The use of CMM techniques to assess the functionality of total knee replacements

Wear analysis of total joint replacements is an important means in determining failure mechanisms and improving longevity of these devices. In developing such joints the industry standard wear measurement methodology is to use gravimetric methods only. This method can introduce significant errors to the wear measurement when there is material transfer from the metal component to the plastic component or when trying to consider the effects of fluid uptake into the ultra high molecular weight polyethylene (UHMWPE) counterface. It is also impossible to accurately measure a cemented component throughout a test by using purely gravimetric means for similar reasons. Consequently there is a need for an alternative measurement method that accounts for this factor. The purpose of the present study is to establish the effectiveness of the CMM technique for assessing volumetric material loss during simulated life testing of a replacement knee joint. In this study a standard design total knee replacement joint was chosen and samples were tested in a knee simulator to simulate 5 years worth of wear from walking. At the start of the test the bearing surface was scanned and digitised into CAD data, by use of a CMM, to use as a baseline comparator. Further measurements were then taken at various points throughout the test and compared with simultaneously taken gravimetric data. The implications of the results are discussed here in terms of assessment of joint functionality and development of standardised CMM based product standards.