Abstract:
A prosthesis has prosthetic components principally comprising metallic materials, in which a portion of an articular interface between respective components is a metallic-nonmetallic interface. At least a portion of the articular surface of a femoral head may include a ceramic material defining an articulation zone, such as at a polar region of the femoral head, so that the ceramic articulates with a metallic acetabular liner. The area covered by the ceramic may be engineered to optimize the contact conditions between the femoral head and the acetabular liner, such as by providing two clearances therebetween. A relatively smaller, polar articulation clearance is defined by the gap between the ceramic coating and the metallic acetabular liner. A relatively larger, equatorial non-articulation clearance between the femoral head and the acetabular liner is defined by the gap between the portion of the femoral head not covered by the ceramic coating.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to joint replacement surgery, and more particularly to a prosthesis with a modified articular surface. 
     2. Brief Description of the Related Art 
     Joint replacement surgery is used to replace one or more damaged articular joint surfaces or components, thereby allowing the joint to function normally when it would otherwise not be possible to do so. For example, hip arthroplasty is a common joint replacement surgery in which a diseased or damaged femoral head and/or acetabulum is removed and replaced with one or more artificial components. In a typical hip arthroplasty procedure, a femoral component is secured to the femur after resection of the natural femoral head, and a prosthetic femoral head is affixed to the femoral component to approximate the location and orientation of the natural femoral head. The acetabulum of the hip joint may also be resected and replaced with a prosthetic acetabular cup designed to articulate with the prosthetic femoral head. The prosthetic femoral head and acetabular cup may be made of metallic, ceramic and/or polymer components, for example. 
     Longevity is desirable in the components used in a hip arthroplasty procedure, to reduce or eliminate the need for revision surgery and enhance functionality during the service life of the hip prosthesis. In addition to increasing longevity, minimizing wear of the hip prosthesis components also curtails the potential release of particulate material from prosthetic components into the patient&#39;s body. One method of reducing wear in hip prostheses is to select a femoral head component having a different hardness as compared with the acetabular cup component. For example, the acetabular cup may be made of a metallic material and the femoral head may be made of a ceramic material, creating a bearing surface in which respective bearing surface components have disparate material properties, such as hardness (i.e., the relatively softer metallic acetabular cup and the relatively harder ceramic femoral head). These disparate material properties may prolong the service life of the hip prosthesis. 
     SUMMARY 
     The present disclosure provides a prosthesis, and a method for making the same, in which the prosthetic components principally comprise metallic materials, but at least a portion of an articular interface between respective components is a metallic-nonmetallic interface. For example, at least a portion of the articular surface of a femoral head may include a ceramic material defining an articulation zone, such as at a polar region of the femoral head, so that the ceramic articulates with a metallic acetabular liner. The area covered by the ceramic may be engineered to optimize the contact conditions between the femoral head and the acetabular liner, such as by providing two clearances therebetween. A relatively smaller, polar articulation clearance is defined by the gap between the ceramic coating and the metallic acetabular liner (i.e., an articulation zone having a ceramic-on-metal interface). A relatively larger, equatorial non-articulation clearance between the femoral head and the acetabular liner is defined by the gap between the portion of the femoral head not covered by the ceramic coating (i.e., an non-articulation zone, which has a ceramic-on-metal or metal-on-metal interface, depending on whether the remainder of the surface of the femoral head is also coated with ceramic). 
     The geometry of the transition area between the articulation zone and the non-articulation zone can be engineered to optimize lubrication during ambulation. In addition, channels or grooves may be formed in the ceramic of the articulation zone to promote entrainment of lubricating fluid from the non-articulation zone to the articulation zone. 
     The ceramic area of the femoral head can be produced by various methods, including coatings, coverings, inlays, encapsulation and chemical processes such as oxidation, nitriding, and other processes. A coating or covering may be applied to an unmodified femoral head, or may cooperate with a modified area, such as an indented or surface-treated area, for example, to bond with the femoral head. 
     In one embodiment, a prosthesis for a ball and socket joint is provided. The prosthesis includes a body having a curved portion, and the body has a polar region with a radial center. The prosthesis further includes a cup with an interior wall defining a concave cavity sized to receive the body, and a coating covering between 60 degrees and 120 degrees of the polar region of the body when viewed in cross-section and measured from the radial center. The coating is disposed between the body and the interior wall of the cup when the body is disposed within the cavity. 
     In one aspect, the coating may include a transition extending between an outer surface of the coating and an outer surface of the body. The coating may have a thickness between 50 microns and 200 microns. 
     In another aspect, the coating may be harder than the interior wall of the cup. For example, the coating may be harder by forming the interior wall of the cup from a metallic material and forming the coating from a ceramic material. 
     In another aspect, with the body disposed within the cavity, a gap may be formed between the body and the interior wall of the cup at a location of the body that is not covered by the coating. 
     In yet another aspect, the coating may include at least one groove formed in coating, the groove extending from a first location at a junction between the coating and body, across a portion of the coating, to a second location at the junction that is different from the first location. 
     In still another aspect, the body may be a femoral head and the cup may be an acetabular liner. 
     In another aspect, the curved portion of the body may be spherical. 
     In another embodiment, a prosthesis is provided which includes a body with a curved portion, with the body having a polar region with a radial center. The prosthesis also includes a cup with an interior wall defining a concave cavity sized to receive the body in which the interior wall has a first hardness, and a coating disposed on the body, the coating having a thickness of between 50 microns and 200 microns. The coating has a second hardness that is greater than the first hardness. 
     In one aspect, the coating covers between 60 degrees and 120 degrees of the polar region when viewed in cross-section and measured from the radial center. 
     In another aspect, the coating may include a transition extending between an outer surface of the coating and an outer surface of the body. 
     In another aspect, with the body disposed within the cavity, a gap may be formed between the body and the interior wall of the concave cup at a location of the body that is not covered by the coating. 
     In yet another aspect, the interior wall of the cup may include a metallic material and the coating may include a ceramic material. 
     In still another aspect, the coating may include at least one groove formed in the coating, the groove extending from a first location at a junction between the coating and body, across a portion of the coating, to a second location at the junction that is different from the first location. 
     In another aspect, the body may be a femoral head and the cup may be an acetabular liner. 
     In yet another embodiment, a prosthesis is provided which includes a femoral component adapted to be fixed to a bone, and a femoral head attached to the femoral component. The femoral head includes: a polar region having a first radius, the polar region disposed opposite the femoral component and defining an angular span of between 60 degrees and 120 degrees as viewed in cross-section; an equatorial region having a second radius, the equatorial region disposed between the femoral component and the polar region, so that the first radius is greater than the second radius by at least 50 microns; and a transition region disposed at a junction between the polar region and the equatorial region, the transition region having a width of at least 20 microns. The transition region provides a gradual transition from the first radius to the second radius. 
     In one aspect, the first radius may be greater than the second radius by less than 200 microns. 
     In another aspect, the prosthesis may include an acetabular liner with an interior wall defining a concave cavity sized to receive the femoral component, with the interior wall having a first hardness, and the polar region of the femoral component having a second hardness that differs from the first hardness. 
     In another aspect, at least a portion of the femoral head may be spherical. 
     In yet another aspect, the polar region may have a surface including a ceramic material, and the equatorial region may have a surface including a metallic material. 
     In yet another aspect, the polar region includes at least one groove formed therein, the groove extending from the transition region at a first location on the femoral head, across a portion of the polar region, and to the transition region at a second location on the femoral head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a portion of a hip prosthesis, illustrating a femoral component and a femoral head in accordance with the present disclosure; 
         FIG. 2  is an elevational, sectional view of the femoral head shown in  FIG. 1 , illustrating coatings on the femoral head; 
         FIG. 3  is an elevational, sectional view of a portion of the femoral head shown in  FIG. 2 , illustrating a chamfered transition region; 
         FIG. 3A  is an elevational, sectional view of a portion of the femoral head shown in  FIG. 2 , illustrating a concave transition region; 
         FIG. 3B  is an elevational, sectional view of a portion of the femoral head shown in  FIG. 2 , illustrating a convex transition region; 
         FIG. 3C  is an elevational, sectional view of a portion of the femoral head shown in  FIG. 2 , illustrating a hybrid concave/convex transition region; 
         FIG. 4  is a partial perspective view of a portion of the hip prosthesis shown in  FIG. 1 , with an alternative femoral head coating; and 
         FIG. 4A  is a top plan view of the portion of the hip prosthesis shown in  FIG. 4 . 
     
    
    
     The exemplification set out herein illustrates an exemplary embodiment of the present invention, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , prosthesis  10  includes femoral component  11  with femoral head  12  attached at a proximal end thereof. Femoral head  12  includes substrate  13  ( FIG. 2 ) including polar region  22  forming a “cap” on femoral head  12  proximate pole  25  ( FIG. 2 , discussed below) and equatorial region  23  forming an annular “band” disposed between femoral component  11  and polar region  22 . Substrate  13  is made of a metallic material, such as titanium, titanium alloys, and other alloys including cobalt chromium and zirconium materials such as cobalt chrome molybdenum, for example. Femoral head  12  also includes polar coating  18  made of a ceramic material, with coating  18  disposed at polar region  22  of femoral head  12 . An inner, optional coating  16  may extend over a portion of an outer surface of femoral head  12 , such as over more than half of femoral head  12 . Polar coating  18  is disposed at polar region  22  of femoral head  12  and extends over less than half of femoral head  12 , as discussed below. 
     As used herein, “coating” refers to a dissimilar material applied to a substrate by any of a variety of methods and modalities. For example, coating  18  may be applied to substrate  13  of femoral head  12  by a coating process in which a flowable or particulate material is deposited on the solid substrate  13 , and then cured or solidified in bonding engagement with substrate  13 . Coating methods in accordance with the present disclosure include encapsulation and chemical processes such as oxidation, nitriding, and other processes. Alternatively, coating  18  may be deposited on substrate  13  by forming coating  18  separately from substrate  13 , i.e., as a covering, and then bonding coating  18  to femoral head  12 . 
     Coating  18  may cover or be deposited on an unmodified substrate  13 , as shown in  FIG. 2 , or coating  18  may be applied to a modified area of substrate  13 . For example, femoral head  12  may include a depression or divot formed in substrate  13  sized to receive coating  18 , so that coating  18  takes the form of an inlay. The modified area of substrate  13  may also include, for example, a surface treatment differing from the remainder of substrate  13  to promote bonding of coating  18  with substrate  13  of femoral head  12 . 
     Referring now to  FIG. 2 , femoral head  12  is shaped and sized to articulate with an acetabular cup or liner  14  in the manner of a ball and socket joint. Radius of curvature  15  of femoral head  12  (which originates at a radial center, or origin “O”) varies depending on the presence or absence of one or more coatings. For example, radius  15  of femoral head  12  is effectively increased by optional coating  16  (where coating  16  is present) by an amount equal to thickness T′ of coating  16 . Radius of curvature  15  of femoral head  12  is further increased by thickness T of polar coating  18  in places where polar coating  18  is present. 
     Optional coating  16  may cover a portion of femoral head  12  that is available for articulation, i.e., a portion of femoral head  12  that may be potentially come in to contact with acetabular liner  14  at the limits of motion of the ball and socket joint. Alternatively, optional coating  16  may cover substantially all of the remaining exposed surfaces of femoral head  12 , i.e., the surfaces that are not covered by polar coating  18 . In yet another alternative, optional coating  16  may be a surface treatment or process applied to substrate  13  of femoral head  12 , so that optional coating  16  provides little or no increase in radius of curvature  15  of femoral head  12 . 
     Acetabular liner  14  includes a concave interior surface with a radius of curvature generally corresponding with the increased radius of curvature created by coatings  16 ,  18  on femoral head  12 . Polar coating  18  defines an articular surface of femoral head  12  when femoral head  12  is received within a cavity defined by concave interior surface  19  of acetabular liner  14 . When so received, polar coating  18  engages with concave surface  19  of acetabular liner  14  to define articular interface  24 , while uncoated areas at equatorial region  23  of femoral head  12  define a diametrical space or gap  26  between optional coating  16  and acetabular liner  14  or, if optional coating  16  is not present, between substrate  13  and acetabular liner  14 . Polar coating  18  may be made of a different material than acetabular liner  14 , thereby establishing a bearing surface between polar coating  18  and acetabular liner  14  in which the respective bearing surface components have disparate material properties. For example, and as discussed below, polar coating  18  may be made from a relatively harder ceramic material while acetabular liner  14  may be made from a relatively softer metallic material, which may serve to mitigate wear at articular interface  24 . Alternatively, polar coating  18  and liner  14  may be made from the same or similar materials, and may establish different hardnesses for respective bearing components through material treatments such as alloying, heat treatment or the like. 
     Referring still to  FIG. 2 , polar coating  18  is generally disposed about polar region  22  of femoral head  12 , and extends away from pole  25  of femoral head  12  to define an angular extent θ. In the illustrated embodiment, pole  25  is defined as the point on femoral head  12  furthest from rim  17  of acetabular liner  14  when prosthesis  10  and acetabular liner  14  are oriented so as to correspond to a standing position. In alternative embodiments, pole  25  may be defined as the point on femoral head  12  that is furthest from the interface between substrate  13  and femoral component  11 , or may be defined as the point on femoral head  12  intersecting nominal axis A-A ( FIG. 1 ) extending through neck  11 ′ of femoral component  11 . Moreover, it is contemplated that pole  25  may defined at any suitable location on femoral head  12 , as required or desired for a particular application. 
     In an exemplary embodiment, polar coating  18  covers polar region  22  of femoral head  12  so that an area of contact at articular interface  24  is optimized for a particular application. For example, polar coating  18  extends downwardly from pole  25  of femoral head  12  to form a “cap” or covering defined by angular extent θ, in which the bottom of the “cap” formed by polar coating is a generally annular structure. As viewed in the cross-sectional plane of  FIG. 2 , angular extent θ may be as little as 60° or 90°, or as much as 120° (measured as the included angle, as viewed in section, between the two sides extending from radius center O to the end of coating  18 ), or any value within any range defined by any of the foregoing values. Put another way, the coverage of polar coating  18  may be expressed as a percentage of an overall surface area of femoral head  12 . If femoral head  12  is spherical, this percentage P can be approximated by the following formula: 
             P   =         1   -     cos   ⁢           ⁢   θ       4     ×   100.           
Thus, if angular extent θ is between about 60° and about 120°, the percentage of femoral head  12  covered by polar coating  18  is between about 12.5% and 37.5%. However, it will be understood that a typical femoral head is not a complete sphere, so that the actual percentage of polar coating coverage may be somewhat larger. Moreover, the varying of angular extent θ and/or percentage P ensures that the area of contact defined by articular interface  24  is optimized so that the area is small enough to minimize wear and any concomitant potential release of particulate material into the body of the patient, but is large enough to distribute forces experienced at articular interface  24  during joint articulation.
 
     Polar coating  18  defines thickness T ( FIG. 2 ), which may be varied to optimize wear characteristics and force translation between components of femoral head  12 . In an exemplary embodiment, thickness T may be as little as 10 microns or 50 microns, and as much as 200 microns or 500 microns, or may be any value within any range defined by any of the foregoing values. Thickness T of polar coating  18  ultimately determines radius of curvature  15  of femoral head  12  at articular interface  24 , as discussed above. Therefore, for any given radius of concave internal surface  19  of acetabular liner  14 , thickness T is one variable that determines a clearance between femoral head  12  and concave internal surface  19  of acetabular liner  14  at articular interface  24 . Variation of thickness T of polar coating  18  can be used to ensure an optimal tightness of fit between femoral head  12  and acetabular liner  14  at articular interface  24 , thereby promoting precise articular movement and high joint stability, while also leaving sufficient clearance to minimize the risk of seizure between femoral head  12  and acetabular liner  14 . Further, and as discussed in more detail below, thickness T can also be varied to optimize the entrainment of lubrication and/or synovial fluid at articular interface  24 . 
     Polar coating  18  provides two distinct clearances between femoral head  12  and acetabular liner  14 . While the articulation clearance at articular interface  24  may be relatively small, as discussed above, the diametrical clearance at gap  26  is comparatively large. Gap  26  is disposed generally opposite polar coating  18  on femoral head  12 , so that gap  26  is spaced away from pole  25  and occupies the portion of femoral head  12  that is not covered by polar coating  18  (i.e., equatorial region  23 ). In an exemplary embodiment, a ratio of the articulation clearance at articular interface  24  to the diametrical clearance at gap  26  is between about 0.1 and about 1.0, or any value between these two ratios. That is to say, gap  26  may be small enough to render the diametrical clearance approximately equal to the articulation clearance, or gap  26  may be large enough to render the diametrical clearance approximately ten times larger than the articulation clearance. 
     Advantageously, lubricating fluid may be present in relatively large amounts in gap  26 , which may promote entrainment of the lubricating fluid into articular interface  24  during articulating movement of prosthesis  10  with respect to acetabular liner  14 , as discussed in more detail below. Also advantageously, the larger clearance created by gap  26  near rim  17  of acetabular liner  14  results in a minimized risk of acetabular liner  14  “clamping” on femoral head  12 , i.e., becoming fixedly coupled to femoral head  12  so that movement of prosthesis  10  urges a corresponding movement of acetabular liner  14 . 
     Polar coating  18  may include transitional chamfer  20  ( FIGS. 2 and 3 ) extending around the perimeter of the terminus of polar coating  18 . Transitional chamfer  20  provides a smooth transition at the junction between polar coating  18  and substrate  13  (or between polar coating  18  and optional coating  16 , if present). Transitional chamfer  20  thus prevents or reduces any “step” or ledge that may be present between a relatively larger radius  15  (such as where femoral head  12  includes thickness T of polar coating  18 ) and a relatively smaller radius  15  of femoral head  12  (such as where polar coating  18  is absent). This smooth transition enhances formation of a film of lubricating fluid at articular interface  24  by facilitating fluid flow from gap  26  to articular interface  24 . As shown in  FIG. 3 , chamfer  20  has width W defined as the shortest distance along chamfer  20  between polar coating  18  and optional coating  16  (or, where optional coating  16  is not present, between polar coating  18  and substrate  13  of femoral head  12 ). A larger width W provides a more gradual transition angle between polar coating  18  and optional coating  16  (or substrate  13  of femoral head  12 ), while a smaller width W provides a sharper, less gradual transition angle. In exemplary embodiments, width W may be as little as 40% of thickness T of coating  18  and as much as 60% of thickness T of coating  18 , or any value within this range. For example, where coating  18  has thickness T equal to 50 microns, width W may be between 20 and 30 microns. Similarly, for a where coating  18  has thickness T equal to 200 microns, width W may be between 80 and 120 microns. 
     In an exemplary embodiment, the transition angle and geometrical configuration of chamfer  20  may be optimized to promote favorable lubricant entrainment into articular interface  24 . For example, polar coating  18  and chamfer  20  may be made asymmetrical with respect to polar region  22  by extending chamfer  20  further from pole  25  along some portions of a circumferential extent of chamfer  20  as compared with other portions. This asymmetry of chamfer  20  may provide directionality to lubricating fluid by promoting entrainment of fluid into certain areas of polar region  22  during movement of femoral head  12  relative to acetabular liner  14 . For example, in one exemplary embodiment (not shown), coating  18  may have a generally oval profile with the major axis of the oval corresponding to a flexion and extension movement path. Chamfer  20  may also be symmetrical about polar region  22  to allow even fluid entrainment throughout polar coating  18 , such as by defining a fixed distance of chamfer  20  from pole  25  around the entire circumferential extent. 
     Referring now to  FIGS. 3A-3C , the geometry of the junction between polar coating  18  and femoral head  13  (or between polar coating  18  and optional coating  16 , if present) may take alternative forms to the planar illustration of transitional chamfer  20 . For example, concave transition  20   a  may be provided, as shown in  FIG. 3A . Another alternative is convex transition  20   b , as shown in  FIG. 3B . Yet another alternative is hybrid transition  20   c , including both concave portion  20   c ′ and convex portion  20   c ″. These alternative transition geometries may be used to enhance or modify the entrainment characteristics of fluid from gap  26  to articular interface  24 , for example. 
     Optionally, grooves or channels may be provided in polar coating  18  to aid in the introduction of lubricating fluid to articular interface  24 . In the illustrated embodiment of  FIGS. 4 and 4A , eight equally-spaced grooves  28  extend across a portion of polar coating  18  to provide a plurality of pathways for lubricating fluid, such as synovial fluid, to travel from gap  26  ( FIG. 2 ) to various portions of articular interface  24  ( FIG. 2 ). Each of grooves  28  extends from chamfer  20  at a first location on femoral head  12 , and across a portion of polar coating  18 . In the illustrated embodiment, grooves  28  all extend upwardly toward pole  25  of substrate  13  of femoral head  12 , while leaving the portion of polar coating  18  near pole  25  intact. Thus, grooves  28  present an unobstructed pathway for fluid to migrate from gap  26  to articular interface  24 . More particularly, fluid that enters a respective groove  28  can travel along the extent of the groove  28  to convey lubricating fluid from gap  26  toward pole  25 . 
     In an exemplary embodiment, grooves  28  extend partially into, but not through, polar coating  18 . For example, grooves  28  may define a radiused or arcuate cross-sectional profile extending into polar coating  18  by about 40-60% of thickness T. In addition, grooves  28  may vary in width. In the illustrated embodiment of  FIG. 4A , for example, each of grooves  28  defines a maximum width at chamfer  20 , and tapers to a minimum width near pole  25 . 
     However, it is contemplated that the number, cross-sectional profile and geometrical arrangement of grooves  28  on polar coating  18  may be varied as desired or required for a particular application. For example, grooves  28  may extend entirely across a portion of femoral head, i.e., from chamfer  20  at a first location, across a portion of polar coating  18 , and back to chamfer  20  at a second location. Grooves  28  may also be in fluid communication with one another, i.e., by a connecting groove or by overlapping portions of grooves  28 . Moreover, grooves  28  may be take any number of forms in order to provide a desired amount and profile of fluid entrainment from gap  26  to articular interface  24 . 
     In the case of an asymmetrical perimeter of chamfer  20 , it is contemplated that femoral head  12  may include a structure (not shown) to ensure a proper rotational orientation of femoral head  12  on femoral component  11 . This structure may be a key or keyway extending from or formed in femoral head  12 , for example, which corresponds with a keyway or key formed in or extending from femoral component  11  to ensure that chamfer  20  is properly oriented. Advantageously, chamfer  20  provides a ramped surface or entrance plane for lubrication fluid, and increases the entrainment velocity thereof to promote entrainment of lubricating fluid throughout articular interface  24 . 
     Advantageously, femoral head  12  with polar coating  18  allows for an optimal area of contact between dissimilar materials (defined by articular interface  24 ) while retaining the ability to use a relatively large femoral head. A large femoral head offers certain advantages over a smaller counterpart, such as improved range of motion and reduced likelihood of dislocation. With the area of articular interface  24  optimized via polar coating  18 , the benefits derived from the use of a large femoral head (i.e., a femoral head having a diameter of 22 mm to 60 mm or larger) are retained while wear between femoral head  12  and acetabular liner  14  is reduced. 
     Also advantageously, femoral head  12  has improved strength, in that metal substrate  13  of femoral head  12  ( FIG. 2 ) is resistant to fracture. Thus, femoral head  12  has strength characteristics consistent with an entirely metallic femoral head, as well as the desirable wear characteristics of a ceramic femoral head. 
     In an exemplary embodiment, polar coating  18  and optional coating  16  are made of a ceramic material, while substrate  13  of femoral head  12  and acetabular liner  14  are made of a metallic material. Coatings  16 ,  18  may minimize patient exposure to the metallic substrate  13 . However, it is within the scope of the present disclosure that other materials may be used for optional coating  16  and/or polar coating  18 , such as alumina, or non-ceramic materials such as nitrides and metallic compounds, and that coating  16  may be eliminated entirely. It is also contemplated that one or more coatings may be placed on the interior surface of the acetabular liner, rather than on the femoral head. Further, although the illustrated embodiment discloses a spherical or spheroidal articular surface, it is contemplated that other surface geometries may be used. For example, the coated outer surface of substrate  13  and/or the surface of the polar coating  18  itself may be conicoidal, or may have any complex curved surface. 
     It is further contemplated that coatings in accordance with the present disclosure may also be used in other contexts, such as for glenoid prostheses or in other anatomical surfaces. Such coatings may also be used in the context of non-ball and socket joints, such as in knee prostheses for articulation between the tibia and femur or femur and patella, for example. Moreover, such coatings may be used in any context in which disparate material properties, such as differing hardnesses, are required or desired, or in which it is desired or required to define an articular surface with less than the entire available surface area of the surface. 
     While this invention has been described as having an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.