Most of the CMM machines are used to measure a dimension, the distance between two points in space. This is how the CMM specified in the MDL (multi district litigation) protocol is to be used...to give the spherical diameter of the femoral head and the spherical diameter of the acetabular cup. Where is the volume of wear though?
It is now possible with the newer machines to measure many thousands of points accurately so you can take the CMMs dimensional measurements and convert these to a volume. The first publication related to the ware measurement of these hips was back in 1996..some 15 years ago. Essentially we are back to the following conclusion from my sources: