Techniques of posterior subaxial cervical fusion

Techniques of Posterior Subaxial Cervical Fusion Paul D. Sawinl MD and Volker K. H. Sonntag, MD

Techniques of posterior cervical fixation have been augmented and refined over the past century to treat more effectively cervical instability arising from trauma, neoplasia, inflammatory conditions, and degenerative disease. Although adequate for many indications, posterior subluxation stabilization constructs perform best when used primarily to address flexion-type injuries. The integrity of the dorsal tension band is thereby reestablished. Posterior cervical fixation techniques may be subdivided into three general categories: (1) wire or cable constructs (including various interspinous wiring, facet wiring, sublaminar wiring, and wire/polymethylmethacrylate techniques; (2) interlaminar clamps; and (3) articular mass platescrew devices. This article details these techniques and their variations and presents a rationale for choosing a construct. Copyright 9 1998 by W.B. Saunders Company

he past century has witnessed major advances in the operative management of cervical instability. The earliest cervical fusion procedures involved autogenous bone grafts applied to the posterior elements in onlay fashion. As these constructs possessed little inherent stability, proper alignment and immobility had to be maintained for extended periods by means of traction, bed rest, and/or external orthoses until bony fusion occurred. In 1891, Hadra ushered in the era of internal spinal fixation using wire to stabilize adjacent cervical vertebrae rendered unstable by trauma or Pott's disease. 1 This technique represented a significant conceptual leap in the treatment of spinal instability as it heralded the use of nonbiological materials to restore spinal stability before osseous union. Subsequently, numerous posterior fusion techniques incorporated wire as a means to secure the spinous processes, laminae, and/or articular processes of the cervical spine. Many of these time-honored methods remain viable options for the treatment of cervical instability in modern neurosurgical practice. During the past decade, surgical treatment of the unstable cervical spine has been revolutionized by the advent of implantable fixation devices that are capable of affording immediate internal stability to a cervical construct. Instrumentation systems such as osteosynthetic plate and screw devices, hook-rod fixators, and interlaminar clamps have been designed specifically for spinal application to protect the neural elements from trauma and deformity until the bony fusion matures and can assume this role. In doing so, these implants minimize or even obviate the need for external orthotic immobilization during the postoperative period and thereby

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enhance patients' comfort, ensure compliance, and facilitate early mobilization and rehabilitation. Additionally, internal fixation devices facilitate fusion of shorter segments by providing intrinsic strength and load-sharing properties to the construct, thus preserving cervical motion and limiting the resultant moment arm created by the fusion mass. An overview of standard posterior subaxial cervical fusion techniques follows, encompassing surgical indications, operative techniques, complications, and outcomes. As the focus of this review is, by intent, technical, the rationale for selecting one mode of stabilization over another is addressed only cursorily. In general, the choice of construct is predicated upon the surgeon's familiarity and preference as well as on the specific nature and extent of the patient's instability.

Operative Indications In general terms, the rationale for spinal stabilization encompasses the following goals: (1) to restore stability to the structurally compromised spine, (2) to maintain alignment after correction of a deformity, (3) to prevent progression of a deformity, and (4) to alleviate pain. 2 Posterior cervical fusion constructs are most frequently applied to restore stability to the structurally incompetent spine. Instability of the subaxial cervical region may arise from trauma, degenerative disease, infection, inflammatory conditions, or neoplasia, or it may occur as a consequence of previous surgical intervention (Table 1). Irrespective of cause, however, the surgeon must carefully gauge both the. nature and the extent instability before instituting treatment. 3,4 The nature of an instability is established by assessing the integrity of specific anatomical structures that normally confer stability upon each cervical motion segment. The extent of instability is a product of the number of affected motion segments and the number of spinal columns involved for each segment. Determining the nature and extent of instability dictates first whether operative stabilization is required and, if so, which technique(s) would suffice.

Traumatic Instability

From the Division of Neurological Surgery, Barrow Neurological Institute, Mercy Healthcare Arizona, Phoenix, AZ. Address reprint requests to Volker K.H. Sonntag, M.D., c/o Neuroscience Publications, Barrow Neurological Institute, 350 West Thomas Road, Phoenix, AZ 85013. Copyright 9 1998 by W.B. Saunders Company 1092-440X/98/0102-000558.00/0

Instability resulting from trauma is the most frequent indication for posterior fixation of the subaxial cervical spine. 3-~ However, many patients with cervical spine injuries do not require surgery. In almost all cases, initial management consists of spinal realignment (if required), neural decompression (when indicated), and stabilization. Frequently, external immobilization alone is sufficient to protect the neural elements while healing occurs, provided that spinal alignment is acceptable and neural compression is absent. Conservative management is most appropriate when the injury is mainly osseous; primary ligamentous incompetence is often refractory to nonoperative measures. 6,7 Internal fixation is considered when conservative management is inappropriate or ineffectual. Posterior techniques are

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Operative Techniquesin Neurosurgerg,Vol 1, No 2 (June), 1998: pp 72-83

TABLE 1. Indications for Posterior Cervical Arthrodesis Type of Traumatic Cervical Instability

Location

Fractures

Articular facet fractures Fractures of the laminae, pedicles Vertebral body fractures*

Ligamentous injury

Articular facet dislocation (unilateral, bilateral) Posterior ligamentous incompetence Anterior ligamentous incompetence*

Degenerative cervical instability

Spondylosis Articular facet arthropathy Intervertebral disc degeneration

latrogenic cervical instability

Postlaminectomy, reversal of lordosis Postlaminectomy, established kyphosis* Postfacetectomy

Inflammatory/infectious instability

Rheumatoid arthritis Ankylosing spondylitis Osteomyelitis

Neoplasia

Benign Malignant

*Consider anterior stabilization.

most effective when used to address posterior disease. Posterior ligamentous instability, facet dislocations, and posterior element fractures are injuries well suited to a posterior approach. In most cases, fixation across the injured motion segment is sufficient to restore stability in patients with either isolated posterior ligamentous injuries or in those with concomitant facet dislocations. When, however, the instability is severe or when less rigid fixation techniques are used, additional levels may need to be incorporated. Fractures of the posterior elements typically necessitate incorporation of at least one intact level above and below the injury, respectively, to achieve a stable construct. Many of the posterior wiring techniques require intact posterior elements to be effective. As such, fractures of the spinous processes, laminae, or articular facets may preclude use of some methods and favor others. These nuances are addressed as the individual fixation techniques are described. In general, anterior injuries are difficult to stabilize with posterior constructs. Anterior ligamentous incompetence, vertebral body fractures, and/or intervertebral disc injuries are most appropriately addressed via an anterior approach, particularly in patients with compromise of the ventral spinal canal and incomplete neurological deficits. In such cases, a ventral approach permits neural decompression and spinal stabilization, thus optimizing neurological o u t c o m e / If the ventral spinal canal is not compromised or neurological deficits are complete, a posterior stabilization construct may be acceptable. 5,9 However, posterior stabilization should be attempted only when the articular facets at the involved level are intact because these structures must bear substantial axial loads under these conditions. If a posterior approach is selected, multiple segments above and below the level of injury should be incorporated to minimize the risk of progressive kyphosis. 10-12 Posterior fixation may also supplement an anterior arthrodesis when instability is severe or extensive or when the anticipated load upon a ventral construct alone is deemed excessive. The 360 ~ or circumferential fusion procedure is reserved for the management of severe three-column instabilPOSTERIOR CERVICAL FUSION TECHNIQUES

ity, for which a single approach would not adequately restore stability. 13,14 This approach restores stability in almost all motion planes and may prevent complications such as progressive kyphosis and graft dislodgement associated with a ventral procedure alone) 5,16 If a circumferential fusion is required, we typically prefer to perform both the ventral and the dorsal procedures under a single anesthetic, although each may be performed separately in a staged fashion if desired.

Nontraumatic Instability Although most posterior fixation techniques were initially developed to treat traumatic instability, many of these methods are equally effective in the treatment of instability unrelated to trauma (Table 1). Spinal neoplasia can create instability as a consequence of the local destruction of load-bearing elements. Further destabilization may result from attempts at surgical resection. Malignant tumors, whether primary or metastatic, typically mandate multilevel fixation. In all cases, the construct must incorporate disease-free segments both rostral and caudal to the involved levels. Frequently, a circumferential arthrodesis is required to restore cervical stability in this setting. Posterior cervical fusion is also useful for the treatment of spondylotic instability. Segmental instability resulting from degenerative disease may be addressed effectively with a variety of posterior techniques. For patients with cervical stenosis, internal fixation undertaken concomitantly with posterior neural decompression can reduce the incidence of postlaminectomy kyphosis and should be considered in selected cases, particularly when the normal cervical lordosis is absent. Posterior fixation is much less effective as a primary treatment of an established kyphosis; such deformities are best managed with ventral reconstruction.m7 Occasionally, internal fixation is undertaken to safeguard against anticipated instability that could arise as a consequence of future disease progression (as with spinal neoplasia) or from potentially destabilizing iatrogenic maneuvers (eg, decompressive laminectomy). Structural embarrassment created by tumor, infection, inflammatory conditions, degenerative disease, or previous attempts at neural decompression may be progressive, necessitating sound preoperative planning to address not only the existent instability but also that which is anticipated to occur over time. The surgeon must be aware of increased demands placed on the construct by the disease process, both in terms of the degree of support required from the implant and the duration of that requirement. In certain settings, the implant must bear 100% of the load for the remainder of the patient's life. Ultimately, these considerations profoundly influence the selection of an appropriate construct; thus, solutions for most instability problems must be formulated on an individual basis.

Construct Selection and Design Often, a variety of fixation methods may be acceptable alternatives to address a given instability problem. In such cases, the rationale for selecting one technique over another should be predicated upon the following variables: 1. The surgeon's preference and familiarity 2. The cost of implant 73

3. The requirement for postoperative bracing 4. Compatibility with postoperative imaging 5. The patient's comfort or compliance

TABLE 3. Denis' Three-Column Classification Scheme for Acute Spinal Injuries*l-

Additionally, selection of a stabilization strategy is influenced by a patient's general medical condition, neurological status, and bone quality. The nature, location, and extent of the pathological process also influence operative planning because the long-term structural demands placed on an internal fixation construct are often determined by the progression (or remittance) of the underlying disease state.

Column

Anatomical Constituents

Anterior

Anterior longitudinal ligament Anterior annulus fibrosis Anteriorvertebral body

Middle

Posterior longitudinal ligament Posteriorannulus fibrosis Posterior vertebral body

Posterior

Posterior neural arch Articular facets Supraspinous/intraspinal ligaments

General Considerations There are three essential questions in the evaluation of cervical pathological disease: 1. Is the spine stable or unstable? 2. If it is unstable, how severe is the structural compromise? 3. Is surgical intervention justified? To answer the first query, the concept of clinical instability must be addressed. White and Panjabi define clinical instability as the "loss of the ability of the spine under physiological loads to maintain its pattern of displacement so that there is no initial or additional neurological deficit, no major deformity, and no incapacitating pain. ''2 In practice, the presence of instability is suggested by static radiographs and, if necessary, confirmed with dynamic studies. The former include plain cervical radiographs, computed tomography (CT), magnetic resonance imaging (MRI), and occasionally, myelography. The latter consist primarily of flexion-extension radiographs or pluridirectional tomography. Based on imaging data, the extent of spinal stability can be estimated. Several methods have been developed to facilitate the process of identifying and quantifying cervical instability. In an effort to identify unstable cervical injuries, White and colleagues proposed a point system predicated upon the radiographic appearance of the fracture and the presence or absence of neurological deficit (Table 2). 18 Denis' three-column classification scheme, initially developed to evaluate acute thoracolumbar fractures, is also often extrapolated to assess cervical spine injuries (Table 3). 19Although these grading strategies are often quite useful in gauging the presence and degree of structural compromise, none have been subjected to stringent evaluation in randomized outcome studies that compare surgical and/or nonsurgical treatment methods. Thus, they are merely guidelines that must be used in the context of each patient's specific injury.

TABLE 2. Criteria for Instability in Subaxial Cervical Spine Injuries* Criterion Anterior elements nonfunctional Posterior elements nonfunctional Sagittal plane translation >3.5 mm Sagittal plane angulation >11 ~ Positive stretch test Spinal cord injury Nerve root injury Abnormal disc-space narrowing Dangerous loading anticipated *Data from White et al. 18 tClinical instability if point total >5.

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Point Valuer

*Data from Denis. 19 tOlinical instability with injury to two or more columns.

Biomechanical Considerations Once surgical stabilization is deemed appropriate for a given patient, two decisions must be made: (1) Which approach (anterior or posterior) should be used? (2) What fixation method should be used? These questions are best answered by attempting to match the implant with the site and type of instability. Typically, posterior fixation techniques are most effective when used to treat dorsal, not ventral, pathological conditions. Nonetheless, some authors have successfully stabilized vertebral body fractures and anterior ligamentous injuries with posterior constructs. 5,9,1~As a rule, however, it is unreasonable to expect any implant to provide optimal stability when placed in a biomechanically disadvantageous position. Posterior fixation devices provide various degrees of internal stability to the injured cervical spine. They do not, however, confer long-term structural integrity. The latter depends on bony fusion. Bone and certain dynamic fixators such as wire or cables exhibit plastic properties at implant-bone interfaces, deforming and reforming as stress is applied. 2~Over time, even the most rigid construct permits some segmental motion across the site(s) of fixation. Repetitive loading will ultimately induce implant failure unless osseous fusion ensues. Thus, the long-term stability of any construct depends on the quality of the bony fusion. Unlike thoracolumbar devices, which may be applied in distraction, compression, neutral, flexion, extension, or lateral bending modes, cervical implants are typically applied in neutral mode. Consequently, reduction of deformity and spinal realignment are essential before stabilization and are most frequently accomplished with axial traction. As a rule, posterior cervical implants are used to maintain alignment, not to accomplish reduction. The reductive forces that can be applied by such devices are sinai1 and usually insufficient to achieve physiological spinal alignment. Posterior cervical constructs provide stability through several mechanisms of load bearing. All function as tension-band fixators, restoring the posterior tension band to resist flexion. Certain techniques such as simple interspinous wiring afford little else, providing almost no stability in extension, lateral bending, or axial rotation. 21 Osteosynthetic articular mass plates, the most rigid posterior cervical devices currently available, provide three-point bending and nonfixed momentarm cantilever-beam fixation in addition to their tension-band function. These more complex mechanisms of load bearing SAWlN AND SONNTAG

confer stability in flexion, extension, lateral bending, axial rotation, and axial loading. Selecting an appropriate posterior fixation construct requires understanding these implant characteristics. In many cases, if the degree of instability is mild, restoring the tension band may be all that is required. More substantial instability usually necessitates a more aggressive operative strategy. Again, selection of a construct is facilitated by matching the implant with the instability on an individual case basis.

Operative Technique Anesthesia and Spinal Cord Monitoring In almost all instances, posterior cervical stabilization is performed with the assistance of general anesthesia. In patients with cervical instability, laryngoscopy and endotracheal intubation must be approached with extreme caution. Awake fiberoptic laryngoscopy may be performed with little or no neck manipulation, affording maximal protection from iatrogenic spinal cord injury while the airway is secured. The awake patient may then be positioned and neurological function assessed before anesthesia is induced. In light of these attributes, this technique should be strongly considered in all patients with known or suspected cervical instability. 1~ However, awake laryngoscopy requires a patient's compliance and thus should be avoided in subjects who are unwilling or unable to cooperate. In such cases, direct laryngoscopy under intravenous anesthesia may be undertaken cautiously, provided that the head and neck are adequately stabilized in neutral posture throughout the intubation sequence3 2 Pharmacological paralysis with neuromuscular blocking agents should be undertaken with great caution or avoided entirely in patients with cervical instability. The cervical musculature maintains some resting tone even under profound general anesthesia, functioning as a physiological splint that contributes to the overall maintenance of alignment. Neuromuscular blockade abolishes the inherent stabilizing influence of the cervical musculature and thus may exacerbate instability. Intraoperatively, spinal cord function may be assessed by somatosensory and/or motor evoked potentials. Continuous spinal cord monitoring is particularly effective as an adjunct to decompressive surgery in the patient with at least partial preservation of neurological function in whom inadvertent manipulation of the spinal cord may be heralded by an alteration in wave morphology. Although often employed, the use of these monitoring tools for cervical pathological conditions other than intrinsic spinal cord tumors or vascular malformations remains controversial. If the spinal cord is to be monitored, baseline studies before surgery are very beneficial.

Positioning Positioning the patient with cervical instability must be undertaken with great care because inadvertent cervical manipulation can have devastating consequences. The surgeon should directly supervise all aspects of the turn and, with few exceptions, be responsible for maintaining neutral cervical alignment throughout the positioning process. This strategy is imperative when positioning anesthetized patients because such individuals are unable to assist with this task. Although the sitting position is occasionally used, most POSTERIOR CERVICAL FUSION TECHNIQUES

posterior stabilization procedures are performed with the patient prone. Ideally, the neck is maintained either in neutral posture or in slight extension. 3,1~,~2 If axial cervical traction was used preoperatively, it is typically continued during the operative procedure) ,5,~~ The awake patient may be positioned before general anesthesia is induced, affording the opportunity to assess neurological function after the patient is turned. Any change in neurological status mandates a prompt reappraisal of cervical alignment, both by inspection and with radiographic studies. The luxury of a neurological examination after positioning is foregone when patients are intubated under general anesthesia. Instead, these patients must be evaluated with intraoperative radiographs after final positioning to ensure acceptable cervical alignment.

Exposure The surgical procedure may begin once the patient has been properly positioned, general anesthesia induced, and acceptable cervical alignment verified. Almost without exception, the dorsal aspect of the subaxial cervical spine is exposed via a posterior midline approach. The site is prepared and draped in the usual sterile manner. If desired, the dermis and subcutaneous tissues may be infiltrated with a dilute epinephrine solution to effect vasoconstriction. A midline incision is carried sharply to the ligamentum nuchae, and self-retaining retractors are gently placed. The ligamentum is divided with monopolar electrocautery. The underlying paraspinous musculature is dissected from the spinous processes and laminae in a subperiosteal manner and retracted laterally. Sharp dissection techniques are preferable to avoid inadvertent cervical manipulation. Caution must be used when the dorsal cervical spine is exposed, particularly in the presence of laminar fractures. When possible, all supporting soft tissue elements (ie, interspinous/supraspinous ligaments and facet capsules) are preserved. The dimensions of surgical exposure are dictated by the extent of the cervical pathology and the type of fixation selected. Some techniques, such as articular mass plate fixation, mandate extensive lateral exposure to fully uncover the lateral masses at the levels to be fused. Others, including simple interspinous wiring, require less lateral dissection. Exposure of adjacent levels not intended for incorporation into the fusion construct should be avoided to limit the risk of creating iatrogenic instability rostral or caudal to the arthrodesis. Once adequate exposure has been achieved, the offending pathology may be addressed. Persistent spinal deformity should be promptly corrected. As noted previously, aggressive efforts should be taken to restore anatomical alignment with axial traction before surgery. Occasionally, however, irreducible injuries necessitate open reduction with cautious intraoperative manipulation and/or partial facetectomy. If neural compression persists despite adequate reduction, a decompressive procedure is performed. Resection of key load-bearing elements may be an adverse consequence of adequate decompression, thus exacerbating segmental instability. The surgeon must be prepared to reassess the degree and extent of instability after neural decompression and to alter the stabilization strategy accordingly. '75

Subaxial Fixation

Wire and Cable Techniques During the last 50 years, many wire and cable techniques for posterior subaxial fixation have been introduced. Although each technique has its own unique nuances, all may be subclassified into three general categories: (1) spinous process wiring, (2) facet wiring, and (3) sublaminar wiring techniques. Still a viable option in the atlantoaxial region, the latter has largely been abandoned as a means to secure the subaxial motion segments due to the risk of creating or exacerbating neurological injury as a consequence of sublaminar wire passage, s,25,26As other standard methods provide biomechanical stability that is equivalent or superior to that achieved with sublaminar wiring, these risks are rarely justified. 26 We do not advocate the routine use of sublaminar wiring in the subaxial cervical spine; consequently, these techniques are not further discussed. In general, posterior wiring techniques are simple to perform, require no special skills or equipment, and employ inexpensive materials. These methods effectively reconstitute the posterior tension band and, as such, perform optimally in resisting flexion. 21 In most cases, wiring alone does not provide substantial immediate internal stability and thus must be supplemented with bone graft, methylmethacrylate, or external bracing to augment the construct until bony fusion occurs. Wire constructs may be created with single-strand wire, twisted wire, or braided cables. Most of these methods were initially described using a single-strand, small gauge, stainless steel wire. Currently, most surgeons prefer a larger gauge wire (16-, 18-, or 20-gauge) for additional strength, although these thicker wires are less flexible and more difficult to manipulate. A compromise between strength and malleability may be found by using twisted (braided) wires. Two strands of 22-gauge wire braided together are not only easier to handle but also stronger than a single strand of 18-gauge wire. 27 Care, however, must be taken to avoid sawing through bone as the braided wire is passed. Braided multistrand cables have supplanted traditional singlestrand or twisted wires for most applications. These devices have the advantages of higher tensile strength, relatively uniform distribution of applied tension, and ease of handling. Braided cables are available in both stainless steel and titanium alloy; the latter produces less artifact on MRI and CT. These cables are more expensive than wire, although the aforementioned advantages may justify the added cost in many situations.

Spinous Process Wiring Techniques Rogers' wiring. Interspinous wiring was initially described by Rogers in 1942 for treatment of traumatic cervical instability.28 Although multiple modifications have been espoused over the years, the fundamental technique remains unaltered. In its most basic form, this method is designed to stabilize a single motion segment (two adjacent vertebral levels). Additional segments may be incorporated by repeating the maneuver at contiguous levels. The spinous processes and laminae at the levels to be stabilized are exposed as previously described. With a right-angle dental drill, a single hole is drilled through the base of the spinous process just dorsal to the spinolaminar junction at each level. A single cerclage wire or cable is looped in opposite directions through the hole in each spinous process, with the superior loop passing above and the inferior loop passing below their respective processes (Fig 1A-C). The free ends of the wire are then twisted to the desired tightness (Fig 1D). Provisions are made for bone fusion, with thorough decortication of the spinous processes and laminae at the instrumented level(s) and onlay of autogenous corticocancellous bone graft. Graft material may also be insinuated between the wire's parasagittal limbs (as Rogers initially recommended) or within the facet joint space. In Rogers' original series, all 11 patients achieved solid arthrodesis with this technique. 28 This method is technically simple, fast, inexpensive, and biomechanically sound for the treatment of many posterior cervical injuries, provided that the instability is not too complex or severe. A prerequisite for successful Rogers' wiring is the presence of intact posterior elements at the levels to be incorporated into the construct. If the posterior elements are pristine, wiring across a single motion segment will often suffice. A posterior element fracture necessitates incorporation of an additional level above or below the motion segment to be stabilized. Care must be taken to avoid overtightening the wires, which can induce cervical hyperextension and resultant stenosis of the spinal canal or neural foramina. Whitehill modification. Whitehill et a129 described a variation of the Rogers' technique. Again, the fundamental method involves fixation of a single motion segment. A single cerclage wire is passed through a hole in the base of the spinous process of the upper vertebra and looped around the inferior edge of the spinous process at the adjacent caudal level, creating a simple interspinous loop (Fig 2A). Equalization twists are then used to distribute tension equally on either side of the posterior elements. If additional levels are to be incorporated into the construct, the wire loops are placed in overlapping

Fig 1. In the Rogers' interspinous wiring technique, (A) a single cable is looped through a hole in the base of the superior spinous process with the loop passing above its respective process. (B) The free end of the cable is passed through the inferior spinous process. (C) The inferior loop encircles the inferior spinous process. (D) The cable is then tightened as desired. 76

SAWIN AND SONNTAG

Fig 2. In the Whitehill interspinous wiring modification, (A) a single cable is passed through a hole in the base of the superior spinous process and looped around the inferior margin of the spinous process at the next caudal level. Additional levels may be incorporated in similar fashion, overlapping the wires as shown in (B) posteroanterior and (C) lateral projections.

thus minimizing the risk of cervical hyperextension. 27 In practice, this method is associated with a high rate of successful fusion (98% in the authors' series of 50 patients) and long-term stability in individuals with unstable cervical injuries. 30 Bohlman triple-wire technique. The Bohlman triple-wire technique is another common modification of the Rogers' interspinous wiring. 31-33This method may be used to stabilize a single motion segment, but it is also effective for treating multilevel instability. As implied by the title, Bohlman's technique requires three separate wires to stabilize each motion segment. Holes are created in the spinous processes as previously described. The first (tethering) wire incorporates the two adjacent spinous processes in the manner of Rogers (Fig 4A). Single wires are then passed separately through the holes in each of the two spinous processes so that the horizontally oriented wires parallel one another (Fig 4B). Autogenous bone is harvested and split longitudinally into two corticocancellous grafts. The horizontal wires are passed through holes placed midposition in the grafts and tightened, thereby securing the grafts against the decorticated spinous processes and laminae on each side (Fig 4C and 4D). Similar to the Benzel-Kesterson modification, the Bohlman triple-wire technique integrates the bone graft into the construct, with use of the corticocancellous struts as buttresses to augment torsional stability. The grafts are placed under compression, thus optimizing conditions for bony incorporation. The soundness of this construct has been verified by biomechanical testing. 32 Its clinical utility has been confirmed in several studies, most notably in the series of Weiland and McAfee,33 in

fashion (Fig 2B and 2C). Provisions are made for bone fusion as previously described. In Whitehill's series of 22 patients, all achieved solid arthrodeses. 29 Benzel-Kesterson modification. Benzel and Kesterson 3~ described yet another modification of Rogers' basic interspinous wiring technique. This method is also suitable for stabilizing a single motion segment and may be repeated to provide multilevel fixation. A double-stranded (braided) 22-gauge cerclage wire is passed through a hole in the base of the rostral spinous process and looped around the inferior aspect of the caudal spinous process in a manner similar to the Whitehill wiring method (Fig 3A). Before the braided wire is tightened, a single strand of 22-gauge wire (compression wire) is passed through the interspinous space beneath the cerclage wire (Fig 3B). The braided interspinous wire is then tightened. A tricortical iliac crest bone graft is split longitudinally, yielding two matched corticocancellous halves. The spinous processes and laminae are decorticated, and the grafts are fashioned to press-fit against the spinous processes (cortical sides out). The compression wire is passed around the outside of both grafts and seated in ventrally placed notches. As the compression wire is tightened, the grafts are secured against the spinous processes and laminae, and the cerclage wire is tightened further (Fig 3C). Theoretically, this technique has several advantages. The bone grafts are incorporated as an integral part of the construct, acting as buttresses to provide a measure of torsional stiffness. The second wire compression-loads the grafts, thereby enhancing the probability of achieving a successful bony fusion. Furthermore, an adequate interspinous distance is maintained by the grafts as the compression wire is tightened,

C

h

I

1

m

Fig 3. In the Benzel-Kesterson interspinous wiring modification, (A) a simple interspinous cerclage cable is passed in the manner of Whitehill. (B) A compression cable is passed through the interspinous space beneath the cerclage cable, and the interspinous cable is tightened. (C) The compression cable secures the bone graft against the decorticated spinous processes and laminae and is tightened, augmenting the tension of the interspinous cerclage cable. POSTERIOR CERVICAL FUSION TECHNIQUES

77

Fig 4. In the Bohlman triple-wire technique, (A) the first cable creates an interspinous loop after the manner of Rogers. (B) Two separate cables are passed through holes in the superior and inferior spinous processes, respectively. (C) The ends of these cables are passed through holes in the two autologous bone grafts, and (D) the cables are tightened, securing the grafts against the decorticated spinous processes and laminae. which all patients with traumatic cervical instability achieved successful arthrodeses with this technique. Murphy-Southwick modification. Murphy and Southwick modified Rogers' interspinous wiring technique to facilitate incorporation of two adjacent motion segments (three vertebral levels) into a single fusion construct. 34 Drill holes are created in the three spinous processes to be incorporated as previously described. The first wire is passed between the upper and middle vertebrae, creating a simple interspinous loop (Fig 5A). A second wire secures the middle and lower vertebrae in a similar fashion (Fig 5B). A third wire is then passed between the upper and lower vertebrae and tightened (Fig 5C). The laminae are decorticated, and corticocancellous bone grafts are applied in onlay fashion. This technique is effective when two contiguous motion segments require stabilization; it is not applicable to single-segment instabilitTr

Facet Wiring Techniques Oblique facet wiring. In 1983, Cahill et al described a technique of oblique facet-to-spinous process wiring for the treatment of segmental cervical instability after trauma. 35 This method was advocated primarily for fixation of subaxial flexion-compression injuries or facet fracture-dislocations because the rotational stability provided by traditional interspinous wiring techniques may be inadequate to maintain reduction after surgical fixation. This technique is equally useful for stabilization of posterior element fractures that involve the rostral lamina or spinous process of an unstable motion segment, without the need to incorporate an additional vertebral level into the construct.

After routine exposure of the dorsal cervical spine at the level of injury, the inferior articular processes of the rostral vertebra are isolated and the investing facet capsules removed. The facet joint spaces are opened with a small elevator, and articular cartilage is removed with a drill or fine curette. Drill holes are created through the midportion of the inferior articular processes, perforating the bone at right angles to the articular surfaces (Fig. 6A). A Penfield dissector protects the underlying superior articular process, nerve root, and vertebral artery during drilling. A wire or cable is passed through the hole in the articular process and looped beneath an intact spinous process one or two levels below and tightened (Fig 6B). The maneuver is then repeated on the contralateral side (Fig 6C). The laminae and spinous process are decorticated, and corticocancellous bone grafts are placed in onlay fashion. The facet joint spaces may be packed with additional graft substrate to further stimulate fusion. In their series of 18 patients with bilateral facet-to-spinous process wiring, Cahill et al reported a 100% fusion rate and stable alignment after 3 to 4 months. 35 No additional neurological morbidity was conferred by the procedure. The authors recognized the importance of bilateral fixation even in the context of unilateral facet injury: A unilateral facet-to-spinous process wiring construct is not biomechanically sound and may allow redislocation because of persistent rotatory instability Facet wiring. Occasionally, the cervical spinous processes and laminae are unavailable for incorporation into a fusion construct, either because of extensive fracture or previous surgical removal. When subaxial instability is present under

Fig 5. In the Murphy-Southwick interspinous wiring modification, (A) the first cable is passed through holes in the spinous processes of the superior and middle vertebrae, creating a simple interspinous loop. (B) A second cable incorporates the middle and inferior spinous processes in a similar fashion. (C) A third cable is passed between the superior and inferior vertebrae and tightened. 78

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Fig 6. In the Cahill oblique facet wiring technique, (A) holes are drilled through the midportion of the inferior articular facets of the most rostral level to be incorporated, along a trajectory perpendicular to the articular surfaces. (B) A cable is passed through the hole in the facet and looped beneath an intact spinous process at the level below. (C) A second cable completes the construct by repeating the facet-to-spinous process wiring maneuver on the contralateral side.

these conditions, the articular processes may be used as alternative sites for multilevel segmental fixation. In the 1970s, Callahan et al described a method of facet wiring initially intended for use after laminectomy to mitigate against postlaminectomy kyphosis) 6 Although effective for this indication, facet wiring also can be used when injury to the posterior elements spans multiple levels, rendering the previously described wire-based fixation methods infeasible. This technique is relatively simple to perform, utilizes inexpensive materials, and provides reasonable stability in multiple planes, including axial rotation and translation. Facet wiring requires dorsal exposure of all levels selected for fusion. The entire lateral mass should be exposed at each level and the facet capsular ligaments removed. The facet joint is opened with a small dissector, and the articular cartilage is removed. A hole is drilled in the inferior articular process at each level to be incorporated, oriented at right angles to the articular surfaces (Fig 7A). A wire or cable is passed through each hole, rostral to caudal, exiting through the joint space (Fig 7B). Two corticocancellous strut grafts of sufficient length to span the entire construct are harvested, either from a rib or from the posterior iliac crest. The former site may be preferable for this indication as rib possesses a native curvature that conforms well to the cervical lordosis, has a circumferentially intact cortex for added strength, and may be harvested in long segments for multilevel constructs) 7 Holes are placed through the grafts at intervals corresponding to the spacing of the inferior articular processes. The dorsal surfaces of the lateral masses are decorticated, and the facet wires are passed through

comparable sites in the graft. The wires should always be passed medial to the graft to prevent displacement of the strut into the spinal canal. The struts are then advanced over the wires and segmentally transfixed against the lateral masses (Fig 7C). Alternatively, the struts may be secured by wrapping the facet wires around (instead of through) the grafts. In lieu of autogenous bone struts, small contoured metal rods or a Luque rectangle may be incorporated into the construct and secured with the segmental facet wires) 8 Additional bone graft may be packed into the facet joints or placed alongside the longitudinal members. Typically, the facet fusion construct should extend at least one vertebral level above and below the rostral and caudal extent of the laminectomy, respectively. The caudal end of the graft may be secured to the first intact spinous process rather than through the articular mass to avoid the chronic pain that occasionally results from violating an unfused facet joint.

Interlaminar Clamps Subaxial cervical stabilization by means of interlaminar clamps was first described in 1975. 3~ These devices have been used effectively in the treatment of flexion injuries to reestablish the integrity of the posterior tension band. They perform optimally when used to stabilize a single motion segment. Implants are available in a variety of sizes and configurations to conform to a given patient's anatomical constraints. Initially manufactured of stainless steel, interlaminar clamps are now available in a titanium alloy to facilitate postoperative imaging. Interlaminar clamps are relatively quick and easy to apply.

Fig 7. In the Callahan facet wiring technique, (A) holes are drilled through the inferior articular masses at the levels to be incorporated, at right angles to the articular surfaces. (B) A separate cable is passed through each articular mass, rostral-to-caudal, exiting through the joint space. (C) The cables are passed through autogenous strut grafts and tightened, transfixing the grafts against the articular masses. POSTERIOR CERVICAL FUSION TECHNIQUES

79

After the posterior elements are exposed at the levels to be fused, the leading edge of the lamina above and the trailing edge of the lamina below are thinned bilaterally to augment the interlaminar spaces rostral and caudal to the unstable motion segment (Fig 8A). The appropriate size clamp is selected and :separated into its two major components, which are, in esser/ce; fitted laminar hooks. The upper (threaded) half-clamp is hooked over the leading edge of the upper lamina, while the lower (unthreaded) half-clamp is hooked under the trailing edge of the lower lamina (Fig 8B). A machine screw is placed through the unthreaded hook and engages the threads of the hook above. Tightening of the screw apposes the two laminae by drawing the hooks together (Fig 8C). Provisions are made for bone fusion by lightly decorticating the laminae and lateral masses and applying autogenous bone graft. 4~ Although good results have been reported for unilateral implants in selected patients, 41 most surgeons advocate bilateral placement of interlaminar clamps to optimize fixation and multiplanar stability.4~ Multilevel fixation with these devices should be avoided because the incidence of failure in this setting is excessive. 4~ With these caveats in mind, interlaminar clamps have demonstrated efficacy in restoring long-term stability to the compromised subaxial cervical spine. The principal disadvantages of this method are the requirement for intact laminae at the level to be instrumented and the risk of increased neurological deficit caused by metal stenosis from the sublaminar hooks. 42

avoid disruption of the facet capsular ligaments during the dissection. Holes are created in the bases of the spinous processes at all levels to be incorporated; as if preparing for an interspinous wiring. Interspinous wires or cables span the injured segments in a standard configuration and are tightened to achieve adequate reduction (Fig 9A). Stainless steel pins (Kirschner wires are suitable for this purpose), approximately 1.5 mm in diameter, are cut into 3.0-cm lengths, passed through the holes in the spinous processes, and bent slightly to prevent them from backing out (Fig 9B). These pins provide multiple points of fixation to secure the PMMA to the individual vertebral elements, enhancing interdigitation interface-bonding among bone, wire, and cement. The entire construct is then encased in PMMA (Fig 9C). In a series of almost 100 patients with traumatic cervical spine injuries, Branch et al reported "long-term" stability (mean follow-up, 1.5 years) in 97% of patients treated in this manner. 43 Wire-reinforced PMMA is an inexpensive, technically simple, and rapid means of restoring the posterior tension band and of providing immediate internal stability in multiple motion planes. Despite these attributes, PMMA constructs have failed to gain widespread acceptance in the treatment of cervical instability arising from benign causes. Unlike bone, PMMA is simply an inert filler that is never incorporated into a viable fusion; thus, the ultimate stability of such a construct must be questioned.

Articular Mass Plates Polymethylmethacrylate and Wire Fixation Traditionally, stability has been restored to the incompetent subaxial cervical spine with wire fixation in conjunction with autogenous bone grafting. As a general rule, long-term spinal stability relies on successful osseous integration of the construct; thus, adequate provisions for bone fusion are mandatory. Nonetheless, however, some surgeons advocate the use of polymethylmethacrylate (PMMA) as an alternative to bone grafting in the treatment of posterior cervical instability, even when traumatic in origin. 43 Although this technique remains controversial, PMMA and wire constructs are touted by some as viable options in managing traumatic subaxial instability. The basic technique for posterior subaxial fixation with PMMA and wire is described by Branch et al,43 although many permutations exist. The posterior elements are exposed in standard fashion; at least one level above and one below the level of injury must be fully visualized. Care should be taken to

Posterior stabilization of the subaxial cervical spine has been revolutionized by the advent of articular mass osteosynthetic plates. These devices provide immediate internal stability, are appropriate for single or multilevel fixation, and do not require intact posterior elements for application. Presently in the United States, three articular mass plate systems are in widespread clinical use (Table 4): the AME Haid Universal Bone Plate System (American Medical Electronics, Inc, Richardson, TX), the Axis Fixation System (Sofamor-Danek, Inc, Memphis, TN); and the AO Small Notched Reconstruction Plate (SNRP) System (Synthes Spine, Inc, Paoli, PA). All systems are available in titanium alloy, thus facilitating postoperative imaging. The general operative technique for articular mass plate implantation is similar for all systems and consists of four fundamental steps: (1) sizing and contouring the plate, (2) drilling the articular masses, (3) selecting the appropriate screws, and (4) securing the plate.

Fig 8. Interlaminar clamps. (A) The leading edge of the lamina above and the trailing edge of the lamina below are thinned bilaterally to increase the interlaminar spaces above and below the segment to be fused. (B) The upper (threaded) half-clamp is hooked over the rostral lamina and the lower (unthreaded) half-clamp is hooked under the caudal lamina. (C) A machine screw transfixes the clamp halves and is tightened, apposing the laminae by drawing the two hooks together. 80

SAWlN AND SONNTAG

Fig 9. PMMA and wire fixation. (A) The unstable motion segments are incorporated with a standard interspinous wiring technique. (B) Stainless steel pins are cut about 3-mm long, passed through holes in the spinous processes, and bent slightly to resist backing out. (C) The entire construct is then encased with PMMA.

Sizing and contouring the plate. Plates of identical length are selected to span congruous segments on either side of the spine. When possible, they should be implanted bilaterally and symmetrically. All major plating systems are replete with plates of different dimensions, affording versatility of application. The shortest plate that will allow screw purchase in each articular mass to be instrumented should be selected. 44Approximating the posterior elements with a clamp or interspinous wiring often enables the application of a plate that initially appeared to be too short) This maneuver may also confer biomechanical advantage because it preloads the construct. 44 Additionally, compression-loading of the facet joint may enhance the probability of successful fusion. Ideally, alignment of the instrumented cervical spine should approximate the normal lordotic posture. However, articular mass plates are in situ fixators and should not be relied on to alter cervical alignment. The Axis and SNRP plates may be readily bent into lordosis; the Haid plate is more stout and does not bend easily. The contact surface of the latter device is contoured to maintain lordosis as the plate is secured. If lordosis cannot be achieved by preoperative traction or intraoperative manipulation, the cervical spine may be instrumented in neutral alignment. It is more appropriate to modify the plate to fit the patient than to attempt to alter the patient's anatomy to conform to the plate. One exception to this rule is that lateral mass plates should never be bent into kyphosis. If an irreducible kyphotic deformity is encountered, ventral reconstruction should be considered in lieu of, or before, posterior stabilization. Drilling the articular masses. Once a plate tandem is selected and custom contoured, holes may be drilled into the articular masses in preparation for screw placement. Various screw trajectories have been described for lateral mass fixation from C3 to C7. Roy-Camille and others have advocated engaging the drill bit at the center of the lateral mass, TABLE 4. Selected Features of the Haid, SNRP, and Axis Systems* Feature

AME-Haid

Synthes-SNRP

Axis

Plate strength Plate malleability Screw placement variability Screw/plate variety Complexity

+++ + + + +

+ + + + +

+ + + + +

++ + ++ +

+ ++ ++ ++ +

NOTE: + = low; + + = intermediate; + + + = high. Sawin and Traynelis. 3 (Reprinted with permission from McGraw-Hill.)

*Modified from

P O S T E R I O R C E R V I C A L FUSION T E C H N I Q U E S

proceeding along the axial plane while angling 10 ~ laterally) TM The widely used Magerl technique involves drilling from a point 1 to 2 m m medial and rostral to the center of the lateral mass along a trajectory 25 ~ lateral and 40 ~ cephalad. 4s The sagittal angulation is intended to orient the screw parallel to the facet joint. Several variations of the Magerl technique have been described. 3,1~ In general, a lateral screw trajectory is compulsory to avoid both nerve root and vertebral artery injury. 46 We advocate a screw placement technique similar to that described by An et al. 46 Drilling is initiated at a point 1 m m medial to the midportion of the lateral mass and proceeds along a course 15 ~ cephalad and 20 ~ to 30 ~ lateral (Fig 10A). This trajectory affords reasonable protection from neurovascular injury while attaining sound bicortical screw purchase in the articular masses. Acceptable screw placement may be accomplished in this manner from C3 to C6. Screw trajectory is often altered somewhat at C7, because of the relatively small size of its articular mass. If lateral mass fixation at C7 is desired, a slightly more lateral and cephalad trajectory accommodates this anatomical constraint. Because the articular mass is small, it is often preferable to obtain pedicular fixation at C7 and T1. The pedicle may be entered 1 m m caudal to the facet joint along a trajectory directed medially 25 ~ to 300. 3 Each screw hole must be positioned optimally in its articular mass. Thus, the holes are oriented with reference to the patient's anatomy, not placed according to the lie of the plate. To minimize this latter tendency, we avoid drilling screw holes through the plate. All articular mass plate designs are sufficiently versatile to accommodate properly positioned screws. Once the drill bit entry site and trajectory have been determined, the outer cortex of the lateral mass is pierced with an awl or a cutting burr to facilitate initial drilling. The articular masses may be drilled with an unprotected drill or a K-wire; however, it is preferable to use a drill bit with a depth stop (typically at 15-16 mm) to avoid overpenetration. During drilling, toggling, which can result in an irregular or oversized hole, must be minimized. The use of a low-speed drill may reduce these concerns. 21 Bicortical screw purchase in the articular mass is desirable, and often the drill bit may be felt to penetrate its anterior cortex. If this penetration is detected before the predetermined depth set by the drill stop has been reached, drilling should cease immediately. Screw holes should be placed unilaterally in all articular masses to be instrumented before the contralateral side is 81

Fig 10. Articular mass plates. (A) Holes are drilled in the inferior articular masses at each level to be incorporated, along a trajectory 15 ~ cephalad and 20 ~ to 30 ~ lateral. (B) The plate is sized, contoured, and secured against the lateral masses with bicortical screws at each level. (C) The process is then repeated on the contralateral side.

addressed. When a three- or four-hole plate is implanted, the rostral and caudal holes are usually drilled first. If a three-hole plate is used to bridge a fractured facet or pedicle, no screw should be placed at the site of i n j u ~ If the corresponding contralateral elements are intact, the center hole should be drilled and a screw placed on that side. Screw selection. Primary screws, 3.5 to 4.5 m m in diameter and 14 to 19 m m long, are usually sufficient to fixate the lateral masses. Typically, safe bicortical fixation is obtained with 3.5 • 15-mm to 16-ram screws. Bicortical fixation is preferable, although unicortical screw purchase is acceptable. Cancellous screws provide better purchase than those with cortical threads. Articular mass screws may or may not be self-tapping. If the screw is not seE-tapping, the posterior cortex of the articular mass at least should be tapped before the screw is placed. Securing the plate. After all articular masses to be instrumented on one side have been drilled, the plate is secured with appropriate screws by tightening to about 80% of final torque in a sequential fashion (Fig 10B). The contralateral articular masses are then drilled, and the corresponding plate is applied and secured with partially tightened screws (Fig 10C). Final screw tightening may then be performed on both sides. The screws will seat into the plate and become snug with two-finger torque. Caution must be exercised to avoid overtightening, as this may strip the screw bed. Before the final plate is applied, the facet joints at all instrumented levels are cleared of soft tissue and packed with autogenous corticocancellous graft material to stimulate fusion. Unacceptable screw purchase may result from osteoporosis, irregular or oversized drill holes, or stripping of threads from tapping or overzealous tightening of the screw. When the purchase is inadequate, a salvage technique must be used. The primary screw is removed and may be replaced by a rescue screw of a slightly larger diameter to improve bony purchase. These screws are not placed without peril, however, as the articular mass may fracture. This risk is increased when the articular mass is small or when the entry site is lateral to the facet midline. Alternatively, the stripped screw hole may be filled with PMMA and the primary screw reinserted. Articular mass plates provide unsurpassed immediate stability to the posterior subaxial cervical spine, often obviating postoperative orthotic immobilization. With this technique, successful arthrodesis has been reported in 98% of cases with low operative morbidity. 47 In almost 500 literature-derived cases reviewed by Traynelis, 47 the incidence of neurovascular injury was substantially less than 1%. Hardware failure may be anticipated in fewer than 1.5% of cases. 3 82

Wound Closure and Postoperative Care Irrespective of the fixation technique used, intraoperative radiographs should be obtained to confirm acceptable alignment and hardware position before the wound is closed. The neck may be gently flexed and extended under direct vision and/or fluoroscopy to assure that stability has been restored. The wound is then reapproximated in anatomical layers with absorbable suture unless local irradiation is anticipated, in which case nonabsorbable suture should be considered. Immediately after anesthesia is reversed, a detailed neurological examination is mandatory. New neurological deficits must be promptly investigated with imaging studies and/or surgical exploration. Titanium implants facilitate imaging, and this factor should be considered when designing a stabilization construct. The need for postoperative orthotic immobilization is dictated by the extent of preoperative instability, the nature of the underlying disease process, and the quality of the internal fixation. With more rigid implants such as articular mass plates, orthoses are seldom required. Less rigid constructs must be supplemented by external bracing. Typically, orthotic immobilization is maintained until there is radiographic evidence of bony fusion.

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9. Capen DA, Nelson RW, 7igler J, et al: Surgical stabilization of the cervical spine: A comparative analysis of anterior and posterior spine fusions. Paraplegia25:111-119, 1987 10. Anderson PA, Henley MB, Gradey MS, et al: Posterior cervical arthrodesis with AO reconstruction plates and bone graft. Spine 16:72-79, 1991 11. Cherney WB, Sonntag VKH, Douglas RA: Lateral mass posterior plating and facet fusion for cervical spine instability. BNI Quarterly 7:2-11, 1991 12. Cooper PR, Cohen A, Rosiello A, et al: Posterior stabilization of cervical spine fractures and subluxations using plates and screws. Neurosurgery 23:300-306, 1988 13. McAfee PC, Bohlman HH: One-stage anterior cervical decompression and posterior stabilization with circumferential arthrodesis. A study of twenty-four patients who had a traumatic or a neoplastic lesion. J Bone Joint Surg Am 71:78-88, 1989 14. McNamara MJ, Devito DP, Spengler DM: Circumferential fusion for the management of acute cervical spine trauma. J Spinal Disord 4:467-471,1991 15. Stauffer ES, Kelly EG: Fracture-dislocations of the cervical spine. Instability and recurrent deformity following treatment by anterior interbody fusion. J Bone Joint Surg Am 59:45-48, 1977 16. Van Peteghem PK, Schweigel JF: The fractured cervical spine rendered unstable by anterior cervical fusion. J Trauma 19:110-114, 1979 17. Herman JM, Sonntag VKH: Cervical corpectomy and plate fixation for postlaminectomy kyphosis. J Neurosurg 80:963-970, 1994 18. White AA, Johnson RM, Panjabi MM, et al: Biomechanical analysis of clinical stability in the cervical spine. Clin Orthop 109:85-96, 1975 19. Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine 8:817-831, 1983 20. Benzel EC: Qualitativeattributes of spinal implants, in Benzel EC (ed): Biomechanics of Spine Stabilization: Principles and Clinical Practice. New York, McGraw-Hill, 1995 21. Roy-Camille R, Saillant G, Mazel C: Internal fixation of the unstable cervical spine by posterior osteosynthesis with plates and screws, in Cervical Spine Research Society Editorial Committee (ed): The Cervical Spine. Philadelphia, Lippincott, 1989 22. Sawin PD, Todd MM, Traynelis VC, et al: Cervical spine motion with direct laryngoscopy and orotracheal intubation: An in vivo cinefluoroscopic study of subjects without cervical pathology. Anesthesiology 85:26-36, 1996 23. Murphy MJ, Daniaux H, Southwick WO: Posterior cervical fusion with rigid internal fixation. Orthop Clin North Am 17:55-65, 1986 24. Savini R, Parisini P, Cervellati S: The surgical treatment of late instability of flexion-rotation injuries in the lower cervical spine. Spine 12:178-182, 1987 28. Geremia GK, Kim KS, Cerullo L, et al: Complications of sublaminar wiring. Surg Neuro123:629-634, 1985 26. Sutterlin CE, III, McAfee PC, Warden KE, et al: A biomechanical evaluation of cervical spinal stabilization in a bovine model. Static and cyclical loading. Spine 13:795-802, 1988 27. Osenbach RK, Moores LE: Subaxial wire and cable techniques in the cervical spine. Tech Neurosurg 1:128-138, 1995 28. Rogers WA: Treatment of fracture dislocations of the cervical spine. J Bone Joint Surg Am 24:245-258, 1942

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29. Whitehill R, Reger SI, Fox E, et al: The use of methylmethacrylate cement as an instantaneous fusion mass in posterior cervical fusions: A canine in vivo experimental model. Spine 9:246-252, 1984 30. Benzel EC, Kesterson L: Posterior cervical interspinous compression wiring and fusion for mid to low cervical spinal injuries. J Neurosurg 70:893-899, 1989 31. Bohlman HH: Acute fractures and dislocations of the cervical spine. An analysis of three hundred hospitalized patients and review of the literature. J Bone Joint SurgAm 61:1119-1142, 1979 32. McAfee PC, Bohlman HH: The triple wire fixation technique for stabilization of acute cervical fracture-dislocations. Orthop Trans 9:142, 1985 33. Weiland DJ, McAfee PC: Posterior cervical fusion with triple-wire strut graft technique: One hundred consecutive patients. J Spinal Disord 4:15-21, 1991 34. Murphy MJ, Southwick WO: Surgical approaches and techniques: Posterior approaches and fusions, in Cervical Spine Research Society Editorial Committee (ed): The Cervical Spine. Philadelphia, Lippincott, 1989 35. Cahill DW, Bellegarrigue R, Ducker TB: Bilateral facet to spinous process fusion: A new technique for posterior fusion after trauma. Neurosurgery 13:1-4, 1983 38. Callahan RA, Johnson RM, Margolis RM, et al: Cervical facet fusion for control of instability following laminectomy. J Bone Joint Surg Am 59:991-1002, 1977 37. Sawin PD, Traynelis VC, Menezes AH: Rib and iliac crest: A comparative analysis of fusion rates and donor site for autogenous bone grafts in posterior cervical fusions. J Neurosurg 88:255-265, 1998 38. Garfin SR, Moore MR, Marshall LF: A modified technique for cervical facet fusion. Clin Orthop 230:149-153, 1988 39. Tucker HH: Technical report: Method of fixation of subluxed or dislocated cervical spine below C1-C2. Can J Neurol Sci 2:381-382, 1975 40. Aldrich EF, Weber PB, Crow WN: Halifax interlaminar clamp for posterior cervical fusion: A long-term follow-up review. J Neurosurg 78:702-708, 1993 41. Holness RO, Huestis WS, Howes WJ, et al: Posterior stabilization with an interlaminar clamp in cervical injuries: Technical note and review of the long term experience with the method. Neurosurgery 14:318-322, 1984 42. Schulder M: Interlaminarclamps: Indications, techniques, and results, in Menezes AH, Sonntag VKH (eds): Principles of Spinal Surgery. New York, McGraw-Hill, 1996 43. Branch CL, Jr, Kelly DL, Jr, Davis CH, Jr, et al: Fixation of fractures of the lower cervical spine using methylmethacrylate and wire: Technique and results in 99 patients. Neurosurgery 25:503-513, 1989 44. Gill K, Paschal S, Corin J, et al: Posterior plating of the cervical spine. A biomechanical comparison of different posterior fusion techniques. Spine 13:813-816, 1988 45. Heller JG, Carlson GD, Abitbo JJ, et al: Anatomic comparison of the Roy-Camille and Magerl techniques for screw placement in the lower cervical spine. Spine 16:552-557, 1991 46. An HS, Gordin R, Renner K: Anatomic considerations for plate-screw fixation of the cervical spine. Spine 16:548-551, 1991 47. Traynelis VC: Anterior and posterior plate stabilization of the cervical spine. Neurosurg Quart 2:59-76, 1992

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