Pelvic retroversion is the key protective mechanism of L4–5 degenerative spondylolisthesis

Purpose To explore the role of spinopelvic sagittal alignment in the pathological mechanism of degenerative spondlylolisthesis (DS) development. Method A total of 52 asymptomatic volunteers, 32 single segment L4–5 DS and 29 lumbar spinal stenosis (LSS) without spondylolisthesis patients were enrolled. All subjects had standard lumbar spine X-ray films with standard position along with lumbar spine magnetic resonance image. Comparative analysis of sagittal parameters and disc degeneration grades among asymptomatic volunteers and patients with the two disorders were performed. Results Compared to normal population (NP) and LSS, DS showed significantly greater pelvic incidence (PI), sacral slope (SS) and lumbar lordosis (LL), while LSS showed significantly smaller PT and PT/SS. DS showed significantly greater L5 slope than NP and LSS. In both Great-PI group and Small-PI group, all above differences between DS and LSS remained. LSS showed significantly higher degenerative grade of each adjacent disc than DS. Population with adjacent segment degeneration showed higher incidence of pelvic retroversion (PT/SS ≥1), and LSS showed greater proportion of adjacent segment degeneration than DS. Conclusions Lumbar spine morphology of great LL determined by great PI is a risk factor of L4–5 DS. L5 slope is a parameter that can be used to predict the risk of L4–5 DS. Pelvic retroversion is the key protective mechanism from DS. Adjacent segment degeneration is a driving factor of pelvic retroversion for compensation of lumbar sagittal malalignment.