bubble_chart Overview

Atlantoaxial dislocation is the most common injury in the upper cervical spine. Without timely treatment, the degree of dislocation often progressively worsens, leading to high spinal cord compression and life-threatening conditions. Due to its high potential risk, active treatment should be pursued.

bubble_chart Etiology

Traumatic dislocation

combined with odontoid fracture refers to the displacement of the atlas along with the odontoid fracture. The distance from the posterior superior corner of the axis vertebral body or the posterior edge of the fracture line to the anterior edge of the posterior arch of the atlas represents the effective space available for the spinal cord. This measurement can be used to assess the narrowing of the buffer space and the degree of spinal cord compression.

Simple anterior dislocation of the atlas refers to atlantoaxial dislocation without an odontoid fracture, which inevitably involves extensive ligamentous injury between the atlas and axis. Due to the presence of the odontoid process, the spinal cord is compressed between the odontoid and the posterior arch of the atlas, making it more susceptible to injury.

Congenital malformation dislocation

Individuals with developmental abnormalities in the occipitocervical region are more prone to acute atlantoaxial dislocation following trauma compared to normal individuals. In most cases, atlantoaxial instability develops gradually after adolescence. Two common types are: (1) segmentation disorders, manifested as fusion of the occipital bone and atlas or fusion of the C_2–3 vertebral bodies; (2) odontoid hypoplasia.

Spontaneous dislocation

In adults, it is often secondary to rheumatoid arthritis, while in children, it is usually secondary to deep neck infections.

Atlantoaxial rotatory fixation is essentially a chronic dislocation. Fielding (1977) defined the condition of atlantoaxial rotatory subluxation that occurs spontaneously or after grade I trauma as atlantoaxial rotatory fixation. Later (1983), he referred to it as rotatory displacement.

Pathological dislocation

This is also a slowly developing dislocation, differing from spontaneous dislocation in that it involves actual bone destruction of the atlas and/or axis due to pathological changes. In China, atlantoaxial tuberculosis is the most common cause, though it may also occur rarely with tumors or osteomyelitis of the atlas and axis.

bubble_chart Pathogenesis

Anatomical Features and Injury Mechanisms:

The atlantoaxial joint consists of: (1) The lateral atlantoaxial joints, formed by the inferior articular surfaces of the atlas on both sides and the superior articular surfaces of the axis; (2) The anterior and posterior joints of the dens, located between the anterior surface of the dens and the dental fovea of the anterior arch of the atlas, and between the posterior surface of the dens and the transverse ligament of the atlas, forming two synovial cavities. The surrounding ligaments and membranes of the atlantoaxial joint include the transverse ligament of the atlas, the apical ligament of the dens, the alar ligaments, the tectorial membrane, and the ligamenta flava between the posterior arch of the atlas and the vertebral arch of the axis. Approximately 90% of head rotation occurs at this joint, which is not only highly mobile but also connected by numerous ligaments to the occipital bone, atlas, axis, and other cervical vertebrae. When the head is subjected to sudden flexion, the kinetic energy is largely concentrated on the transverse ligament, with the dens positioned at its center, creating a "shearing" force that can lead to transverse ligament rupture. Additionally, vertical forces causing burst fractures of the atlas (Jefferson fracture) can displace the lateral masses and vertebral arches of the atlas, also resulting in transverse ligament tears. The transverse ligament attaches to the anterior aspects of the lateral masses of the atlas and, together with its anterior arch, forms a fibro-osseous structure that encircles and restricts excessive movement of the dens, maintaining atlantoaxial stability. When the transverse ligament is injured or ruptured, atlantoaxial dislocation or subluxation can occur. This is a severe injury, often accompanied by spinal cord injury, which can be immediately fatal.

bubble_chart Clinical Manifestations

The clinical manifestations mainly depend on the severity of the transverse ligament injury, the degree of atlas anterior dislocation, and whether spinal cord compression occurs. Local symptoms primarily include suboccipital and occipitocervical pain, as well as restricted movement. If combined with spinal cord injury, three scenarios may occur: (1) If the respiratory center is affected, it can be fatal at the injury site; (2) Transient neurological symptoms may appear after the injury, manifesting as brief limb paralysis or weakness, but recovery can be rapid and even complete; (3) Quadriplegia, severe urinary incontinence, and respiratory dysfunction, which is the most severe condition. Without timely and effective treatment, atlas dislocation worsens, and spinal cord compression intensifies; (4) Delayed neurological symptoms. The injury does not manifest immediately or in the early stages but gradually appears with increased head and neck movement. The typical clinical presentation of atlantoaxial dislocation is head and neck tilting. In cases of unilateral anterior displacement, the head tilts away from the affected side toward the healthy side, accompanied by neck pain, stiffness, and greater occipital neuralgia. Spinal cord compression symptoms and signs are rare. Sometimes, even minor trauma can cause rotational displacement of the atlantoaxial joint, where the head remains in a rotated position, replacing the normal motion of the atlas over the axis, with only minimal movement possible between the two.

bubble_chart Diagnosis

A clear history of trauma can help differentiate from subluxation caused by inflammation. To exclude injuries in other areas of the upper cervical spine, X-ray imaging is necessary. The main characteristic feature on an open-mouth X-ray is asymmetry in the spacing between the odontoid process of the axis and the two lateral masses of the atlas. However, poor cooperation during open-mouth imaging may cause projection deviation, leading to abnormal spacing or unsatisfactory visualization of the anatomical structures in this region. If necessary, multiple images should be taken to rule out misdiagnosis due to improper projection positioning. A lateral X-ray can clearly show changes in the distance between the odontoid process and the posterior arch of the atlas. Normally, this distance should be within 3mm. If needed, a CT scan can help differentiate from fractures of the atlas arch and upper cervical spine deformities. Severe chronic subluxation should also be noted, presenting as torticollis, limited movement, pain during neck motion, and potential facial asymmetry. Torticollis may lead to spasms in the contralateral sternocleidomastoid muscle. Additionally, the transverse ligament is a soft tissue structure that does not appear on standard X-rays, so its injury must be assessed indirectly. The distance between the midpoint of the posterior edge of the anterior tubercle of the atlas and the odontoid process (ADI) is particularly useful. (1) Increased atlanto-dental interval: On a lateral view, the distance between the posterior edge of the anterior arch of the atlas and the corresponding point on the odontoid process is normally 3mm in adults and 4mm in children. In adults, an ADI of 3–5mm often indicates a transverse ligament tear. An ADI of 5–10mm suggests a transverse ligament rupture with partial auxiliary ligament tears, while an ADI of 10–12mm confirms complete ligament rupture. (2) Functional lateral flexion-extension radiographs of the occipitocervical region: A "V"-shaped gap between the anterior arch of the atlas and the odontoid process during flexion indicates partial tearing of the transverse ligament fibers, allowing the atlas and axis to pivot on the remaining intact fibers, resulting in a V-shaped separation of the atlanto-dental interval. (3) Functional lateral flexion-extension radiographs of the occipitocervical region showing signs of anterior-posterior instability confirm ligament injury.

bubble_chart Treatment Measures

The treatment method primarily depends on whether the transverse ligament is partially torn or completely ruptured. In cases of partial tearing, skull traction or occipitomandibular traction is typically employed, with a weight of 1–3 kg. After 3 weeks of traction, a head-neck-thorax plaster cast is applied. For diagnosed transverse ligament ruptures, most scholars believe that non-surgical treatment cannot restore stability and advocate for early surgical intervention. Delaying treatment may hinder reduction. The goal of surgery is to achieve reduction and restore the anatomical stability of the atlantoaxial joint. Typically, atlantoaxial fixation is performed under skull traction, with the Gallie method being the primary approach. This method involves posterior fixation of the atlas posterior arch to the axis spinous process using wire and bone grafting for fusion. The Brook method involves passing wires around both sides of the atlas posterior arch and beneath the axis lamina, with bone grafts placed on each side and secured with wires. In recent years, many modified techniques have been adopted, and transoral atlantoaxial joint fusion has been reported. A tracheotomy is performed, followed by a transoral or transnasal incision, with a rubber strip passed from the nostril through the oral cavity to firmly secure the uvula. A longitudinal incision is made in the posterior pharyngeal wall to reach the anterior arch tubercle of the atlas, which, along with the odontoid process, is removed using a power drill. The articular cartilage on both sides is excised, and autologous iliac bone grafts are placed on the anterior surface of the axis body and the anterior arch of the atlas to promote bony fusion between the atlas and axis. This procedure is technically challenging, and postoperative infection prevention is difficult. Treatment for atlantoaxial subluxation is relatively easier and includes traction reduction and fixation. Some cases may reduce spontaneously within days without any treatment. Typically, occipitomandibular Glisson traction is applied in a neutral position, with the traction weight adjusted based on age—2.5–3 kg for adults and 1.5–2 kg for children. Radiographs are taken during traction to monitor reduction, and adjustments to the traction weight and direction are made accordingly. Reduction is usually achieved within 2–3 days, followed by 2 weeks of maintained traction and immobilization with a head-neck-thorax plaster cast or cervical brace. For refractory or chronic subluxation, skull traction may be used, and atlantoaxial fusion can be considered after reduction.