Symptomatic disc degeneration is believed to be a common cause of chronic low back pain. Although the exact pathogenesis remains unknown, it is believed that degenerative disc disease is genetically determined, mechanically induced, and biologically mediated, while natural aging also plays a role. Lumbar disc degeneration is more commonly found in the highly loaded lower lumbar spine. It should be emphasized that back pain is not necessarily correlated or associated with morphologic or biomechanical changes in the disc. Further studies are needed to elucidate the mechanism of pain in patients with disc degeneration.
Fortunately, most people with low back pain improve or respond to nonoperative treatments, such as antiinflammatory medications, exercise, and physical therapy. For patients who have severe pain despite prolonged nonoperative treatment, operative intervention may be considered. Options for management of discogenic low back pain include a variety of fusion techniques, including fusion cage devices and, more recently, intradiscal electrothermal therapy (IDET) and disc replacements. The premise behind fusion procedures is that the pain is largely associated with continued motion at the affected disc level, and stabilization of the affected disc or motion segment will provide pain relief. Conversely, the proponents of disc prosthesis theorize that “dynamic stabilization” of the motion segment provides pain relief, while maintaining some motion. Additionally, disc prostheses are thought to decrease the rate of adjacent segment degeneration and provide better long-term outcomes. IDET proponents argue that it is a minimally invasive method of relieving pain by denervating nociceptors in the posterior anulus fibrosus and stabilizing the disc by collagen modulation. At present, all of these concepts remain unsubstantiated theories.
The indications for these invasive procedures include unrelenting pain and disability despite conservative therapy for at least 6–12 months. Patient selection is the key determinant in successful outcome of these procedures. Factors associated with improved outcome include patients not claiming worker’s compensation, those with single-level degenerative disc on magnetic resonance imaging (MRI), positive concordant pain reproduction on discography, and favorable psychosocial factors. Fusion techniques include posterolateral fusion with or without pedicle screw instrumentation, posterior lumbar interbody fusion (PLIF), anterior lumbar interbody fusion (ALIF), combined anterior and posterior fusion, cage devices, and minimally invasive fusion techniques. Among the available fusion procedures, interbody fusion cages through anterior approaches have shown good fusion and clinical results, while avoiding extensive dissection of the paraspinal muscles through the posterior approach. More recent cage devices restore the sagittal balance better by lordotic or tapered designs. However, cage devices are not biomechanically stable in extension and cyclic loading and are associated with a higher pseudarthrosis rate in multiple level cases. Furthermore, PLIF with cages alone increases instability by removal of a significant part of the facet joints, and a higher rate of failures and pseudarthrosis is expected. The accurate diagnosis of pseudarthrosis in cage cases is difficult, mainly because of the interference of titanium with radiographic or computed tomography imaging. Critical patient selection criteria and improved cage designs and materials may help in assessing the fusion process better, while achieving a high fusion rate and good outcome.
Although the use of disc prosthesis allows motion and theoretically prevents adjacent segment degeneration, the long-term stability, endurance, and strength of the prosthesis and clinical outcomes are unknown for the majority of disc prostheses. Significant facet joint osteoarthritis is a contraindication to this procedure; however, it is difficult to identify in its earliest stages. Furthermore, the fate of the facet joints after a prosthetic disc implantation is unknown, and facet joint hypertrophy with accelerated spinal stenosis may be a potential long-term complication of motion preservation. There are numerous types of disc prostheses and designs under study or in development, and well-designed prospective randomized controlled studies are needed before approval and wide application. The long-term benefits and complications may not be known for many years. These statements apply to disc nucleus replacements or any other technologies that replace a part of or the entire disc. The exact role of disc replacement in our treatment armamentarium remains to be determined.
Intradiscal Electrothermal Therapy
Intradiscal electrothermal therapy is a relatively new technique in which a spinal catheter is inserted at the posterior anulus fibrosus to deliver heat that denervates the painful nociceptors and contracts collagen fibers in the anulus fibrosus. Some authors have reported encouraging preliminary results, whereas others have found inferior results and declining outcome measures after 6 months. Few studies have involved concurrent control patients and randomization. Well-designed long-term trials must be performed to better assess the merit of IDET. The mechanism of action is still controversial, and more basic science studies should be performed to elucidate biologic effects of IDET. In fact, IDET may cause undesirable effects to the intervertebral disc cells and matrices over time. The literature on IDET demonstrates responders and nonresponders, and more clinical research may shed light to better define the patient selection criteria.
It should be emphasized that all of the aforementioned procedures for low back pain have unpredictable outcomes; therefore, these procedures should be only considered after failure of conservative therapy of at least 6 months and with the full understanding of patients who are well informed about the potential advantages, disadvantages, and unpredictable outcomes. It is not established in the literature that any of these procedures, including fusion techniques, are superior to natural history or nonoperative treatment. The clinicians’ challenge in treating patients with persistent low back pain is the absence of a test to accurately diagnose discogenic pain and the absence of reliable patient selection criteria favoring a good outcome. Better diagnostic tools for making the diagnosis of discogenic pain will help designing clinical studies with more meaningful outcomes. More research to determine the pathogenesis of disc degeneration and the mechanism or source of low back pain will guide the logical choice of therapeutic strategies or interventions in the future.