[Os Odontoideum - Post-traumatic Instability of C1-2.].

The case report presents a patient (male, 21 years old) who had a car accident and sustained an injury of upper cervical spine. X-ray examinations showed instability of C1-2 segment with a simultaneous os odontoideum. Subsequently CT and MRI examinations were performed to specify the anatomical relations. The patient had only a neurological lesion revealed by the examination by motor evoked potentials (MEP). The metric measurements on standard and functional radiographs showed a minimal sagittal dimension (Dmin) of 13 mm, the distance between os odontoideum and C1 arch (Datl) was 24 mm, the instability index after Penning was 20 mm, the distance between os odontoideum and proc. spinosus C2 in extension (Dext) after Shirasaki et al. was 30 mm, the sagittal rotational angle was 25 degrees and the instability index after Watanabe et al. 56.7 %. CT examination showed an exact shape of os odontoideum but was of minor significance for this anomaly. MRI did not prove simptoms of myelopathy. The final finding was os odontoideum of conical type after the Matsui et al. classification. Based on these examinations and the marked instability of C1-2 segment found a fusion of C1-2 after Magerl was performed in combination with Gallie technique dorsally. Key words: os odontoideum, instability, myelopathy.