Abstract
Metal on Metal bearings have functioned satisfactorily for up to 30 years without a full understanding of what shape the components were at the start, or as they were worn over time.
Modern metrology methods can use elegant computer driven coordinate measuring machines in both manufacturing and retrieval analysis as most manufacturers and laboratories do. This method however is fl awed in its ability to look at the much higher levels of resolution required for analysing these bearings. A method has been developed which will analyse to the sub micron deviances from roundness that the subleties of manufacturing can achieve, and allows an analysis of wear of explanted implants.
Both new and explanted Ring, McKee, Stanmore, Metasul and Resurfacing implants have demonstrated initial form variances that range from 1 to over 100 microns out of round. Roundness trace analysis has allowed linear wear on explanted implants to be measured from 0.5 to 150 microns. This represents combined head and cup linear annual wear rates from 2 to 65 microns per year.
Metal heads and cups are not manufactured as round as has been believed. The average wear rates over time for different Metal on Metal systems is not the same.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
