Atlantoaxial instability is abnormal, excessive movement between the first and second vertebrae in your neck, called the atlas (C1) and the axis (C2). This joint is uniquely designed: the axis has a bony peg that sits inside the ring of the atlas, and this “peg in ring” arrangement is what allows you to turn your head side to side. In fact, the atlantoaxial joint provides about 50% of all rotation in the cervical spine. When the ligaments holding these two bones together become damaged, weakened, or malformed, the joint can slip out of alignment and potentially compress the spinal cord.
How the C1-C2 Joint Stays Stable
The most important structure keeping this joint in place is the transverse ligament, a thick band of tissue that wraps behind the bony peg of C2 and binds it tightly against the front arch of C1. This ligament is the primary restraint against the atlas sliding forward on the axis. The bony peg itself prevents the atlas from sliding backward. Together with the joint capsules on either side, these structures keep the two vertebrae moving in a controlled, predictable way.
The atlas and axis can only shift out of alignment relative to each other in two scenarios: either the ring of C1 fractures, or the transverse ligament ruptures. Because the injury is usually to the ligament rather than the bone, and ligaments in this area don’t heal reliably on their own, atlantoaxial instability often becomes a permanent problem once it develops.
What Causes It
The causes fall into three broad categories: congenital conditions you’re born with, inflammatory diseases that erode the joint over time, and acute trauma or infection.
Down Syndrome and Other Congenital Causes
Down syndrome is the most well-known congenital cause. Roughly 10% to 30% of people with Down syndrome show radiologic signs of atlantoaxial instability, though some studies have reported rates exceeding 60%. The underlying problem is ligamentous laxity, a looseness in the connective tissue that affects the transverse ligament’s ability to hold C1 and C2 together. This is why children with Down syndrome are routinely screened with neck X-rays, and those with significant instability are advised to avoid contact sports and activities that risk forceful bending of the neck.
Other congenital conditions involving skeletal or connective tissue abnormalities can also cause instability at this joint. In one such condition, chondrodysplasia punctata, atlantoaxial instability is the primary cause of disability and death. About 20% of affected individuals develop weakness, and another 20% develop overactive reflexes from spinal cord compression, sometimes at a very early age.
Rheumatoid Arthritis
In rheumatoid arthritis, chronic inflammation gradually erodes the ligaments and joint surfaces at C1-C2. Anterior atlantoaxial subluxation, where C1 slides forward on C2, is the most common cervical spine deformity in people with RA. The longer someone has poorly controlled rheumatoid arthritis, the greater the risk. This makes regular monitoring of the cervical spine important for people living with RA long-term.
Infection in Children (Grisel Syndrome)
Children can develop atlantoaxial instability after an upper respiratory infection or head and neck surgery through a process called Grisel syndrome. Infectious material from the throat area spreads through nearby blood vessels to the tissues around C1 and C2. The resulting inflammation weakens the ligaments and joint capsule, allowing the joint to subluxate. The hallmark sign is a child who suddenly develops a head tilt (torticollis) after a recent throat infection or adenoid/tonsil surgery, without any history of trauma.
Symptoms and Warning Signs
Many people with mild atlantoaxial instability have no symptoms at all, which is why it’s sometimes discovered incidentally on imaging. When symptoms do appear, they typically start with neck pain and stiffness, particularly with rotation. Some people notice a grinding or clicking sensation when turning their head.
The more concerning symptoms emerge when the shifting vertebrae start compressing the spinal cord. This is called myelopathy, and it can develop gradually or suddenly after a fall or jolt. Signs include clumsiness in the hands, difficulty with fine motor tasks like buttoning a shirt, unsteady walking, and a feeling that your legs don’t move the way you expect them to. You might also notice an electric shock sensation running down your spine or limbs when you bend your neck forward.
Overactive reflexes, loss of position sense in the hands and feet, and an abnormal reflex in the sole of the foot (where the big toe extends upward instead of curling down when the sole is stroked) are neurological signs that indicate the spinal cord is being compressed. These findings can be subtle early on and tend to worsen progressively if the instability isn’t addressed.
How It’s Diagnosed
The key measurement is the atlantodental interval (ADI), the gap between the front arch of C1 and the bony peg of C2. In adults, a gap greater than 3.5 millimeters on a flexion-extension X-ray suggests instability. In children, the normal ADI is less than 4 millimeters, and anything beyond 4 to 5 millimeters is considered abnormal.
Standard X-rays taken with the neck in a neutral position can miss the problem entirely because the bones may look aligned when the neck isn’t moving. That’s why dynamic flexion-extension X-rays are the preferred initial test. You’re asked to gently bend your head forward and then backward while lateral X-rays are taken in each position. If the gap between C1 and C2 changes significantly between the two views, that confirms instability.
When instability is confirmed or when neurological symptoms are present, MRI is typically the next step. It shows the spinal cord directly and can reveal whether compression, swelling, or damage has occurred. CT scans provide detailed views of the bony anatomy and are useful for surgical planning or when a fracture is suspected.
Treatment Approaches
Treatment depends entirely on the severity of the instability and whether the spinal cord is involved.
For people without neurological symptoms, management is often conservative. In Down syndrome, for example, those with an ADI greater than 5 millimeters are advised to avoid contact sports and high-risk activities but don’t necessarily need surgery. Activity modification and regular monitoring with imaging can be enough when the spinal cord isn’t threatened.
Surgery becomes necessary when instability is severe or the spinal cord is at risk. The standard procedure is a posterior C1-C2 fusion, which permanently connects the two vertebrae using hardware and bone graft. This eliminates the dangerous movement but also eliminates much of the neck’s rotational range. Because the injury in most cases is purely ligamentous and unlikely to heal on its own, fusion is often the definitive solution rather than a last resort.
Specific surgical thresholds vary by condition. In rheumatoid arthritis, surgery is indicated when the atlantoaxial subluxation exceeds 8 millimeters with evidence of cord compression, or when there’s a progressive neurological decline. In Down syndrome, fusion is recommended when the ADI exceeds 10 millimeters or when neurological findings develop. For traumatic injuries classified as unstable (types III and IV on the Fielding and Hawkins scale), surgical stabilization is the standard recommendation.
What Happens Without Treatment
Mild, stable cases may never progress, particularly when activity modifications are followed. But when significant instability is left unaddressed, the risk is progressive myelopathy, a slow deterioration of spinal cord function that worsens over months to years. Fine motor skills decline, walking becomes increasingly difficult, and bladder or bowel function can eventually be affected. In severe cases, a sudden trauma, even a minor fall, can cause acute spinal cord compression with devastating consequences. This is why regular monitoring matters even for people who currently have no symptoms, particularly in high-risk populations like those with Down syndrome or long-standing rheumatoid arthritis.