Paradoxical motion in spondylolisthesis due to two-segment instability.

We report here a paradoxical motion in unstable spondylolytic spondylolisthesis: a more forward displacement of the L5 vertebral body on the sacrum on the standing extension view than on the standing flexion view. An axial loading through the inferior articular process of the posteriorly displaced L4 on extension appears to be the cause, while the instability in the two contiguous segments may be an important contributing factor.