Posterior Cervical Approach

Michael Y. Wang, Mick J. Perez-Cruet, Dino Samartzis, and Richard G. Fessler

Abstract

The posterior approach to the cervical spine is one of the most common approaches to the spine. Accessing the spinal column and its neural elements through the posterior musculature of the neck has been performed for nearly a hundred years. Nonetheless, there are important elements of preparation and technical nuances that have to be considered when performing the posterior cervical approach. This chapter summarizes these considerations.

Keywords: neck, laminectomy, laminoplasty, musculature, posterior approach, cervical

19.1 Introduction

Compression of the spinal cord from cervical spondylosis was first described in 1911 by Bailey and Casamajor ¹ and in 1928 by Stookey,² who described a patient with quadriplegia resulting from spinal cord compression due to cervical spinal stenosis. However, it was not until 1952, when Brain, Northfield, and Wilkinson described the role of vascular supply to the spinal cord and the manifestation of myelopathic symptoms, that cervical stenosis causing myelopathy was identified as a distinct entity. Lees and Turner³ further noted the lengthy clinical course of the disease accompanied by a long duration of nonprogressive disability.

Cervical spondylosis may be the most common underdiagnosed spine disorder whose true incidence is unknown.⁴ Most patients present in the fifth decade of life, but the condition is not limited therein and is indiscriminate of age and degree of disease manifestation.⁵^,⁶^,⁷ Levels below C3–C4 are primarily affected, with predominance at C5–C6 followed by C6–C7 and C4–C5.⁸ Radiographically, spondylotic manifestations are evident in 25 to 50% of the population by age 50, and they are seen in as many as 85% of individuals by their mid-60 s.⁹^,¹⁰ Treatment is performed using an anterior, posterior, or combined surgical decompression approach( Fig. 19.1).

A number of disorders may mimic the symptoms of cervical spondylosis and need to be differentiated for proper diagnosis.

Fig. 19.1 (a) Preoperative axial view; (b) preoperative sagittal CT/myelogram in patient presenting with weakness and myelopathy showing significant multilevel cervical stenosis. Patient has multiple bridging osteophytes consistent with the diagnosis of diffuse idiopathic skeletal hyperostosis (DISH). (c) Postoperative plain AP view; (d) postoperative lateral radiographs following multilevel posterior decompression, fusion, and lateral mass instrumentation. (e) Sagittal and (f) axial MRIs in a patient with myelopathy primarily from multiple anterior cervical disc herniations compressing spinal cord. (g) Postoperative lateral radiograph following multilevel anterior cervical decompression, fusion, and instrumentation.

Differential Diagnosis: Cervical Spondylosis

Disorders exhibiting some of the same symptomology include:

• Torticollis.

• Athetosis.

• Chronic dystonia.

• Cerebral palsy.

• Syringomyelia.

• Low-pressure hydrocephalus.

• Cerebral hemisphere lesions.

• Amyotrophic lateral sclerosis.

• Down’s syndrome.

• Multiple sclerosis.

• Neoplastic lesions.

Cervical spondylosis is an insidious, progressive disease process that presents with a number of symptoms that are frequently refractory to conservative nonoperative management.

Nevertheless, controversy exists over the optimal choice of surgical treatment for this disorder. For patients with progressive symptoms, despite nonoperative treatment, posterior decompression is an appropriate treatment option. However, traditional methods of cervical decompressive laminectomy require stripping of the posterior cervical muscular, as well as ligamentous, attachments to the spine. In so doing, patients experience considerable postoperative pain from muscle spasms and muscle injury. Some patients will go on to develop iatrogenic swan neck deformity, which is particularly prevalent in younger individuals undergoing multilevel posterior cervical decompression.¹¹^,¹²

Relatively recent improvements in posterior spinal instrumentation, namely, lateral mass plates and/or screw-rod constructs, have made fusion more attractive. However, this method adds considerable cost to the procedure, does nothing to reduce iatrogenic muscle and ligamentous injury, and can result in considerable complications from misplacement of screws.

The laminoplasty technique attempts to widen the diameter of the spinal canal by preserving the posterior spinal elements, namely, the spinous processes and laminae, while maintaining the dynamic motion of the cervical spine. However, this technique still requires extensive muscle dissection and retraction. Recent studies have shown that despite attempts to maintain spinal mobility using the laminoplasty technique, many patients develop progressive limitation of cervical range of motion similar to that seen after laminectomy and fusion and/or continue to experience significant axial neck pain.¹¹ Cervical laminectomy may result in instability and progressive kyphotic deformity in some adults, particularly when extensive resection of the facets has been performed.¹²^,¹³ In particular, progressive kyphotic deformity and cervical instability are common in children after laminectomy.¹²^,¹⁴^,¹⁵ Bone grafting and internal fixation using lateral mass plates have been reported to prevent the development of postlaminectomy instability and deformity.¹⁶ Some investigators believe that development of a postlaminectomy membrane may yield late deterioration after laminectomy with or without fusion.¹⁷

To address many of the issues encountered with more traditional posterior cervical decompressive approaches, namely, significant muscle, ligament, and bone removal, postoperative pain, and iatrogenic instability, a minimally invasive microendoscopic cervical laminectomy technique was developed. The effectiveness of this technique was first tested in cadaveric specimens before being applied in the clinical setting (see Cadaveric Studies (p.199) later in this chapter). This chapter will review both the results of the cadaveric study and recent surgical experiences with this technique. It should be emphasized that the clinical application of this technique portends a steep learning curve, and our initial clinical experience only includes those patients with one- to three-level stenosis, despite the effectiveness of applying this technique to four-level decompression in the cadaveric model.

19.2 Anatomy and Pathophysiology

The cervical spine consists of seven vertebrae in which the first two vertebrae, the atlas and the axis, compose the high cervical region and are considered integral components of the craniovertebral junction. These vertebrae are unique in structure and are rarely involved in the degenerative process of cervical spondylosis. Cervical vertebrae C3 through C7, otherwise known as the subaxial spine, are distinguished from the high cervical vertebrae by the presence of uncovertebral joints and the morphology of the vertebral bodies. The lateral masses of the cervical spine are composed of superior and inferior articular processes, which are thinnest at the C6–C7 level and have dimensions that increase from depth to height to width.¹⁸ The spinal canal has a triangular configuration with a varied sagittal diameter of approximately 17 to 18 mm from C3–C6 to 15 mm at C7.¹⁹^,²⁰^,²¹^,²² In vitro biomechanical testing has determined that the anterior column of the cervical spine transmits 36% of the applied load, whereas each pair of facets transmits 32% of the total load, stressing the importance of the posterior structures in cervical stability.²³

The diameter of the spinal cord is not uniform in the cervical spine region. At the C1 level, the spinal cord occupies one-half of the spinal canal. At the C5–C7 level, the cord expands and occupies three-fourths of the canal diameter, which increases the incidence of cord compression at the lower cervical spine. Cailliet²⁴ determined that cervical spinal canal stenosis is more uniform and restrictive in the diagonal anteroposterior diameter than the transverse. The size of the spinal canal is a predisposing factor in symptomatic cervical stenosis. Those patients with congenitally smaller canals are potentially at increased risk. A canal anteroposterior diameter of less than 13 mm has been established as a diagnostic standard of medullary symptoms from spondylotic encroachment.²⁴^,²⁵

The anterior spinal artery provides 60 to 70% of the vascular supply of the cervical spinal cord.²⁶ Although the midsagittal position of the anterior spinal artery is at risk for direct compression from disc protrusion or degenerative hypertrophies, its segmental medullary feeders provide collateral blood flow to enhance cord perfusion. Mannen ²⁷ and Jellinger²⁸ also reported that the lower cervical arteries are also more predisposed to atherosclerotic changes.

The normal cervical spine has a sagittal lordotic curvature mainly attributed to the intervertebral disc height, which accounts for 22% of the overall length of the cervical spine. The disc space height is greatest at the anterior aspect of the interspace and accounts for the natural lordotic curvature of the cervical spine. Motion is greatest at the intervertebral discs, and compressive and tensile forces are distributed throughout the cervical spine. Loss of cervical lordosis is often attributed to dehydration of the intervertebral discs and may contribute to altered biomechanical forces throughout the cervical spine, resulting in reactive hyperostotic changes in the adjacent vertebral endplates and development of a spondylotic bar due to posterior disc protrusion. Posterior disc herniation may contribute to impingement of the exiting nerve root and produce radiculopathic symptoms. In combination, these physiologic alterations may contribute to overriding of the uncinate processes leading to destruction of the uncovertebral joint space. Further progression of this degenerative process may lead to subsequent hypertrophy of the facet joints, ligamentum flavum, and ossification of the posterior longitudinal ligament. Moreover, manifestation of pain is attributed to compression or stretching of the sinuvertebral nerve, and altered integrity or distortion of the apophyseal facet joints, ligamentous elements, and cervical musculature.²⁹ Superimposed hypermobility at the adjacent levels may further induce hypertrophy of the respective motion segment and threaten encroachment on the spinal cord and nerve roots.

Additionally, the pincher phenomenon also contributes dynamically to instability in addition to physical cord encroachment. In flexion, the anteroposterior diameter decreases 2 to 3 mm, and the superior rim of the posterior vertebral body produces tension on the spinal cord. Although spinal cord tension is minimized in extension, the cord is still susceptible to compression by buckling of the ligamentum flavum. Lateral motion of the neck on the side of compression may further reduce foraminal diameter and produce or exacerbate symptoms. In addition, compensatory subluxation may develop above the level of the rigid spondylotic segment.

19.3 Surgical Approach

Surgical treatment can be through either an anterior or posterior approach. The anterior approach consists of decompression and interbody or strut grafting with or without instrumentation.³⁰^,³¹^,³²^,³³ The posterior approach consists of laminoplasty or laminectomy with fusion with or without internal fixation (see Fig. 19.1). Selection of an approach is often dependent on:

• Surgeon preference.

• Source of the cord compression (i.e., either anterior or posterior).

• Age of patient.

• Number of vertebral levels involved.

• Maintenance of lordotic alignment.

The anterior approach is recommended for patients with kyphotic curvature. The posterior approach is preferred in patients with preserved lordosis and more than three levels of involvement. The principal advantage of the posterior approach is its relative ease and familiarity among spine surgeons. This approach has been clearly established as a safe and effective means of decompressing the cervical spinal cord and nerve roots.³⁴^,³⁵^,³⁶ Nonetheless, the objective of any surgical approach is to maintain spinal stability and provide sufficient decompression without compromising sagittal balance.

19.4 Surgical Technique

19.4.1 Cadaveric Studies

Five cadaveric specimens were imaged preoperatively and postoperatively with CT myelography. Approximately 50 mL of Omnipaque contrast agent was injected into the subdural space of the cervical spine followed by CT imaging. In the cadaver laboratory, the cadaver specimen was placed on a radiolucent table in the prone position with the lateral fluoroscopic C-arm in place. The approach to the posterior cervical spine was achieved by means of the microendoscopic approach previously described for treating herniated discs of the lumbar spine as well as lumbar stenosis.³⁷ The C4–C5 level was first identified with a spinal needle and lateral fluoroscopic imaging, and a small incision was made approximately 2 cm lateral to the midline. A Kirschner wire (K-wire) was then passed under fluoroscopic visualization and docked on the C4–C5 laminofacet junction. The initial muscle dilator was passed over the K-wire, docked securely on bone, and then the K-wire was removed. Subsequent dilators were then passed, and an 18-mm-diameter tubular retractor was passed over the final dilator and locked in place. The muscle dilators were removed and the endoscopic assembly was white balanced, focused, and passed down the tube for visualization during the operation. Using a Bovie cautery, the soft tissue overlying the lamina and medial facet was removed. Fluoroscopic imaging both in the anteroposterior and lateral projections was used to aid in proper surgical orientation and location with wanding of the endoscopic assembly to perform a multilevel decompression ( Fig. 19.2). With the endoscopic assembly facing away from the surgeon, the ipsilateral lamina was removed using a high-speed drill and Kerrison punch ( Fig. 19.3a). This allowed for good visualization using the 30-degree angulation of the endoscope and also allowed for ipsilateral cervical foraminotomy as previously described.³⁸^,³⁹ Wanding the tubular retractor–endoscope assembly rostrally and caudally, up to a four-level laminectomy was performed.¹⁹ Once the ipsilateral laminectomy (or laminectomies) was performed, the endoscope was repositioned facing the surgeon on the tubular retractor.¹⁹ This allowed for contralateral cervical decompression ( Fig. 19.3b). The spinous process was identified, and drilling of the underside of the spinous process and contralateral lamina was performed ( Fig. 19.3c). This maintains the bony integrity of the spinous process and contralateral lamina with no removal of any muscle or ligament ( Fig. 19.3d). Therefore, much of the bone, muscular, and ligamentous integrity was maintained. This may reduce the incidence of postoperative muscle pain, spasms, and iatrogenic instability that is seen in more traditional approaches. Postoperative CT myelography confirmed adequate cervical decompression ( Fig. 19.4a–d). Subsequent open cadaveric dissection revealed adequate spinal cord decompression, while much of the posterior bony and muscular attachments of the cervical spine were preserved ( Fig. 19.4e).