Cervical motion segment percent contributions to flexion-extension during continuous functional movement in control subjects and arthrodesis patients.

STUDY
DESIGN: Case control study.
OBJECTIVE: To quantify precisely and compare intervertebral segmental contributions to cervical spine flexion-extension during continuous, functional flexion-extension in asymptomatic subjects with patients who underwent single-level anterior arthrodesis. SUMMARY OF BACKGROUND DATA: Segmental contributions to cervical flexion-extension have traditionally been determined using single images collected at full flexion and full extension. These calculations neglect midrange motion and assume that percent contributions to motion remain constant throughout the entire flexion-extension range of motion (ROM).
METHODS: Six patients with single-level (C5-C6) anterior arthrodesis and 18 asymptomatic control subjects performed flexion-extension while biplane radiographs were collected at 30 images per second. A previously validated tracking process determined 3-dimensional vertebral position with submillimeter accuracy during continuous flexion-extension. Mixed-effects models of segmental percent contribution to C2-C7 flexion-extension were developed to identify differences in percent contribution within each motion segment, among motion segments, and between controls and patients who underwent arthrodesis over the full ROM.
RESULTS: The C2-C3, C3-C4, and C4-C5 motion segments made their maximum contributions during the midrange of motion. The C5-C6 and C6-C7 motion segments, in contrast, made their maximum contributions near the start and end of the ROM. Arthrodesis patients' contribution from the C4-C5 motion segment increased significantly over the range of motion from 30% to 95% of the total flexion-extension ROM (average increased contribution of 5.1%) and arthrodesis patients' contribution from the C6-C7 motion segment increased significantly over the entire flexion-extension ROM (average increased percentage contribution of 8.9%) in comparison to controls.
CONCLUSION: Cervical motion segment contributions to flexion-extension change significantly during the flexion-extension motion. The largest change in percent contribution to motion, relative to controls, occurs at the C6-C7 motion segment, over the entire ROM, suggesting a potential mechanical mechanism for the clinical observation of increased incidence of adjacent segment degeneration at C6-C7 rather than at C4-C5 after C5-C6 arthrodesis.