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.

Effect of compressive follower preload on the flexion-extension response of the human lumbar spine.

Traditional experimental methods are unable to study the kinematics of whole lumbar spine specimens under physiologic compressive preloads because the spine without active musculature buckles under just 120 N of vertical load. However, the lumbar spine can support a compressive load of physiologic magnitude (up to 1200 N) without collapsing if the load is applied along a follower load path. This study tested the hypothesis that the load-displacement response of the lumbar spine in flexion-extension is affected by the magnitude of the follower preload and the follower preload path.