Introduction: The C0-C2 junction is a highly mobile region of the spine. The occipito-atlantal (OA) joint primarily permits flexion-extension angular motion and the atlanto-axial joint (AA) axial rotation angular motion. A cadaveric study was used to identify range of motion patterns during primary bending, the secondary contribution of off-axis rotations during primary bending, and the translational component of the OA and AA joint complexes.
Methods: Intervertebral motions were tested in seven cadavers on a spine simulator. Primary bending moments of 2.5 N-m were applied to create flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Primary rotations, secondary rotations (coupled), and translations(anterior-posterior (AP), cranial-caudal (CC), and medial-lateral (ML) planes) were recorded.
Results: During applied FE, the OA joint accounted for 16°, 4°, and 3°. The AA joint accounted for 10°, 3°, and 14° (during primary FE in FE, LB, and AR, respectively). AP and ML translations were the principal translations, with CC translations mainly at AA joint.
During applied LB, the OA joint accounted for 5°, 5°, and 4°. The AA joint accounted for 4°, 4°, and 18° (during primary LB in FE, LB, and AR, respectively). All translation planes played a role during LB.
During applied AR, the OA joint accounted for 12°, 6°, and 9°. The AA joint accounted for 12°, 12°, and 69° (during primary AR in FE, LB, and AR, respectively). Overall, the AA joint contributes to the highest amount of translation with AP being the greatest and ML being the least.
Conclusions: Occipito-atlantal and atlanto-axial movements are not just pure rotations, but highly complex coupled motions that display translations. Secondary motions may exceed primary motions at both joints. Although it was not noted in previous studies, this study found that the dominant motion patterns of each joint combine as coupled motion to perform lateral bending at the craniocervical junction.
Patient Care: This research will help to better understand the amount of both primary and secondary rotation along with normal range of translations in the craniocervical junction. Due to the unique configuration of the anatomy, understanding the amount of motion and translation may assist in distinguishing trauma, degeneration, or deformities that occur.
Learning Objectives: The goal of this biomechanical study is to identify normal range of motion patterns during primary bending, the secondary contribution of off-axis rotations during primary bending, and the translational component of the occipito-atlantal joint and the atlanto-axial joint.