Abstract
A new hip simulator has been developed at the University of Portsmouth and manufactured at Simulation Solutions, Ltd. (UK) for the purpose of fatigue testing of implanted acetabula. Although hip simulators for in vitro wear testing of prosthetic materials in total hip arthroplasty (THA) have been available for many years, similar equipment has yet to appear for endurance testing of fixations in cemented THA, despite of considerable evidence of late aseptic loosening as one of the most singnificant failure mechanisms in acetabular replacements [1].
In this study, a new four-station hip simulator designed for in vitro fatigue testing of implanted acetabula is described. The four-station machine has spacious test cells that can accommodate full hemi-pelvic bones with implants. The machine was designed to simulate the direction and the magnitude of the hip contact force relative to the acetabular cup coordinate system, as reported by Bergmann et al. [2], under typical physiological loading conditions, including stair climbing as well as walking. The controls were designed as such that each station may operate independently with a loading waveform that is fully programmable. The motions were achieved through two encoded servomotors suitably connected to gearboxes; while the loading was realised through a close-looped pneumatic system. The motions and the resultant hip contact force of the new hip simulator were evaluated, and found to be satisfactory in reproducing the typical physiological loading waveforms including normal walking, ascending and descending stairs.
Experiments have been carried out using third generation composite bones (Pacific Research Laboratories, Inc.) and bovine bones. Both hip simulator and conventional fatigue testing were carried out. The implanted acetabula were CT scanned periodically to monitor the damage development in the fixation. Preliminary results seem to suggest that both magnitude and direction of the hip contact force influence the integrity of the fixa-tion, and failures appear to occur earlier in samples tested using the hip simulator. The predominant failure mechanism appears to be interfacial fracture, consistent with clinical observation of radiolucent lines and bone-cement interfacial failure.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
