Atlantoaxial Instability

Etiology

The atlas is a ring-shaped structure devoid of a vertebral body, consisting of anterior and posterior arches bridged by two lateral masses.[1] The lateral masses of C1 consist of superior and inferior articulating facets. The superior facets are medially angled, paired structures that articulate with the occipital condyles. The inferior facets articulate with the axis. Immediately posterior to the anterior arch of C1 is the odontoid process of C2. This configuration allows for a significant degree of flexion, extension, and rotation at the craniocervical junction. Several ligaments bridging the occipitoatlantoaxial complex produce the stability required to prevent devastating neurologic injury, the most important of which is the cruciate ligament.[1]

The cruciate ligament is composed of a transverse axial ligament that runs horizontally between the lateral masses of C1 (considered the strongest ligamentous attachment in the cervical spine), posterior to the odontoid process and a vertical ligament extending from the basion to the C2 vertebral body. Additional stability is provided by paired alar ligaments running from the tip of the odontoid process to the foramen magnum and the apical ligament connecting the tip of the odontoid process to the basion. Finally, the anterior longitudinal ligament, the rostral extension of the posterior longitudinal ligament (tectorial membrane), facet joint capsules, and the posterior myoligamentous complex complete the cohesive occipitoatlantoaxial junction. The list of etiologies associated with atlantoaxial instability includes the following

• Traumatic
• Congenital: Down syndrome, skeletal dysplasia, congenital osseous abnormalities
• Inflammatory: rheumatoid arthritis.[5]

Epidemiology

In individuals without any predisposing factors, atlantoaxial instability is extremely rare. Radiographic atlantoaxial instability is seen in up to 30% of patients with Down syndrome (DS), but only 1% of patients with DS have symptomatic atlantoaxial instability. Patients with rheumatoid arthritis (RA) are also susceptible to cervical spinal instability and atlantoaxial instability. Ranges of 25% to 80% have been seen, and newer disease-modifying RA drugs may change the course of the disease and decrease the incidence of atlantoaxial instability. Atlantoaxial instability can occur equally in both genders and present at any age. A high risk of atlantoaxial instability is present in patients with Down syndrome and older patients with rheumatoid arthritis.[6]

Pathophysiology

Atlantoaxial instability can be classified into three general categories: inflammatory, congenital, and traumatic.[5] RA is the most common inflammatory disease affecting the craniovertebral junction. This is due to a large number of synovial joints and ligaments contributing to the overall stability of the upper cervical spine. The chronic inflammation of RA leads to laxity and stretching of the transverse ligament, development of granulation tissue, and erosion of the bony structures, resulting in atlantoaxial subluxation and instability. 

Several congenital disorders are associated with atlantoaxial instability. The most well-known of these is DS. Patients with DS are more prone to atlantoaxial instability due to ligamentous laxity and osseous abnormalities. Os odontoideum is the separation of the odontoid process from the body of C2. Previously it was thought to be a congenital lesion, but it is more likely due to a traumatic insult early in life. Patients with os odontoideum require dynamic studies to demonstrate the degree of movement and canal compromise accurately. Trauma as the sole cause of atlantoaxial instability is a unique entity, usually resulting from a disruption of the transverse, alar, or apical ligaments. This type of injury is commonly associated with head trauma. Fractures of C1 or C2 also are traumatic causes of atlantoaxial instability.

History and Physical

Patients presenting with atlantoaxial instability can suffer from a spectrum of clinical signs and symptoms, although most are asymptomatic.[7] These include: 

• Neck pain
• Restricted neck movements
• Pyramidal signs and myelopathy
• Lower cranial nerve palsies
• Respiratory failure
• Vertebral artery dissection
• Quadriplegia
• Death[5]

Atlantoaxial instability is seldom discovered incidentally during radiologic evaluation for acute neck injuries. Clinicians must obtain a complete history to guide adequate management. The history should include a review of any current or past neck trauma, head injury, or fall. This is especially crucial when evaluating children, where prior spinal trauma may result in an improperly healed odontoid process injury leading to mechanical instability and/or neurological symptoms in later years.

Evaluation

The alignment of facets on lateral imaging with the head kept in neutral positioning is the mainstay of dichotomizing the variants of AAI. The National Emergency X-Radiography Utilization Study (NEXUS) study group advocates an open-mouth odontoid, anterior-posterior, and cross-table lateral view. Computed tomography (CT) has higher diagnostic specificity.[5]

Criteria for AAI

• Atlanto-dens interval of more than 5 mm
• Overriding of the anterior arch of the atlas over the odontoid
• Space available for the cord (SAC) of less than 13 mm
• Violation of the Steel’s rule of thirds (one-third cord, one-third odontoid, and one-third safe space)
• Translation of the tip of the odontoid of more than 4 mm of the basion.[8]

Types of AAI

• Greenberg —reducible and irreducible
• Fielding and Hawkins—anterior, posterior, lateral, and rotational.
• Wang classification system- instability (type I), reducible dislocation (type II), irreducible dislocation (type III), and bony dislocations (type IV).[5]

Types of AAI depending on the alignment of facets on lateral imaging with the head kept in neutral positioning:

• Type 1: the facet of the atlas is dislocated anterior to that of the axis
• Type 2: the facet of the atlas is dislocated posterior to that of the axis with rotatory atlantoaxial dislocation
• Type 3: though the facets are aligned, instability is confirmed by clinical and specific radiological cues.[1]

Types 2 and 3 are chronic and ‘central’ or ‘axial’ atlantoaxial instability.[1] 

Treatment / Management

Non-operative Treatment

This includes the application of cervical halter traction with active range of motion exercises. This is followed by ambulatory immobilization with active range-of-motion exercises.[5]

Indications for Surgery

For Adults

1. ADI of more than 5 mm
2. Moderate displacement or instability in dynamic films[5][8]
(A1)

For Children (when one or more of the following is present)

1. Neurological deficits
2. Persistent ADI of more than 4 mm
3. Persistent deformity of more than three months
4. Recurrent deformity despite six weeks of immobilization[5][8]
(A1)

Brief skeletal traction (starting with 7 to 8% of body weight and increasing to 7 kg) under general anesthesia is initially advised.[5] Decompression via the posterior approach alone is advocated if it is reducible, but the anterior approach is recommended in case the reduction is not possible, which includes:

• Transoral odontoidectomy
• Transoral anterior release and
• Transoral anterior reduction plate[5]

These have shown good anatomical reduction as well as improved neurological outcomes.[8](A1)

Methods of Posterior Arthrodesis

• Gallie or Sonntag or Brook’s methods

These procedures used wiring alongside bone grafts—however, these required prolonged postoperative immobilization alongside a high risk of complications and nonunion. There are also high graft site complications.[8](A1)

• Transarticular C1-2 (Magerl), C1 lateral mass to C2 pedicle screw (Goel-Harms), and C1 pedicle to C2 pedicle implants

They can be applied to children as young as 1.7 years.[9][10][11] These are solid constructs and have high fusion rates.[12] They also have shown good global as well as patient-reported outcomes.[13] They do not require structural grafts. Goel Harms construct can achieve joint reduction after placement of the screw, and there is minimal risk of injury to the vertebral artery compared Magerl technique. The bleeding from the venous plexus while exposing C1 lateral mass is a major concern while operating on children.(B2)

The screws are most often placed via Pedicle drill-guided, fluoroscopy-assisted methods.[14][15] An accurate screw positioning and avoidance of any vertebral artery injury can now be prevented by spinal navigation.[16] Meta-analysis has confirmed that all constructs provided significant stabilization in all axes of rotation, except for the C1 lateral mass and C2 translaminar (C1LM-C2TL) construct in lateral bending.[17](A1)

Occipitocervical fusion is indicated for: 

• Occipitocervical instability
• Concomitant basilar invagination,
• An osseous anomaly of C1, and
• Failed previous atlantoaxial fusions - occipitocervical fixation restricts head movements[8][18]
(A1)

Other long-term complications of the posterior fusion include:

• Graft subsidence
• Instability of the subaxial spine- cervical lordosis may increase with 1 degree per fused level per year till adulthood. Subaxial kyphosis usually corrects itself via Toyama remodeling.[8]
(A1)

Differential Diagnosis

• Torticollis
• Atlantoaxial rotary fixation, and
• Odontoid fractures without atlantoaxial dislocation[5]

Staging

A four-stage classification scheme based on rotatory displacement has been developed:

• Type I - Simple rotatory displacement with an intact transverse ligament
• Type II - Anterior displacement of C1 on C2 of 3 to 5 mm with one lateral mass serving as a pivot point and a deficiency of the transverse ligament
• Type III - Anterior displacement exceeding 5 mm
• Type IV - Posterior displacement of C1 on C2

Type III and type IV are highly unstable, and emergent treatment is recommended.[19]

The Wang classification is the most useful system that aids in dichotomizing the patients based on their imaging characteristics and their patterns of reducibility following traction. The instability (type I) and reducible dislocation (type II) variants can be managed with the posterior-only approach. However, the irreducible dislocation (type III) and bony dislocations (type IV) require ventral decompression before the posterior fusion.[5]

Prognosis

The prognosis in symptomatic patients who are treated is good. Early decompression and fusion can help halt and restore cord dysfunctions.[20] However, in some patients, pain is a common symptom that can be disabling. Other individuals may develop the following neurological complications:

• Restricted neck movements
• Pyramidal signs and myelopathy
• Lower cranial nerve palsies
• Respiratory failure
• Vertebral artery dissection
• Quadriplegia
• Death[5]

Single breath count <10 and single breath-hold <10 s are major determinants of morbidity and mortality in patients with congenital AAI.[8] An accurate screw positioning and avoiding vertebral artery injury can be prevented by spinal navigation compared to free-hand or fluoroscopic-guided methods.[16]

Complications

The most significant risk of atlantoaxial instability is neural compression which can present with the following complications:

• Restricted neck movements
• Pyramidal signs and myelopathy
• Lower cranial nerve palsies
• Respiratory failure
• Vertebral artery dissection
• Quadriplegia
• Death[5]

The major complications relating to the surgical approaches include:

• Anterior approach-pharyngeal wound dehiscence, CSF leak, meningitis, and need for tracheostomy.[21]
• Posterior approach- venous plexus bleed, vertebral artery injury, implants related, instability of the subaxial spine, graft subsidence, non-union, graft-site complications.[8]