A square to indicate the unstable zone in severe spondylolisthesis.

The reduction and stabilisation of high-grade dysplastic developmental spondylolisthesis by means of modern internal fixators can correct slip, but can leave the sagittal alignment unbalanced, causing instability, e.g. in the adjacent, non-fused lumbar segments. Through analysis of the modifications of imbalance in the spine and pelvic ring due to surgical correction, this study defines the unstable zone of high-grade dysplastic developmental spondylolisthesis and proposes a simple radiographic method to identify it.