Patient Selection

6.1.1 Pathophysiology

Atlantoaxial instability (AAI) or atlantoaxial subluxation (AAS) is characterized by excessive motion at the junction between the atlas (C1) and axis (C2) owing to either a bony or a ligamentous abnormality. The three patterns of AAI most commonly seen are described as flexion-extension, distraction, and rotational. AAI is defined radiographically as an atlantodental interval (ADI, the distance between the odontoid process and the posterior border of the anterior arch of the atlas) of > 3.5 mm in adults and > 5 mm in children as measured on plain lateral radiography.

Symptomatic AAI occurs when subluxation of the odontoid process or posterior arch of the atlas causes pain or compression of the spinal cord, with associated neurologic deterioration. Traumatic injuries, such as an odontoid fracture, may disrupt the function of the transverse atlantal ligament, leading to instability. Extension of infections from an upper respiratory source (Grisel’s syndrome) or from an operative procedure involving the pharynx or sinuses may lead to inflammation of the ligament and subsequent instability. Transoral resection of the odontoid process for craniocervical junction compression also disrupts the C1–2 complex and causes instability. Other syndromes, such as rheumatoid arthritis, with its effects on the synovial-lined joints of the upper cervical spine and the type II cartilage of the transverse ligament, as well as Down’s syndrome, may lead to laxity of the transverse atlantal ligament causing hypermobility and subsequent instability.

6.1.2 Epidemiology

AAI is rare in patients without predisposing factors or traumatic injury. No data exist regarding prevalence in the absence of known risk factors. In general, congenital anomalies do not become symptomatic before the third decade of life.

Several disorders are known to be associated with AAI. Among patients with Down’s syndrome, the frequency of asymptomatic AAI was estimated to be 13.1% on the basis of a study that reviewed the radiographs of 404 patients with the syndrome. This study found that 13.1% of patients with Down’s syndrome will have symptomatic AAI, and 1.5% will have neurologic symptoms stemming from this instability. The risk of AAI in Down’s syndrome actually seems to decrease with age, possibly owing to stiffening of the transverse atlantal ligament associated with the aging process. Spondyloepiphyseal dysplasia congenita (SED) is another risk factor for AAI and is associated with a 40% risk of AAI; SED tarda usually does not manifest AAI. Odontoid hypoplasia, seen in Morquio’s syndrome and metatropic dysplasia, usually results in AAI. Many surgeons recommend prophylactic stabilization once the diagnosis of odontoid hypoplasia is made.

Chondrodysplasia punctata is associated with AAI, and AAI is the primary cause of disability and death in these patients; 20% will manifest with weakness, and 20% will manifest with hyperreflexia; spinal cord compression often manifests at an early age. Rheumatoid arthritis is one of the more common diagnoses associated with AAI. The reported rate of AAI in the rheumatoid population ranges from 20 to 49%, depending on disease severity and the population studied.

There is no age, race, or sex predisposition to the development of AAI, but the pediatric population seems more prone to ligamentous laxity resulting from oropharyngeal infection (Grisel’s syndrome).

6.1.3 History and Physical Examination

The history and physical examination should focus on the risk factors previously mentioned. AAI may be manifest by subtle or unusual findings. For example, the presence of torticollis may indicate atlantoaxial rotatory fixation but not overt instability. Look for signs of nasopharyngeal infection, lymphadenopathy, or palpable tenderness overlying the cervical spinous processes. The patient’s voice may be nasal if the nasopharynx shrinks or the odontoid process translates anteriorly. Many patients complain of occipital neuralgia or headaches. Others may develop vertigo, brainstem signs, lower cranial nerve palsies, or overt myelopathy as evidence of ongoing instability. Sudek’s sign (displacement of the axis in the direction of head tilt) may be present.

6.2 Preoperative Preparation

6.2.1 Imaging Studies

Plain cervical radiographs are an initial point of evaluation. Standard views include open-mouth odontoid, anteroposterior (AP), and lateral cervical spine. In the setting of a Jefferson fracture, the combined overhang of the lateral masses of C1 over C2 on the AP image should not exceed 6.9 mm (rule of Spence). A measurement of > 6.9 mm would indicate probable rupture of the transverse atlantal ligament. An atlantoaxial distance or an ADI > 3.5 mm, as demonstrated by lateral radiographs, is diagnostic of AAI. The normal ADI in children is < 5 mm on a neutral position lateral cervical spine radiograph. The presence of prevertebral swelling, indicative of soft tissue injury, is also considered an important finding in cases of possible upper cervical spine trauma.

When the diagnosis is questionable, the use of computed tomography (CT) (axial and reconstruction views) may provide additional information regarding the stability of the atlantoaxial joint as well as continuity of the transverse atlantal ligament. Occasionally, small bone fragments may be seen at the origin of the transverse ligament indicating an avulsion of the ligament from its attachment to the inner wall of the atlas.

Additionally, three-dimensional, multiplanar reformatted images of fine cut CT data are used in preoperative planning to determine the safety and feasibility of transarticular screw placement. In a parasagittal plane, the pars interarticularis of C2 must be visualized to be sure that the vertebral artery does not loop upward and compromise the integrity of the pars interarticularis (isthmus) of C2 or the body of C1 along the proposed trajectory of the screws. If the vertebral artery does so, avoid screw fixation on the compromised side and consider an alternate means of supplemental fixation. ( ▶ Fig. 6.1) Magnetic resonance imaging (MRI) may identify joint effusions and soft tissue edema not typically seen on conventional radiographs.

6.2.2 Classification of Atlantoaxial Instability

Briefly, in the evaluation of AAI, Fielding and Hawkins suggest a four-part classification scheme. Type I is a simple rotatory displacement with an intact transverse ligament. Type II injuries involve an anterior displacement of C1 on C2 of > 3.5 mm, with one lateral mass serving as a pivot point and an obvious deficiency of the transverse ligament. Type III injuries involve > 5 mm of anterior displacement. Type IV injuries are diagnosed by identifying a posterior displacement of C1 on C2. Both type III and type IV patterns are considered biomechanically unstable.

6.2.3 Anesthetic Management

In all patients with radiographic or clinical evidence of cervical instability, careful attention must be paid to correction of rotational abnormalities and maintenance of both coronal and sagittal balance. A preoperative discussion between the surgical and anesthetic teams as to airway management and monitoring is necessary. Extension often reduces the degree of subluxation in most forms of AAI and allows such patients to be safely intubated using normal anesthetic techniques. We ask that the anesthesiologist use short-acting neuromuscular blockade and have a difficult airway cart in the room until intubation is successfully accomplished. If direct laryngoscopy is unsuccessful, the patient may be ventilated manually until the paralytic wears off. Indirect or fiberoptic laryngoscopy may be required in some patients. Invasive blood pressure monitoring is generally used because the patient’s position may elevate peak airway pressures, make blood pressure cuff readings inaccurate, and make intraoperative access to the upper extremities impossible. Intraoperative neurophysiological monitoring, if used, generally affects the choice of anesthetic agent and the extent of muscular relaxation used.

6.2.4 Operative Positioning

After intubation and adequate anesthesia have been obtained, the patient is placed in three-point cranial pin fixation using the Mayfield device. At this point, baseline neurophysiological data should be obtained if monitoring is used. The neck is kept in a neutral position as the patient is rotated prone onto chest rolls. The extremities are temporarily secured at the patient’s sides using the draw sheet, and the fluoroscopy unit is used to obtain lateral images of the craniocervical junction. Under guidance, the atlantoaxial articulation is realigned. Most commonly, a “military tuck” posture (neutral head posture, extension of the lower cervical spine, posterior translation of the occiput–C1 complex) will usually reduce AAS while optimizing the surgical exposure ( ▶ Fig. 6.2). Screw trajectory can be estimated by holding a long needle-holder or K-wire next to the patient’s neck under x-ray. Once the Mayfield is secured in position, a more careful padding of the extremities can be done, keeping them tucked at the side with either tape or sleds. We advise against using arm boards; they make it much more difficult to get close to the patient. AP fluoroscopy is used to confirm adequate alignment before skin preparation. If use of a biologic bone graft substitute is not planned, the posterior superior iliac spine should also be prepared for autograft harvest. We elevate the foot end of the table to reduce stress on the Mayfield and place the patient in a reverse Trendelenburg position so that the neck is parallel to the floor to optimize screw trajectory.

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