The influence of spinal sagittal balance on acetabular component orientation and its clinical relevance

Total hip arthroplasty (THA) is a well developed technique to which the spatial orientation of acetabular component is crucial. Since the idea of "safe zone" being proposed, such idea has long been obeyed by hip surgeons but has also been challenged in recent years. This is because post-operative instability could happen even if acetabular components are placed within "safe zone" . The latest researches have demonstrated that sagittal spinal balance and spino-pelvic mobility can affect the spatial orientation of acetabular components. Factors including spinal degenerative changes, long lumbosacral changes and spinal ankyloses can lead to sagittal spinal imbalance and/or influence spino-pelvic mobility, which subsequently affect pelvic retroversion. Proper pelvic retroversion is a way to compensate for sagittal spinal imbalance and is beneficial to post-operative stability. It is necessary for hip surgeons to rethink "safe zone" before performing THA and understand spinal balance as well as spinopelvic mobility. The goal is to reduce the rate of dislocation. This review started by introducing the related spinopelvic parameters frequently seen in papers published recently and the definitions of acetabular cup orientations. After the spinal sagittal balance and its compensatory mechanisms session, discussed its clinical relevance, as well as the significance of spinal mobility, in spinopelvic motion under three different postures.