Characteristics of unicortical and bicortical lateral mass screws in the cervical spine

Study Design. A biomechanical study evaluating the safety and efficacy of unicortical versus bicortical lateral mass screws in the cervical spine. Objectives. To analyze the safety, pullout strength and radiographic characteristics of unicortical and bicortical screws placed in cadaveric spines and to evaluate the influence of level of training on the positioning of these screws. Summary of Background Data. Lateral mass plating for posterior cervical spine fusion is an effective method for the treatment of traumatic and degenerative instability. The initial description of the technique included bicortical screw purchase. The added benefit of bicortical purchase must be weighed against the increased risk of injury to nerve roots and the vertebral artery. Methods. In 21 cadaveric spines (mean age 78.9 years), 3.5-mm anterior oblique lateral mass screws were placed bilaterally from C3 to C6 (n = 168) using a modification of the Magerl technique. In the right side (unicortical) only 14-mm screws (effective length of 11 mm) were used, whereas on the left side, bicortical purchase was obtained. All screws were evaluated clinically and radiographically for safety and zone placement. Pullout force was determined for all screws. Results. Most screws (92.8%) were rated satisfactory. There were no injuries to the spinal cord. On the right side (14 mm) 98.9% of the screws were satisfactory, and on the left side (bicortical) 68.1% were satisfactory. There was a 5.8% incidence of direct artery injury (compression of vessel wall) and a 17.4% incidence of direct nerve root injury by the bicortical screws. There were no direct injuries with the unicortical screws. Most of the direct-injury-bicortical screws were placed by the surgeon with the least experience. The mean pullout force for all screws was 542.9 ± 296.6 N. There was no statistically significant difference between the pullout force for unicortical (519.9 ± 286.9 N) and bicortical (565.2 ± 306 N) screws (P< 0.05). There were no significant differences in pullout strengths in association with zone placement. Conclusions. Fourteen-millimeter lateral mass screws (effective length, 11 mm) placed in a superolateral trajectory in the adult cervical spine provide an equivalent strength with a much lower risk of injury than the longer bicortical screws placed in a similar orientation.