Method of providing a threaded bore in a prosthetic implant
FIELD OF THE INVENTION 
     This invention relates to a method of forming a threaded bore within a prosthetic implant. 
     BACKGROUND OF THE INVENTION 
     It is common in a prosthetic implant, specifically in the femoral portion of a knee joint prosthesis, to provide the option to the surgeon of connecting attachments such as augmentation blocks to the implant. A wide variety of attachment mechanisms and methods are known in the industry for connecting such attachments. One method is to provide a bore for accommodation of a peg carried by the augment. The bore can be formed with threads to accommodate a screw securing the augment and implant together. 
     It is common for a femoral portion of a knee joint prosthesis to include at least the posterior condyle having an interior surface. The general U-shape of the femoral knee prosthesis forms a limited area between the interior surfaces of the condyles and the anterior wall of the prosthesis. Due to this limited space, it is difficult to properly position automated machining equipment to drill a blind bore into the posterior condylar wall. Providing threads within this bore is also very difficult. 
     SUMMARY OF THE INVENTION 
     This invention eliminates the difficulties discussed above by providing a method wherein the attachment bore is drilled through the articular surface of the posterior condyle. The bore is internally threaded and then a plug is welded in place to seal the opening of the bore from the articular surface. The weld bead is polished so that there is no interruption in the articular surface. By drilling and threading the bore from the articular surface, there is no restriction on space, and automated machining equipment can be readily used. 
     Accordingly, it is an object of this invention to provide a method of providing a blind bore within an implant. 
     Another object of this invention is to provide for a novel method of providing a blind bore in the posterior condyle of a femoral knee joint prosthesis. 
     Another object of this invention is to provide a method of forming a threaded blind bore in the posterior condylar wall of a femoral knee joint prosthesis by drilling through the condylar articulating surface, threading the bore and back plugging the bore at the articular surface. 
     Still other objects of the invention will become apparent upon a reading of the following description taken with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a elevational view of a femoral knee joint prosthesis having a bore formed through the posterior condyle in accordance with the method of this invention. 
     FIG. 2 is a partial elevational view of the posterior condyle of FIG. 1 illustrating a thread formed in the bore of FIG. 1. 
     FIG. 3 is the partial elevational view of FIG. 2 illustrating the counter bore step of the method of the invention. 
     FIG. 4 is the partial elevational view of FIG. 3 illustrating the counter bore chamfered. 
     FIG. 5 is the partial elevational view of FIG. 4 illustrating the chamfered counter bore plugged and welded. The plug used in the method of the invention is illustrated. 
     FIG. 6 is the partial elevational view of FIG. 5 after the articular surface of the condyle has been polished to remove any interruptions caused by the welding process. 
     FIG. 7 is an enlarged view of the area circled in FIG. 6 designated by the numeral 7. 
    
    
     DESCRIPTION OF THE PREFERRED METHOD 
     The preferred method herein described is not intended to be exhaustive or to limit the invention to the precise method disclosed. Rather, it is chosen and described to best explain the invention so that others skilled in the art might utilize its teachings. 
     Referring now to FIG. 1, a femoral portion of a knee joint prosthesis is illustrated and designated numeral 10. As is common, prosthesis 10 includes an anterior wall 12, distal wall 14 and posterior condyles 16 (only one shown). Posterior condyle 16 includes an interior surface 18 and an articular surface 20. Interior surface 18 is adapted to contact the prepared surface of the femur bone while articular surface 20 is adapted to contact an articular plate. The specific function and geometry of prosthesis 10 is known and is not needed to understand the invention. 
     To form a threaded blind bore extending from the interior surface of the posterior condyle, a through bore 22 is drilled through the posterior wall of condyle 16 from the articular surface out through the interior surface (See FIG. 1). Next, as illustrated in FIG. 2, a helical thread is formed within the bore 22 by a known method. In the next step in the process, a counter bore 24 is drilled into the articular surface concentric with bore 22 as illustrated in FIG. 3. The counter bore 24 is beveled or chamfered by appropriate milling instrumentation to yield the finished through bore as illustrated in FIG. 4. A plug 26, shaped as illustrated in FIG. 4 to substantially conform to the counter bore, is inserted in the counter bore such that the outer end 28 of the plug is substantially flush with or extending slightly beyond the articular surface of the condyle. As illustrated, plug 26 includes champhered edges 25 and 27. With plug 26 inserted, the junction between champhered edge 27 of the plug and the counter bore 24 forms a V-shaped notch 29 around the plug (see FIG. 5). Plug 26 is mechanically secured to the articular surface of the condyle by welding about the outer perimeter of end 28. Welding the plug into position fills in the V-shaped notch. It should be noted that at this point, the remaining portion of original bore 22 forms a threaded blind bore 30 for the securement of an augmentation block as by a screw. The final step in preparing the implant is to polish the articular surface of the condyle to remove all interruptions in the articular surface of the condyle caused by the welding process. 
     FIG. 7 illustrates the resulting welded junction between the plug 26, the filled in V-shaped notch 29 and the posterior condyle 16. 
     It should be understood that the practice of the invention is easily accomplished using computer assisted machining operating from the articular surface of prosthesis. It should be also understood that to ensure accuracy, the prosthesis would be secured in a jig to prevent its movement during the operations. 
     Finally, it should be understood that the invention is not to be limited to the above forms but may be modified within the scope of the appended claims.