Atlantoaxial Instability
What is atlantoaxial instability?
Atlantoaxial instability (AAI) refers to excessive or abnormal movement specifically at the joint between the atlas (C1) and the axis (C2) — the first and second vertebrae of the cervical spine, sitting immediately below the base of the skull. This joint is responsible for approximately 50% of the rotational movement of the head, and it is stabilised primarily by the transverse ligament, which holds the dens (the bony peg of C2) in close relationship with the arch of C1. When this stabilising system is compromised, the joint can move beyond safe limits, potentially placing the surrounding spinal cord and vertebral arteries at risk.
Atlantoaxial instability is distinct from the broader concept of cervical instability, which can affect any level of the cervical spine, and from cranio-cervical instability, which involves the junction between the skull and C1. AAI specifically involves the C1-C2 articulation and carries its own characteristic presentation, causes and management considerations.
What causes atlantoaxial instability?
AAI has a broader range of underlying causes than either CCI or general cervical instability, and understanding the cause is essential to managing it safely.
Connective tissue disorders — particularly Ehlers-Danlos Syndrome, Marfan Syndrome and hypermobility spectrum disorders — produce systemic ligament laxity that can affect the transverse ligament and other stabilising structures at C1-C2, resulting in instability that is part of a broader pattern of generalised hypermobility. The Ehlers-Danlos Society provides detailed information on how connective tissue disorders affect the cervical spine.
Down syndrome (trisomy 21) is associated with atlantoaxial instability in approximately 10 to 30% of individuals, due to ligamentous laxity inherent to the condition. This is why children with Down syndrome are routinely screened for AAI before contact sports participation or general anaesthesia.
Rheumatoid arthritis can cause erosion of the dens and destruction of the transverse ligament as part of the inflammatory process, producing a distinct form of AAI that is managed very differently to ligamentous instability.
Trauma — including significant whiplash or direct injury to the upper cervical spine — can rupture or stretch the transverse ligament, resulting in acute or chronic AAI.
Congenital abnormalities of the dens or the C1-C2 joint structures can produce instability from birth, sometimes not presenting symptomatically until later in life.
What are the symptoms?
The symptom profile of AAI overlaps with both cervical instability and CCI, but there are some features more characteristic of C1-C2 involvement specifically. Upper cervical pain and stiffness — often felt at the base of the skull and into the upper neck — is common.
Headaches originating at the occiput and referring into the head are frequently reported. A sensation of the head feeling heavy, unstable or difficult to support is characteristic.
More significant instability can produce dizziness, visual disturbances, tinnitus, difficulty swallowing, and symptoms consistent with vertebrobasilar insufficiency — a reduction in blood flow through the vertebral arteries that run through the upper cervical spine. Neurological symptoms including weakness, sensory disturbance or coordination problems indicate potential spinal cord involvement and require urgent medical assessment.
A clinical feature more specific to AAI than general cervical instability is a rotational component — patients often describe their symptoms being provoked by or associated with rotation of the head, and some report a clunking or catching sensation with turning.
How is it diagnosed?
Clinical assessment by a physiotherapist can raise suspicion of AAI, but definitive diagnosis requires imaging. The Sharp-Purser test and Alar ligament stress tests are clinical screening tools used to assess upper cervical stability, though they have known limitations in sensitivity and specificity. Imaging — including flexion-extension X-rays, CT scan to assess bony anatomy, and MRI to evaluate ligamentous and neural structures — is required to confirm the diagnosis and quantify the degree of instability. Upright or dynamic MRI is particularly valuable for patients in whom instability is only apparent under load.
For patients with Down syndrome or rheumatoid arthritis, regular surveillance imaging is standard practice given the known risk of progressive instability in these conditions.
How can physiotherapy help?
Physiotherapy for AAI requires a conservative, carefully graded approach. Manipulation and high-velocity thrust techniques at the upper cervical spine are absolutely contraindicated — these techniques carry genuine risk of neurological injury in the presence of significant C1-C2 instability and should never be applied.
The goal of physiotherapy is to build the active muscular stabilisation of the upper cervical spine to the point where the muscles compensate meaningfully for the deficient passive restraints. The deep cervical flexors — longus colli and longus capitis — are the primary targets, along with the suboccipital musculature and the upper trapezius and sternocleidomastoid as secondary stabilisers. This work is slow, requires careful progression, and is measured in months rather than weeks.
Postural education is important — many patients with AAI adopt head and neck positions that increase rather than decrease load on the upper cervical structures, often as a protective response that has become habitual. Correcting these patterns while simultaneously building strength forms the core of the rehabilitation approach.
For patients with co-occurring POTS, autonomic dysfunction, chronic pain or fatigue — which are common in the hypermobility population — we integrate management of these conditions into the overall treatment plan rather than treating the neck in isolation. Hypermobility Connect Australia is a valuable resource for patients navigating AAI in the context of a systemic connective tissue disorder.
Our physiotherapists Yulia Khasyanova and Mauricio Bara both have specific experience in upper cervical conditions and hypermobility-related presentations. Yulia holds multiple certifications through the Ehlers-Danlos Society and is one of a small number of physiotherapists in Brisbane with specialist training in hypermobility spectrum disorders and their cervical manifestations. Both are members of the Australian Physiotherapy Association.
If you or a loved one has questions about Atlantoaxial Instability and how our physiotherapists might be able to help please call us on 07 3706 3407 or email [email protected]. We would love to work with you!
Atlantoaxial instability (AAI) refers to excessive or abnormal movement specifically at the joint between the atlas (C1) and the axis (C2) — the first and second vertebrae of the cervical spine, sitting immediately below the base of the skull. This joint is responsible for approximately 50% of the rotational movement of the head, and it is stabilised primarily by the transverse ligament, which holds the dens (the bony peg of C2) in close relationship with the arch of C1. When this stabilising system is compromised, the joint can move beyond safe limits, potentially placing the surrounding spinal cord and vertebral arteries at risk.
Atlantoaxial instability is distinct from the broader concept of cervical instability, which can affect any level of the cervical spine, and from cranio-cervical instability, which involves the junction between the skull and C1. AAI specifically involves the C1-C2 articulation and carries its own characteristic presentation, causes and management considerations.
What causes atlantoaxial instability?
AAI has a broader range of underlying causes than either CCI or general cervical instability, and understanding the cause is essential to managing it safely.
Connective tissue disorders — particularly Ehlers-Danlos Syndrome, Marfan Syndrome and hypermobility spectrum disorders — produce systemic ligament laxity that can affect the transverse ligament and other stabilising structures at C1-C2, resulting in instability that is part of a broader pattern of generalised hypermobility. The Ehlers-Danlos Society provides detailed information on how connective tissue disorders affect the cervical spine.
Down syndrome (trisomy 21) is associated with atlantoaxial instability in approximately 10 to 30% of individuals, due to ligamentous laxity inherent to the condition. This is why children with Down syndrome are routinely screened for AAI before contact sports participation or general anaesthesia.
Rheumatoid arthritis can cause erosion of the dens and destruction of the transverse ligament as part of the inflammatory process, producing a distinct form of AAI that is managed very differently to ligamentous instability.
Trauma — including significant whiplash or direct injury to the upper cervical spine — can rupture or stretch the transverse ligament, resulting in acute or chronic AAI.
Congenital abnormalities of the dens or the C1-C2 joint structures can produce instability from birth, sometimes not presenting symptomatically until later in life.
What are the symptoms?
The symptom profile of AAI overlaps with both cervical instability and CCI, but there are some features more characteristic of C1-C2 involvement specifically. Upper cervical pain and stiffness — often felt at the base of the skull and into the upper neck — is common.
Headaches originating at the occiput and referring into the head are frequently reported. A sensation of the head feeling heavy, unstable or difficult to support is characteristic.
More significant instability can produce dizziness, visual disturbances, tinnitus, difficulty swallowing, and symptoms consistent with vertebrobasilar insufficiency — a reduction in blood flow through the vertebral arteries that run through the upper cervical spine. Neurological symptoms including weakness, sensory disturbance or coordination problems indicate potential spinal cord involvement and require urgent medical assessment.
A clinical feature more specific to AAI than general cervical instability is a rotational component — patients often describe their symptoms being provoked by or associated with rotation of the head, and some report a clunking or catching sensation with turning.
How is it diagnosed?
Clinical assessment by a physiotherapist can raise suspicion of AAI, but definitive diagnosis requires imaging. The Sharp-Purser test and Alar ligament stress tests are clinical screening tools used to assess upper cervical stability, though they have known limitations in sensitivity and specificity. Imaging — including flexion-extension X-rays, CT scan to assess bony anatomy, and MRI to evaluate ligamentous and neural structures — is required to confirm the diagnosis and quantify the degree of instability. Upright or dynamic MRI is particularly valuable for patients in whom instability is only apparent under load.
For patients with Down syndrome or rheumatoid arthritis, regular surveillance imaging is standard practice given the known risk of progressive instability in these conditions.
How can physiotherapy help?
Physiotherapy for AAI requires a conservative, carefully graded approach. Manipulation and high-velocity thrust techniques at the upper cervical spine are absolutely contraindicated — these techniques carry genuine risk of neurological injury in the presence of significant C1-C2 instability and should never be applied.
The goal of physiotherapy is to build the active muscular stabilisation of the upper cervical spine to the point where the muscles compensate meaningfully for the deficient passive restraints. The deep cervical flexors — longus colli and longus capitis — are the primary targets, along with the suboccipital musculature and the upper trapezius and sternocleidomastoid as secondary stabilisers. This work is slow, requires careful progression, and is measured in months rather than weeks.
Postural education is important — many patients with AAI adopt head and neck positions that increase rather than decrease load on the upper cervical structures, often as a protective response that has become habitual. Correcting these patterns while simultaneously building strength forms the core of the rehabilitation approach.
For patients with co-occurring POTS, autonomic dysfunction, chronic pain or fatigue — which are common in the hypermobility population — we integrate management of these conditions into the overall treatment plan rather than treating the neck in isolation. Hypermobility Connect Australia is a valuable resource for patients navigating AAI in the context of a systemic connective tissue disorder.
Our physiotherapists Yulia Khasyanova and Mauricio Bara both have specific experience in upper cervical conditions and hypermobility-related presentations. Yulia holds multiple certifications through the Ehlers-Danlos Society and is one of a small number of physiotherapists in Brisbane with specialist training in hypermobility spectrum disorders and their cervical manifestations. Both are members of the Australian Physiotherapy Association.
If you or a loved one has questions about Atlantoaxial Instability and how our physiotherapists might be able to help please call us on 07 3706 3407 or email [email protected]. We would love to work with you!
