Clinical Instability

The most frequent indication for cervical instrumentation is instability. To paraphrase an oft-quoted general definition, instability requires the loss of spinal biomechanical integrity such that the spine is unable to prevent initial or additional neurologic deficit, major deformity, or incapacitating pain under physiologic loads.³ The precise definition of spinal instability is difficult to establish and may vary according to the specific clinical setting.

In practice, it is essential to precisely determine the nature and extent of spinal instability. The nature of instability refers to the status of specific structures that normally confer stability on each motion segment in the cervical region. This concern addresses the competency of the ligamentous structures, bony elements, and anulus fibrosus of the intervertebral disc. Identification of the incompetent elements allows the severity of segmental spinal instability to be estimated. The extent of instability denotes the number of unstable motion segments, as well as whether the instability is predominantly ventral, dorsal, or both. Defining these concepts precisely is of fundamental importance and affects the decision to instrument the spine and also dictates the selection of an appropriate construct.

The etiology of spinal instability is important. Symptomatic cervical instability may result from trauma, degenerative disease, neoplasia, or infection. Iatrogenic instability may also occur, particularly after cervical laminectomy for spondylotic disease. Construct design is influenced by the nature of the disease process that produced the instability. The long-term structural demands placed on a construct are often determined by the progression or remittance of the underlying disease. Posttraumatic instability may demand the least of a construct. Short-term immobilization is often all that is required to promote adequate healing. After the injury heals, the load-bearing and load-sharing properties of the construct are no longer required to maintain stability. Spondylotic and iatrogenic instability may require more from a construct, owing to the slowly progressive nature of the process. Instability arising from spinal neoplasia often mandates long-term instrumentation to maintain structural integrity. Bone fusion may not be attainable, because of the rapid progression of disease. In these situations, the instrumented construct must be designed to bear physiologic loads for the remainder of the patient’s life.

Maintenance of Alignment

Cervical constructs are often required to maintain spinal alignment. Internal fixation may be indicated to prevent deformity or to preserve normal alignment after reduction. Unlike thoracolumbar instrumentation, cervical constructs are generally applied in the neutral mode. Thus, deformity reduction is essential before stabilization. This is often accomplished by applying axial skeletal traction. Many constructs designed for use in the thoracolumbar spine can apply significant compressive, distractive, translatory, and rotatory forces to a region of spinal deformity, thus affecting reduction. As a rule, most cervical instrumentation systems cannot apply the magnitude of force required to reduce a deformity. Internal cervical fixation may be used to maintain reduction, but application of reductive forces with these constructs should be avoided in most instances.

Prevention of spinal deformity may also be accomplished by the timely use of internal fixation. Progressive kyphosis or spondylolisthesis may result from spinal decompression procedures. If individuals at risk for this complication are identified preoperatively, cervical deformity may be preventable. Patients exhibiting a loss of the normal cervical lordotic configuration are prone to develop postlaminectomy kyphosis.⁴ This complication may be avoided by proper internal stabilization at the time of decompression. Similarly, operative resections that compromise principal load-bearing elements may render the spine incompetent to withstand physiologic loads. Progressive postoperative deformity may be prevented by spinal reconstruction, using bone graft and instrumentation to reconstitute the axial spine.

Pain Management

Spinal stabilization may be indicated to relieve incapacitating pain by reducing motion between spinal segments. This concept has been applied more extensively in the lumbar spine, particularly for treatment of mechanical low back pain arising from spondylolysis and subsequent degenerative spondylolisthesis. Fusion of the cervical spine purely for amelioration of axial pain may benefit selected patients greatly. Such a procedure should be carefully considered and only be performed after conservative treatment measures have failed.

General Considerations

In most cases cervical constructs are used to maintain clinical stability. This may be accomplished most efficiently by matching the implant with the major site of instability. That is, if the instability is primarily dorsal in location, a dorsal construct should be considered for stabilization. Similarly, ventral instability, created by incompetence of the anterior longitudinal ligament (ALL), vertebral body, or intervertebral disc complex, is most effectively treated by the application of a ventral construct. It is unreasonable to expect that a construct will function with optimal stability when implanted in a biomechanically disadvantageous position.

Internal fixation systems provide immediate postoperative stability to the instrumented region but do not provide long-term stability due to the “plastic” properties of bone at the implant-bone interface. As with most biologic materials, bone deforms and reforms in response to stress.⁵ Eventually, even the most rigid construct allows a small degree of motion. Repetitive loading gradually increases the amount of movement and can ultimately lead to implant failure, unless bony fusion occurs. The long-term stability of all constructs thus depends on osseous fusion. No internal fixation system currently available can compensate for a poorly designed bone graft.⁶

Cervical spine implants may be considered as rigid, semirigid, or dynamic.⁵ Rigid implants attempt to achieve complete immobilization of the instrumented motion segments. Ventral plate systems, with locking screws and dorsal screw-rod and hook-rod systems, provide rigid fixation. Luque rods and rectangles (Zimmer, Warsaw, IN), secured with segmental sublaminar or facet wires, and most lateral mass plate devices are examples of semirigid cervical implants. Rigid immobilization may be potentially detrimental to bone fusion because of stress shielding and stress-reduction osteopenia.^5,7 This concern has led to the development of dynamic instrumentation, such as nonfixed moment arm cantilever beam screw-plate implants and axially dynamic ventral fixators.⁸

Modes of Application

The modes of application available for cervical constructs are more limited than those available for use in other spinal regions. Thoracolumbar implants may be placed in distraction, compression, neutral, translation, flexion, extension, and lateral-bending modes. In contrast, cervical spine constructs are generally applied in the neutral mode. This is not universally true, because certain cervical plate systems and wire constructs may provide a modest degree of compression. Theoretically, cervical rod-hook devices can be placed in the compression or distraction modes as well. However, the vast majority of cervical constructs in clinical use are applied in the neutral mode at the time of surgery. Biomechanical conditions change as the spine is loaded after surgery. Most “neutral” implants must resist axial compression when the upright posture is assumed. These constructs then function in a distraction mode.⁵

Cervical construct designs are also more limited in their mechanism of load bearing than their thoracolumbar counterparts. Generally, cervical constructs conform to one of five fundamental load-bearing types: (1) distraction fixation, (2) tension band fixation, (3) three-point bending, (4) fixed moment arm cantilever beam, and (5) nonfixed moment arm cantilever beam fixation.² Applied moment arm cantilever beam fixation, a technique occasionally applied in the thoracolumbar spine, is not used in the cervical spine. Assigning an implant to one of these fundamental load-bearing types is somewhat artificial, because a given construct may exhibit features of several mechanical types. However, it permits classification of implants by their principal biomechanical attributes.

Construct Materials

A variety of biologic and prosthetic materials have been used for cervical spine stabilization. Most constructs are composed of a bone graft, coupled with a metal prosthesis. Occasionally, bone and/or metal components may be supplemented or replaced by methyl methacrylate.

Construct Application

Cervical spine integrity may be restored by either ventral or dorsal stabilization techniques. The application of both may be indicated in cases of severe instability creating a “360-degree” construct. The rationale for selecting one approach over another is case dependent and relies on the degree and extent of instability. If the site of major instability is ventral, a ventral construct should be created to restore structural integrity to the ventral spine. Dorsal instability is treated most effectively through dorsal stabilization. This general rule is valid for all causes of cervical instability. The underlying disease process does influence the selection of specific construct components and the method by which they are applied.

Neural compression often accompanies cervical instability and must be alleviated before stabilization. Neurologic deficit may result from direct neural compression by the disease process itself or by attendant spinal instability. Decompressive procedures may exacerbate segmental instability as a result of key load-bearing structures. This is particularly important when disease involvement is extensive. The underlying pathology may predispose to postoperative instability by rendering other load-bearing elements incompetent.

The requirements of neural element decompression influence the approach that is selected for stabilization. Generally, ventral compressive or invasive pathology should be dealt with via a ventral approach. If dorsal neural compression is encountered, a dorsal decompressive procedure is indicated. Internal fixation techniques should attempt to restore the structural integrity of the elements made incompetent by the disease process or surgical resection.

The surgeon must be wary and avoid exacerbation of neural compromise by the process of spinal stabilization. For example, dorsal tension band fixation may increase ventral neural compression resulting from traumatic intervertebral disc herniation or neoplastic disease. This may produce additional neurologic deficit. Constructs must be designed with consideration for the structural alterations that they may induce and the effect that this may have on the neural elements. If this is not appreciated, disastrous consequences may follow.

Interbody Strut Graft

By definition, a simple interbody strut graft implanted after a ventral cervical discectomy constitutes a ventral cervical construct. Larger grafts are often used for vertebral body replacement after corpectomy for trauma, neoplasia, and spondylotic disease. Ventral strut grafts function predominantly in the simple distraction mode, reconstituting the ventral load-bearing column of the cervical spine. This construct offers excellent resistance to axial compressive loads (Fig. 140-1). It also imparts some stability in flexion, extension, axial rotation, and lateral bending.¹³ In most cases, however, immediate postoperative stability is not provided with a simple strut graft.

FIGURE 140-1 Coronal (A) and lateral (B) views of an osseous strut graft. This construct functions in a simple distraction mode (black arrows), providing resistance to axial compression (white arrows).

Some means of fixation, whether external or internal, is usually required to provide temporary stability until osseous fusion occurs. The extent of supplemental fixation is dictated by the degree of instability that remains after placement of the bone graft. The instability created by a single-level ventral cervical discectomy may be managed adequately with interbody strut graft placement and immobilization in a cervical collar. More significant instability requires more rigid fixation while the fusion matures. This may be accomplished internally with instrumentation or externally with an orthosis. In the setting of multilevel corpectomy, some studies suggest that ventral corpectomy with instrumentation be supplemented with dorsal instrumentation to prevent postoperative graft and instrumentation complications.^17,18 However, this is a point of controversy; others have advocated ventral cervical fixation as sufficient for multilevel corpectomy up to four levels.¹⁹

Ventral Cervical Plate and Screw Constructs

Ventral cervical plate and screw constructs were developed to provide immediate internal stability before osseous integration of a strut graft. When used in this context, these devices often eliminate the need for postoperative external bracing. All ventral plate constructs reconstitute the ventral tension band, thereby providing the most stabilization in extension.²⁰ Some of these devices also provide fixed moment arm cantilever beam fixation, thereby sharing some of the axial load with the strut graft. Rigid implants with fixed-angle locking screw mechanisms function in this capacity. Plating systems that use variable-angle screws or translationally mobile screws are more dynamic implants and provide less axial load sharing. These devices act as nonfixed moment arm cantilever beam fixators in addition to their tension band attributes (Fig. 140-2). Dynamic implants theoretically allow for the graft to be exposed to continuous axial loading, which may facilitate bone fusion.⁸

FIGURE 140-2 Coronal (A) and lateral (B) views of a ventral cervical plate (bicortical, unlocked) construct. The plate-screw device reconstitutes the ventral tension band (black arrows), thereby resisting ventral distraction and extension (white arrows). Axial compressive forces (not shown) are resisted by the strut graft, as depicted in Figure 140-1.

Biomechanical studies have demonstrated that ventral plates can restore stability to the injured spine in essentially all motion planes, although this is most significant in flexion and extension.^13,21 An interbody bone graft must supplement the instrumentation to effectively stabilize an injured motion segment. The load-bearing capacities of ventral cervical plates are temporary, so all plated segments must be fused to achieve long-term stability.

Ventral cervical plates are affixed with screws at the implant-bone junction. Some devices use screws with bicortical purchase, whereas others use unicortical fixation. Bicortical screw purchase confers greater holding strength to the construct.¹¹ Placement of bicortical screws is slightly more perilous than implantation of unicortical screws, because the dorsal cortex of the vertebral body must be drilled to accept these screws. To avoid drilling into the spinal canal and traumatizing the spinal cord, bicortical screw placement must be monitored using fluoroscopy.²²

Unicortical screws may be applied with less hazardous results. Fluoroscopy is not mandatory, because the dorsal cortex is not violated. Imaging, however, is recommended as a confirmatory intraoperative study.

The indications for ventral cervical plating are extensive. Traumatic lesions that produce persistent instability may require surgical stabilization. Unstable injuries involving the vertebral body or intervertebral disc are managed most efficiently by ventral stabilization. This is particularly important when the ventral spinal canal is compromised by bone fragments or herniated intervertebral disc material. Cervical burst fractures may require ventral decompression and internal fixation. A strut graft for vertebral body replacement and a ventral plate for immediate internal stability are appropriate construct designs for this indication. Ventral plates should be applied to intact vertebral bodies above and below the involved levels, spanning the instability.

Other traumatic lesions may be stabilized ventrally. Irreducible facet dislocations are generally approached dorsally. However, when facet dislocation is complicated by concomitant disc herniation, decompression and reduction may be undertaken via a ventral approach. Stabilization is then accomplished with an interbody bone graft and a ventral plate. Neural decompression must precede reduction of the spinal deformity, thereby minimizing the risk of producing or exacerbating a neurologic deficit.

Spinal neoplasms often involve the vertebral body, potentially causing spinal instability and neurologic dysfunction. Ventral cervical plates may be applied after decompression to reconstruct the axial spine. In these instances, screw fixation must be performed in vertebrae that are free of disease.

Cervical spondylotic disease may also be treated by ventral decompression and stabilization, using ventral plates. Kyphotic deformities almost always require a ventral approach. Discectomy(ies) and/or corpectomy(ies), grafting, and ventral plate stabilization usually forms the first-line therapy for this type of spinal deformity.

Ventral cervical plate systems are extremely versatile. They provide substantial immediate postoperative stability, limit the extent of instrumentation, and facilitate aggressive reconstruction of the ventral spine. They enhance fusion rates and permit early patient mobilization. As with all devices that use screw fixation, the performance of ventral plating systems is Y-fixation. Supplemental fixation or additional bracing may be necessary in the setting of osteoporotic bone.

Cervical cages are another means of stabilizing the anterior column. These devices may be made of titanium, carbon, or other materials. Threaded cages appear to provide greater initial stiffness than the nonthreaded devices.^23–25 The results from acute biomechanical testing may be misleading since subsequent subsidence may lead to a subsequent decrease in stiffness.

Wire Constructs

Dorsal stabilization with wire or braided cables usually entails incorporation of the spinous processes or articular facets, with or without bone autograft. These constructs function primarily by reconstituting the dorsal tension band (Fig. 140-3). Dorsal wire constructs provide some stability in flexion, minimal stability in extension, and add little to rotatory or translatory stability.²⁶ If translational instability exists, dorsal tension band fixation implants may be inadequate to prevent the “parallelogram effect.”³ This may result in translatory displacement and further spinal deformity. Wiring alone does not provide sufficient immediate internal stability in most cases. It must be supplemented with bone graft, methyl methacrylate, or external bracing to augment the construct until bony fusion occurs. Still, this is an inexpensive, rapid, and relatively safe method to reconstitute the dorsal tension band, particularly in cases of isolated dorsal ligamentous injury.

FIGURE 140-3 Coronal (A) and lateral (B) views of interspinous cable fixation. This construct reconstitutes the dorsal tension band (black arrows) and resists dorsal distraction (white arrows). As flexion and dorsal distraction are coupled the flexion moment (curved arrow) is resisted across the fixed level.

Wire constructs may be created with single-strand wire, twisted wire, or braided cables. The latter have the advantage of higher tensile strength, relatively uniform distribution of applied tension, and ease of application. Braided titanium alloy cables are available, and these produce less CT or MRI artifact. Titanium cables are more expensive than wire, although the aforementioned advantages may justify the added cost in many situations.

Interlaminar Clamps

The dorsal tension band may also be re-created by application of interlaminar clamps. These devices are used rarely because they are somewhat unwieldy to apply and may be hazardous.⁵ These clamps function by reconstituting the dorsal tension band and may be adequate to restrict flexion. No stability is provided in extension or axial rotation. Extension is prevented by placing the bone graft between the spinous processes. Interlaminar clamps require intact laminae at the levels to be instrumented. They are also prone to experience the parallelogram effect if translatory instability is present.

Luque L-Rods and Rectangles

Originally used for thoracolumbar instability, Luque L-rods and rectangles may also be incorporated into dorsal cervical constructs. These devices are usually applied over multiple spinal segments and are secured with sublaminar or facet wires. Alternatively, braided cables may be used to affix the construct at the implant-bone junction. They act principally as semirigid implants, reconstructing the dorsal tension band. Additionally, they provide a significant degree of three-point bending fixation (Fig. 140-4). These implants stabilize in flexion, extension, and lateral bending modes.

FIGURE 140-4 Coronal (A) and lateral (B) views of a Luque rectangle construct. This device provides three-point bending fixation. Dorsally directed forces are applied at both ends of the construct, with an equal but opposite force applied at the fulcrum (straight arrows). Torsional stability (curved arrows) is imparted by the strong cross-fixation of the rectangle. Distraction and flexion are also resisted, as this device reconstructs the dorsal tension band. Although the illustration depicts fixation with sublaminar cables, the authors do not recommend sublaminar cable passage in the midcervical spine.

The use of a rectangle rather than two L-rods is biomechanically advantageous because of the strong cross-fixation provided by the rectangle configuration. Torsional stability is enhanced by this design, and “telescoping” is less likely. These concerns may be partially alleviated by cross-fixation of L-rod constructs.

Luque L-rods and rectangles have been used for the treatment of instability from spinal neoplasia. After dorsal neural decompression, a Luque construct may be applied if instability or progressive spinal deformity is anticipated. Additionally, this type of construct may be advantageous in patients with poor bone quality because the construct has an extremely low profile, which may be important in thin patients. It is fairly simple to apply, inexpensive, and requires no special instrumentation to install. However, a Luque construct must often incorporate several motion segments to achieve an adequate biomechanical advantage, thus creating a long fusion. The risk of passing sublaminar wires in the subaxial cervical spine should not be underestimated, particularly if the canal diameter has been further compromised by spondylotic disease, traumatic injury, or neoplasm. In such cases, fixation to the spinous processes, facets, or lateral masses is preferable.

Lateral Mass Fixation

Dorsal cervical stabilization has been revolutionized by the development of lateral mass screw fixation systems. These include screws fixed to plates and screws fixed to rods. These devices provide a high degree of immediate internal stability, often eliminating the need for postoperative external immobilization or bracing. Lateral mass plates are dynamic implants and behave primarily as nonfixed moment arm cantilever beam fixators. They also provide some dorsal tension band fixation (Fig. 140-5). Biomechanical studies have demonstrated the ability of these devices to restore stiffness to the injured spine in flexion, extension, and torsion.^27,28 Similar to other constructs that restore the dorsal tension band, lateral mass plates are probably weakest in extension.^5,24 Lateral mass screw-rod systems are rigid devices and as such provide more stability than screw-plate systems.

FIGURE 140-5 Coronal (A) and lateral (B) views of a lateral mass plate construct. This device provides stabilization in all motion planes, using tension band (small, black vertical arrows in A), three-point bending (large, black horizontal arrows in B), and cantilever beam fixation (small, black vertical arrows in B).

Lateral mass fixation may be used to treat instability from C2 to T1. Posttraumatic instability provides the most common indication for the use of lateral mass plates.^26,29 Dorsal ligamentous injury and irreducible unilateral or bilateral facet dislocations may be stabilized effectively with this type of construct after reduction of malalignment. In these cases instrumentation across the affected segment alone is usually adequate to restore stability. A longer construct may be required if instability of a single motion segment is severe, or if the dorsal elements at the level of instability are not intact. Multiple segments of instability may also mandate instrumentation of additional levels to achieve an adequate biomechanical advantage.

Unstable fractures of the dorsal cervical elements may be treated with lateral mass screw constructs. Fractures of the articular facet, pedicle, and/or lamina at a given level usually require a multilevel construct to restore stability. An intact level above and below the site of injury should be instrumented. Instability arising from vertebral body fractures has been successfully treated with lateral mass plates, usually incorporating multiple levels into the construct. This should only be attempted when the articular facets at that level are intact because they must contribute to axial load bearing in this situation. In most cases of vertebral body injury, a ventral approach is indicated. Certainly, if ventral spinal canal compromise is present, decompression and stabilization with a ventral construct should be considered.

Lateral mass fixation may also be used to reestablish stability after spinal tumor resection. The extent of instrumentation depends on the size and site of the tumor, but in general, several segments above and below the affected levels should be incorporated.⁶ Lateral mass screws should be placed only in bone that is free of disease.

Instability created by degenerative disease may be treated with lateral mass fixation. This is particularly effective when the dorsal elements are incompetent. Additionally, these devices may be applied at the time of laminectomy to prevent progressive kyphotic deformity in patients who are deemed to be at risk. This is more effective than attempting to treat an established kyphotic deformity with dorsal instrumentation because constructs fixed to the articular pillars are at a mechanical disadvantage in the latter. Ventral decompression and stabilization should be considered in this situation.

Excellent results have been reported using lateral mass plates without fusion.^26,29 However, long-term structural stability is augmented by incorporating bone graft into the construct. This may be accomplished by denuding the articular processes at the unstable level(s) and packing cancellous bone graft into the joint space. Often, adequate material for bone autograft may be obtained from adjacent spinous processes or from bone removed during decompression.

Advantages of lateral mass fixation over other dorsal construct designs include superior biomechanical stability in essentially all planes and applicability to a variety of clinical settings. These devices may be applied in the presence of extensive laminectomies or dorsal element fracture. They provide immediate postoperative stability without passage of sublaminar wires. Their installation may prolong the operative procedure somewhat and requires special equipment and technical expertise. These devices should be used with caution in patients with inferior bone quality because screw fixation systems perform suboptimally in this setting. If screws are used in this situation, postoperative immobilization with a rigid orthotic should be considered.

Cervical hook-rod systems have many of the same advantages of lateral mass plates but may be less easy to apply. Hook constructs are probably not the best choice in the setting of stenosis since the hooks require some space within the canal. Polyaxial screws allow the lateral mass screws to be applied in the appropriate trajectory, with the appropriately conformed rod applied after screw application. This is in contrast to lateral mass plates, in which the shape of the plate and placement of screw holes can occasionally dictate screw placement. As mentioned earlier, these systems provide rigid fixation that is biomechanically superior to that of the semirigid lateral mass plates.³⁰

Cervical pedicle fixation has also been shown to provide appropriate dorsal stabilization for the cervical spine.^31–33 However, cervical pedicle screws are significantly more difficult to place than lateral mass screws, and they carry a higher risk of injury to the nerve root and the vertebral artery. Lateral mass screws are probably equally effective in biomechanical stabilization and are safer in most situations.

360-Degree Constructs

Occasionally, cervical spinal instability is so severe that it warrants both ventral and dorsal stabilization, creating a “360-degree” construct. This approach is usually reserved for situations of ventral and dorsal instability. A 360-degree construct may also be indicated when extensive instability is anticipated from progression of underlying disease. This may be encountered in cases of advanced malignancy or extensive degenerative disease. To justify a 360-degree procedure, there must be a reasonable concern that instability will persist or recur, despite stabilization via an isolated ventral or dorsal approach.

Constructs that use wire-reinforced methyl methacrylate may be used for treatment of instability created by neoplastic disease. The biomechanical stability of methyl methacrylate reconstruction is optimized when a 360-degree construct is applied. Ventral stabilization is usually performed after decompression.

A methyl methacrylate strut may be used as a vertebral body replacement. More commonly, fibular strut grafts are used ventrally to span multiple vertebral levels. Struts function in a simple distraction mode, effectively resisting compressive axial loads (Fig. 140-6A). This can be supplemented by threaded Kirschner wires (K-wires), which are embedded into the vertebral bodies above and below the involved region. These devices prevent strut migration and provide some resistance to translatory forces.

FIGURE 140-6 Ventral (A), dorsal coronal (B), and sagittal (C) views of a 360-degree wire-reinforced methyl methacrylate construct. The ventral strut provides simple distraction fixation (black arrows), resisting axial compressive loads (white arrows [A]). The dorsal construct imparts torsional stability (curved arrows [B]). It also functions as a dorsal tension band (black arrows), resisting distraction and flexion (white arrows [C]).

A dorsal polymethylmethacrylate (PMMA) or fibular strut construct should be applied, in conjunction with ventral stabilization, which completes the 360-degree construct. Several techniques have been described for application of this material to the dorsal cervical spine. With one such method, K-wires are passed through the spinous processes, are bent, and then are encased in PMMA. These wires provide an increased surface area for interdigitation junction bonding between bone, wire, and cement.³ The net result is an increase in the tensile load-bearing capacity of the construct.

The dorsal methyl methacrylate-wire construct functions primarily as a dorsal tension band fixator (Figs. 140-6B and 104-6C). Some three-point bending fixation is also provided. When applied in this manner, 360-degree methyl methacrylate constructs provide a substantial degree of immediate internal stability in virtually all motion planes.³⁴ It must be remembered that, in contrast to an osseous fusion, the strength of a methyl methacrylate construct is maximal initially and decreases over time. This type of construct is generally reserved for treatment of cervical instability associated with progressive spinal malignancy. This is a purely palliative procedure and is not intended for long-term stabilization. It should be used only in patients with a limited life expectancy. Methyl methacrylate has been used for the treatment of traumatic instability,³⁴ although this material probably should be avoided because other constructs are superior in this context.

Methyl methacrylate constructs are relatively simple and safe to apply. They provide significant immediate internal stability in multiple planes. Special instrumentation is not required, and the materials used are inexpensive. The major shortcoming of this construct is that its long-term stability is suspect. There is no attempt to create an osseous fusion. As a result, the full extent of load bearing must be assumed indefinitely by the cement-wire construct.

A 360-degree construct is occasionally used for treatment of instability created by benign disease. If the nature and extent of instability are such that a single approach would not adequately restore structural integrity, a 360-degree construct must be considered. This situation is occasionally encountered with multilevel procedures in which there is preexisting instability and/or deformity. When an osseous strut graft is internally stabilized with a ventral plate, only two motion segments are actually fixed. The intervening motion segments may require dorsal fixation to provide optimal stability to the construct in cases of extreme instability.

Ventral and dorsal cervical plating systems may be applied concurrently in conjunction with appropriate bone grafting. In most cases these devices should confer an optimal biomechanical advantage to the construct, providing immediate internal stability in all motion planes. Other constructs may be devised if screw fixation is contraindicated by poor bone quality. These situations are rare and necessitate individualized management. However, fundamental construct design concepts should guide the selection and application of hardware systems, just as in less complex problems of instability.