ONGENITAL anomalies of the posterior arch of C-1 are very rare. They may appear as clefts in the arch, and their location and size vary. 2,15,21 According to the classification of Currarino et al. 2 there are five types of anomalies: Type A defects are defined as failure of the two hemiarches to fuse at the posterior midline (that is, failure of the two lateral ossification centers to unite posteriorly in the midline); Type B, unilateral clefts, ranging from a small defect to a complete absence of one hemiarch; Type C, bi- lateral clefts of the lateral aspects associated with preserva- tion of the most dorsal portion of the arch; Type D, the complete absence of the posterior arch with a persistent posterior tubercle; and Type E, the complete absence of the posterior arch and posterior tubercle. 2 Few cadaveric, imag- ing, and case studies have been reported on the variations of such C-1 arch defects, and their clinical significance for neurosurgical practice is unclear. We therefore investigated the incidence of these congenital defects in the cervical spine on CT studies, in fresh human cadaveric cervical spines, and in dried C-1 specimens. Anatomical and imag- ing features of these defects and their clinical significance for neurosurgical practice were the focus of this study. Clinical Material and Methods To evaluate the annual incidence of congenital C-1 arch defects, we reviewed the institutional database and retro- spectively evaluated consecutive CT scans obtained in 1104 patients (664 males and 440 females; mean age 41 years, range 10–106 years) who presented to our institution between January 2006 and December 2006. The patients presented with various medical problems, including head and neck trauma, cervical radiculopathy, posterior inferior artery aneurysms, and head and neck pain. When a con- genital defect of the C-1 posterior arch was identified on a CT scan, the patient’s medical record was also reviewed to determine his or her neurological status. Dried C-1 speci- mens from 166 adults that had previously been used in another study were evaluated, 18 but the sex and age of the specimens were unavailable. Fresh human cadaveric cervi- cal spines from 84 adults (52 males and 32 females; mean J. Neurosurg: Spine / Volume 7 / October, 2007 J Neurosurg Spine 7:399–402, 2007 The frequency and clinical significance of congenital defects of the posterior and anterior arch of the atlas MEHMET SENOGLU, M.D., 1 SAM SAFAVI-ABBASI, M.D., PH.D., 1 NICHOLAS THEODORE, M.D., 1 NICHOLAS C. BAMBAKIDIS, M.D., 1 NEIL R. CRAWFORD,PH.D., 2 AND VOLKER K. H. SONNTAG, M.D 1 1 Division of Neurological Surgery and 2 Spinal Biomechanics Laboratory, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona Object. In this study the authors investigated the anatomical, clinical, and imaging features as well as incidence of congenital defects of the C-1 arch. Methods. The records of 1104 patients who presented with various medical problems during the time between January 2006 and December 2006 were reviewed retrospectively. The craniocervical computed tomography (CT) scans obtained in these patients were evaluated to define the incidence of congenital defects of the posterior arch of C-1. In addition, 166 dried C-1 specimens and 84 fresh human cadaveric cervical spine segments were evaluated for anomalies of the C-1 arch. Results. Altogether, 40 anomalies (2.95%) were found in 1354 evaluated cases. Of the 1104 patients in whom CT scans were acquired, 37 (3.35%) had congenital defects of the posterior arch of the atlas. The incidence of each anomaly was as follows: Type A, 29 (2.6%); Type B, six (0.54%); and Type E, two (0.18%). There were no Type C or D defects. One patient (0.09%) had an anterior arch cleft. None of the reviewed patients had neurological deficits or required surgical intervention for their anomalies. Three cases of Type A posterior arch anomalies were present in the cadaveric specimens. Conclusions. Most congenital anomalies of the atlantal arch are found incidentally in asymptomatic patients. Congenital defects of the posterior arch are more common than defects of the anterior arch. (DOI: 10.3171/SPI-07/10/399) KEY WORDS • agenesis • arch • atlas • congenital defect C 399 Abbreviation used in this paper: CT = computed tomography. 5-16909 8/29/07 11:30 AM Page 399