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

Study Design. The three-dimensional flexibility of six human lumbar functio nal spinal units was measured after the anterolateral insertion of an inter body cage. Objectives. To determine whether an interbody cage inserted from an anterol ateral direction stabilizes the spine with respect to the intact state and to compare the finding with that from the same cage inserted from an anteri or direction. Summary of Background Data. Several biomechanical studies have shown that i nterbody cages do not stabilize the spine in extension. It is suspected tha t this may be caused by the destruction of the anterior longitudinal ligame nt and anterior anulus fibrosus. Methods. Six human cadaveric lumbar functional spinal units were tested und er pure moments of flexion, extension, bilateral axial rotation, and bilate ral lateral bending to a maximum of 10 Nm. The relative intervertebral moti ons were measured by an optoelectronic camera system with the spinal units in the intact condition, after discectomy, after anterolateral interbody ca ge stabilization, and with additional translaminar screw fixation. The impl ant used was a central, porous, contoured im-plant with endplate fit. The r esults were compared with those of a previous study, which used the same im plant inserted from an anterior direction. Results. The anterolateral cage insertion significantly decreased the motio n in comparison with the intact situation in flexion and lateral bending, b ut not in extension or axial rotation. No differences were found between th e anterior and anterolateral insertion approaches in flexion or extension, but differences were observed in axial rotation and lateral bending, in whi ch the anterolateral approach resulted in more motion, Additional translami nar screw fixation reduced motion to below intact levels in all loading dir ections. None of the surgical procedures introduced asymmetrical behavior. Conclusions. Anterolateral cage insertion did not stabilize the spine in ex tension or axial rotation and was not different from the anterior approach in flexion and extension. Additional translaminar screw fixation stabilized in all directions.