Atlantoaxial subluxation in two dogs with cervical block vertebrae.

Atlantoaxial (AA) subluxation is an uncommon disorder that can cause various degrees of neurological deficits in dogs. Block vertebra is a congenital deformation involving the fusion of two or more vertebrae. This report describes two dogs with cervical block vertebrae from C(2) to C(5) and C(2) to C(4), respectively. We hypothesize that the fused cervical vertebrae created a "fulcrum effect" at the AA joint and predisposed these dogs to traumatic AA subluxation.