The Split Atlas Anomaly: A Comprehensive Review

Abstract

The "split atlas" is a rare congenital finding occurrence arising from the non-fusion of the anterior and posterior arches of the C1 vertebra. In most cases, this anomaly is asymptomatic and is discovered incidentally. However, it may be difficult to distinguish between fractures of the atlas, hence further imaging modalities are required to analyse the atypical appearance of the anterior and posterior arches of C1. Using standard search engines, the embryology, classification, clinical presentation, diagnosis, and treatment of the split atlas are reviewed. Clinicians who treat patients with issues near the cervical spine or interpret imaging of the craniocervical junction should be familiar with variations involving the first cervical vertebrae. Spine Scholar 1:37-44, 2017. INTRODUCTIONThe atlas is an atypical vertebra distinguished by its ring shape and lack of a vertebral body. Its main function is to support the weight of the skull and to allow a great degree of movement between the head and neck at the atlanto-occipital joint (Standring, 2016). It consists of two lateral masses connected anteriorly by a short thick anterior arch and posteriorly by a longer and usually thinner posterior arch (Standring, 2016;; Drake et al., 2015). Extending from the lateral masses are the transverse processes with their transverse foramina, which contain the vertebral arteries, venous plexus and sympathetic nerve fibers.A split or bipartite atlas is a congenital abnormality that results in a coexisting anterior and posterior schisis due to the failure of fusion of the anterior and posterior arches (Fig. 1).

Cervical Myelopathy Caused by a Split Atlas Anomaly: A Case Report

“…Split atlas (or bipartite atlas) is a rare congenital malformation consisting of the association of partial aplasia (or hypoplasia or defect or dehiscence) of the posterior and the anterior arches of C1 vertebrae [2]. It was considered by several authors as a “benign anatomical variation,” without clinical impact, and indeed almost all of the cases reported in the literature were discovered incidentally [3-6].…”

“…Malformations of the OCJ have embryological origin [2]. Several theories have been proposed to explain the split atlas, but the exact mechanism remains undetermined.…”

A rare triple skeletal bone variation, including a median cleft of the posterior arch of the atlas, a sternal foramina and a bifid xiphoid process

“…The prevalence of an SF rises to 4.5%, marking this bone defect as a rare one [7]. Congenital spondyloschisis of the C1 is usually found medially or paramedially in the posterior arch, whereas 97% of the CPAs tend to be located in the midline with only 3% occurring laterally [4,5]. Bony CPAs of the atlas present an incidence that varies between 0.69% and 4% in the literature [4].…”

“…Two ossification centers form the lateral masses, while the median center builds the anterior arch. The ossification proceeds dorsally from the lateral centers to the posterior arch, while the fusion of the ossified atlas parts occurs between 5 to 9 years of age [4,5].…”

“…A solitary median cleft of the posterior arch (CPA) of the Atlas may be explained as a failure of chondrogenesis and an ossification anomaly. Its distribution differs between populations, with higher incidence of congenital atlas posterior arch defects in Caucasian population [4,5].…”