A method for the evaluation of the change in volume of retrieved acetabular cups

Studies on retrieved hip prostheses are currently performed in order to assess the wear mechanisms and the overall wear rate of such artificial joints. Several reported studies on the survival of artificial hips have been based on the measurement of the amount of worn material directly on retrieved acetabular cups. The estimation of the change in volume, V, of the cup cavity is particularly difficult in the case of slight wear due to several factors of which the most critical is the lack of information on the unworn geometry of the cup. This paper presents a new measuring technique, which is described in detail and has been applied to estimate the wear of 65 acetabular cups harvested from revised hip arthroplasty. The coordinate data of several points on the articular surface are sampled using a coordinate measuring machine (CMM). The value of V is calculated mathematically from the measurements by using the hypothesis that the actual shape of both slightly worn and highly worn surfaces has a small departure from a truly spherical shape. The wear volume is estimated with reasonable accuracy mainly on those cups showing penetration depths greater than 0.2 mm, corresponding to an amount of wear greater than 100 mm(3) for a 32 mm cup. The uncertainty in the results is estimated for each cup. The repeatability of the technique is studied for a case showing very slight wear. The advantages, disadvantages and limitations of the method are presented and discussed.