The incidence and dimensions of the retroarticular canal of the atlas vertebra.

Abstract

The retroarticular canal has been implicated in compression of the vertebral artery, where it passes over the posterior arch of the atlas vertebra, during extreme rotational movements of the head and neck. The incidence of this retroarticular canal is not known in the South African population. The aim of the present study was to record the incidence and the dimensions of the retroarticular canal in South African white and black adults, aged 20-80 years. In a total of 1,354 atlas vertebrae, 9.8% of sides (264 left and/or right sides) were classified as having complete retroarticular canals, of which 11.7% (31 sides) were right-only, 24.6% (65 sides) left-only and 31.8% (84 left plus right sides) bilateral canals. This incidence did not increase with age and was lower in whites than blacks, with white males having the lowest and white and black females alike having the highest incidence of the canal. Measurements of the retroarticular canal showed that there was no difference between left and right sides. However, the superoinferior diameter was significantly less than the anteroposterior diameter, in all but the right canals in the white female subgroup. This difference in the dimensions of the retroarticular canal will decrease the cross-sectional area of the space available for the vertebral artery passing through it and may compromise blood flow in the vessel.