In all, 145 of 160 hips (90%) were considered safe from impingement. Patients with highest risk are those with biological or surgical spinal fusion; patients with dangerous spinal imbalance can be safe with correct acetabular component position. The clinical relevance of the study is that it correlates acetabular component position to spinal pelvic mobility which provides guidelines for total hip arthroplasty. Cite this article: Bone Joint J 2017;99-B(1 Supple A):37-45.

* 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° to 70° and pelvic posterior tilt of 20°. Because the acetabulum 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 occurs with 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) can 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.

Background: Late dislocations after total hip arthroplasty (THA) are challenging for the hip surgeon because the cause is often not evident and recurrence is common. Recently, decreased spinopelvic motion has been implicated as a cause of dislocation. The purpose of this study was to assess the mechanical causes of late dislocation, including the influence of spinopelvic motion. Methods: Twenty consecutive patients were studied to identify the cause of their late dislocation. Cup inclination and anteversion were measured on standard pelvic radiographs. Lateral standing and sitting spine-pelvis-hip radiographs were used to measure pelvic motion, femoral mobility, and sagittal cup position by assessing sacral slope, pelvic-femoral angle, and cup ante-inclination. Spinopelvic motion was defined as the difference between the standing and sitting sacral slopes (Δsacral slope). A new measurement, the combined sagittal index, which measures the sagittal acetabular and femoral positions, was used to assess the functional motion of the hip joint and risk of impingement. Results: There were 9 anterior dislocations (45%) and 11 posterior dislocations (55%) at a mean of 8.3 years after a primary THA. Eight of the 9 patients with an anterior dislocation had spinopelvic abnormalities such as fixed posterior pelvic tilt when standing, increased standing femoral extension, and an increased standing combined sagittal index. Ten of the 11 patients with a posterior dislocation had abnormal spinopelvic measurements such as decreased spinopelvic motion (average Δsacral slope [and standard error] = 9.0° ± 2.4°), increased femoral flexion, and a decreased sitting combined sagittal index. For every 1° decrease in spinopelvic motion, there was an associated 0.9° increase in femoral motion and, in some patients, this resulted in osseous impingement and dislocation. Conclusions: Patients with a late dislocation have abnormal spinopelvic motion that precipitates the dislocation, especially when combined with cup malposition or soft-tissue abnormalities. Spinopelvic stiffness is associated with increased age and increased femoral motion, which may lead to impingement and dislocation. Lateral spine-pelvis-hip radiographs may predict the risk and direction of dislocation. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

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