Congenital absence of the posterior arch of the atlas associated with a fracture of the anterior arch

Abstract

Structural defects of the posterior arch of the atlas are rare, and range from clefts of variable location and size to more extensive defects such as complete agenesis. These abnormalities are usually incidental radiological findings. We present a case of a fracture of the anterior arch of the atlas associated with a congenital abnormality of the posterior arch.

The Split Atlas Anomaly: A Comprehensive Review

“…This complete bipartition of the atlas has been reported in only 0.1% of the population (Hummel et al, 2013;; Tachibana et al, 2010). Similar but less striking variations include an isolated posterior arch cleft reported in 1.5% to 5% of the population, and a less common isolated anterior arch cleft that has been reported in 0.1% to 0.7% of the population (Hummel et al, 2013;; Tachibana et al, 2010;; Park et al, 2011;; Corominas et al, 2010) (Fig. 2).…”

“…This classification was widely used in numerous case reports to identify which type of posterior arch defect that was present in the patients (Fig 5). Type A clefts were found to be the most common and accounted for 4% of the population and represented 97% of all posterior arch defects, whereas types B-E were reported in 0.69% of the population (Corominas et al, 2010;; Gangopadhyay et al, 2003). Types C and D which both involve an isolated posterior tubercle is of clinical importance as these anomalies can often cause acute neurological deficits such as transient quadriparesis, paraparesis, chronic neck pain, headaches and L'Hermitte's sign (Geipel,1955).…”

“…During the 7 th week of gestation, an ossification center from each lateral mass begins to extend and grow dorsally forming the posterior arch of the atlas (Corominas et al, 2010;; Klimo et al, 2003;; Schulze et al, 1980). By birth, the two hemiarches of the posterior arch are separated by a narrow segment of several millimeters of cartilage, which is expected to completely fuse and form a union by an average age of three to five years (Corominas et al, 2010;; Childers et al, 1971;; Dorne et al, 1986). If this does not occur successfully, a posterior cleft would be the result.…”

“…These include median clefts of the posterior arch and varying degrees of aplasia of the posterior arch (Corominas et al, 2010;; Klimo et al, 2003;; Torriani et al, 2002). These findings can be classified into five subtypes based on the varying presentations of the possible defects within the posterior arch, and is listed below (Corominas et al, 2010;; Currarino et al, 1994;; Geipel,1955). This classification was widely used in numerous case reports to identify which type of posterior arch defect that was present in the patients (Fig 5).…”

“…(Illustration by Katie Yost ©2017, provided under CC-BY). It in unclear whether the bipartite atlas anomaly is familial or not although there have been reports documenting the existence of a split atlas between a mother and son, and between a mother and daughter (Corominas et al, 2010;; Currarino et al, 1994;; Motateanu et al, 1991;; Caminos et al, 2008). Anterior and posterior clefts of the atlas have varying radiological appearances.…”