Sagittal alignment of cervical flexion and extension: lateral radiographic analysis.

STUDY DESIGN: An analysis of lateral radiographs in the upright, flexion-extension position. OBJECTIVES: To document and define the differences in cervical flexion-extension kinematics as they relate to changes of alignment in upright cervical lordosis. SUMMARY OF BACKGROUND DATA: No previous study has reported the association between sagittal plane cervical rotation kinematics and changes of alignment in upright cervical lordosis. METHODS: Lateral radiographs were classified into five groups (Group A, lordosis; Group B, straight; Group C, kyphosis; Group D, S-curve with lordotic upper cervical and kyphotic lower cervical spine; and Group E, S-curve with kyphotic upper cervical and lordotic lower cervical spine) by changes of alignment in upright position. Sagittal cervical rotation angles were measured by a computer-assisted method in the fully flexed and extended positions. RESULTS: Group A revealed the largest angle of lordosis at extension and the smallest angle of kyphosis at flexion, whereas Group C revealed the smallest angle of lordosis at extension and the largest angle of kyphosis at flexion. When Group D adopted the flexion-extension position, the curvature of the upper cervical spine was the same as Group A, whereas the lower cervical spine showed the same curve as Group C. Similarly, the cervical rotation kinematics in Group E were a combination of motion of upper cervical spine in Group C and that of lower spine in Group A. CONCLUSION: The results suggest that alterations in the static alignment of the cervical curvature cause alterations in the dynamic kinematics of the cervical spine during cervical flexion-extension. This information should aid in the interpretation of kinematic studies of the cervical spine.