Bipartite Atlas or Jefferson Fracture? A Case Series and Literature Review.

STUDY
DESIGN: Case series and literature review.
OBJECTIVE: We present a new case of a C1 anterior arch fracture after suboccipital craniotomy and C1 laminectomy to add to 3 previously reported cases, thus increasing awareness of this postoperative complication. We also present 2 adult cases of presumably congenital bipartite atlas. The radiographical similarity of these cases raises the possibility of a traumatic cause for some instances of bipartite atlas rather than a congenital defect. SUMMARY OF BACKGROUND DATA: The bipartite atlas is a rare entity that has previously been presumed to represent a congenital variant. Its imaging appearance may be mistaken for a Jefferson fracture in the setting of trauma.
METHODS: The first case is a 4-year-old female who underwent suboccipital craniotomy and C1 laminectomy and was monitored over the course of the subsequent year. Two cases of bipartite atlas in young males without a history of recent trauma were observed over a similar time period and are presented for comparison.
RESULTS: In the first case, imaging demonstrated the development of an anterior arch fracture after the C1 laminectomy surgery. Correlation of the pediatric and adult cases revealed that it may be difficult to differentiate a subacute or chronic anterior arch fracture from a congenital cleft, leading to the hypothesis that the etiology of the bipartite atlas may be traumatic rather than congenital in some cases.Review of the literature revealed that isolated anterior arch defects are extremely rare and they are more commonly seen in conjunction with posterior arch defects, which further supports our hypothesis from a biomechanical perspective.
CONCLUSION: C1 laminectomy may increase the risk of an anterior C1 arch fracture. A traumatic mechanism is proposed as a cause of the bipartite atlas. In addition, the bipartite atlas may have a higher prevalence than previously reported. LEVEL OF EVIDENCE: 4.