In vivo three-dimensional kinematics of the upper cervical spine during upright head rotation in patients with atlantoaxial dislocation.

Abstract

To quantify in vivo six-degree-of-freedom (6DOF) kinematics of the upper cervical spine during active head rotation in patients with atlantoaxial dislocation (AAD) and to compare the motion patterns with healthy adults. Fifty-one imaging-confirmed AAD patients (23 men, 28 women; mean age 52 ± 15 years) performed left- and right-head rotation under dynamic fluoroscopic imaging system (DFIS). Subject-specific 3-D models of the skull and C1-C3 were reconstructed from computed tomography (CT), registered frame-by-frame, and expressed with an extrinsic Y-X-Z Euler sequence. Intersegmental range of motion (ROM) were contrasted with published normative datasets. At C0-C1, flexion-extension (5.7 ± 3.6°) and lateral bending (4.6 ± 3.5°) were roughly 60% lower than controls (both P < 0.001), whereas axial rotation was similar (5.9 ± 5.3°, P = 0.67). Primary C1-C2 rotation fell to 38.1 ± 22.8° (controls 61.9 ± 13.5°, P < 0.001) accompanied by 3-fold increases in vertical and anteroposterior translations (both P < 0.001). C2-C3 showed compensatory gains: axial rotation 6.2 ± 8.5° vs. 3.1 ± 1.6° (P = 0.004) and larger translations on all axes (all P < 0.001). Motion was bilaterally symmetrical at C1-C2 and C2-C3, with only mild asymmetry in coupled lateral bending and mediolateral translation at C0-C1. AAD markedly restricts C1-C2 rotation, stiffens occipito-atlantal nodding, and provokes hypermobility at C2-C3. These segment-specific 6DOF benchmarks enhance biomechanical insight into AAD pathomechanics and may refine surgical stability assessment.