Consideration of non-bearing surface condition and its potential effect on hip wear simulation test results

Material loss during hip simulator studies is commonly attributed to wear occurring within the bearing surfaces. Potential mass loss from the non-bearing surfaces and its contribution to the overall gravimetric measurement results are seldom mentioned in the literature. In this study, CoCrMo alloy disc samples and resurfacing cups with various surface conditions were immersion tested in serum solution for up to 1200 h. Gravimetric measurement results showed that the static immersion and cleaning induced weight loss for a cup alone could be in the same order of magnitude as a typical hip simulator tested wear loss reported in the literature for a metal-on-metal hip joint. Since in a hip wear simulator the parts to be tested are generally immersed in serum or other physiologically relevant solutions and cleaned periodically before gravimetric measurement, it is highly possible that material loss can also occur from the non-bearing surfaces, affecting the accuracy, repeatability and comparability of hip simulation tested wear results. Accordingly, non-bearing surface conditions and the potential material loss from the non-bearing surfaces have to be considered in designing hip simulator test protocols and in analysing wear results. The results presented in this paper pertain to in vitro wear simulator studies and have little clinical relevance to the performance of any implant in vivo.