Does Pelvic Orientation Influence Wear Measurement of the Acetabular Cup in Total Hip Arthroplasty-An Experimental Study

Roentgen stereophotogrammetric analysis (RSA) is the gold standard to detect in vivo material wear of the bearing couples in hip arthroplasty. Some surgical planning tools offer the opportunity to detect wear by using standard a.p. radiographs (2D(wear)), whilst RSA (3D(wear)) needs a special radiological setup. The aims of this study are to prove the interchangeable applicability of a 2D(wear) approach next to RSA and to assess the influence of different pelvic positions on measurement outcomes. An implant-bone model was used to mimic three different wear scenarios in seven pelvic-femur alignment positions. RSA and a.p. radiographs of the reference and a follow-up (simulated wear) pose were acquired. Accuracy and precision were worse for the 2D(wear) approach (0.206 mm; 0.159 mm) in comparison to the 3D(wear) approach (0.043 mm; 0.017 mm). Changing the pelvic position significantly influenced the 2D(wear) results (4 of 7, p < 0.05), whilst 3D(wear) results showed almost no change. The 3D(wear) is superior to the 2D(wear) approach, as it is less susceptible to changes in pelvic position. However, the results suggest that a 2D(wear) approach may be an alternative method if the wear present is in the range of 100-500 mu m and a.p. radiographs are available with the pelvis projected in a neutral position.