Brady Huang, MD and Per-Lennart Westesson, MD, PhD, DDS
Clinical
Presentation:
A 14-year-old male with a clinical history of fall with trauma to the cervical spine. The patient is said to have known C1 fracture. CT scan of the cervical spine is requested for evaluation of this fracture.
Radiological Findings:
Figure 1. Scout image from the cervical spine CT demonstrates absence of the posterior arch of the atlas (arrow), with an apparent persistent posterior tubercle.
Figure 2. An axial reformatted CT image in the anatomical plane of the atlas confirms absence of the posterior arch of the atlas with a persistent posterior tubercle.
Figure 3A-C. 3D volume renderings of the atlas demonstrate the finding viewed from its superior, inferior, and lateral aspects.
Figure 4. A 3D volume rendering of the cervical spine demonstrates its relationship with the remainder of the cervical spine.
Figure 5. A schematic of the ossification centers of the atlas demonstrates open synchondroses (arrows) that can be normally found in children. These should not be mistaken for fractures.
Diagnosis: Congenital absence of the posterior arch of the atlas, with persistent posterior tubercle
Discussion: Identification of congenital abnormalities is important and should not be mistaken for fractures of the atlas, which comprise approximately 6% of cervical vertebral column injuries [1]. The atlas, or C1, is formed by three primary ossification centers, namely the anterior arch and the two neural arches. The two neural arches surround the anterior arch and eventually fuse to form the posterior arch. The neural arches normally fuse posteriorly by 3 years of age. The anterior arch subsequently fuses by 7 years of age. Occasionally, the anterior arch ossification center never develops and the neural arches attempt to fuse anteriorly [2].
In general, congenital abnormalities of the arch are classified into aplasias and clefts. Clefts are rare, but are observed more frequently than aplasias. In 1613, Geiple performed 2,749 post mortem exams of the atlas vertebra. Posterior clefts were found in 4% atlas specimens versus 0.1% anterior arch clefts [3]. Of the posterior clefts, 97% were midline and 3% were through the sulcus of the vertebral artery. No aplasias were found in that series. Posterior mid-line clefts are alternatively recognized as rachischisis or spine bifida occulta, and are due to defective or absent development of the cartilaginous preformation of the arch, rather than a disturbance of ossification [4]. Anterior mid-line clefts are rarer, and are due to failure of the ossification in the anterior arch to develop. They are usually not found in isolation and are more often associated with posterior mid-line clefts. Aplasias are even rarer, and are limited to case reports in the literature [5]. Congenital aplasias of the posterior arch of the atlas were classified by Von Toklas and Gehle in 1972 [6]. These include: A) Total aplasia B) Aplasia with persistent posterior tubercle, C) Aplasia with paramedian unilateral posterior arch remnant, D) Aplasia with paramedian bilateral posterior arch remnant and rachischisis, E) Hemiplasia, and F) Unilateral posterior arch partial aplasia. More recently, Curriano et al, proposed a generalized classification to include both aplasias and clefts [7]. According to the Curriano classification, there are six types of congenital anomalies of the posterior arch of the atlas: A) Failure of posterior midline fusion of the two hemiarches, B) Unilateral defect, C) Bilateral defects, D) Absence of the posterior arch, with persistent posterior tubercle, and E) Absence of the entire arch, including the tubercle.
The clinical presentation of reported cases of congenital anomalies of the posterior arch of the atlas is variable. Curriano et al, reported that approximately one-third of patients were asymptomatic [7]. Some patient present with transient or chronic neck pain. It has been hypothesized that cases of aplasia with an isolated posterior arch remnant may predispose some patients to neurologic morbidity. Sharma et al describe inward mobility of the posterior fragment during extension of the cervical spine on lateral radiographs [8]. Autopsy studies of cases such as these demonstrate that the bony gap in the posterior arch is bridged by loose connective tissue rather than cartilage, as mentioned above. Thus the posterior bony fragment, may be mobile with respect to the anterior arch, moving anteriorly during extension with narrowing of the spinal canal during extension. Interestingly, no cases to date have been reported directly demonstrating cord compression with cervical extension on MR imaging. Congenital anomalies of the atlas can mimic Jefferson fractures, and it is clinically important to differentiate the two. On AP open-mouth views the atlanto-axial offset is assessed by examining the amount of distance between the lateral aspects of the lateral masses of the atlas and axis. The typical radiographic findings for a Jefferson fracture is bilateral atlanto-axial lateral offset of 3-9 mm with offset greater 7 mm likely indicating rupture of the transverse ligament of the atlas [1]. With anterior and posterior atlas defects, there may be associated atlanto-axial offset by 1-2 mm, but this does not indicate abnormal spread of the atlas and is not clinically significant [1, 4].
References:
Gehweiler JA Jr. Daffner RH. Roberts L Jr. Malformations of the atlas vertebra simulating the Jefferson fracture. AJR. American Journal of Roentgenology. 1983;140(6):1083-1086. [Medline]
Lustrin ES. Karakas SP. Ortiz AO. Cinnamon J. Castillo M. Vaheesan K. Brown JH. Diamond AS. Black K. Singh S. Pediatric cervical spine: normal anatomy, variants, and trauma. Radiographics. 2004;23(3):539-60. [Medline]
Geipel P. Zur Kenntnis der spanltbildungen des Atlas und Epistopheus. Teil IV. [Studies on the fissure formation of the atlas and epistropheus. IV]. Zentralbl Allg Pathol 1955;94:19-84. [Medline]
Prempeh RC. Gibson JC. Bhattacharya JJ. Mid-line clefts of the atlas: a diagnostic dilemma. Spinal Cord. 2003;40(2):92-93. [Medline]
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Von Torklus D, Gehl W. The Upper Cervical Spine. New York; Grune & Stratton, 1972.
Curriano, G, Rollings N, Diehl JT. Congenital defects of the posterior arch of the atlas: a report of seven cases including an affected mother and son. AJNR Am J Neuroradio 1994;15:249-254. [Medline]
Sharma A. Gaikwad SB. Deol PS. Mishra NK. Kale SS. Partial aplasia of the posterior arch of the atlas with an isolated posterior arch remnant: findings in three cases. AJNR Am J Neuroradio. 2000;21(6):1167-1171. [Medline]
