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Abstract. In a series of 400 patients with fractures and dislocations of the cervical spine, 5% involved the atlas. There are nearly equal numbers of fractures of the posterior arch, bursting fractures of Jefferson, and horizontal fractures of the anterior arch. Atlas fractures are rarely associated with neurologic deficit, but they may be the cause of severe pain. Hypotheses concerning the mechanism of injury in each type of fracture are discussed. The characteristic roentgenographic findings and the differential diagnoses are reviewed.Key words: Atlas, fractures -Cervical spine, injuries.Fractures of the atlas vertebra are rare. In a series of 400 patients with fractures and dislocations of the cervical spine collected over the past 8 years at the Duke University Medical Center (DUMC), we find that atlas fractures account for 5% of the total.Our purpose is to review the pertinent anatomy, to classify the types of fractures, to discuss the mechanisms of injury, and to review the differential diagnosis for each type of fracture. AnatomyThe first cervical vertebra, or atlas, differs from all other vertebra in that it lacks a body and a spinous process (Figs. 1, 2). To simplify its description, we divide the ring-shaped atlas into three parts : the anterior arch (20%), the lateral masses (40%), and the posterior arch (40%). The ventral surface of the anterior arch is convex and possesses a midline anterior tubercle to which the anterior longitudinal ligament and the tendons of the superior oblique portion of the longus colli muscles attach. The posterior surface of the anterior arch is slightly concave. In its midportion there is a smooth oval or circular facet articulation with the dens of the axis.The curved posterior arch of the atlas lacks a spinous process. It has a small, roughened posterior tubercle which gives attachment to the nuchal ligament and the rectus capitus posterior minor muscle. On each side, the posterior arch is grooved on its cranial surface by the sulcus of the vertebral artery. Structurally, this is the weakest portion of the atlas ring. Crossing the vertebral artery sulcus are the vertebral artery and, below it, the first cervical spinal nerve.The lateral masses are quite large since they must bear the weight of the skull. Each lateral mass carries a superior and an inferior facet. The superior articular facets are large, concave, and reniform. Each facet faces superiorly, medially, and posteriorly to accomodate the occipital condyle above. The inferior articular facets are smaller than the superior facets. They are slightly concave, circular, and obliquely oriented so that they face downwards, medially, and backward. They articulate with the superior articular surface of the axis. A small tubercle for the attachment of the strong transverse ligament of the atlas projects medially from each lateral mass. This transverse ligament divides the vertebral foramen of the atlas into two uneven compartments: a small anterior compartment which accommodates the dens and a larger posterior compartment...