The micromechanisms of fatigue fracture in bone cements were explored and characterized in this investigation. Fatigue crack initiation sites were identified and the microscopic features responsible for crack initiation are illustrated. Differences in the fracture morphology of PMMA beads, in early crack growth and rapid crack growth stages, are brought out. Based on the fractographic features, a micromechanism of PMMA bead fracture is proposed.

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The performance of modular head/taper joints in total hip replacements now in use for nearly two decades can be easily termed as a clinical success. However, associated with concerns of metal particulates found in retrieved tissues, a critical review of modularity in hip joints has recently emerged. Evaluation of retrieved modular joints has shown some surface morphological changes and debris generated at the mating surfaces which has not been demonstrated equally by any in vitro laboratory testing. In this investigation, an experimental setup was developed to conduct fretting corrosion tests of modular head/taper assemblies in an aggressive environment to accelerate the in vivo phenomena. The fretting corrosion tests were conducted in a low pH Ringer's solution at 50°C. A maximum cyclic load of 5.34 kN was applied at 5 Hz on simulated modular head/taper assemblies mounted in a 15° valgus anatomic orientation. Ti-6Al-4V and Co-Cr- Mo alloy tapers and Co-Cr-Mo alloy (cast or wrought) and zirconia ceramic femoral heads in various combinations were evaluated. The morphology evaluation of taper and head bore surfaces after 10 million cycles revealed features such as etching of structure, preferential leaching and fretting in congruence to those reported for retrieved modular hip joints. Nitrogen diffusion surface hardening of Ti-6Al-4V alloy tapers resulted in an increase in resistance to fretting corrosion-induced changes, and a reduction in generated debris. A zirconia ceramic femoral head tested on either a Ti-6Al-4V or a Co-Cr-Mo alloy taper showed an enhanced resistance to both mechanical and chemical phenomena.

Polyethylene wear and associated osteolysis can limit the longevity of total hip replacement. In recent years, many improvements have been made in the consolidation, manufacture, and sterilization of polyethylene acetabular components. These improvements provided reduced polyethylene wear and prolonged usefulness of total hip replacement. Recent advances in extensively cross-linking polyethylene offer the possibility to substantially further reduce wear in total hip replacement. Hip simulator wear testing demonstrates an order of magnitude reduction in wear resulting from cross-linking GUR 1050 polyethylene by exposure to 100 kGy of electron beam radiation followed by annealing to encourage cross-linking and to reduce residual free radicals. Clinical investigation will be required to validate the wear advantage of these materials in vivo. (Hip International 2002; 2: 103-7).