Abstract
INTRODUCTION
Hip wear simulator test results could be affected by many non-bearing related factors such as fixation surface conditions, equipment calibration and component set-up. In an effort to improve the accuracy, reliability and repeatability of hip simulator test, a quality management system has been established at the IDC hip tribology laboratory, which has been accredited by UKAS (United Kingdom Accreditation Service) in accordance with the recognised international standard ISO17025. This study demonstrates that under well-controlled laboratory and testing conditions, satisfactory repeatability can be achieved during hip simulator studies.
METHODS
Between 2008 to 2010, ten 50 mm Birmingham Hip Resurfacing (BHR) devices were tested by the IDC tribology laboratory using two ProSim hip wear simulators in three different tests (T1, T2 and T3). All tests were performed following the same IDC testing protocols at 1 Hz frequency for 5 million cycles (Mc) or until after a steady state was reached. Paul type stance phase loadings with a maximum load of 3 kN and a swing phase load of 0.3 kN was used. The flexion and extension angles were 30 and 15 degree. The internal/external rotation angel was ±10 degree. Wear was measured gravimetrically using an analytical balance (Mettler, Toledo xp504) with an accuracy of 0.1 mg.
RESULTS
Results showed that wear of all the components tested followed the typical biphasic trend of wear for MoM hip joint device with a high “running-in” wear during the initial stage followed by a low “steady state” wear in the later stage. The data points scattered in a narrow range following the same trend line throughout the tests. The maximum difference in accumulated volume loss between any two tests at any given cycle was less in 0.2 mm3.
The average running-in wear rates (±SD) were 0.72 ±0.17, 0.86 ±0.01 and 0.71 ±0.19 mm3/Mc respectively for test T1, T2 and T3. The running-in wear rates for test T1 and T3 were identical, whilst that for T2 was slightly higher. In spite of this, the wear rates for the bearings tested in T2 were still in the same range as those tested in T1 and T3 during the running-in stage. The steady state (>1.0Mc) wear rate of three different tests was practically the same, being 0.18 ±0.02, 0.18 ±0.06 and 0.16 ±0.04 mm3/Mc respectively for test T1, T2 and T3. Overall, the mean running-in wear rate was 0.74 ±0.16 mm3/Mc, and the steady state wear rate was 0.17 ±0.04 mm3/Mc for the 10 BHR devices tested in three different studies.
DISCUSSIONS
This study shows that satisfactory repeatability can be achieved during hip simulator test of metal on metal hip joint devices. Well controlled test procedures including calibration of testing and measurement equipment, correct and consistence test set-up and attention to non-bearing surfaces are some of the most important prerequisites for reliable and accurate hip simulator test results.
