INTRODUCTION

Although many patients benefit from surgical discectomy with or without a fusion, these open procedures are associated with known risks, including early or late failures secondary to symptomatic adhesions, pseudoarthrosis, and adjacent level instability.¹ These complications have prompted the development and introduction of less invasive percutaneous intradiscal procedures that have the ability to chemically or mechanically remove intradiscal nuclear material. Chemonucleolysis using the intranuclear injection of chymopapain was introduced several decades ago and a variety of other intradiscal and endoscopic procedures have followed. Among the intradiscal procedures are endoscopic nucleotomy, nucleoplasty (coblation), percutaneous lumbar discectomy methods of decompression.

Most of the devices removed nuclear material using mechanical incision by surgical instruments or suction, but in 1986 Choy et al.² introduced nonendoscopic percutaneous laser disc decompression and nucleotomy (PLDN) with the Nd:YAG laser (wavelength 1064 nm). Even though the 1064 nm wavelength Nd:YAG laser technique in this context is not to be understood as the use of soft laser technology,³ many consider this first introduction of intradiscal laser technology a pioneering achievement.

PRINCIPLES

Several postulates have been proposed to explain why intradiscal laser decompression can help relieve pain caused by a herniated disc. The first and more apparent reason is the potential beneficial effect of lowering the intradiscal pressure by vaporizing nuclear material. The Nd:YAG 1064 nm irradiation of discal tissue creates a small vaporization defect lined with a carbonized margin (Fig. 29.1)⁴ and minimal ablation of disc tissue.⁵ Greater ablation is possible with the Nd:YAG laser 1320 nm with a pressure drop of up to 55.6% recorded (Fig. 29.2).⁶ This reduction in pressure appears to be independent of age or the degree of disc degeneration.⁷ Even though the amount of tissue removed and the ablation defect is less than that achieved with a mechanical discectomy, clinical results are similar using the Nd:YAG laser.

Fig. 29.1 Controlled vaporization of a disc. Note the zone of thermocoagulation surrounding the defect.

(Courtesy W. Siebert, M.D.)

Fig. 29.2 Intradiscal pressure reduction after intradiscal irradiation by Nd:YAG laser

(modified from Choy et al.⁶).

Laser ablation may have other beneficial effects. Some believe that laser provides benefits other than mechanical decompression. One such potential beneficial effect is the shrinkage of collagen fibrils caused by laser-generated heat^8–10 followed by a subsequent reduction of intradiscal volume. We postulated this mechanism after observing sudden shrinking of a resected meniscus irradiated by the Nd:YAG laser 1064 nm.⁸ Our basic studies showed a reduction in disc diameter up to 14% in explanted bovine discs, even though comparative studies⁸ using the holmium:YAG laser caused 1% or less diameter shrinkage. Consistent with our findings regarding disc shrinkage (Fig. 29.3), Turgut et al.¹¹ demonstrated water loss, and proteoglycan and collagen changes in animal discs following laser treatment. Kosaka et al.¹² showed size reduction of several millimeters of extruded discs by open surgical Nd: YAG laser disc decompression and intraoperative monitoring. As well, Mayer¹³ documented disc shrinkage on video during endoscopic Nd:YAG laser-assisted percutaneous discectomy and Grönemeyer¹⁴ with computed tomography (CT) intraoperatively.

Fig. 29.3 Diameter shrinkage of bovine discs and local shrinking effect (arrow) after intradiscal Nd:YAG 1064 nm shooting up to 750 joules (15 watts, 1 sec).

Reductions in the density of disc protrusions and extrusions have been documented with CT scans performed on the first postoperative day following Nd:YAG PLDN.¹⁵ Brat et al.¹⁶ similarly documented reduced size of extrusions on postoperative magnetic resonance imaging (MRI) scans. Similar findings were seen on MRI myelograms performed on the first postoperative day¹⁷ where improved cerebrospinal fluid (CSF) flow was demonstrated at the site of the previously constricted dural sac (Fig. 29.4). The authors concluded that the improved CSF flow was the evidence of reduced venous congestion, reduced arteriole compression, and reduced dural sympathetic nerve fiber compression. Because only minimal venous congestion will adversely affect dorsal root neurons¹⁸ and compression often occurs at two adjacent segments,¹⁹ proximal and distal root decompression over two segments is recommended.

Fig. 29.4 (A) Pre- and (B) postoperative MRI myelogram with evidence of improved dural sac contours and cerebrospinal fluid flow.

Contrary to open and most percutaneous surgical disc interventions, the Nd:YAG 1064 nm laser technique may not cause postoperative instability.²⁰ On the contrary, Wittenberg and Steffen²¹ have even reported an increase in translation stability of spinal segments, which they felt was due to a change of collagen tissue formation.²² Scar tissue was seen at 6 weeks intradiscally, but takes a year to mature.¹⁰ Thal et al.²² reported a late shrinking effect of the intervertebral disc without any detectable increased segmental movement.

Finally, another theoretical but unproven potential beneficial side effect of the heat generated during laser ablation is the destruction of nociceptors in the outer anulus, destruction of nociceptors that follow the ingrowth of vascular tissue into radial and concentric annular tears, and the denaturing of inflammatory chemokines.

SAFETY OF USE

Siebert²³ in the lumbar spine and Schmolke et al.²⁴ in the cervical spine have provided safety data for the Nd:YAG 1064 nm laser system. At an irradiation time of one shot per second at 20 watts the laser beam will penetrate to a depth of 6 mm in disc tissue. When the laser beam was not directed at the endplate or within the spinal canal, temperatures above the coagulation threshold of proteins were not reached in either the endplate or the epidural tissue adjacent to the disc. Their findings prompted the recommendation that a posterolateral approach be used to access the dorsolateral third of the lumbar and thoracic disc and an anterior approach involving the ventral third of the cervical disc. Based on the experimental studies, the recommended maximal dose per disc is 1600 joules in the lumbar spine, 1000 joules in the thoracic spine, and 300–400 joules in the cervical spine.

Pilot studies showed that patients under regional anesthesia and analgesic sedation will empirically tolerate single shots of 15 watts at 1 second intervals in both the lumbar and thoracic spine, and in the cervical spine 20 watts every 0.3 seconds with a 5 second interruption after five shots. During the past 15 years, the authors have gradually reduced the lumbar dose to 900–1000 joules without any difference in clinical outcome. Although endplate injury has been reported during the use a holmium:YAG laser, we have observed no cases of endplate injury using the Nd:YAG laser 1064 nm. We have, however, seen sporadic edematous lesions in the adjacent vertebrae, but these lesions are often seen after open surgery as well and are of uncertain clinical significance.

INDICATIONS AND PATIENT SELECTION

We define pain by its quality, quantity, topology, and chronology and classify discogenic pain as causing local, pseudoradicular, radicular, medullary, or autonomic symptoms. In addition, pain symptoms may be associated with neurological deficits such as dysesthesias, hypo- or hypersensitization, or even paralysis. The cause of these pain syndromes can be diagnosed by a characteristic history and physical examination substantiated by MRI or CT evidence of disc bulging, protrusions, extrusions, or sequestrations.^33,34 Abnormal structural findings alone without corresponding symptoms are not, however, a reason for surgical intervention.³⁵

Patients who fail 6 weeks of conservative care and continue to have disabling discogenic pain are first offered Nd:YAG PLDN before an open surgical procedure. Although progressive neurologic deficits, paralysis, conus medullaris, and cauda syndrome require immediate intervention, the severity of pain most often dictates when we determine that surgery will be offered. With the exception of a patient with a hemostatic disorder or untreated infection, there are few contraindications to discal laser intervention. Because shrinkable collagen fibers are always present in the fibrous ring, even age is not a contraindication. In fact, as long as we clinically believe the patient’s axial or extremity pain is due to a disc bulge, protrusion, or extrusion and it is seen on a CT or MRI scan, the patient will be offered laser decompression. In particular, laser decompression is not contraindicated when a disc protrusion is aggravating the pain of spinal instability or is contributing to stenosis.³⁶ Furthermore, many patients who have unrelieved pain for over 6 weeks will begin to develop somatization symptoms, but unlike other authors^20,37 we believe these symptoms are not a contraindication to surgery. In addition, although many physicians including Siebert²⁰ and others^16,37,38 prefer restricting percutaneous laser decompression to patients with monoradicular pain, the authors have shown that patients with monoradicular pain alone account for only 20% of patients presenting with discogenic pain.⁴ Consequently, the authors believe that axial pain alone is not a contraindication to the performance of laser disc decompression, similar to radiofrequency (RF) application. Finally, the authors do not routinely use discography to confirm the source of pain,³⁹ but will use discography to help position the cannula during the operation. In practice, the majority of patients who are offered a percutaneous laser decompression have mono- or polyradicular pain associated with a disc bulge, protrusion, or contained and noncontained extrusions. When selection is limited to patients with radicular pain, patients who will eventually require an open procedure may be less than 10%^4,40 and more than 90% would choose the procedure again for a recurrence in pain. In a consecutive prospective series, 85% of the operated patients had either protrusions or contained extrusions and 15% had noncontained extrusions with either caudal, cranial or buttonhole dislocations. The failure rate was 3% in the protrusion-contained extrusion group and even though the failure rate in the noncontained extrusions was 20%, open surgery was avoided in four of the five cases. Although most surgeons rarely operate on sequestered fragments that are free-floating in the epidural space, many sequestered fragments remain within the protrusion or trapped in the fibrous ring.⁴¹ Furthermore, when there is both a free-floating epidural sequestered fragment and a foraminal disc protrusion, decompression of the protrusion alone may provide enough radicular pain relief such that the patients are satisfied. In the authors’ series of 3970 lumbar Nd:YAG PLDN procedures between 1989 and 2002 there were seven patients with both protrusions and free-floating sequestered fragments. Six of the seven patients were satisfied with their outcome following discal decompression alone, and only three had residual local axial pain. Two patients with dorsiflexor foot weakness had full recovery.

Finally, most consider cauda equina syndrome as a contraindication for percutaneous disc decompression.^20,42 The authors have, however, successfully performed Nd:YAG PLDN on 30 cases of cauda equina syndrome. Moreover, in only one case of a recurrent herniation with a free-floating sequestered fragment was open surgery necessary.⁴³ While this chapter has emphasized lumbar disc disorders, it should be understood that laser decompression may also be considered for symptomatic disc herniations in the cervical and thoracic spine.⁴⁴

TECHNIQUE

In the lumbar and thoracic spine, the patient is positioned in the lateral decubitus position with the painful side up, and a posterolateral approach is used. In the cervical spine, the patient’s neck is placed in hyperextension and access is made on the right side between the vessels and the trachea. Local anesthetic is infiltrated in the skin and subcutaneous muscles, and analgesic sedation is given by the anesthetist. Direct and continuous fluoroscopic visualization using intermittent anteroposterior (AP) and lateral projections is used during needle insertions. Because the 400–600 μ bare fiber of the Nd:YAG laser extends 2 mm beyond the cannula tip. The laser beam penetrates to a depth of 6 mm. Accurate and specific placement within the disc is important. Although rarely needed, puncture laser osteotomy⁴⁵ through the edge of the vertebral body, osteophyte, or superior articular process will facilitate access to the intervertebral disc. On rare occasions, transdural puncture will be needed to access the L5–S1 disc.