Etiology

Nonspecific lower back pain is, by definition, pain without a clearly defined etiology. A specific identification of the exact source of pain is often not possible; nevertheless, nonspecific lower back pain is a well-recognized clinical entity. Evidence suggests etiologies include intervertebral disks, vertebrae, facet joints, fascia, ligaments, and/or muscles.[2]​

Pathophysiology

The pathophysiology of nonspecific lower back pain can arise from many anatomic structures of the lower back. The basic principle behind our understanding of pain sources is that only innervated spinal structures can be sources of lower back pain.

Intervertebral disk: consists of a fluid- and proteoglycan-filled nucleus pulposus in the center surrounded by the outer annulus. Whereas the nucleus pulposus is noninnervated, the external annulus is innervated by the sinuvertebral nerve and can act as a pain source.

Annular tear: can be an acute event involving inflammatory factors. One study using gadolinium-diethylenetriaminepenta-acetic acid (DTPA)-enhanced T1 weighted magnetic resonance imaging reported enhancement in the posterior annulus of the intervertebral disk (outer annular tear) in 48% of patients with sudden-onset nonspecific lower back pain. When the enhancing disk was injected with local anesthetic, 73% of these patients experienced pain reduction.[19]

Degenerative disk disease: a longstanding process due to mechanical and genetic factors. Degeneration involves inflammation, apoptosis, abnormalities in proteoglycan content and collagen, and vascular ingrowth.[20]

Facet joint- (or zygapophysial-) mediated pain: due to repetitive mechanical strain or trauma. The medial branch of the dorsal primary rami provides innervation.[21] Various inflammatory factors have been identified in this degenerative process, including interleukin (IL)-1B, IL-6, and tumor necrosis factor (TNF)-alpha.[22][23]

Levels of immune-inflammatory and hypothalamic-pituitary-adrenal axis biomarkers may differ in people with nonspecific low back pain compared with healthy controls; however, evidence is limited, and results inconclusive.[24]

Muscular, fascial, and ligamentous sprain/injury can cause lower back pain. These structures are innervated by the dorsal primary rami.

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