In a patient presenting for a total hip arthroplasty with severe, multi-level degenerative scoliosis and a completely stiff, fused lumbar spine, how does this profound lack of normal spinopelvic mobility alter the dynamic biomechanics of the pelvis during the transition from a standing to a seated position?

Explanation:
The correct option is c. Normal spinopelvic kinematics are crucial for THA stability. When a healthy person moves from standing to sitting, the lumbar spine flexes (flattens), which allows the pelvis to roll backward (retrovert). This pelvic retroversion naturally 'opens up' the acetabulum, increasing its functional anteversion by 10 to 20 degrees, safely accommodating the flexing femur and preventing impingement. In a patient with a stiff or fused spine, this compensatory pelvic retroversion cannot occur. The acetabular cup remains locked in its standing orientation (relatively 'closed' or under-anteverted). When the femur flexes up to 90 degrees for sitting, the neck crashes into the anterior rim of the cup (anterior impingement), forcefully levering the head out posteriorly. Reference: Lazennec PR et al. Pelvis and total hip arthroplasty: a new dynamic concept. J Bone Joint Surg Br.