Atlantoaxial instability. Definition

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The first and second cervical vertebrae are called the atlas and the axis. A medical condition characterized by an excess in movement between these vertebrae is called atlantoaxial instability.
Atlantoaxial instability is extremely rare in people who do not have predisposing factors, but 50 percent of patients with rheumatoid arthritis have this medical condition.

The spine is made up of 33 individual bones called vertebrae. The vertebrae are numbered and are divided into four regions. The cervical region of the spine is made up of seven vertebrae, which are designated as C1 through C7.

The cervical region of the spine is very mobile. Mobility of the neck is ensured by the facet joints, the intervertebral discs, and the spinal ligaments.

The first two cervical vertebrae, the atlas and the axis, are unlike any of the other spinal bones. The unique joints between these vertebrae, combined with the structure of the cervical spine allow for rotation and side-to-side movement of the head.