Atlantoaxial joint

The atlantoaxial joint is a specialized articulation within the upper cervical spine that provides significant mobility while maintaining stability around the craniovertebral junction. It plays a key role in head rotation and is clinically significant due to its susceptibility to congenital anomalies, inflammatory conditions, and traumatic injuries.

Gross Anatomy of the Atlantoaxial Joint

Location and General Features

The atlantoaxial joint lies between the first cervical vertebra (atlas, C1) and the second cervical vertebra (axis, C2). It is situated just below the atlanto-occipital joint and forms an essential connection between the skull and the remainder of the cervical spine. This joint is classified as a pivot type of synovial joint, permitting rotational movements of the head.

Bony Components

• Atlas (C1): A ring-shaped vertebra lacking a vertebral body and spinous process, designed to cradle the dens and articulate with the occipital condyles above.
• Axis (C2): Characterized by the presence of the odontoid process (dens), which projects superiorly and forms the axis of rotation for the atlas.
• Odontoid Process (Dens): A tooth-like projection of the axis that articulates with the anterior arch of the atlas, allowing pivotal motion.

Joint Type and Classification

The atlantoaxial complex consists of three distinct synovial articulations: one median atlantoaxial joint and two lateral atlantoaxial joints. Collectively, they permit approximately 50% of the total cervical rotation. The median joint functions as a pivot joint, while the lateral joints are plane synovial joints that allow gliding movements.

Articulating Surfaces

Median Atlantoaxial Joint

The median joint is formed between the anterior surface of the dens of the axis and the posterior surface of the anterior arch of the atlas. Additionally, the posterior surface of the dens articulates with the transverse ligament of the atlas, forming a synovial articulation. This unique arrangement allows the atlas and skull to rotate around the dens.

Lateral Atlantoaxial Joints

Each lateral atlantoaxial joint is formed by the articulation between the inferior articular facets of the atlas and the superior articular facets of the axis. These facets are nearly flat, enabling sliding and gliding movements that complement the pivoting action of the median joint.

Surface Morphology

The articulating surfaces are covered with hyaline cartilage, ensuring smooth movement. The congruency of the facets and the supporting ligaments provide stability to the joint despite its wide range of motion.

Ligaments and Supporting Structures

Transverse Ligament of the Atlas

The transverse ligament is the most important stabilizing structure of the atlantoaxial joint. It extends between the medial surfaces of the lateral masses of the atlas, firmly holding the dens against the anterior arch of the atlas. This prevents posterior displacement of the dens, which could compress the spinal cord.

Alar Ligaments

These paired ligaments extend from the lateral sides of the dens to the medial margins of the occipital condyles. They restrict excessive rotation and side bending of the head, acting as strong stabilizers of the craniovertebral junction.

Apical Ligament of the Dens

The apical ligament runs from the tip of the dens to the anterior margin of the foramen magnum. Although relatively weak compared to the transverse and alar ligaments, it contributes to the vertical stability of the dens.

Tectorial Membrane

The tectorial membrane is a continuation of the posterior longitudinal ligament. It runs from the body of the axis to the internal surface of the occipital bone, covering the dens and its associated ligaments. It reinforces the craniovertebral junction from behind and limits hyperflexion.

Accessory Ligaments

Additional supportive structures include the atlantoaxial capsular ligaments surrounding the synovial joints and small accessory bands that assist in restricting abnormal movements.

Relations

Anterior Relations

Anterior to the atlantoaxial joint lie the pharynx, prevertebral fascia, and the longus colli muscles. These soft tissue structures separate the joint from the aerodigestive tract.

Posterior Relations

Posteriorly, the joint is related to the spinal cord, meninges, and posterior atlantoaxial membrane. The close relationship with the spinal canal underscores the clinical importance of stability in this region.

Lateral Relations

Laterally, the atlantoaxial joint is related to the vertebral arteries as they ascend through the transverse foramina of the cervical vertebrae and curve medially to enter the foramen magnum. The proximity of these vessels makes them vulnerable in cases of severe rotational injuries or instability.

Neurovascular Supply

Arterial Supply

The atlantoaxial joint receives its arterial supply primarily from branches of the vertebral arteries. Small twigs from the ascending pharyngeal and deep cervical arteries may also contribute. The close anatomical relationship of the vertebral arteries with the joint makes their integrity crucial during surgical procedures or in cases of instability.

Venous Drainage

Venous blood from the atlantoaxial region drains into the vertebral venous plexus. This extensive plexus communicates with intracranial venous sinuses and other cervical venous networks, allowing collateral circulation in cases of obstruction.

Nerve Supply

The nerve supply to the atlantoaxial joint is derived from the dorsal rami of the C1 and C2 spinal nerves. These nerves mediate proprioception and nociception, which explains the referral of pain from this joint to the occipital and suboccipital regions in clinical disorders.

Biomechanics and Movements

Flexion and Extension

The atlantoaxial joint permits limited flexion and extension, primarily occurring at the lateral joints. This movement is minor compared to the atlanto-occipital joint, which contributes more significantly to nodding motions of the head.

Rotation

Rotation is the principal movement at the atlantoaxial joint. The atlas, carrying the skull, pivots around the dens of the axis. Approximately 30 to 45 degrees of rotation to each side is possible, accounting for nearly half of the cervical spine’s total rotational capacity.

Contribution to Cervical Spine Mobility

Together with the atlanto-occipital joint, the atlantoaxial articulation allows a wide range of head movements, including nodding and shaking motions. It provides a balance between flexibility and stability, enabling functional motion while protecting the spinal cord.

Stability Mechanisms

Stability at the atlantoaxial joint is ensured by the strong ligamentous structures, congruence of the articulating surfaces, and the surrounding musculature. The transverse and alar ligaments are particularly critical in preventing excessive displacement of the dens, which could result in neurological compromise.

Development and Ossification

Embryological Development of Atlas and Axis

The atlas and axis develop from the cartilaginous precursors of the cervical vertebrae during the embryonic stage. The dens of the axis originates from the body of the first cervical vertebra, which fuses with the second cervical vertebra during development. This unique embryological shift explains the atypical anatomy of the axis compared to other vertebrae.

Fusion and Growth Patterns

The dens is initially separated from the body of the axis by a cartilaginous plate, known as the subdental synchondrosis, which usually fuses by the age of 6 to 8 years. Secondary ossification centers also appear at the apex of the dens during childhood and fuse in adolescence. The proper fusion of these centers is critical to joint stability.

Congenital Variations

Certain congenital anomalies may affect the atlantoaxial joint, including non-union of the dens with the body of the axis (os odontoideum) and persistence of the subdental synchondrosis into adulthood. These conditions can predispose individuals to instability and neurological complications.

Clinical Significance

Atlantoaxial Instability

Instability can result from trauma, congenital anomalies, or pathological weakening of ligaments. It leads to abnormal mobility between the atlas and axis, increasing the risk of spinal cord compression. Clinical manifestations may include neck pain, restricted movement, and neurological deficits.

Congenital Anomalies (e.g., Os odontoideum)

Os odontoideum is a condition in which the dens is separated from the body of the axis. This anomaly may be asymptomatic or present with instability, making individuals vulnerable to neurological injury even after minor trauma.

Rheumatoid Arthritis and Inflammatory Disorders

Rheumatoid arthritis frequently affects the cervical spine, particularly the atlantoaxial joint. Synovial inflammation may erode ligaments and bone, leading to instability. Early recognition is vital to prevent serious complications such as myelopathy.

Traumatic Injuries (Fractures and Dislocations)

High-impact trauma can cause fractures of the dens or dislocations of the atlantoaxial joint. These injuries are considered highly unstable and often necessitate urgent immobilization or surgical stabilization to protect the spinal cord.

Neurological Implications (Cord Compression, Myelopathy)

Neurological involvement occurs when instability or fracture compromises the spinal canal, leading to cord compression. Symptoms may range from paresthesia and weakness to severe myelopathy with paralysis. Prompt diagnosis and treatment are critical for preventing permanent disability.

Radiological Evaluation

X-ray (Open-mouth Odontoid View)

The open-mouth odontoid view is a standard radiographic technique used to visualize the atlantoaxial joint. It allows direct assessment of the alignment between the atlas and axis as well as visualization of the odontoid process. This view is particularly useful for detecting fractures, asymmetry, or abnormal spacing suggestive of instability.

CT Scan

Computed tomography (CT) provides high-resolution imaging of the bony anatomy of the atlantoaxial joint. It is considered the gold standard for evaluating fractures of the dens, congenital anomalies, and fine details of articular surfaces. Three-dimensional reconstructions can assist in preoperative planning.

MRI

Magnetic resonance imaging (MRI) is invaluable for assessing soft tissue structures such as ligaments, spinal cord, and intervertebral discs. It is particularly useful in detecting ligamentous injuries, inflammatory pannus formation in rheumatoid arthritis, and spinal cord compression due to instability or trauma.

Surgical and Therapeutic Considerations

Conservative Management

Mild cases of instability or minor injuries without neurological compromise may be managed conservatively. Treatment strategies include cervical collars, immobilization, and physical therapy. Regular monitoring is essential to ensure stability is maintained.

Atlantoaxial Fusion Techniques

Surgical fusion is indicated in cases of persistent instability, severe trauma, or progressive neurological deficits. Techniques include posterior wiring, screw fixation, and transarticular screw fixation. Modern instrumentation has improved fusion rates and reduced complications.

Complications and Outcomes

Complications of surgery may include vertebral artery injury, infection, or hardware failure. However, with careful planning and modern techniques, outcomes are generally favorable, providing long-term stability and neurological improvement in most patients.

References

1. Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Elsevier; 2021.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 8th ed. Wolters Kluwer; 2018.
3. Drake RL, Vogl W, Mitchell AWM. Gray’s Anatomy for Students. 4th ed. Elsevier; 2019.
4. White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Lippincott Williams & Wilkins; 1990.
5. Goel A, Shah A. Atlantoaxial joint instability: Principles of management. Neurol India. 2019;67(Supplement):S82-S91.
6. Fielding JW, Hawkins RJ. Atlanto-axial rotatory fixation. J Bone Joint Surg Am. 1977;59(1):37-44.
7. Harrop JS, Sharan A, Przybylski GJ. Epidemiology of spinal cord injury after acute odontoid fractures. Neurosurg Focus. 2000;8(6):e4.
8. Menezes AH. Craniovertebral junction abnormalities: A comprehensive surgical approach. Clin Neurosurg. 2005;52:96-119.