Twelve specimens of the upper cervical spine were functionally examined by using radiography, cineradiography and computerized tomographic (CT) scan. The range of rotation was measured from CT images after maximal rotations to both sides. The left alar ligament was then cut and the examination repeated. The alar and transverse ligaments could be differentiated on CT images in axial, sagittal, and coronal views. Rotation at occiput-atlas was 4.35deg; to the right and 5.9 deg; to the left and at atlas - axis it was 31.4deg; to the right and 33deg; to the left. After one-sided lesion of the alar ligament, there was an overall increase of 10.8 deg; or 30percnt; of original rotation to the opposite side, divided about equally between the occiput - atlas and the atlas - axis. It is concluded that a lesion (irreversible overstretching or rupture of alar ligaments) can result in rotatory hypermobility or instability of the upper cervical spine.
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