Causes of Instability in the Upper Cervical Spine

Instability of the upper cervical spine can be caused by congenital, traumatic, inflammatory, or neoplastic involvement.⁸ Of the several congenital abnormalities that occur in the upper cervical spine, basilar impression (or cranial settling) is the most common. Basilar impression may be progressive and lead to cervicomedullary spinal cord compression. Congenital ligamentous laxity in the upper cervical spine (e.g., Down syndrome, Morquio syndrome) can cause instability and subluxation. Upper cervical spine involvement in patients with rheumatoid arthritis can also lead to atlantoaxial instability and cranial settling, both of which may require surgical decompression and stabilization. Trauma, however, is the most common cause of instability in the upper cervical spine. Most upper cervical spine injuries result from blows to the head (e.g., motor vehicle accidents and falls). The direction of the force vector determines the type of injury (i.e., blows to the head versus deceleration of the torso). Injuries of the occipitoatlantal junction are usually fatal and are only detected postmortem. Atlantoaxial instability, on the other hand, can result from disruption of the bony or ligamentous elements or both. Bony fractures often involve the dens. Fractures of the odontoid process of C2 are classified into three types: I, II, and III.⁹ Type II odontoid fractures are unstable and notorious for nonunion with conservative management. Published reports of nonunion rates for conservatively managed type II odontoid fractures range from 30% to 60%. Type I fractures are always stable, and type III fractures may be unstable in 10% to 15% of cases.

Craniocervical instability can result from destructive tumors as well as iatrogenically, following resection of such craniocervical tumors. Instability may depend on the extent of condylar destruction or resection in these cases.

Instability at the Craniocervical Junction

Several definitions of instability at the craniocervical junction have been proposed. Craniocervical instability can be vertical or horizontal. Vertical instability or cranial settling usually occurs from a congenital or chronic inflammatory process. Destructive arthropathy of the occipital condyles and the lateral masses of the atlas and axis leads to progressive cranial settling in rheumatoid arthritis. Horizontal (ventrodorsal) instability usually results from acute traumatic situations that cause ligamentous disruption and/or bony disruption. A rotary component may or may not be present.

Common radiologic criteria used to document instability at the craniocervical junction include (1) predental distance greater than 5 mm in a child (<8 years of age) and greater than 3 mm in adults, (2) separation of more than 7 mm of the lateral masses of the atlas on the open mouth view, (3) greater than one third of the rostral dens above the ring of the foramen magnum, and (4) “bare occipital condyles,” indicating an occipitoatlantal dislocation.

There is a paucity of reports in the literature regarding resection of the occipital condyle or the destruction of the condyle with tumors of the craniocervical junction causing instability and the necessity for stabilization in this group of patients. The recommendations to date have been to stabilize the occipitocervical junction when greater than 70% of the occipital condyle has been removed.^4,10

Management of Instability in the Upper Cervical Spine

Patients with instability of the upper cervical spine run the potential risk of fatal injury to the cervical spinal cord. Early recognition, reduction, immobilization, and stabilization are the goals of treatment.

Patients with upper cervical spine instability from congenital causes should be further evaluated for other associated congenital defects (e.g., Chiari malformation, spinal dysraphic lesions, and hydrocephalus). An MRI should be obtained to assess the soft tissue pathology. In addition, CT and plain radiographs (with or without flexion-extension views) should also be obtained when appropriate.

Patients with rheumatoid arthritis should have an MRI to ascertain the presence of inflammatory pannus and/or bony encroachment of the neural elements (Fig. 143-1). Patients who have evidence of neural compromise with occipitocervical instability should be considered for a ventral decompressive procedure before undergoing dorsal stabilization.¹¹

FIGURE 143-1 MRI of the cervical spine in a patient with rheumatoid arthritis showing atlantoaxial instability with pannus formation.

Patients with radiologic evidence of subluxation on neutral radiographs should also be evaluated with dynamic, lateral flexion-extension radiographs to assess the reducibility of the subluxation. Patients with nonreduced subluxations should undergo a trial of cervical traction to reduce the dislocation before a decision is made about the appropriate surgical treatment. Patients with chronic instability should remain in traction for 4 to 5 days with muscle relaxation before being considered for surgical treatment. Patients with a reducible subluxation, reduction achieved with flexion-extension, or with axial traction (Fig. 143-2) and without compromise of the cervicomedullary cord, may be safely stabilized by a dorsal approach.¹² In contrast, patients who have a ventral spinal cord compression with a nonreducible subluxation may require a ventral transfacial decompression before undergoing occipitocervical stabilization. Usually, patients with instability related to rheumatoid arthritis or with pannus, but without neurologic deficit, can be stabilized from a dorsal approach without undergoing an initial ventral decompression.¹³ The pannus typically resolves in 6 to 12 months after abnormal movement has been eliminated.

FIGURE 143-2 A, Plain lateral cervical spine radiograph in flexion showing ventrolisthesis of C1 on C2. B, Plain lateral cervical spine radiograph in extension showing reduction of the subluxation.

Indications for Instrumentation in Occipitoatlantal Instability

The majority of traumatic occipitoatlantal dislocations are fatal. Some patients who arrive in the emergency department are treated effectively in cervical traction followed by occipitocervical instrumentation and fusion. Chronic instability at the occipitoatlantal junction occurs with rheumatoid arthritis, with tumors, and following tumor resection.¹⁴ Chronic instability at the craniocervical junction has been described as “glacial” instability in patients with chronic instability erosion of the occipitoatlantal articulation. This instability may result in cranial settling with associated rotary subluxation. In patients presenting with cranial settling with minimal or no neurologic deficits, reduction with cervical traction may be attempted, and if successful, dorsal occipitocervical instrumentation and fusion can be performed.^15,16 In patients with significant neurologic deficits, a ventral decompression may be necessary before dorsal stabilization and fusion. Patients with rheumatoid arthritis, atlantoaxial subluxation, and associated cranial settling are candidates for occipitocervical instrumentation and fusion.

Patients who have a traumatic atlantoaxial subluxation with fractured dorsal elements of C1 and C2 may also be candidates for an occipitocervical fusion because most of the current techniques require intact dorsal elements of C1 and C2 for stabilization.