Introduction: The intricate biomechanical function of the alar ligaments in the craniocervical articular complex has received considerable scientific attention. While allowing the greatest range of axial rotation of the entire spine with 40° to each side, definitive restraint at the extremes of motion by the alar ligaments is of vital importance. Detailed knowledge of the function of these ligaments is essential for comprehending the factors leading to potentially devastating instability.

Methods: Bilateral alar ligaments including the bony entheses were removed from six adult cadavers aged 65–89 years within 48 hours of death. All specimens were judged to be free of abnormalities with the exception of non-specific degenerative changes. Dimensions of the alar ligaments were measured. Schematic multipla-nar reconstruction of axial atlanto-axial rotation was done in the transverse and frontal planes for the neutral position and for rotation to 30° and 40° in the neutral plane to assess schematic fibre elongation during axial rotation and to determine the change in the angle of insertion at the odontoid and condylar entheses. This was repeated with a 1mm descending translation of the occipital condyles at 30° and 3mm descending translation of the occipital condyles at 40° rotation.

Results: The average diameter of the odontoid process measured in the sagittal plane was 10.6 mm (SD 1.1). The longest fibre length was measured from the posterior border of the odontoid enthesis to the posterior border of the condylar enthesis with an average of 13.2 mm (SD 2.5) and the shortest between the lateral (anterior) border odontoid enthesis and the anterior condylar enthesis with an average of 8.2 mm (SD 2.2). Attachment areas of the enthesis revealed an average of 60 mm2 (SD 12.4) for the odontoid and 50,6 mm2 (SD12.6) for the condylar enthesis. Schematic fibre elongation reaches 27,1% for the longest fibres at 40° axial rotation. This is reducible to 7,8% elongation by 3mm caudal translation of the atlas.

Conclusions: This theoretical model confirms that the bi-convex shape of atlanto-axial joint allows for rotation when modelled with oblique alar ligaments. This provides baseline for further research with functional MRI which will be useful for rheumatoid and post traumatic spine.