Posterior bilateral supernumerary atlantoaxial facets: true or false joint?

STUDY
DESIGN: Case report.
OBJECTIVE: To describe a symptomatic patient with bilateral posterior accessory C1-C2 joints and their differences from the true synovial joints. SUMMARY OF BACKGROUND DATA: Accessory joints between C1-C2 are rarely described. Origin and functional importance of such joints is unknown. Moreover only gross anatomic features of such joints were discussed in the past. The emphasis here is to review the morphology of such joints and to differentiate it from normal synovial joints.
METHODS: A 17-year-old male presented with progressive spastic ataxia precipitated by episodic transient quadriparesis secondary to trivial trauma. Radiology revealed bilateral posterior C1-C2 accessory joints compressing the cervicomedullary junction. Both C1-C2 posterior arches were removed, excising the accessory joints. Fusion was achieved using C1-C2 lateral mass screws and is doing well at follow-up.
RESULTS: The surfaces of these joints were smooth. Histopatholgy showed opposing osseous surfaces with articular cartilage but no synovium or articular capsule.
CONCLUSION: Accessory C1-C2 joints are possibly a result of genetic aberration giving rise to abnormality in segmentation of C1 sclerotome. These joints are dysmorphic and partially formed as evidenced by the absence of synovial membrane and capsule. The function of such joints remains questionable.