Craniovertebral Junction Instabilities and Surgical Fixation Techniques

Overview

Occipitocervical (OC) instability affects a number of patients every year and is associated with significant morbidity. It is due to the lack of firm articulation between the occipital bone of the skull and the cervical vertebrae and is a potentially dangerous condition that can progress to dislocation or subluxation and subsequent damage to the spinal cord, medulla, and cervical or cranial nerve roots. Instability can arise from many different disorders, including but not limited to primary conditions such as congenital anomalies, spontaneous disassociation of the atlantoaxial junction, traumatic dislocation, rheumatoid arthritis, degenerative bone disease, inflammatory or infectious lesions, neoplasms, and secondary conditions arising from cervical laminectomy, decompression, fusion, or other surgical intervention.

Patients with OC instability frequently require operative intervention to stabilize the craniovertebral junction. Foerster documented the first OC fusion in 1927, and many fusion techniques have been described since. Stabilization techniques have evolved from the use of bone graft with titanium wire and external halo orthoses to more recent techniques that have used screws, rods, and plates to obtain internal fixation without the need for external stabilizers. Internal fixation may prove to be superior, because external ring halo stabilization is not without complications and morbidity, and it may not provide adequate stabilization.

OC instability can result from various congenital, degenerative, inflammatory, infectious, neoplastic, or traumatic processes. In pediatric patients, laxity and immaturity of the craniovertebral ligaments predisposes this population to OC instability. Patients with OC instability are usually treated primarily with conservative measures, including external bracing, physical therapy, and activity modification. However, many patients will have persistent instability that frequently requires operative fixation and stabilization through OC fusion. Additionally, patients with atlantoaxial instability who are not candidates for atlantoaxial fixation or who have failed previous attempts at atlantoaxial fixation may also require OC fusion.

The evolution from OC instability to dislocation or subluxation can result in considerable damage to the spinal cord, medulla, and nerve roots. Thus in patients suffering OC instability, the rigidity and durability of surgical fixation is of supreme importance. Over time, stabilization methods have evolved from the use of semirigid techniques and titanium cable and wire constructs with external halo stabilizers to more recent rigid internal fixation techniques, as previously mentioned.

Foerster first pioneered OC fusion techniques in 1927. Hamblen expanded on Foerster’s and others’ techniques to develop a procedure that used iliac crest autograft and wire to stabilize the occipitocervical junction (OCJ). This Hamblen technique called for prolonged external stabilization with a Minerva plaster jacket for 4 to 6 weeks and a halo collar for an additional 3 to 6 months. In the early 1990s, Jain and colleagues obtained occiput–C2 stability by placing a bridge of bone posterior to the foramen magnum. This bridge, or artificial atlas, was used to mediate fusion with the C2 lamina using conventional wiring techniques.

In an effort to obtain better internal fixation, several authors developed techniques using some combination of titanium cables, wires, hooks, rods, and bone grafts. In 1993, Sonntag and Dickman described a technique that used U-shaped threaded rods to facilitate stabilization of the OCJ. The rod was attached with wires passed through burr holes in the occiput with sublaminar wires at C1 and C2. The levels fused were then decorticated, and morcellized bone was placed to assist arthrodesis. Fehlings and colleagues described a similar technique but also used bone grafts and interspinous wires to help facilitate fusion. However, these techniques offered only semirigid fixation and required postoperative external stabilization.

In the late 1990s, Faure and colleagues pioneered a technique that used hooks attached to the occiput, laminar hooks made to facilitate a lamina-to-lamina clamp, and contoured rods to facilitate the remaining stabilization. Paquis and colleagues described a similar technique in which hooks were screwed to a rod to facilitate the stabilization of the OCJ. Alternatively, they stated that screws and rods or plates could also be used to facilitate OC fusion. In 2003, Singh and colleagues studied a technique that used a precontoured titanium loop (OMI Loop, Ohio Medical Instrument Company, Cincinnati) to facilitate fusion of the OCJ. The loop construct was found to provide immediate fixation of the OCJ with high bone fusion and low failure rates. The Hartshill-Ransford loop and Luque rods with a Hartsfill rectangle also performed similar functions. These techniques were among the first that did not require postoperative stabilization.

Screws and plates or rods have become a prominent technique in OC fusion and provide further rigidity to the OCJ. Pait and colleagues described an “inside out” technique, in which a lateral mass plate was contoured to match the occipital bone and cervical lordosis. The plate was secured with flathead screws placed under the occipital bone in the epidural space with the threads protruding from burr holes and by nuts placed on the outside of the occiput. This technique led to 100% fusion in patients with rheumatoid arthritis. In addition, Vale and Cahill and colleagues described a technique using a T-shaped plate that facilitated rigid stabilization of the OCJ and which can be attached to both the occipital plate and lateral mass screws on cervical vertebrae. Several screw-and-rod systems are available today, and many of these can transition to the occiput using a plating system. These systems offer immediate rigid stabilization without the need for prolonged postoperative external stabilization.

Several biomechanical studies have been conducted to assess the stability of different fusion techniques. In 1999, Abumi and colleagues demonstrated that occipital plate, rod, and screw systems provide a high rate of fusion and sufficient correction of misalignment in the OCJ region.

The use of bone morphogenic proteins (BMPs) has been shown to improve fusion rates in the lumbar spine. Fifty-three patients in the study received BMP intraoperatively as an adjunct to allograft bone placement and instrumentation, and 45 patients had traditional iliac crest autograft. Of the BMP patients, 88% had successful fusion operations with good early fusion and no evidence of fusion failure at the most recent follow-up, whereas the autograft iliac crest group had only 73% fusion. As BMPs become more widely used, it is possible that the rate of early fusion will be increased in patients undergoing OC fusion. This could reduce the rate of fusion failure and lead to fewer reoperations and fewer delayed complications.

Regardless of the technique used to fuse the OCJ, additional precautions must be taken in the operating room to ensure successful fusion. Specifically, the surgeon must understand the optimal position of the OCJ before attempting fusion. Phillips and colleagues wrote that OC “neutral” is considered to be the most functional position of the cranium on the cervical vertebrae and further posited that “neutral position” is defined radiologically as the position in which the subject looks straight ahead during a standard lateral cervical radiograph. They used the occipital cervical angle (using the McRae line and the superior end plate of C3) and distance to estimate the position of the OCJ. In addition, Takami and colleagues stated that proper alignment of the craniovertebral junction angle could help prevent postoperative complications such as dysphagia, dyspnea, and subaxial subluxation.

Furthermore, the decision to proceed with OC fusion should be carefully considered, because the procedure often results in the loss of 10 to 15 degrees of sagittal rotation. OC fusion in the pediatric population is also complicated by the potential for limitation of future growth, long-term construct failure requiring reoperation, and the development of secondary deformities at adjacent subaxial levels. However, two independent studies with long-term postoperative follow-up showed no growth limitation in patients who underwent atlantoaxial fixation. Potential intraoperative complications include venous hemorrhage, vertebral artery injury, and dural tear. Delayed complications include wound infection, loss of reduction as a result of construct slippage, and pseudoarthrosis.

Anatomy Review

The OCJ is an anatomically and biomechanically complex region of the spine that presents unique challenges in operative management. The discussion of surgical techniques will be limited to posterior approaches for the purposes of this chapter. The posterior approach to the upper cervical spine and OCJ first begins with incision of the skin and dissection of the subcutaneous tissues in the avascular midline raphe. Once the bony structures are encountered, subperiosteal dissection is performed to expose the occipital bone, posterior rim of the foramen magnum, posterior arch of the atlas, and the spinous process, lamina, and lateral mass of the axis ( Fig. 10-1 ). With these elements exposed, it is then possible to proceed with decompression and fixation as indicated.

The bony anatomy of the OCJ is defined by the occipital bone, the atlas, and the axis. The occipital bone makes up the most posterior and inferior portions of the cranial vault. Its curved posterior surface, or squamosa, serves as the point of attachment for the semispinalis and rectus capitis muscles, which define the superior and inferior nuchal lines, respectively. The occipital bone is thickest in the midline, where a thick keel lies along the intracranial surface below the transverse sinus; this thickness is greatest at the level of the occipital protuberance (10 to 18 mm) and decreases inferiorly toward the foramen magnum (3 to 8 mm). Lateral to the midline and inferior to the external occipital protuberance, the occipital bone quickly becomes thinner, where it is 2 to 8 mm. Therefore the midline occipital keel provides the thickest surface for screw purchase in OC fixation ( Fig. 10-2 ).

The inferior surface of the occipital bone carries the occipital condyles at either side of the foramen magnum. The condyles articulate with the superior articulating facets of the atlas. Although occipital plates are commonly used for OC fixation, it is sometimes not possible or desirable to place instrumentation in the occipital bone, either because of anatomic irregularities, previous occipital surgery, or lack of surface area for bony fusion. In these cases, some authors have advocated placement of occipital condyle screws. Morphometric analysis of the occipital condyle has revealed that it is a boxlike structure situated at approximately a 20-degree medial angle from posterior to anterior. The average dimensions of the condyles have been shown in studies of cadaveric specimens and computed tomography (CT) to be approximately 10 mm in height (craniocaudal), 10 mm in width (mediolateral), and 22 mm in length (anteroposterior). Therefore, the occipital condyle can provide an additional or alternate fixation point for OC fixation ( Fig. 10-3 ).

Below the occiput, the first cervical vertebra, or atlas, articulates with the occipital condyles on either side of the foramen magnum. The posterior arch of the atlas extends posteriorly and, along with the spinous process of the axis, serves as an important anatomic landmark when performing the initial exposure. The atlantooccipital membrane is a thin layer encountered deep to the cervical musculature; it connects superiorly to the posterior rim of the foramen magnum and inferiorly to the upper border of the arch of the atlas.

Several important vascular structures are found in the vicinity of the OCJ, most notably the paired vertebral arteries. In the subaxial spine, the vertebral arteries are transmitted through the transverse foramina of the cervical vertebrae as the arteries ascend cranially. At the level of the axis, the arteries turn laterally and posteriorly before ascending to enter the transverse foramen of the atlas. After exiting the transverse foramen of the atlas, the arteries turn medially and posteriorly to travel in a groove along the superior surface of the posterior arch of the atlas, the sulcus arteriosus; this constitutes the horizontal segment of the third portion of the vertebral artery, and this sulcus can usually be identified 15 to 18 mm lateral to the midline. This horizontal portion is located posterior and slightly inferior to the atlantooccipital joint. The artery then courses medially and anteriorly to pierce the atlantooccipital membrane and enter the dura ( Fig. 10-4 ).

Several venous structures are also encountered in the OCJ. At both the atlantooccipital and atlantoaxial junctions, the epidural venous plexus is found posterior to the joint space and adjacent to the vertebral artery, and it can be the source of significant intraoperative bleeding. At the foramen magnum the marginal sinus is occasionally encountered, and farther on laterally, the emissary vein of the occipital condyle is found ( Fig. 10-5 ). The major intracranial dural sinuses must also be considered when planning OC fusions. The transverse sinuses are usually found at the level of the external occipital protuberance and course laterally along the intracranial surface of the occipital bone. Occasionally, the occipital sinus is found along the midline of the occipital bone, running from the foramen magnum to the confluence of the sinuses, and it is sometimes found within the occipital bone itself. This must be considered in OC fixation, because it can be injured and may produce bleeding during midline occipital plating.

Indications

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Occipitocervical instability ( Fig. 10-6 )

Figure 10-6

Lateral radiograph of an 88-year-old male demonstrates destruction of the C2 vertebral body from metastatic prostate cancer and anterior subluxation of the cranium.
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Atlantoaxial instability with previously failed fixation or need for cranial extension because of a high risk of fusion failure with atlantoaxial fixation

Operative Technique

Equipment

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Radiolucent operating table
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Mayfield head holder (consider radiolucent head holder)
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Intraoperative fluoroscopy or CT
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Headlamps and optical loupes
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Monopolar and bipolar electrocautery
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High-speed drill
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Straight and angled curettes
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2- to 5-mm Kerrison punches
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Instrumentation (plate, screws, wires, rods)
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Bone graft (local or distant autograft, allograft, etc.)
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Three-dimensional radiographic study (CT or magnetic resonance imaging [MRI]) to assess the anatomy of the craniovertebral junction, especially the suitability of C1, C2, and occipital bone for instrumentation and the location of the vertebral artery

Positioning

In cases of OC instability, the patient should be brought to the operating room in some form of external orthosis, rigid cervical collar, or halo–vest, depending on the degree of instability. The patient is intubated without extending the neck to avoid subluxation and subsequent neurologic injury. This can be accomplished with the patient awake or sedated and with the aid of fiberoptic laryngoscope visualization. Once the patient is intubated, a Foley catheter is placed in the bladder, and an arterial line is placed for intraoperative hemodynamic monitoring. If intraoperative neuromonitoring is to be used, the leads should be placed at this point. The patient’s head is then fixed in the Mayfield head holder, and the patient is carefully turned to the prone position. It is important during this maneuver to maintain neutral position of the OCJ to avoid causing subluxation. If the patient is in a halo-vest orthosis, the halo and vest are left in place, and the patient is carefully turned to the prone position. Once the patient has been properly positioned, the back of the vest is removed to facilitate surgery. Intraoperative neuromonitoring can detect neurologic changes during patient positioning, which could indicate spinal cord injury from abnormal cervical motion. With the patient in the prone position, the head is fixed in the Mayfield holder, and the alignment of the OCJ is confirmed with lateral fluoroscopy. It is important to achieve neutral anatomic alignment at this point to avoid improper alignment during arthrodesis. The patient is then secured to the operative table, pneumatic compression devices are placed, and the field is prepped and draped. The posterior iliac crests should be draped into the field if autograft bone is needed for the arthrodesis ( Fig. 10-7 ).