Patent Publication Number: US-7914580-B2

Title: Prosthetic ball-and-socket joint

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part of application Ser. No. 12/714,288, filed Feb. 26, 2010, which is currently pending, which is a Continuation-in-Part of application Ser. No. 11/936,601, filed Nov. 7, 2007, which is currently pending. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to medical implants, and more particularly to prosthetic joints having conformal geometries and wear resistant properties. 
     Medical implants, such as knee, hip, and spine orthopedic replacement joints and other joints and implants have previously consisted primarily of a hard metal motion element that engages a polymer contact pad. This has usually been a high density high wear resistant polymer, for example Ultra-High Molecular Weight Polyethylene (UHMWPE), or other resilient material. The problem with this type of configuration is the polymer eventually begins to degrade due to the caustic nature of blood, the high impact load, and high number of load cycles. As the resilient member degrades, pieces of polymer may be liberated into the joint area, often causing accelerated wear, implant damage, and tissue inflammation and harm. 
     It is desirable to employ a design using a hard member on a hard member (e.g. metals or ceramics), thus eliminating the polymer. Such a design is expected to have a longer service life. Extended implant life is important as it is now often required to revise or replace implants. Implant replacement is undesirable from a cost, inconvenience, patient health, and resource consumption standpoint. 
     Implants using two hard elements of conventional design will be, however, subject to rapid wear. First, a joint having one hard, rigid element on another will not be perfectly shaped to a nominal geometry. Such imperfections will result in points of high stress, thus causing localized wear. Furthermore, two hard elements would lack the resilient nature of a natural joint. Natural cartilage has a definite resilient property, absorbing shock and distributing periodic elevated loads. This in turn extends the life of a natural joint and reduces stress on neighboring support bone and tissue. If two rigid members are used, this ability to absorb the shock of an active lifestyle could be diminished. The rigid members would transmit the excessive shock to the implant to bone interface. Some cyclical load in these areas stimulates bone growth and strength; however, excessive loads or shock stress or impulse loading the bone-to-implant interface will result in localized bone mass loss, inflammation, and reduced support. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides a prosthetic joint having wear-resistant contacting surfaces with conformal properties. 
     According to one aspect of the invention, a prosthetic joint includes: (a) a first member of rigid material including a nominal interior cup surface, the interior including: (i) first flange defining a wear-resistant first contact rim protruding inward relative to the cup surface, and located at or near the apex of the cup; and (ii) a second flange defining a wear-resistant second contact rim protruding inward relative to the cup surface, and second flange located at or near an outer periphery of the first member; (b) a second member comprising rigid material with a wear-resistant, convex third contact surface; (c) where the first and second contact rims bear against the third surface, so as to transfer axial and lateral loads between the first and second members, allowing pivoting motion therebetween; and (d) wherein the flanges are configured so as to permit the first and second contact rims to conform in an irregular shape to the contact surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a cross-sectional view of a portion of a resilient contact member constructed in accordance with the present invention; 
         FIG. 2  is an enlarged view of the contact member of  FIG. 1  in contact with a mating joint member; 
         FIG. 3  is a side view of a resilient contact member in contact with a mating joint member; 
         FIG. 4  is a cross-sectional view of a cup for an implant according to an alternate embodiment of the invention; 
         FIG. 5  is an enlarged view of a portion of the cup of  FIG. 4 ; 
         FIG. 6  is a perspective view of a finite element model of a joint member; 
         FIG. 7  is a cross-sectional view of an implant joint including a flexible seal; 
         FIG. 8  is an enlarged view of a portion of  FIG. 7 ; 
         FIG. 9  is a side view of a prosthetic joint constructed in accordance with an aspect of the present invention; 
         FIG. 10  is a cross-sectional view of the prosthetic joint of  FIG. 9  in an unloaded condition; 
         FIG. 11  is a cross-sectional view of one of the members of the prosthetic joint of  FIG. 9 ; 
         FIG. 12  is an enlarged view of a portion of  FIG. 10 ; 
         FIG. 13  is a cross-sectional view of the prosthetic joint of  FIG. 9  in a loaded condition; 
         FIG. 14  is an enlarged view of a portion of  FIG. 13 ; 
         FIG. 15  is a cross-sectional view of an alternative joint member; 
         FIG. 16  is an enlarged view of a portion of  FIG. 15 ; 
         FIG. 17  is a cross-sectional view of another alternative joint member; 
         FIG. 18  is a cross-sectional view of another alternative joint member including a filler material; 
         FIG. 19  is a cross-sectional view of another alternative joint member including a wiper seal; 
         FIG. 20  is a cross-sectional view of another alternative prosthetic joint; 
         FIG. 21  is a cross-sectional view of a prosthetic joint constructed in accordance with another aspect of the present invention; 
         FIG. 22  is a cross-sectional view of a prosthetic joint constructed in accordance with yet another aspect of the present invention; and 
         FIG. 23  is a perspective view of a joint member having a grooved surface. 
         FIG. 24  is a exploded perspective view of two mating joint members; 
         FIG. 25  is a top plan view of one of the joint members shown in  FIG. 24 ; 
         FIG. 26  is a cross-sectional view of one of the joint members shown in  FIG. 24 ; 
         FIG. 27  is a contact stress plot of the joint member shown in  FIG. 26 ; 
         FIG. 28  is a perspective view of a rigid joint member used for comparison purposes; 
         FIG. 29  is a cross-sectional view of the joint member shown in  FIG. 28 ; and 
         FIG. 30  is a contact stress plot of the joint member shown in  FIG. 29 ; 
         FIG. 31  is a cross-sectional view of a prosthetic joint constructed in accordance with another aspect of the present invention; 
         FIG. 32  is an enlarged view of a portion of the joint shown in  FIG. 31 ; 
         FIG. 33  is a cross-sectional view of a cup member of the joint shown in  FIG. 31 ; 
         FIG. 34  is a greatly enlarged cross-sectional view of a portion of the joint shown in  FIG. 31  in an initial condition; 
         FIG. 35  is a greatly enlarged cross-sectional view of a portion of the joint shown in  FIG. 31  after an initial wear-in period; 
         FIG. 36  is a graph showing contact pressure of the joint of  FIG. 31  compared to the number of operating cycles; 
         FIG. 37  is a cross-sectional view of a prosthetic joint constructed in accordance with another aspect of the present invention; and 
         FIG. 38  is an enlarged view of a portion of the joint shown in  FIG. 37 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a specialized implant contact interface (implant geometry). In this geometry, an implanted joint includes two typically hard (i.e. metal or ceramic) members; however, at least one of the members is formed such that it has the characteristics of a resilient member, such as: the ability to absorb an impact load; the ability to absorb high cycle loading (high endurance limit); the ability to be self cleaning; and the ability to function as a hydrodynamic and/or hydrostatic bearing. 
     Generally, the contact resilient member is flexible enough to allow elastic deformation and avoid localized load increases, but not so flexible as to risk plastic deformation, cracking and failure. In particular, the resilient member is designed such that the stress levels therein will be below the high-cycle fatigue endurance limit. As an example, the resilient member might be only about 10% to about 20% as stiff as a comparable solid member. It is also possible to construct the resilient member geometry with a variable stiffness, i.e. having a low effective spring rate for small deflections and a higher rate as the deflections increase, to avoid failure under sudden heavy loads. 
       FIG. 1  illustrates an exemplary contact member  34  including a basic resilient interface geometry. The contact member  34  is representative of a portion of a medical implant and is made of one or more metals or ceramics (for example, partially stabilized Zirconia). It may be coated as described below. The geometry includes a lead-in shape, Z 1  and Z 2 , a contact shape, Z 3  and Z 4 , a lead-out shape, Z 5  and Z 6 , and a relieved shape, Z 7 . It may be desired to vary the cross-sectional thickness to achieve a desired mechanical stiffness to substrate resilience characteristic. The presence of the relieved region Z 7  introduces flexibility into the contact member  34 , reduces the potential for concentrated point contact with a mating curved member, and provides a reservoir for a working fluid. 
     The Z 7  region may be local to the contact member  34  or may be one of several. In any case, it may contain a means of providing fluid pressure to the internal contact cavity to produce a hydrostatic interface. A passive (powered by the regular motion of the patient) or active (powered by micro components and a dedicated subsystem) pumping means and optional filtration may be employed to provide the desired fluid interaction. 
     A hydrodynamic interface is desirable as, by definition, it means the contact member  34  is not actually touching the mating joint member. The lead-in and lead-out shapes Z 1 , Z 2 , Z 5 , Z 6  are configured to generate a shear stress in the working fluid so as to create the fluid “wedge” of a hydrodynamic support. 
       FIG. 2  shows a closer view of the contact member  34 . It may be desirable to make the contact radius (Z 3  and Z 4 ) larger or smaller, depending on the application requirement and flexural requirement. For example,  FIG. 3  illustrates the contact member  34  in contact with a mating joint member  38  having a substantially larger radius than the contact member  34 . The radius ratio between the two joint members is not particularly critical, so long as one of the members exhibits the resilient properties described herein. 
     The contact member  34  includes an osseointegration surface “S”, which is a surface designed to be infiltrated by bone growth to improve the connection between the implant and the bone. Osseointegration surfaces may be made from materials such as TRABECULAR METAL, textured metal, or sintered or extruded implant integration textures. TRABECULAR METAL is an open metal structure with a high porosity (e.g. about 80%) and is available from Zimmer, Inc., Warsaw, Ind. 46580 USA. 
       FIGS. 4 and 5  illustrate a cup  48  of metal or ceramic with two integrally-formed contact rings  50 . More contact rings may be added if needed. As shown in  FIG. 5 , the volume behind the contact rings  50  may be relieved. This relieved area  52  may be shaped so as to produce a desired balance between resilience and stiffness. A varying cross-section geometry defined by varying inner and outer spline shapes may be desired. In other words, a constant thickness is not required. A material such as a gel or non-Newtonian fluid (not shown) may be disposed in the relieved area  52  to modify the stiffness and damping characteristics of the contact rings  50  as needed for a particular application. The cup  48  could be used as a stand-alone portion of a joint, or it could be positioned as a liner within a conventional liner. The contact ring  50  is shown under load in  FIG. 6 , which depicts contour lines of highest compressive stress at “C 1 ”. This is the portion of the contact ring  50  that would be expected to undergo bending first. The bearing interface portion of the resilient contact member could be constructed as a bridge cross-section supported on both sides as shown or as a cantilevered cross-section depending on the desired static and dynamic characteristics. 
       FIGS. 7 and 8  illustrate an implant  56  of rigid material which includes a wiper seal  58 . The wiper seal  58  keeps particles out of the contact area (seal void)  60  of the implant  58 , and working fluid (natural or synthetic) in. The seal geometry is intended to be representative and a variety of seal characteristics may be employed; such as a single lip seal, a double or multiple lip seal, a pad or wiper seal made from a variety of material options. Different seal mounting options may be used, for example a lobe in a shaped groove as shown in  FIGS. 7 and 8 , a retaining ring or clamp, or an adhesive. The wiper seal  58  may also be integrated into the contact face of the interface zone. 
     It may be desirable to create a return passage  62  from the seal void region  60  back into the internal zone  64  in order to stabilize the pressure between the two and to allow for retention of the internal zone fluid if desired. This is especially relevant when the hydrostatic configuration is considered. 
       FIGS. 9-14  illustrate a prosthetic joint  100  comprising first and second members  102  and  104 . The illustrated prosthetic joint  100  is particularly adapted for a spinal application, but it will be understood that the principles described herein may be applied to any type of prosthetic joint. Both of the members  102  and  104  are bone-implantable, meaning they include osseointegration surfaces, labeled “S”, which are surfaces designed to be infiltrated by bone growth to improve the connection between the implant and the bone. Osseointegration surfaces may be made from materials such as TRABECULAR METAL, textured metal, or sintered or extruded implant integration textures, as described above. As shown in  FIG. 10 , a central axis “A” passes through the centers of the first and second members  102  and  104  and is generally representative of the direction in which external loads are applied to the joint  100  in use. In the illustrated examples, the first and second joint members are bodies of revolution about this axis, but the principles of the present invention also extend to shapes that are not bodies of revolution. 
     The first member  102  includes a body  106  with a perimeter flange  116  extending in a generally radially outward direction at one end. Optionally, a disk-like base  108  may be disposed at the end of the body  106  opposite the flange  116 , in which case a circumferential gap  111  will be defined between the base  106  and the flange  116 . The first member  102  is constructed from a rigid material. As used here, the term “rigid” refers to a material which has a high stiffness or modulus of elasticity. Nonlimiting examples of rigid materials having appropriate stiffness for the purpose of the present invention include stainless steels, cobalt-chrome alloys, titanium, aluminum, and ceramics. By way of further example, materials such as polymers would generally not be considered “rigid” for the purposes of the present invention. Generally, a rigid material should have a modulus of elasticity of about 0.5×10 6  psi or greater. Collectively, one end of the body  106  and the flange  116  define a wear-resistant, concave first contact surface  118 . As used herein, the term “wear-resistant” refers to a material which is resistant to surface material loss when placed under load. Generally the wear rate should be no more than about 0.5 μm (0.000020 in.) to about 1.0 μm (0.000040 in.) per million cycles when tested in accordance with ASTM Guide F2423. As a point of reference, it is noted that any of the natural joints in a human body can easily experience one million operating cycles per year. Nonlimiting examples of wear-resistant materials include solid metals and ceramics. Known coatings such as titanium nitride, chrome plating, carbon thin films, and/or diamond-like carbon coatings may be used to impart wear resistance to the first contact surface  118 . Optionally, the first contact surface  118  could comprise a separate face layer (not shown) of a wear-resistant material such as ultra-high molecular weight (UHMW) polyurethane. 
     The first contact surface  118  includes a protruding peripheral rim  120  (see  FIG. 11 ), and a recessed central portion  122 , which may also be considered a “pocket” or a “relief”. As used herein, the term “recessed” as applied to the central portion  122  means that the central portion  122  lies outside of the nominal exterior surface of the second member  104  when the joint  100  is assembled. In one configuration, shown in  FIGS. 9-14 , and best seen in  FIG. 11 , the rim  120  is concave, with the radius of curvature being quite high, such that the cross-sectional shape of the surface of the rim  120  approaches a straight line.  FIGS. 15 and 16  show another configuration of a joint member  102 ′ in which the rim  120 ′ has a convex-curved cross-sectional shape. The cross-sectional shape of the rim may be flat or curved as necessary to suit a particular application. 
     The annular configuration of first contact surface  118  with the protruding rim  120  results in a configuration which permits only pivoting and rotational motion, and is statically and dynamically determinate for the life of the joint  100 . In contrast, prior art designs employing mating spherical shapes, even very accurate shapes, quickly reach a statically and dynamically indeterminate condition after use and wear. This condition accelerates wear, contributes to the fretting corrosion wear mechanism, and permits undesired lateral translation between the joint members. 
     The second member  104  is also made from a rigid material and has a wear-resistant, convex second contact surface  124 . The first and second contact surfaces  118  and  124  bear directly against each other so as to transfer axial and lateral loads from one member to the other while allowing pivoting motion between the two members  102  and  104 . 
     Nominally the first and second members  102  and  104  define a “ring” or “band” contact interface therebetween. In practice it is impossible to achieve surface profiles completely free of minor imperfections and variations. If the first and second members  102  and  104  were both completely rigid, this would cause high Hertzian contact stresses and rapid wear. Accordingly, an important feature of the illustrated joint  100  is that the flange  116  (and thus the first contact surface  118 ) of the first member  102  is conformable to the second contact surface  124  when the joint is placed under load. 
       FIGS. 10 and 12  show a cross-sectional view of the flange  116  in an unloaded condition or free shape. It can be seen that the distal end of the rim  120  contacts the second contact surface  124 , while the inboard end of the rim  120  (i.e. near where the flange  116  joins the body  106 ) does not.  FIGS. 13 and 14  show the flange  116  in a deflected position or loaded shape, where substantially the entire section width of the rim  120  contacts the second contact surface  124 , resulting in a substantially increased contact surface area between the two members  102  and  104 , relative to the free shape. The rim  120 ′ of the joint member  102 ′ (see  FIG. 16 ) is similarly conformable; however, given the curved cross-sectional shape, the total amount of surface contact area remains substantially constant in both loaded and unloaded conditions, with the rim  120 ′ undergoing a “rolling” or “rocking” motion as the loading changes. 
     The conformable nature of the flange  116  is explained in more detail with reference to  FIGS. 24 through 30 . As noted above, the first member  102  has a flange  116  and a concave first contact surface  118 . The second member  104  has a convex second contact surface  124 . When assembled and in use the joint  100  is subject, among other loads, to axial loading in the direction of the arrows labeled “F” in  FIG. 24  (i.e. along axis “A” of  FIG. 10 ). As previously stated, it is impossible in practice for either of the contact surfaces  118  or  124  to be perfect surfaces (i.e. a perfect sphere or other curve or collection of curves). It is believed that in most cases that a defect such as a protrusion from the nominal contact surface of just 0.00127 mm (0.00005 in.), that is, 50 millionths of a inch, or larger, would be sufficient to cause fretting corrosion and failure of a metal-on-metal joint constructed to prior art standards. A defect may include a variance from a nominal surface shape as well as a discontinuity in the contact surface. Defects may arise through a variety of sources such as manufacturing, installation, and/or operating loads in the implanted joint. 
       FIG. 25  shows the second member  104  which in this particular example varies from a nominal shape in that it is elliptical rather than circular in plan view. The elliptical shape is grossly exaggerated for illustrative purposes. For reference, the dimensions of the second member  104  along the major axis labeled “X” is about 0.0064 mm (0.00025 in.) larger than its dimension along the minor axis labeled “Y”. When assembled and loaded, the flange  116  conforms to the imperfect second contact surface  124  and deflects in an irregular shape. In other words, in addition to any uniform deflection which may be present, the deflected shape of the flange  116  includes one or more specific locations or portions that are deflected towards or away from the nominal free shape to a greater or lesser degree than the remainder of the flange  116 . Most typically the deflected shape would be expected to be non-axisymmetric. For example, the deflection of the flange  116  at points located at approximately the three o&#39;clock and nine o&#39;clock positions is substantially greater than the deflection of the remainder of the flange  116 . As a result, the contact stress in that portion of the first contact surface  118  is relieved.  FIG. 27  is a plan view plot (the orientation of which is shown by arrow in  FIG. 26 ) which graphically illustrates the expected contact stresses in the first contact surface  118  as determined by analytical methods. The first contour line “C 2 ” shows that a very low level of contract stress is present around the entire perimeter of the first contact surface  118 . This is because the entire first contact surface  118  is in contact with the second contact surface  124 . Another contour line “C 3 ” represents the areas of maximum contact stress corresponding to the protruding portions of the elliptical second contact surface  124 . 
     For comparative purposes,  FIGS. 28 and 29  depict a member  902  constructed according to prior art principles. The member  902  has a contact surface  918  with an identical profile and dimensions of the first contact surface  118  of the first member  102 . However, consistent with the prior art, the member  902  has a massive body  920  behind the entire contact surface  918 , rendering the entire member  902  substantially rigid.  FIG. 30  graphically illustrates the expected contact stresses in the contact surface  918  as determined by analytical methods, when the member  902  is assembled and placed in contact with the second member  104 , using the same applied load as depicted in  FIG. 27 . Because of the rigidity of the member  902 , a “bridging” effect is present wherein contact between the contact surfaces (one of which is circular in plan view, and the other of which is elliptical) effectively occurs at only two points, located at approximately the three o&#39;clock and nine o&#39;clock positions. A first contour line “C 4 ” shows two discrete areas where the lowest level of contract stress is present. These lines are not contiguous because there is no contact in the remaining area of the contact surfaces (for example at the six o&#39;clock and twelve o&#39;clock positions). Another contour line “C 5 ” represents the areas of maximum contact stress. Analysis shows a peak contact stress having a magnitude of two to twenty times (or more) the peak contact stress of the inventive joint as shown in  FIG. 27 . 
     To achieve this controlled deflection, the flange  116  is thin enough to permit bending under working loads, but not so thin as to allow material yield or fatigue cracking The deflection is opposed by the elasticity of the flange  116  in bending, as well as the hoop stresses in the flange  116 . To achieve long life, the first member  102  is sized so that stresses in the flange  116  will be less than the endurance limit of the material, when a selected external load is applied. In this particular example, the joint  100  is intended for use between two spinal vertebrae, and the design average axial working load is in the range of about 0 N (0 lbs) to about 1300 N (300 lbs.). These design working loads are derived from FDA-referenced ASTM and ISO standards for spinal disc prostheses. In this example, the thickness of the flange  116 , at a root  126  where it joins the body  106  (see  FIG. 12 ) is about 0.04 mm (0.015 in.) to about 5.1 mm (0.200 in.), where the outside diameter of the flange  116  is about 6.4 mm (0.25 in.) to about 7.6 cm (3.0 in.). 
     The joint members may include multiple rims. For example,  FIG. 17  illustrates a joint member  202  where the first contact surface  218  includes two protruding rims  220 , with a circumferential groove or relief area  228  therebetween. The presence of multiple rims increases the contact surface areas between the two joint members. 
     If present, the circumferential gap between the flange and the base of the joint member may be filled with resilient nonmetallic material to provide damping and/or additional spring restoring force to the flange.  FIG. 18  illustrates a joint member  302  with a filler  304  of this type. Examples of suitable resilient materials include polymers, natural or synthetic rubbers, and the like. 
     As discussed above, the joint may incorporate a wiper seal. For example,  FIG. 19  illustrates a joint member  402  with a resilient wiper seal  404  protruding from the rim  420  of the first contact surface  418 . The wiper seal  404  keeps particles out of the contact area (seal void), while containing working fluid (natural or synthetic). The seal geometry is intended to be representative and a variety of seal characteristics may be employed; such as a single lip seal, a double or multiple lip seal. A pad or wiper seal may be made from a variety of material options. Different seal mounting options may be used, for example a lobe in shaped groove as shown in  FIG. 18 , a retaining ring or clamp, adhesion substance. The seal may also be incorporated into the contact face of the interface zone. 
     The joint construction described above can be extended into a three-part configuration. For example,  FIG. 20  illustrates a prosthetic joint  500  having first, second, and third members  502 ,  504 , and  506 . The first and second members  502  and  504  are similar in construction to the first member  102  described above, and each includes a body  508 , an optional disk-like base  510 , and a flange  512 . The flanges  512  define wear-resistant concave first and second contact surfaces  514  and  516 , each of which includes a protruding peripheral rim, and a recessed central portion as described above. The third member  506  has a double-convex shape defining opposed wear-resistant, convex third and fourth contact surfaces  524  and  526 . The first and second  514  and  516  bear against the third and fourth contact surfaces  524  and  526 , respectively, so as to transfer axial (i.e. compression) and lateral loads between the first and second members  502  and  504  through the third member  506 , while allowing pivoting motion between the members  502 ,  504 , and  506 . The first and second contact surfaces  514  and  516  are conformal to the third and fourth contact surfaces  524  and  526  as described in more detail above. 
       FIG. 21  illustrates an alternative prosthetic joint  600  comprising first and second members  602  and  604  constructed from rigid materials. Both of the members  602  and  604  are bone-implantable, meaning they include osseointegration surfaces, labeled “S”, as described in more detail above. 
     The first member  602  is hollow and includes a disk-like base  606  and a cup  608 , interconnected by a peripheral wall  610 . An interior cavity  612  is defined between the base  606  and the cup  608 . The cup  608  is constructed from a rigid material and defines a wear-resistant, concave first contact surface  614 . The first contact surface  614  includes a protruding peripheral rim  616 , and a recessed central portion  618 , which may also be considered a “pocket” or a “relief”. The rim  616  may have a conical or curved cross-sectional shape. 
     The second member  604  is constructed from a rigid material and has a wear-resistant, convex second contact surface  620 . The first and second contact surfaces  614  and  616  bear directly against each other so as to transfer axial and laterals loads from one member to the other while allowing pivoting motion between the two members  602  and  604 . 
     As described above with reference to the prosthetic joint  100 , the cup  606  of the first member  602  is thin enough to permit bending under working loads, but not so thin as to allow material yield or fatigue cracking. The first contact surface  614  is thus conformable to the second contact surface  620  when the prosthetic joint  600  is placed under external load. 
     An inverted configuration of hollow members is also possible. For example,  FIG. 22  illustrates a prosthetic joint  700  comprising first and second members  702  and  704 , both constructed of rigid materials. The first member  702  is solid and includes a wear-resistant, concave first contact surface  708 . The first contact surface  708  includes a protruding peripheral rim  710 , and a recessed central portion  712 , which may also be considered a “pocket” or a “relief”. 
     The second member  704  is hollow and includes a dome  714  connected to a peripheral wall  716 . An interior cavity  718  is defined behind the dome  714 . The dome  714  defines a wear-resistant, convex second contact surface  720 , which is shaped and sized enough to permit bending under working loads, but not so as to allow material yield or fatigue cracking The second contact surface  720  is thus conformable to the first contact surface  708  when the prosthetic joint  700  is placed under external load. 
     The first and second contact surfaces  708  and  720  bear directly against each other so as to transfer axial and lateral loads from one member to the other while allowing pivoting motion between the two members  702  and  704 . 
     Any of the contact surfaces described above may be provided with one or more grooves formed therein to facilitate flow of fluid or debris. For example,  FIG. 23  illustrates a joint member  800  including a concave contact surface  802 . The contact surface  802  includes a circular groove  804 , and plurality of generally radially-extending grooves  806  which terminate at the center of the contact surface  802  and intersect the circular groove  804 . 
       FIGS. 31-33  illustrate an alternative prosthetic joint  1000  comprising first and second members  1002  and  1004 . The illustrated prosthetic joint  1000  is particularly adapted for a ball-and-socket joint application such as is found in a human hip joint (i.e. the acetabulofemoral joint) or shoulder joint (i.e. the glenohumeral joint), but it will be understood that the principles described herein may be applied to any type of prosthetic joint. Both of the members  1002  and  1004  are bone-implantable, meaning they include osseointegration surfaces, labeled “S”, which are surfaces designed to be infiltrated by bone growth to improve the connection between the implant and the bone. 
     Osseointegration surfaces may be made from materials such as TRABECULAR METAL, textured metal, or sintered or extruded implant integration textures, as described above. As shown in  FIG. 31 , a nominal central axis “A” passes through the centers of the first and second members  1002  and  1004  In the illustrated examples, the first and second joint members  1002  and  1004  are bodies of revolution about this axis, but the principles of the present invention also extend to non-axisymmetric shapes. 
     The first member  1002  is constructed from a rigid material as described above. The first member  1002  is concave and may generally be thought of as a “cup”, although it need not have any particular degree of curvature. Its interior defines a nominal cup surface  1006  shown by the dashed line in  FIG. 33 . The interior includes an annular first flange  1008  which is located relatively near an apex  1010  of the first member  1002  and which extends in a generally radial direction relative to the axis A. The first flange  1008  is defined in part by an undercut groove  1012  formed in the first member  1002 . A ramped surface  1014  forms a transition from the groove  1012  to the nominal cup surface  1006 . The first flange  1008  includes a protruding first contact rim  1016 . As used herein, the term “protruding” as applied to the first contact rim  1016  means that the first contact rim  1016  lies inside of the nominal cup surface  1006  when the joint  1000  is assembled. The first contact rim  1016  may have a curved or toroidal cross-sectional shape. 
     The interior also includes an annular second flange  1018  which is located at or near an outer peripheral edge  1020  of the first member  1002  and which extends in a generally axial direction relative to the axis A. The second flange  1018  is defined in part by an undercut groove  1022  formed in the first member  1002 . The second flange  1018  includes a protruding second contact rim  1024 . As used herein, the term “protruding” as applied to the second contact rim  1024  means that the second contact rim  1024  lies inside of the nominal cup surface  1006  when the joint  1000  is assembled. The second contact rim  1024  may have a curved or toroidal cross-sectional shape. Depending on the particular application, joint  1000  may include more than two flanges defining more than two contact rims. 
     In the illustrated example, the first member  1002  includes a face layer  1026  of a known coating such as titanium nitride, chrome plating, carbon thin films, and/or diamond-like carbon coatings, and/or a another wear-resistant material such as ultra-high molecular weight (UHMW) polyurethane. This face layer  1026  is used to impart wear resistance, as described above. The face layer  1026  may be extraordinarily thin. In this particular example, its as-applied thickness is about 0.0041 mm (0.00016 in.), or 160 millionths of a inch thick. The face layer  1026  is applied at a substantially uniform thickness over the surface profile which is defined by machined or formed features of the substrate. Alternatively, and especially if a much thicker face layer were used, the face layer could be profiled so as to define both the nominal cup surface  1006  and the first and second contact rims  1016  and  1024 . 
     The second member  1004  is also made from a rigid material and has a wear-resistant, convex contact surface  1028 . In the specific example illustrated, the second member  1004  includes a face layer  1030  of a known coating such as titanium nitride, chrome plating, carbon thin films, and/or diamond-like carbon coatings, and/or a another wear-resistant material such as ultra-high molecular weight (UHMW) polyurethane. This face layer  1030  is used to impart wear resistance, and may be quite thin, as described above. The first and second contact rims  1016  and  1024  bear directly against the contact surface  1028  so as to transfer axial and lateral loads from one member to the other while allowing pivoting motion between the two members  1002  and  1004 . 
     The annular configuration of contact rims  1016  and  1024  results in a joint configuration which permits only pivoting and rotational motion, and is statically and dynamically determinate for the life of the joint  1000 . In particular, the presence of the relatively widely-spaced contact rims  1016  and  1024 , and the peripheral positioning of the second contact rim  1024  is highly effective in resisting any translation of the first and second members  1002  and  1004  lateral to the axis A. 
     Nominally the first and second contact rims  1016  and  1024  define two separate “ring” or “band” contact interfaces with the contact surface  1028  of the second member  1004 . In practice it is impossible to achieve surface profiles completely free of minor imperfections and variations. If the first and second members  1002  and  1004  were both completely rigid, this would cause high Hertzian contact stresses (i.e. non-uniform contact) and rapid wear. Accordingly, an important feature of the illustrated joint  1000  is that the flanges  1008  and  1018  (and thus the contact rims  1016  and  1024 ) of the first member  1002  are conformable to the contact surface  1028  when the joint  1000  is placed under load. The flanges  1008  and  1018  can conform to the imperfect contact surface  1028  and deflect in an irregular shape. In other words, in addition to any uniform deflection which may be present, the deflected shape of the flanges  1008  and  1018  can include one or more specific locations or portions that are deflected towards or away from the nominal free shape to a greater or lesser degree than the remainder of the flanges  1008  and  1018 . To achieve this controlled deflection, the flanges  1008  and  1018  are thin enough to permit bending under working loads, but not so thin as to allow material yield or fatigue cracking, or to exceed the endurance limit of the material. The deflection is opposed by the elasticity of the flanges  1008  and  1018  in bending, as well as the hoop stresses in the flanges  1008  and  1018 . 
     The contact rims  1016  and  1024  are designed in conjunction with the contact surface  1028  to create a wear characteristic that is constantly diminishing (similar to an asymptotic characteristic). With reference to  FIG. 32 , the as-manufactured or initial curvatures (e.g. radii) of the first and second contact rims  1016  and  1024 , denoted “R” are different from the curvature (e.g. radius) of the contact surface  1028 , denoted “r”. It is noted that the direction of curvature (i.e. the convexity or second derivative shape) of the first and second contact rims  1016  and  1024  may be the same as, or opposite to, that of the contact surface  1028  upon initial manufacture. In this example they are opposite. When assembled and placed under load, the annular interface between each of the contact rims  1016  and  1024  and the contact surface  1028  will have a characteristic width denoted “W”, (effectively creating a contact band). The initial dimensions R and r are selected such that, even using highly wear-resistant surfaces or coatings, some wear takes place during an initial wear-in period of movement cycles. As a result, the contact band width W increases during the initial wear-in period. This increases contact area and therefore decreases contact stress for a given load. After the initial wear-in period (which preferably occurs before the joint is implanted), the contact band reaches a post wear-in width at which the contact stress is below a selected limit, below which the rate of wear in the contacting surfaces approaches a very low number or zero, consistent with a long life of the joint  1000 .  FIG. 36  illustrates this wear characteristic, with the limit “L” depicted as a horizontal line. 
       FIGS. 34 and 35  are schematic views showing the initial wear-in of the surface of the contact rim  1016  at a microscopic (or nearly microscopic) level. It will be understood that these figures are greatly exaggerated for the purposes of illustration. On initial manufacture, as shown in  FIG. 34 , the curvatures R and r of the contact rim  1016  and the contact surface  1028  have opposite directions. When assembled, the contact band width W is some nominal value, for example about 0.03 mm (0.001 in.), and the total thickness “T” of the face layer  1026  is at its as-applied value of about 0.0041 mm (0.00016 in.) for example. The action of the wear-in period described causes the face layer  1026  to wear to a shape complementary to the contact surface  1028 . After this wear-in period the curvature of the portion of the contact rim  1016  within the contact band, denoted “R′”, and the curvature r of the contact surface  1028  are in the same direction, and the values of the two curvatures are substantially the same. For example, the thickness T at the location of the contact band may decrease by about 0.0004 mm (0.000014 in.), with a corresponding increase in the width of the contact band W to about 0.2 mm (0.008 in.). Analysis shows that this increase in contact band width and surface area can reduce mean contact pressure by over 80%. 
     The configuration of the flanges  1008  and  1018  are important in developing the constantly diminishing wear characteristics described above. In particular, the flanges  1008  and  1018  are sized and shaped so that deflections of the contact rims  1016  and  1024  under varying load are always essentially normal to their respective tangent points on the opposing contact surface  1028 , as the joint  1000  is loaded and unloaded. This ensures that the position of each of the contact bands remains constant and that the contact bands remain substantially uniform around the entire periphery of the joint  1000 . 
     An inverted configuration of the joint described above may be used. For example,  FIGS. 37 and 38  illustrate a prosthetic joint  1100  having first and second members  1102  and  1104  which are substantially similar in general construction to the members of the joint  1000  described above in terms of materials, coatings, and so for forth. However, in this joint  1100 , the concave member  1102  has a contact surface without protruding rings. The convex member  1104  has first and second flanges  1108  and  1118  which define first and second contact rims  1116  and  1124  which function in the same manner that the flanges and contact rims described above. 
     As noted above, known coatings such as titanium nitride, chrome plating, carbon thin films, and/or diamond-like carbon coatings may be used to impart wear resistance or augment the wear resistance of any of the contact surfaces and/or contact rims described above. To the same end, it may be desirable to surface treat either or both interfaces of any of the above-described implants or joints with a laser, shot peen, burnishing, or water shock process, to impart residual compressive stresses and reduce wear. The benefit could be as much from surface annealing and microstructure and microfracture elimination as smoothing itself 
     The foregoing has described prosthetic joints with wear-resistant properties and conformal geometries. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.