Background
As humans age, they endure both macrotraumas and microtraumas and undergo changes in body habitus that alter and redistribute biomechanical forces unevenly on the lumbar spine. Natural progression of degeneration of the lumbar segment with motion proceeds with characteristic anatomic, biomechanical, radiologic, and clinical findings in lumbar degenerative disk disease (LDDD). [1, 2] Magnetic resonance imaging (MRI) is currently the criterion standard imaging modality for detecting disk pathology. Physical rehabilitation with active patient participation is a key approach to treatment of patients with diskogenic pain.
Descriptions of treatment for low back pain (LBP) date to Hippocrates (460-370 BCE), who reported joint manipulation and use of traction. Onset of LBP often is associated with bipedal ambulation. Theories propose that this transformation in the mechanics of locomotion is the inciting evolutionary event that made the lumbar spine susceptible to degenerative disease. Degeneration is universal to structures that make up the functional spinal unit, composed of two adjacent vertebral bodies and the intervertebral disk. The disk and two zygapophyseal joints at the same level function as a trijoint complex.
Functional Anatomy
The spine is composed of a series of spinal functional units, wherein each unit consists of three joints with an anterior and posterior segment. The anterior segment consists of two adjacent vertebral bodies and the intervertebral disk between them. The posterior segment consists of the laminae and their processes. One joint is formed between the two vertebral bodies, while the other two joints are formed by the articulation of the superior articular processes of one vertebra with the inferior articular processes of the vertebra above. The intervertebral disk consists of an inner core of gelatinous material called the nucleus pulposus. The nucleus pulposus is enclosed by a ring called the annulus fibrosus.
The individual lumbar nerve roots exit laterally through the intervertebral foramen, located on each side of the spinal functional unit. Each intervertebral foramen is bound anteriorly by the vertebral column and intervertebral disk. The intervertebral foramen is bound superiorly and inferiorly by a pedicle, while posteriorly it is bound by the vertebral lamina and zygapophyseal joint. The outer one third of the intervertebral disk is an innervated structure, while the remainder of the disk, including the nucleus pulposus, lacks any innervation.
The sinuvertebral nerves are recurrent branches of the ventral rami that reenter the intervertebral foramina to be distributed within the vertebral canal. These nerves are mixed nerves, formed by a somatic root from a ventral ramus and an autonomic root from a gray ramus communicans. The sinuvertebral nerve supplies the posterior margin of the annulus fibrosus, anterior dura mater, dural sleeve, posterior vertebral periosteum, and the posterior longitudinal ligament. The anterior longitudinal ligament and the lateral aspect of the annulus fibrosus are innervated by ventral rami and gray ramus communicans. The posterior rami of the spinal nerves supply zygapophyseal joints above and below the nerve, as well as the paraspinous muscles at multiple levels.
The spinal motions that frequently are encountered in many sports and other activities include axial compression/distraction, flexion and extension, torsion (rotational forces), and lateral flexion. Additionally, certain sports can subject the spine to tensile stress; shear forces in the anterior and posterior position, as well as to compressive forces in the craniocaudal direction. Protection of the functional units of the spine requires unrestricted and efficient motions between adjacent vertebral segments. Simple flexion/extension movements and even moderate axial compression forces are relatively well tolerated by the disk and the associated joint complexes of the spine.
Rotational forces and combined motions, such as forward flexion with rotation, have been shown to be the most injurious to the disk. Therefore, it is crucial that the supporting stabilizers of the spine, both static and dynamic, are sufficiently strong to offset some of these potentially injurious forces.
The static stabilizers of the lumbar spine include the longitudinal ligaments, the intervertebral disks between the vertebral bodies, and the zygapophyseal joint capsules connecting the posterior elements of the spine. The dynamic stabilizers are made up of not only the musculature surrounding the lumbar spine, but also the abdominal and hip muscles, including the hip flexors, extensors, and abductors.
The relationship of the pelvis to the spine is an important consideration when assessing problems in the lumbar spine. Increased lumbar lordosis may result from anterior pelvic tilt, caused by weak abdominals and/or tight hip flexors. Decreased lumbar lordosis may result from posterior pelvic tilt, caused by weak paraspinal extensor muscles and/or tight hamstring muscles. The dynamic stabilizers of the spine, when acting synergistically, can directly or indirectly reduce the shear forces to the intervertebral disk and to the zygapophyseal joints of the spine.
Pathophysiology
Deterioration of the spinal structures is a universal phenomenon with progression of age, occurring in athletic and nonathletic populations. As mentioned, the intervertebral disk is part of a three-joint complex; therefore, damage at the level of either the zygapophyseal joints or the disk affects the function of the entire unit.
Posterior elements of the lumbar spinal functional unit typically bear less weight than anterior elements in all positions. Anterior elements bear over 90% of forces transmitted through the lumbar spine in sitting; during standing, this portion decreases to approximately 80%. As the degenerative process progresses, relative anterior-to-posterior force transmission approaches parity. The spine functions best within a realm of static and dynamic stability. Bony architecture and associated specialized soft tissue structures, especially the intervertebral disk, provide static stability. Dynamic stability, however, is accomplished through a system of muscular and ligamentous supports acting in concert during various functional, occupational, and avocational activities.
The overall mechanical effect of these structures maintains the histologic integrity of the trijoint complex. Net shear and compressive forces must be maintained below respective critical minima to maintain trijoint articulation integrity. Persistent, recurrent, nonmechanical, and/or excessive forces to the motion segment beyond minimal thresholds lead to microtrauma of the disk and facet joints, triggering and continuing the degenerative process. [3] Degenerative cascade, described by Kirkaldy-Willis, is the widely accepted pathophysiologic model describing the degenerative process as it affects the lumbar spine and individual motion segments. [4] This process occurs in three phases that make up a continuum with gradual transition, rather than three clearly definable stages.
Phase I
The dysfunctional phase, or phase I, is characterized histologically by circumferential tears or fissures in the outer annulus. Tears can be accompanied by endplate separation or failure, interrupting blood supply to the disk and impairing nutritional supply and waste removal. Such changes may be the result of repetitive microtrauma. Since the outer one third of the annular wall is innervated, tears or fissures in this area may be painful. Strong experimental evidence suggests that most episodes of LBP are a consequence of disk injury, rather than musculotendinous or ligamentous strain. Circumferential tears may coalesce to form radial tears.
The nucleus pulposus may lose its normal water-imbibing abilities as a result of biochemical changes in aggregating proteoglycans. Studies suggest proteoglycan destruction may result from an imbalance between the matrix metalloproteinase-3 (MMP-3) and tissue inhibitor of metalloproteinase-1 (TIMP-1). [5, 6, 7] This imbalance results in diminished capacity for imbibing water, causing loss of nuclear hydrostatic pressure and resulting in buckling of the annular lamellae. This phenomenon leads to increased focal segmental mobility and shear stress to the annular wall. Delamination and fissuring within the annulus can result. Annular delamination has been shown to occur as a separate and distinct event from annular fissures.
Microfractures of collagen fibrils in the annulus have been demonstrated with electron microscopy. MRI at this stage may reveal desiccation, disk bulging without herniation, or a high-intensity zone (HIZ) in the annulus. Structural alteration of the facet joint following disk degeneration is acknowledged widely, but this expected pathologic alteration does not necessarily follow. Changes associated with zygapophyseal joints during the dysfunctional phase may include synovitis and hypomobility. The facet joint may serve as a pain generator.
Phase II
The unstable phase, or phase II, may result from progressive loss of mechanical integrity of the trijoint complex. Disk-related changes include multiple annular tears (eg, radial, circumferential), internal disk disruption (IDD) and resorption, or loss of disk-space height. Concurrent changes in the zygapophyseal joints include cartilage degeneration, capsular laxity, and subluxation. The biomechanical result of these alterations leads to segmental instability. Clinical syndromes of segmental instability, IDD syndrome, and herniated disk seem to fit in this phase.
Phase III
The third and final phase, stabilization, is characterized by further disk resorption, disk-space narrowing, endplate destruction, disk fibrosis, and osteophyte formation. Diskogenic pain from such disks may have a higher incidence than that of the pain from the disks in phases I and II; however, great variation of phases can be expected in different disks in any given individual, and individuals of similar ages vary greatly.
Epidemiology
Frequency
International
Globally, the prevalence of LBP was reportedly 619 million in 2020, with that figure projected to reach 843 million by 2050. However, for persons with LBP, the worldwide, age-standardized rates of prevalence and years living with disability (YLDs) had, between 1990 and 2020, fallen by 10.4% and 10.5%, respectively, the YLDs for 2020 being 832 per 100,000. [8]
Mortality/Morbidity
The natural history of LBP has been reported to be favorable in some studies and is frequently quoted to patients. Reports indicate that 40-50% of patients are symptom-free within 1 week and that up to 90% of symptoms resolve without medical attention in 6-12 weeks.
Deyo and Tsui-Wu reported that 33.2% of patients with LBP reported symptoms for less than 1 month, 33% reported pain for 1-5 months, and 32.7% reported pain for longer than 6 months. [9] Later, over a 2-year follow-up, 44% of patients reported chronic symptoms (defined as back pain for >90 d in the previous 6 mo). Most patients had low levels of back pain, with 20% rating their pain at 4 or greater on a scale of 0-10 (where 0 indicates no pain), 13% rated their pain as 5 or greater, and 8% reporting pain of 6 or greater.
Von Korff and colleagues reported that 15-20% of primary care patients with LBP had moderate-to-severe limitations in activity during a 1-year follow-up after their initial episode resolved. Recurrence rates of 60-85% have been reported in the first 2 years after an acute episode of LBP. [10]
Sex
LBP secondary to degenerative disk disease affects men and women equally. Gautschi et al found in a cohort of 214 patients with LDDD that preoperatively, females scored worse than males on measurement of subjective functional impairment but that males and females scored similarly in terms of objective functional impairment. The investigators also found that postoperative results did not differ between the sexes at 6-week, 6-month, and 1-year follow-up. [11]
Age
LBP secondary to degenerative disk disease is a condition that affects young to middle-aged persons, with peak incidence at approximately 40 years. With respect to radiologic evidence of LDDD, the prevalence of disk degeneration increases with age, but degenerated disks are not necessarily painful.
Patient Education
An education-based paradigm for the patient with LBP can be inexpensive, beginning with providing reassuring information to patients.
Seeds of the educational approach exist in back schools, functional restorative programs, and innovative prevention and rehabilitation strategies.
LaCroix found that 94% of patients with a good understanding of their condition returned to work, whereas only 33% of patients with a poor understanding of their condition returned to employment.
Reassurance that activity is helpful promotes return to function.
