Posterior Cervical Microforaminotomy and Discectomy

18 Posterior Cervical Microforaminotomy and Discectomy

Carter S. Gerard, Lee A. Tan, and Richard G. Fessler

Abstract

Posterior cervical microforaminotomy and discectomy is an effective minimally invasive surgical technique that can result in decompression of cervical nerve roots and improvement of cervical radiculopathy in patients with foraminal stenosis or lateral disc herniation. This chapter provides a step-by-step guide for this important minimally invasive technique, along with valuable clinical pearls from the senior author (R. G. F.).

Keywords: posterior cervical, foraminotomy, discectomy, minimally invasive, tubular dilator, cervical disc herniation

18.1 Introduction

Multiple degenerative pathologies of the cervical spine can be successfully treated with posterior decompressive techniques.¹^,²^,³^,⁴ While anterior cervical procedures are well established as treatments for cervical disc herniation, posterior cervical laminoforaminotomy consistently shows symptom improvement of 90 to 97% for patients with foraminal stenosis or lateral disc herniation.³^,⁵^,⁶^,⁷^,⁸ Posterior decompressive procedures avoid the complications associated with anterior approaches such as esophageal injury, recurrent laryngeal nerve paralysis, dysphagia, and adjacent level disease after fusion.⁹^,¹⁰^,¹¹^,¹²

While standard open approaches achieve excellent decompression of the lateral recess and neural foramen and are effective, minimally invasive approaches have been developed in order to avoid the extensive subperiosteal stripping of paraspinal musculature, which can result in significant postoperative pain, muscle spasm, and dysfunction in 18 to 60% of patients.⁴^,⁹^,¹³^,¹⁴ Furthermore, preoperative loss of lordosis combined with long segment decompression can contribute to the risk of sagittal plane deformity, a known complication that often obliges fusion at the time of decompression.¹⁵^,¹⁶^,¹⁷^,¹⁸ The use of a posterior fusion technique increases operative time, blood loss, surgical risk, and early postoperative pain, and potentially contributes to adjacent level disease.

The principal tenet of minimal access techniques is to reduce approach-related morbidity. To that end, the advent of muscle-splitting tubular retractor systems and associated instruments has allowed for the application of minimally invasive techniques to posterior cervical decompressive procedures.¹⁴^,¹⁹ The microforaminotomy/discectomy (MF/D) was first described in a cadaver model and has subsequently been shown to have clinical efficacy equal to open procedures in addition to having less blood loss, shorter hospital stay, and decreased postoperative pain.⁷^,²⁰^,²¹^,²²^,²³^,²⁴^,²⁵

18.2 Indications

Radicular symptoms in the upper extremities most commonly occur due to degenerative changes in the cervical spine such as disc herniation or osteophytes within the intervertebral foramen. Regardless of the pathology, it must be lateralized without significant canal stenosis to be amenable to MF/D. Patients with persistent radicular pain refractory despite at least 6 weeks of effective nonoperative therapy, those with debilitating pain, or those who have developed muscle weakness are appropriate candidates for surgical intervention. The prototypical patient will present to the neurosurgeon with a history of neck pain and unilateral cervical monoradiculopathy, having already undergone initial pain management with first- and second-line analgesics. Determining the proper candidate for surgery is straightforward in patients who present with a clear history of isolated radicular pain in a classical distribution and imaging, which shows foraminal stenosis at the expected level. When the clinical evaluation is not supported by MRI findings, further testing including dynamic imaging or electromyography (EMG) may be helpful.

18.3 Contraindications

Relative contraindications to the MF/D include those patients with central pathology or cervical instability.⁷^,⁸ When an MF is properly performed, the surgeon gains access to the foramen and the most lateral portion of the spinal canal. Therefore, patients with central disc herniations and resulting myelopathy are best treated with an anterior approach. Patients with a progressive kyphotic deformity or suspected instability should not undergo MF/D. While recent series have failed to show an increased risk of postoperative instability in well-selected patients, patients with increased mobility are at elevated risk of frank destabilization and are unlikely to benefit from this procedure.²²^,²³^,²⁴^,²⁵^,²⁶^,²⁷^,²⁸

18.4 Preoperative Evaluation

A preoperative radiographic evaluation follows a detailed history and physical examination and should include magnetic resonance imaging (MRI) or postmyelographic computed tomography (CT), and anteroposterior (AP), lateral, and flexion/extension cervical radiographs. Preoperative EMG and nerve conduction studies may also assist in the neurological localization of specific radiculopathy. Those patients with radicular symptoms that correlate with electrophysiologic and radiographic findings may be well suited for MF/D, depending on the underlying pathology. Fig. 18.1a shows a lateralized disc herniation without spinal cord compression on preoperative MR scan. In contrast, Fig. 18.1b shows moderate cord and nerve root compression due to a herniated disc. The former would be an ideal candidate for MF/D, while an anterior approach would be safer and more effective in the latter. Regardless of the pathology, whether a soft disc or an osteophyte, it must be lateralized without significant canal stenosis to be amenable to MF/D.

18.5 Surgical Technique and Instrumentation

Appropriately selected patients, following routine presurgical evaluation and medical clearance, are brought to the operating suite. Endotracheal intubation ensues under general anesthesia. Somatosensory and motor evoked potentials are recorded to follow intraoperative spinal cord integrity. Electromyograms of appropriate muscle groups can also be monitored as additional intraoperative feedback. Use of neuromuscular paralytic agents is minimized to better assess intraoperative nerve irritation. A single dose of antibiotics (either cephazolin or vancomycin) is routinely administered before skin incision. Intravenous steroids are not used routinely for patients undergoing micro-endoscopic discectomy/foraminotomy (MED/F). A Foley catheter is generally not needed.

Fig. 18.1 Axial T2-weighted magnetic resonance imaging (MRI) scans of the cervical spine demonstrate: (a) laterally herniated disc to the right with compression of the exiting nerve root; (b) a centrally located disc/osteophyte causing both spinal cord and nerve root compression.

With the patient supine on the operating table, the head is secured with a Mayfield Skull Clamp (Integra LifeSciences) (this refers to the skull clamp with three pins that secure the head in place during the procedure) and the patient is brought to the semi-sitting position; the knees are level with the heart. The Mayfield Spine Table Adaptor (Integra LifeSciences) (this refers to the adapter that connects the skull clamp to the operating room [OR] table) is attached to the table between the hip and knees and arched back as necessary to accommodate the patient’s body habitus. Care is taken to maintain the neck in a comfortable, neutral position. The arms are folded and secured across the chest; pressure points are padded to prevent nerve palsies and pressure sores. The fluoroscope is positioned to obtain a lateral view of the cervical spine. Precordial Doppler monitoring is used to monitor the right atrium for air emboli; a central venous catheter is not necessary because of the brevity and minimal blood loss associated with the procedure.

After segmental localization with fluoroscopy, a small incision (approximately 1.8 cm) is made 1 cm from midline on the symptomatic side. Soft tissue is bluntly dissected, and the underlying cervical fascia is sharply dissected with scissors or Bovey cautery. Failure to do this places undue strain on the cervical spine and makes serial tissue dilation difficult. The first dilator is then passed through this pathway and docked on the facet complex ( Fig. 18.2a). Docking onto the facet avoids the risk of slipping off the lamina into the canal and adds a margin of safety to the approach. Sequential tissue dilators are then passed to obtain a working corridor ( Fig. 18.2b,c) with each docking onto the facet. Note that use of a Steinmann pin to localize level is not recommended. Care is taken in advancing the Steinmann pin toward the spine to avoid passing it between the interlaminar space and into the spinal canal. The first dilator is passed using slow careful rotation toward the facet complex. Once the first dilator is passed, the Steinmann pin is removed to avoid migration during passage of subsequent dilators. The working channel is then passed over the final muscle dilator and adjusted to the desired position under fluoroscopy ( Fig. 18.2d). If the tubular retractor is not docked directly on the facet, dissection of the paraspinal muscle can be done under direct endoscopic or microscopic visualization and then advanced again using the serial muscle dilators. The tubular retractor is then fixed to the flexible arm, which is mounted to the operating table, opposite to the approach, and the endoscope is secured to the tubular retractor ( Fig. 18.3). Alternatively, a microscope can be used which provides excellent 3D visualization of the anatomy. Using the microscope, we typically perform the procedure in the sitting position. An arm rest can be used if arm support is helpful by bending the head rest down.

A Bovie cautery unit on a long handle is used to circumferentially remove muscle from the operative field. We prefer to work from lateral to medial. The surgeon should always be cognizant of the interlaminar space. The bony anatomy is defined with a curette to appreciate the interlaminar space and facet relationship ( Fig. 18.4a). A 1- or 2-mm Kerrison rongeur is used to begin the hemilaminotomy ( Fig. 18.4b). After the hemilaminotomy is completed, the foraminotomy can be continued with either the Kerrison rongeur or the Midas Rex Legend Stylus high-speed surgical drill with the hooded telescoping attachment. In either case, the foraminotomy is usually completed using the drill. Whether drilling cephalad, caudal, or lateral to the nerve root, the root itself is protected by holding the hooded telescoping attachment toward the nerve to prevent any soft tissue surrounding the nerve from getting caught up in the rotating drill bit. The nerve is visualized well because of the 30-degree angle on the endoscope. When performing a posterior discectomy, drilling approximately 2 mm of the superior medial pedicle facilitates safe removal of herniated disc fragments and limits nerve root manipulation ( Fig. 18.4c–f). Once again the hooded telescoping attachment is used to protect the nerve root (located cephalad to the drill bit) and the medially located dura. The space created by drilling the pedicle also can be used to reduce anterior osteophytes contributing to foraminal stenosis with a down-angled curette. Radiographic and tactile confirmation of the extent of discectomy or foraminotomy is obtained before closing.