FLEXION FAILURE OF POSTERIOR CERVICAL LATERAL MASS SCREWS - INFLUENCEOF INSERTION TECHNIQUE AND POSITION

Study Design. The strength of posterior cervical lateral mass fixation was evaluated in a cadaver model for two techniques of screw insertio n. Objective. To compare the flexion failure strengths of posterior ce rvical plate fixation for two techniques of screw placement at the sup erior and inferior screw hole positions, and to evaluate the effect of bone mineral density on fixation strength. Summary of Background Data . Biomechanical analyses of various screw insertion techniques for pos terior cervical lateral mass fixation have never evaluated the effect of screw position along the plate. Methods. Individual C3-C6 segments of 24 human cadaveric cervical spines were used. The spinous process a nd lamina were removed to simulate a postlaminectomy situation. Verteb ral body bone mineral density for each specimen was determined by dual -energy radiograph absorption scanning. In each lateral mass, a bicort ical 3.5-mm screw was placed using either the Magerl or Roy-Camille in sertion technique through an end hole of a titanium bone plate. For '' superior'' screws, the plate was directed caudally; for ''inferior'' s crews, the plate was directed cranially. Screw voilation of the surrou nding facet joints was noted. An increasing flexion moment was applied by loading the plate 4 cm from the screw head at a rate of 10 cm/min using a servohydraulic testing machine until screw failure. Results. F or the superior screw hole position, the Magerl screw sustained a sign ificantly higher average moment to failure (190.2 Ncm) than the Roy-Ca mille screw (138.7 Ncm; P < 0.05). For the inferior screw hole positio n, there was no significant difference in flexion failure strength bet ween the two techniques (Magerl screws, 287.7 Ncm; Roy-Camille screws, 308.2 Ncm). For each insertion technique, inferior screws were nearly twice as strong as superior screws (P < 0.01). Violation of the infer ior articular process occurred with 53% of Roy-Camille screws and with none of the Magerl screws. Lateral mass fracture on screw insertion o ccurred with 6% of the Roy-Camille screws and with 7% of the Magerl sc rews. Significant correlation between screw path length and load to fa ilure was found only at the superior screw hole position. Correlation with vertebral body bone mineral density was significant at both posit ions. Conclusions. The Magerl technique has advantages over the Roy-Ca mille technique for placing the end screws when performing posterior c ervical lateral mass plate fixation, providing greater strength superi orly and not violating unfused facet joints inferiorly. Evaluation of bone mineral density by dual-energy radiographic absorption scanning i s predictive of failure strength for both test modes.