Aspherical Femoral Head with Highly Cross-Linked Ultra-High Molecular Weight Polyethylene Surface Cracking

Abstract

Hip simulator and early clinical studies of highly cross-linked ultra-high molecular weight polyethylene have demonstrated less wear and less femoral head penetration when compared with conventional ultra-high molecular weight polyethylene1-6. However, cross-linking also alters some of the mechanical properties of ultra-high molecular weight polyethylene, including its ultimate tensile strength, strain to failure, fracture toughness, and fatigue crack propagation resistance7-9. Analyses of early retrieved highly cross-linked components have shown initiation of surface cracking, which is possibly related to the decrease in ductility caused by cross-linking10. A reduction in ductility, fracture, and fatigue properties is the hallmark of material embrittlement. Accordingly, newer so-called second-generation highly cross-linked polyethylenes have been developed in an effort to better retain the desirable mechanical properties of conventional ultra-high molecular weight polyethylene as well as the benefits of cross-linking11. Wear behavior of conventional and highly cross-linked ultra-high molecular weight polyethylene acetabular liners can be affected by the geometry of the femoral head. Geometrical factors include not only roughness associated with asperities and machining marks but also asphericity of the implant. Roughening of cobalt-chromium femoral heads occurs in vivo and may increase wear of the liner12-14. Root-mean-square roughness values of clinically retrieved cobalt chromium components heads have been reported to be between 0.15 and 0.20 μm13,14. Asphericity, or out-of-roundness, of the femoral head influences the bearing wear of the implant. It is possible to have an extremely smooth, polished femoral head surface that has relatively large-size scale deviations from the optimal implant shape. Asphericity can manifest as a region of relative flatness or as a protrusion15. Substantial asphericity of the head has been associated with increased acetabular wear16. This case report focuses on a retrieved …