The author reported an extremely rare case of intraspinal synovial cyst at C1-C2 junction, located posteriorly to the
odontoid process, compressing the upper cervical spinal cord. The patient had a history of spinal trauma 40 years ago without prior definitive treatment, presenting with two months of left leg and arm numbness with neck pain. Imaging studies including plain radiographs, computed tomography (CT), and magnetic resonance imaging (MRI) of the cervical spine showed a large synovial cyst associated with atlantoaxial instability from non-union fracture of the odontoid process. The atlantoaxial fusion was performed without a direct excision of the synovial cyst, the patient showed significant improvement of neck pain and numbness of all extremities. In addition to the fracture stabilization, the follow-up post operative MRI revealed spontaneous regression of the intraspinal synovial cyst after atlantoaxial fusion alone.

Keywords: Synovial cyst, Atlantoaxial instability, Odontoid fracture