Molded UHMWPE direct compression molding

At a conceptual level, polyethylene consists only of carbon and hydrogen, as was described in the previous chapter. However, if the discussion of polyethylene is to proceed from ideal abstractions to actual physical implants, three "real world" steps need to occur. First, the ultra-high molecular weight polyethylene (UHMWPE) must be polymerized from ethylene gas. Second, the polymerized UHMWPE, in the form of resin powder, needs to be consolidated into a sheet, rod, or near-net shaped implant (Figure 2.1). Finally, in most instances, tire UHMWPE implant needs to be machined into its final shape (Figure 2.1). A small subset of implants are consolidated into their final form directly, in a process known as direct compression molding (DCM), witiiout need of additional machining. 

The current system consists of cobalt chrome components with a titanium porous coating on the ulnar component and the distal half of the stem (Figure 10.17). The UHMWPE is direct compression molded and sterilized by gamma irradiation in argon. It may be used either cemented or uncemented. While the Kudo system has had high survival rates at some institutions, the rate of reported loosening of the humeral and ulnar components has been varied [18, 43, 46-53]. The Kudo Elbow System has been used extensively in Europe and Japan, but it is not currently marketed in the United States. 

One carbon-UHMWPE composite, commonly known as Poly II (Zimmer, Inc., Warsaw, Indiana, USA), was developed commercially and used clinically. The CFR-UHMWPE was reinforced by chopped, randomly oriented carbon fibers in a direct compression molded UHMWPE matrix [4]. The carbon fiber reinforcement was initially considered to be responsible for improved wear behavior relative to UHMWPE during initial experimental testing conducted by the manufacturer [5]. However, further studies ultimately revealed that such performance came at the expense of the ductility, decreased crack resistance, and the fiber-matrix interface of the composite [6]. Subsequent wear studies also showed evidence of fiber disruption at the surface and abrasive wear of the metallic counterface [7]. Furthermore, occasional difficulties in the manufacture of the composite material resulted in incomplete consolidation of the powder and carbon fibers in certain implants [8]. Thus, after its clinical introduction, Poly II was found to exhibit wear, fracture, and extensive delamination [2, 3,9], and, as a result, the material was eventually withdrawn [ 10]. 

As described in Chapter 2 of this Handbook, the mechanical properties of compression molded and direct compression molded UHMWPE depend on the molding conditions (time, temperature, pressure) used to consolidate the part. Incomplete consolidation, as evidenced by residual UHMWPE powder particle boundaries in microtome shces of the consolidated product, can result in inferior mechanical properties. 

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