Moment-rotation relationships of the ligamentous occipito-atlanto-axial complex

The relationships between applied pure moments at the occiput (C0) and the resulting rotations at the atlanto-occipital (C0–C1) and atlanto-axial (C1–C2) joints are quantified. In axial twist, with a moment of 0.3 Nm, a mean rotation of about 2.5° and 23.3° was observed at C0–C1 and C1–C2 units respectively. Both the atlas and axis contributed to produce lateral bending motion. The ratio between extension and flexion rotations at C0–C1 was 2.5 : 1. Lateral bending and axial rotations were strongly coupled to each other. The occipito-atlanto-axial complex exhibited a large ‘neutral zone’ compared to lower cervical spine segments. The likely clinical significance of these findings are discussed.