Dorsal Subaxial Cervical Instrumentation Techniques

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Chapter 147 Dorsal Subaxial Cervical Instrumentation Techniques

Subaxial cervical instability has many causes, including trauma, degenerative disease, neoplasm, and infection. Instability may also develop after spinal canal or foraminal decompression or in conjunction with tumor resection. Historically, the management of such instability first consisted of extended immobilization with traction or an orthosis to maintain proper alignment until bony and/or ligamentous healing transpired. Despite the usefulness of these treatment modalities, they predispose patients to a variety of medical complications. Furthermore, such management does not always result in long-term spinal stability. In 1891, Hadra1 described the role of spinous process wiring to treat traumatic and inflammatory cervical instability. Subsequently a multitude of cervical fusion techniques were reported that used wires secured to the spinous processes, laminae, and/or facets. Cervical wiring techniques are important in the management of cervical instability.

Sophisticated cervical instrumentation has expanded the surgical capabilities for spinal reconstruction. Cervical fixation devices are particularly useful to treat multiplanar or multisegmental instability. Rigid internal stabilization usually provides excellent neural protection until fusion occurs, lessens the number of segments that require fusion, facilitates immediate postoperative mobilization, and minimizes the need for external orthoses. Several dorsal cervical fixation devices have been developed, and each has unique advantages and disadvantages.

This chapter discusses issues pertinent to the application of dorsal cervical instrumentation, including the indications for their use and operative implantation techniques. Specifically, wire fixation, Luque L-rod and rectangle constructs, laminar compression clamps, semirigid and rigid lateral mass fixation, hook-rod instrumentation, and pedicle screw fixation are reviewed. Concordant with the overall theme of this text, complication avoidance and management are emphasized here. Although biomechanical concerns are extremely important in the selection of the proper method of stabilization, they are discussed only briefly in this chapter.

Indications for Surgery

The decision to perform surgery, the operative approach, the need for fusion, and the method by which it is accomplished must be determined on an individual basis. Factors that influence the decision-making process include the patient’s overall medical and neurologic condition, the particular pathologic process, the location of the pathology, the degree of instability, and the number of levels affected. These issues, as they pertain to trauma, neoplasia, and degenerative disease, are addressed briefly in this section.

Trauma

Trauma is a common indication for dorsal cervical stabilization.2 The primary management of cervical spine injuries consists of realignment (when necessary), decompression of the neural elements (when indicated), and stabilization. In the setting of trauma, if the spine is in good alignment and no decompression is necessary, external immobilization may be all that is required to protect the neural elements while healing occurs. This is particularly true when the major cause of the instability is bony injury. Primary ligamentous instability is much less likely to resolve after immobilization; hence early surgical stabilization is often an appropriate consideration in the management of these injuries.

Instrumentation of the dorsal cervical spine should be considered seriously in all trauma victims who require an open reduction or a dorsal cervical decompression. Persistent dorsal ligamentous instability is most appropriately treated by dorsal surgical stabilization; in fact, it is not unreasonable to offer patients with severe ligamentous injuries internal fixation as an alternative to halo immobilization. Fixation across the afflicted level only is usually successful in achieving long-term stabilization in patients with dorsal ligamentous injuries; however, consideration should be given toward incorporating additional levels into the construct in the setting of severe instability3 (Fig. 147-1). Bony cervical spinal injuries may also be stabilized by using dorsal instrumentation. In particular, cervical lateral mass instrumentation may be used in the presence of laminar and spinous process fractures that often preclude the use of many other types of dorsal fixation.

Extension instability and injuries of the ventral axial spine have been managed successfully by using multilevel dorsal fixation; however, a ventral approach is usually more appropriate. This is particularly true if the spinal canal is compromised from bone or disc fragments or when a burst fracture is associated with 25-degree or greater kyphosis.3,4

General Considerations

Imaging

A complete radiographic workup is essential to properly plan and execute any spinal stabilization procedure. This does not mean that every imaging modality must be employed in every patient. MRI is extremely important in the evaluation of cervical pathology because of its excellent multiplanar visualization of the spinal cord, nerve roots, and surrounding soft tissue. Gadolinium contrast MRI studies should be used for imaging possible tumors and infections but have limited benefit in spondylosis. Static plain radiographs provide information concerning segmental and overall alignment and bone quality and should always be obtained. Considering the widespread use of MRI and CT, preoperative radiographs should still be ordered almost routinely. Preoperative radiographs serve as standards against which alignment can be judged after prone positioning and surgery.

Dynamic studies (i.e., flexion-extension lateral views) often provide valuable information, particularly in terms of assessing stability. Although dynamic films should be obtained in most patients, they are not universally appropriate, and judgment must be exercised before obtaining flexion-extension radiographs. Specifically, flexion-extension radiographs should not be obtained in the trauma patient until the potential for significant instability has been ruled out with static films and/or scans.

CT provides better bony detail than MRI and therefore is more useful to define fractures. MRI often complements CT in the trauma setting because of its ability to define ligamentous injury.7 Both modalities are useful in assessing the extent of tumor involvement in patients with metastatic malignancies. CT myelograms should be considered in patients who are unable to have MRIs or when the MRI is equivocal, such as when a previous instrumentation artifact obscures adequate visualization. CT allows for evaluation of the transverse foramina and, by proxy, the vertebral artery. Localization of the vertebral artery is important in surgical planning for placement of screws in the cervical spine.

Bony Fusion

Fusion is always part of a cervical instrumentation procedure, and the segments to be fused should be properly prepared. Complete removal of the soft tissues and periosteum from all bone surfaces is required for fusion. The cortex should be scraped with a curet or may be eburnated with a bur. If a drill technique is used, the bur should be of cutting design rather than a diamond. Copious irrigation should be employed while drilling to prevent scorching temperatures, which may inhibit bony fusion. The facet joint is frequently the site of fusion when using dorsal instrumentation. Each facet joint is prepared for fusion by removing all cartilage and scraping or using a bur on the bony joint surfaces. If a dorsal decompression is to be performed as part of the operative procedure, the facet joint is dissected and denuded of cartilage before the laminectomy (or laminectomies) is performed. Theoretically, the longer the spinal cord is protected by the bony and ligamentous dorsal elements, the less the chance of inadvertent intraoperative trauma. Approximation of the bony articular surfaces will result in a successful arthrodesis. Frequently, the lateral mass joint space is packed with autogenous bone to facilitate fusion.

Corticocancellous bone may be obtained from the cervical laminae if a laminectomy is performed. If spinal canal decompression is not warranted, adequate bone for a facet fusion may be obtained from the cervical or upper thoracic spinous processes. Another alternative is to harvest bone from the dorsal iliac crest or a rib.11 Corticocancellous bone is placed over the dorsal elements.

Dorsal Subaxial Cervical Instrumentation Techniques

Luque Instrumentation

Stainless steel pediatric Luque L-rods and Luque rectangles (Zimmer, Warsaw, IN) may be used to stabilize the cervical spine.13 The rectangular construct provides greater torsional stability than the L-rods and is therefore preferable. These devices are not indicated for one- or two-level fixation but rather multilevel stabilization procedures. Ideally, both the rods and the rectangles are segmentally secured to every level traversed; however, this is not always necessary. Luque instrumentation can be used to bridge dorsal element defects, such as may occur with metastatic malignancies; however, when using this technique, at least two levels of segmental fixation must be obtained above and below the incompetent region. These devices are most useful for fixation extending to the upper cervical spine or crossing the cervicothoracic junction.

For insertion, the majority of the facet should be exposed at each level one wishes to instrument. It is important to choose a rod or rectangle of correct length. The device should not extend above or below the segments at which arthrodesis is desired. When the proper size is selected, the instrumentation is bent to conform to the normal cervical lordotic curve. After contouring is performed, the surgeon should verify again that the length is appropriate. Luque instrumentation can be secured by using wires or braided cables. Cable is stronger and easier to work with than wire. An effort should be made to obtain segmental fixation at every level to undergo arthrodesis.

Laminar, facet, or spinous process purchase may be used. Cervical sublaminar cables are relatively easy to pass, but their use is associated with risk of neurologic injury. Sublaminar wires should be passed with trepidation in the region of the cervical enlargement of the cord; therefore, sublaminar fixation is often limited to the upper cervical segments (C1, C2), C7, and the upper thoracic spine. Spinal canal stenosis is an absolute contraindication to the use of sublaminar wires and cables. Safe passage of sublaminar wires requires opening the ligamentum flavum and directly visualizing the dura mater.

Sublaminar wires and cables should be passed very carefully by using two hands to push and pull simultaneously. When the wires or cables are passed, they should be held taut with heavy clamps hung over the side of the wound. These maneuvers minimize the risk of ventral displacement of the wire or cable. All wires and cables should be passed without the Luque rods or rectangle in the wound. Some epidural bleeding may occur with the dissection and passage of sublaminar wires and cables, but this often stops as the cables or wires are tightened. If the epidural bleeding persists, hemostatic agents such as thrombin-soaked Gelfoam should be employed.

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