Introduction: Lateral mass fixation stabilizes the cervical spine with minimal morbidity and high fusion rates. With 2-year follow-up, ~6% of posterior cervical fusions have worsening kyphosis or symptomatic adjacent segment disease. Based on length of the construct, the question of whether to extend fixation to undisrupted levels has not been answered in the cervical spine. We sought to quantify the role of construct length and the terminal dorsal musculoligamentous interspinous support in the adjacent segment kinematics of the subaxial cervical spine.

Methods: In vitro flexibility testing performed on six human cadaveric specimens, C2-T7, with upper thoracic rib cage, osseous and ligamentous integrity intact. An industrial robot was utilized to apply pure moments with measurement of segmental motion at each level. We tested the intact state, followed by nine post-surgical permutations of laminectomy and lateral mass fixation spanning C2-C7 (hybrid model). Tukey-Kramer analysis was utilized for multiple comparisons.

Results: Constructs spanning a single level exerted no significant effects on immediately adjacent segment motion. The addition of a second immobilized segment, however, created significant changes in flexion-extension range of motion at the supradjacent level (+39%, p=0.013). Regardless of construct length, resection of the terminal dorsal ligaments did not greatly affect adjacent level motion except at C2/3 and C7/T1 (increasing by +316%, p=0.06 and 107%, p=0.01, respectively).

Conclusions: Dorsal musculoligamentous support was found to contribute significant stability to the C2/3 and C7/T1 segments only. Construct length was found to play a significant role when fixating two or more segments. The addition of a fused segment to support an undisrupted cervical level is not suggested by our data, except potentially at C2/3 and C7/T1. Our study emphasizes the importance of the C2/3 segment and its dorsal support and illustrates the importance of global cervical alignment even in the setting of short cervical constructs.

Patient Care: The importance of long-term outcomes after spinal fusion are becoming increasingly apparent. The role of sagittal balance has been well described following posterior cervical fusion, however adjacent segment pathology is not completely understood. A fundamental understanding of adjacent segment kinematics is necessary as we move towards more durable solutions, reducing long-term morbidity and improving patient outcomes. The role of the cervicothoracic junction as a vulnerable transition point has been well described, however the importance of the C2/3 segment, the role of construct length, and the role of the dorsal musculoligamentous support are also emphasized by our study.

Learning Objectives: By the conclusion of this session, participants should be able to 1) Describe the affect of posterior cervical construct length on adjacent segment kinematics, 2) Describe the affects of the interspinous musculature and supraspinous ligaments at the terminal ends of a cervical construct, 3) Understand the importance of the C2/3 cervical segment as well as the cervicothoracic junction when considering adjacent segment disease.

References: Sekhon LHS: Posterior cervical decompression and fusion for circumferential spondylotic cervical stenosis: review of 50 consecutive cases. J Clin Neurosci 13:23–30, 2006.