Dynamic in vivo 3D atlantooccipital kinematics during multiplanar physiologic motions.

Normal biomechanics of the upper cervical spine, particularly at the atlantooccipital joint, remain poorly characterized. The purpose of this study was to determine the intervertebral kinematics of the atlantooccipital joint (occiput-C1) during three-dimensional in vivo physiologic movements. Twenty healthy young adults performed dynamic flexion/extension, axial rotation, and lateral bending while biplane radiographs were collected at 30 images per second. Motion at occiput-C1 was tracked using a validated volumetric model-based tracking process that matched subject-specific CT-based bone models to the radiographs. The occiput-C1 total range of motion (ROM) and helical axis of motion (HAM) was calculated for each movement. During flexion/extension, the occiput-C1 moved almost exclusively in-plane (ROM: 17.9 ± 6.9°) with high variability in kinematic waveforms (6.3°) compared to the in-plane variability during axial rotation (1.4°) and lateral bending (0.9°) movements. During axial rotation, there was small in-plane motion (ROM: 4.2 ± 2.5°) compared to out-of-plane flexion/extension (ROM: 12.7 ± 5.4°). During lateral bending, motion occurred in-plane (ROM: 9.0 ± 3.1°) and in the plane of flexion/extension (ROM: 7.3 ± 2.7°). The average occiput-C1 axis of rotation intersected the sagittal and coronal planes 7 mm to 18 mm superior to the occipital condyles. The occiput-C1 axis of rotation pointed 60° from the sagittal plane during axial rotation but only 10° from the sagittal plane during head lateral bending. These novel results are foundational for future work on upper cervical spine kinematics.