This work examined the treatment of discogenic LBP in patients with degenerative disc disease treated with a discectomy and interbody fusion via lateral access. Isolated axial low back pain rapidly resolved after surgery and disability more gradually improved, as would be expected. Radiolographic analysis revealed improvements in segmental lordosis at treated levels and a high rate of solid fusion. Additionally, few complications occurred, as would be expected using a modern minimally invasive approach, and the patients were generally treated successfully through removal of the pathological intervertebral disc and by stabilizing and fusing the level.
This work represents a retrospective study on prospectively collected data in a small case series with midterm followup, so conclusions are limited to the study design drawbacks. The primary reason for a small sample size was the relative infrequency of surgical candidates for lumbar spine fusion surgery without radicular symptoms (only 10% of all cases in this series). This strengthens the results through sample homogeneity, but greatly limited the sample.
Intervertebral disc morphology continuously changes from birth to late stages of the human life [20
]. Disc degeneration is a natural phenomenon, detectable in individuals as early as 11 to 16 years old. By the age of 50, approximately 10% of lumbar intervertebral discs would be classified as degenerated to some extent on MRI and severely degenerated in as many as 60% of 70-year-old discs [21
]. Macroscopical changes during this process have been described [23
]: the nucleus is the first to change and goes from exhibiting fluid-like to solid-like behavior; the annulus suffers a decrease in the number of layers, decrease in radial permeability, defects in the structure, and microfailure; subchondral bone/nucleus junction calcification, exhibition of focal defects and Modic changes culminate to display the ongoing inflammatory process.
Various phenomena are involved in lumbar disc disease. Genetics, trauma, nutrient pathways, cell death, and matrix synthesis can be primary degeneration inductors [24
] and biomechanical matters also greatly contribute to the disease [25
]. Impaired neuromuscular control of the paraspinal and abdominal muscles (muscle hypo- or hyperfunctionality) and external forces (e.g., sustained and repetitive loading) can additionally cause disc damage [25
], to the point where only a narrow safe window remains between hypermobility (wear and tear) and underuse (immobilization).
Although in normal anatomy, intradiscal nerve terminations have a limited distribution (mostly on the posterolateral annulus), disc degeneration has been shown to have a massive ingrowth of nerves fibers [27
]. These growths seem to penetrate from outside to inside the annulus, along the edges of annular fissures, dependent of the inflammation process and dependent upon specific markers like substance P and receptor to CGRP-ir nerve growth factor [29
]. Nociceptive information is transmitted primarily by small neurons associated with inflammatory pain and some specific proinflammatory mediators (NGF; PGE2, IL-1, IL-6; IL-8) [29
]. And importantly, these networks tend to resultantly function under peripheral and central sensitization [9
One of the most challenging factors of discogenic low back pain is an accurate differential diagnosis. Morphological and functional statuses of apophyseal joints, ligaments and musculature and spine biomechanics must be analyzed [9
]. Additionally, external forces and postural behavior also interfere in symptoms onset [25
]. Psychosocial factors such as depression, anxiety, and worker's compensation act an positive feedback in pain modulation and may be a drawback in diagnosis and treatment [11
Classically discs are innervated segmentally and discogenic pain pathways flow through the sinuvertebral nerve into the corresponding dorsal root ganglion and into the spinal cord, generating symptoms located at the index level [29
]. More recently, an alternative pathway through the grey ramus communications has been described [41
]. The signal travels into the upper lumbar dorsal root ganglion (especially at the L2 level), when a L4-5 disc pathology may generate signals in an L2 dermatome, like a groin and anterior tight pain during a L4-5 provocative discography procedure [41
Identification of signs and symptoms of discogenic back pain includes continuous axial low back pain persistent in extended period deep in the central line of the spine, usually with no irradiation (few times with diffuse or inguinal irradiation), possible relief when lying, no significant worsening with movements, and worsened with axial load and long standing or sitting periods. In radiolographic analysis, low signal intensity of the disc on sagittal T2W, high-intensity zones, annular damages, and especially Modic changes corroborate clinical findings [9
Provocative discography is one of the possible tests to contribute in the diagnosis of discogenic pain, but a few studies have shown equivocal results for discography [44
] and the procedure can also accelerate progression of degeneration changes in the lumbar disc [47
]. False-positive rates were once reported to reach up to 40% [46
], and the presence of many confounding factors can limit its potential: speed and pressure control; low/high pressure provocation; quiescent phase of the illness; somatization disorder; regular medications; abnormal psychometric scores; worker's compensation.
When a degenerated intervertebral disc is determined to be the primary pain generator, surgical removal must be considered. Nucleus replacement was one attempt to treat discogenic pain and maintain movement and function, but the ideal indication window is too narrow and several unwanted complications have occurred [48
]. Lumbar fusion has been widely used for different pathological conditions resulting from idiopathic changes, degeneration, trauma, infection, or neoplasia. As reviewed elsewhere [53
], lumbar fusion has more high-quality studies testifying favorable comparative outcomes [54
] than with nonoperative care [57
For a painful disc, discectomy and interbody fusion intend to remove the pathologic tissue, which presents itself as nonfunctional fibrotic structure, soaked with inflammatory mediators and nerve ingrowth, and to fuse the segment. Additionally, index motion is related to pain occurrence and can be treated with lumbar level stabilization, and the addition of interbody fusion has show the favorable results in lumbar fusion [56
], especially for discogenic pain.
Lateral interbody fusion has been shown to significantly increase foramen and disc height [19
], impact sagittal [60
] and coronal plane reconstruction [15
], and provide indirect decompression and relief of low back and irradiated symptoms [65
]. With true 90° lateral access, satisfactory results have also been shown in thoracic access for the treatment of tumor [67
], trauma [68
], spondylolisthesis [61
], and disc herniation [69
]. Moreover, artificial discs placed laterally have been an advance in lumbar arthroplasty due to anterior and posterior longitudinal ligament preservation [70
If the affected lumbar level does not present with gross instability, a stand-alone interbody construction may be considered. In this instance, posterior muscle damage is prevented as well as posterior instrumentation complications. Biomechanical studies [71
] have shown lateral interbody implants provide the largest reduction in range of motion in a stand-alone construct, with this stability increasing when moving from 18
mm cages (anteroposterior dimension), to wider ones (22 and 26
Payment and reimbursement for lumbar fusion, especially for degenerative disc disease, are being rigorously reviewed by North American and worldwide institutions with the premise that it is ineffective. In this study, however, at 2 years postoperatively over 70% improvement in VAS and patient outcomes was demonstrated, much higher than previous studies on treatment for degenerative spine condition [55
]. This study, while somewhat limited, has shown that, in carefully selected patients, MIS lumbar fusion can be effective in treating isolated axial discogenic low back pain. The spine community must continue to debate the benefits and drawbacks of lumbar fusion for degenerative disc disease.