Comparison of cervical spine kinematics using a fluoroscopic model for adjacent segment degeneration - Invited submission from the Joint Section on Disorders of the Spine and Peripheral Nerves, March 2007

Object. In this cervical spine kinematics study the authors evaluate the motions and forces in the normal, degenerative, and fused states to assess how alteration in the cervical motion segment affects adjacent segment degeneration and spondylosis. Methods. Fluoroscopic images obtained in 30 individuals (10 in each group with disease at C5-6) undergoing flexion/extension motions were collected. Kinematic data were obtained from the fluoroscopic images and analyzed with an inverse dynamic mathematical model of the cervical spine that was developed for this analysis. Results. During 20' flexion to 15' extension, average relative angles at the adjacent levels of C6-7 and C4-5 in the fused patients were 13.4' and 8.8' versus 3.7' and 4.8' in the healthy individuals. Differences at C3-4 averaged only about 1'. Maximum transverse forces in the fused spines were two times the skull weight at C6-7 and one times the skull weight at C4-5, compared with 0.2 times the skull weight and 0.3 times the skull weight in the healthy individuals. Vertical forces ranged from 1.6 to 2.6 times the skull weight at C6-7 and from 1.2 to 2.5 times the skull weight at C4-5 in the patients who had undergone fusion, and from 1.4 to 3.1 times the skull weight and from 0.9 to 3.3 times the skull weight, respectively, in the volunteers. Conclusions. Adjacent-segment degeneration may occur in patients with fusion due to increased motions and forces at both adjacent levels when compared with healthy individuals in a comparable flexion and extension range.