Incidence and Variants of Posterior Arch Defects of the Atlas Vertebra

In order to describe the incidence and existing variants of congenital anomalies of the atlas vertebrae in a Caucasian population, we examined 1069 CT scans of the upper cervical spine. We found 41 cases with altered atlas vertebrae, representing 3.8% of all analyzed patients. With 83% of all found anomalies, the predominant type is characterized by a small dorsal cleft (3.2% of all patients). Rare varieties feature unilateral or bilateral dorsal arch defects, combined anterior and posterior clefts (0.2% of all patients) or total erratic atlas vertebra malformation (0.1% of all patients). Atlas arch defects are found nearly 4% at the time. Most anomalies affect the posterior arch, whereas the anterior arch or both are rarely affected. Totally irregular C1 vertebrae are extremely infrequent. 1. Introduction Atlas arch anomalies are found mostly coincidentally. The predominant defect involves the posterior arch [1–4]. Currarino et al. proposed 5 types of atlas posterior arch defects referring to Torklus [2, 5]. The anomalies vary from unifocal clefts to total absence of the posterior arch and posterior tubercle. Less common are anterior atlas arch defects and the combination of both [3, 4]. Accompanying anomalies include an enlarged anterior arch, cephalad elongation of the spinous process of the axis, and a dense fibrous membrane forming a posterior atlanto-occipital membrane [2]. These altered anatomical findings exhibit natural adaption in order to maintain stability and function. In cervical spine trauma, profound knowledge of congenital atlas defects is crucial. Malformations, where C1/C2 junction might be compromised, have to be distinguished from fractures. We therefore conducted this study to further describe defects of the atlas vertebra and to estimate their incidence. The found anomalies were examined and grouped. 2. Materials and Methods The institutional review board approved this retrospective study waiving the need for patient consent. We retrospectively reviewed 1069 consecutive cervical CT scans from our trauma database. Indication for the CT scans was adequate trauma with the risk of a cervical spine injury and/or the presence of clinical symptoms. Cases with atlas fractures, severe degeneration, and previous operations were excluded. Multiplanar CT reconstructions (axial and sagittal) in 1.5？mm slices were evaluated (Siemens Somatom Definition Dual Source). For each subject, anatomical alteration of the atlas vertebra of any kind was analyzed. The CT scans were examined by 2 independent reviewers. The atlas anomalies were studied, and