WEAR OF XLPE CUPS: THE IMPORTANCE OF WEAR VOLUME CALCULATION

Abstract

Introduction: Although acetabular cup wear is usually reported in terms of penetration (measured from radiographs), true wear – wear volume – depends on several variables. This study examined how these variables affect the calculation of the theoretical wear volume at the low wear penetrations found with highly cross-linked polyethylene (XLPE) cups.

Method: A computerised numerical analysis technique was used to calculate the “exact” theoretical wear volume of an acetabular cup under a variety of circumstances, including: variations in wear direction, head size, and initial radial discrepancy (i.e. initial difference between socket and head sizes). The validity of published wear volume formulae was assessed. The effects of creep and wear measurement error were also assessed.

Results: For a given wear penetration, as the wear direction (relative to the cup base) increased, the wear volume increased – almost doubling as the direction reached 60°. The initial radial discrepancy made a substantial difference to the calculated wear volume at penetrations less than 1 mm. At low penetrations, its neglect caused an overestimation of wear volume of well over 100%. Creep volume was substantially overestimated because of this. An analysis of wear measurement error showed that the calculation of wear direction (an important variable in the calculation of wear volume) was severely affected at low penetrations by the precision of penetration measurements. For a penetration precision of ±0.25 mm (as reported for the Martell Hip Analysis Suite), the maximum wear direction error was ±39° at a penetration of 0.4 mm. When the precision was ±0.1 mm (as with RSA), this reduced to ±14°.

Discussion: Many studies have shown the superior wear performance XLPE acetabular cups compared with standard PE cups. In those studies, comparison in terms of wear penetration was possible because of the large difference in penetrations between the two groups. This study has shown that true wear (wear volume) is significantly affected by wear direction (relative to the cup), the initial radial discrepancy, and the femoral head size. The differences in penetrations when comparing two types of XLPE cups would not be so large and it is therefore necessary to compare the two groups in terms of wear volume. Published formulae for estimating the wear volume of acetabular cups do not take the initial radial discrepancy into account, and they can substantially overestimate the wear volume in the penetration range encountered with XLPE cups. Creep volume is also greatly overestimated. Since wear volume varies with wear direction, the wear measurement technique must be capable of accurately determining the wear direction. This analysis has shown that only RSA might have sufficient precision to determine the wear direction at the medium-term penetrations encountered with XLPE cups.