Atlantoaxial dislocation, or atlanto-axial dislocation, refers to the loss of normal alignment between the first (atlanto-axial) and second (pivotal) segments of the cervical spine. It is a rare but serious disorder that can cause compression of the medulla oblongata and high cervical spinal cord, leading to tetraplegia and even death from respiratory failure in severe cases. Because of its high rate of disability and death, it must be diagnosed and managed promptly. The stability of the atlantoaxial joint depends on the following structures: the anterior arch of the atlantoaxial spine, the transverse ligament and the dentate process of the pivot; and the lateral block joint between the atlantoaxial spine. Disruption of the integrity of these structures, or their disuse for some reason, may result in atlantoaxial instability or dislocation. There are many causes of atlantoaxial instability or dislocation, such as old dentate fractures due to trauma, congenital deformities of the dentate, infection or inflammation of the transverse ligament or the lateral mass joint, or even tuberculosis or tumor invasion of the atlantoaxial joint. The most common clinical causes are traumatic causes and congenital deformities. In early pathological states, the atlantoaxial joint loses its normal alignment, but in some positions (e.g., neck supination) the atlantoaxial joint can be repositioned, and this condition should be called instability. If the atlantoaxial joint cannot be repositioned in any position with a longer history, the condition is called atlantoaxial dislocation.