Atlantoaxial dislocation.

Atlanto-axial dislocations (AADs) may be classified into four varieties depending upon the direction and plane of the dislocation i.e. anteroposterior, rotatory, central, and mixed dislocations. However, from the surgical point of view these are divided into two categories i.e. reducible (RAADs) and irreducible (IAADs). Posterior fusion is the treatment of choice for RAAD. Transarticular screw fixation with sub-laminar wiring is the most stable& method of posterior fusion. Often, IAAD is due to inadequate extension in dynamic X-ray study which may also be due to spasm of muscles. If the anatomy at the occipito-atlanto-axial region {O-C1-C2; O: occiput, C1: atlas, C2: axis} is normal on X-ray, the dislocation should be reducible. In case congenital anomalies at O-C1-C2 and IAAD are seen on flexion/extension studies of the cervical spine, the C1-C2 joints should be seen in computerized tomography scan (CT). If the C1-C2 joint facet surfaces are normal, the AAD should be reducible by cervical traction or during surgery by mobilizing the joints. The entity termed "dolichoodontoid" does not exist. It is invariably C2-C3 (C3- third cervical vertebra) fusion which gives an appearance of dolichoodontoid on plain X-ray or on mid-saggital section of magnetic resonance imaging (MRI) or CT scan. The central dislocation and axial invagination should not be confused with basilar invagination. Transoral odontoidectomy alone is never sufficient in cases of congenital IAAD, adequate generous three-dimensional decompression while protecting the underlying neural structures should be achieved. Chronic post-traumatic IAAD are usually Type II odontoid fractures which get malunited or nonunited with pseudoarthrosis in dislocated position. All these dislocations can be reduced by transoral removal of the offending bone, callous and fibrous tissue.