Biomechanical effects of progressive anterior cervical decompression

STUDY DESIGN: A repeated-measures in vitro flexibility test was performed. OBJECTIVES: To determine the biomechanical functions of tissues resected during anterior cervical decompression of various extents. SUMMARY OF BACKGROUND DATA: The biomechanical consequences of discectomy have been studied in vitro, and uncovertebral joint removal has been modeled numerically. No studies have assessed the relative biomechanical contributions of different anterior column structures. METHODS: In seven human cadaver C4-T1 specimens, 20 motion segments were studied. After each destructive step, including discectomy, unilateral uncinate process removal, bilateral uncinate process removal, and posterior longitudinal ligament transection, torques were applied to four-level specimens while the angular motion was measured at each level. RESULTS: Angular range of motion and neutral zone increased by variable but statistically significant amounts after each progressive resection, most notably in flexion and extension. Each resection step caused progressively larger shifts (up to 23 mm) in the location of the axis of rotation. Uncovertebral joint resection caused the most significant changes in the observed angular coupling. CONCLUSIONS: Anterior cervical decompression significantly increases the instability and alters the kinematics of cervical motion segments. Each structure resected contributes to normal stability and kinematics, so as many structures as possible should be left intact during anterior decompression without fusion. Because flexion and extension were the modes of motion that increased most significantly after decompression, the primary function of a grafting technique or fixation device should be to limit these motions.