Single- and Multiple-Level Interbody Fusion Techniques

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Chapter 40 Single- and Multiple-Level Interbody Fusion Techniques

Cervical discectomy via a ventral approach, better known as anterior cervical discectomy (ACD) or anterior cervical discectomy and fusion (ACDF), is one of the most common procedures performed by spine surgeons. Complication rates are low and the clinical results are gratifying. Some surgical complications are treatable at the time of their detection intraoperatively or in the immediate postoperative period, and other complications may have no reasonable treatment once detected. Avoiding irreversible complications is the only logical solution to their management. Overall, complication rates for ACDF operations vary from approximately 5%14 to 15%.59 The operation itself can be divided into stages, including general surgical considerations, discectomy, donor site considerations, and bony fusion.

Surgical complications may be categorized as occurring in the preoperative, intraoperative, or postoperative period. A majority of the complications that occur during an ACDF are avoidable with appropriate patient selection, careful preoperative planning, meticulous surgical technique, and close follow-up and monitoring of the clinical and radiographic conditions of the treated patient.

A brief history of ACD and ACDF is useful. More than 400 years ago, Vesalius described the intervertebral disc.10 It was not until 1928 that Stookey described a number of clinical syndromes that resulted from disc protrusions. These protrusions were thought to be neoplasms of notochordal origin and were incorrectly identified as chondromas.11 During this same era, other investigators provided a more precise understanding of the pathophysiology of the intervertebral disc.1214

In the 1950s, the first reports of ventral approaches to cervical disc pathology appeared. The two most common methods for ACDF were described by Robinson and Smith in 195515 and by Cloward in 1958.16 Robinson and Smith described an operation for removal of cervical disc material with replacement of a rectangular bone graft, obtained from the iliac crest, to allow for the development of a cervical fusion.15 With the Cloward technique, the discectomy was performed by a cylindrical dowel technique.16 Although numerous modifications have been developed since the 1950s, the great majority of spine surgeons currently use either the Cloward or the Smith-Robinson technique.9,1726

Preoperative Considerations

The best predictor of a good postoperative clinical result is proper preoperative patient selection. ACD and ACDF are indicated for myelopathy, radiculopathy, and degenerative disc disease with mechanical pain. The presence of clinical symptoms, a consistent physical examination, and confirmatory imaging studies lead to the best postoperative result. In addition, a meticulous evaluation of the general overall medical condition of the patient is mandatory. Postoperative mortality may be caused by myocardial infarction,6,10,27,28 respiratory failure,29 pulmonary embolism,30 or laryngeal edema,28 among many other potential complications.

General considerations that may directly affect ACDF include the presence of diabetes mellitus or immunocompromised states such as AIDS, autoimmune disturbances, or systemic medical conditions that require corticosteroid administration. A history of smoking is clearly associated with diminished postoperative fusion rates.27,3137

The deleterious effects of smoking are manifested by inhibition of the neovascularization necessary for incorporation of a bone graft.3840 A current preoperative recommendation is cessation of smoking for a minimum of 8 weeks before surgery and for a minimum of 12 weeks postoperatively. A preoperative dependence on narcotic analgesics has been associated with suboptimal outcome. This is particularly true if the clinical surgical indication is axial neck (mechanical) pain in the absence of radiculopathy or myelopathy. An important concern is preoperative difficulty with swallowing, which is more common in the elderly; it should be investigated, as necessary, before surgical intervention. If possible, the use of estrogen replacements or oral contraceptive pills in female patients should be discontinued preoperatively. These medications are known to increase the development of deep vein thromboses in the postoperative period. In addition, corticosteroids and nonsteroidal anti-inflammatory agents have a known deleterious effect on spine fusions and should be discontinued 10 days before surgery, if possible.

Preoperative radiographic imaging studies are necessary to confirm the history and physical examination findings. Plain radiographs remain a cornerstone of the preoperative radiographic evaluation. Lateral cervical spine radiographs allow for an assessment of the sagittal plane alignment and a rough assessment of bone mineralization. Flexion and extension views are useful to establish the presence of spine instability that may alter the surgical decision-making process. Finally, the dorsal elements should be assessed for splaying of the spinous processes or for facet joint abnormalities.

For many years, the gold standard imaging study for ventral cervical surgery was the myelogram, followed by a postmyelogram CT scan. This study provides excellent anatomic detail of both the spinal cord and the cervical nerve root sleeves. Recently, MRI has become more popular. MRI allows for greater soft tissue detail and is useful for identifying disc degeneration. However, MRI is extremely sensitive and may overestimate the extent of surgical pathology. A recent study has demonstrated a significant incidence of abnormal MRI findings in asymptomatic patients.41 As a result, it is important to remember that an abnormal MRI is not necessarily an indication for surgery. However, note that MRI allows for the evaluation of pathology in both the axial and sagittal planes. In some cases of previously instrumented cervical spine surgery, CT myelogram may be preferable to MRI because it is less affected by metallic artifacts. Finally, reports of lower cervical spine ventral surgery performed in patients with significant pathology of the foramen magnum and the upper cervical spine should increase the surgeon’s index of suspicion for such lesions.2,5

Intraoperative Considerations

The majority of intraoperative complications may be avoided by careful preoperative planning and meticulous intraoperative technique. If intraoperative complications occur, they are usually best managed at the time of detection. However, some may not be detected until the postoperative period. Thus, there is considerable overlap between the management of intraoperative and postoperative complications. Intraoperative considerations include positioning, incision, dissection, retraction, distraction, discectomy, donor site considerations, and fusion.

Positioning

The patient is positioned supine on the operating table, and general endotracheal anesthesia is administered. If significant spinal cord compression or myelopathy is present, consideration should be given to a fiberoptic nasotracheal intubation on a patient who is awake. After successful intubation, the patient’s neurologic examination results are confirmed to be unchanged before the induction of general anesthesia.

The patient’s head should be supported with either a foam donut or a Mayfield horseshoe headrest. The neck should be supported dorsally with a firm support to prevent intraoperative motion. In addition, an attempt at achieving a normal lordotic cervical curvature should be made to optimize the postoperative sagittal plane alignment. Ordinarily, a degree of neck extension is preferable to improve the lordotic curvature, as well as to aid in the dissection process. This is particularly true for upper cervical dissections. It is important to evaluate the patient’s ability to extend the neck preoperatively and to not exceed this degree of extension intraoperatively. Hyperextension of the neck in a narcotized patient may lead to spinal cord compression.4

The operating table is flexed slightly at its midpoint, and a sandbag or other bolster is placed beneath the iliac crest to facilitate bone graft harvesting. All bony prominences must be padded, with particular attention paid to the protection of the ulnar nerves at the elbow. The knees are flexed and the heels are padded. Antiembolic stockings may be placed, and sequential compression devices are used to prevent the development of an intraoperative deep vein thrombosis.

After patient positioning and before preparation, the endotracheal cuff is deflated for 5 seconds and then reinflated. This maneuver was described by Apfelbaum42 and has been used to limit compression of the vocal cords at the level of the arytenoid cartilage in the larynx. The recurrent laryngeal nerve (RLN) terminates at the arytenoid cartilage, and if it is compressed by the endotracheal tube, an RLN palsy may result.

A Doppler probe may be used to auscultate a baseline signal for the superficial temporal artery. During intraoperative retraction (of the carotid artery), the Doppler pulse can be reevaluated. If skeletal traction is to be used, the tongs are placed at this stage.

Incision

The selection of the ideal side for approach is controversial, with advocates for both right- and left-sided approaches. As a general rule a right-handed surgeon can approach the operation more easily from the patient’s right side, but the more variable anatomic course of the right RLN may render the nerve more vulnerable to injury during a right-sided approach.4345 This vulnerability is particularly true with lower cervical dissections (Fig. 40-1). The reported incidence of postoperative RLN palsies presenting as postoperative hoarseness varies between 0.8% and 3.7%.47,9,28,4351 From a left-sided approach, the RLN has a longer course and may be less likely to be injured, but the thoracic duct is vulnerable with left-sided approaches to the lower cervical spine9 (Fig. 40-2). In addition, the thoracic duct may be bifid, and injury to one of the limbs of the thoracic duct may not be recognized intraoperatively. If chyle is observed, simple ligation of the thoracic duct is usually all that is necessary. With lower cervical discectomies there is a theoretical risk of pneumothorax or mediastinitis with approaches from either side.10,16,52,53

It is essential to make the skin incision at the proper level. The most common error is to place the incision too caudal, thereby obligating the physician to operate at an awkward, upward oblique angle. This can limit visibility during the discectomy. It is much easier to gain access caudally from a rostrally placed incision than the converse.

With one- or two-level discectomies a transverse incision is most commonly used. This is placed in a skin fold that allows for a more cosmetic postoperative result. If discectomies at three or more levels are to be performed, an oblique incision that parallels the medial border of the sternocleidomastoid muscle is preferable. This incision is commonly used for carotid endarterectomies because it allows for a better exposure of multiple spine levels. We prefer to make our transverse incisions for lower cervical approaches at the level of the upper border of the crossing omohyoid muscle. This incision will be at the C5-6 level and provides comfortable access to both the C5-6 and C6-7 intervertebral discs. Maintaining the dissection plane rostral to the omohyoid muscle and depressing it inferiorly as necessary has resulted in an extremely low rate of postoperative RLN palsy in our practice.

Dissection

The dissection is carried sharply through the subcutaneous tissue and the platysma muscle. The platysma muscle may be sharply divided in a transverse fashion or split longitudinally for access to the subplatysmal space. As a general rule, transecting the platysmal muscle is preferable for exposures of two or more levels. If access to multiple levels of the upper cervical spine is necessary, a generous subplatysmal dissection is used to limit the extent of soft tissue retraction required to gain adequate exposure. After the subplatysmal dissection is completed the fascia overlying the medial border of the sternocleidomastoid muscle is sharply divided, and the deep dissection is performed, either sharply or bluntly. The plane of the deep dissection is between the sternocleidomastoid muscle and carotid sheath laterally and the trachea, esophagus, and strap muscles of the neck medially. Careful dissection, with identification of the carotid artery by palpation and gentle finger dissection, is required to avoid carotid artery injuries.6,30,54,55 This trajectory allows for exposure of the prevertebral fascia. In patients who have not undergone previous ventral cervical surgery, blunt dissection is easily and safely accomplished. Excessive soft tissue stretching should be avoided because occasional RLN injury has been hypothesized to be secondary to stretching. In this case, avoiding high endotracheal cuff pressures may reduce the incidence of such injuries.42

In patients who undergo reoperation sharp dissection may be necessary. It is important to confirm that the dissection remains dorsal to the hypopharynx and the esophagus. With reoperation, a nasogastric (NG) tube should be placed. This may be palpated to confirm the location of the esophagus and hypopharynx. The incidence of hypopharynx perforation during upper cervical discectomies varies between 1%8,46,47,56 and 5%.48,57,58 Esophageal perforation has also been reported in cervical discectomies.47,5964 If the hypopharynx or esophagus is penetrated, a drain should be placed, a layered closure performed, and an NG feeding tube inserted. The latter must be maintained for at least 1 week postoperatively to allow for the soft tissue to heal and prevent the development of a fistula.

If there is a question of perforation of the alimentary tract, the NG tube should be withdrawn so that the tip of the tube is in the esophagus. After this maneuver, instillation of a colored inert dye, such as methylene blue or indigo carmine, should assist with demonstration of the violation. Unrecognized esophageal perforations can lead to the development of deep soft tissue infections (including mediastinitis). These manifest as high fevers, severe retrosternal pain, and subcutaneous emphysema. Other severe complications of esophageal perforation include esophagocutaneous fistula57 and even death.61

Retraction

On entering the prevertebral space a radiographic marker must be placed and a lateral cervical spine radiograph obtained. This mandatory step ensures that the operation is performed at the correct level. There have been reports of ACDs being performed at the wrong level.2,9

Establishing that the correct level is being operated on is readily accomplished by placing a radiopaque 18-gauge spinal needle into the intervertebral disc space and obtaining a lateral fluoroscopic image. Alternatively, the authors prefer to place the spinal needle into the ventral aspect of the vertebral body cortex rather than the intervertebral disc, which avoids perforation of the anulus. Should the first attempt at localization demonstrate exposure of an intervertebral disc not to be operated upon, perforation of the outer anulus with a spinal needle may lead to accelerated adjacent-segment disc degeneration at this level. Thus, placement of the spinal needle through the ventral vertebral body cortex potentially avoids this complication.

When the appropriate level has been identified, it is useful to mark the true anatomic midline. This is best accomplished by marking a point midway between the most medial borders of the longus colli muscles. After the midline is identified, the longus colli muscles are elevated from the vertebral bodies and discs bilaterally. Longus colli dissection should be limited laterally to 3 mm of muscle. If the longus colli muscles are dissected excessively, a Horner syndrome—the triad of ipsilateral ptosis, myosis, and anhydrosis—may result. The incidence of postoperative Horner syndrome varies from 0.2% to 2%2,46,46,47,6567 after ventral cervical spine surgeries.

After the longus colli muscles are elevated, a self-retaining retractor system is used. Toothed retractors are placed under the longus colli muscles bilaterally. With single-level discectomies there is rarely a need to place vertical self-retaining retractors. When operating on the lower cervical spine, it is essential to avoid retraction of the RLN, which is particularly problematic when a right-sided cervical approach is used. If necessary, vertical retractors should be smooth at the tips, and care should be taken to avoid excessive retraction.

The self-retaining lateral retractors should be carefully placed to avoid excessive retraction on the esophagus, which may lead to postoperative dysphagia. The exact mechanism responsible for the development of postoperative dysphagia is unknown; however, it is thought that retraction-induced pressure on the esophageal wall leads to local ischemia with subsequent hyperemia and swelling.68 This in turn may lead to postoperative dysphagia. A mild, transient, postoperative dysphagia is common after ventral cervical surgery. However, in the majority of patients this resolves within 3 months.6971 Dysphagia rates have been reported to vary from 1.8% to 9.5%,1,7,10,17,27,28,48,53,6972 to between 21.2% and 35%.5,9,46 Intermittently releasing the retractor pressure during prolonged surgical procedures helps to avoid this complication. Most dysphagia episodes are transient and do not require a gastrostomy tube. In cases of severe postoperative dysphagia, a gastrostomy tube may be needed for enteral feedings.

Excessive lateral retraction may also compress the carotid sheath. In patients with significant preoperative atherosclerosis, prolonged pressure against the carotid artery can lead to thrombosis with cerebral ischemia. To avoid this problem, after the lateral self-retaining retractors have been placed, the pulse of the superficial temporal artery above the level of the zygoma may be auscultated with a Doppler probe or palpated by the anesthesiologist intraoperatively. This measure confirms blood flow in the external carotid artery. Because the common carotid artery bifurcates into its external and internal branches at the C3-4 level, this maneuver indirectly increases the degree of confidence that blood flow in the internal carotid artery has not been significantly compromised. In addition, the retractors may alter the position of the endotracheal tube. Release of the endotracheal tube cuff for 5 seconds, followed by re-inflation to the lowest pressure that eliminates air leak, confirms that the vocal cords are not being excessively compressed.42

Discectomy

The adequacy of neural decompression is directly related to the completeness with which the discectomy itself is performed. In addition, most neurologic complications occur at this stage of the procedure. The disc is removed, including the entirety of the dorsal anulus fibrosus in the midline. The depth of the dissection necessary to achieve this may be estimated from the preoperative MRI and CT scans. Small, upbiting microcurettes and rongeurs allow for removal of the anulus fibrosus. Hemostasis is achieved with judicious use of bipolar electrocautery, hemostatic gelatin (Gelfoam) soaked in thrombin, and cotton patties. After removal of the anulus fibrosus, the underlying posterior longitudinal ligament (PLL) will be seen.

The need to open the PLL is debated. Numerous authors recommend routine opening of the PLL after removal of the dorsal anulus fibrosus.42,48,51,7375 However, others do not agree with the routine sectioning of the PLL after good quality preoperative radiographic imaging studies.7678 Although preoperative imaging studies may suggest that the disc material has not protruded dorsally, the PLL may be safely sectioned to allow for entry into the epidural space. On entry a blunt nerve hook may be used to search for disc material. In addition, the PLL itself may be thickened and may be responsible for ongoing neural compression. As a result, if there is any doubt about the adequacy of decompression, the PLL should be opened sharply to allow a direct look at the underlying dura mater. Any disc fragments dorsal to the PLL are removed. Likewise, ridges from dorsal osteophytes may compress the spinal cord or nerve roots. If osteophytes are detected, either by preoperative imaging studies or during the surgical procedure, they should be resected using small Kerrison rongeurs.1,7,24,50,76,7982

Tearing of the underlying dura mater is possible during the opening of the PLL. This is particularly likely in cases of ossification of the PLL and in patients who have undergone previous ventral procedures.83,84 In a series of 450 patients who underwent ventral cervical surgery, Bertalanffy and Eggert reported 8 patients (1.8%) who sustained damage to the dural sac. Of these 8 patients, 1 developed meningitis.46 If a dural tear occurs, it is usually impossible to repair the defect primarily. The methods used to prevent egress of cerebrospinal fluid (CSF) include placing free muscle and fascial grafts and using Gelfoam soaked in thrombin or fibrin glue. Additionally, newer dural substitutes made from synthetic materials, bovine grafts, and collagen can also be used.8588 With a dural tear, placement of a lumbar subarachnoid drain must be considered to divert CSF in the immediate postoperative period. Once the PLL is opened, instead of electrocautery, thrombostatic agents such as Gelfoam and cotton patties should be used for hemostasis. Our philosophy has been to routinely open the PLL in all cases of radiculopathy and myelopathy. In our practice, only surgery for axial mechanical neck pain, which constitutes less than 5% of our cases, is performed without opening the PLL.

The width of the decompression is determined on a case-by-case basis. Care must be taken to maintain the orientation of the midline, which is essential when determining the width of decompression. Useful techniques include referring to the marking of the true bony midline made before the longus colli muscle dissection, as well as being aware of the anatomic bony structures, such as the uncovertebral joints. As a general rule a 15-mm bony dissection centered over the midline is necessary for an adequate decompression.89 If nerve root compression is present, the dissection may be extended laterally. The medial border of the uncovertebral joint serves as a bony anatomic marker of the lateral extent of a cervical discectomy. Limiting the dissection to this point will allow for a good decompression of the shoulder of the nerve root. Once again, the majority of intraoperative neurologic injuries that occur are the result of loss of orientation of the bony anatomic midline. A useful intraoperative maneuver to prevent an excessively wide discectomy is frequent placement of a cotton patty in the discectomy defect. A standard cotton patty measures 13 mm and allows for reorientation throughout the procedure.

As mentioned earlier in this chapter, the majority of neurologic injuries occur during the deep portion of the discectomy procedure. The most common complications include dural tears, damage to the neural elements, and vertebral artery injuries. Intraoperative nerve root injuries and spinal cord contusions occur in less than 1% of ACDs.2,4,7,16,28,46,90,91

If the discectomy is too wide, the vertebral artery may be injured. The vertebral artery and its accompanying venous plexus are at risk during removal of the lateral disc material.6,8,54,55,92,93 Profuse arterial bleeding occurs after a vertebral artery injury. If the patient’s head was rotated as part of the initial operative positioning, the head should be immediately returned to the midline before attempts are made to control bleeding.94 Immediate tamponade should be used for the initial management of vertebral artery injuries. If the tamponade maneuver is unable to curtail bleeding successfully, either direct ligation or primary repair of the vertebral artery may be necessary.54 These maneuvers are technically demanding and require extension of the exposure in both a rostral and a caudal direction. More recently, neuroendovascular treatment, performed immediately after a suspected vertebral artery perforation, has been used to successfully treat these injuries.

In patients with two functional vertebral arteries and an intact circle of Willis, the majority of vertebral artery injuries are asymptomatic. As a result, the actual incidence of vertebral artery injuries may be underappreciated. However, if one vertebral artery is thrombosed, or if a hypoplastic artery is present, occlusion of the dominant vertebral artery may be catastrophic.84 Shintani and Zervas reviewed the results of 100 patients whose vertebral arteries were ligated for a variety of reasons and found a 12% mortality rate.95

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