Isolated fracture of the medial portion of the lateral mass of the atlas: a previously undescribed entity.

Four cases of fracture of the medial aspect of the lateral mass of the atlas are presented. Unlike most cases of atlantal fracture, definite history of a blow to the vertex could not be established. While the exact mechanism of injury is unknown, it is postulated that the bone fragment is produced by a combination of stretching the ligamentum transversarium and pressure on the lateral mass due to contraction of the neck muscles. All four patients were neurologically intact and complained of pain in the distribution of the greater occipital nerve. It is suggested that this fracture might be identified more often in patients with similar histories and complaints by use of laminagraphy, since part or all of the atlas is frequently obscured on routine open mouth roentgenograms of the upper cervical spine.