Abstract
Metal-on-Metal devices generate significantly lower volumetric wear than conventional total hip replacements. However, clinically some patients may suffer some form of laxity in their joints leading to subluxation of the joint, which in turn may cause edge loading of an implant thereby increasing the chances of failure due to higher than expected wear.
In this study, the effect of subluxation on MoM implant wear was investigated on a hip joint simulator.
Materials & Methods
Two groups of 44 mm MoM devices were tested, n=3 in each group. The devices were subjected to 1 and 2 mm of subluxation. The flexion/extension was 30° and 15° respectively, internal/external rotation was ±10°, and cup inclination was 35°. The force was Paul type stance phase loading with a maximum load of 3 kN, with ISO swing phase load of 0.3 kN, run at 1 Hz.
The test was carried out on a ProSim deep flexion & subluxation hip wear simulator (SimSol, UK). Rather than separating the head and the cup (microseparation), or reducing the swing phase load, this simulator is equipped with a novel mechanism to achieve translation of the head, while subjecting the devices to subluxation. During the swing phase, a controlled lateral force necessary for the translation of the head is applied by a cam mechanism, head retraction will then take place on heel strike.
The lubricant used was new born calf serum with 0.2 wt. % sodium azide concentration diluted with de-ionised water to achieve average protein concentration of 20 g/l. Lubricant was changed every 250k cycles. Gravimetric wear measurements have been taken at 0.25 & 0.5 Mc stages.
Results
Tests conducted with 1mm (Group 1) and 2mm (Group 2) subluxation significantly increased volumetric wear compared to standard hip simulator tests [1]. At 0.5 million cycles, group 1 and 2 produced an average volume loss of 4.38±0.98 mm3 (95% CL) and 7.07±1.64 mm3 (95% CL) respectively.
Discussion/conclusion
Well positioned and well-fixed hip implants perform well in vitro and in vivo; however optimal performance a device can be affected by a number of factors from design, technical factors, patient factors, surgical technique to position of the device in vivo.
The study presents test results of a hip joint simulator with a subluxation mechanism to simulate clinically relevant subluxation during the swing phase of a gait cycle under the ISO swing phase load of 0.3kN, with differing levels of luxation. Increasing the level of subluxation in turn increased volumetric wear due to greater head contact at the superior rim of the cup. Further tests will be conducted with high cup inclination angles (>45°) and subluxation to determine the effect upon wear. Tests which can simulate the (ideal and adverse) conditions clinically can help to improve the design and understanding of implant behaviour in vivo.
