[A malformation to know: os odontoideum. Report on two pediatric cases]. / Une malformation à connaître : l'os odontoideum. À propos de deux cas pédiatriques.

The os odontoideum, or mobile odontoid apophysis, is a malformation of the cervico-occipital hinge caused by missing unity in the ossification center of the dens on the body of the axis. This malformation induces atloaxial instability and exposes the subject to the risk of bulbar-medullary compression. The true incidence of this condition is difficult to determine because many cases are asymptomatic. The pathogenesis is discussed. Congenital and traumatic theories have been advanced. Clinically, the defect can be discovered incidentally or revealed by neck pain, a stiff neck, dizziness, or deficient syndrome. CT and MRI imaging can detect the defects, and allow one to study the impact on the contents of the spinal canal and detect lesions. We report two new os odontoideum observations in children aged 5 and 14 years who were referred for equilibrium disorders and tetraparesis, respectively.