Traumatic transverse atlantal ligamentous injury with posterior atlantoaxial instability in os odontoideum: Magnetic resonance features

The os odontoideum is an anomaly in which the odontoid is divided by a transverse gap, leaving the apical segment without support from its base. Because of its detachment from the axis body, os odontoideum can lead to hypermobility and instability. The following case report demonstrates rare magnetic resonance documentation of transverse atlantoaxial ligamentous injury with posterior atlantoaxial instability in a patient with os odontoideum.