Abstract:
A prosthetic device that may be utilized as an artificial meniscus is disclosed. The prosthetic device can restore shock absorption, stability, and function to the knee after the damaged natural meniscus is removed and replaced with the prosthetic device. In some embodiments, the meniscus includes an integral fixation anchor and additional features that minimize the requirement for modification of the implant for proper fit during surgery.

Description:
PRIORITY 
     This is a continuation application claiming priority to U.S. patent application Ser. No. 13/481,338 filed May 25, 2012, now U.S. Pat. No. 9,320,606, which claims priority to U.S. patent application Ser. No. 11/868,254 filed Oct. 5, 2007, now U.S. Pat. No. 8,192,491, which claims priority to U.S. Provisional Application No. 60/828,770 filed Oct. 9, 2006, each of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD/BACKGROUND 
     The present disclosure generally relates to medical prosthetic devices that replace the functionality of the natural meniscus. Each knee has two menisci, a lateral meniscus and a medial meniscus. Each meniscus is a crescent-shaped fibrocartilaginous tissue attached to the tibia at an anterior and a posterior horn. Damage to the meniscus can cause debilitating pain and arthritis. In some instances the prosthetic devices of the present disclosure are configured to be surgically implanted into a knee joint to replace the natural meniscus. 
     SUMMARY 
     In one embodiment, a meniscus prosthesis is disclosed. 
     In another embodiment, a meniscus prosthesis is disclosed that achieves good tribology by maintaining precise contact surfaces between the meniscus, the femoral condyle cartilage, and the tibial plateau cartilage. This is accomplished through the use of a hollow-like structure that is capable of the deformation necessary to accommodate the cartilage surfaces. The prosthesis can advantageously utilize modern material technology in a configuration that, in addition to its outstanding physical characteristics, significantly reduces the need for customization and fitting of the prosthesis during the implantation procedure. The meniscus prosthesis can be manufactured in a sufficient range of sizes to fit all applications. In some embodiments, the means for mechanical fixation of the implant to the tibial platform may be integrated into the meniscus body eliminating the requirement for the meniscus to be connected to an intermediary, separate fixation means such as a bone bridge. The fixation anchor configuration can utilize a keyhole cross section geometry that provides secure position control while minimizing lateral stresses that may result from the bone screw securing techniques of prior approaches. 
     In another embodiment, a meniscus prosthetic device comprises a semi-ellipsoidal solid body structure having top and bottom surfaces. The top surface is concavely shaped to mate with a surgically prepared femoral condyle and the bottom surface is shaped to mate with a surgically prepared tibial plateau. The top and bottom surfaces define a shelf comprising a membrane section that extends between the interior of the semi-elliptical walls of the body structure. The thickness of the membrane section may be less than approximately 2 millimeters and the height of the semi-elliptical walls may be less than approximately 15 millimeters. The cross section of the walls of the body structure functionally duplicates the nominal cross section of the natural meniscus. A fixation anchor may be integrated into and extend from the bottom surface of the prosthetic device. 
     In some embodiments, the fixation anchor comprises a keel having a keyhole shaped cross section. The anchor may extend substantially parallel to the bottom surface of the prosthetic device. The anchor may be in close proximity to the edge of the shelf in some embodiments. The keel may either continuously or discontinuously extend across the width of the bottom surface. In addition, the fixation anchor may comprise one or more tabs projecting from and perpendicular to the bottom surface. In a further embodiment, the meniscus may comprise a non-biologically derived material such as a pliable polyurethane based polymer. In another implementation, the meniscus may further include a deformation control element integrated into the meniscus that may, for example, be a filament wound into a machined undercut in the lower surface. In another embodiment, the bottom surface of the meniscus may be coated with a bioactive coating applied for the purpose of encouraging the in-growth of natural tissue into the meniscus. Such in-growth may improve fixation of the replacement meniscus to the tibial plateau. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other features and advantages of the present disclosure will become apparent in the following detailed description of embodiments of the disclosure with reference to the accompanying of drawings, of which: 
         FIG. 1  is a diagrammatic perspective view of an embodiment of a prosthetic device according to one embodiment of the present disclosure. 
         FIG. 2  is an alternative diagrammatic perspective view of the prosthetic device of  FIG. 1 . 
         FIG. 3  is an alternative diagrammatic perspective view of the prosthetic device of  FIGS. 1 and 2 . 
         FIG. 4  is an alternative diagrammatic perspective view of the prosthetic device of  FIGS. 1, 2, and 3 . 
         FIG. 5  is a diagrammatic cross-sectional view of the prosthetic device of  FIGS. 1, 2, 3 , and  4 . 
         FIG. 6  is a diagrammatic side view of an arrangement showing the prosthetic device of  FIGS. 1, 2, 3, and 4  inserted into a surgically prepared knee joint. 
         FIG. 7  is a diagrammatic front view of the arrangement of  FIG. 6 . 
         FIG. 8  is a diagrammatic cross-sectional view of a prosthetic device similar to  FIG. 5 , but showing an alternative embodiment. 
         FIG. 9  is a diagrammatic cross-sectional view of a prosthetic device similar to  FIGS. 5 and 8 , but showing an alternative embodiment. 
         FIG. 10  is a diagrammatic cross-sectional view of a prosthetic device similar to  FIGS. 5, 8, and 9 , but showing an alternative embodiment. 
         FIG. 11  is a diagrammatic cross-sectional view of a prosthetic device similar to  FIGS. 5, 8, 9, and 10  but showing an alternative embodiment. 
         FIG. 12  is a diagrammatic cross-sectional view of a prosthetic device similar to  FIGS. 5, 8, 9, 10, and 11 , but showing an alternative embodiment. 
         FIG. 13  is a diagrammatic cross-sectional view of a prosthetic device similar to  FIGS. 5, 8, 9, 10, 11, and 12  but showing an alternative embodiment. 
         FIG. 14  is a diagrammatic cross-sectional view of the prosthetic device of  FIG. 13  with an insert according to one embodiment of the present disclosure. 
         FIG. 15  is a diagrammatic cross-sectional view of a prosthetic device  FIG. 13  with an insert similar to  FIG. 14 , but showing an alternative embodiment of the insert. 
         FIG. 16  is a diagrammatic perspective view of a prosthetic device according to another embodiment of the present disclosure. 
         FIG. 17  is a diagrammatic cross-sectional view of the prosthetic device of  FIG. 16 . 
         FIG. 18  is a diagrammatic perspective view of a prosthetic device according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is intended. Any alterations and further modifications in the described devices, instruments, methods, and any further application of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. In particular, it is fully contemplated that the features, components, and/or steps described with respect to one embodiment may be combined with the features, components, and/or steps described with respect to other embodiments of the present disclosure. 
     Referring now to  FIGS. 1, 2, 3, 4, and 5  shown therein is a prosthetic device  10  according to one aspect of the present disclosure. In particular,  FIGS. 1, 2, 3, and 4  are various perspective views of the device  10 .  FIG. 5  is a cross-sectional view of the device  10 . Generally, the prosthetic device  10  is for the replacement of a meniscus that has been damaged, ruptured, disintegrated, diseased, or is otherwise in need of replacement. For illustrative purposes, the prosthetic device  10  will be described for use with a left knee, lateral meniscus replacement. However, corresponding embodiments may be utilized for replacement of any of the other menisci, such as the left knee medial meniscus, right knee lateral meniscus, and/or right knee medial meniscus. In that regard, the position, size, shape, and/or other properties of the fixation anchor may be configured for each particular application. Similarly, the size, shape, thickness, material properties, and/or other properties of the prosthetic device may be configured for each particular application. 
     The prosthetic meniscus  10  comprises an outer body portion  12 , a central body portion  14 , a fixation member  16 , and a fixation device  18 . Generally, the outer body portion  12  has an increased thickness and height relative to the central body portion  14 . In some instances the outer body portion  12  has a thickness between 5 mm and 15 mm. In some instances, the central body portion  14  has a thickness between 0.1 mm and 5 mm. In one particular embodiment, the outer body portion  12  has a thickness of approximately 10 mm and the central body portion  14  has a thickness of approximately 2 mm. Further, in some instances the outer body portion  12  has an increased stiffness relative to the central body portion  14 . As discussed in greater detail below, this increased stiffness may be a result of different material properties, geometries, support features, and/or other mechanisms for varying the stiffness between the central body portion  14  and the outer body portion  12 . 
     Generally, the central body portion  14  defines an upper articulation surface  20  and a lower fixation surface  22 . The fixation member  16  extends from the lower fixation surface  22 . The upper articulation surface  20  is bounded by the outer body portion  12  on several sides. In that regard, the outer body portion  12  comprises a rim or wall having an increased height relative to the central body portion  14  such that the central body portion is recessed with respect to the outer body portion. In the current embodiment, the outer body portion  12  defines a substantially convex upper surface  24  that tapers down in to the upper articulation surface  20  on one side and to an outer surface  26  of the prosthetic device  10  on the other side. Accordingly, the upper surface  20  of the central body portion  14  and the taper of the upper surface  24  of the outer body portion  12  define a concave recess configured for receiving a portion of the femur such as the femoral condyle. The outer body portion  12  has a semi-ellipsoidal shape in some embodiments. In one particular embodiment, the outer body portion  12  is shaped to substantially match the shape of a natural meniscus. 
     While the majority of the central body portion  14  is bounded by the outer body portion  12 , one side of the body portion  12  defines an edge or boundary  28 . In the current embodiment, the boundary  28  is a substantially planar surface having a thickness approximately equal to the thickness of the central body portion  14 . In other embodiments, the boundary  28  may have an increased thickness relative to the central body portion  14  (e.g., see  FIGS. 16 and 17 ). In some embodiments, the boundary  28  has a thickness greater than the central body portion  14 , but less than the outer body portion  12 . In one particular embodiment, the boundary  28  may have a thickness such that it extends above the upper surface  20  approximately one-half of the distance of the upper surface  24  of the outer body portion  12 . 
     As noted above, the fixation member  16  extends down from the lower surface  22  of the prosthetic device  10 . In the current embodiment, the fixation member  16  extends from the lower surface  22  adjacent to and substantially parallel to the boundary  28 . In other embodiments, the fixation member  16  may extend from other portions of the prosthetic device  10  and/or in other directions, including directions substantially perpendicular to the boundary  28  and/or oblique to the boundary  28 . Alternative positioning and orientations of the fixation member  16  are used to accommodate alternative surgical approaches, patient specific anatomical attributes, meniscus specific orientations, physician preference, and/or other factors. 
     In the current embodiment, the fixation member  16  comprises a keel structure having a first portion  30  extending directly from the lower surface  22  and a second portion  32  extending from the first portion  30 . The second portion  32  has an increased profile or thickness relative to the first portion  30 . In the current embodiment, the fixation member  16  has a keyhole cross section that can engage a complementary keyhole shaped groove that has been surgically incised a portion of the tibia, such as the tibia plateau, according to a keyhole surgical approach. In that regard, the fixation member  16  is configured to engage an opening extending substantially in a direction from the anterior to the posterior of the tibia. In other embodiments, the fixation member  16  may have other structural geometries to encourage engagement with the tibia. In one particular embodiment, the fixation member  16  comprises a dovetail configured to engage a dovetailed groove prepared in the tibia. 
     The fixation member  16  is manufactured as an integral part of the prosthetic device in some embodiments. In that regard, the fixation member  16  may be molded simultaneously with the other portions of the prosthetic device  10  and/or permanently attached to the other portions of the prosthetic device. Implementing the fixation member  16  as an integral component of the prosthetic device  10  provides several distinct advantages. First, the configuration and dimensions of the fixation member  16  are precisely controlled prior to the surgical procedure and may be maintained even for different size prosthetic devices  10 . Accordingly, a single size of standardized surgical tools, including bores, rasps and guides, etc., may be employed for the implantation of any size prosthetic device  10 . This results in a more precise engagement and mating between the prosthetic device  10  and the tibia, which improves the fixation properties and overall performance of the prosthetic device. Second, by implementing the fixation member  16  as an integrated section of the overall prosthetic device  10 , the possibility of separation of the fixation member  16  from the other portions of the prosthetic device  10  is virtually eliminated. Third, the keyhole and/or dovetail configurations of the fixation member  16  do not require the use of bone screws or other means to tighten or secure the fixation member  16  to the tibia. Accordingly, the integral fixation member  16  minimizes or eliminates the need to customize the fixation member, thereby reducing the time required to perform the implantation procedure. 
     In some embodiments, a fixation device  18  is utilized in combination with the fixation member  16  to secure the prosthetic device  10  to the tibia. It should be noted, however, that in other embodiments, the fixation member  16  is the sole fixation means utilized to secure the prosthetic device  10  to the tibia. The fixation device  18  extends from the bottom surface  22 . As shown, in some embodiments the fixation device  18  extends substantially perpendicularly to the bottom surface  22 . The fixation device  18  is utilized to provide further fixation of the prosthetic device  10  to the tibia. In that regard, in the current embodiment the fixation device  18  includes an opening  34  for receiving a fixation member. The fixation member may be a bone screw, staple, or other device configured to secure the prosthetic device  2  to the tibia through the opening  34 . In one particular embodiment, the fixation device  18  may be fastened to the tibia by means of a bone screw extending through the opening and securely engaging the tibia. 
     When the prosthetic device  10  is implanted and secured to the tibia, the central body portion  14  bounded by the outer body portion  12  serves to isolate the femoral condyle from the tibial plateau when implanted into a patient. In that regard, the outer body portion  12  serves to limit the movement of the femoral condyle relative to the prosthetic device. In particular, in the current embodiment the outer body portion  12  prevents the portion of the femur movingly engaged with the prosthetic device  10  from moving laterally beyond outer body portion. In other embodiments, the outer body portion  12  limits movement of the femur relative to the prosthetic device  10  in the medial direction. Further, the prosthetic device  10  provides shock absorption and a desirable tribology between the femur and tibia thereby attributing to the overall therapeutic value of the prosthesic device. 
     The prosthetic device  10  may be manufactured in various sizes, so that any given application can be satisfied by a “stock” unit. Accordingly, a surgeon could, during an implantation procedure, select a correctly sized device from the selection of stock units. Alternatively, in another embodiment, a replacement meniscus could be custom manufactured for a particular patient utilizing characteristics determined by medical imaging techniques, such as MRI, coupled with computer aided manufacturing (CAM) techniques. 
     In some embodiments, the bottom surface  22  of the prosthetic device  10  is coated with a bioactive coating to encourage the in-growth of natural tissue to further improve fixation of the prosthetic device to the tibial plateau. In some embodiments, the coating is formed by grit blasting or spraying the bottom surface  22 . The bioactive coating may be any suitable material for encouraging tissue growth and, in some embodiment, may be specifically adapted for promoting bone growth between the tibia and the prosthetic device  10 . 
     Referring generally to  FIGS. 8-15 , in some embodiments the outer body portion  12  of the prosthetic device  10  includes a deformation control element to limit the deformation of the outer body portion. As will be described in greater detail with respect to the specific embodiments shown in  FIGS. 8-15 , the deformation control element may be a material property, a structural property, an additional component, and/or combinations thereof. It should be noted that the various deformation control elements described herein may be combined to further limit or define the amount of deformation of the outer body portion  12 . 
     Referring more particularly to  FIG. 8 , shown therein is a cross-sectional view of the prosthetic device  10  wherein the outer body portion  12  is comprised of a reinforced material relative to the central body portion  14 . For example, in one embodiment the outer body portion  12  includes carbon fibers providing additional strength and limiting the flexibility of the outer body portion. In some embodiments the carbon fibers are injected prior to the curing of the outer body portion  12 . In other embodiments, the outer body portion  12  is formed or molded around the carbon fibers. In other embodiments, other additives are utilized to reinforce the material of the outer body portion  12 . The particular additives that are used depend upon the material(s) used for forming the outer body portion  12 . As shown in  FIG. 8 , in the current embodiment the entire outer body portion  12  is formed from a substantially uniform materials and/or the additives are equally distributed throughout the outer body portion. However, in other embodiments the deformation control element may comprise only a portion of the outer body portion  12 . In that regard, the deformation control element may extend along only a portion of the outer body portion  12 , the deformation control element may be positioned within a particular portion of the outer body portion, and/or combinations thereof. 
     For example, referring more particularly to  FIG. 9 , shown therein is a cross-sectional view of the prosthetic device  10  wherein the outer body portion  12  includes a wire, cable, or filament  38  extending therethrough. The filament increases the stiffness of the outer body portion  12  to limit the flexibility and/or deformity. In some embodiments, the filament  38  comprises a carbon fiber. In other embodiments, the filament  38  comprises a metal, polymer, or other material having an increased hardness and/or stiffness relative to the material comprising the central body portion  14 . In some embodiments, the outer body portion  12  is formed around the filament  38 . In other embodiments, the filament  38  is inserted into the outer body portion  12  prior to curing of the prosthetic device  12 . In some embodiments, the filament  38  is inserted into an opening in the body portion  12  and then additional material is inserted into the opening to close the opening and secure the filament therein. In the current embodiment, the filament  38  is shown having a substantially circular or cylindrical cross-section. However, in other embodiments the filament  38  may have other geometrical cross-sections and/or varying cross-sections along its length. The cross-section(s) of the filament  38  are configured to provide the desired stiffness and deformation properties to the outer body portion  12 . 
     Further in the current embodiment, the outer body portion  12  has a total thickness or height H 1 , the filament  38  is positioned a distance H 2  from the peak of the upper surface  24 , and the filament has a height H 3 . In some embodiments, the total thickness H 1  of the outer body portion  12  is between 5 mm and 15 mm. In some embodiments, the thickness H 3  of the filament  38  is between ¼ and ½ of the total thickness H 1  of the outer body portion  12 . The distance H 2  from the peak of the upper surface  24  to the filament  38  varies from 0 (i.e., the filament  38  is positioned at the top of the outer body portion  12 ) to ¾ of the total thickness H 1  (i.e., the filament is positioned at the bottom of the outer body portion). In that regard, in some embodiments the filament  38  may engaged a recess in the upper surface  24  or the lower surface of the outer body portion  12  configured to receive the filament  38 . In one particular embodiment, the total thickness H 1  is approximately 10 mm, the distance H 2  is approximately 3.3 mm, and the thickness H 3  is approximately 3.3 mm. 
     Similarly, in the current embodiment the outer body portion  12  has a total thickness or width W 1 , the filament  38  is positioned a distance W 2  from the outer surface  26 , and the filament has a width W 3 . In some embodiments, the total width W 1  of the outer body portion  12  is between 5 mm and 15 mm. In some embodiments, the width H 3  of the filament  38  is between ¼ and ½ of the total width W 1  of the outer body portion  12 . The distance W 2  from the peak of the upper surface  24  to the filament  38  varies from 0 (i.e., the filament  38  is positioned at the very inside of the outer body portion  12 ) to ¾ of the total thickness W 1  (i.e., the filament is positioned at the outside of the outer body portion). In that regard, in some embodiments the filament  38  may engaged a recess in the outer surface  26  or the inner surface of the outer body portion  12  configured to receive the filament  38 . In one particular embodiment, the total width W 1  is approximately 10 mm, the distance W 2  is approximately 3.3 mm, and the width W 3  is approximately 3.3 mm. In some embodiments, the outer body portion  12  may include multiple filaments  38  positioned therein. In that regard, the multiple filaments  38  may be spaced equally about the outer body portion  12  and/or grouped into specific areas of the outer body portion. 
     Further, in the illustrated embodiment the fixation member  16  includes a filament  39  extending therethrough. The filament  39  increases the stiffness of the fixation member  16 . In the current embodiment, the filament  39  extends substantially along the length of the fixation member  16  through portion  32 . In other embodiments, the fixation member  16  may include other features that increase the stiffness of the fixation member. In particular, the fixation member  16  may include features similar to those described with respect to the deformation control elements of the outer body portion  12 . 
     Referring now to  FIG. 10 , shown therein is a cross-sectional view of the prosthetic device  10  wherein the outer body portion  12  includes area  40  of increased stiffness and/or hardness. In the current embodiment the area  40  extends substantially across the entire width of the outer body portion  12 . The area  40  may comprise a different material from the rest of the outer body portion, the same material as the rest of the outer body portion with additives, and/or an insert piece configured to be secured within the outer body portion. As mentioned with respect to the other deformation control elements, the position, size, and shape of the area  40  is configured to achieve the desired deformation properties for the outer body portion  12 . For example, referring to  FIG. 11 , shown therein is a cross-sectional view of the prosthetic device  10  wherein the outer body portion  12  includes an area  42  of increased stiffness and/or hardness extending substantially along the entire thickness or height of the outer body portion. 
     Referring now to  FIG. 12 , shown therein is a cross-sectional view of the prosthetic device  10  wherein the outer body portion  12  includes a recess  44  for receiving a component  46  for defining the deformation properties of the outer body portion. For example, in some instances the component  46  may be a wire, cable, or filament similar to the filament  38  described above. In other instances, the component  46  may be a material that is injected or otherwise introduced into the recess  44  in the outer surface  26 . Generally, the size of the recess  44  and the properties of the component  46  are tailored to achieve the desired deformation properties of the outer body portion  12 . In some embodiments, the recess  44  comprises between ⅛ and ⅔ of the height of the outer body portion  12  and between ⅛ and ⅔ of the width of the outer body portion. In many embodiments, the component  46  substantially fills the entire recess  44 . However, in some embodiments the component  46  is sized such that it fills only a portion of the recess  44 . In such embodiments, the remaining portion of the recess  44  may remain vacant or be filled with another material. In some embodiments, the component  46  is secured in the recess  44  by the introduction of additional material into the open space remaining in the recess. 
     Referring now to  FIGS. 13 and 14 , shown therein is a cross-sectional view of the prosthetic device  10  wherein the outer body portion  12  includes an undercut  48  extending upward from the bottom surface of the outer body portion. In some instances the undercut  48  is configured to receive a component  50  for increasing the stiffness and limiting the deformation of the outer body portion  12 . For example, in some instances the component  50  may be a wire, cable, or filament similar to the filament  38  described above. In other instances, the component  50  may be a material that is injected or otherwise introduced into the recess  48  in the outer surface  26 . Generally, the size of the recess  48  and the properties of the component  50  are selected to achieve the desired deformation properties of the outer body portion  12 . In some embodiments, the recess  48  comprises between ⅛ and ⅞ of the height of the outer body portion  12  and between ⅛ and ⅞ of the width of the outer body portion. In many embodiments, the component  50  substantially fills the entire recess  48 , as shown in  FIG. 14 . However, in some embodiments a component  52  is sized such that it fills only a portion of the recess  48 , as shown in  FIG. 15 . In such embodiments, the remaining portion of the recess  44  may remain vacant—as shown in  FIG. 15 —or the remaining open portion may be filled with another material or component. In some embodiments, the component  52  is secured in the recess  48  by the introduction of additional material into the open space remaining in the recess. 
     Referring now to  FIGS. 16 and 17 , shown therein is a prosthetic device  60  according to another embodiment of the present disclosure. In some instances, the prosthetic device  60  is substantially similar to the prosthetic device  10  described above. Accordingly, similar reference numerals may be utilized and the description limited for particular aspects of the prosthetic device  60 . However, it should be noted that the prosthetic device  60  does not include a fixation member  16  or a fixation device  18  as described with respect to the prosthetic device  10 . Rather, the prosthetic device  60  is configured to be implanted without rigid fixation to either the femur or tibia. In this regard, the prosthetic device  60  may be implanted into a patient without causing permanent damage to the patient&#39;s tibia or other bone structure(s) engaged by the prosthetic device. Accordingly, the prosthetic device  60  may be implanted in an attempt to alleviate the patient&#39;s knee problems while avoiding permanent destruction of the patient&#39;s anatomy, such as cutting or reaming a large opening in the tibia. more invasive procedures. 
     To this end, the prosthetic device  60  includes a boundary  62  extending between the ends of the outer body portion  12  having an increased height relative to the central body portion  14 . In that regard, the boundary  62  extends substantially above an upper surface  63  of the central body portion  14 . Accordingly, the outer body portion  12  and the boundary  62  completely surround the central body portion  14 . Accordingly, when positioned between the femur and tibia the outer body portion  12  and the boundary  62  define the outer limits of movement for the femur relative to the prosthetic device  60 . That is, the increased height of the outer body portion  12  and the boundary  62  along with the contact pressure on the prosthetic device  60  from being positioned between the femur and the tibia prevents the femur from escaping the cavity defined by the outer body portion  12  and the boundary  62 . In some embodiments, the boundary  62  has a thickness that is between ¼ and ¾ the total thickness or height of the outer body portion  12 . In one particular embodiment, the boundary  62  has thickness such that it extends above the upper surface  63  approximately one-half of the distance of the upper surface  24  of the outer body portion  12 . 
     Further, the upper surface  63  and the lower surface  64  are both articulating bearing surfaces in the current embodiment. In particular, the upper and lower surfaces  63 ,  64  are configured to movingly engage with the femur and tibia, respectively. In that regard, the prosthetic device  60  can translate and rotate with respect to the femur and/or tibia. Translation is possible in both the anterior-posterior and medial-lateral directions. In some embodiments, the upper surface  63  includes both a vertical and a horizontal bearing component. To that end, in some embodiments the upper surface  63  comprises a concave surface that defines the vertical and horizontal bearing components. Similarly, in some embodiments the lower surface  64  includes both a vertical and horizontal bearing component. In particular, in some embodiments the lower surface  64  comprises a convex surface. In other embodiments, the lower surface  64  comprises only a vertical bearing component and is substantially planar. In such embodiments, the tibia may be prepared to mate with the substantially planar lower surface. In some embodiments, the upper surface  63  and/or the lower surface  64  are shaped such that the prosthetic device  10  is biased towards a neutral position in the knee. For example, the arcuate profiles of the upper surface  63  and/or the lower surface  64  are shaped such that the interaction between the surfaces and the bone encourages the bone to a particular orientation relative to the surfaces. 
     Referring to  FIG. 18 , in some embodiments the prosthetic device  10  includes one or more recesses  66  in the upper surface  20  that provide for the accumulation of synovial fluid. In some embodiments, the recesses  66  are positioned at the most prevalent contact points of the femur with the upper surface  20 . In such embodiments, the synovial fluid lubricates the upper articulation surface  20  of the prosthetic device. The recesses  66  may have various shapes within the upper surface  20 . In that regard, the recesses  66  may comprises a sloping depression that creates a concave recess in some embodiments. The concave recess may comprises a substantially circular profile, an elongated profile, an irregular shape, and/or combinations thereof. In the current embodiment, the recesses  66  are shown as comprising a D-shaped profiles spaced equally about a midline of the prosthetic device  10 . However, this depiction is not to be limiting as many shapes, sizes, and transitions may be utilized for the recesses  66 . In other embodiments, the prosthetic device  10  includes a greater or fewer number of recesses  66 . In some embodiments, the prosthetic device  10  does not include any recesses in the upper surface  20 . 
     In some instances, the femoral condyle may be surgically prepared to permit near-normal knee joint flexion after implantation. Similarly, the tibial plateau may be surgically prepared to fixedly engage with the prosthetic devices. 
     A variety of materials are suitable for use in making the prosthetic meniscus. Medical grade polyurethane based materials especially suitable for use in the embodiments described include, but are not limited to the following: 
     Bionate®, manufactured by POLYMER TECHNOLOGY GROUP PTG, a polycarbonate-urethane is among the most extensively tested biomaterials ever developed. Carbonate linkages adjacent to hydrocarbon groups give this family of materials oxidative stability, making these polymers attractive in applications where oxidation is a potential mode of degradation, such as in pacemaker leads, ventricular assist devices, catheters, stents, and many other biomedical devices. Polycarbonate urethanes were the first biomedical polyurethanes promoted for their biostability. Bionate® polycarbonate-urethane is a thermoplastic elastomer formed as the reaction product of a hydroxyl terminated polycarbonate, an aromatic diisocyanate, and a low molecular weight glycol used as a chain extender. The results of extensive testing encompassing Histology, Carcinogenicity, Biostability, and Tripartite Biocompatibility Guidance for Medical Devices verifies the cost effective material&#39;s biocompatibility. 
     Another group of potentially suitable materials are copilymers of silicone with poplyurethanes as exemplified by PurSil™, a Silicone Polyether Urethane and CarboSil™, a Silicone Polycarbonate Urethane. Silicones have long been known to be biostable and biocompatible in most implants, and also frequently have the low hardness and loq modulus useful for many device applications. Conventional silicone elastomers can have very high ultimate elongations, but only low to moderate tensile strengths. Consequently, the toughness of most biomedical silicone elastomers is not particularly high. Another disadvantage of conventional silicone elastomers in device manufacturing is the need for cross-linking to develop useful properties. Once cross-linked, the resulting thermoset silicone cannot be redissolved or remelted. In contrast, conventional polyurethance elastomers are generally thermoplastic with excellent physical properties. Thermoplastic urethane elastomers (TPUs) combine high elongation and high tensile strength to form tough, albeit fairly high-modulud elastomers. Aromatic polyether TPUs can hica excellent flex life, tensile strength exceeding 5000 psi, and ultimate elongations greater than 700 percent. The are often used for continuously flexing, chronic implants such as ventricular-assist devices, intraaortic balloons, and artificial heart components. TPUs can easily be processed by melting or dissolving the polymer to fabricate it into useful shapes. 
     The prospect of combining the biocompatibility and biostability of conventional silicone elastomers with the processability and toughness of TPUs is an attractive approach to what would appear to be a nearly ideal biomaterial. For instance, it has been reported that silicone acts synergistically with both polycarbonate- and polyether-based polyurethanes to improve in vivo and in vitro stability. In polycarbonate-based polyurethanes, silicone copolymerization has been shown to reduce hydrolytic degradation of the carbonate linkage, whereas in polyether urethanes, the covalently bonded silicone seems to protect the polyether soft segment from oxidative degradation in vivo. 
     POLYMER TECHNOLOGY GROUP PTG synthesized and patented silicone-polyurethane copolymers by combining two previously reported methods: copolymerization of silicone (PSX) together with organic (non-silicone) soft segments into the polymer backbone, and the use of surface-modifying end groups to terminate the copolymer chains. Proprietary synthesis methods make high-volume manufacturing possible. 
     Other potentially applicable materials include PurSil™ silicone-polyether-urethane and CarboSil™ silicone-polycarbonate-urethane which are true thermoplastic copolymers containing silicone in the soft segment. These high-strength thermoplastic elastomers are prepared through a multi-step bulk synthesis where polydimethylsiloxane (PSX) is incorporated into the polymer soft segment with polytetramethyleneoxide (PTMO) (PurSil) or an aliphatic, hydroxyl-terminiated polycarbonate (CarboSil). The hard segment consists of an aromatic diisocyanate, MDI, with low molecular weight glycol chain extender. The copolymer chains are then terminated with silicone (or other) Surface-Modifying End Groups™ Aliphatic (AL) versions of these materials, with a hard segment synthesized from an aliphatic diisocyanate are also available. 
     Many of these silicone urethanes demonstrate previously unavailable combinations of physical properties. For example, aromatic silicone polyetherurethanes have a higher modulus at a given shore hardness than conventional polyether urethanes—the higher the silicone content, the higher the modulus (see PurSil Properties). Conversely, the aliphatic silicone polyetherurethanes have a very low modulus and a high ultimate elongation typical of silicone homopolymers or even natural rubber (see PurSil AL Properties). This makes them very attractive as high-performance substitutes for conventional cross-linked silicone rubber. In both the PTMO and PC families, certain polymers have tensile strengths three to five times higher than conventional silicone biomaterials. 
     Further examples of suitable materials include Surface Modifying End Groups™ (SMEs) which are surface-active oligomers covalently conded to the base polymer during synthesis. SMEs—which include silicone (S), sulfonate (SO), fluorocarbon (F), polyethylene oxide (P), and hydrocarbon (H) groups—control surface chemistry without compromising the bulk properties of the polymer. The result is key surface properties, such as thromboresistance, biostability, and abrasion resistance, are permanently enhanced without additional post-fabrication treatments or topical coatings. This patented technology is applicable to a wide range of PTG&#39;s polymers. 
     SMEs provide a series of (biomedical) base polymers that can achieve a desired surface chemistry without the use of additives. Polyurethanes prepared according to PTG&#39;s development process couple endgroups to the backbone polymer during synthesis via a terminal isocyanate group, not a hard segment. The added mobility of endgroups relative to the backbone is though to facilitate the formation of uniform overlayers by the surface-active (end) blocks. The use of the surface active endgroups leaves the original polymer backbone intact so the polymer retains strength and processability. The fact that essentially all polymers chains carry the surface-modifying moiety eliminates many of the potential problems associated with additives. 
     The SME approach also allows the incorporation of mixed endgroups into a single polymer. For example, the combination of hydrophobic and hydrophilic endgroups gives the polymers amphipathic characteristics in which the hydrophobic versus hydrophilic balance may be easily controlled. 
     Other suitable materials, manufactured by CARDIOTECH CTE, include ChronoFlex® And Hydrothane™. 
     The ChronoFlex®, polycarbonate aromatic polyurethanes, family of medical-grade segmented biodurable polyurethane elastomers have been specifically developed by CardioTech International to overcome the in vivo formation of stress-induced microfissures. 
     HydroThane™, hydrophilic thermoplastic polyurethanes, is a family of super-absorbent, thermoplastic, polyurethane hydrogels ranging in water content from 5 to 25% by weight. HydroThane™ is offered as a clear resin in durometer hardness of 80 A and 93 Shore A. The outstanding characteristic of this family of materials is the ability to rapidly absorb water, high tensile strength, and high elongation. The result is a polymer having some lubricious characteristics, as well as being inherently bacterial resistant due to their exceptionally high water content at the surface. HydroThane™ hydrophilic polyurethane resins are thermoplastic hydrogels, and can be extruded or molded by conventional means. Traditional hydrogels on the other hand are thermosets and difficult to process. 
     Additional suitable materials manufactured by THERMEDICS include Tecothante®(aromatic polyether-based polyurethane), Carbothane®(aliphatic polycarbonate-based polyurethane), Tecophilic®(high moisture absorption aliphatic polyether-based polyurethane) and Tecoplast®(aromatic polyether-based polyurethane). Tecothane® is a family of aromatic, polyether-based TPU&#39;s available over a wide range of durometers, colors, and radiopacifiers. One can expect Tecothane resins to exhibit improved solvent resistance and biostability when compared with Tecoflex resins of equal durometers. Carbothane® is a family of aliphatic, polycarbonate-based TPU&#39;s available over a wide range of durometers, colors and radiopacifiers. This type of TPU has been reported to exhibit excellent oxidative stability, a property which may equate to excellent long-term biostability. This family, like Tecoflex, is easy to process and does not yellow upon aging. Tecophilic® is a family of aliphatic, polyether-based TPU&#39;s which have been specially formulated to absorb equilibrium water contents of up to 150% of the weight of dry resin. 
     Polyurethanes are designated aromatic or aliphatic on the basis of the chemical nature of the diisocyanate component in the formulation. Tecoflex, Tecophilic and Carbothane resins are manufactured using the aliphatic compound, hydrogenated methylene diisocyanate (HMDI). Tecothane and Tecoplast resins use the aromatic compound methylene diisocyanate (MDI). Tecoflex® is a family of aliphatic, polyether-based TPU&#39;s. These resins are easy to process and do not yellow upon aging. Solution grade versions are candidates to replace latex. All formulations, with the exception of Carbothane, are formulated using plytetramethylene ether glycol (PTMEG) and 1,4 butanediol chain extender. Carbothane is specifically formulated with a polycarbonate diol (PCDO). These materials represent the major chemical composition differences among the various families. Aromatic and aliphatic polyurethanes share similar properties that make them outstanding materials for use in medical devices. In general, there is not much difference between medical grade aliphatic and aromatic polyurethanes with regard to the following chemical, mechanical and biological properties: high tensile strength (4,000 to 10,000 psi); high ultimate elongation (250 to 700%); wide range durometer (72 Shore A to 84 Shore D); good biocompatibility; high abrasion resistance; good hydrolytic stability; can be sterilized with ethylene oxide and gamma irradiation; retention of elastomeric properties at low temperature; good melt processing characteristics for extrusion, injection molding, etc. 
     With such an impressive array of desirable features, it is no wonder that both aliphatic and aromatic polyurethanes have become increasingly the material of choice in the design of medical grade components. There are, however, distinct differences between these tow families of polyurethane that could dictate the selection of one over the other for a particular application: 
     In their natural states, both aromatic and aliphatic polyurethanes are clear to very light yellow in color. Aromatics, however, can turn dark yellow to amber as a result of melt processing or sterilization, or even with age. Although the primary objection to the discoloration of aromatic clear tubing or injection molded parts is aesthetic; the yellowing, which is caused by the formation of a chromophore in the NMI portion of the polymer, does not appear to affect other physical properties of the material. Radiopaque grades of Tecothane also exhibit some discoloration during melt processing or sterilization. However, both standard and custom compounded radiopaque grades of Tecothane have been specifically formulated to minimize this discoloration. 
     Aromatic polyurethanes exhibit better resistance to organic solvents and oils than do aliphatics—especially as compared with low durometer (80 to 85 Shore A) aliphatic, where prolonged contact can lead to swelling of the polymer and short-term contact can lead to surface tackiness. While these effects become less noticeable at higher durometers, aromatics exhibit little or no sensitivity upon exposure to the common organic solvents used in the health care industry. 
     Both aliphatic and aromatic poly-ether based polyuretanes soften considerably within minutes of insertion in the body. Many device manufacturers promote this feature of the urethane products because of patient comfort advantage as well as the reduced risk of vascular trauma. However, this softening effect is less pronounced with aromatic resins than with aliphatic resins. 
     Tecothane, Tecoplast and Carbothane melt at temperatures considerably higher than Tecoflex and Tecophilic. Therefore, processing by either extrusion of injection molding puts more heat history into products manufactured from Tecothane, Tecoplast and Carbothane. For example, Tecoflex EG-80A and EG-60D resins mold at nozzle remperatures of approximately 310 degrees F. and 340 degrees F. respectively while Tecothane and Carbothane products of equivalent durometers mold at nozzle temperatures in the range of 380 degrees F. and 435 degrees F. 
     Additional materials of interest include Tecogel, a new member to the Tecophilic family, a hydrogel that can be formulated to absorb equilibrium water contents between 500% to 2000% of the weight of dry resin, and Tecoplast®, a family of aromatic, polyether-based TPU&#39;s formulated to produce rugged injection molded components exhibiting high durometers and heat deflection temperatures. 
     Additional potentially suitable materials include four families of polyurethanes, named Elast-Eon™, which are available from AorTech Biomaterials. 
     Elast-Eon™1, a Polyhexamethylene oxide (PFMO), aromatic polyurethane, is an improvement on conventional polyurethane in that it has a reduced number of the susceptible chemical groups. Elast-Eon™2, a Siloxane based macrodiol, aromatic polyurethane, incorporates siloxane unto the soft segment. Elast-Eon™3, a Siloxane based macrodiol, modified hard segment, aromatic polyurethane, is a variation of Elast-Eon™2 with further enhanced flexibility due to incorporation of siloxane into the hard segment. Elast-Eon™4 is a modified aromatic hard segment polyurethane. 
     Bayer Corporation also produces candidate materials. Texin 4210 and Texin 4215 are thermoplastic polyurethane/polycarbonate blends for injection molding and extrusion. Texin 5250, 5286 and 5290 are aromatic polyether-based medical grade materials with Shore D hardness of approximately 50, 86, and 90 respectively for injection molding and extrusion. They each comply with 21 CFR 177.1680 and 177.2600. 
     It should be appreciated that the devices described hereinabove, while preferably formed by injection molding of polyurethane, may also be formed by any suitable manufacturing method and may be formed of any suitable medical grade elastomers. It is further appreciated that any of the following manufacturing methods may be utilized: injection molding including inserting inserts, compression molding including inserting inserts, injection-compression molding including inserting inserts, compression molding of prefabricated elements pre-formed by any of the above methods including inserting inserts, spraying including inserting inserts, dipping including inserting inserts, machining from stocks or rods, machining from prefabricated elements including inserting inserts. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure. 
     While the present disclosure has been particularly shown and described with reference to particular embodiments, it will be fully appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations of improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.