Background:Many cases of ossiculum terminale in Down’s syndrome have been reported. We describe an unusual case of sudden atlantoaxial instability in a previously healthy woman without a history of trauma due to a dystopic ossiculum of the dens that could be classified as an ossiculum terminale persistens or as os odontoideum according to different classification proposals.Case Report: A 36-year-old woman was admitted with neck pain and Lhermitte’s sign. Neurological examination revealed spasticity of both upper and lower limbs. Radiological evaluation of the cervical spine showed an ossiculum close to the anterior arch of the atlas. Flexion and extension films revealed an atlantoaxial instability. Additional findings using computerized tomography and magnetic resonance Imaging provided significant information about the soft tissue structures. The patient underwent a C1-C2 posterior cervical wiring and interspinous fusion with the Sonntag technique. Postoperatively, the patient was neurologically intact and without any symptoms.Conclusions: In the present report we address the notion that, in cases of atlantoaxial instability and myelopathy due to a dens anomaly, surgical intervention is required and that there is no need to distinguish which type of anomaly really exists according to different classification schemes.

Keywords: Atlanto-Axial Joint - pathology, Spinal Cord Diseases - diagnosis, Spinal Cord Diseases - radiography, Atlanto-Axial Joint - pathology, Magnetic Resonance Imaging, Spinal Cord Diseases - surgery, Tomography, X-Ray Computed