Types
- Aplasia
- rare
- Hypoplasia
- eg in achondroplasia, spondyloepiphyseal dysplasia
- Os Odontoideum
- may be congenital or traumatic
Clinically
- instability of atlanto- axial
- joint pain
- torticollis
- neurologic compromise
- vertebral artery compression seizures, mental deterioration, syncope, vestibular disturbances
Investigations
- flexion – extension films
- show hypermobility- atlanto-dens interval more than 5mm in children
- MRI
- shows cord damage
- CT
- demonstrate the degree of displacement
Treatment
- C1-2 fusion
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.