Fig. 2

A 70-year-old woman with right hip osteoarthritis. On plain pelvic radiographs taken before right total hip arthroplasty (THA), the pelvis tilts backward by 18° in the supine position (a) and 34° in the standing position (b). Because the lumbolordotic angle had decreased to 5° in this case, the target reference plane for preoperative planning was set at retroversion of 30°. On radiographs taken at 1 year after THA, the pelvis tilts backward by 24° in the supine position (c) and 38° in the standing position (d), and combined anteversion (CA) is 31.3° in the supine position and 39.2° in the standing position. Because the ideal CA value of 37.3° is situated between the CA values in the supine and standing positions, the pelvic tilt is less likely to deviate from 37.3° even with further progression of posterior pelvic tilt in the future