Abstract:
A prosthetic head for a femoral component has a metal shell with a tapered cavity. The shell has a part-spherical outer surface defining an inner portion terminating in an open end. A polymeric material completely fills the inner portion of the hollow shell extending from an inner surface of the shell to the open end. The polymeric material includes a conically tapered socket centered about the polar axis intermediate ends of the open end wherein the shell is a hollow titanium shell having an inner surface with a porous structure for receiving a portion of the polymeric material. The hollow titanium shell inner surface has at least one rib extending inwardly toward the conically tapered socket.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a femoral prosthesis for use in hip joint replacement procedures. More specifically the invention relates to a novel modular prosthetic head for a femoral prosthesis. 
         [0002]    In one embodiment, the present invention includes as components a hollow ball component having a polymeric filler with a molded tapered socket for connecting the ball component to a prosthetic femoral component stem having a conically tapered trunion. 
         [0003]    Artificial joint prostheses are widely used to restore joint mobility to patients affected by a variety of conditions, including degeneration of the joint and bone structure. Typically, the degenerated bone structure is replaced with an orthopedic implant that mimics, as closely as possible, the structure of the natural bone and performs its functions. The satisfactory performance of these implants can be affected not only by the design of the component itself, but also by the surgical positioning of the implanted component and the long-term fixation of the implant. Improper placement or positioning of the implant can adversely affect the goal of satisfactorily restoring the clinical bio-mechanics of the joint as well as impairing adequate fixation of the component when implanted. 
         [0004]    Orthopedic implants are constructed from materials that are stable in biological environments and withstand physical stress with minimal or controlled deformation. Such materials must possess strength, resistance to corrosion, biocompatibility, and good wear properties. Also, the implants include various interacting parts, which undergo repeated long-term physical stress inside the body. 
         [0005]    For these reasons, among others, the bone/implant interface and the connection between various parts of the implant must be durable and resistant to breakdown. This is especially important since installation of an orthopedic implant often involves an extensive medical procedure, and therefore replacement or revision of the installed implant is typically even more difficult. The requirements for the useful life of the implant continue to grow with the increase in human life expectancy and use in younger patients. As implants improve, more younger patients are considered as implant candidates. It is therefore desirable to develop implants that, while durable in their own right, minimize the difficulty of replacement or revision surgery should the implant eventually fail. 
         [0006]    The strength and longevity of implants in large part depend on the bone/implant interface. Various methods of connection are known in the art. For example, a hip joint is a ball-in-socket joint, and includes a rounded femoral head and a cup-like socket (acetabular cup) located in the pelvis. The surfaces of the rounded femoral head and the acetabula cup continually abrade each other as a person walks. The abrasion, along with normal loading, creates stress on the hip joint and adjacent bones. If the natural femoral head or the acetabular cup is replaced with an implant, this stress must be well tolerated by the implant&#39;s bearing surfaces to prevent implant failure. 
         [0007]    One type of hip prosthesis is a resurfacing femoral head. In this design, the natural bone neck is preserved and a hollow metal shell is implanted on the prepared head area of the femur. The inside or distal surface is covered by cancellous bone. The metal shell replaces diseased natural cartilage. Because the acetabular cartilage will wear out over time, this resurfacing is considered temporary solution and total hip replacement is eventually conducted. Hollow metal shell designs are well-known (UK 719,308, US 2010/0121458 A1). The shell is typically made of CoCr alloy. Especially, UK 719308 discloses a resurfacing prosthesis that is composed of CoCr shell filled with plastic core, where the plastic core directly contacts cancellous bone. 
         [0008]    The total hip replacement replaces diseased cartilage with both a prosthetic femoral head and acetabular cup. One design of the femoral head is a one piece component integral with a femoral stem. In this design, the neck length is fixed, however a patient&#39;s femoral neck length may vary from person to person. Another femoral prostheses comprises a separate stem part and a head part. The neck length is easily adjusted by different offsets to fit different patient neck lengths. The modular design is easy to use, and has become popular over the past 20 years. The stem part comprises a body section for location in the intramedullary cavity of the patient&#39;s femur, and a neck section at its proximal end for carrying the modular head part. The head part comprises an outer bearing surface formed as part of a sphere, which is for articulation in a cup component implanted into the patient&#39;s acetabulum. The stem part and the head part usually are separate components. The proximal end of the neck section of the stem part comprises a conically tapered trunion and a distal side of the head part comprises a tapered bore adapted to receive the trunion. This configuration allows for the femoral stem to be more readily implanted into the femur initially, and for a particular head part to then be chosen to suit during the procedure. Multiple femoral prosthetic head parts may be provided with, for example, different offsets and diameters for mating with different diameter acetabular cups. Modular prosthetic heads are typically made of solid metal ground into a spherical shape and include a tapered socket, such as a Morse taper in a distally facing surface thereof. However, the traditional metal-on-metal taper junction may have corrosion and fretting problems, especially with a titanium stem against a CoCr head. 
         [0009]    One solution for the taper corrosion is to change the metal head into ceramic head. Because ceramic is non-electric conducting material, there is no galvanic corrosion problem. However, ceramic heads are brittle compared to metal heads. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    One purpose of this invention is to provide a modular femoral head that engage a metal trunion on a femoral component which improves the performance and is free of corrosion. The femoral ball head has a polymeric tapered cavity and a metal or ceramic outer shell. The polymer cavity is non-electrically conductive, thus intrinsically eliminating galvanic corrosion when it mates a metallic stem taper. Also, the selected polymer is tough and wear resistant; there is no brittle fracture issue. The metal or ceramic shell provides wear resistance when it articulates against the acetabulum such as against natural cartilage or a prosthetic bearing made of UHMWPE or ceramic. Therefore, the present invention has the overall advantage over the current metal-on-metal and ceramic-on-metal modular design. 
         [0011]    Use of a polymer such as Polyetherether ketone (PEEK) produced a low corrosion current of a PEEK-on-metal taper trunion and reduced crevice corrosion in certain conditions. In an alternate embodiment a PEEK sleeve was placed on a Ti6Al4V stem taper. The PEEK sleeve mated with a CoCr female trunion. In another embodiment a PEEK coated Ti6Al4V stem taper was a mirror polished PEEK surface. In another embodiment PEEK was machined into a sleeve and polished. 
         [0012]    The above aspects of the invention are provided by a prosthetic head for a femoral component having a hollow, preferably titanium, shell having a thickness between 0.2 and 3.0 mm. The shell has a part-spherical outer surface defining an inner portion terminating in a flange extending inwardly towards a polar axis of the hollow shell. A polymeric material, such as PEEK, fills the inner portion of the hollow titanium shell extending from an inner surface of the shell to an inwardly facing surface of the flange or open end. The polymeric material includes a conically tapered socket formed therein centered about the polar axis intermediate radially spaced ends of the flange on the hollow titanium shell. The hollow shell may have an inner surface with a porous structure for lockingly receiving a portion of the polymeric material. The hollow shell inner surface may have at least one rib extending from an inner surface inwardly toward the conically tapered socket for lockingly receiving the polymer. The outer surface of the hollow titanium shell may be polished or coated with a ceramic. The polymeric material may be polyetheretherketone or polyetherketoneketone or other suitable polymers. The polymeric material may be glass or carbon fiber reinforced such as with chopped fibers. The conically tapered socket may have a polished inner surface or may be molded. The at least one rib on the inner spherical surface of the head may be generally perpendicular to the inner surface of the hollow titanium shell. The ribs may extend in the longitudinal or latitude direction and are spaced at predetermined intervals. The rib may include an inner flange extending perpendicular to the rib, these being “T” shaped. 
         [0013]    The above designs can be more expressed in the following table as combination of the options. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Shell 
                   
                 Core 
                 Bearing 
               
               
                 Example 
                 material 
                 Interlock structure 
                 material 
                 surface 
               
               
                   
               
             
             
               
                 1 
                 Ti6Al4V 
                 3D foam by additive 
                 PEEK 
                 TiO2* 
               
               
                   
                   
                 manufacturing 
               
               
                 2 
                 Ti6Al4V 
                 2D foam by PVD CoCr 
                 PEEK 
                 CoCr 
               
               
                 3 
                 CoCr 
                 3D by E-beam 
                 PEK 
                 CoCr 
               
               
                 4 
                 316 L 
                 3D by E-beam 
                 PEEK 
                 316L 
               
               
                 5 
                 Zr alloy 
                 3D foam by additive 
                 PPS 
                 ZrO2 
               
               
                   
                   
                 manufacturing 
               
               
                 6 
                 Ti6Al4V 
                 2D surface by CVD 
                 PEEK 
                 Al2O3 
               
               
                 7 
                 Ti6Al4V 
                 2D surface by PVD CrN 
                 PEI 
                 CrN 
               
               
                 8 
                 Al2O3 
                 2D surface by PVD CrN 
                 PEKK 
                 Al2O3 
               
               
                 9 
                 ZrO2 
                 2D surface by PVD CoCr 
                 PEK 
                 Zr2O3 
               
               
                   
               
               
                 *This can be done by the process of U.S. Pat. Nos. 8,268,099 and 8,266,810 
               
             
          
         
       
     
         [0014]    As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention will be described in more detail below with reference to the embodiments shown in the drawings wherein  FIG. 1  shows an isometric view of a typical modular prosthetic hip implant including a femoral component having a stem, a neck and a hollow spherical prosthetic femoral head; 
           [0016]      FIG. 2  shows a cross-sectional view of a spherical head of  FIG. 1  including an outer shell and an inner polymer filled part spherical cavity including a conically tapered recess and a rib structure; 
           [0017]      FIG. 3  shows a cross-sectional of an alternate embodiment of a part spherical head including an outer metal shell filled with a polymeric material including a trunion formed therein with a retaining flange extending from a distal end of the prosthetic femoral head; 
           [0018]      FIG. 4  shows a comparison of surface roughness profile (a) as-machined PEEK sleeve surface (top picture) and (b) polished PEEK sleeve surface (bottom picture); 
           [0019]      FIG. 5  shows the tensile bond strength of a PEEK bonding titanium results soaked in simulated body fluid at 37° C.; and 
           [0020]      FIG. 6  shows the mechanical locking provided by PVD CrN coating for PEEK bonding to chemically pure titanium (CP-Ti). 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In describing the preferred embodiments of the subject matter illustrated and to be described with respect to the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
         [0022]    Referring to the drawings, wherein like reference numerals represent like elements, there is shown in  FIGS. 1-3 , in accordance with various embodiments of the present invention, more particularly in  FIG. 1 , a femoral implant generally denoted as  110  used during a total hip replacement. Implant  110  includes a modular femoral head  10  having an outer surface  12  which is part-spherical in shape and adapted to engage a part-spherical cavity in a prosthetic acetabular implant. As is typical in modular implants  110 , the femoral component includes a distal stem portion  114 , a mid-stem portion  116  and a proximal stem portion  118 . A neck  122  extends proximally from upper surface  120  of proximal stem portion  118  and has a conically tapered trunion or spigot  124  adapted to form a taper lock within a conically tapered cavity within ball or head  10 . The conically tapered cavity is shown in  FIGS. 2 and 3  and denoted as  14 ,  14 ′ respectively. 
         [0023]    As best shown in  FIG. 2 , femoral head  10  is preferably formed from two separate parts a metal or ceramic shell  16  and a polymeric support  18 . Support  18  is interposed within shell  16  and forms a tapered trunion engaging cavity or socket  14 . The outside surface  20  of polymeric support  18  is designed to substantially mate with inside surface  22  of shell  16 . Shell  16  forms articulating surface  12 , and preferably has a thin, substantially uniform thickness such that the shape of inside surface  22  substantially matches that of articulating surface  12 . Accordingly, polymeric support  18  provides a majority of the appropriate spacing between articulating surface  12  and trunion engaging surface  14 . 
         [0024]    Various materials can be used in formation of part-spherical shell  16  and inner support  18 . Acceptable materials for shell  16  include various metals, such as titanium, CoCr, stainless steel (SS) and titanium alloys, or a ceramic material, such as alumina. Also a metal shell can be coated with ceramic such as alumina. If a polymeric material is used to form support  18 , the polymer may be reinforced with carbon or glass fiber, including long, short or micro fibers, as they are known in the art. Preferably, shell  16  is formed from a metal, such as titanium or titanium alloy, CoCr or SS having a thickness between about 0.2 mm and about 3.0 mm. In a preferred embodiment, shell  16  is formed from titanium and has a thickness of about 2 to 3 mm. 
         [0025]    Various materials may also be used in the formation of polymeric support  18 . Acceptable materials for support  18 , include polymeric materials including ULTEX®, PEEK, polycarbonate, polysulphone, XYLAR®, LEXAN® and PEKK. In one embodiment of the present invention, support  18  can be made from a fiber-reinforced polymeric material. Such materials may include PEEK reinforced with carbon fibers, which may comprise long, short or micro fibers. Further, shell  16  is preferably formed with a series of inwardly extending ribs  24  therein. Ribs  24  may increase the overall strength of shell  16  and, thus, of femoral head implant  10 , allowing for less-rigid and, possibly, less expensive materials to be used for support  18 . Still further, the inclusion of ribs  24  allows the material from which support  18  is formed to have a more uniform thickness. This is advantageous when forming support  18  using an injection molding process because uniform material thickness allows the material throughout the entire part to cool (and thus, shrink) uniformly. This helps prevent the part from warping during cooling. 
         [0026]    In a preferred embodiment of implant  10 , shell  16  is formed from a metal, preferably titanium or titanium alloy and support  18  is formed from a polymeric material, preferably PEEK. In such an arrangement, shell  16  is more preferably formed using a hydroform process. Hydroforming is a process that is generally known in the art and is useful for imparting complex, three-dimensional (“3D”) shapes into metal. Preferably, shell is formed using a vertical hydraulic hydroforming press. Such a process can be carried out by Aero Trades Manufacturing, located at 65 Jericho Turnpike, Mineola, N.Y. It is preferred that a metal subjected to a hydroform process is thin enough to be accurately formed by the process. It is also preferred that the material be thick enough to retain the shape imparted therein. The ideal thickness for shell in this embodiment will vary by the material and specific geometry used and will be known by those having reasonable skill in the art as indicated above 0.2 to 3 mm is acceptable. The use of a hydroform process to form shell  16  reduces the need for the additional process steps of CNC grinding or polishing, as are needed with a casting process. 
         [0027]    Generally, the combination of a shell  16  made from hydroformed metal and a support  18  made from a polymeric material allows for an implant  10  which is appropriately shaped and sufficiently rigid to provide an acceptable femoral component head, while being lightweight and cost-effective from a manufacturing standpoint. Further, the cost-effective manufacture of such implants makes it reasonable to use each of such implants in only one surgical procedure or, alternately, as a trial femoral head. The provision of such disposable trial implants eliminates the need to design such an implant to withstand multiple autoclave cycles, and to withstand multiple trial reductions, further lowering the manufacturing cost thereof. 
         [0028]    Shell  16  may be affixed to support  18  by a variety of methods, including using adhesives. Additionally, fixation may be done by providing inner surface of shell  16  with a porous metal coating  28  such as described in U.S. Pat. No. 7,537,664 or U.S. Publication No. 2011/0014081, the disclosures of which are incorporated herein by reference, to secure shell  16  to support  18 . Further, in addition to ribs  24 , corresponding tabs may be formed in appropriate portions of shell  16  and support  18  to achieve fixation therebetween. 
         [0029]    An integral formation of rib or ribs  24  on the inner surface  22  of shell  16  increases the rigidity of shell  16 , and accordingly of implant  10  overall. Rib  24  may be formed in a metal shell  16  by hydroforming. 
         [0030]    As shown in  FIG. 2 , shell  16  further includes a flange portion  32  extending perpendicularly from the end surface of rib  24 . Portion  32  further increases the rigidity of shell  16  and implant  10 . Additionally, flange portion  32  provides for an additional means of affixation between support  18  and shell  16 . In particular, in a preferred embodiment of the present invention, shell  16  is formed from hydroformed metal, preferably and support  18  is formed from PEEK. In this embodiment of  FIGS. 2 and 3 , support  18  is formed by molding the polymeric material onto the inner surface of shell  16 . In such a process, support  18  may be formed by injection-molding of a polymeric material into an appropriately shaped mold into which a pre-formed shell  16  has been inserted. Because the molten polymeric material can easily flow into and around any geometry formed in the shell, including ribs  24  or porous portion  28  and flange portion  32 , direct contact between the polymeric support  18  and the shell  16  may be the primary method of attachment therebetween. Incorporation of rib  24  and flange portion  32  furthers this attachment because the polymer flows into the shell, fully encasing the flange portion  32 . This direct contact between the two materials along the periphery of the shell provides sufficient purchase to fully affix the support  18  to shell  16 . The cavity is then machined into the head. 
         [0031]    Referring to  FIG. 3 , an alternative embodiment of the present invention is shown as an implant  10 ′ generally similar in structure to those discussed with respect to  FIG. 2 . Head  10 ′ a thin metal shell  16 ′ similar to that described above with regard to  FIG. 2  with the exception that the distal portion of the shell includes an annular flange  40  extending radially inwardly towards a polar axis  42  of head  10 ′. Ends  44  and  46  of flange  40  are spaced sufficiently to allow for the formation of a conically tapered socket  14 ′. Again a polymeric support  18 ′ is utilized to make shell  16 ′ rigid and allow for the formation of socket  14 ′. As discussed above, although not shown in  FIG. 3 , head  10 ′ can include ribs  24  with or without end section  32  extending perpendicular to the longitudinal extent of ribs  24 . 
         [0032]    As stated above, the inner surface of the socket  14 ,  14 ′ may be polished to allow the formation of a locking taper with the conically tapered trunion of the femoral component. The locking tapers may be a Morse taper junction within the contacting smooth tapered surfaces providing a sufficient locking force to couple the head to the femoral component trunion. Support  18  is made from polymeric reinforced carbon fiber. Carbon fiber is a reinforcing fiber known for its lightweight, high strength and high stiffness. Carbon fiber is produced by a high-temperature stretching process of an organic precursor fiber based on polyacrylonitrile (“PAN”), rayon, or pitch in an inert atmosphere at temperatures above 1,800 degrees, Fahrenheit. Fibers can be transformed by removing more non-carbon atoms via heat treating above 3,000 degrees Fahrenheit. After these fibers are produced, they can be utilized in many different forms. They can be woven into long, dry fabric, pre-impregnated with PEEK resin, wound onto spools for use in filament winding, or braided and chopped into small fibers. There are several ways in which to produce components using carbon fiber; however, all of such processes require the use of a mold to impart the necessary geometry into the carbon fiber. The mold used in such a process defines the shape of the component. Accordingly, any component that can be molded can be formed from carbon fiber reinforced PEEK. 
         [0033]    Molding processes used to form support  18  from carbon fiber include injection molding, autoclave molding, compression molding, bladder molding or resin transfer molding (“RTM”). Any of these methods can be used to produce knee femoral trials for total hip replacement (THR) and hip stem trials for THR. All of these types of molding processes force the carbon and resin to conform to the desired shape using heat and/or pressure. Once the part has cured, it maintains its shape permanently and the composite construction provides sufficient rigidity to allow the implant  10  to perform equivalently to a solid metal prosthetic femoral head having a metal conically tapered bore for receiving a conically tapered trunion on a femoral component neck/stem. 
         [0034]    The socket  14 ,  14 ′ can be made during the molding operation by placing a conically tapered metal thimble in the mold. The thimble may have a machined surface of may have a polished tapered outer surface. The thimble tapered outer surface which, after injection molding the head/shell with PEEK, forms a socket with a similar surface finish. Alternately, the tapered socket can be machined and optimally polished. 
         [0035]    A white light microscopy was used to measure surface roughness profile ( FIG. 4 ) of the socket. The as-machined PEEK surface profile was peak height 9.4 μm, valley −9.6 μm, average surface roughness (Ra) 1.3 μm, and the sum of peak and valley (PV) 19.0 μm. The polished PEEK surface profile was peak height 0.4 μm, valley −0.3 μm, average surface roughness (Ra) 0.04 μm, and the sum of peak and valley (PV) 0.7 μm. A fatigue galvanic corrosion experiment was conducted in simulated body fluid at room temperature under normal body walking load profile in MTS machine. After a 5 million cycle test, the electropolarization of corrosion current was recorded and the components were visually inspected. It was shown that (1) as-machined PEEK sleeve significantly decreased corrosion current as compared to a no sleeve situation; (2) the titanium alloy stem taper had crevice corrosion when using a PEEK sleeve made by direct coating and the PEEK with a polished surface, had black colored areas in the taper area. However, the as-machined PEEK sleeve did not show any black color areas, i.e., no crevice corrosion occurred. These results indicated the as-machined PEEK surface is better than the polished surface. Intimate contact between PEEK and titanium is not good in the taper/trunion area. This may be attributed to the as-machined surface has high (at least 5 μm) peaks and low (at least 5 μm) valleys when compared to a polished surface, which provide a channel allowing body fluid to enter and leave the trunion/taper area. Thus crevice corrosion was prevented. 
         [0036]    The second finding related to interface hydrolysis. Two metal plugs were bonded with PEEK by melting PEEK film at about 385° C. in between the two metal plug in an Ar atmosphere. After cooling to room temperature, the bonded plugs were soaked in simulated body fluid (Phosphate Buffered Saline) at 37° C. The results are summarized in  FIG. 5 . It was found that the bonding strength was as high as 110 MPa before soaking. After soaking however, the bonding strength dropped quickly over soaking time. For Example, for alumina grit blasted Ti6Al4V plugs, bond strength decreased to 20 MPa at 4 weeks soaking. The grit blasted CoCr decreased the bond strength to about 60 MPa at 52 weeks soaking. PEEK bonding to sintered and polished alumina (&gt;99% purity, &gt;98% density) was also tested. Bonding strength dropped to about zero MPa after soaking in Phosphate buffered saline at 37° C. about 3 weeks (not shown in  FIG. 5 .) It was unexpectedly found that using a physical vapor deposition (PVD) CoCr or Chromium Nitride (CrN) coating on grit blasted titanium plugs, that the tensile bonding strength did not drop at all at 25 weeks soaking. Each data point in  FIG. 5  is an average of at least two test results.  FIG. 6  shows a SEM picture of PVD CrN coating on chemically pure titanium with PEEK bonding Note that the PVD CoCr has average surface roughness about 5 micrometer. 
         [0037]    Based on the above, a femoral head was designed that has two unique features, a PEEK trunion with as-machined surface roughness and a metal or ceramic shell with mechanical locking feature in the inner surface. The as-machined trunion surface prevents crevice corrosion of the titanium metal stem taper, while the shell inner mechanical locking feature will overcome the hydrolysis issue. 
         [0038]    In one embodiment of the invention a shell is made first and then PEEK is molded into the interior space of the shell. The shell may be made of a metal or sintered ceramic with a thickness of 0.2 to 3.0 mm. The metal shell may be made by forging, laser or E-beam additive manufacturing or by Metal Injection Molding (MIM) or hydroforming or any other relatively inexpensive process. The ceramic shell may be made by slip casting, injection molding, and cold isostatic pressing followed by normal sintering or a hot isostatic press sintering process. The outer head surface is ground/machined then polished. The metal surface may be a neat metal alloy that has good biocompatability, good wear resistance, and corrosion resistance, such as CoCr and special stainless steel. If the metal shell does not have good wear resistance such as Titanium alloy and zirconium alloy, the metal shell should be coated with a wear resistant coating. The wear resistant coating may be selected from a ceramic or metal. The ceramic coating may be alumina, zirconia, titanium oxide, chromium oxide, silicon nitride, titanium nitride, chromium nitride, diamond-like coating (DLC), diamond, titanium carbide, zirconium carbide, chromium carbide, etc. The metal coating may be CoCr, stainless steel, chromium. In addition to coating, the outer surface of the metal shell can be transformed to create a ceramic coating with a surface treatment (Sonova LLC, Long Island City, N.Y.)) or transformed with an additional ceramic sprayed or PVD coated with nano-ceramic. A process disclosed in U.S. Pat. No. 5,037,438 or 7,582,117 may also be used. 
         [0039]    The metal or ceramic shell should have a mechanical locking feature. The locking feature should have an average surface roughness of at least 5.0 micrometer, which is the current grit-blasted metal surface roughness. The method to achieve the mechanical locking feature may be selected from selective laser or e-beam additive manufacture process, which can generate a three-dimensional (3D) interlock structure. The 3D structure has interconnected pores that allowed polymer to be molded into it, thus form strong mechanical locking. Another method is physical vapor deposition (PVD). A CoCr, Mo, Ta, Cr, W, Pt, Au, Ag coating can be coated by a PVD method to create two dimensional (2D) surface roughness which has some undercuts to form mechanical interlocks. Grit blasting does not produce undercuts only increased surface area. 2D surface has porosity on its surface only, while the surface pores hold polymer well to form strong mechanical lock. The inner surface may have some anti-rotation fins or ribs around the inside as well. This mechanical locking feature has been disclosed in US 2007/0233266. A 3D porous structure, 2D porous layer, and ribs can provide enough mechanical bonding strength to polymer and overcome hydrolysis issue. 
         [0040]    The interlocking feature may vary according to the prosthesis size, shell thickness, and trunion diameter and depth. The 3D porous structure may have interconnected porosity 30-80%, preferably 40-70% and more preferred 50-65%. The 2D locking feature may have a surface roughness minimum of 5 micrometer, preferred more than 10 microns. More preferred more than 20 micrometer. The rib structure can be designed to different patterns. The preferred pattern has T-shape thus that can hold polymer from shrinking back after molding. 
         [0041]    No matter what kind of locking feature or design details, the shell-core structure should not have any gap between polymer and shell after manufacturing. It is desirable that the tensile bonding strength between the shell and polymer should more than 35 MPa after soaking in body fluid for life time. 
         [0042]    The polymer may be injection molded to form the interior volume and taper junction and the inside of the shell can be textured by grit blasting or left smooth. Advantages are low cost of manufacturing (avoid grinding taper geometry). The polymer taper bore eliminates fretting corrosion concerns at the junction with the trunion on the neck. Light weight saves on shipping and packaging. Burst (fracture) concerns for head are eliminated and fewer metal ions are produced during use. Such a polymer may be selected from polyaryletherketone (PAEK, including PEEK, PEKK, PEK, PEKKEKK), polyphenylene sulfide (PPS), polyacetal (POM), polyamideimide (PAI), liquid crystalline polymers (LCP), polyaryether (PAE), polyarylsulfone (PAS), polyether sulfone, polyetherimide. Of all these polymers, PAEK is the preferred polymer because its semicrystalline nature, POM, and PPS, whose hardness and fracture toughness can be adjusted by controlling crystallinity. The more preferred PAEK polymer is PEEK, which can be sterilized by current popular methods such as gamma ray, X-ray, e-beam, ETO, and autoclave. The desired PAEK polymer will have molecular weight (Mw) more than 80,000 g/mol, more preferred Mw more than 100,000 g/mol, the most preferred Mw more than 110,000 g/mol. The higher the molecular weight, the higher the polymer toughness. Because the polymer socket mates with metal taper, a micromotion and wear may occur at taper junction, high toughness of the polymer is required, which guarantees no catastrophic fracture which can occur with a bulk ceramic head. If the molecular weight is lower than 80,000 g/mol, the polymer will be brittle and is not preferred application for the taper trunion, whose behavior is more similar to ceramic. 
         [0043]    For semi-crystalline polymers, the toughness can be as high as metal, or as low as ceramic. The key controlling factor is crystallinity. For a head application, the preferred crystallinity is lower than 50%. The corresponding polymers are LTI 1, LT2, and LT3 (Invibo Ltd, UK) or equivalent PAEK. More preferred crystallinity is lower than 40%, such as LT1 and LT 2 PEEK or equivalent PAEK. The most preferred polymer is less than 31%, which is corresponding to LT1 PEEK or equivalent PAEK. 
         [0044]    Because the polymer is used against a metal tapered trunion, a neat polymer is preferred, i.e., not carbon fiber or glass fiber, or Barium sulfate reinforced polymer, especially not reinforced PAEK. The reason is that the reinforcement acts as a third body wear surface, which speeds up the damage to the metal taper in wet and warm corrosive body fluid. 
         [0045]    As discussed above, in order to eliminate crevice corrosion, the mold used for injection molding PAEK should have non-polished surface. As-machined mold surface is preferred. This rough mold surface will result in a rough PEEK surface. When mating with a metallic stem taper, the body fluid and oxygen can enter and leave the taper junction freely. 
         [0046]    Another embodiment of the invention is to make plastic (PEEK) core first, then applying a metal or ceramic shell second. In this embodiment, the inner shell mechanical locking feature and plastic trunion surface roughness may keep the same of the first embodiment, just manufacturing order was reversed. 
         [0047]    Although the various embodiments of the present invention have been discussed as they apply either to the human knee and hip joints, one having reasonable skill in the art upon reading this disclosure would understand that the present invention can be used to form other joints of human or animal bodies. Such joints may include the elbow, wrist, shoulder, etc. 
         [0048]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.