Atlantoaxial Instability

Atlantoaxial Instability (AAI)

What is the Atlantoaxial Joint?

The atlantoaxial joint involves the upper cervical vertebra (C1), known as atlas, and second cervical vertebra (C2), called the axis. The atlantoaxial joint is formed by 3 separate synovia joints:

One middle joint is the pivot joint, where the dens on the second vertebrae (C2/axis) fits into a ring formed by the front arch of the first vertebrae (C1/atlas), and a strong band of tissue called the transverse ligament. This structure acts like a pivot, allowing the head to move from side to side, as in a “no” motion. There are two alar ligaments that attach to the dens of the c2/axis to the occipital condyles at the base of the skull.

Two side (lateral) joints are on each side (right and left) of the middle joint (above). They are formed by flat-like surfaces (called facets) of the two vertebrae meet and slide past each other. These joints help guide the rotation movement and help provide stability.

Together, these 3 joints allow for a wide range of head rotation to the right and left.

Atlantoaxial joint anatomy video.

What is Atlantoaxial Instability?

Atlantoaxial instability (AAI) is characterized by excessive or “too much” movement of the atlantoaxial joints. This is typically the result of a fracture, degeneration of the joints or injury to the ligament(s). The excess motion of the atlantoaxial joint can lead to AAI.

What Causes Atlantoaxial Instability?

Atlantoaxial instability can originate from congenital conditions, but in adults, it is primarily seen in the setting of acute trauma or degenerative changes due to an inflammatory response. Infection has been found to be an additional cause of instability, with the rich arterial supply and venous plexus in this region of the body providing a route for infectious sequelae…” Ref. AAI can also happen in connective tissue disorders.

What are some common symptoms of AAI?

AAI can be symptomatic or asymptomatic. AAI places a lot of mechanical stress on the central nervous system. Neurologic symptoms along with fatigue and brain fog can occur when the spinal cord, vertebral artery or adjacent nerve roots are compressed.

Patients presenting with atlantoaxial instability can suffer from a spectrum of clinical signs and symptoms, although most are asymptomatic. These include:

- - Neck pain
  - Vertigo and/or Tinnitus
  - Face, neck and shoulder pain
  - Severe headaches and migraines
  - Ears full or plugged
  - Restricted neck movements
  - Pyramidal signs and myelopathy
  - Lower cranial nerve palsies
  - Respiratory failure
  - Vertebral artery dissection
  - Quadriplegia

How is AAI Diagnosed?

A neurosurgical consultation and assessment with the use of radiographic diagnostic imaging with key measurements help determine if there is a excessive movement of the atlantoaxial joint.

Are there different types of Atlantoaxial Instability?

Atlantoaxial subluxation can be anteroposterior (front-back), rotatory (rotating head left and right), lateral horizontal (C1-C2 vertebrae slipping / overhanging to the left or right) or vertical (vertebrae slips downward relative to the one below it).

Atlantoaxial dislocation (AAD) is a recognized and serious medical condition that involves the potentially fatal misalignment between the atlas and the axis.

Rotatory Atlantoaxial Instability

Rotatory or Rotational atlantoaxial instability can be due to ligament or facet injury. It can range from from muscle spasm to a lack of movement of the atlantoaxial joint due to a structural or mechanical obstruction.

Fielding and Hawkins Classification

Fielding and Hawkins suggested a four-part classification for evaluating rotatory displacement as follows:

- Type I – Simple rotatory displacement with an intact transverse ligament
- Type II – Anterior displacement of C1 on C2 of 3-5 mm with one lateral mass serving as a pivot point and a deficiency of the transverse ligament
- Type III – Anterior displacement exceeding 5 mm
- Type IV – Posterior displacement of C1 on C2

**Both type III and type IV are highly unstable.

Vertical Atlantoaxial Instability

Vertical AAI is when the dens is often above the McGregor line by over 8 mm in men and 9.7 mm in women.

Horizontal Atlantoaxial Instability

Horizontal AAI is when there is increased lateral (side) movement to the left or right of atlas over axis.

References:

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