Statistical analysis of VEXLPE wear against alumina produced by a new 200-station, multidirectional pin-on-disk device

With low wear rates shown by contemporary bearing materials of total hip prostheses, the standard deviation of wear rate is relatively high. Therefore, large sample sizes are needed for an adequate power of test. Because wear tests take a long time, it is practical to test several samples simultaneously. A new high-capacity, multidirectional wear test device, called the SuperCTPOD-200, was introduced. A 3 million-cycle wear test with an unprecedented sample size of 200 was performed for VEXLPE. The duration of the test was 6 weeks. The wear factor was normally distributed with a mean +/- SD of 1.64 x 10-7 mm3/Nm +/- 0.22 x 10-7 mm3/Nm (n = 200). The observation that SD was 13.1% of the mean can be useful in power analyses of future tests with other highly cross-linked polyethylenes. Burnishing was the most typical feature on the worn pins, which was in agreement with clinical findings on retrieved acetabular liners. The present study emphasizes statistics that often plays a minor role only in wear studies.