The most common cause of symptomatic atlantoaxial subluxation is acute trauma to the cervical spine.
Atlantoaxial subluxation (also known as atlantoaxial instability or AAI) occurs when there is excessive movement between the first cervical vertebra (atlas, C1) and the second cervical vertebra (axis, C2), leading to potential compression of the spinal cord or nerve roots. While various factors can contribute, acute traumatic injuries are the primary culprits for cases where symptoms arise.
How Acute Trauma Leads to Atlantoaxial Subluxation
Acute trauma typically involves forceful movements or impacts that place excessive stress on the ligaments and joint structures supporting the atlantoaxial joint. These mechanisms include:
- Cervical Hyperflexion: This occurs when the head is forced excessively forward, beyond its normal range of motion. This can stretch or tear the posterior ligaments and joint capsules, leading to instability.
- Cervical Hyperextension: This involves the head being forced excessively backward. Such a motion can injure the anterior structures, including the anterior longitudinal ligament and the anterior atlantoaxial membrane.
- Direct Axial Load on the Head or Cervical Spine: This is a force applied directly down the length of the spine, often seen in falls or impacts where the top of the head sustains a direct hit, compressing the cervical vertebrae and potentially injuring the atlantoaxial joint.
These types of injuries can happen in various scenarios, including sports accidents, motor vehicle collisions, or falls.
Predisposing Factors
While acute trauma is the direct cause of symptomatic atlantoaxial subluxation, certain conditions can predispose an individual to this injury, making them more vulnerable even to less severe trauma. These conditions often affect the integrity of the ligaments or bones around the atlantoaxial joint, increasing the risk of instability. Examples of such predisposing conditions include:
- Rheumatoid Arthritis: This autoimmune disease can cause inflammation and erosion of the joint surfaces and ligaments, particularly in the cervical spine.
- Down Syndrome: Individuals with Down syndrome often have ligamentous laxity, including in the neck, which makes the atlantoaxial joint inherently less stable.
- Congenital Anomalies: Certain birth defects affecting the bones or ligaments of the upper cervical spine can lead to chronic instability.
- Ehlers-Danlos Syndrome: This group of inherited connective tissue disorders can result in generalized joint hypermobility and ligamentous laxity throughout the body, including the neck.
Understanding both the direct traumatic mechanisms and any underlying predisposing conditions is crucial for accurate diagnosis and management of atlantoaxial subluxation.