Abstract
Dislocation and accelerated wear have been the nemesis of hip surgeons. No study has been able to correlate cup position to instability. In recent years the influence of the spine-pelvis-hip construct has emerged as important to understand the shift in component position with postural change.
Using measurements familiar to spine surgeons, we have correlated the pelvic incidence (PI), a static measurement of pelvic width and hip position; the static tilt, a dynamic measure of pelvic-spine mobility. For THR we have measured the sagittal cup position as the fixed angular change of the cup shifts with pelvic tilt, and this is named anteinclination; and the sacral acetabular angle (SAA) which is the relationship of the acetabulum to the absolute value of sacral tilt (ST) in both standing and sitting. The pelvic femoral angle (PFA) is a measure of femur/hip flexion/internal rotation correlated to pelvic mobility.
Dislocation is most common in patients with low PI combined with an ST change <15 degrees. With normal PI and high PI, it occurs much less commonly and only in patients with ST change <5 degrees (very stiff). In patients with stiff pelvis (ST<13) the cup needs increased inclination and anteversion (45/20–25) to compensate for absence of cup opening by posterior tilt of pelvis. For patients with low PI and stiff pelvis we recommend constraint (such as dual mobility articulation).
