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.

The only scientific evidence for the assessment of instability at the craniocervical junction following tumor surgery comes from the biomechanical study by Shin et al.¹⁷ Based on our clinical series, the biomechanical studies available, and new concepts of instability applied to the CVJ,¹⁸ we recommend that occipitocervical stabilization and fusion be performed when 50% or more of one condyle is resected or noted to have been destroyed by the tumor. A strong argument supporting this guideline can be made when using the extreme lateral transcondylar approach¹⁸ for the resection of these tumors. This approach removes the thickest parts of the capsule of the occipitoatlantal joint, and therefore renders the joint unstable (glacial instability). Thus, patients who have the condyle resected for surgical access and patients with tumor destruction of the condyle equal to or greater than 50% (but <70%) who do not undergo stabilization and fusion may progress to a glacial instability and eventually to overt instability with severe neck pain and torticollis.

Occipitocervical Techniques

Several techniques exist for upper cervical spine stabilization, with or without instrumentation. Bony arthrodesis usually is the long-term goal of these techniques. With occipitocervical junction arthrodesis, a bony ridge must be established between the occiput and the upper cervical spine. Techniques may be divided into those that use bone alone and those that use internal fixation with bone.

In 1959, Perry and Nickel described a simple onlay graft for neck fusion for instability after severe poliomyelitis. In 1969, Newman and Sweetnam¹⁹ described the technique for occipitocervical fusion in atlantoaxial instability. Fusion was achieved by decortication and laying down strips of corticocancellous bone obtained from the iliac crest. Patients were kept in cervical traction for 6 weeks and then placed in a high plastic cervical collar until bone fusion was observed.

The combination of internal fixation with onlay bone grafting has reduced the need for postoperative traction and rigid immobilization. Pseudarthrosis rates in a series of 302 occipitocervical fusions and 98 atlantoaxial fusions have been reported to be as low as 1%.⁷

Occipitocervical Fusion with Internal Fixation

Rigid metallic implants to obtain immediate fixation with generous onlay bone graft have produced successful fusion without the need for postoperative halo-vest immobilization. Techniques with contoured rods, cables, plates, and screws have been described.

Contoured Rod and Cable Fixation

Different smooth and threaded rods have been used for internal fixation in occipitocervical fusions. We use a titanium rod, contoured similar to the “Ransford loop,” with cross-linkage at the caudal end to conform to the craniocervical angle, with the patient’s head in a neutral position (Fig. 143-3).

FIGURE 143-3 Contoured 5-mm Cotrel-Dubousset rod with cross-fixation linkage.

More recently, screw systems have become more popular and provide a more rigid fixation. Cortical screws are placed in the occiput close to the midline where the bone is thickest; this is combined with a rod and or plate system at the occiput and lateral mass and C2 pedicle screws (Figs. 143-4A and B).

FIGURE 143-4 A case of occipitocervical fusion using a contoured rod, occiptal screw fixation, and lateral mass and sublaminar wire fixation is depicted via an intraoperative photograph (A) and radiograph (B).

Lateral cervical spine radiographs are obtained after the patient is positioned prone, as described previously. The head is maintained in a neutral position. The contoured rod is placed on the back of the occiput, and sites for the bur holes are marked on either side of the occipital portion of the contoured rod bilaterally. Four bur holes are thus made in the suboccipital bone—two on either side of the midline—ensuring an adequate bridge of bone between the bur holes.

The posterior fossa dura mater is separated from the inner table of the bridge of bone between the bur holes, and a double cable is passed from one bur hole to the other. This is repeated on the opposite side of the occiput. The use of a double cable on either side of the occiput provides two cables on either side to hold down the rod. Alternatively, a single bur hole on either side of the midline, with the cable passed around the rim of the foramen magnum, can be used.

After cable placement for cranial fixation, sublaminar placement for spinal fixation is achieved by passing a double cable under the dorsal arch of C1 and the lamina of C2. When separated, a single cable is available on either side of the dorsal arch of C1 and C2. At C3 and C4, a hole may be made at the spinolaminar junction using a right-angled drill. Single cables threaded through Drummond buttons are passed from either side of the spinolaminar junction (Fig. 143-5). Alternatively, sublaminar cables can be passed with due care under the subaxial cervical spine. The contoured rod is placed in the bed, and the cables are torqued sequentially. The distal end of the rod may be cross-fixed, just caudal to the C3 or C4 spinous process, to increase the rigidity of the construct (Fig. 143-6). After decortication, bone graft is obtained through a separate incision from the dorsal iliac crest and is laid as an onlay graft of corticocancellous bone. Care must be taken during head positioning before internal fixation and fusion to obtain the best subluxation reduction with optimal head and neck position, compatible with normal function. Care must be taken to avoid dural perforation during wire passage at the suboccipital bur holes and at C1 or C2. Bevelling the edges of the bur holes and meticulous dissection of the dura mater from the inner table should prevent dural laceration. The leader wire of the single cable should be bent back on itself to present a smooth surface during sublaminar passage. Monitoring the bony surfaces carefully during torquing can help prevent overtightening of the cable. Sequential cable tightening with torque not exceeding 4 to 5 inch-pounds with titanium cables and 8 inch-pounds with stainless steel cables is appropriate in healthy adults. In patients with soft bones and in those with rheumatoid arthritis, the torque is reduced as appropriate (i.e., when cable begins to cut into the bone).

FIGURE 143-5 Operative photograph showing the spinolaminar junction wires in place. Note the use of Wisconsin wires and Drummond buttons.

FIGURE 143-6 Intraoperative photograph of a contoured rod with occipital and sublaminar cables for occipitocervical stabilization.

All patients are treated postoperatively in a Philadelphia collar for 6 to 12 weeks or until radiologic fusion is observed.

Plate Fixation

Plates and screws have been used for occipitocervical fixation; however, their limiting factor is the variable thickness of the occipital bone. Heywood et al.¹ reported that occipital bone thickness measured 9 to 16 mm in the midline and only 3 to 9 mm in the lateral suboccipital bone. No room exists between the dura mater and the cerebellum. Penetration of the inner table could lead to cerebellar injury. Therefore, it is possible that with bicortical purchase during plate fixation, especially laterally in the suboccipital bone, the tip of the screw could perforate the dura mater and the underlying cerebellum.

With the Y-plate system of plate and screws for occipitocervical fusion, the stem of the Y is laid over the midline of the occiput (Fig. 143-7). The occipital screws are 2.7 mm in diameter and 8 to 10 mm long. If subaxial fusion is necessary, transfacet screws are placed in the C1-2 facet joint (Magerl technique) and in the lateral masses of the subaxial spine.

FIGURE 143-7 Illustration of a Y plate.

Because of the risk of screws perforating the dura matter and the cerebellum, the use of the Y-plate and the lateral occipitocervical plate (see Fig. 143-4) is limited due to the need to place screws in the lateral suboccipital bone. Occipital plates with screws placed in the midline and connected to rod systems that connect to the screws placed in the C2 pedicle and lateral masses in the lower cervical spine are more rigid biomechanically than the wiring techniques.

Atlantoaxial Fusion

In 1910, Mixter and Osgood described the first atlantoaxial stabilization procedure using internal fixation. Gallie popularized wiring and bone grafting techniques for dorsal atlantoaxial arthrodesis.²⁰ A loop of wire is passed beneath the dorsal arch of the atlas from caudal to rostral. The loop of wire is then pulled caudally over the spinous process of C2. The free ends of the wire are run downward and around the spinous process of C2 and are twisted together. The dorsal surfaces of C1 and C2 are decorticated, and onlay bone graft from the iliac crest is placed to achieve bony union. This construct provides minimal rotational stability. In 1978, Brooks and Jenkins modified this technique.¹⁰ They described passing two wires on either side, underneath the dorsal arch of C1 and the lamina of C2. Two unicortical wedges of iliac crest bone are placed between C1 and C2. This improves rotational stability. A modification of the Gallie technique described by Papadopoulos et al.²¹ uses a bicortical H-graft placed between the dorsal arch of C1 and the spinous process and lamina of C2.

Preoperative evaluation and management for this technique is identical to that described with occipitocervical fusion. In the modified Gallie technique, a loop of cable is passed under the dorsal arch of C1 from caudal to rostral. The caudal edge of the dorsal arch of C1 and the rostral edge of the C2 lamina and spinous process are decorticated. A bicortical graft, notched on its caudal surface to hug the contour of the spinous process and lamina of C2, is placed between C1 and C2. The cancellous surfaces of the graft abut the decorticated areas of C1 and C2. The cable passes as with the Gallie technique. The loop is dorsal to the graft, and the free ends run ventral to the graft. This ensures that the graft does not migrate dorsally or ventrally while providing the necessary rotational stability. Further decortication and onlay bone grafting of the dorsal surfaces of C1 and C2 is carried out. Overtightening of the cable around the graft can lead to graft erosion and loosening. In the presence of dorsal subluxation of the dens, there is a risk of pulling the dens dorsally into the spinal cord during wire tightening. This risk is minimized by using a larger wedge of bone between the dorsal elements of C1 and C2. The tip of the leader wire should be doubled back to present a smooth surface during sublaminar wire passage to prevent dural laceration.

Several techniques have been described for wiring the atlas to the axis vertebra, with varying degrees of success. The disadvantage for patients undergoing wire and cable techniques alone is that in the presence of significant instability they must be placed in a halo body vest postoperatively for 3 to 4 months. The patient should undergo transfacet fixation of C1 to C2 when there is significant instability or fractured or absent dorsal elements of C1 or C2 or when a laminectomy of C1 and C2 is indicated.

Summary

Upper cervical spine stabilization can be achieved using a variety of techniques, depending on the surgical indications. Controversy still abounds regarding the use of occipitocervical fusion versus atlantoaxial fusion in the presence of atlantoaxial instability. Some surgeons continue to perform occipitocervical fusions in patients with instability at the atlantoaxial joint, arguing that the loss of 15 to 20 degrees in flexion-extension by including the occiput in the fusion is negligible. However, the 15 to 20 degrees of movement at the occipitoatlantal joint is responsible for the nodding or chin-tuck movement in the cervical spine, and the loss of this movement may be a cause of significant morbidity. We recommend that occipitocervical fusions be reserved for patients with occipitoatlantal instability or cranial settling with atlantoaxial instability. All other patients with instability at the atlantoaxial articulation alone should undergo the appropriate stabilization technique at C1-2. Rigid internal fixation techniques reduce the need for cumbersome and rigid external orthotics and increase the rate of bone fusion.

Guidelines for occipitocervical stabilization following resection of tumors at the CVJ have been based on anecdotal evidence with very little reported in the literature. Based on our clinical series²¹ and the biomechanical studies available, we recommend that occipitocervical stabilization and fusion be performed where 50% or more of one condyle is resected or noted to have been destroyed by the tumor.