Multiple biological and mechanical factors may be responsible for the failure of fixation in cemented total hip replacements (THRs). Although the eventual failure of THRs may appear to be biological, the initiation of the failure during early period post operation may well be mechanical. It is in this area that mechanistic analysis is of particular significance.

This study builds on work by Rapperport et al, Dals-tra and Huiskes on stress analysis of implanted acetabulum, while focuses on fracture mechanics analyses of fracture of cement and of bone-cement interface. Specifically, finite element models were developed where cracks of most favourable orientations in the cement mantle were simulated. Possible crack path selections were explored. A simplified multilayer experimental model was also developed to represent the implanted acetabulum, and fatigue tests were carried out on the model. The experimental results were compared with those from the FE model.

Furthermore, interfacial crack growth at bone-cement interface was simulated from the superior edge of the acetabulum, as suggested from the clinical observations. The strain energy release rates were computed for typical hip contact forces during gait and as a function of crack length. Associated phase angles were also computed to account for the materials mismatch. The results were evaluated against the interfacial fracture toughness of the bone-cement interface, measured using sandwich Brazilian disk specimens. The results show that although interfacial fracture seems to be unlikely for large phase angles where shear component is most active, the strain energy release rates are comparable with the values of the interfacial fracture toughness when mode I is predominant, suggesting interfacial fracture.

The study also shows that the fracture toughness of cement is much higher than the interfacial fracture toughness of bone-cement, this may explain the reason why interfacial fracture is favoured even if the crack driving force at bone-cement interface appears to be weaker than that in the cement mantle.

Retrieval studies based on revision operations at King Edwards VII Hospital reveal that, although micro-cracks develop in the cement mantle, it is the debonding between cement and bone that often defines the final failure of cemented acetabular replacements. This was illustrated at the revision surgeries by the easy removal of the acetabular cups with cement mostly attached to the cup. It is felt that a fundamental understanding of the mechanisms that initiate and propagate the interfacial failure at the bone-cement interface is the key towards solving the problem.

In this work, in-vitro fatigue tests were carried out on cemented acetabular replacements using third-generation of composite pelvic bones. Standard Charnley cups were implanted using common bone cement, CMW, following the standard surgical procedures. The implanted hemi-pelvic bone model was then constrained at the sacro-iliac and pubic joints to represent the anatomic constraint conditions. Cyclic loads representing the maximum range of the hip contact force during normal walking were used and the direction of the maximum hip contact force was achieved by using angled plates. In addition to standard cup position, open cup and retroverted cup positions were also examined to assess the significance of cup orientation under fatigue loading conditions.

Damage development in the reconstruction was monitored using CT scanning at regular intervals. Permanent records were collected and the sample was eventually sectioned and polished for microscopic studies. Results show excellent correlations between the results from the CT images and the microscopic studies, indicating progressive bone-cement interfacial failure in the posterior-superior quadrant.

The significance of the work in the studies of ‘aseptic loosening’ will be discussed.

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.