Biomechanical analysis of transpedicular screw fixation in the subaxial cervical spine

Study Design. An in vitro biomechanical study to compare 2 different dorsal screw fixation techniques in the cervical spine with respect to primary stability and stability after cyclic loading. Objectives. To investigate if the biomechanical stability is better in pedicle screw or in lateral mass fixation. Summary of Background Data. In patients with poor bone quality who require multisegmental fixations, the current dorsal stabilization procedures in the subaxial cervical spine using lateral mass screws are often insufficient. Cervical pedicle screw fixation has been suggested as an alternative procedure, but there are still limited data available on the biomechanical differences between pedicle screw and lateral mass fixation. Methods. A severe multilevel discoligamentous instability was created in 8 human cervical spine specimens (C2-C7). Dorsal stabilization was performed with the assistance of computer navigation (SurgiGate, Medivison, Switzerland) using either lateral mass or pedicle screw fixation. In the first part of the study, primary stability was measured by means of a multidirectional flexibility test. Then, specimens were divided into 2 groups, randomized for bone mineral density. Cyclic loading was applied with sinusoidal loads in flexion/extension (1000 cycles, +/-1.5 Nm, 0.1 Hz). Mechanical behavior of the specimens was determined by a flexibility test before and after the application of cyclic loads. Data analysis was performed by calculating the ranges of motion, and statistical differences were determined with the t test for group comparison. Results. Pedicle screw fixation showed a significantly higher stability in lateral bending (pedicle screw range of motion 0.86+/-0.31degrees; lateral mass range of motion 1.43+/-0.62degrees; P=0.037). No significant differences were seen in flexion/extension and axial rotation. After cyclic loading, the decrease in stability was less with pedicle screw fixation in all load directions. Differences in the decrease of stability were statistically significant in flexion/extension (pedicle screw 95.4+/- 9.4%; lateral mass 70.5+/- 9.8%; P=0.010) and lateral bending (pedicle screw 105.3+/-5.0%; lateral mass 84.2+/-13.6%; P=0.046), whereas there was no significant difference in axial rotation. Conclusions. The major finding of the current study was the higher stability of pedicle screws over lateral mass fixation with respect to primary stability and stability after cyclic loading. From a biomechanical point of view the use of pedicle screws in the subaxial cervical spine seems justified in patients with poor bone quality and need for multisegmental fixation.