Approximately 25% of cervical arthrodesis patients require reoperation within 10 years of the initial surgery due to degeneration of motion segments adjacent to the arthrodesis. Adjacent segment degeneration is believed to result from one or more of the following distinct causes:​ 1) the natural history of the adjacent disc;​ 2) biomechanical stress on the adjacent level following the fusion;​ and 3) disruption of the adjacent segment anatomy due to the initial surgery. The overarching hypothesis of this study is that, after fusion, mechanical factors initiate disc degeneration by exposing the disc tissue to novel, excessive loads. The aims of this study were to identify kinematic and arthrokinematic characteristics of cervical spine motion that differentiate asymptomatic subjects from single-level anterior fusion patients during in vivo functional loading, and to verify and validate a single-level, subject-specific finite element model of the sub-axial cervical spine. Twenty asymptomatic control subjects and 17 single-level ACDF patients of similar age performed dynamic flexion-extension of the cervical spine while biplane radiographs were collected at 30 images per second. A previously validated volumetric modelbased tracking process matched subject-specific vertebral bone models to each pair of radiographs with sub-millimeter accuracy. Adjacent segment kinematics (total range of motion, contributions to motion, path of the center of rotation) were not significantly different between fusion and control groups. Adjacent segment arthrokinematics (disc and facet joint capsule deformation) were significantly different between groups. Inverse dynamics and finite element computational models indicated that, relative to the static neutral position, the force applied to the C56 motion segment increased by five times head weight during full extension.

This study has identified differences in the mechanics of adjacent segments during dynamic functional loading. The results suggest that in order to evaluate the effects of fusion on adjacent segments, from a mechanical perspective and clinical perspective, it may be most beneficial to assess arthrokinematic factors such as disc deformation and facet joint capsule deformation, rather than more traditional kinematic parameters such as range of motion and center of rotation.