Abstract:
A method of making a non-modular prosthetic device for a joint arthroplasty. The method comprises molding a polymer interlayer between a porous metal structure and a polymer insert, wherein the insert generally comprises conventional or cross-linked ultra high molecular weight polyethylene (“UHMWPE”).

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention is related generally to prosthetic orthopedic implants, particularly to joint components such as for use with knees, hips, shoulders, elbows, toes, fingers, wrists, ankles, spinal discs and the like. More specifically, the present invention relates to a method of making a non-modular prosthetic joint component having a polymer, ceramic, or metal bearing component bonded to a polymer, ceramic, or metallic substrate having at least one porous surface. 
   2. Description of the Related Art 
   Orthopedic implant devices known to those of skill in the art often comprise a backing component and bearing component attached thereto. Furthermore, it is often desirable for the metal backing component to comprise a porous structure or surface suitable for bone ingrowth after the prosthetic devise is implanted. For example, a typical prosthetic acetabular cup comprises a hemispherical metal backing having a porous convex exterior and a solid concave interior. A similarly hemispherical, but smaller, polymer bearing surface is inserted into to the concave interior of the backing. In another example, a typical tibial component for use during a knee arthroplasty comprises metal tibial plateau having a porous bone contacting surface and a polymer bearing component attached to an opposing surface. 
   In many instances, prosthetic joint devices are modular. A modular device comprises a backing component, generally comprising a biocompatible metal having a porous structure or surface, and a separate bearing surface component, generally comprising a polymer. For example, a modular acetabular cup comprises a metal backing component and a polymer bearing surface fixedly inserted therein. Such fixation may be achieved via any of one or more of a variety of known mechanical means, such as snap fitting the components, press fitting the components, threadably connecting the components, using a locking ring, etc. 
   Those of skill in the art recognized that these additional mechanical retaining means could be avoided by using non-modular (“monoblock”) joint components. Monoblock joint components comprise a metal backing such as a metal acetabular shell or a metal tibial plateau with the bearing surface integrally attached thereto. Unlike a modular component, the bearing surface of a monoblock is integral with the bearing component and need not be mechanically attached to the metal backing of an implant during an intraoperative step. There are several monoblock prosthetic devices presently available. These devices are generally produced by directly compression molding a thermoplastic polymer bearing component onto a backing component. However, this method of producing monoblock devices has disadvantages. 
   More recently, the bearing components of traditional monoblock prosthetic devices often comprise cross-linked ultra high molecular weight polyethylene (“UHMWPE”). Cross linking can be accomplished chemically, but it is usually accomplished via gamma or electron beam irradiation after the monoblock device is assembled. A problem with this process is that the metal component of the monoblock device can shield the bearing component from the electron beam radiation used to initiate cross linking, thereby making cross linking of the bearing component more difficult and time consuming or possibly having areas within the polymer remaining uncrosslinked. 
   Another problem with monoblock processes known in the art is that such processes do not accommodate using non moldable materials such as metals or ceramics for the bearing surface, as the same cannot be compression or injection molded onto a backing component. 
   Thus, a need exists for a method of making a monoblock orthopedic joint device, wherein the polymer component can be cross-linked separately from the backing component and subsequently connected to thereto to form a monoblock device. 
   A still further need exists for a method of making a monoblock orthopedic joint utilizing a metal, ceramic or other non-flowable material for the bearing surface. 
   SUMMARY 
   The present invention comprises a novel method of making a monoblock prosthetic joint device having a polymer, metal, or ceramic bearing component fixedly attached to a porous metal component or a metal component. In such devices, the metal component is generally in communication or contact with an adjacent bone. The bearing surface (or articular surface) is generally in movable contact with another bone or an articular surface from an adjacent implant. 
   An advantage of the present invention is that a cross-linked polymer component may be attached to a metal component rather than attaching a non-cross-linked polymer and subsequently irradiating the same to create cross-links as the metal component may make such subsequent irradiation difficult. 
   Another advantage of the present invention is that the bearing surface may comprise a variety of materials, such as, thermoplastics, thermosets, metals, and ceramics, yet still be bonded the porous metal component. 
   These and other advantages and features of the present invention will be apparent to those skilled in the art upon review of the appended claims and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention, and the manner of obtaining 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 side cross-sectional view of a prosthetic acetabular cup according to the present invention. 
       FIG. 2  is a side cross-sectional view of prosthetic knee tibial component according to the present invention. 
       FIG. 3  is a diagrammatic view of a first embodiment of the present invention. 
       FIG. 4  is a front view of an interface according to the present invention. 
       FIG. 5  is a front view of an interface according to an alternative embodiment of the present invention. 
       FIG. 6  is a diagrammatic view of a second embodiment of the present invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent an exemplary embodiment of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explaining the invention. The exemplification set out herein illustrates an exemplary embodiment of the invention only. 
   DETAILED DESCRIPTION 
   The present invention comprises a method of making a monoblock prosthetic device, having a porous metal component.  FIGS. 1 and 2  show exemplary devices that can be made using the present method, including an acetabular cup for a hip prosthesis and a tibial plateau for a knee prosthesis. It will be appreciated by those of skill in the art that other prosthetic devices comprising a metal component and bearing component, such as, glenoid components for shoulder prostheses and the like could also be made by the present method. 
   Referring now to  FIG. 3 , there is shown a diagrammatical view of a first embodiment  300  of the present method. The method comprises the steps of: providing metal backing component  120  of desired shape; providing a bearing component  160  of desired shape, said component having a plurality of grooves  165  disposed thereon; placing metal component  120  and bearing component  160  into an injection molding device, such that a desired gap exists between bearing component  160  and porous metal component  120 ; and injection molding a polymer interlayer  140  between porous metal component  120  and bearing component  160 , such that polymer interlayer  140  is in communication with the porous structure of metal component  120  and grooves  165  of bearing component  160 . 
   As used herein, the terms backing component  120 , polymer interlayer  140 , and bearing component  160 , shall apply to such components generically without regard to a particular shape or prosthetic implant application. For example, the term bearing component  160  has equal application to the meniscus component of a knee prosthesis and to the articular surface of a prosthetic acetabular cup. 
   Referring again to  FIG. 3 , there is shown step  310  of method  300  comprising providing metal backing component  120 . Metal backing component  120  comprises a textured surface  121  to which polymer interlayer  140  can attach. Preferably, metal backing  120  and surface  121  are entirely porous or surface  121  is porous and attached to metal backing  120  by means known commonly in the art. Alternatively, surface  121  of metal backing component  120  comprises a knurled surface, a roughened surface, or a grooved surface such that a mechanical bond can be created between interlayer  140  and surface  121 . Metal component  120  comprises a biocompatible metal material selected from the group consisting of tantalum, titanium, cobalt chrome, and stainless steel. Metal backing component  120  is sufficiently porous to allow a polymer interlayer  140  to interdigitate therein (as shown in  FIG. 6 ) during the molding process, described subsequently herein. Preferably, metal component  120  comprises a tantalum porous metal or a tantalum porous metal surface. An example of a suitable tantalum porous metal is disclosed in U.S. Pat. No. 5,282,861, entitled Open Cell Tantalum Structures for Cancellous Bone Implants and Cell and Tissue Receptors, issued on Feb. 1, 1994 to Richard B. Kaplan and assigned to Ultramet of Pomona, Calif., the disclosure of which is hereby incorporated by reference herein. Those of skill in the art will recognize that any biocompatible material having a surface of sufficient porosity and suitable mechanical properties to avoid being adversely affected by the present method can be used in the present invention. Some exemplary biocompatible materials include: stainless steel, cobalt chrome alloy, titanium, and titanium alloys. 
   Metal backing component  120  further comprises a shape appropriate for use in a particular orthopedic implant. For example, metal backing component  120  of step  310  could be shaped into a hemispherical shell for use in an acetabular cup implant as shown in  FIG. 1 . Alternatively, metal component  120  could be shaped into a plate for use as a tibial plateau, as shown in  FIG. 2 . 
   Referring still to  FIG. 3 , the method of the present invention further comprises step  320 , wherein bearing component  160  of desired shape is provided. Bearing component  160  comprises a material selected from the group consisting of thermosets, thermoplastics, metals and ceramics, including, for example, polyurethane, polyethylene, and cross-linked polyethylene, titanium alloy, cobalt alloy, alumina, and zirconia. Bearing component  160  is shaped in a manner suitable for a particular orthopedic implant. For example, as shown in  FIG. 1 , bearing component  160  comprises a hemispherical shape that can be disposed within the concave interior of an acetabular cup shell. Alternatively, as shown in  FIG. 2 , bearing component  160  may, by way of example and not limitation, comprise a shape suitable for a prosthetic knee meniscus. 
   Referring now to  FIG. 4 , there is shown bearing surface  160 . Bearing surface  160  comprises grooves  165  to which polymer interlayer  140  can attach or “interdigitate.” In a first embodiment of the present invention, grooves  165  are disposed around a portion of the exterior of bearing component  160  such that metal component  120  may be at least partially disposed therearound as shown in  FIG. 6 , and described in more detail below. In other embodiments of the invention ( FIG. 5 ) bearing component  160  may comprise in place of grooves, a textured surface, a knurled surface, or a surface having a plurality of machined or molded indentations, as shown in  FIG. 5 . 
   Referring again to  FIG. 3 , there is shown step  330  of method  300 . Step  330  comprises placing porous backing component  120  and bearing component  160  into an injection molding device. Any injection molding device capable of transporting and curing a chosen polymer material is satisfactory. The molding device should also be capable of accommodating metal backing component  120  and bearing component  160  in its mold. Components  120  and  160  should be placed in the mold such that the at least one porous surface of metal component  120  faces a textured surface of bearing component  160 . A gap of desired distance should exist within the mold between components  120  and  160 . 
   Referring still to  FIG. 3 , there is shown step  340  of method  300 , wherein a polymer interlayer  140  is thereafter injection molded into the gap. Interlayer  140  comprises any biocompatible thermoplastic polymer, including, for example, polyethylene, PEEK (a trademark polyketone of the Vitrex company); other polyketones; and polyurethane. During the injection molding process, interlayer  140  flows at least partially around the interlock means of bearing component  160  and at least partially into the porous surface of metal component  120 . Upon curing, interlayer  140  mechanically locks itself to bearing component  160  and metal component  120 , as illustrated in  FIG. 6 . Thus, a bond is created between components  120  and  160  via interlayer  140 . Those of skill in the art will appreciate that it is preferable for interlayer  140  to comprise a polymer that is miscible with the material used for bearing component  160 . For example, a polyethylene interlayer  140  should be used with cross-linked ultrahigh molecular weight polyethylene bearing components  160  because such interlayer  140  will provide an adhesive as well as a mechanical bond between interlayer  140  and bearing component  160 . 
   Turning now to  FIG. 6 , there is shown another embodiment of the present invention, method  600 . Method  600  comprises the steps of: providing a porous backing component  120  of desired shape; providing a bearing component  160  of desired shape, said component having polymer interlock means  165 ; placing the porous backing structure and the bearing component into a compression molding device, such that a desired gap exists between the bearing component and the porous structure; placing a layer of polymer resin in the gap between the porous surface of the backing component and the textured surface of the bearing component, such that the polymer layer is in communication with the porous structure and the textured surface of the bearing component, thereby forming an assembly; and subjecting the assembly to a compression molding cycle such that the polymer layer forms a solid interlayer, wherein the interlayer bonds the porous structure and the bearing component. 
   The steps of method  600  are as described above with regard to other embodiments of the present invention except for steps  640  and  650  described subsequently herein. Step  640  of method  500  comprises placing a polymer resin in a compression molding device adjacent to and in communication with a porous surface of backing component  120  and in communication with a textured surface of bearing component  160 . This polymer resin will act as polymer interlayer  140 . Those of skill in the art will appreciate that interlayer  140  may be provided in any usable form, including for example flakes or powder. Polymer interlayer  140  may, as disclosed previously herein, comprise any biocompatible thermoplastic polymer, including PEEK (a trademarked polyketone of the Vitrex company); poly ethylene, UHMWPE, polyurethane, and the like. The combination of interlayer  140  in communication with metal component  120  and bearing component  160  is referred to herein as assembly  180 . 
   Referring still to  FIG. 6 , there is shown step  650  of method  600  in which assembly  180  is subjected to a compression molding cycle such that interlayer  140  flows into the porous surface of metal component  120  and around interlock means  165  of bearing component  160 , respectively. The compression molding cycle generally comprises utilizing a pressure from about 100 psi to about 600 psi for a time of about 1 to about 6 hours; and a temperature from about 150° C. to about 200° C. Interlayer  140  mechanically secures itself into these surfaces as illustrated in  FIGS. 4–5 . After molding is complete, a near finished orthopedic implant has been produced. 
   In prior art methods of creating a monoblock prosthetic device, non-cross-linked material was molded directly to the porous surface of the metal component. In order to produce an implant having a cross-linked polymer bearing surface material the cross-linking is performed after the implant is assembled by irradiating the part. However, it is difficult to achieve uniform cross-link density using such practices because the metal component disrupts electron beam or gamma radiation that is generally used to initiate cross-linking in a polymer. An advantage, therefore, of the present method is that a bearing component comprising a cross-linked material may be bonded to a metal component without having to subsequently irradiate the part to cross-link the bearing component. 
   Another advantage of the present method is that it provides a means by which non-flowable materials may be used to form the bearing surface in that the bond between components  120  and  160  exists via interlayer  140 . For example, the present method can bond a ceramic bearing component  160  having at least one textured or porous surface with a porous metal component  120 . 
   It will be appreciated by those skilled in the art that the foregoing is a description of a preferred embodiment of the present invention and that variations in design and construction may be made to the preferred embodiment without departing from the scope of the invention as defined by the appended claims.