Crossing the Cervicothoracic Junction: A Biomechanical Investigation of C7 vs T1 Caudal Selection's Effect on Adjacent Segment Motion in Posterior Cervical Fusion.

Abstract

Biomechanical Study. This study aims to evaluate the biomechanical adjacent segment effects of multi-level posterior cervical fusion constructs that terminate at C7 compared to those that terminate at T1 in cadaveric specimens. The cervicothoracic junction poses unique challenges for spine surgeons. Deciding to terminate multi-level posterior cervical fusion constructs at C7 or extend them across the cervicothoracic junction remains a controversial issue. Six cadaveric specimens underwent biomechanical testing in the intact state and after instrumentation with constructs from C3 and terminating at either C7 or T1. Range of motion (ROM) was assessed in flexion-extension, lateral bending, and axial rotation globally and at cranial and caudal adjacent segments. There was a significant decrease in overall flexion/extension by both C7 (-35.5°, P=0.002) and T1 (-39.8°, P=0.002) instrumentation compared to the intact spine. T1 instrumentation had significantly lower (-4.3°, P=0.008) flexion/extension ROM compared to C7 instrumentation. There were significant decreases in axial rotation by both C7 (-31.4°, P=0.009) and T1 (-36.8°, P=0.009) instrumentation compared to the intact spine, but no significant differences were observed between the two. There were also significant decreases in lateral bending by both C7 (-27.9°, P=0.022) and T1 (-33.7°, P=0.022) instrumentation compared to the intact spine, but no significant differences were observed between the two. No significant differences were observed in ROM at cranial or caudal adjacent segments between constructs terminating at C7 and those extending to T1. This biomechanical investigation demonstrates that constructs that cross the cervicothoracic junction experience less overall spinal motion in flexion-extension compared to those that terminate at C7. However, contrary to prior studies there is no difference in cranial and caudal adjacent segment motion. Surgeons should make clinical decisions regarding the caudal extent of fusion in multi-level posterior cervical fusions without major concerns about adjacent segment motion.