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What is Atlantoaxial Instability (AAI)?

Craniocervical instability and atlantoaxial instability are diagnoses that often cause an immense amount of disability and confusion. Let’s review all of that here to make sure you know what these terms mean, common symptoms of these problems, how these diagnoses are made, and what the treatment options entail. Let’s dig in.

What is Atlanto-Axial Instability (AAI), also known as Craniocervical Instability (CCI)?

CCI stands for craniocervical instability and AAI means atlantoaxial instability. Before we get into what these terms mean, let’s review the anatomy of the craniocervical junction, or where the cervical spine meets the skull.

The upper neck where the spine meets the head is called the craniocervical junction or CCJ (see Figure 1). The seven spine bones or vertebrae in your neck are numbered from C1-C7. The skull is numbered as C0. The C0-C2 bones make up the CCJ. In fact, another name for instability in this area is CCJ Instability.

While the lower neck bones have a cushioning disc in the front and two facet joints in the back at each level, the upper neck is built differently.

The C1 bone is called the atlas and the C2 bone has a part that sticks up (dens) that fits into the atlas:

This CCJ area has many ligaments that connect C1 to C2 and C2 to the skull. These main ligaments are:

  • Alar
  • Transverse
  • Accessory

Altogether, the elements outlined here work as a functioning unit to hold the skull onto the neck. This is how those ligaments fit together:

Atlantoaxial instability and craniocervical instability basically mean the same thing – that these ligaments that hold the head onto the neck are damaged, which leads to instability – but refer to different parts of the same functioning unit, the CCJ.

What is Instability and Why Is It a Big Deal?

Instability simply means that bones move around too much, usually due to damaged ligaments. In the spine, this can cause nerves to get banged into and joints to get damaged. In the craniocervical junction, instability can cause the upper cervical spinal nerves to get irritated, leading to headaches. In addition, the C0-C1 and C1-C2 facet joints can also get damaged. In addition, there are other nerves that exit the skull here that can get irritated, like the vagus nerve, which can cause rapid heart rate.

What’s the Difference Between CCI and AAI? CCI refers to instability in any part of the craniocervical junction while AAI refers to instability at C1-C2. The only reason to differentiate them is usually that atlantoaxial instability patients can be treated with a less invasive C1-C2 screw fixation while CCI patients may need more extensive surgery. However, as you’ll see below, surgery is not always the best treatment for these conditions.

What are the Symptoms of AAI?

The symptoms of CCI / AAI include:

  • Headaches
  • Dizziness/Imbalance/Vertigo
  • Visual disturbances
  • Brain fog
  • Rapid heart rate (tachycardia)
  • Random area of numbness or pain or other symptoms such as stomach upset.

How do you test AAI? (Testing & Diagnosis)

CCI and AAI are often diagnosed via the patient’s symptoms and either a hands-on manual test by the doctor or therapist or diagnostic imaging. However, realize that for many patients, getting the right testing to cinch the diagnosis is difficult. Let’s dive in.

Highly trained upper cervical chiropractors and manual physical therapists can often diagnose CCI by manual techniques. This means that the patient is seeing an upper cervical chiropractic specialists (NUCCA or Atlas-Orthogonal) and the patient gets only temporary relief from manual adjustments. Or if the patient is seeing a manual physical therapist (very specialized PT focused on spine), then specific hands on diagnostic tests are performed (alar stress test).

There are many different types of imaging that can be performed to help make a diagnosis:

  • Dynamic Motion X-ray (DMX)
  • CT Scan with motion
  • Static Upper Neck MRI
  • Dynamic MRI

Let’s dive into each a bit:

Dynamic Motion X-ray (DMX)

This is a real-time x-ray of the neck. The goal is to allow the patient to move and look for how the bones stay aligned or move too much. For more information, see my video below:

CT Scan with Motion

A CT scan is a specialized 3D x-ray and in this case, it’s performed with the patient’s head turned right or left. Excessive motion, usually at C1-C2 can be used to make a diagnosis of CCI.

Static Upper Neck MRI

This test can be performed in two ways. The first is a routine MRI where different measurements can be performed such as:

To learn more about Grabb-Oakes, see my video below:

Powers Ratio:

Dynamic MRI

MRIs are usually performed with the patient lying very still, face-up in a tube, but they can also be performed with the patient sitting with movement. This can be used to see if the upper neck bones move too much with motion.

How is Atlantoaxial Instability Treated?

There are three main types of treatment:

  • Conservative Care-Chiropractic and PT
  • Injections
  • Surgery

Let’s dive into each Treatment Option

Conservative Care

There are two major disciplines in chiropractic which are NUCCA (National Upper Cervical Chiropractic Association) and AO (Atlas-Orthogonal). These generally involve measuring the angles of the upper neck bones on x-ray and then specific manipulations to get them back in place. Patients who only get temporary relief are then more likely to be chronically unstable. There are also highly trained physical therapists and osteopaths who specialize in the upper neck. These providers can also offer manipulative care with the same caveats as above.


There are a number of different type of injections here which include:

  • Intra-articular facet injections
  • Posterior prolotherapy
  • PICL procedure

Intra-articular facet injections of the upper cervical facet joints are highly specific and difficult to perform. These are joints in the neck that can get beat up or damaged when there is CCI or AAI (AAI impacting C1-C2). There are only a handful of physicians in the United States with much experience in injecting these joints using x-ray guidance and contrast to ensure placement. These joints can be injected with substances like PRP or bone marrow concentrate, which can help to reduce the pain or heal the damage.

Posterior prolotherapy involves injecting the ligaments in the back of the neck (not the alar, transverse, or accessory ligaments). While this can help patients with CCI or AAI, regrettably, in our experience, most patients don’t get relief with posterior prolotherapy.

The PICL procedure involves direct injection of the ligaments that hold the head on (alar, transverse, or accessory ligaments) from the front. This is a highly complex procedure that involves endoscopy for direct visualization of the back of the throat area, specialized anesthesia, a specialized mouth piece, and x-ray guidance. In our experience, this procedure can help many patients avoid the need for neck fusion. See our video below for more information:


The surgical approach here involves fusion, which means that the doctor inserts screws and plates to fuse various parts of the upper cervical bones. These are all higher risk procedures than a routine mid or lower cervical fusion.  Here are the main surgeries:

  • C1-C2 screw fixation
  • C0-C2 fusion (Goel and Harms Technique)
  • Occipital-cervical Fusion (Skull Base to C2)

C1-C2 screw fixation is likely the most common procedure we see. This involves placing a screw across the C1-C2 joint and is involved in atlanto-axial instability (AAI). The main problems have been:

  • Lack of fusion of the C1-C2 joint. If this joint doesn’t fuse (grow together with bone), the joint is now destroyed due to the surgery and cause pain. In that case a repeat procedure may need to be performed.
  • Sacrifice of the occipital nerve. This is the nerve that goes up to your head, so injury to this nerve causes chronic headaches.
  • Misplacement of screws. The most common thing we see is that the screw is placed too deep and into the C0-C1 joint which leads to arthritis in that joint.

A C0-C2 fusion is when the doctor inserts screws into the vertebral bodies instead of across the joint itself. This is a bigger procedure than C1-C2 screw fixation. It has many of the same possible complications including lack of fusion and injury to the nerve.

An occipital-cervical fusion involves extending the hardware up to the skull base. This is the largest of the three surgeries. It has all of the same possible complications, which are generally highest for this procedure since it’s the most invasive.