Os Odontoideum

Definition

• dens incompletely fused to body of C2
• smaller than the normal dens but size may vary
• it is located usually in position of the normal odontoid tip (orthotopic) or near the foramen magnum (dystopic)
• often fixed to the anterior ring of atlas and the two move as a unit
• instability common

Incidence

• rare - not known

Aetiology

• unknown
• failure of fusion of the base of odontoid
• results from traumatic process

Associated syndromes

• Morquio sy.
• Multiple epiphyseal dysplasia

Clinical

• often asymptomatic & discovered incidentally
• local neck symptoms (neck pain, torticollis, or headache)
• transitory episodes of paresis following trauma
• myelopathy (cord compression) or cervical & brain stem ischemia d/t vertebral artery compression (seizures, syncope, vertigo, visual disturbances)
• sudden death

X-rays

• os odontoideum may be overlooked without tomograms
• radiolucent oval or round ossicle with a smooth, dense border
• moves with atlas in flexion & extension

Specific characteristics

• gap between os & hypoplastic dens is wide & lies well above the level of superior articular facets of axis
• not normal shape or size of dens - half size, rounded or oval with a smooth uniform cortex
• if os is in area of foramen magnum - little dg. problem

Diff Dx

• odontoid hypoplasia
• non union

Treatment

• controversial

Indications for C1-C2 fusion

• ADI > 10 mm or SAC < 13 mm
• neurological involvement (even if transient)
• progressive instability
• isolated local symptoms (pain,torticollis) or brain stem symptoms (diplopia, nystagmus vertigo, dysarthria, dysphagia) are Not indications

C1-C2 fusion - Gallie type

• followed by Minerva cast for 6-12/52