Chronic low back pain is a problem of immeasurable medical and economic magnitude.1,2 A common cause of chronic low back pain is degenerative lumbar disc disease. The term internal disc disruption (IDD) was coined by Crock to identify the syndrome of low back pain and nonradicular referred pain in the setting of degenerative disc disease.3 IDD has been described as a distinct clinical entity to be distinguished from other painful processes such as degenerative disc disease and segmental instability.4 Its treatment has traditionally been limited to either conservative management or surgical fusion. This is a very limited treatment continuum that must be approached judiciously. Significant variability in outcomes after arthrodesis have been reported.5 This is not surprising considering the inherent variability of information obtained from primarily subjective, pain-based diagnostics. Given the prevalence of this problem and the limited treatment options, the development of alternative treatment methods seems to be a logical progression. Other than biomechanical solutions such as fusion, there exist the general concept categories of biochemistry and biophysics. The field of biochemistry has been rapidly evolving with recent developments in disc regeneration. Biophysics has witnessed an emergence of various intradiscal methods to address discogenic back pain (mostly thermal), including intradiscal electrothermal therapy (IDET), radiofrequency ablation (RFA), cryotherapy, and percutaneous endoscopic laser discectomy (PELD).6–13
Pathophysiology of Discogenic Pain
Discogenic back pain may be attributable to any cause that offends the sensory nerve endings of the disc. Pain in IDD is thought to be caused by mechanical and chemical mediation of nociceptors within the anulus.14,15 In IDD, there is no associated instability or herniation/prolapse of the disc material. The hallmark radiographic changes of degenerative disc disease, such as disc space narrowing, osteophyte formation, endplate sclerosis, and gas formation within the disc space, are not present in IDD.
The innervation of the anulus fibrosis and the intervertebral disc (IVD) has been the subject of much research.16,17 Recent histologic techniques have clarified the innervation more specifically. Surrounding the IVD is a plexus of interlacing nerve fibers.18 Contributions to this network occur ventrally from a plexus surrounding the anterior longitudinal ligament and dorsally from a plexus surrounding the posterior longitudinal ligament. The dorsal nerve plexus is supplied by the recurrent sinuvertebral nerve (nerve of Luschka). Origin of the sinuvertebral nerve has been debated in the literature, and contributions can be made from the rami communicantes, dorsal root ganglion, sympathetic trunk, or the spinal nerve. The sinuvertebral nerve then courses medially to enter the spinal canal where it divides into two main branches. A shorter branch travels caudally to innervate the anulus of the level immediately below. A longer branch travels cranially along the posterior longitudinal ligament and innervates the anulus of the level above.12,15 In the normal IVD, nerve endings are confined to the outer third of the anulus fibrosis. According to Groen et al, branches of the sinuvertebral nerve can ascend or descend more than one segment, thus providing anatomic rationale for the poor localization of a painful disc.19 The superficial layers of the anulus are richly innervated with small nerve fibers that correspond with the A-delta and C fibers, which have been implicated in the generation of pain originating from the IVD.20–22
The diagnosis of IDD is rarely apparent based on routine clinical evaluation. History reveals chronic complaints of lumbar pain and occasional buttock pain without significant radiculopathy. Magnetic resonance imaging (MRI) is a valuable diagnostic tool in assessing the potential presence of IDD.23 If an MRI is completely normal, IDD is typically not the source of pain. MRI allows determination of the proton density of the disc and the presence of anular tears. Radial fissures are thought to correlate strongly with the production of pain on discography. Internal tears of the anulus can also be visualized on MRI. Aprill and Bogduk described the high intensity zone, which they believe to be a highly specific finding for IDD.23 The high intensity zone can be seen on spin echo T2-weighted images as a high intensity signal located in the substance of the posterior anulus fibrosis, clearly distinguished from the nucleus pulposus, and surrounded on all sides by the low intensity signal of the anulus fibrosis.
Discography with or without computed tomography remains the most important, albeit most controversial, tool in the diagnosis of IDD.4,24,25 The discogram provides critical information consisting of four components: the morphology of the disc being injected, the disc pressure and volume of fluid accepted by the disc, the subjective pain response, and the pain response at the adjacent disc levels. Of these, the subjective pain response is the most important. Subjective severity and similarity to the patient’s chronic symptoms on injection are essential to a positive test. The use of CT after discography has been reported to increase the ability to diagnose radial tears of the anulus. However, discography appears to have a very low specificity and sensitivity for making a diagnosis of IDD.24,25
Intradiscal Treatment Methods
The application of thermal energy to affect structural change has been studied in peripheral joints. Hayashi et al investigated the effect of nonablative laser energy on joint capsular properties.26,27 In histologic studies, the authors noted a shrinkage of collagen fibers. In a companion study, these authors noted that granulation tissue was cauterized as well. On the basis of these biologic changes, an intradiscal catheter was developed.28,29
Using fluoroscopic guidance, a flexible intradiscal catheter with a temperature-controlled thermal resistive coil is passed through the trocar into the nucleus.7,30,31 The electrode is then inserted through the nucleus until it penetrates the inner layers of the anulus (Figure 1). The outer layers of the anulus deflect the electrode, guiding it in a circumferential course toward the affected side. The electrode is advanced while in the substance of the anulus, approximately 5 mm from the outer lamellas. At its final position, the electrodes traverse the posterior midline of the anulus and fully involve the affected side. After positioning, the thermal resistive coil is heated to 90 C per protocol provided by the manufacturer.7,30,31 At this stage, the patient’s pain may be reproduced and analgesia may be required. After appropriate heating, the catheter is removed. Patients are encouraged to walk and perform light stretching. Activities such as bending, lifting, and prolonged sitting are restricted for 8 to 12 weeks. Athletic activities and recreational sports are delayed until the third or fourth month after surgery.32
Biologic Effects of IDET
The biologic effects of IDET on the IVD are not well understood. To destroy nociceptors in the anulus, temperatures must be raised to a minimum of 42 C to 45 C. Radiofrequency and resistive heat coils have been used for this purpose.33,34 Collagen modulation and shrinkage of the disc with potential stabilization have been proposed as a possible mechanism of action of IDET.
Histologic studies of IVD material after IDET have been performed to determine the extent of collagen denaturation. Shah et al found IDET-induced histologic changes of collagen fibril denaturation in the posterior anulus fibrosis.35 In addition, the authors observed an impressive local effect of IDET on examination of the treated anulus fibrosis in cadavers. In a similar yet conflicting study, Kleinstueck et al concluded that the effects of IDET do no significantly alter collagen architecture and that temperatures necessary to achieve collagen denaturation occur only at the site of the catheter itself.36
Spinal stability has also been measured after IDET. In a study of five cadaveric specimens with the posterior elements intact, Lee et al found an overall decrease in motion of the spinal segments in all planes of testing with no significant difference in spinal stability.37 Kleinstueck et al tested spinal stability with IDET after removing the posterior bony structures and found a decrease in stiffness of 6% to 12% depending on the plane of motion.38 Both studies reflect the initial effect of heat on collagen, which is to break the collagen bonds. The long-term effect of collagen shortening and restructuring still remains unstudied.
There is a paucity of well-controlled studies in the literature regarding the efficacy of IDET. Most reports represent short-term follow-up of patients. Saal and Saal (developers of the technology) reported early results of 58 patients with a minimum of 2-year follow-up.28,29,39 These patients were selected from an intake cohort of 1,116 patients with low back pain of greater than 3 months duration. The authors reported statistically significant improvements in VAS measurements and Short Form 36 (SF-36) scores. Using the criterion of a “successful” outcome to be a minimum 7-point change between pretreatment and post-treatment SF-36 scores, the IDET procedure achieved a 71% success rate.
Wetzel et al reported the preliminary results of a multicenter prospective controlled cohort study.40 This study of 75 patients included those with degenerative lumbar disc disease on MRI and concordant provocative discography. Patients with back pain for at least 3 months who failed to respond after 6 weeks of conservative care and with a normal neurologic examination were selected. They reported 14.7% failure of treatment, with the majority of the patients requiring a subsequent arthrodesis procedure. Mean change in VAS score from 6.0 to 3.6 (P = 0.001) was observed at 12 months. At the 1-year follow-up evaluation, 56.2% of patients rated their health as “somewhat better.”
Pauza et al reported their 6-month results of a randomized prospective double-blind study of 55 patients undergoing IDET.34 The authors reported a statistically significant improvement in the IDET group versus the placebo control. Pain was worse for 6% of the treatment group and 30% of the placebo group. Bodily pain scale demonstrated a mean improvement of 61.1% in the treatment group and 29.8% in the control group.
In a review of the published effects of IDET, it becomes obvious that there is no clear consensus regarding the effects on neuronal deafferentation, collagen modulation, or spinal stability.40,41 Research thus far has been limited because of the few investigators and cadaveric specimens. Until further evidence of the biologic effects of IDET in vivo is established, the usefulness of the procedure can only be based on available clinical results.
Radiofrequency electrodes have been used in a number of applications where precision heat denervation is required.6 The spread of heat from radiofrequency electrodes is well understood, and various electrode designs over the years have incrementally improved the accuracy of these devices.42 The first attempt at direct radiofrequency heating of intervertebral discs was by Sluijter and subsequently reported by van Kleef et al43,44 Radiofrequency needle electrodes were placed in the nucleus and improvement was reported in approximately 50% of patients. However, a recent trial of nuclear heating at 70 C has shown no beneficial effect. Improved outcomes have been claimed by multiple placements of electrodes within the vicinity of the posterolateral anulus.7
Two varying types of thermoresistive catheters for RFA have been used in the IVD. A flexible, seminavigable, thermoresistive catheter has been introduced, which is placed at the nuclear-anular interface (SpineCATH; Oratec, Menlo Park, CA) via a circuitous approach.39 It is introduced from a posterolateral direction in the same manner as the insertion of a discogram needle. The catheter is then navigated between the anulus and the nucleus from the anterior portion of the disc to the posterior. In a recent placebo-controlled trial, 6-month outcomes were improved for the group treated with this device.34
A novel flexible radiofrequency electrode has been developed (discTRODE, Radionics, Boulder, CO).45 Unlike the SpineCATH, the discTRODE can be directly placed in the posterior and posterolateral midannulus. The electrode is inserted on the contralateral side to the anular tear and is navigated directly across the posterior anulus between the anular lamellas. The depth of insertion is controlled by use of electrical impedance values and radiologic positioning. No data have been presented to date on efficacy and adverse events.
Oh and Shim described good clinical outcomes using RF thermocoagulation of the ramus communicans nerve in patients suffering from chronic discogenic low back pain.12 Forty-nine patients were observed at 4 months after surgery. The patients reported VAS pain scores that were significantly lower by 3.3 points and a mean increase of 11.3 points on the SF-36 bodily pain subscale. The authors concluded that RF neurotomy of the ramus communicans nerve is a safe, uncomplicated procedure for the treatment of intractable chronic discogenic pain.
Most recently, Finch et al reported on a group of 31 patients with chronic discogenic low back pain treated with RF ablation of the anulus at 1-year follow-up.45 The authors noted that patients had a sustained, significant reduction of the VAS and the ODI (Oswestry Disability Index) at 1 year after the intervention. This series was the first outcome data published on the use of RF electrodes placed directly across the posterior anulus and between the lamellas. The small numbers in the trial, the short-term follow-up, and the absence of a sham procedure are all inherent weakness in the study.
PELD aims to reduce the size of the prolapsed disc by ablating the nucleus pulposus with laser energy (Figure 2). The use of the Nd:YAG laser as an alternative to conventional surgical methods for IVD herniation was first described in 1985.46 The early clinical results suggested that this was an effective method of relieving pain. Like other intradiscal therapies, the main goal of PELD is to reduce intradiscal pressure in an attempt to release the compression of the nerve root by the herniated disc.
Choi et al evaluated the thermal, mechanical, and morphologic changes in bovine nucleus pulposus subjected to Nd:YAG laser irradiation.47 The authors noted that photothermal heating resulted in an irreversible matrix alteration, causing shape change and volume reduction. With cessation of the laser irradiation, a sustained increase in tissue tension was observed, which was consistent with changes in specimen length and volume. Higher laser power resulted in a faster heating rate and subsequently an accelerated tension change.
A probe is inserted into the disc through an incision in the patient’s back. Visualization within the disc space is achieved by the use of an endoscope. For the percutaneous endoscopic laser decompression procedure, the laser most commonly coupled with the endoscope is the holmium:yttrium-aluminum-garnet (Ho:YAG) laser. Occasionally, a neodymium (Nd):YAG laser is used, and the procedure has been performed using a 1,064-nm Nd:YAG laser with at 30o or 70o rigid endoscope. Techniques vary according to whether mechanical instrumentation is used together or whether laser ablation is the only method of treatment.
Tonami et al reported on the success of MRI in evaluating intervertebral discs after PELD.48 The authors demonstrated that MRI is not a predictor of surgical outcome. For the 26 patients in the study, 3 required open surgery, a rate of 11.5%. The case series described by Mayer et al differs technically in that a combination of laser ablation and mechanical shavers were used to remove disc material with all patients reporting resolution of their radiculopathy.49
Choy et al described the largest series of PELD incorporating a 17-year experience.50 2,400 PELD procedures were performed in 1,275 patients. The overall success rate according to the MacNab criteria was 89%. The only complication noted was discitis at a rate of 0.4%. The recurrence rate was 5% with the vast majority of reherniations being associated with traumatic injury.49–51
McMillan et al also reported on the efficacy of PELD in patients with discogenic lumbar pain and sciatica with an early follow-up of 3 months; the authors noted that of the 30 patients treated, 24 patients (80%) reported an improvement in their preoperative symptoms at 3 months.52 Using the American Academy of Orthopedic Surgery’s symptom scoring scale, an average pain improvement of 44% was noted in all patients at the 3-month follow-up period.52
The quality of information about PELD is limited to small case series with relatively short follow-up periods. At present, the associated use of an endoscope and laser forms a small percentage of the total number of laser procedures on the IVD conducted in the United States. Although satisfactory clinical outcomes have been demonstrated with PELD, it requires a highly experienced endoscopic surgeon. The learning curve is relatively steep, and the clinical outcomes can be affected by the surgeon’s technique. If the herniated disc is overtly calcified or combined with severe spinal stenosis, the effect of an endoscopic removal can be limited. As such, the use of lasers in the treatment of discogenic back should be limited to select cases until other clinical studies demonstrate a significant improvement in patient outcomes.
Treatment with cooling energy has paralleled thermal and radiofrequency uses in other organ systems. Cryoablation has been used in pain management, rehabilitation, and for posterior spinal pain syndromes emanating from the facet joint or sinuvertebral nerve region.11,53 The successful use of cryosurgery to ablate vertebral body spine lesions has also been reported.54 When cryotherapy is compared with intradiscal electrothermal and radiofrequency anuloplasty, a greater range of cooling energy is available. This suggests the potential for reversible tissue damage (as seen with cryoablation in other organ systems) by application of cooling energy for the acute treatment of discogenic back pain.
Histology of Cryoablation.
In an unpublished in vivo analysis by Carl et al, eight castrated, male, 55-kg Alpine-Nubian crossbred goats (Capra hircus) were obtained and randomly assigned to differing cryotherapies (−20 C, −60 C, or −120 C cryoenergy) at either the T13–L1 or L1–L2 disc space.55 Animals were randomly killed either 2 days following surgery or at 2, 12, or 24 weeks after surgery. Gross inspection of the treated motion segments at necropsy indicated granular tissue superficial to where the probe entered the disc space, the quantity of which was proportional to decreasing probe temperature. MRI images (and gross inspection) of the excised segments showed few appreciable structural changes in the treated discs compared with controls but did indicate sclerotic subchondral bone proportional to decreasing probe temperature (Figure 3). Histology indicated no cellular changes to the spinal cord or to the intervertebral disc but consistently showed (endplate changes and) patchy necrosis of marrow cellular elements and sclerotic subchondral bone (Figure 4).
Previous reports have indicated that the degree of tissue damage from cryoablation is dependent on temperature.11,56 Carl et al assessed three different temperature ranges for their effect on the intervertebral disc and surrounding tissues.55 Results from the current study indicate that some degree of tissue necrosis resulted from application of all three temperatures. However, the necrotic tissue was limited to the cellular, more vascular marrow elements in the subchondral bone. There was no evidence of change in collagen makeup or structural properties and no evidence of damage to the spinal cord or exiting nerves.
It has been reported that acute damage caused at −20 C ultimately may not result in tissue necrosis, whereas temperatures below −60 C are likely to do so.56 Nonpublished reports of partial cell death and metaplastic growth have been described recently for cardiac muscle and vascular recannulation technology.
There are a multiplicity of factors that determine the potential effect of cryotherapy on treated tissues, including temperature, exposure time, probe size, and tissue vascularity. Preliminary data from Carl et al suggest that these changes are morphologic, affecting vascular and cellular structures, rather than structural changes affecting collagen-based tissue as seen with IDET.55 In other applications, morphologic changes have yielded therapeutic benefits. Further research is necessary to elucidate the potential therapeutic effects of cryotherapy on discogenic back pain.
Low back pain is an extremely common and potentially debilitating problem.4 The IVD is a likely contributor to symptomatology. Internal disc disruption and discogenic back pain must be considered and properly investigated in the appropriate setting. On the basis of the literature to date, it might be concluded that intradiscal lesioning is promising. The attractiveness of a minimally invasive option for treating discogenic lumbar pain, the apparent technical ease of the procedure, and the low complication rate must be balanced against economic and medical reality. Adding biophysical methods to well-tested biomechanical and newly investigated biomolecular solutions allows for multiple avenues of therapeutic interventions. With future clinical and basic science studies regarding intradiscal therapies forthcoming, we may soon alter our current treatment algorithms for the management of discogenic back pain.
- Biophysical modalities such as heat and cooling energy can be applied safely to the intervertebral disc without affecting the spinal cord or nerve tissues.
- IDET, RFA, and PELD are thought to affect the structural condition of the disc.
- Cryotherapy results were morphologic, affecting cellular and vascular structures rather than structural changes affecting collagen-based tissue.
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