Congenital absence of the C1 vertebral arch.

Congenital absence of part of the anterior arch of the atlas, a rare disorder, was observed in two patients. The defect in the C1 cervical vertebrae, caused by congenital nonfusion of the arch of C1, may be confused with a cervical spinal fracture on plain radiographs of the cervical spine. The diagnosis is established by characteristic radiological changes, especially in the CT scan and bone scan. Because congenital anomalies of the cervical spine may predispose to instability of the cervical spine and can be confused with traumatic lesions, thorough evaluation is warranted. Treatment should be based on signs of instability, if present.