Abstract
Introduction
Damage development in cemented acetabular replacements has been studied in bovine pelvic bones under long-term physiological1 loading, albeit dry, conditions, using a specially designed hip simulator2. In this work we report further experimental results from testing in wet condition in a new custom designed environmental chamber. Damage was detected and monitored using mCT scanning at regular intervals of the experiments. Two dimensional projections in the axial, sagittal and coronal planes were extracted from the 3D data for fatigue damage identification. The simulated mechanical and biological effects on the initiation and evolution of the damage of cemented acetabular reconstructs were examined and compared with those under dry condition.
Materials and methods
Bovine bones were treated and reamed to receive a cemented polyethylene cup (Charnley ogee, Depuy Int) in the standard position. Standard cementing technique was utilised to apply the cement (CMW1, DePuy CMW) into the socket, with an average cement mantle thickness of 2–3 mm. The combined loading block included four routine activities, as measured by Bergmann et al.1, was programmed into a specially designed 4-station hip simulator for endurance testing of cement fixation2. A body weight of 125 kg was assumed to represent an upper bound load case and to accelerate the tests. A custom made environmental chamber (Fig. 1) was designed and built to accommodate saline solution (0.9% NaCl), where the temperature was kept constantly at 37°C. The implanted bone samples were removed from the test rig at regular intervals (100,000 and 200,000 cycles) and examined using a mCT scanner.
Results and discussion
For the tests under dry condition2, μCT images showed progressive development of radiolucent lines, usually in the superior-posterior quadrant near the dome region which led to gross failure; and the number of cycles to failure seems to be related to the type of physiological loadings in that the worst case was found to be descending stairs, followed by combined loading and normal walking. For the tests conducted under wet condition, debonding was detected at the bone-cement interface along the rim of the acetabulum (Fig. 2), as opposed to near the dome in the cases under dry condition. Under the same load magnitude, the survival life in cycles under wet condition is also drastically reduced (∼200,000) compared to ∼ 2,000,000 in dry condition.
Conclusion
Preliminary endurance testing in physiological wet condition seems to suggest that interfacial debonding at the bone-cement interface near the rim is responsible for earlier crack initiation and failure of the acetabular fixation, as opposed to debonding near the dome region in dry condition.
