The os odontoideum has been described as an upper cervical ossicle with corticalized margins and no bony continuity to the body of C2 (1, 2 and 3). The earliest reports of an os odontoideum has been attributed to either Giacomini (4) or Cunningham (5), both in 1886. Though many reports have described this condition, the exact incidence and prevalence of os odontoideum remains unclear. Sankar et al. (6), in a retrospective review of 519 abnormal pediatric radiographs, reported a 3.1% incidence of os odontoideum. Broader population-based statistics have yet to be identified.

The etiology of the os odontoideum has been a widely discussed topic with little consensus to date. The two most commonly reported etiologies are congenital and posttraumatic. Proponents of a congenital basis for os odontoideum highlight the coexistence of other congenital upper and subaxial cervical anomalies, as well as genetic conditions and familial associations (6, 7, 8, 9, 10, 11, 12 and 13). Additionally, the lack of a known history of injury is thought to refute a posttraumatic cause (6,14). To the contrary, other authors have proposed that the os odontoideum is an acquired lesion after cervical trauma (1,14, 15, 16, 17, 18 and 19). In a retrospective review of 35 patients, Fielding et al. (16) reported eleven patients having a known history of cervical trauma prior to the age of four and nine patients with prior normal radiographs. The exact etiology of os odontoideum may be multifactorial with genetic, developmental, and environmental factors all contributing.

Ultimately, the etiology of os odontoideum is of little consequence to the diagnosis and treatment decision making. Defining the optimal treatment of os odontoideum remains difficult. Only case reports and small case series exist, providing weak supporting evidence (level IV and level V data). Published guidelines, hampered by this limited evidence, have offered only practice options with no strong recommendations (20). Patients with severe symptoms such as disabling neck pain, myelopathy, or spinal cord injury are considered surgical candidates and have been optimally treated with a dorsal cervical fusion (6,14,16,19, 20, 21, 22 and 23). The asymptomatic patient presents a more difficult clinical scenario. The purpose of this chapter is to discuss the management of patients with an asymptomatic os odontoideum.