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Atlanto-axial joint

Original Editor - Rachael Lowe

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Description

Atlanto-occipital joint (anterior)
Atlanto-occipital joint (posterior)

The atlantoaxial joint (AAJ) links the first cervical vertebra, known as the atlas (C1), to the second cervical vertebra, termed the axis (C2), forming the most flexible section of the spine. This joint consists of three synovial joints: a single median pivot joint and two lateral plane joints. [1]. This intricate structure allows for significant flexibility, contributing to about 50% of cervical rotation, while still providing stability[2]. However, its unique anatomy also makes it prone to various pathologies, including instability, degenerative changes, and dislocation[3].

[4]


Articulating Surfaces

There are two lateral atlanto-axial joints which are concave in an anterior-posterior direction, this allows rotation.

The median atlantoaxial joint is the articulation of the:

  • posterior surface of the anterior arch of atlas and the front of the odontoid process
  • anterior surface of the transverse ligament and the back of the odontoid process

Between the articular processes of the two bones there is on either side an arthrodial or gliding joint.

Capsule

The atlantoaxial articular capsules are thick and loose, and connect the margins of the lateral masses of the atlas with those of the posterior articular surfaces of the axis.

Each is strengthened at its posterior and medial part by an accessory ligament, which is attached below to the body of the axis near the base of the odontoid process, and above to the lateral mass of the atlas near the transverse ligament.

Ligaments

The ligaments connecting these bones are:

Muscles

  • Rectus Capitis Posterior Major: Extends the head and rotates it to the same side.[5][6].
  • Obliquus Capitis Inferior: Extends the head and rotates it to the same side.[7]
  • Semispinalis Cervicis: Extends and stabilizes the neck; aids in rotation.[7].

Motions Available

Rotation is the primary movement at this joint - 60% of cervical rotation (50°) comes from the atlanto-axial articulation.  This is allowed by the pivot articulation between the odontoid process of the axis and the ring formed by the anterior arch and the transverse ligament of the atlas.

Flexion (10°) and Extension is limited by tectoral membrane.

Side flexion is around 5°.

Clinical Signs and Diagnosis

Neurological problems, limited mobility, and neck pain are common manifestations of pathological alterations in the atlanto-axial joint. Cervical Myelopathy, respiratory compromise, and in severe cases, quadriplegia or death, can result from severe instability or dislocation.[3].

Radiological investigations are essential for diagnosing pathology of the atlanto-axial joint. A widened atlantodental interval (ADI) > 3 mm on dynamic flexion-extension on X-rays often indicates instability[3]. Advanced imaging, such as Dynamic X-rays, CT scans, and MRI which helps identify soft tissue abnormalities and spinal cord compression ​are crucial for diagnosing AA pathologies such as degenerative changes, fractures, and soft tissue abnormalities[8].

Biomechanics and Resulting Pathology

The atlanto-axial joint, consisting of the atlas (C1) and axis (C2), plays a vital role in enabling cervical spine rotation but lacks the bony stability found in the lower cervical vertebrae. Its significant mobility and reliance on ligamentous structures for support make it especially susceptible to instability, dislocation, and various pathologies, such as degeneration, trauma, congenital abnormalities, and inflammatory conditions[5].

Degenerative Changes

Age, mechanical stress, and cumulative wear are the main causes of the atlanto-axial joint's degeneration, which results in calcific synovitis, intraosseous cyst formation, and shrinking of the joint space. The anterior atlantodens joint interval progressively reduces with age, significantly increasing the risk of odontoid fractures in the elderly. Research indicates that osteoarthritis of the atlanto-axial joint affects approximately 44.6% of adults, demonstrating a clear correlation with increasing age[8].

Atlanto-Axial Instability

Atlanto-axial instability (AAI) arises from the compromise of stabilising structures, including the transverse ligament or underlying bony abnormalities. Primary aetiological factors encompass trauma, rheumatoid arthritis (RA), and congenital malformations. In Rheumatoid Arthritis (RA), persistent synovitis contributes to ligamentous laxity and bony erosion, frequently resulting in anterior displacement of the atlas[9][3].

Traumatic Pathology

Trauma to the atlanto-axial joint commonly results in odontoid fractures or ligamentous injuries. Damage to the transverse ligament, alar ligaments, or apical ligament can compromise joint stability, potentially leading to significant instability and neurological deficits. Such injuries are frequently associated with high-impact events, including motor vehicle accidents and sports-related trauma.[9]​[3].

Congenital Pathology

Congenital conditions like Down Syndrome, skeletal dysplasias and Morquio syndrome, can increase the risk of atlanto-axial instability. Key contributing factors include ligamentous laxity, underdevelopment of the odontoid process, and structural abnormalities, such as atlas assimilation.[5].

Pathological Dislocations

Inflammatory conditions like rheumatic fever or infections like Tuberculosis can cause pathological dislocation of the atlanto-axial joint. Even though they are uncommon, these disorders can damage the joint structures, resulting in deformity and instability.[9]

Management

Conservative Management This is taken into consideration when the instability is mild or when there are substantial risks associated with surgical intervention. it involves the use of cervical traction or collar for pain relief, limit movement and prevent further complications[5].

Pharmacological Management

While NSAIDs or corticosteroids are used to relieve pain during flare-ups, disease-modifying antirheumatic medications (DMARDs) like methotrexate and tumor necrosis factor (TNF) inhibitors are used to reduce inflammation and slow the progression of common conditions like rheumatoid arthritis[10].

Surgical Management

This is indicated when there is:

  • Failure of conservative measures.
  • Significant atlantoaxial instability.
  • Neurological compromise due to spinal cord compression.

Common surgical interventions include:

  • Transoral Decompression: This method is utilised for anterior spinal cord compression induced by odontoid disease. Post-decompression, posterior fusion is frequently indicated.
  • Occipitocervical Fusion: in a situation where concurrent occipital-cervical junction instability occurs, This procedure offers the necessary stabilisation​[3].
  • Posterior Stabilisation: Techniques such as transarticular screw fixation (Magerl procedure) and the Goel-Harms construct offer dependable atlas and axis stability and fusion[11].

Postoperative rehabilitationThis is essential for recovery and involves:

  • Gradual and gentle neck mobilisation under the guidance of a physiotherapist to restore strength and flexibility.
  • Tailored strengthening and stabilisation exercises to the neck muscles.
  • Education on safe neck movements and posture to avoid strain[5].

References

  1. ↑ Magee, D. Orthopedic Physical Assessment. Elsevier
  2. ↑ Forbes, J., & Das, J. M. (2024). Anatomy, head and neck: Atlantoaxial joint. In StatPearls. StatPearls Publishing.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Yang SY, Boniello AJ, Poorman CE, Chang AL, Wang S, Passias PG. A review of the diagnosis and treatment of atlantoaxial dislocations. Global spine journal. 2014 Aug;4(3):197-210.​
  4. ↑ Ninja Nerd. Atlas & Axis Cervical Vertebrae (C1-C2) Anatomy. Available from:https://www.youtube.com/watch?v=U3wx14CWPCQ
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Ganapathy S, Lingaraju T. Atlanto-axial instability and its management. Document: Atlanto-Axial Instability and Its Management. 2019;39:1–18.
  6. ↑ Sendić G, McLaren N. Atlantoaxial joint: anatomy, function, movements. Kenhub; 2023
  7. ↑ 7.0 7.1 Cattrysse E, Provyn S, Kool P, et al. Morphology and kinematics of the atlanto-axial joints. Manual Therapy. 2011;16(5):481-486​
  8. ↑ 8.0 8.1 Betsch MW, Blizzard SR, Shinseki MS, Yoo JU. Prevalence of degenerative changes of the atlanto-axial joints. The Spine Journal. 2015 Feb 1;15(2):275-80.
  9. ↑ 9.0 9.1 9.2 Wang C, Yan M, Zhou H, Wang S, Dang G. Atlantoaxial transarticular screw fixation with morselized autograft and without additional internal fixation: technical description and report of 57 cases. Spine. 2007 Mar 15;32(6):643-6.
  10. ↑ Di Muzio C, Conforti A, Bruno F, Currado D, Berardicurti O, Navarini L, Pavlych V, Di Cola I, Biaggi A, Di Donato S, Marino A. The assessment of atlantoaxial joint involvement in patients with rheumatoid arthritis, results from an observational “real-life” study. Scientific Reports. 2023 Nov 17;13(1):20146.
  11. ↑ Yin QS, Wang JH. Current trends in management of atlantoaxial dislocation. Orthopaedic Surgery. 2015 Aug;7(3):189-99.
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