The vertebral artery, in its course from the subclavian artery to the basilar artery, is vulnerable to damage or distortion from external factors such as bony, ligamentous or muscular structures (Mehalic & Farhat, 1974; Parkin et al. 1978; Schellhas et al. 1980; Braun et al. 1983; Dunne et al. 1987; Fast et al. 1987). In the atlas vertebra, the retroarticular canal and the lateral bridge are examples of bony outgrowth or exostosis which may cause external pressure on the vertebral artery as it passes from the foramen transversarium of the vertebra to the foramen magnum of the skull. If this pressure is severe enough, as may occur during the extreme rotatory movements carried out during therapeutic manipulation of the cervical spine, the vertebral artery may be compressed (Lamberty & Zivanovich, 1973), reducing its cross-sectional area, and compromising its blood flow (Taitz & Nathan, 1986). Vertebrobasilar ischaemia from compression of the vertebral arteries by osteophytes is an uncommon occurrence under normal circumstances (Warlow, 1996).

There are few studies of the lateral bridge of the atlas reported in the literature (MacAlister, 1869, 1893; Lamberty & Zivanovic, 1973; Saunders & Popovich, 1978; Taitz & Nathan, 1986). The lateral bridge was first described by MacAlister (1869, 1893) as a variety of the ‘posterior glenoid process’ (the retroarticular canal), which he termed the ‘gleno-transverse bony arch’. As its name implies, it is a lateral outgrowth of bone from the superior articular facet or lateral mass to the posterior root of the transverse process of the atlas (MacAlister, 1869, 1893; Lamberty & Zivanovic, 1973; Saunders & Popovich, 1978; Taitz & Nathan, 1986). The retroarticular canal is formed by an exostosis passing from the posterior surface of the lateral mass to the posterior margin of the vertebral artery groove of the atlas. Thus the lateral bridge forms another arch, secondary to the retroarticular canal, through which the vertebral artery must pass (MacAlister, 1869).