Analysis of adjacent-segment cervical kinematics: the role of construct length and the dorsal ligamentous complex

OBJECTIVE Lateral mass fixation stabilizes the cervical spine while causing minimal morbidity and resulting in high fusion rates. Still, with 2 years of follow-up, approximately 6% of patients who have undergone posterior cervical fusion have worsening kyphosis or symptomatic adjacent-segment disease. Based on the length of the construct, the question of whether to extend the fixation system to undisrupted levels has not been answered for the cervical spine. The authors conducted a study to quantify the role of construct length and the terminal dorsal ligamentous complex in the adjacentsegment kinematics of the subaxial cervical spine. METHODS In vitro flexibility testing was performed using 6 human cadaveric specimens (C2-T8), with the upper thoracic rib cage and osseous and ligamentous integrity intact. An industrial robot was used to apply pure moments and to measure segmental motion at each level. The authors tested the intact state, followed by 9 postsurgical permutations of laminectomy and lateral mass fixation spanning C2 to C7. RESULTS Constructs spanning a single level exerted no significant effects on immediate adjacent-segment motion. The addition of a second immobilized segment, however, created significant changes in flexion-extension range of motion at the supradjacent level (+164%). 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 +794% and +607%, respectively). CONCLUSIONS Dorsal ligamentous 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 the present data, except potentially at C2-3 and C7-T1. The study findings emphasize the importance of the C2-3 segment and its dorsal support.