Two-point fixation of the lumbar spine. Differential stability in rotation.

Single vertebral motion segments were tested in torsion. The adjacent vertebrae were transfixed by two 3-mm Steinman pins placed vertically. These were applied in five different positions: between the anterior vertebral bodies, posterior vertebral bodies, pedicles, transverse processes, and lamina. Rotational displacement was limited the most by transfixation between the vertebral bodies (position one or two). Disrupting the anulus fibrosus significantly increased rotation in all positions except those in the vertebral body. These findings may imply that spinal fixation devices that engage the vertebral bodies may offer inherent advantages over purely posterior devices in stabilizing a vertebral motion segment. In addition, an interbody arthrodesis may prevent intervertebral motion better than a posterior or posterolateral fusion.