Three-dimensional intervertebral kinematics in the healthy young adult cervical spine during dynamic functional loading.

The objective of this study was to determine the intervertebral kinematics of the young, healthy cervical spine during dynamic, three-dimensional, functional loading. Intervertebral motion was characterized by the range of motion (ROM) and the helical axis of motion (HAM). Biplane radiographs of the cervical spine were collected at 30 images/s as 29 participants (20-35 yr) performed dynamic flexion\extension, axial rotation, and lateral bending. Vertebral motion (C1-T1 in flexion\extension, C3-T1 in lateral bending and axial rotation) was tracked with sub-millimeter accuracy using a validated volumetric model-based tracking process that matched subject-specific CT-based bone models to the radiographs. Flexion\extension ROM was smallest at the C2-C3 motion segment (12.7±2.6°) and largest at the C5-C6 motion segment (19.7±3.7°). During head lateral bending and axial rotation, the intervertebral bending ROM was greater than the rotation ROM at every motion segment. The HAM demonstrated differences among motion segments and among movements. During flexion\extension, the helical axis of motion was directed nearly perpendicular to the sagittal plane for the C2-C3 through C7-T1 motion segments. During lateral bending, the angle between the HAM and the transverse plane progressively increased from the C6-C7 motion segment (approximately ±22°) to the C3-C4 motion segment (approximately ±40°). During axial rotation, the angle between the transverse plane and the HAM was approximately ±42° at the C3-C4 through C5-C6 motion segments, and approximately ±32° at the C6-C7 motion segment. This study provides valuable reference data for evaluating the effects of age, degeneration, and surgical procedures on cervical spine kinematics during three-dimensional dynamic functional loading.