Fig. 7

Paradoxical patient #5. This was a 76-year-old female with a balanced spine with SVA 2.2 cm. The trunk anterior shift from standing (A) to sitting (B) was 4.9 cm (orange arrow). An extensive spinal fusion of the lumbar spine (T12-S2) caused a fixed lumbosacral spine (∆LL: 0°). The hip motion was − 78.6° (black arrows), which caused pelvic anterior tilt for 2.4° (yellow arrow). The anterior tilt of the pelvis caused a decrease in acetabular anterior clearance in the sitting position. Moreover, as the THA cup version was not large enough (anteversion: 21°; inclination: 45°), posterior dislocation occurred 4 months after surgery. SVA, C7-sagittal vertical axis; LL, lumbar lordosis