/or control features described above. 
     Referring now to  FIGS. 20A-20E , it is noted that certain embodiments of the present invention relate to use of an offset pivot component (e.g., using an offset axial post) in association with other components which may otherwise be configured for use with a non-offset pivot component. 
     For example (which example is intended to be illustrative and not restrictive), an asymmetric component (e.g., offset axial post) may be utilized in association with a symmetric tibial tray and symmetric bearing (e.g., polyethylene bearing) to operate on a “cam” concept.  FIG. 20A  shows a plan view of a polyethylene bearing (under a no rotation condition) according to this embodiment and  FIG. 20B  shows a plan view of a tibial tray (under a no rotation condition) according to this embodiment. Further,  FIG. 20C  shows the eccentric center of rotation (at point A) associated with external rotation (right knee) and  FIG. 20D  shows the eccentric center of rotation (at point B) associated with external rotation (left knee). 
     Further,  FIG. 20E  shows a plan view utilizing a “Beam” shape. 
     Further still, it is noted that the embodiments of these  FIGS. 20A-20E  may include one or more internal stop mechanisms (e.g., for rotational constraint and/or control). Moreover, the embodiments of these  FIGS. 20A-20E  may be used in connection with a tibial insert and/or a tibial tray having an interface surface which includes flat, concave and/or convex portions. 
     Further still, it is noted that the embodiments of these  FIGS. 20A-20E  may, of course, include some or all of the various pivoting, translating, locking, rotational constraint and/or control features described above. 
     Of note, for the same size of knee prosthesis (e.g., size 3), the contact area between the tibial insert and the tibial tray may be higher for a “wave” design than for a flat design (e.g., ten percent higher contact area). Under certain circumstances, it may be desired to minimize this contact area. 
     Thus, in one embodiment, this contact area between the tibial insert and the tibial tray may be minimized by reducing the congruence factor of the second bearing. It is noted that this solution is not possible for a flat design, for which the congruence factor is always equal to one. It is further noted that an advantage of the congruence factor approach is when contact only occurs on the loaded area. 
     In another embodiment, wear due to contact between the post and the hole may be decreased because shear stress is absorbed by the tibial tray (e.g., by the medial part of the “wave”). 
     In another embodiment, a mobile bearing knee prosthesis is provided, comprising: a tibial tray for interfacing with a tibia of a patient; and a tibial insert disposed adjacent the tibial tray; wherein the tibial insert is capable of movement relative to the tibial tray and the movement of the tibial insert relative to the tibial tray includes at least pivotal movement; wherein the pivotal movement is around an axis of rotation defined by a post extending from a bottom surface of the tibial insert into a hole provided on an upper surface of the tibial tray; wherein the hole provided on the upper surface of the tibial tray is provided at a substantially circular raised location; wherein the substantially circular raised location curves downward away from the hole along both a medial-lateral axis of the tibial tray and an anterior-posterior axis of the tibial tray; wherein at least a portion of a medial edge of the tibial tray and at least a portion of a lateral edge of the tibial tray curve upward as the medial edge and the lateral edge are approached; and wherein the bottom surface of the tibial insert is configured to be substantially complementary to the top surface of the tibial tray when the tibial insert and the tibial tray are aligned in both the medial-lateral axis and the anterior-posterior axis. 
     In one example, an upper surface of the tibial insert may be configured to receive a femoral component which interfaces with a femur of the patient. 
     In another example, a mechanism for providing at least one of rotational control and rotational constraint to movement of the tibial insert relative to the tibial tray may be provided. 
     In another example, the mechanism for providing at least one of rotational control and rotational constraint to movement of the tibial insert relative to the tibial tray may comprise a cross-pin disposed within the hole and a groove disposed on the post, wherein the cross-pin is received within the groove. 
     In another example, the mechanism for providing at least one of rotational control and rotational constraint to movement of the tibial insert relative to the tibial tray may comprise at least a first tab disposed along the medial edge of the tibial tray and at least a second tab disposed along the lateral edge of the tibial tray, wherein the first tab contacts the tibial insert to provide at least one of the rotational control and the rotational constraint during rotation in a first direction and the second tab contacts the tibial insert to provide at least one of the rotational control and the rotational constraint during rotation in a second direction. 
     In another example, the first tab may include a first undercut for receiving a first edge of the tibial insert and the second tab may include a second undercut for receiving a second edge of the tibial insert. 
     In another example, the mobile bearing knee prosthesis may further comprise a mechanism for substantially preventing lift-off of the tibial insert from the tibial tray. 
     In another example, the mechanism for substantially preventing lift-off of the tibial insert from the tibial tray may comprise a raised portion on the post cooperating with an indentation in a portion of the tibial tray defining the hole therein. 
     In another example, the mechanism for substantially preventing lift-off of the tibial insert from the tibial tray may comprise at least a first tab disposed along the medial edge of the tibial tray and at least a second tab disposed along the lateral edge of the tibial tray, wherein the first tab includes a first undercut for receiving a first edge of the tibial insert and the second tab includes a second undercut for receiving a second edge of the tibial insert. 
     In another example, at least one of the hole and the post may be substantially circular. 
     In another example, the hole and the raised location may be disposed at a substantially central location along the medial-lateral axis of the tibial tray. 
     In another example, the hole and the raised location may be disposed at a substantially central location along the anterior-posterior axis of the tibial tray. 
     In another example, the upper surface of the tibial tray may comprise metal and the lower surface of the tibial insert may comprise polyethelene. 
     In another example, the tibial tray may be formed of metal and the tibial insert may be formed of polyethelene. 
     In another embodiment, a mobile bearing knee prosthesis is provided, comprising: a tibial tray for interfacing with a tibia of a patient; and a tibial insert disposed adjacent the tibial tray; wherein the tibial insert is capable of movement relative to the tibial tray and the movement of the tibial insert relative to the tibial tray includes at least pivotal movement; wherein the pivotal movement is around an axis of rotation defined by a first cooperating element associated with a bottom surface of the tibial insert and a second cooperating element associated with an upper surface of the tibial tray; wherein the second cooperating element is provided at a substantially circular raised location on the upper surface of the tibial tray; wherein the substantially circular raised location curves downward away from the second cooperating element along both a medial-lateral axis of the tibial tray and an anterior-posterior axis of the tibial tray; wherein at least a portion of a medial edge of the tibial tray and at least a portion of a lateral edge of the tibial tray curve upward as the medial edge and the lateral edge are approached; and wherein the bottom surface of the tibial insert is configured to be substantially complementary to the top surface of the tibial tray when the tibial insert and the tibial tray are aligned in both the medial-lateral axis and the anterior-posterior axis. 
     In one example, an upper surface of the tibial insert may be configured to receive a femoral component which interfaces with a femur of the patient. 
     In another embodiment, a mobile bearing knee prosthesis is provided, comprising: a tibial tray for interfacing with a tibia of a patient; and a tibial insert disposed adjacent the tibial tray; wherein the tibial insert is capable of movement relative to the tibial tray and the movement of the tibial insert relative to the tibial tray includes at least pivotal movement; wherein the pivotal movement is around an axis of rotation provided by a first cooperating element associated with a bottom surface of the tibial insert and a second cooperating element associated with a top surface of the tibial tray; wherein the tibial tray includes at least a first diamond bearing surface; wherein the tibial insert includes at least a second diamond bearing surface; and wherein at least a portion of the first diamond bearing surface and at least a portion of the second diamond bearing surface bear against one another. 
     In one example, an upper surface of the tibial insert may be configured to receive a femoral component which interfaces with a femur of the patient. 
     In another example, the first diamond bearing surface may include a plurality of diamonds. 
     In another example, the second diamond bearing surface may include a plurality of diamonds. 
     In another example, the first diamond bearing surface may include a plurality of diamonds and the second diamond bearing surface may include a plurality of diamonds. 
     In another example, at least one of the diamonds of the first diamond bearing surface may extend beyond the other diamonds of the first diamond bearing surface such that the raised diamond cooperates with at least a portion of the second diamond bearing surface to provide rotational constraint to movement of the tibial insert relative to the tibial tray. 
     In another example, at least one of the diamonds of the second diamond bearing surface may extend beyond the other diamonds of the second diamond bearing surface such that the raised diamond cooperates with at least a portion of the first diamond bearing surface to provide rotational constraint to movement of the tibial insert relative to the tibial tray. 
     Reference will now be made to an artificial axis of rotation (“AAR”) concept, under which the shape of the second bearing creates a desired axis of rotation (e.g., one and only one axis of rotation). 
     More particularly, the following description applies to a mobile bearing knee prosthesis associated with one rotational degree of freedom between the tibial insert and tibial tray. The AAR may be used for tibial insert rotation relative to the tibial tray; may be substantially perpendicular to the bottom surface of the tibial tray; and may or may not be in a substantially central position of the tibial tray. Because the AAR of various embodiments is sufficient to guide the tibial insert in rotation relative to the tibial tray, a central peg may no longer be necessary.  FIGS. 22A-22D  show various examples (which examples are intended to be illustrative and not restrictive), of how the AAR concept may be applied such that the bottom surface of the tibial insert and the upper surface of the tibial tray provide a unique axis, which is characterized as the axis of revolution (or neutral axis) for these surfaces (such a unique axis may be contrasted, for example, with the spherical surface (concave or convex) of  FIG. 21 , which provides an infinite number of axes of revolution). 
     In any case, as seen in  FIGS. 22A-22D , various surfaces associated with an AAR may be obtained by rotating any desired geometric form around a desired axis of revolution. 
     More particularly, as seen in  FIG. 22A , the upper surface of the tibial tray may be obtained by rotating a circle around an axis (the geometry of this Fig. is named “wave”), but the center of the circle+ does not belong to the axis (if the center of the circle belongs to the axis, the result is  FIG. 21 ). 
     Further, as seen in  FIG. 22B , the upper surface of the tibial tray may be obtained by rotating a concave form around an axis, but the center of the circle of the form of this example does not belong to the axis (if it did, there would be no unique AAR—like depicted in  FIG. 21 ). 
     Further still, as seen in  FIG. 22C , the upper surface of the tibial tray may be obtained by rotating a free form around an axis. 
     Further still, as seen in  FIG. 22D , the upper surface of the tibial tray may be obtained by rotating different form around an axis. 
     As mentioned above, the unique axis of the examples of  FIGS. 22A-22D  represents the axis of revolution of the tibial insert relative to the tibial tray. In other words, a post and hole mechanism may be unnecessary (unlike the typical MBKs available on the market). This is because the shape of the tibial tray upper surface provides a unique rotational axis. Also, because the bottom surface of the tibial insert is configured to be substantially complementary to the upper surface of the tibial tray, both components self-align. 
     Of course, in another example, if a post and hole mechanism exists between the tibial insert and tibial tray (e.g., to prevent dislocation) the AAR could be parallel to the axis of revolution of the post or hole. 
     Further, in another example, if a post and hole mechanism exists between the tibial insert and tibial tray (e.g., to prevent dislocation), the AAR may or may not be collinear to the axis of revolution of the post or hole. 
     Reference will now be made to a discussion of various advantages of the AAR concept (e.g., as provided by a constraint shape second bearing). The examples used to illustrate some of these advantages are obtained by the revolution of a spherical part around an axis, which is co-linear with the axis of a central peg, to the extent used (of course, these examples are intended to be illustrative and not restrictive). 
     The aforementioned advantages will be explained based on three points: (a) Self centered design; (b) Predictable kinematics; and (c) Low wear expectation. 
     Referring first to self-centered design, it is noted that a conventional MBK typically uses a flat second bearing. As a consequence of this, the constraint between the tibial insert and the tibial tray is typically provided by a rotational guide device. The following are two conventional designs for this rotational guide device: (a) UHMWPE central peg inside a tibial tray hole (so-called “cone-in-cone” design), such as the LCS or ΣRP from DePuy; and (b) tibial tray post inside a cavity specially designed in the UHMWPE tibial insert, such as LPS mobile bearing knee from Zimmer, Rotaglide from Corin ( FIG. 23  shows an example of such a conventional tibial tray post that engages in a cylindrical cavity specially designed on the bottom surface of the UHMWPE tibial insert). 
     Typically, such a rotational guide device covers two functions. First of all, this device defines a fixed axis of rotation for the tibial insert relative to the tibial tray. In addition, it provides an anti-dislocation function in case of large subluxation or distraction. 
     However, there is typically a tradeoff required in performing these two functions. That is, the “cone-in-cone” design is believed to be primarily a safety anti-dislocation system (due to its length), but because the second bearing is flat, essentially all the shear forces are absorbed by the central peg (which increases UHMWPE wear and fracture risk). On the other hand, the tibial tray post solution is believed to offer a safer environment for the UHMWPE wear (e.g., less risk of breakage), but results in a poor anti-dislocation device (due to the short length of the post). 
     Thus, under various embodiments of the present invention, a unique constraint second bearing is provided (which differs significantly from the conventional flat second bearing). That is (as discussed above), the second bearing shape according to this embodiment of the present invention could be described as a wave formed by the revolution of a spherical tool around an axis, wherein the axis may be collinear to an axis of a tibial tray hole designed to receive a central peg of a UHMWPE tibial insert (as seen in  FIG. 24 , a “wave” may be obtained by the revolution of the spherical tool around an axis which is collinear to the neutral axis of the tibial tray hole designed to receive the UHMWPE central peg). 
     One benefit of this characteristic is that the tibial insert rotates around the AAR provided by the wav shape and not around the neutral axis of the tibial tray hole. 
     Given this, the UHMWPE central peg may be designed only to prevent dislocation in cases of severe subluxation or distraction (such that during common activities, essentially no shear stress is anticipated on the central peg, so the peg diameter may be smaller than in conventional “cone-in-cone” designs). Further, the self-centering design of the second bearing may provide an artificial axis of rotation distinct from the UHMWPE central peg axis, which allows the tibial insert to rotate relative the tibial tray. 
     Referring now to predictable kinematics (e.g., in the context of predictable rotation), it is noted (as stated previously) that conventional MBKs are typically associated with a flat second bearing. Because a flat second bearing provides essentially no constraint in the transverse plane, the gap of the rotational guide device (e.g., “cone-in cone” design or tibial tray post) is typically relatively tight in order to avoid any unwanted motion in the transverse plane, under the action of the shear forces (see  FIG. 25 , showing that by using a typically tight gap between the central peg and the hole, excess friction can cause binding over time). 
     In this regard, in-vivo, the UHMWPE could absorb fluid over time. This absorption might increase the UHMWPE central peg diameter (in “cone-in-cone” designs) or decrease the cavity diameter (in tibial tray post designs). In this case, due to the tightened gap between the components, the friction between the tibial components will increase and could lead to tibial insert-tibial tray binding (see again,  FIG. 25 ). 
     Of course, a purpose of a MBK is to allow tibial insert mobility relative to the tibial tray during surgery and postoperatively. This may even be a critical feature of conventional MBKs, because these prostheses typically offer a highly congruent first bearing. As a result, when the tibial insert binds with the tibial tray, the torque is transmitted more readily to the implant-bone interface, increasing the potential for implant loosening. 
     Because of the self-centering design of the second bearing provided by various AAR embodiments of the present invention, the tibial insert rotates around the artificial axis of revolution created by the wave surface. One benefit to this design is that the UHMWPE central peg of the tibial insert (if such central peg is even used) may be essentially limited to the anti-dislocation function, so the gap between the tibial insert central peg and tibial tray hole may be slightly increased to help avoid risk of tibial insert-tibial tray binding over time (see  FIG. 26 , which shows that in comparison with typical conventional MBKs with flat second bearings, the prosthesis according to an embodiment of the present invention may provide a larger gap between the central peg and the hole (to the extent they are used) to help avoid tibial insert-tibial tray binding. 
     Referring now to predictable kinematics (e.g., in the context of predictable torque), it is noted that certain in-vivo fluoroscopic data suggested that the kinematics at the second bearing for a large subset (˜50%) of patients might not have had the intended motion. One potential reason is that the maximum resistance torque needed to initiate rotation at the second bearing in these patients is higher than the range of peak torques developed in a TKA patient. 
     With conventional MBKs (e.g., associated with a flat second bearing), the resistance torque is typically substantially dependent on the surface contact between the tibial insert and the tibial tray. In these devices, it appears that the location of the surface contact seems essentially dependent on the flatness of the tibial surfaces in contact (see  FIGS. 27A-27C , showing the location of the contact between the tibial components for a conventional MBK associated with a flat second bearing—the contact could be medial (i.e. near the central peg) as in  27 A or be lateral as in  27 B;  FIG. 27C  is an illustration of a conventional MBK with a flat second bearing showing a very lateral contact between the tibial insert and the tibial tray.). 
     More particularly, a prosthesis showing medial contact ( FIG. 27A ) will require significantly less torque to initiate rotation at the second bearing than one with lateral contact ( FIG. 27B ). When the test specimen shows significant lateral contact ( FIG. 27C ), the torque defined by ISO 14243-1 (i.e. 6 N.m) is inferior to the resistance torque. As a result, no rotation occurs at the second bearing during the kinematic test. 
     Due, for example, to the wave shape and an appropriate congruency ratio between the tibial components according to the an embodiment of the present invention, the surface contact between the tibial insert and tray occurs along the deepest line of the wave (see  FIG. 28 , showing that in this embodiment of the present invention the location of the contact between the tibial components occurs along the deepest line of the second bearing—because this location is predictable, the resisting torque is also predictable.). 
     Thus, at least one advantage of this “controlled” surface contact location is predictable resistance torque (this has been well demonstrated by a wear test, as all three test specimens showed similar kinematics in response to tibial rotational torque—a similar observation resulted from an analysis of the tibial inserts according to an embodiment of the present invention after an in-vitro test; both tibial inserts showed essentially identical contact area on the second bearing). 
     Referring now to low wear expectation, it is noted that with the conventional MBK, associated with a flat second bearing, the shear component of the eccentric joint loading has a considerable effect on the wear. In fact, because the second bearing is flat, the shear component of the load is totally transmitted to the central peg causing high peg stresses, and potential source of wear (see  FIGS. 29A and 29B , showing (in connection with a conventional MBK associated with a flat second bearing) that the shear component of eccentric load is transmitted to the central peg ( FIG. 29A ), which creates high stress on the peg due to the low contact area between the central peg and the hole ( FIG. 29B ). 
     In contrast, because of the wave shape of various embodiments of the present invention, which wave shape absorbs the shear component of the joint load (e.g., until about 30° of valgus-varus angle) on its internal curvature, essentially no shear stress is transmitted to a central peg (to the extent that such central peg is even used). In this regard, see  FIG. 30 , showing that in this example (which example is intended to be illustrative and not restrictive), until about 30°, the shear component of the joint load is borne by the internal curvature of the wave. There are at least two advantages to this—a potential source of wear is essentially eliminated, and the central peg diameter may be significantly reduced (to the extent that a central peg is even used), because essentially no load is anticipated on the peg. A smaller UHMWPE central peg means that the cavity in the tibia for the stem is also smaller, which may help preserve the cancellous bone stock. 
     In fact, this advantage has been shown by a wear test performed by Endolab (DE) according to the standard ISO14243-1. For this, the total knee joint prosthesis according to an embodiment of the present invention was mounted in an apparatus, which applied a cyclic variation of flexion/extension angle and contact force to the interface between tibial and femoral components, simulating normal human walking. The tibia component was free to move relative to the femoral component under the influence of the applied contact forces, this motion having all degrees of freedom except for the flexion/extension angle, which follows a specified cyclic variation. 
     The applied contact force actions were:
         Axial force   Anterior-posterior force   Tibial rotation torque       

     Line of action of the axial force was offset by 0.07*W in the medial direction from the tibial axis, where W was the overall width of the tibial component. 
     Analysis of the test specimens after the completion (of about 5 million cycles) of the wear test confirmed the initial hypothesis: no evidence of wear showed on a central peg (see  FIG. 31 ), this is in contrast to a conventional MBK associated with a flat second bearing (see  FIG. 29B ). 
     According to another aspect of the wear test specimens analyzed, it was interesting to verify that the contact between the tibial insert and the tibial tray occurred along the deepest line of the wave, as was expected. The shear stress created by the anterior-posterior force during the wear test was absorbed by the internal curvature of the posterior portion of the tibial tray (see FIG.  32 —the polished area is well defined within the expected region of the wave; the shear stress (due to the anterior-posterior force) was absorbed by the internal curvature of the wave (on the posterior portion) as shown in this Fig. by the stripped area). 
     Of course, it is well established that the contact stress decreases with the thickness of the tibial insert: thicker is the tibial insert, lower is the contact stress. In comparison with a conventional FBK, a conventional MBK typically has no out profile ring. In order to compensate this, the conventional MBK associated with a flat second bearing typically utilizes a thicker tibial tray. This increasing of thickness of the tibial tray is made at the detriment of the tibial insert thickness. In other words, for the same composite (i.e. tibial tray—tibial insert assembly) thickness, the conventional MBK tibial insert will be thinner than one associated with a conventional FBK. 
     Another drawback of the flat second bearing is the thickness of the tibial insert is typically thinnest at the site of the maximum load (see FIG.  33 —showing that for the conventional MBK associated with a flat second bearing, the thickness of the tibial insert is typically thinnest at the site of the maximum load.). 
     In contrast, due at least in part to the wave shape of various embodiments of the present invention, the thickness of the tibial insert is essentially constant along the medio-lateral axis (see FIG.  34 —showing that the thickness of the tibial insert of this example of the present invention is essentially constant along the medio-lateral axis (e.g., due at least in part to the wave shape) and that the thickness of the tibial insert of this example of the present invention is higher than typically found with a conventional MBK tibial insert, at the site of the maximum load). 
     In comparison with the typical conventional tibial trays associated with a flat second bearing, the tibial tray of this example of the present invention is thinner at the level of the deepest portion of the wave, and is thicker at the level of the internal and external curvatures of the wave. 
     Advantageously, the thicker portion of the tibial tray provides an increasing of the mechanical strength, while the thinner portion allows an increasing of the thickness of the tibial insert. This is well demonstrated in Table 1 by a comparison of the thickness distribution between one example of the present invention and a well-known conventional MBK currently available on the market (for a 10 mm composite assembly): 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Example of the 
               
               
                   
                 Thickness at the site of 
                 Conventional 
                 present 
               
               
                   
                 the maximum load (in mm) 
                 MBK 
                 invention 
               
               
                   
                   
               
             
            
               
                   
                 Tibial insert 
                 5.2 
                 6.5 (+25%) 
               
               
                   
                 Tibial tray 
                 4.8 
                 3.5 
               
            
           
           
               
               
               
               
            
               
                   
                 Thickness of the composite 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     Thus, as seen above, due at least in part to the wave shape, the thickness of the tibial insert of this example of the present invention is 25% higher than in a conventional MBK, while there is essentially no sacrifice in tibial tray strength. 
     In another example, the arbitrary shape may comprise a surface which includes a plurality of convex portions along the upper surface of the tibial tray. 
     In another example, the arbitrary shape may comprise a surface which includes a plurality of concave portions along the upper surface of the tibial tray. 
     In another example, the arbitrary shape may comprise a surface which has a stepped configuration along the upper surface of the tibial tray. 
     In another example, the arbitrary shape may comprise a surface which has a sawtooth configuration along the upper surface of the tibial tray. 
     In another example, the arbitrary shape may comprise a surface which is spherical along the upper surface of the tibial tray. 
     In another example the arbitrary shape may comprise a surface which is elliptical along the upper surface of the tibial tray. 
     While a number of embodiments of the present invention have been described, it is understood that these embodiments are illustrative only, and not restrictive, and that many modifications may become apparent to those of ordinary skill in the art. For example, one or more appropriate fasteners may be used to assemble the mobile bearing knee prosthesis of the present invention (e.g., a screw or bolt to hold the tibial insert in correct orientation relative to the tibial tray). Further, the mobile bearing knee prosthesis of the present invention may provide a bearing which predicts position, self-aligns, and/or self-centers. Further still, the metal may be polished using any desired technique (e.g., a drill with polishing compound). Further still, the tibial insert may be smaller than the tibial tray (at least in certain dimensions) to prevent overhang during rotation (this may be accomplished, for example, by reducing the size of the medial/lateral aspect of the tibial insert). Further still, one or more of the mating articulating surfaces may be formed of poly, metal, diamond (distinct pieces and/or “coating”), ceramic, polyether ether ketone (“PEEK”) and/or any other desired low friction articular materials. Further still, the tibial tray, the tibial insert and/or the femoral component may utilize, for example, a molded-on-metal configuration (e.g., UHMWP molded-on-metal). Further still, the tibial tray, the tibial insert and/or the femoral component may comprise, for example, cobalt chrome and/or titanium. Further still, the femoral component may interface with (e.g., be attached to) the femur of the patient using any desired mechanism (e.g., cement, one or more undercuts and matching protrusions, mechanical fasteners (e.g., screws), etc.). Further still, the tibial tray may interface with (e.g., be attached to) the tibia of the patient using any desired mechanism (e.g., cement, one or more undercuts and matching protrusions, mechanical fasteners (e.g., screws), etc.). Further still, one or more parts of the mobile bearing knee prosthesis according to the present invention may be used to “retrofit” existing prosthesis/components. Further still, the term “mobile bearing knee prosthesis” is, of course, intended to include (but not be limited to) “rotating platform” type mechanisms and “mensical bearing” type mechanisms.