In the anteroposterior view of the atlanto-axial joint, there is very frequently seen a notch-like radiolucent defect in the medial portion of the articular mass of the atlas. While in some individuals this may be entirely absent, in others it may be so prominent that it can be mistaken for a fracture or even a destructive process (Fig. 1). However, the frequency with which this shadow is encountered and the fact that it is almost invariably bilateral make it apparent that it is produced by normal anatomic structures. Since no previous description of this shadow could be found, this study was undertaken to identify the structures by which it is produced (1, 2). Method Lead strips were placed on various portions of the superior articular facets of the lateral masses of the atlas (Fig. 2): (A) the anterior edge; (B) the posterior edge and posteromedial surface; (C) the nadir of the concavity of the facet and medial aspect of the lateral mass; (D) the medial tubercle of the lateral mass. Radiographs were the...