Preoperative Preparation and Positioning

After informed consent for the surgical procedure has been obtained, patients are brought to the operating room wearing antiembolic stockings. Intravenous and intra-arterial access are secured, and a single prophylactic dose of antibiotic is administered 30 minutes before the skin is incised. In patients with evidence of myelopathy or significant compromise of the vertebral canal, baseline evoked potentials (somatosensory and motor) are measured before the patient is intubated or positioned. Muscle relaxants are avoided during surgery to provide an immediate indication of neural irritation. Patients with posttraumatic cervical instability or preexisting myelopathy undergo fiberoptic or awake intubation with the surgeon in attendance. Patients with preexisting myelopathy may receive methylprednisolone in accordance with the NASCIS III protocol immediately before the procedure begins.²¹ Drug infusion is continued throughout the procedure and is discontinued after surgery when the patient’s neurological examination is stable. A urinary catheter is placed after general anesthesia is induced if the procedure is anticipated to exceed 3 hours. When patients are positioned, bony and soft tissue prominences are carefully padded to prevent pressure sores or peripheral neuropathies.

Before surgery patients are counseled regarding the choice of autograft or allograft for their fusion substrate. Given the slightly lower but comparable rate of anterior cervical fusion using fibular allograft instead of autograft (particularly for single-level procedures) and the opportunity to prevent the complications associated with harvesting a tricortical segment of autologous iliac crest, most of our patients choose an allograft fusion procedure.^20,22 Among the choices for allograft sources, we prefer fibular allograft given its greater compressive strength compared with tricortical iliac crest allograft.^6,20,23,24 An alternative to bone grafts is now available with the emergence of polyetheretherketone (PEEK) implants, which are associated with fusion rates similar to those of bone grafts when used in conjunction with the correct dose of recombinant human bone morphogenic protein-2 placed inside the implant.²⁵

Reported donor site complications after autologous iliac crest harvest include acute and chronic pain, infection, arterial hemorrhage, peripheral nerve injury, pelvic instability, cosmetic deformity, hernia formation, and postoperative hematoma.²⁶ The risk of transplanting HIV-infected bone has been estimated at less than 1 in 1 million when stringent screening criteria are used.^6,24,27 If an autograft is to be harvested from the iliac crest, the appropriate hip is elevated with a towel roll. Patient preference determines the side from which the autograft is removed. An intrascapular roll, tape along the lateral aspect of the arms, and soft wrist ties that can be manipulated by the circulating nurse are helpful adjuncts that facilitate intraoperative radiographic visualization of the distal vertebral column.

We prefer the Caspar head holder (Aesculap, Center Valley, PA) to support the patient’s head and cervical spine (Fig. 297-1). The head is maintained in a neutral position, and the neck is maintained neutrally or is extended minimally with the assistance of a chin strap. The evoked potentials should be observed carefully for any changes, and intraoperative fluoroscopy should be available to confirm the maintenance of cervical alignment (anatomic or best attainable) after positioning is completed. At our institution, the convenience and cost-effectiveness of using cross-table fluoroscopy from the beginning of the procedure are well-established compared with the relative expense and delays associated with obtaining intraoperative plain film radiographs. The importance of fluoroscopy in confirming the operative level, selecting an appropriately sized cervical plate, directing screw trajectories, assessing final screw positions, and evaluating plate alignment relative to the vertebral column cannot be overstated.

FIGURE 297-1 Patient positioning with the Caspar head holder. The patient’s head is maintained in a neutral position by means of an elastic chin strap. The cervical spine is carefully maintained in either a neutral or minimally extended posture to recreate cervical lordosis. Adhesive tape is run along the lateral margin of the shoulder joint and arm and affixed to the foot of the bed to assist with intraoperative fluoroscopic visualization of the distal cervical spine. The tape should not be run directly over the clavicle to avoid a pressure injury to the brachial plexus. An intrascapular roll facilitates operative access by allowing the shoulders to fall below the coronal plane of the cervical spine. Both the scalp leads for evoked potential monitoring and the endotracheal tube (not shown) would be rostral in the operative field.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Skin Incision

The operative approach is directed from the side that is most comfortable for the surgeon and usually corresponds to the patient’s right side in a right-handed surgeon. It was thought that the recurrent laryngeal nerve is more susceptible to injury given its relatively anterolateral course outside the tracheoesophageal groove when compared with the left side.²⁸ Consequently, some surgeons may prefer an approach directed from the patient’s left side. However, injury to the thoracic duct has been reported as a complication of approaching the caudal cervical spine from the left side.²⁹ Studies have shown that there is no difference in the incidence of recurrent laryngeal nerve injury associated with a right- or left-sided approach.^30,31 If the patient has already undergone a cervical procedure, we pursue operative access from the ipsilateral side. Although this strategy requires contending with scar tissue and altered anatomic planes, it avoids the more daunting possibility of incurring bilateral injury to vagal nerve branches, the unilateral manifestations of which may be subtle and otherwise undetected unless specifically evaluated for after the previous procedure. If a contralateral approach to a previous scar is pursued, we recommend that the patient undergo preoperative examination of the vocal cords by an otolaryngologist to avoid the possibility of bilateral injury to vagal nerve branches.

A general orientation along the cervical spine can be estimated by external anatomic landmarks (Fig. 297-2), but intraoperative fluoroscopy ensures a more precise placement of the skin incision and limits the amount of soft tissue dissection necessary to obtain adequate visualization of the targeted segment of the vertebral column. Preoperative use of the fluoroscope to define the most rostral and caudal levels of exposure necessary for decompression and stabilization also assists in selecting the optimal orientation for the incision. We have found that a transverse incision located within a skin crease is cosmetically superior to a longitudinal incision that follows the medial border of the sternocleidomastoid muscle. When extended adequately (beyond the midline and laterally across the sternocleidomastoid muscle) and accompanied by generous undermining of the platysma muscle, a transverse incision rarely fails to provide sufficient access and visualization to enable multiple corpectomies to be performed with an accompanying fusion and plating procedure. However, the longitudinal incision may be more functional in patients with difficult anatomy or in whom a particularly long fusion construct is required.

FIGURE 297-2 Orientation to the vertebral column may be estimated by palpating superficial anatomic structures. The hyoid bone sits roughly at the level of the C2-C3 disk space. The top of the thyroid cartilage can be estimated as the C3-C4 disk space. The inferior border of the thyroid cartilage can be estimated as the C4-C5 level. The cricoid ring approximates the level of the C5-C6 disk space. The C7-T1 disk space sits approximately one finger’s breadth above the clavicle. The incision is planned according to the level and extent of the pathology. Single-level procedures may be performed with a transverse incision (dashed line) through a skin fold. Multilevel procedures are undertaken with a longitudinal incision (solid line) along the anterior border of the sternocleidomastoid muscle. Our preference is to expose the vertebral column through a transversely oriented skin incision.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Soft Tissue Dissection and Exposure of the Vertebral Column

After the patient has been prepared and draped, the skin is sharply incised to the level of the platysma muscle. Hemostasis is obtained with electrocauterization, and the platysma layer is traversed or split longitudinally. Early attention to broad undermining of this subcutaneous muscle is greatly rewarded by the rostral and caudal extents of surgical exposure that can be attained. The underlying sternocleidomastoid muscle and tracheoesophageal bundle are identified, and the avascular plane between these structures is developed with careful, blunt dissection. A trajectory medial to the carotid sheath is followed, and the underlying vertebral column is palpated (Fig. 297-3). Comparing the osteophytic topography to preoperative or intraoperative radiographs can often help orient the surgeon along the cervical column, but fluoroscopy is used for definitive localization along the cervical spine. The prevertebral fascia is opened, and the ventral aspect of the anterior longitudinal ligament is cleaned of overlying soft tissue. The medial insertions of the longus colli muscles are elevated bilaterally from the vertebral column.

FIGURE 297-3 The relative anatomy and trajectory for an anterior transcervical retropharyngeal approach to the cervical vertebral column. A plane of dissection is maintained lateral to the tracheol-esophageal bundle and medial to the carotid sheath. Bilaterally, the longus colli muscles are dissected subperiosteally to provide a submuscular pocket for seating the toothed retractor blades. Failure to seat these blades beneath the longus colli muscles properly places the esophagus and adjacent vascular structures at risk for perforation. When the vertebral column is approached anterolaterally, the tendency is to direct decompression eccentric to the contralateral side. Thus, regardless of whether the operative approach is conducted from the patient’s left or right side, it is important to ensure that the ipsilateral neural foramen receives adequate attention and decompression. When anterior cervical plates are applied, the tendency is to place the plates slightly eccentric to the side from which dissection is directed. Ideally, the plates should be applied in as midline of a position as possible to minimize the risk of injuring the vertebral artery and to provide as optimal a biomechanical construct as possible.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Several self-retaining retractor systems are available for enhancing exposure of the anterior vertebral column. Fundamental to all of these retractors is the requirement to seat their laterally directed blades deep to the longus colli musculature to prevent injury to the adjacent tracheoesophageal bundle or carotid artery. Injury to the vagal innervation of the larynx can result in silent aspiration (superior laryngeal branch) or hoarseness (recurrent laryngeal nerve). Apfelbaum and Kriskovich⁸ have reported that injury to the recurrent laryngeal nerve during anterior cervical surgery is most likely because of compression of the endolaryngeal segment of this nerve against the shaft of the endotracheal tube after the retractor is placed rather than direct injury from dissection. To protect against this neuropraxia, they describe transiently deflating the cuff of the endotracheal tube after placing the self-retaining retractors and then reinflating the cuff to “just seal” pressure. This allows the endotracheal tube to recenter within the larynx and remove pressure from the laryngeal wall. This technique improved the incidence of injury to the recurrent laryngeal nerve, which decreased from 6.4% in 250 patients undergoing anterior cervical plating without this maneuver to 1.7% of the next 650 consecutive patients in whom it was used.

Rostral-caudal exposure can be improved by placing a second retractor perpendicular to the first (Fig. 297-4). Use of the high-speed drill to remove obstructing osteophytes, in conjunction with the superior illumination and magnification offered by the surgical microscope, has significantly diminished the amount of active cervical distraction necessary to confirm adequate neural decompression during a diskectomy procedure. Distraction posts are usually inserted in the vertebrae to improve exposure of the disk space during decompression of the spinal cord and neural foramina. Alternatively, some surgeons maintain vertebral distraction by placing patients in axial traction using Gardner-Wells tongs at the start of the procedure³² or through gentle, even traction applied by the anesthesiologist during graft impaction. If the patient is maintained in tongs during the procedure, it is crucial to remember to release this distraction after the grafting material has been inserted and before the fixation hardware is placed.

FIGURE 297-4 A second, perpendicularly oriented Caspar retractor system assists with rostral-caudal exposure of the vertebral column. These longitudinally oriented retractor blades are without teeth. Alternatively, the Caspar vertebral body distraction posts can be used to help retract soft tissue in this plane. If the latter system is used, the posts are preferentially placed in the vertebral bodies rostral and caudal to those intended for screw placement.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Discectomy With or Without Corpectomy

Once orientation at the level of pathology has been confirmed, annulotomies are performed and superficial diskectomies are initiated with straight and angle curets (Fig. 297-5A). If an interval corpectomy is necessary, a bone rongeur can be used to resect the anterior half of the vertebral body and the Midas Rex drill (Medtronic, Memphis, TN.) used to complete the deeper aspect of bone removal (Fig. 297-5B). If placement of an allograft strut (fibular) is planned, autologous bone from the vertebrectomy site is saved for packing the hollow center of the allograft shaft. The operative microscope is routinely used to assist with the removal of deeper bone and soft tissues to facilitate safe exposure of the dura. The epidural space is inspected, and posteriorly based osteophytes are removed from the vertebral bodies and foramen to ensure adequate decompression of the spinal cord and nerve roots. Where possible, generous diskectomies are substituted for a corpectomy if adequate neural decompression can be achieved through wide and deep undercutting of the offending posterior vertebral body surfaces. The benefits of avoiding a complete corpectomy include preserving additional sites for screw fixation along the plate and circumventing the higher risk of nonunion or hardware failure that accompanies fusion constructs involving multiple corpectomy segments.^33,34 This latter point is particularly relevant for patients who require multilevel decompressions.

FIGURE 297-5 A, Anterior view of partially completed C3-C4 and C4-C5 diskectomies. The lateral extent of disk removal is the uncovertebral joints bilaterally. If a corpectomy is planned, the adjacent disk spaces are similarly first defined, and superficial diskectomies are performed before the vertebral body is resected. The superficial aspect of the corpectomy can then be performed easily with either a bone rongeur or a Midas Rex (Medtronic, Minncapolis, MN) drill. Typically, we prefer to use the AM-8 Midas bit to perform the corpectomy. B, Sagittal view of a C4 corpectomy. The deeper aspects of the diskectomies and corpectomy are performed under the operating microscope to ensure the safe exposure and decompression of the epidural space. Once the epidural space has been identified at the level of the rostral and caudal disk spaces, the remaining posterior cortex of the interval vertebral body is removed with bone punches.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

When completed, the typical lateral extent of tissue removed for diskectomies or corpectomies spans up to 18 to 20 mm. Reliable identification of the vertebral midline is crucial to ensure adequate decompression of neural tissue and to prevent vascular complications related to injury of the vertebral artery.^35–37 Frequently, severe degenerative disease, traumatic disruption, or scarring from previous surgical procedures result in the loss of the otherwise apparent anatomic midline. Typically, however, several anatomic cues remain and can be used to provide orientation to the midline for both decompressive maneuvers and plate positioning (Table 297-1). Marking the midline of the vertebral bodies with monopolar cauterization before the longus colli muscles are elevated can also provide helpful reference as the procedure progresses. It is also worthwhile to confirm with the anesthesiologist that the patient’s head has not deviated from the midline position established at the beginning of the case.

TABLE 297-1 Anatomic Cues Available for Maintaining Midline Orientation

From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.

Bone Grafting and Plate Fixation

Screw-plate application provides immediate rigid fixation of the cervical spine and functions analogously to an internal halo brace. By ideally imparting some combination of load bearing to the vertebral column and load-sharing properties through the graft site, plating systems protect the neural elements from trauma while facilitating the development of a fusion response, respectively. Bone and instrumentation deform and reform as stress is applied.³⁸ Over time, even the most rigid constructs permit some segmental motion across the site of fixation. In the absence of an osseous union, repetitive loading will fatigue an implant to the point of failure through loosening or breakage. Consequently, perhaps the most fundamental principle related to performing rigid internal fixation is that the presence of instrumentation does not substitute for a carefully conceived and meticulously prepared fusion site.^39,40 In the absence of an associated arthrodesis, hardware failure is a time-dependent certainty (Fig. 297-6).

FIGURE 297-6 A, Anteroposterior and (B) lateral views of a patient treated at an outside facility with a failed anterior cervical plate. Note the transversely oriented fracture at the plate’s midline. The rostral and caudal screw sets maintain good position. There is no evidence of arthrodesis spanning the diskectomy site.

We typically use the Robinson-Smith technique for interbody fusion after a cervical diskectomy.⁴¹ Techniques to optimize the chances for successful arthrodesis (Table 297-2) at the operative site can be subdivided into those that (1) enhance the natural capacity for bone healing to occur, (2) minimize the extent of iatrogenically induced impediments to bone graft incorporation, and (3) maximize the biomechanical advantage of the hardware construct.

TABLE 297-2 Techniques to Optimize Fusion-Hardware Construct

From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.

Enhancing the Natural Capacity for Bone Healing

Bone grafts are incorporated with the greatest success when the fusion construct is maintained under a compressive load (Wolff’s law).⁴² Consequently, the vertical dimension of the graft material being used is typically sized a few millimeters longer than the measured diskectomy or corpectomy defect to ensure mechanical compression of the graft within the recipient bed. At the time the graft is placed, the adjacent vertebral bodies can be distracted mildly through several techniques (disk space spreader, vertebral body distraction posts, axial traction, or gentle vertical distraction by the anesthesiologist) to ensure that the graft ultimately experiences a compressive load. After the posterior half of the graft has been tamped into place, the vertebral body distraction is released and the remainder of the graft advanced. These maneuvers promote seating and prevent excessive impaction of the graft. When relying on axial traction applied with cranial tongs to achieve vertebral distraction during the decompressive portion of the procedure, it is important not to overlook requesting its release before proceeding to internal fixation.

After final positioning, graft security and an appropriate epidural margin are confirmed by palpating posterior to the graft with a nerve hook and visualizing the posterior border of the graft with fluoroscopy. Before the graft is inserted, all soft tissues are removed from the graft material and fusion interfaces (end plate articular cartilage) to prevent the delayed differentiation of fibrous tissue that might hinder bone formation. The creation of smooth, apposing surfaces along the bone graft and vertebral bodies serve to improve the environment for fusion by maximizing the area of surface contact at the bony interfaces.

Iatrogenic Impediments to Fusion Biology

High-speed drills are convenient for denuding articular surfaces within the fusion bed and for shaping the fusion surfaces of the vertebral bodies and graft. During all drilling, consistent irrigation is needed to prevent thermal injury that can incite subsequent bone resorption and interfere with the fusion response. Similarly, the excessive use of monopolar cauterization to expose the vertebral column can impede healing as a result of direct thermal injury and bone devascularization. The recipient bed (vertebral surfaces) is the source of almost all of the osteogenic components critical to the fusion response. Even when using an autologous bone graft, few of the transplanted osteoblasts and osteocytes actually survive to contribute to healing.²⁰ As such, the surface area of healthy vascularized bone in the fusion bed should be maximized. Using monopolar cauterization for deep tissue dissection also places adjacent soft tissues (e.g., esophagus, recurrent laryngeal nerve) at greater risk. Bone bleeding within the recipient site can be controlled with thrombin-soaked Gelfoam sponge (Upjohn, Kalamazoo, MI.), Avitene powder (MecChem, Woburn, MA.), Floseal (Baxter, Deerfield, IL.) or, if particularly intractable, bone wax. These substances hinder the fusion response and should be used sparingly at the graft-vertebral body interface.

Optimizing the Fusion and Hardware Construct

Most available cervical plates are machined with an established lordotic curve to conform to the contour of the cervical spine. Although most of these plates can be further shaped using special devices at their time of insertion, this action weakens the plate and should be avoided if possible. Ventral osteophytes should be removed to allow a flush application of the plate to the midline of the spinal column. Its position can be confirmed fluoroscopically and by the absence of seesawing when the sides or the rostral and caudal ends of the plate are alternately compressed against the vertebral column. When osteophytes are removed, as much of the cortical bone layer as possible should be retained because it contributes substantially to an individual screw’s resistance to pulling out. This point is particularly important when screw-plate systems that rely only on unicortical bone purchase are implanted.

In contrast to cervical fusion procedures that do not include instrumentation, the graft material is not countersunk away from the ventral margin of the disk space. Instead, the anterior border of the bone plug or strut is left in line with the anterior margin of the adjacent vertebral bodies to maximize the contact surface between the plate and the entire fusion construct. A trough along the posterior aspect of one of the vertebral bodies protects against graft retropulsion and epidural compression. Autograft struts that span more than a one-level corpectomy are secured to the plate with a bicortically anchored screw to prevent graft retropulsion. Typically, fibular allograft is too brittle to accept a bone screw, and posterior troughs are created at the upper and lower vertebral levels to protect against its displacement (Fig. 297-7).

FIGURE 297-7 Sagittal views of corpectomy defects before (A) allograft and (B) autograft fusion procedures. Because a screw can be used to secure the autograft to the anterior cervical plate, posterior troughs are unnecessary to prevent graft retropulsion. Conversely, the allograft material is usually too brittle to accept a screw, and rostral and caudal troughs are created within the respective vertebral bodies to protect the epidural space. The height of the troughs is 1 to 2 mm, and a concerted effort is made to maintain the integrity of the adjacent vertebral end plates to minimize the risk of the graft telescoping through the adjacent vertebral bodies. C, Midlevel axial view through a corpectomy defect after insertion of an autologous bone graft illustrates the preference for orienting autologous graft material. As placed, the cortical margins serve to buttress the anterior and middle columns while minimizing the anteroposterior diameter of the graft. C4 corpectomy site after (D) allograft and (E) autograft fusion with plating procedures. In both cases, the graft material is not countersunk; the anterior margin of the graft material is flush with the undersurface of the plate. The allograft material is packed with autologous bone from the corpectomy site to promote fusion. A bicortical screw fixes the autograft to the cervical plates.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

When the recipient site is prepared for graft placement, the cortical end plates are thinned sufficiently to expose bleeding bone but mostly left intact to minimize telescoping of the graft material through the adjacent vertebral bodies. This step is particularly important when allograft material is implanted because its rigidity makes this complication more likely than when autograft is used, especially in patients with osteopenic or otherwise soft bone. Loss of graft height, either because of settling or resorption, adds further mechanical stress to the screw-plate construct and increases the risk for implant failure and pseudarthrosis. Proper attention to graft harvesting (autograft) or preparation (allograft) optimizes the load-bearing capabilities of these materials. We prefer to harvest autologous iliac crest bone using an oscillating saw to prevent osteotome-related microfractures, which can reduce the compressive strength of the graft.⁴³ Failure to reconstitute the freeze-dried allograft appropriately in saline (minimum of 30 minutes) before it is shaped or implanted compromises its structural integrity.²⁰

Selection of screw type and screw length depends on the pathology being addressed. The interaction of variable angle or fixed angle screws with the plate determines whether a plate acts as a constrained system or semiconstrained system or hybrid system. Use of variable angle screws allows some movement at the screw-plate interface and thus allows settling of the vertebral body end plate on the interbody graft. Such constructs are considered load-sharing or dynamic fixation. Fixed angle screws do not allow movement at the screw-plate interface and act against settling of the vertebral body end plate on the graft. These constructs are considered load-bearing or constrained or static fixation. Using a combination of fixed and variable angle screws is considered semiconstrained or a hybrid construct. Biomechanical studies on cadaveric specimen have shown that variable angle screws provides better load-sharing properties than fixed angle screws when graft subsidence considered.⁴⁴ For degenerative pathology, we prefer a hybrid construct using fixed angle screws at the most caudal vertebral level and variable angle screws at the remaining levels. For traumatic pathology, we prefer a constrained system and thus use fixed angle screws at all levels and aim for bicortical purchase.

Two-finger tightness is the desired final torque when inserting bone screws. Overtightening screws can strip the threads of the screw hole and diminish their resistance to pulling out. If the tapped threads are stripped, there are several options for securing the construct. A larger diameter rescue screw (if available) can be substituted for the initially placed fixation screw. A new screw path can be drilled within the same plate-hole site if the screw trajectory can be varied. The entire plate can be moved so that a new hole can be drilled. Finally, the initial screw’s purchase can be bolstered with methylmethacrylate. Pilot hole drilling and screw placement should always be monitored with intermittent real-time fluoroscopy, irrespective of the surgeon’s experience with screw-plate fixation.

If possible, the screw trajectory should capture the denser bone tissue in the subchondral region of the vertebral body while respecting the end plate of the vertebral body. Crossing the distal vertebral end plate results in a suboptimal screw purchase that can further loosen over time because of motion across the violated disk space. Alternatively, this misplacement risks incorporating a normal motion segment within the fusion construct. If unicortical screw placement is pursued, the longest screws that can be accepted by an individual patient’s anatomy are used for plate fixation because they resist pulling out better than their shorter counterparts. When fixation involves multiple segments, the plate should be secured at as many points as possible. This step is particularly important at the caudal aspect of the plate given the greater failure stresses transmitted to that location.^33,34 Because additional sites for fixation are preserved, partial corpectomies or generous diskectomies are desirable substitutes for a completed corpectomy. Obviously, minimizing bone removal presupposes satisfying the goal of adequate neural decompression. Midsagittal application of the plate fosters optimal load sharing among the fixation points and is facilitated by fluoroscopic guidance, as noted below.

Closure

Final inspection of the operative site includes assessing the alignment of the vertebral column after fixation, the position of the graft with respect to the epidural space, and the length of the plate and position of the screws with respect to the rostral and caudal disk spaces. Midline location and vertical orientation of the plate can be assessed fluoroscopically by observing the parallel overlap of plate holes and screw trajectories on true lateral cross-table views. An anteroposterior view can be obtained as the fluoroscope is removed from the operative field. Location of the plate midway between the vertebral pedicles and in line with the superimposed spinous processes confirms placement in the vertebral midline. The asymmetrical application of a plate most often is a consequence of the caudal aspect of the plate being shifted toward the side of surgical approach (see Fig. 297-6A).

Bacitracin-containing saline is used to irrigate the wound, and hemostasis is obtained with bipolar cauterization. Self-retaining retractors are removed, and the trachea, esophagus, and carotid sheath are inspected for evidence of injury with a hand held retractor. The presence of a carotid pulse above and below the level of self-retaining retractors is confirmed. The platysma muscle and dermis are closed as separate layers using interrupted absorbable sutures, and the skin is further reapproximated with a running subcuticular suture and Steri-Strips or Dermabond (Ethicon Inc, Somerville, NJ). We have found no need to maintain a surgical drain within the operative site, and current evidence does not support the routine use of prophylactic postoperative antibiotics in otherwise immunocompetent patients.

Caspar System

Introduced commercially in 1982, the Caspar osteosynthetic plate requires bicortical screw purchase (Fig. 297-8). The screw trajectory is variable, and no locking mechanism secures the screw head to the plate (nonconstrained/dynamic system). The requirement for bicortical screw purchase makes this the most technically challenging system to use because the depth of posterior cortical penetration must be determined precisely to avoid compromising the epidural space. The plates are machined without contour in the sagittal plane but can be curved as desired. The slotted configuration allows screws to be positioned and redirected as necessary. The recommended screw trajectory is directed 15 degrees medially and parallel to the adjacent vertebral end plate. The appropriate plate length should measure 2 mm from the rostral and caudal end plates of the rostral and caudal vertebral bodies, respectively, that are incorporated into the fusion construct.^34,47 The posterior longitudinal ligament sufficiently shields the dura as long as the blunt-tipped bicortically placed screw does not extend further than the width of one screw thread beyond the posterior cortex.

FIGURE 297-8 Early generation Caspar plate (28 mm) and screw. Bicortical screw purchase prevents the need to fixate the screw head rigidly to the plate. Newer generation Caspar plates are titanium and have midline holes for holding pin placement.

(From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.)

Although bicortical screw placement poses a greater potential risk for dural violation and spinal cord injury than a unicortically positioned screw, the Caspar system has been in clinical use longer than any of the available screw-plate systems. It remains popular with a very low reported rate of complication.⁴⁷ The newer generation Caspar titanium plate has been modified to include midline holes for holding pin placement and is marked to orient surgeons to its rostral and caudal ends. It is the least expensive of the available screw-plate systems (Table 297-3).

Synthes

Introduced in 1986, the Synthes unicortical system (Cervical Spine Locking Plate) employs fixation screws that accommodate a separate internal 1.8-mm expansion locking screw (Fig. 297-9). This combination rigidly secures the screw head to the plate (constrained/static system). The rostral and caudal fixation screws are directed in a fixed, medially convergent trajectory that increases the resistance to pulling out. The upper screws are angled 12 degrees cephalad to approximate the cervical lordosis, and the lower screws are directed perpendicular to the plate in the sagittal plane.^24,48 Initially available only in a 4.0-mm diameter, 14-mm length, the fixation screws are now produced in 12-, 14-, and 16-mm lengths in both standard and self-tapping varieties. The working screw diameter for the set is 4.0 mm, and a 4.35-mm rescue screw is available in standard and self-tapping forms. As is the case with all of the plating systems, self-tapping screws are significantly more expensive than the standard variety (see Table 297-3).

FIGURE 297-9 Synthes plate (28 mm) with fixation (left) and locking (right) screws. The fixation screws are anchored to the plate by means of an expanding head following placement of the axial anchor screw. Each fixation screw receives its own locking screw.

(From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.)

The plate has an established lordotic curve, and a fixed rostral and caudal orientation (indicated by an arrow at the plate’s rostral end). Bending, which further weakens the plate’s structure, should be performed away from the screw holes to avoid altering the integrity of the locking mechanism. Bending also alters the fixed screw trajectory and can compromise the final screw position. The Synthes plates are available in small stature dimensions, but they cost more than the standard sizes.

Orion

This contoured system uses a fixed trajectory screw (constrained/static system) that is medially convergent (6 degrees) with 15 degrees of angulation at the rostral and caudal ends (divergent) (Fig. 297-10). As with the Synthes system, further plate bending alters the cephalad and caudal angulations of screw placement. In contrast to every other locking plate system, the Orion plate accommodates locking screws (separate components) only at its rostral and caudal ends. The head of each locking screw overlaps the tops of the paired 4.0-mm diameter fixation screws at the respective ends of the plate. Additional anchoring screws can be placed through a variable interval slot (4.35-mm diameter screws recommended) that is present within the longer plates. Screws in this location, however, cannot be secured to the plate with a locking screw and preferably are placed with a bicortical purchase. A 4.5-mm rescue screw is available within this system. The 4.0-, 4.35-, and 4.5-mm screws are currently produced only in a self-tapping form.

FIGURE 297-10 Orion plate (27.5 mm) with fixation (right) and locking (left) screws. Only the rostral and caudal fixation screw sites accommodate a locking screw. One locking screw straddles the two fixation screws at these levels.

(From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.)

Insertion of the locking screw requires an orientation that is truly perpendicular to the plate surface. Failure to achieve this angle, which may be particularly difficult in the vicinity of the mandible or clavicle, can prevent successful placement of the locking screw. A self-limiting torque mechanism causes the driver for the locking screw to twist free from the screw head when it is secured appropriately. Plate sizing must allow for the fixed screw angulation of 15 degrees. Because of the vertical angle of the screws, the plate length should span just beyond the margins of the graft site to ensure that the screws do not violate the adjacent end plates.⁴⁹

Slim-Lock Cervical Plate

The slimlock cerrical platie (DePuy Spine) is a contoured titanium plate (Fig. 297-11) that offers variable trajectories for screw insertion and has no specific rostral or caudal end. A locking cam mechanism integrated into the plate resists screw back out and allows unicortical screw fixation. However, the screw head is not rigidly fixed within its plate hole and can change its angulation relative to the plate (nonconstrained). This dynamic aspect of the system allows some load sharing and affords the system some tolerance for delayed vertebral settling that might occur because of graft resorption or telescoping.

FIGURE 297-11 Codman plate (28 mm) with fixation screw. Note the integrated locking cam mechanism. Each fixation screw site has its own individual corresponding locking-cam site.

(From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.)

When the locking cam engages the screw head appropriately, the cam driver clicks. Extremes in screw angulation (>16 degrees) can prevent the cam system from formally engaging the screw head and cause the cam to continue to rotate as directed by the cam driver. In this case, the cam can be left in its lock zone between 240 and 270 degrees of rotation to offer some resistance to the screw backing out. This incomplete contact between the locking cam and screw head further influences the degree of delayed angulation that can occur between the screw head and plate. If the plate is bent to optimize contact with the vertebral column, its curvature should be distributed evenly throughout its length and limited to the thinner, designated bend zones to prevent the locking cam mechanism from failing.

The more commonly used 12- and 15-mm drill bits, taps, and screws are color-coded (blue and gold, respectively) to simplify use of the system. However, a wide range of screw lengths is available and can be inserted using the available variable depth drill guide and taps. All screws in this system are 4.5 mm in diameter, the size typically reserved for rescue screws in other systems. The recommended screw placement is 10 degrees medially and parallel to the orientation of the adjacent disk space in the sagittal plane. The recommended plate length is from the rostral subchondral region of the most rostral vertebral body to the caudal subchondral region of the most caudal vertebral body included in the fusion construct.⁵⁰

Atlantis Vision System

With the Atlantis Vision System, fixed or variable screw trajectories are available for any plate level or screw site (Figs. 297-12 and 297-13). The system includes specific drill guides that either lock in a fixed position in the plate screw hole (12 degrees divergent in sagittal plane, 6 degrees medially convergent) or allow variable angulation through an arc of approximately 31 degrees relative to the axis of the screw hole (Fig. 297-14). The diameter of the holding pin available for hands-off stabilization of the plate during drilling is small enough that an anchoring screw can ultimately be passed along its track (Fig. 297-15). The option of fixed or variable screw trajectories within any one plate-hole site enables the production of constrained, nonconstrained, or hybrid biomechanical constructs based on the underlying pathology of the vertebral column (Fig. 297-16).

FIGURE 297-12 Atlantis plate (27.5 mm) with variable (left) and fixed (right) trajectory fixation screws. Fixation screw heads at any location on the plate are within access of one of the integrated locking screws.

(From Baskin JJ, Vishteh AG, Dickman CA, et al. Techniques of anterior cervical plating. Oper Tech Neurosurg. 1998;1:90-102.)

FIGURE 297-13 The fixed and variable trajectory screws of the Atlantis system. The configuration of the head and proximal shaft of the fixed screw results in a rigid or constrained interface with the Atlantis plate in contrast to the nonconstrained interface between the variable screw and the Atlantis plate.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 297-14 A, Anterior and sagittal views of the Atlantis plate. The sagittal view shows the fixed screw trajectory angled 12 degrees rostral to a line that is perpendicular to the plate at the rostral screw site and the range of angulation available with the variable screw at the caudal screw site. Because the screw hole is eccentric on the plate, the arc of rotation possible in the sagittal plane is 6 degrees toward the plate relative to a line perpendicular to the plate, or 25 degrees from this line. B, Axial view of the Atlantis plate showing the fixed medial (convergent) trajectory of 6 degrees relative to a line drawn perpendicular to the plate and the angulation available with the variable screw (4.0-mm diameter) relative to the same perpendicular line. The degree of angulation possible in the axial and sagittal planes is less with the larger diameter 4.5-mm screw.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 297-15 A holding pin is available to stabilize the plate while other screw sites are drilled and tapped. The individual plate holes in the Atlantis system can accommodate either a fixed or variable trajectory drill guide or screw. A locking screw is available for all plate-hole sites.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 297-16 Atlantis plate system with a fixed trajectory screw (medially directed) used at one position and a variable trajectory screw at the neighboring site. This flexibility allows the surgeon to compensate for a patient’s aberrant anatomy or for suboptimal screw positions or purchases. Consequently, the biomechanical stability of the fixation construct can be optimized.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

The locking mechanism of the Atlantis plate consists of a locking screw and washer, which compress and cover the fixation screw heads in pairs (Fig. 297-17). Similar to the Codman system, extremes in screw angulation can result in an incomplete interface between the anchor screw and locking mechanism. At the time of screw insertion, it is important to make certain that the locking washer remains above the level of the fixation screw head. Due to its mobility while in an unlocked position, the locking washer can be caught under the head of the anchor screw as the latter is advanced into the vertebral body. The anchor screw would then be prevented from fully seating within its plate hole, and locking the pair of anchor screws to the plate would be precluded. The Atlantis plate is manufactured with all of the locking screws in the open, elevated position. Before the wound is closed, all locking screws should be recessed to minimize the profile of the fixation construct.

FIGURE 297-17 Comparison of the screw-locking mechanisms offered by the (A) Synthes, (B) Orion, (C) Codman, and (D) Atlantis cervical plating systems.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

ABC System

Released for use in 1999, this system consists of precontoured, slotted, trapezoidal plates and a variable trajectory screw (Fig. 297-18). The screw head consists of five compressible petals that snap into the plate slot and prevent the screw from backing out (Fig. 297-19). The screws also have a self-contained locking mechanism: An internal pin, engaged by a locking tool, can be pulled up into the screw head and functions to block the petals in their respective positions and to secure the screw head rigidly to the plate (Fig. 297-20). Although the angle between the screw head and plate is fixed (constrained), the screws can migrate vertically within their slots. This capacity to maintain axial loading despite graft subsidence permits load sharing by minimizing the presence of stress shielding and promotes fusion.⁵¹

FIGURE 297-18 ABC trapezoidal plate (Aesculap). The slotted configuration promotes load sharing by allowing vertical settling of the fixation construct. Midline holes are for the holding pins.

(From Aesculap Anterior Plating System, South San Francisco, CA.)

FIGURE 297-19 Axial view of the ABC screw head (Aesculap). The head consists of five compressible petals that snap into the ABC plate slot. The top of the internal “blocking pin” is recessed within the screw shaft (unlocked position).

(From Aesculap Anterior Plating System, South San Francisco, CA.)

FIGURE 297-20 Longitudinal view of the ABC screw (Aesculap) with internal blocking pin in the (A) unlocked and (B) locked positions. When engaged, the blocking pin prevents compression of the screw head’s petals, thus rigidly locking the screw head to the plate (fixed pitch).

(From Aesculap Anterior Plating System, South San Francisco, CA.)

The screws are available in self-tapping unicortical and nontapping flat-tipped bicortical versions. The system design allows angulation of ±30 degrees in the vertical axis and ±8 degrees medially and laterally in the coronal axis. The optimal ABC plate length is one in which the midpoints of the upper and lower most screw slots overlie the adjacent graft-vertebral body interface. In other words, 50% of the screw slot should overlap the graft and 50% should overlap the adjacent vertebral body. To further accommodate settling, the individual screws should be positioned at the most distal aspect of their respective slots. These positions should correspond to an entry site at the most caudal end of the rostral vertebral body and the rostral end of the most caudal vertebral body. Temporary fixation pins can be placed through designated holes along the plate’s midline axis to stabilize it during insertion.

Atlantis Translational System

An example of translational plates is the new Atlantis Translational system, which became available in 2007. The plate features fixed and variable angle screws allowing constrained, nonconstrained, or hybrid biomechanical constructs. It also features an integrated locking mechanism (Fig. 297-21). The intention of this new plate is to maintain constant graft compression throughout the range of motion of the cervical spine by allowing unidirectional translation up to 2 mm per level in 1-mm increments. Unlike bidirectional translational plates that allow graft unloading during extension, unidirectional translational plates maintain constant graft loading even during extension. In addition, this plate allows in situ compression of the plate to maintain active compression of the interbody graft. This feature could be particularly helpful for the treatment of traumatic distraction injuries. After the graft is placed, the plate is locked in place with fixed angle screws. The plate can be compressed up to 2 mm per level using a plate compressor that fits in the locking mechanism (Fig. 297-22).

FIGURE 297-21 A, Lateral and (B) anterior views of the Atlantis translational plate in the open position and (C) anterior view in the closed position.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

FIGURE 297-22 Lateral view of Atlantis translational plate being compressed with the compressor seated in the locked screw mechanism.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

UNIPLATE System

The UNIPLATE system has been available since 2005. Compared with all the systems discussed previously, it features one column of screws instead of two columns with one screw hole per vertebral body level. It is available with two or three screw holes and thus can be applied after a one- or two-level anterior cervical discectomy and fusion (Fig. 297-23). Similar to the Slim-Lock plate, the UNIPLATE features a cam locking mechanism to lock the screw. There is a paucity of biomechanical data on the efficacy of the UNIPLATE design in stabilizing the cervical spine. As a result, the UNIPLATE can be regarded as an effective graft-holding device to prevent complications from interbody kick-out after an anterior cervical discectomy and fusion.

FIGURE 297-23 Oblique view of a two level Uniplate system showing the single column of screw holes and the featured cam-lock mechanism.

(Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Complications

The risks related to screw-plate stabilization of the cervical spine include all risks intrinsic to a routine anterior cervical discectomy and fusion procedure^52,53: injuries to vertebral and carotid arteries, injury to branches of the vagus nerve (recurrent or superior laryngeal nerves), dysphagia, radicular or myelopathic injury, cerebrospinal fluid leakage, infection, graft migration, postoperative hematoma, stroke and, if autologous graft is used, donor site morbidity. The development of pseudarthrosis (fibrous union) and instrumentation failure are intimately related. The primary occurrence of either can result directly in the other and necessitate revision of the entire fusion and fixation construct. Alternatively, successful arthrodesis can develop in the presence of failed instrumentation,^54,55 thus illustrating the race between bony healing and implant fatigue. In this circumstance, the need to remove the hardware is not absolute and would have to be considered in the context of patients’ symptoms (if any) and their risk for tracheoesophageal or neurovascular injury.