Does anterolateral cage insertion enhance immediate stabilization of the functional spinal unit? A biomechanical investigation.

Abstract

The three-dimensional flexibility of six human lumbar functional spinal units was measured after the anterolateral insertion of an interbody cage. To determine whether an interbody cage inserted from an anterolateral direction stabilizes the spine with respect to the intact state and to compare the finding with that from the same cage inserted from an anterior direction. Several biomechanical studies have shown that interbody cages do not stabilize the spine in extension. It is suspected that this may be caused by the destruction of the anterior longitudinal ligament and anterior anulus fibrosus. Six human cadaveric lumbar functional spinal units were tested under pure moments of flexion, extension, bilateral axial rotation, and bilateral lateral bending to a maximum of 10 Nm. The relative intervertebral motions were measured by an optoelectronic camera system with the spinal units in the intact condition, after discectomy, after anterolateral interbody cage stabilization, and with additional translaminar screw fixation. The implant used was a central, porous, contoured implant with endplate fit. The results were compared with those of a previous study, which used the same implant inserted from an anterior direction. The anterolateral cage insertion significantly decreased the motion in comparison with the intact situation in flexion and lateral bending, but not in extension or axial rotation. No differences were found between the anterior and anterolateral insertion approaches in flexion or extension, but differences were observed in axial rotation and lateral bending, in which the anterolateral approach resulted in more motion. Additional translaminar screw fixation reduced motion to below intact levels in all loading directions. None of the surgical procedures introduced asymmetrical behavior. Anterolateral cage insertion did not stabilize the spine in extension or axial rotation and was not different from the anterior approach in flexion and extension. Additional translaminar screw fixation stabilized in all directions.