An experimental model of adult-onset slip progression in isthmic spondylolistesis.

Progressive spondylolisthesis may occur in adults with isthmic spondylolysis with an estimated incidence of 20%. This biomechanical study showed that in adult spines with bilateral L5 pars fractures, an anterior slip of L5 depends on the extent of the load-bearing deficiency of the disc and the presence of shear force across L5-S1. A combination of disc degeneration and muscle dysfunction may be responsible for converting a stable isthmic spondylolysis into an unstable progressive spondylolisthesis.