Anterior and Posterior Endoscopic Approaches to the Cervical Spine

Overview

Over the past decade, minimally invasive treatment of spinal disorders has become feasible with the application of percutaneous technologies. These percutaneous approaches to spinal problems have the merits of preservation of healthy tissues, shorter hospital stays, less postoperative pain, and consequently faster patient recovery. Percutaneous endoscopic diskectomy of the lumbar spine has been widely used for lumbar soft-disk herniation. This technology and its clinical success have inspired and paved the way for similar minimally invasive approaches to the cervical spine.

Percutaneous endoscopic cervical diskectomy (PECD) could be considered a good alternative to the standard anterior cervical diskectomy and fusion (ACDF) in dealing with soft cervical disk herniations. The goal of this procedure is to decompress the spinal nerve root by percutaneous removal of the herniated mass and shrinkage of the nucleus pulposus under local anesthesia. Even though the ACDF still remains as the mainstay of surgical options for cervical disk herniation, it still requires entrance into the spinal canal with the accompanying risk of complications such as epidural bleeding, perineural fibrosis, graft-related problems, dysphasia, and hoarseness. The minimally invasive PECD under local anesthesia can avoid these complications with the maintenance of stability of the intervertebral mobile segment; it also provides the patient with excellent cosmetic effect and early recovery, and it does not preclude further open procedures, even in cases of failure.

In the early stage, the main target of this intradiskal procedure was the center of the disk, and its clinical application was limited. Since the first description of cervical percutaneous diskectomy by Tajima and colleagues, a remarkable evolution has occurred of various minimally invasive techniques for cervical disk diseases. These include chemoneucleolysis using chymopapain, automated percutaneous cervical diskectomy (APCD), and the combination of chymopapain injection followed by APCD. However, with the progress of endoscopic and instrumental technology, the extent of exploration with working channels has shifted to the posterior subannular portion, even to the epidural space, allowing definite removal of noncontained herniated fragments.

Anatomic Considerations

Lee and colleagues previously reported on the “safety zone” for the percutaneous cervical approach, which was determined by the sum of two distance calculations: the distances from the operator’s fingertip to the digestive tract on the contralateral side and to the carotid artery on the ipsilateral side. This was ascertained at each cervical level after obtaining computed tomographic (CT) scans of the cervical spine at each level of the intervertebral disk, from C3–C4 to C6–C7, by manually pushing the airway in the same position and manner of diskography ( Fig. 16-1 ). They also identified the anatomic structure at risk by simulated needle insertion toward the center of the disk through the safety zone. Their result was that at C3–C4, the safety zone was measured at 18.9 ± 6.6 mm. The superior thyroidal artery (STA) was located in the safety zone of C3–C4 in 86.7%. At C4–C5, the safety zone was measured at 23.5 ± 6.5 mm. The STA and the right lobe of the thyroid gland (TG) were located in the safety zone in 26.7% and 30%, respectively. At C5–C6, the safety zone was measured 33.7 ± 6 mm. The TG was located in the safety zone of C5–C6 in 76.7%. At C6–C7, the safety zone was 29.2 ± 4.5 mm. The TG was located on the approach plane in 90%. They concluded that the safety zone was wider at the distal level (C5–C6, C6–C7) than at the proximal level (C3–C4, C4–C5). The safest needle entry point should be between the pushing point of the airway and the pulsating point of the carotid artery. In addition, the needle should be approached toward the center of the disk, and reducing the finger distance (FD) to less than 5 mm from the ventral surface of the vertebral body is crucial to allow a low risk of pharyngoesophageal structure injury during percutaneous approach to the cervical spine.

When performing the cervical disk puncture, the surgeon must pay careful attention to the carotid artery medial to the sternocleidomastoid (SCM) muscle laterally and the tracheoesophageal trunk medially. The pretracheal fascia is fused on either side with the prevertebral fascia, completing a compartment composed of the larynx, trachea, TG/parathyroid gland, and pharynx-esophagus. When moved medially, all of these components move together, increasing the safety zone for the initial disk puncture. Laterally the carotid artery has an almost vertical path, overlying the SCM muscle obliquely. The carotid artery is placed more medial from the medial edge of the SCM at the C3–C4 level and more laterally at the C6–C7 level. A more lateral puncture increases the risk of carotid puncture, whereas a more medial puncture increases the risk of injury to the hypopharynx and esophagus. The safest needle entry point is between the airway and the pulsating point of the carotid artery.

Surgical Indications and Relative Contraindications

Most patients with cervicobrachial neuralgia as a result of disk herniation respond well to medical treatment. However, symptoms related to perineural cicatricial fibrosis as a result of prolonged pressure on the nerve root could become irreversible. The occurrence or aggravation of a neurologic deficit even after an adequate period of conservative treatment therefore requires the consideration of surgical decompression. PECD is indicated in the surgical treatment of soft cervical disk herniation not contained by the posterior longitudinal ligament (PLL; ligamentous protrusion); this includes the central, lateral, and foraminal disk herniations confirmed by magnetic resonance imaging (MRI) or CT. A very bulky herniation is not a contraindication, as long as the patient has no myelopathic symptoms or signs.

According to our previous series, the two major factors that predict an excellent long-term outcome after PECD were the symptom of radiating arm pain and the location of lateral disk herniation. This observation can be explained by the fact that radiating arm pain and a foraminal or posterolateral soft disk herniation represent root compression by recently developed soft disk herniation; vague numbness and axial pain represent a relatively chronic, hard compression and therefore may be accompanied by other structural problems besides root compression.

In summary, the best indication for PECD seems to be a patient younger than 50 years, having a positive provocative test, without a bony spur greater than 2 mm, and regardless of the herniation size, location, and epidural leakage.

PECD is contraindicated in patients with a severe neurologic deficit, segmental instability, acute pyramidal syndrome, progressive myelopathy, and other pathologic conditions such as tumor, fracture, infection, and nerve entrapment with scar tissue from previous surgery. Also contraindicated were migrated disks, calcified disk protrusions, ossification of the PLL, marked spondylosis with disk space narrowing (less than 3 mm), and neurologic or vascular pathologies that mimic disk herniations.

Instruments and Equipment

Instruments for the Anterior Approach

The oldest working channel made by Tajima in 1981 was 2 mm in diameter and 8 cm in length. He added two dilators, one guide needle for diskography, and three forceps of various sizes: small, medium, and large. The small forceps were principally used for excision of osteophytes. Using medium and large forceps, Tajima performed diskectomy of the posterior disk. However, these instruments had the risk of easily breaking inside the disk during the procedure.

The recently developed cervical working-channel endoscope (WSH endoscopy set; Karl-Storz, Tuttlingen, Germany) is an advanced form of the cervical endoscope used for PECD ( Fig. 16-2 ). It has a working cannula, 4.2 mm in outer caliber with a 1.9-mm central working channel and two additional ports, that has an integrated high-resolution endoscope, illumination, and irrigation. It allows surgeons to selectively remove the herniated disk via a holmium yttrium-aluminium-garnet (Ho:YAG) laser and microforceps under clear endoscopic visualization ( Fig. 16-3 ). This new endoscopic system offers several advantages. First, selective disk removal by microforceps under direct endoscopic vision becomes possible. The working channel allows passage of the microforceps as well as the laser probe. Second, as the resolution and clearness of endoscopic vision is improved, we could explore the intradiskal and epidural anatomy in detail, especially the foraminal area. Better visualization of the operating field may reduce the degree of the learning curve. Finally, the side-firing rigid laser can be applied through the working channel. The side-firing laser is a safe and effective tool that helps to avoid damaging neural tissues, and it is powerful enough to vaporize the pathologic tissues, including fragile osteophytes.

Endoscopic laser foraminoplasty in the cervical spine is also feasible with the WSH working-channel endoscope. With the end-firing laser, bone sculpturing was almost impossible, because high-power lasering may cause critical damage to the adjacent neural tissues. In contrast, the side shot of a laser beam under high-resolution endoscopic vision is tremendously helpful for the elaborate sculpturing of the foraminal osteophytes, uncus, and inflamed fibrotic tissues without causing any neural injury. Moreover, the Ho:YAG laser inherently has a high ablation effect while producing relatively little thermal necrosis. The laser irradiation can be performed with saline irrigation, and the tissue penetration of the Ho:YAG laser is approximately 0.3 to 0.5 mm deep. Therefore the tissue volume heated by the laser is very small, and the heat damage to the surrounding normal tissue is negligible. Endoscopic laser foraminoplasty may play an important role for removing an extruded disk fragment by widening the narrow foramen. However, we do not believe that all the severe cervical foraminal stenosis can be treated by endoscopic laser foraminoplasty with the present technical restrictions.

Instruments for Posterior Approach

The rod-lens optics have an outer caliber of 5.9 mm. This contains an eccentric working channel 3.1 mm in diameter, an optical lens with the connecting light conductor system, and a channel for continuous irrigation ( Fig. 16-4 ). The angle of vision is 25 degrees, and the beveled opening working sheaths are 6.9 mm in outer diameter. All these instruments and optics are Wolf products (Richard Wolf, Knittlingen, Germany).

Surgical Preparations and Techniques

Anterior Approaching Technique

The patient is placed in a supine position with the neck mildly extended on a radiolucent table. A plastic hood over the face enables communication with the anesthesiologist ( Fig. 16-5 ). The C-arm of the fluoroscope is put in front, then in profile, and the level of the operation is carefully marked on the skin with a felt pen using a metallic instrument. The operation is typically conducted under local anesthesia and analgesia by neuroleptics, so the surgeons may immediately become aware of any changes in symptoms and signs of the patient. General anesthesia may be used in a few patients who want it or cannot tolerate the position; but under general anesthesia, the urgent need for conversion to open surgery may not be detected early. A solution of 1% lidocaine is usually used to infiltrate the skin and subcutaneous tissue.

Usually a paramedian approach (2 to 5 mm from the anterior midline) from the contralateral side was chosen. The surgeon gently pushes the trachea or larynx toward the opposite side with the index and middle finger and then applies a firm pressure in the space between the SCM and trachea and points toward the vertebral surface until the prominence of the anterior edge of the disk could be palpated ( Fig. 16-6 ). The trachea and larynx are displaced medially, and the carotid pulsation is palpated on the lateral side. The 18-gauge puncture needle is then inserted through the space between the tracheoesophagus and the carotid artery. After confirming the midline position of this 18-gauge needle on the center of the anterior annulus by intraoperative fluoroscopy, the needle is advanced close to the posterior body line of the posterior disk space ( Fig. 16-7 ). Then diskography with 10 mL Telebrix (Guerbert, France) and indigo carmine (Korean United Pharma, Seoul, Korea) is performed to confirm the presence of soft disk herniation and stain the nucleus blue in contrast with the neural tissue ( Fig. 16-8 ). Up to 0.5 mL of contrast media is injected to specify the posterior part of the disk. Then a guidewire is inserted to replace the puncture needle, and a 3- to 5-mm skin incision is made to allow the passage of a serial progressive dilator (2 to 5 mm) along the guidewire to stretch the soft tissues ( Fig. 16-9 ). Finally, the tip of the working cannula is hammered to reach the posterior part of the disk ( Fig. 16-10 ), and the forceps should be reached at the end of the posterior margin to remove the herniated mass effectively, with care taken not to injure the spinal cord.