Biomechanical comparison of transarticular facet screws to lateral mass plates in two-level instrumentations of the cervical spine

Study Design. In vitro biomechanical comparison of transarticular facet screws to lateral mass plates in two level instrumentations of the cervical spine. Objective. Lateral mass plates are costly, and screw placement is difficult. Facet screws have never been tested as an alternative in the cervical spine. This biomechanical study compared cervical transarticular facet screws to lateral mass plates in two-level instrumentations of human cadaveric cervical spines. Summary of Background Data. Translaminar facet screws have been shown to have similar biomechanical performance to pedicle screw fixation in the lumbar spine, especially in flexion. They have proven to be fast, safe, and effective, with authors reporting 94% to 100% fusion rates in single-level lumbar fusions. However, a biomechanical comparison of transarticular facet screws to lateral mass plates in cervical spine instrumentations has not been reported. Methods. Thirteen human cadaveric cervical motion segments (C2 - C4, C5 - C7) were tested before and after instrumentation, with either transarticular facet screws or lateral mass plates, in flexion, extension, lateral bending, and torsion. Specimens were subjected to six cycles under a load of 2 Nm. Results. Both fixation systems significantly reduced range of motion ( ROM) and increased stiffness compared with the intact state in flexion, extension, lateral bending, and torsion. There were also no significant differences between the facet screws and plates in any of the four directions. To compare the two systems, ROM of each was analyzed relative to the uninstrumented state. Flexion was 0.26 ( or 26% of the intact state) for the transarticular facet screws versus 0.20 for the lateral mass plates ( P = 0.34), extension was 0.10 versus 0.07 ( P = 0.43), lateral bending was 0.17 versus 0.15 ( P = 0.52), and torque was 0.25 versus 0.38 ( P = 0.12). Load to failure testing failed to indicate any differences between the two methods of fixation because all the specimens failed elsewhere. Conclusion. This study proves that transarticular facet screws and lateral mass plates are equivalent in two-level instrumentations of the cervical spine. This is the first biomechanical study to test transarticular facet screws in this context.