INTRODUCTION

Intradiscal electrothermal therapy (IDET) is a recently proposed and investigated treatment for intractable discogenic pain. There is good evidence that IDET denatures collagen and causes changes in disc protein.¹ Yet, these changes do not alter the fundamental biomechanics of the intact motion segment.² Relatively speaking, it is a minimally invasive technique when compared to fusion or total disc arthroplasty. An important attribute of this treatment is that it does not preclude the future application of these more invasive treatments while the corollary is not true. Once disc replacement or fusion is undertaken, IDET is no longer a viable treatment option at that disc level.

Although IDET is a ‘minimally invasive’ intervention, this must not be construed to mean that the procedure is simple to perform or without potential deleterious side effects or complications. Both of these issues can be mitigated and optimal outcomes can be realized if meticulous technique is used. It is important to keep in mind the definition of technique: the systematic procedure by which a complex or scientific task is accomplished. The presentation of such a systematic, step-by-step approach is the aim of this chapter. By adhering to this approach, technical facility in clinical application is attained with every expectation of maximizing successful outcomes.

There exists an interplay between technique and clinical experience whereby good technique impacts clinical experience and vice versa. Undeniably, experience cannot be adequately transmitted through the generation of even the best written manuscript. On the other hand, the hard-earned hindsight that experience provides and which alters the basic procedure is teachable as technique. The technique presented here is tempered with our idiosyncratic experience to which, it is hoped, the future practitioner will add their own.

RELEVANT ANATOMY AND THE IDET PROCEDURE

Intradiscal electrothermal therapy entails the threading of a blunt-tipped thermal catheter through the disc with the goal of traversing the entire posterior anulus midway between endplates. Various aspects of the anatomy of the normal and degenerative disc conspire against the physician attempting to do so. There are small regional variations in disc structure due to the different demands placed upon them in the cervical, thoracic, and lumbar spine. As the main target of the IDET procedure, the lumbar disc will be the focus of discussion.

The intervertebral discs are interposed between adjacent vertebral bodies accounting for approximately 25% of the height of the column. The three main functions of the disc are to allow movement between spinal segments, serve as shock absorbers, and transmit the axial load of the body. The disc is composed of a tough outer anulus and a central nucleus pulposus. The nucleus is a remnant of the embryologic notochord. It is composed of semifluid ground substance and irregularly placed collagen fibers. When compressed, the gelatinous nucleus deforms and distributes load forces in all directions. The anulus is designed to contain these forces. It is composed of 10–20 fibrous lamellae arranged in concentric rings around the nucleus. Coincidental with the eccentric location of the nucleus, which is closer to the posterior anulus, there is a reduction in lamellar thickness posteriorly when compared to its anterior and lateral portions. Lumbar flexion places a stress on this relatively vulnerable area.

Two design features of the disc offset this weakness. First, like the lumbar vertebral bodies, the lumbar disc is kidney shaped with a slight concavity to its posterior border. This concavity increases the surface area of the posterior anulus and serves to strengthen it against distractive and compressive forces. It also increases the length the catheter must navigate as well as the acute angle of the posterolateral corners that must be negotiated to cover the posterior anulus. The passage of the catheter is more difficult as a result of increased resistance from the greater length and sharper angles. Also, this concavity must be considered when attempting to judge the proximity of the catheter to the thecal sac on a lateral fluoroscopic view prior to beginning the heating protocol. It may appear from this lateral orientation that the catheter is well contained within the substance of the anulus if its position is compared to the most dorsal extent of the vertebral body when, in fact, it has escaped at the turn around the posterolateral corner. When this transpires, the catheter will bridge the concavity of the posterior anulus instead of following its contour and, yet, appear to be safely within its confines.

Secondly, each lamella consists of obliquely oriented parallel collagen fibers. The direction of this obliquity is rotated 90° in adjacent lamellae so that the fibers between any two adjacent lamellae form a rough ‘X’ shape. This configuration contributes to the integrity of the anulus under similar forces. While the majority of lamellae form complete rings around the circumference of the disc, as many as 50% of the lamellae at the posterolateral corners are incomplete.³ Where a lamella ends, the superficial and deep rings approximate or fuse together (Fig. 26.1). Advancement of the blunt catheter tip past these fusion points may account for some of the difficulty of passing around the posterolateral corner. The tip, which tracks easily between lamellae when traveling within the anulus, may be unable to penetrate the fused lamellae at the terminus of the layer it is following.

Fig. 26.1 Illustration of normal lumbar disc.

Covering the entire nucleus and a portion of the annular ring is a layer of hyaline and fibrocartilage known as the vertebral endplate. It is bordered by the slightly raised perimeter of the vertebral body called the ring apophysis. The endplate is securely attached to the disc with the fibers of the inner lamellae being continuous with the fibrocartilage of the endplate.

In contrast, the outer annular fibers attach directly to the bony ring apophysis of the vertebral body and serve as the main ligamentous connection between segments. This function is aided by the anterior and posterior longitudinal ligaments (PLL). The PLL is intimately connected to the posterior anulus and serves to separate it from the dural sac. Both PLL and outer anulus are richly supplied with nerve fibers and are common tissue sources of back pain.

With aging, the water content of the disc diminishes and the disc becomes more fibrous. Collagen content of both the nucleus and anulus increases⁴ and the disc becomes increasingly less pliant as a result. The inner anulus expands at the expense of the nucleus and the boundary between them becomes less distinct. With dehydration the nucleus becomes less able to transmit forces equally and certain sections of anulus are subjected to disproportionately greater axial loads. Defects occur at the transition zone between the nucleus and anulus that can develop into radial tears extending through the anulus to the epidural space of the spinal canal. Increasing fibrillation of the lamellae is associated with similar defects.⁵ Circumferential tears of the anulus are the result of splitting between lamellar layers (Fig. 26.2).

Fig. 26.2 Illustration of degenerative disc with disruption of annular fibers.

Fibrotic change within the disc resulting from simple aging or scar formation following injury increases the possibility of obstruction to catheter passage and makes proper placement more difficult. Disruption of the lamellae with attendant loss of smooth tracking of the catheter between layers also increases the resistance to catheter passage. Defects in the nucleus and anulus may capture the advancing catheter tip and redirect it along paths of reduced resistance. These often lead to the epidural space as in complete radial tears or into dead ends such as the PLL or vertebral endplate.

The physician performing the IDET procedure is, in large measure, dependent upon patient anatomy for proper catheter placement; never more so than in the degenerated disc. Strategies for dealing with these problems are discussed below.

PATIENT SELECTION

The suitable candidate for IDET is a patient with a confirmed discogenic source of back pain without predominant leg symptoms unresponsive to aggressive conservative care including medications, activity modification, injection therapy, and an exercise program (Table 26.1). The need to deliver these treatments thoroughly and with expertise prior to considering IDET cannot be overemphasized.

Table 26.1 IDET Criteria

If the judgment has been made that there has been failure to progress, standing X-ray and recent magnetic resonance imaging (MRI) of the lumbosacral spine is required. Acceptable abnormalities for the performance of IDET may include disc space narrowing, disc desiccation and degeneration, or a small, contained disc protrusion. The presence of a high-intensity zone lesion does not preclude employing IDET. These images should also be scrutinized for abnormalities that could lead to the prohibition of the performance of IDET. Higher grade (II–IV) spondylolisthesis, especially isthmic and traumatic cases, will require flexion and extension plain views to rule out instability in the motion segment. Spondylolisthesis also increases the importance of ruling out nondiscogenic causes of axial low back pain. Pain greater with extension versus flexion of the spine with findings on imaging of central canal encroachment or fluid-filled zygapophyseal joints could support the diagnosis of stenosis and facet arthropathy, respectively. A positive discogram without evidence of annular disruption in the presence of a prominent Schmorl’s node may point to the node as the pain generator rather than the anulus. Short of a radical placement of the IDET catheter tip in the node itself, conventional IDET is not indicated. Performing the high heat protocol with a standard catheter placement within the anulus has been shown in a histologic study not to affect the adjacent endplate.¹ Fractures of the pars, significant central canal stenosis, large disc protrusions (>5 mm extrusion beyond the posterior border of the vertebral body), extrusions, and sequestered fragments are all relative contraindications for the procedure.

Patients with prior history of fusion who return with adjacent symptomatic discs or those with multiple symptomatic levels may be suitable candidates (Table 26.2). In experience of the authors, patients tolerate single-visit, three-level IDETs without prolongation of the recovery period. Patients who have had prior partial discectomy and fusion may also be candidates though the fusion mass or instrumentation may prevent an acceptable approach to the disc. Similarly, prior chemonucleolysis, laser decompression, or nucleoplasty at the symptomatic level does not preclude IDET as long as other inclusion criteria, especially adequate preservation of disc height, are present. These intradiscal procedures may increase annular fibrosus with consequent difficulty of catheter placement.

Table 26.2 IDET Potential Indications

A repeat IDET procedure on the same disc may be considered in certain cases. Patients who initially respond with greater than 50% relief, but then have return of similar axial low back pain or those whose lack of response is felt to be due to less than ideal catheter placement may benefit from a second IDET at the same level. With increased experience in performing the procedure, the surgeon finds fewer patients filling the latter category. Further studies are needed to judge the efficacy of repeat procedures, but in the authors’ limited experience, selected patients often experience clinically significant decreases in symptoms. Six months postprocedure is the minimum period required to judge clinical outcome and is necessary prior to consideration of a repeat IDET.

The symptomatic disc should have preserved disc height >50% with reproduction of concordant pain at low pressure and volume on provocative discography. Postdiscography CT reveals an abnormal discogram with disc disruption associated with a radial tear extending to the outer anulus (see Fig. 26.13A below). Extravasation of contrast dye into the extra-annular space indicating full-thickness annular disruption is not uncommon and is not a contraindication for IDET. Patient selection should not rely solely on the results of the discogram. Discography is just one step in the evaluative process and invariably follows a thorough history and physical exam after failure to improve with rehabilitation measures.

Fig. 26.13 Case history 1. (A) Postdiscogram CT scan of L5–S1 disc showing midline posterior annular tear. (B) Lateral and (C) CT views of intradiscal catheter placement.

In general, IDET should be considered in patients in whom fusion is to be avoided. The procedure fills a notable gap in the treatment of discogenic low back pain between nonoperative care outlined above and spinal fusion. Further clinical studies are needed to better define the exact patient subsets that are responsive to IDET.

PREOPERATIVE PREPARATION

The patient should be instructed to stop all nonsteroidal antiinflammatory drugs (NSAIDs) and aspirin-containing compounds 1 week prior to the procedure. The use of anticoagulant medications must be stopped prior to the performance of IDET. An exception is the prophylactic use of 81 mg of aspirin daily in high-risk cardiac patients where the risk of a cardiac event is considered to be greater than the risk of bleeding. Whether the risk of discontinuation of these agents is worth the potential benefit of the procedure is a decision that requires input from the physician managing the relevant medical problem, the surgeon planning to perform IDET, and the patient. In general, when a decision has been made to proceed, the patient is instructed to stop coumadin, antiplatelet drugs such as clopidogrel, and aspirin 5 days prior and nonselective NSAIDs including COX-2 selective drugs 48 hours prior to the procedure. Low molecular weight heparins such as enoxaparin need to be discontinued 24 hours prior. An INR of 1.1 or less and a platelet count of greater than 50 000 should be documented in the chart on the day of the procedure. Bleeding times for chronic antiplatelet medication use are advisable. Medications that treat hypertension, diabetes, and cardiac disease may be taken on the day of the procedure.

Comorbid medical conditions must be under acceptable control. Acute infections of any type are a contraindication to the procedure. Allergies to iodine or latex should be clearly marked on the chart so that gadolinium¹⁰ and latex free gloves and syringes can be used, respectively.

The patient is allowed liquids but no solid food on the day of the procedure. The use of sedative medications precludes driving until the day following the procedure. Arrangements should be made for a relative or friend to accompany the patient to and from the hospital.