Motion Path of the Instant Center of Rotation in the Cervical Spine During In Vivo Dynamic Flexion-Extension Implications for Artificial Disc Design and Evaluation of Motion Quality After Arthrodesis

Study Design. Case-control. Objective. To characterize the motion path of the instant center of rotation (ICR) at each cervical motion segment from C2 to C7 during dynamic flexion-extension in asymptomatic subjects. To compare ICR paths in asymptomatic subjects and patients with single-level arthrodesis. Summary of Background Data. The ICR has been proposed as an alternative to range of motion (ROM) for evaluating the quality of spine movement and for identifying abnormal midrange kinematics. The motion path of the ICR during dynamic motion has not been reported. Methods. Twenty asymptomatic controls, 12 C5-C6, and 5 C6-C7 patients with arthrodesis performed full ROM flexion-extension, while biplane radiographs were obtained at 30 Hz. A previously validated tracking process determined 3-dimensional vertebral position with submillimeter accuracy. The finite helical axis method was used to calculate the ICR between adjacent vertebrae. A linear mixed-model analysis identified differences in the ICR path among motion segments and between controls and patients with arthrodesis. Results. From C2-C3 to C6-C7, the mean ICR location moved superior for each successive motion segment (P < 0.001). The anterior-posterior change in ICR location per degree of flexion-extension decreased from the C2-C3 motion segment to the C6-C7 motion segment (P < 0.001). Asymptomatic subject variability (95% confidence interval) in the ICR location averaged +/- 1.2 mm in the superior-inferior direction and +/- 1.9 mm in the anterior-posterior direction over all motion segments and flexion-extension angles. Asymptomatic and arthrodesis groups were not significantly different in terms of average ICR position (all P >= 0.091) or in terms of the change in ICR location per degree of flexion-extension (all P >= 0.249). Conclusion. To replicate asymptomatic in vivo cervical motion, disc replacements should account for level-specific differences in the location and motion path of ICR. Single-level anterior arthrodesis does not seem to affect cervical motion quality during flexion-extension.