Horizontal fracture of the anterior arch of the atlas associated with a congenital cleft of the anterior arch.

We report a rare combination of a horizontal fracture of the anterior arch of the atlas associated with a congenital cleft of the anterior arch. Sagittally reformatted computed tomography (CT) images demonstrated a fracture of the anterior arch of the atlas and a type II odontoid process fracture. There was also a smoothly marginated midline cleft of the anterior arch just above the fracture consistent with a congenital variant. 3D CT images were helpful in the evaluation of the fractures and the developmental variant. Magnetic resonance imaging showed a spinal cord injury. The patient underwent posterior C1-C2 fusion.