Spine-Pelvis-Hip Relationship in the Functioning of a Total Hip Replacement

Spine-pelvis-hip motion is normally coordinated to allow balance of the mass of the trunk and hip motion with standing and sitting. Normal motion from standing to sitting involves hip flexion of 55 degrees to 70 degrees and pelvic posterior tilt of 20 degrees. Because the acetahulum is part of the pelvis, as the pelvis tilts posteriorly during sitting, the inclination and anteversion increase (the acetabulum opens) to allow clearance of the femoral head and neck during hip flexion- This can be considered the biological opening of the acetabulum. Decreased tilt of the pelvis during movement occurswith stiffness of the spine. Loss of pelvic mobility forces hip motion to increase to accommodate postural change. Increased hip motion combined with change in the opening of the acetabulum increases the risk of impingement. Hip stiffness can also reduce pelvic mobility because pelvic mobility is affected by both the spine and the hip. Relief of hip stiffness with total hip replacement can improve Pelvic mobility Postoperatively. For hip surgeons, the clinical consequences of changes in the mobility of the spine and pelvis (spinopelvic mobility) an be impingement after total hip replacement, with the most obvious complication being dislocation. The reported increased dislocations in patients with surgical spine fusions is a clinical example of this consequence