Acetabular polyethylene wear volume after hip replacement: Reliability of volume calculations from plain radiographs

Polyethylene wear is a main contributor to inflammation, osteolysis, and loosening following hip replacement, and its considerable impact necessitates the effective in vivo assessment of volumetric wear using radiographs. The current study analysed the reliability of assessing linear wear and calculating volume through the use of clinical anterior–posterior (a.p.) radiographs of the pelvis. Twenty-one polyethylene acetabular components retrieved at revision were analysed. Linear penetration depth and direction were measured on clinical a.p. pelvis radiographs. Direct measurements of the cups were made with a 3-dimensional (3D) surface detection method and fluid displacement (FDM). Wear volume was calculated using various previously published formulas. The mean measured linear wear was 1.02 mm (3D), the mean wear volume 777.2 mm3 (FDM). The mean radiographic linear wear and calculated volume were 1.09–1.2 mm and 730.1–813.3 mm3, respectively, depending on method of measurement and calculation. All measurements and calculations agreed well with the 3D and the FDM. Not taking the direction of wear into account led to an overestimation of the wear volume. Errors of measurement and the used formula of calculation were more important than probable wear in the sagittal plane. In conclusion, radiographic measurements of linear wear in the film-plane can provide a reliable estimation of the total wear volume.