Functional anatomy of the alar ligaments.

Nineteen upper cervical spine specimens were dissected to examine the macroscopic and functional anatomy of alar ligaments. They are on both sides, symmetrically placed, approximately 10-13 mm long and elliptical in cross-section 3 X 6 mm in diameter. The fiber orientation is dependent on the height of dens axis, mostly in the cranial caudal direction. In 12 specimens there was a ligamentous connection between dens and lateral mass of the atlas as a part of the alar ligament. In 2 specimens anterior atlanto-dental ligament was identified. The computerized tomographic (CT) images can clearly show alar ligaments in axial, coronal, and sagittal planes. The ligaments limit the axial rotation in the occipito-atlanto-axial complex (to the right by left alar and vice versa) as well as in side bending. The ligament is most stretched, and consequently most vulnerable, when the head is rotated and in addition flexed. This mechanism, common in whiplash injuries, could lead to irreversible overstretching or rupture of the ligaments especially as the ligaments consist of mainly collagen fibers.