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

Total joint replacement is one of the most common elective surgical procedures performed worldwide, with an estimate of 1.5 million operations performed annually.
Currently joint replacements are expected to function for 10-15 years, however, with an increase in life expectancy, and a greater call for knee replacement due to increased activity levels, there is a requirement to improve their function to offer longer term improved quality of life for patients.
Wear analysis of total joint replacements has long been an important means in determining failure mechanisms and improving longevity of these devices. In developing such joints the industry standard and ISO wear measurement methodology is to use gravimetric methods only. The method is not as easy to apply accurately as it would first appear and 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 due to similar fluid uptake and loss of cement material. As a result it is important to have an alternative measurement method that accounts for this factor. The only reasonable answer is to physically measure the component, and using co-ordinate metrology is the first, most reasonable step in this direction.
The effectiveness of the CMM technique for assessing volumetric material loss during simulated life testing of a replacement knee joint has been proved previously by the authors. The purpose of this study is to establish an extension to this through the use of improved geometrical using a Zeiss Prismo CMM methods to measure component wear when there was no pre-wear data taken.
To prove out the method a simulator test was run to 5 million cycles and gravimetric measurements taken throughout the test, such that the components measured had a known wear value. The wear volumes are estimated and compared against gravimetric techniques. The implications of the results are further discussed in terms of assessment of joint functionality and development of standardised CMM based product standards. In addition the results are discussed in terms of their applicability to the measurement of wear on hard on hard knee systems.