Abstract
Introduction
In vitro studies have shown that low clearance metal-on-metal hip joints have the potential of reducing wear during the running-in phase [1]. However, cementless acetabular cup relies on press fitting into the acetabulum, which can generate non-uniform compressive stresses and non-uniform in vivo cup deformation [2, 3]. This could then lead to equatorial contact, resulting in higher wear and friction for a MoM bearing with low clearance. To benefit from low wear generated by low clearance and at the same time to avoid the potential of head seizure and high frictional torque caused by cup deformation, a deflection compensation acetabular cup (DefCom) has been developed based on the Birmingham Hip Resurfacing (BHR) device. The articulating sphere of the DefCom cup provides a low clearance bearing area, whilst the non-articulating sphere maintains the standard BHR clearance. The aim of this study was to evaluate the wear performance of the novel DefCom hip joint using a hip wear simulator.
Materials and Methods
Five pairs of 50 mm DefCom devices were tested in a ProSim hip wear Simulator for 5 million cycles (MC) at a frequency of 1 Hz. The lubricant was new born calf serum with 0.2% sodium azide diluted with de-ionised water to achieve protein concentration of 20 mg/ml. The flexion/extension was 30° and 15° and the internal/external rotation was 10°. The force was Paul-type stance phase loading with a maximum load of 3 kN and a standard ISO swing phase load of 0.3 kN. Five standard 50 mm BHR devices were tested under the same testing conditions for comparison. Statistical analyses were performed at a 95% confidence level (CL) using the statistics function in Excel (Microsoft(r) Excel 2003).
Results
Fig. 1 shows the cumulative volume loss against number of wear cycles for the DefCom and the BHR devices. Similar to the BHR device, the DefCom joints experienced relatively higher amount of material loss during the running-in phase from 0 to 1.0 Mc. However, the running-in wear rate for the DefCom device (0.23 ± 0.06 mm3/MC) was much lower than that for the BHR device (0.72 ± 0.15 mm3/MC). Steady state wear was achieved for all the joints from 1.0 to 5.0 MC. The steady state wear rate was 0.11 ± 0.03 mm3/MC for DefCom and 0.18 ± 0.01 mm3/MC for the BHR joints.
Discussion
This study has shown that the DefCom acetabular cup has the potential to reduce the initial running-in wear by reducing the clearance at the contact area between the head and cup. The device also has the potential to avoid deformation induced head seizure and high frictional torque by maintaining a larger clearance at the periphery of the cup.
