Validation of acetabular cup wear volume based on direct and two-dimensional measurements: hip simulator analysis.

The volumetric wear in retrieved cups can be assessed by mathematical conversion based on linear measurements and by a fluid-displacement method. We used a hip simulator model to produce wear in 22-, 28-, and 32-mm hip implants and then assessed the volumetric wear using a gravimetric wear method. We then compared the findings with those obtained with the linear and fluid-displacement methods. For the linear method, we translated the linear wear to the volumetric wear using the equations developed by Charnley et al., Kabo et al., and Hashimoto et al. The fluid-displacement method showed the strongest correlation with the gravimetric wear method, and it was found to overestimate the volume slightly (by 3%-9%). According to the linear wear conversion, however, the equation by Kabo widely underestimated the volume by 33%-40%. The equation used by Charnley tended to overestimate the volume (by 4%-17%), whereas Hashimoto's equation tended to slightly underestimate the volume (by 2%-12%). The fluid-displacement method demonstrated an average error of 0.34% +/- 13.40% when the wear exceeded 400 mm(3). The linear wear was thus converted to the volume wear most accurately using Hashimoto's equation, with the average error being -3.8% +/- 14.0%. Of the four measurement modalities, the fluid-displacement method showed the most accurate results. We therefore confirmed that the fluid-displacement method is the most accurate way to determine volumetric wear in retrieved cups.