Six-degrees-of-freedom cervical spine range of motion during dynamic flexion-extension after single-level anterior arthrodesis: comparison with asymptomatic control subjects.

BACKGROUND: The etiology of adjacent-segment disease following cervical spine arthrodesis remains controversial. The objective of the current study was to evaluate cervical intervertebral range of motion during dynamic flexion-extension in patients who had undergone a single-level arthrodesis and in asymptomatic control subjects.
METHODS: Ten patients who had undergone a single-level (C5/C6) anterior arthrodesis and twenty asymptomatic control subjects performed continuous full range-of-motion flexion-extension while biplane radiographs were collected at thirty images per second. A previously validated tracking process determined three-dimensional vertebral position on each pair of radiographs with submillimeter accuracy. Six-degrees-of-freedom kinematics between adjacent vertebrae were calculated throughout the entire flexion-extension movement cycle over multiple trials for each participant. Cervical kinematics were also calculated from images collected during static full flexion and static full extension.
RESULTS: The C4/C5 motion segment moved through a larger extension range of motion and a smaller flexion range of motion in the subjects with the arthrodesis than in the controls. The extension difference between the arthrodesis and control groups was 3.8° (95% CI [confidence interval], 0.9° to 6.6°; p = 0.011) and the flexion difference was -2.9° (95% CI, -5.3° to -0.5°; p = 0.019). Adjacent-segment posterior translation was greater in the arthrodesis group than in the controls, with a C4/C5 difference of 0.8 mm (95% CI, 0.0 to 1.6 mm) and a C6/C7 difference of 0.4 mm (95% CI, 0.0 to 0.8 mm; p = 0.016). Translation range of motion and rotation range of motion were consistently larger when measured on images collected during dynamic functional movement as opposed to images collected at static full flexion or full extension. The upper 95% CI limit for anterior-posterior translation range of motion was 3.45 mm at C3/C4 and C4/C5, but only 2.3 mm at C6/C7.
CONCLUSIONS: C5/C6 arthrodesis does not affect the total range of motion in adjacent vertebral segments, but it does alter the distribution of adjacent-segment motion toward more extension and less flexion superior to the arthrodesis and more posterior translation superior and inferior to the arthrodesis during in vivo functional loading. Range of motion measured from static full-flexion and full-extension images underestimates dynamic range of motion. Clinical evaluation of excessive anterior-posterior translation should take into account the cervical vertebral level.