Two-dimensional and three-dimensional cup coverage in total hip arthroplasty with developmental dysplasia of the hip.

We analyzed the mean difference and correlation between 2D cup coverage measured from different projections and three-dimensional (3D) cup coverage to investigate their precise relationship in total hip arthroplasty (THA) among patients with developmental dysplasia of the hip (DDH). We created DDH-THA models on six foam pelvic models. 3D cup coverage was measured using a motion capture system and imaging software. Digitally reconstructed radiographs with predetermined pelvic rotations were simulated using image processing software at three different angles of rotation around the long body axis (0°, 25° and 45°). 2D cup coverage was then measured on these reconstructed radiographs. The 3D technique showed excellent intra-observer (κ>0.98) and inter-observer (κ>0.99) reliability. The 2D technique tended to overestimate the real cup coverage by about 15%. The smallest difference between 2D and 3D cup coverage occurred when 2D measurement was performed on the radiographs with 45° of pelvic rotation toward the operated side (14.50%, P<0.0001), meanwhile, the highest correlation coefficient between 2D and 3D cup coverage was also observed when the 2D measurement was performed on the radiographs at this same pelvic rotation (r=0.67, P=0.0003). Published recommendations regarding the minimum cup coverage based on 2D measurement should be interpreted cautiously. The minimal cup coverage, as an intra-operative 3D parameter related to the long term fixation of the cup component, should be more accurately determined with intra-operative measurement.