The cervical spine provides support for the head and enables a wide range of movements. This complex region consists of seven stacked bones called vertebrae. Among these, the upper two vertebrae are uniquely shaped and play a significant role in head mobility. The atlantoaxial joint facilitates head movement.
Where the Atlantoaxial Joint Sits
The atlantoaxial joint is formed by the first two cervical vertebrae, the atlas (C1) and the axis (C2). The atlas, C1, is the topmost vertebra and is ring-shaped, sitting directly beneath the skull. Unlike most other vertebrae, the atlas does not have a vertebral body or a spinous process.
The axis, C2, is located just below the atlas and is distinguished by a unique upward projection called the dens, or odontoid process. This dens extends superiorly from the body of the axis. The atlas and axis articulate with each other through three synovial joints: one median atlantoaxial joint and two lateral atlantoaxial joints.
Its Unique Design for Movement
The atlantoaxial joint is classified as a pivot joint, also known as a trochoid joint. A pivot joint is a type of synovial joint where one bone rotates within a ring formed by another bone and its connecting ligaments. The ring-shaped atlas rotates around the peg-like dens of the axis.
This design enables rotational movement around a single axis. The atlantoaxial joint is responsible for a significant portion of head rotation from side to side. It contributes approximately 40% to 70% of the total axial rotation of the head. This allows for movements like shaking your head to indicate “no.”
The Role of Supporting Structures
The dens of the axis is central to the function of the atlantoaxial joint. The joint’s stability is maintained by several ligaments that hold the atlas and axis. The transverse ligament of the atlas is a strong, broad band that arches across the ring of the atlas. It keeps the dens in contact with the anterior arch of the atlas, preventing its displacement. This ligament is a strong stabilizer.
Paired alar ligaments also contribute to the joint’s stability. These short, fibrous cords connect the dens posterolaterally to the medial side of the occipital condyles. The alar ligaments limit excessive rotation and side-to-side movements of the head. They ensure that the joint remains secure, protecting the spinal cord from excessive motion.
Common Issues and Their Impact
The atlantoaxial joint’s mobility, while beneficial, also makes it susceptible to various conditions. Atlantoaxial instability (AAI) is an issue, characterized by excessive movement between the atlas and axis. This instability can result from trauma or certain medical conditions. Conditions like rheumatoid arthritis and Down syndrome can lead to ligamentous laxity or bone abnormalities, increasing the risk of AAI.
Subluxation, a partial misalignment, can occur due to injured or loose ligaments. Degenerative changes, such as atlantoaxial osteoarthritis, involve cartilage wear and bony growths. These issues can lead to symptoms like neck pain, stiffness, and a limited range of motion. Neurological symptoms, such as weakness or tingling, can develop if the spinal cord or nerves become affected. Professional evaluation is important for diagnosing these conditions and understanding their impact.