Surgical Treatment of Cervical Myelopathy

CHAPTER 70 Surgical Treatment of Cervical Myelopathy



David H. Kim, Alexander R. Vaccaro



INTRODUCTION


This chapter will review current concepts regarding surgical planning, technique, and perioperative management for patients with cervical myelopathy. The pathophysiology, evaluation, and nonoperative management of this condition have been discussed in previous chapters.



INDICATIONS


Surgical treatment is indicated for patients with moderate to severe myelopathic signs and symptoms such as functional weakness, loss of dexterity, or gait abnormality and who are otherwise sufficiently healthy to withstand the physiological stress of anesthesia and surgery. There is no consensus regarding the optimal treatment of mild forms of myelopathy such as patients with mild sensory alteration and hyperreflexia. Early surgical intervention can be considered a reasonable option for patients with radiographic evidence of static cord compression and clinical signs and symptoms of early myelopathy, with the goal of preserving neurological function and limiting the risk of future spinal cord injury. A period of nonoperative observation is an option for patients with clinical signs of early myelopathy but no significant functional compromise. In the absence of overt clinical signs or symptoms or myelopathy, surgical treatment of radiographic findings such as mild cord compression on magnetic resonance imaging (MRI) should be entered into with caution, given evidence that such findings may be present in as many as 16% of asymptomatic subjects under 64 years of age and in 26% aged 65 years and older.1


Mild, moderate, or severe forms of myelopathy remain clinical impressions without clearly defined criteria. Clinical studies have employed various scales including the Nurick grading system and the Japanese Orthopaedic Association system. Benzel’s modification of the JOA scale (mJOA) is more applicable to Western populations. (These classifications systems are outlined in previous chapters.) Using this system, an mJOA score <12 can be considered moderate myelopathy, while severe myelopathy is reflected by a score <7 (maximum score=18).2



PREOPERATIVE PREPARATION


Patient evaluation includes obtaining a thorough history and examination as well as reviewing updated plain radiographs and advanced imaging studies of the cervical spine. Selection of an anterior or posterior surgical approach as well as the specific surgical technique depends in part on the location of predominant compressive lesions and the patient’s sagittal spinal alignment. The patient’s maximum active flexion and extension arc may suggest whether standard intubation methods will be safe or whether fiberoptic awake intubation should be utilized. Patients with myelopathy frequently have significant medical comorbidities, and a preoperative evaluation by appropriate medical specialists may be necessary. Given the increased risk of catastrophic spinal cord injury, we often provide a hard collar to patients with moderate to severe cervical myelopathy and suggest that they wear it when riding in motor vehicles or are in situations in which they may experience acceleration–deceleration forces to their head and neck.



ANTERIOR SURGERY


An anterior surgical approach is an option when spinal cord compression results from anterior pathology such as herniated disc material, endplate or uncinate process osteophytes, or ossification of the posterior longitudinal ligament (OPLL). The anterior approach is ideal for compression limited in extent to one or two spinal levels, and may provide superior results over posterior approaches by allowing direct excision of the offending pathology (Fig. 70.1). Although a posterior approach can achieve indirect decompression of the spinal cord in many patients, the surgery often must be extended over a greater number of levels. In patients with significant cervical kyphosis and anterior cord compression, a posterior-only approach will not achieve sufficient decompression, and in most cases anterior surgery is required.


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Fig. 70.1 Fourty-five-year-old male with cervical myelopathy and single-level anterior spinal cord compression resulting from herniated intervertebral disc tissue. (A) Sagittal MRI image demonstrates localized anterior cord compression. (B) Axial image through level of compression. (C) Postoperative lateral radiograph following anterior surgical discectomy and instrumented fusion. (D) Postoperative anteroposterior radiograph.


For the majority of anterior surgical procedures, we favor use of a standard operating table. Patients with severe myelopathy or canal stenosis should undergo anesthetic induction on the operating table to minimize manipulation while the patient is unconscious. Fiberoptic intubation is preferred to limit extension of the neck. Neurophysiologic spinal cord monitoring employing both sensory and motor evoked potentials is a potentially useful adjunct. A baseline measurement should be obtained prior to anesthetic induction and patient positioning. The patient is placed supine in 15–30° reverse Trendelenburg to reduce intraoperative bleeding. For one- to two-level discectomy or a one-level corpectomy, a 4 cm transverse skin incision is used from the midline to the medial border of the sternocleidomastoid muscle. For surgery involving three or more levels a transverse incision can be used but for difficult exposure, partial or complete transaction of the omohyoid muscle will improve exposure. A longer oblique incision is an alternative used along the medial border of the sternocleidomastoid muscle but it is less cosmetic. To reduce risk of injury to the recurrent laryngeal nerve, some surgeons prefer a left-sided incision and approach due to the more consistent anatomic location of the nerve on the left side within the tracheoesophageal groove. Superficial dissection consists of incision through skin, subcutaneous tissue, and platysma. Deep dissection then proceeds through a natural anatomic plane between the trachea and esophagus medially and the sternocleidomastoid muscle and the carotid sheath neurovascular bundle containing the internal jugular vein, vagus nerve, and carotid artery laterally. Gentle retraction of the trachea and esophagus medially and blunt dissection through the intervening middle layers of the deep cervical fascia reveals the longus colli muscles and prevertebral fascia overlying the anterior cervical spine. Subperiosteal elevation of this muscle and fascia layer allows placement of self-retaining retractors and sufficient visualization to proceed with decompression. A simple intraoperative maneuver once the retractors are in place and opened is to deflate and then reinflate the endotracheal cuff to more evenly distribute pressures within the endolarynx and possibly minimize excessive pressure on branches of the recurrent laryngeal nerve, especially for a right-sided approach.


Dysphagia and hoarseness are the most frequent immediate postoperative complaints following the anterior approach but typically resolve over the course of several days, although dysphagia may last several weeks to months. Persistent hoarseness or voice change lasting longer than several weeks is less common and can reflect injury to the recurrent or superior laryngeal nerve. Patients who undergoing a second anterior surgery through a contralateral approach should undergo preoperative laryngoscopic evaluation to confirm that vocal cord function is normal and to avoid the risk of bilateral vocal cord paralysis and the requirement of postoperative tracheostomy.


Postoperative wound infections are rare following anterior cervical spine surgery. Significant injury to major vascular structures is also rare, but spinal decompression can lead to persistent bleeding from epidural vessels or decorticated bone surfaces resulting in postoperative hematoma formation. Given the limited space available in the cervical soft tissue, a moderate-sized hematoma can cause life-threatening airway compression, and for this reason, if postoperative bleeding is considered a possibility, patients should be admitted for overnight observation. Esophageal injury is another major concern, and unrecognized intraoperative perforation can lead to delayed life-threatening appearance of mediastinitis.



Anterior decompression


In general, anterior decompression can be performed through the disc space, i.e. discectomy, or the vertebral body and adjacent disc spaces, i.e. corpectomy, or a combination of the two. Selection of the ideal technique for anterior decompression is based largely on the anatomic location of compressive pathology and planned reconstruction strategy.



Discectomy


For stenosis and cord compression mostly limited to the disc space, i.e. disc bulges, disc herniations, and endplate osteophytic ridges, discectomy should provide adequate access for removal of compressive pathology while allowing preservation of the vertebral endplates to allow for stable reconstruction. Following surgical exposure of the anterior cervical spine, an anterior annulotomy is performed with a scalpel, and disc tissue is removed using a combination of curettes and Kerrison rongeurs. The vertebral arteries are located immediately lateral to the uncinate processes, and are vulnerable to injury if the decompression is extended too laterally. The discectomy is continued until the posterior longitudinal ligament (PLL) is visualized. A Kerrison rongeur can then be used to remove osteophytes from the posterior aspect of the adjacent endplates and uncinate processes as required. If imaging studies suggest the presence of extruded disc material behind the PLL, or if there is ossification causing compression, this ligament is resected, otherwise it is often retained to preserve additional stability. A micronerve hook is then used to confirm adequate decompression of the canal and adjoining neuroforamen.



Corpectomy


When compressive pathology is located posterior to the vertebral bodies, discectomy alone may not provide adequate relief, and corpectomy is indicated. Corpectomy allows excision of all sources of anterior compression including vertebral osteophytes, disc material, and OPLL. The surgical approach is identical to that for discectomy. For single-level corpectomy we favor a transverse incision, while multilevel corpectomies are more easily performed through an oblique incision medial to the sternocleidomastoid muscle. Following surgical exposure of the involved vertebral bodies, subtotal discectomies are performed of the disc spaces above and below the planned corpectomy as well as any intervening disc spaces if a multilevel corpectomy is planned. It is vital that the anatomic midline is identified and carefully maintained during subsequent bone removal to avoid straying too far lateral and causing injury to a vertebral artery. A minimum 165 mm corpectomy channel provides adequate decompression of the cervical spinal canal. A Leksell rongeur is used to resect vertebral body bone which can then be morselized and utilized as autologous bone graft during reconstruction. Once the posterior aspect of the vertebral body is approached, a high-speed burr is used to continue through the remaining cancellous bone. Upon reaching posterior cortical bone, a less aggressive diamond-tipped burr may be used to remove remaining bone and reduce the risk of a dural tear. Others find the use of a high-speed cutting burr safe and effective, especially with an intact posterior longitudinal ligament. To complete the decompression, the posterior longitudinal ligament may be removed, if necessary, with use of a micronerve hook, 1-0 and 2-0 Kerrison rongeurs, and a 3-0 cervical curette.


In cases of OPLL, an attempt should be made to remove any ossified tissue contributing to compression of the spinal cord or nerve roots. In cases of severe compression, localized areas of dural erosion may be present when overlying tissue is removed. Even with an apparently intact subarachnoid membrane, postoperative cerebrospinal fluid (CSF) leakage and delayed formation of durocutaneous fistulas may occur, and consideration should be given to patching such defects with muscle or fascia and placing a lumbar subarachnoid shunt.3 Radiographic signs of dural penetration by the ossified posterior ligament have been suggested, including the ‘C’ sign and single-layer sign on plain radiographs or the double-layer sign on computed tomography.4 When these signs are present, an option is to avoid complete removal of overlying ossified tissue and leave free-floating patches attached to small areas of suspected dural penetration. This is accomplished by releasing the posterior longitudinal ligament laterally and allowing the thinned-out remaining ossified ligament to float forward, decompressing indirectly the spinal canal.


An oblique corpectomy is an alternative technique that can be performed to achieve anterior decompression without the need for concomitant fusion. Proponents of this procedure suggest ideal candidates for this procedure as having asymmetric cord compression from spondylotic bars that are predominantly unilateral in location.5 The intervening disc space must be dehydrated and collapsed to limit the risk of postoperative instability. Bilateral foraminal stenosis is a contraindication.


The skin incision and initial soft tissue dissection for oblique corpectomy are the same as for standard corpectomy. Retraction of midline structures, including the trachea and esophagus, is less forceful due to a more lateral surgical approach and may reduce postoperative dysphagia and hoarseness. The sympathetic chain and vertebral artery must be identified and protected. A burr is then used to remove, in an oblique fashion, a wedge-shaped posterior portion of the vertebral body. The central canal and ipsilateral neuroforamen can be decompressed through this approach, but the contralateral neuroforamen cannot be safely accessed. Due to preservation of the anterior half of the vertebral body, no fusion is required.


With this technique, the rate of significant complications approaches 30% and therefore limits its potential application. Horner’s syndrome occurs transiently in up to 57% of patients but can be permanent in up to 9%.6 A prospective study of 26 patients treated by this technique resulted in 76.9% good to excellent results with 84.6% improvement in any preoperative radicular symptoms.5



Discectomy–corpectomy


A combination of discectomy and corpectomy can be utilized for anterior decompression when compressive pathology extends over three motion segments but is localized primarily to the disc spaces and one vertebral body. Preservation of one intervening vertebral body allows application of a plate using segmental screw fixation and significantly enhanced postoperative stability compared with multilevel corpectomy and long anterior plate stabilization without the need for supplemental posterior fixation.



Anterior reconstruction


Following anterior discectomy or corpectomy, restoration of anterior column height allows preservation of sagittal alignment and avoidance of postoperative kyphosis. Reconstruction can be accomplished utilizing autologous structural bone graft, allograft bone, or titanium cage implants filled with autologous bone graft. Structural tricortical iliac crest autograft is well suited for anterior reconstruction following single-level or multilevel discectomy or single-level corpectomy. Following two-level corpectomy or more extensive decompression procedures, structural fibular allograft can be used with good results.


The potential for significant bone graft harvest site morbidity, including chronic pain, numbness, infection, and hematoma, has led to the increased use of allograft bone in place of autologous iliac crest bone graft. Both cortical bone and dense cancellous bone products have been used with success. Titanium cages have the advantage of material strength, but significant mismatch in material properties between titanium and bone may increase the risk of endplate fracture and subsidence, particularly in osteoporotic bone.


A problem with both autologous and allograft bone reconstruction is the need to place these relatively straight grafts in an inherently less stable anterior position to avoid spinal cord impingement. Precontoured titanium mesh cages are available for longer reconstructions that allow more central positioning on endplates while preserving a degree of lordosis.7


The concept of motion-sparing technology as an alternative to spinal fusion is appealing in theory, and numerous artificial disc replacement devices are currently available, mostly for use in the lumbar spine. At this time, spinal disc arthroplasty for the cervical spine is being performed in the United States in the setting of clinical trials only. Preliminary short-term results for some devices have reportedly been positive in the setting of myelopathy, but long-term outcome data are unavailable. A concern is that the theory of allowing continued motion in the setting of a decompressed dysfunctional myelopathic cord may not prevent continued shear injury and therefore continued apoptotic cell death in this setting.



Anterior plating


Supplemental cervical plate and screw fixation is routinely recommended following multilevel anterior discectomies and corpectomy procedures. Plate fixation for single-level discectomy and fusion obviates the use of postoperative cervical brace but there is no clear evidence that it is superior to in situ fusion. If patient bone quality allows adequate fixation strength, instrumentation provides immediate stability, reducing early postoperative pain and in many cases obviating the need for weeks of postoperative hard collar or halo vest immobilization. Long-term outcome studies have demonstrated significantly improved fusion rates and decreased rates of graft-related complications associated with use of anterior plates.8,9 In the absence of instrumentation, 50% of patients undergoing single- or two-level corpectomy experience localized kyphosis of 10° or more as a result of graft subsidence.10 Preservation of cortical endplates can minimize such subsidence but requires the use of anterior plates to prevent graft extrusion and may increase the risk of pseudoarthrosis.



POSTERIOR SURGERY


The two principal surgical options for posterior decompression surgery are laminectomy and laminoplasty. Both techniques require preoperative cervical spinal alignment that is neutral or lordotic because both accomplish indirect decompression of the spinal cord by allowing the cord to migrate posteriorly in an expanded canal. Laminectomy involves removal of posterior laminar bone en bloc while laminoplasty involves elevating the laminar bone on an attached hinge.


For posterior surgical approaches, Mayfield-type cranial tongs are used to immobilize the cervical spine. This allows stabilization of the head and neck in the so-called ‘military position’ with the head slightly forward flexed and in neutral alignment. The arms are typically tucked at the side with protective padding around the elbows to avoid direct compression of the ulnar nerves. As with anterior surgery, a 15–30° reverse Trendelenburg position is utilized to reduce blood loss. Although not yet accepted as the standard of care, intraoperative spinal cord monitoring through both motor and sensory tract evoked potentials is recommended. A baseline reading should be obtained prior to patient positioning and rechecked immediately after the patient has been positioned on the operating table. Potentials should then be monitored regularly throughout the procedure. The use of intraoperative steroids is not routinely recommended, given the lack of clinical evidence that they provide a positive risk–benefit effect in the absence of a discrete spinal cord injury event.


The posterior surgical approach to the cervical spine is standard for both laminectomy and laminoplasty. Following a midline longitudinal skin incision, dissection proceeds in the midline through the subcutaneous tissue and ligamentum nuchae. Subperiosteal exposure of the posterior cervical spine is then performed using a combination of cervical Cobb elevators and electrocautery. Care is taken to avoid injury to any facet joint capsule that will not be fused. Stabilizing muscular attachments to the spinous process of C2 are typically preserved unless the C2 level is to be fused to limit postoperative kyphosis. Alternatively, the muscular attachments to C2 can be removed en bloc with a piece of spinous process and suture repaired at the conclusion of the procedure through a hole in the spinous process base.



Laminectomy


Cervical laminectomy has been associated with a 20% incidence of late postoperative kyphotic deformity in the adult patient.11 Preoperative kyphosis is therefore a contraindication for laminectomy unless concomitant anterior reconstruction can restore at least neutral sagittal alignment. In patients with preoperative neutral or lordotic sagittal alignment, laminectomy is technically straightforward and yields typically good results in conjunction with a posterior fusion procedure (Fig. 70.2). Ideal candidates have multilevel stenosis that would be difficult to reconstruct following anterior decompression. Patients with extensive OPLL are also candidates to avoid the risk of dural tears with anterior decompression. The operation is also well suited for elderly and debilitated patients who may not tolerate a longer anterior procedure with potential for greater blood loss. Patients with significant calcification of the ligamentum flavum causing posterior cord compression are also good candidates.


image

Fig. 70.2 Sixty-year-old male with multilevel cervical spondylotic myelopathy and preserved cervical lordosis. (A) Preoperative MRI. (B,C) Postoperative anteroposterior and lateral radiographs following posterior cervical laminectomy and instrumented fusion.



Technique


The posterior cervical spine is exposed to include all involved levels. Care is taken to avoid injury to the facet capsules of any levels that are not to be surgically fused. If an instrumented fusion is planned, then we recommend preparing screw holes for all implants prior to performing the actual laminectomy in order to minimize the risk of iatrogenic cord injury during instrumentation. The interspinous ligaments are resected proximal and distal to the levels to be decompressed. The laminectomy margins are defined anatomically by the longitudinal border between the lateral masses and the laminar bone. A high-speed burr is used to sequentially divide the laminar bone along this border on one side. Although this step is not typically associated with significant blood loss, an extensive epidural plexus of veins is concentrated near the junction of the lamina and lateral mass, and excessively lateral burring of the troughs can lead to substantial hemorrhage. We recommend placement of the troughs 2 mm medial to the junction of the lamina and lateral mass to minimize this risk. A Kerrison rongeur can be used to complete resection of the inner cortical bone. The contralateral laminar bone is then divided in similar fashion. With division of the ligamentum flavum, the resected laminar bone can be elevated sequentially and removed en masse, completing the laminectomy. Because of the increased incidence of postoperative C5 and C6 root palsy, a concomitant C5 and C6 foraminotomy is routinely prophylactically performed.



Laminoplasty


Development of cervical laminoplasty was pioneered by Japanese surgeons for treatment of OPLL and first reported in the English language by Hirabayashi et al. in 1983.12 Extensive anterior decompression and reconstruction procedures were associated with high failure rates and motivated development of a posterior approach that was less destabilizing and did not require complex reconstruction techniques. The surgical indications were subsequently expanded successfully to include patients with more generic spondylotic myelopathy as well. Laminoplasty results in lower rates of postoperative kyphosis compared to laminectomy without fusion. Elevating the laminar bone on a hinge expands the sagittal diameter of the spinal canal, allowing the spinal cord to migrate posteriorly, accomplishing an indirect decompression (Fig. 70.3). In patients with OPLL, postoperative imaging demonstrates posterior cord translation of up to 6 mm accompanied by increased transverse cord area, both of which correlate with improvements in myelopathy.13 Following the original description of the procedure by Hirabayashi et al., now referred to as ‘open-door’ laminoplasty, several modifications including ‘French-door’ and ‘skip’ laminoplasty have been reported and are currently being practiced.14–16


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Fig. 70.3 (A,B) Preoperative sagittal and axial MRI images and (C,D) postoperative MRI images following C3–7 laminoplasty for cervical spondylotic myelopathy., (E) Axial CT image demonstrating hinged segment with bone graft strut.


In general, the reduced operative time and blood loss associated with laminoplasty compared with both anterior decompression and fusion or laminectomy and fusion make it an appealing option for high-risk surgical patients. Theoretically, retention of the dorsal laminar bone may also limit formation of postoperative epidural fibrosis and tethering of the dura to paraspinal muscles.


Prior to surgery, careful assessment of a lateral radiograph in the neutral position must be made. Kyphotic sagittal alignment is considered by many to be a contraindication for laminoplasty. Either neutral or lordotic alignment is acceptable, although in patients with excessive lordosis there may be a risk of overexpansion of the canal diameter. Due to potentially increased risk of C5 palsy in patients with severe lordosis and excessive posterior cord migration, limiting the amount of laminar elevation to approximately 12 mm may minimize the degree of neural stretch. Preoperative spondylolisthesis is not considered a contraindication for laminoplasty, as 85% of spondylolisthesis cases have been observed to resolve or improve within 1 year following surgery.17 In terms of prognosis, however, posterior spondylolisthesis may be associated with lower rates of postoperative neurological recovery.


Patients with significant axial neck pain complaints may not be ideal candidates for laminoplasty, given relatively high rates of persistent neck pain following this procedure. In such patients, an anterior decompression and fusion for one- to two-level disease or a posterior laminectomy and fusion for three-level or greater disease may be better tolerated.



Technique


During posterior surgical exposure, violation of the intervertebral facet capsules is avoided, as spontaneous fusion may result and preservation of joint motion is a surgical goal. At the beginning of the procedure the tips of the spinous processes may be removed to prevent impingement of these structures during neck extension. A high-speed burr is utilized to create bilateral longitudinal troughs at the junction between the laminae and the lateral masses from C3 through C7. On the side to be opened, the trough is completely extended through both cortices of bone, and the ligamentum flavum is divided with a Kerrison rongeur to open the spinal canal. On the hinge side of the laminae, the trough is extended to but not through the anterior cortex of the laminae. Elevation of the laminae creates a controlled ‘greenstick’ fracture on the hinge side and expands the posterior dimension of the canal, allowing indirect decompression of the cord.


Sutures can be used to maintain the hinged laminae in the open position, or spacers made of various materials, including spinous process bone, allograft bone, or hydroxyapatite, or miniature titanium plates can be utilized to block open the laminae. When fashioning allograft spacers to support the opened posterior arch, an ideal height of 10–15 mm allows adequate cord decompression while at the same time limiting the extent of posterior migration and possibly reducing the risk of C5 root palsy.18


In the setting of coexisting myeloradicular deficits or symptoms, careful assessment of the cervical neuroforamen should be performed on the preoperative MRI or computed tomography (CT)-myelogram images. The presence of significant foraminal stenosis that correlates with anatomic distribution of radiculopathy indicates the need for concomitant foraminotomies. Technical aspects of performing cervical foraminotomies are covered in a separate chapter. Individual nerve roots can be decompressed before or after creating the longitudinal bone troughs for laminoplasty, but should be performed prior to elevation of the lamina. If the number of foraminotomies is limited, performing them prior to creating the longitudinal troughs may be safer and allow greater preservation of stability.


Following completion of the C3–7 laminoplasty, if compression persists proximally at C2 or distally at T1, a laminectomy can be performed at the affected level(s) without significantly compromising spinal stability. In order to retain stabilizing muscular attachments to the C2 spinous process, a dome-shaped laminectomy of C2 can be performed utilizing a burr and Kerrison rongeur.


Spontaneous closure of the hinged laminae can occur in the postoperative period and may compromise surgical results. Several techniques for maintaining laminar elevation have been developed. Small titanium plates and screws can be placed at each level or alternating levels to maintain the laminae in an open position. In patients with normal lordotic alignment, nonabsorbable sutures may be used to secure the opened laminae to the adjacent facet. Titanium plates and screws may be used in patients with neutral sagittal alignment when a more rigid block to hinge closure is desired. Another application is for patients with significant lordosis where more controlled expansion of the canal may be advantageous to avoid C5 root palsy.

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Sep 8, 2016 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Surgical Treatment of Cervical Myelopathy

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