[Post-traumatic os odontoideum]

This is a report about a 20-month-old child with acquired os odontoideum caused by a fall. Diagnosis of atlanto-axial instability was eventually made 29 months later after repeated trauma and transitory tetraparesis. Dorsal C1/2 fusion with an autogenic rib graft and plaster fixation for 8 weeks failed; an autogenic iliac bone graft plus external fixation for 12 weeks finally resulted in solid fusion.