Influence of the X-ray technique on evaluation of cup anteversion after total hip arthroplasty

Aim: Total hip arthroplasty (THA) is a standard procedure in orthopaedic surgery. Planar radiography of the implant is routinely performed to ensure that the operation has achieved the desired result. This study investigates the influence of varying the X-ray beam on the analysis of the radiograph. Method: Twenty-three patients were analysed after implantation of a total hip replacement. Besides a CT scan of the pelvis, symphysis- and hip-centred radiographs were available. The cup anteversion was calculated using two published algorithms. The CT-based data were the gold standard. Results: Our study revealed a significant major error in the analysis of the symphysis-centred radiographs when compared with the hip-centered radiographs. Widmer's algorithm showed a significant superiority over Pradhan's formula. Conclusion: The best approximation of the anteversion angle was achieved using Widmer's technique by centring the X-ray beam onto the hip.