Fig. 6

Paradoxical patient # 1. This was a 70 year old female. Standing SVA was negative (− 1.9 cm), and the trunk anterior shift from standing (A) to sitting (B) was 5.8 cm (orange arrow). The lumbar spine degeneration resulted in only 6° of lumbosacral motion (white arrows). The hip motion was − 77.2° (black arrow), which caused pelvic anterior tilt for 4.6° (yellow arrow). The anteriorly tilted pelvis led to reduced acetabular anterior clearance when sitting down. Moreover, as the THA cup version was not large enough (anteversion: 21°; inclination: 25°), posterior dislocation occurred 1 month after surgery. SVA, C7-sagittal vertical axis