Changes in Midrange and End Range Adjacent Segment Motion 3 Years After One- Or Two-Level Cervical Arthrodesis

Biomechanical cadaver studies consistently find that cervical arthrodesis leads to increased motion adjacent to the arthrodesis. However, in vivo evidence indicates adjacent segment end range motion does not reliably increase at 1 year after one-level or two-level cervical arthrodesis. Because changes beyond 1 year remain unknown, this study assessed the effects of one- and two-level cervical arthrodesis on adjacent segment motion 3 years after arthrodesis.

Sixty-two patients were tested preoperatively (PRE), 1 year postoperatively (1YR-POST), and 3 years postoperatively (3YR-POST) along with 22 asymptomatic controls, who were tested once. A validated CT model-based tracking technique was used to measure three-dimensional (3D) vertebral motion from biplane radiographs acquired during dynamic flexion-extension and axial rotation of the cervical spine. Adjacent segment intervertebral end range of motion (ROM) and midrange ROM (i.e., head within 20° of neutral) were compared between one-level and two-level arthrodesis patients and controls after accounting for head ROM.

End range adjacent segment motion tended to increase over time after arthrodesis; however, significant increases were found only after two-level arthrodesis. In contrast, midrange adjacent segment motion increased significantly after one- and two-level arthrodesis and was greater than in corresponding motion segments in controls.

Midrange adjacent segment motion may be better than end range motion for detecting early mechanical changes after cervical arthrodesis that could contribute to adjacent segment degeneration. In vivo changes in adjacent segment motion are inconsistent across patients up to 3 years after anterior cervical arthrodesis.