A case of fourth atlanto-axial facet joint: anatomical description.

Purpose: To report a rare fourth atlanto-axial joint.

Methods: A cadaveric specimen of a young male adult from occiput to C4 was dissected for anatomical study of craniocervical region. A true fourth atlanto-axial joint was confirmed. Its morphological characteristics were described.

Results: The fourth atlantoaxial joint is rarely seen. The possible embryogenesis is discussed. This case raises an anatomical possibility of a new variant in this region.

Conclusions: A unique case with an anatomically proven fourth atlantoaxial joint is reported. This anomaly can lead to misdiagnosis. CT scan coupled with MRI can facilitate accurate diagnosis.