"Suspended Posterior Tubercle of Atlas" with Atlantoaxial Dislocation.

Introduction: Congenital anomaly of the C1 posterior arch is a well-known entity and is often associated with atlantoaxial dislocation. However, a well-formed C1 posterior tubercle with absence of the remaining posterior arch is rare. Such unusual anomalies pose a surgical challenge as trying to delineate the arch early in the course of surgery could be potentially dangerous. We discuss here a similar case of C1 posterior arch defect with atlantoaxial dislocation and its management.

Case Report: A 17-year-old female presented with progressive spastic quadriparesis and neck pain. Evaluation revealed atlantoaxial dislocation with the presence of a well-formed posterior C1 tubercle in the absence of the rest of the posterior arch. C1-C2 joints were opened, and lateral masses were fused in reduced position. Intraoperatively, a posterior C1 tubercle was found suspended from the lateral masses by fibrous bands. The surgical nuances have been discussed.

Conclusion: Presence of posterior tubercle alone with aplasia of the posterior arch results from a persistent posterior ossification center with nonextension of lateral ossification centers. In the presence of the C1 posterior fibrous arch, the joint spaces must be exposed first before attempting to delineate the posterior arch. This will prevent inadvertent injury to the vertebral artery and dura.