High-grade dysplastic spondylolisthesis and spondyloptosis: report of three cases with surgical treatment and review of the literature.

High-grade dysplastic spondylolisthesis is extremely rare and always involves the L5-S1 level. It is attributed to congenital dysplasia of the superior articular process of the sacrum. It can remain asymptomatic for a long time and can progress to a more severe grade of olisthesis and spondyloptosis.