Aetiology

Non-specific lower back pain is, by definition, pain without a clearly defined aetiology. A specific identification of the exact source of pain is often not possible; nevertheless, non-specific lower back pain is a well-recognised clinical entity. Evidence suggests aetiologies include intervertebral discs, vertebrae, facet joints, fascia, ligaments, and/or muscles.[2]​

Pathophysiology

The pathophysiology of non-specific lower back pain can arise from many anatomical 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 disc: consists of a fluid- and proteoglycan-filled nucleus pulposus in the centre surrounded by the outer annulus. Whereas the nucleus pulposus is non-innervated, 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 disc (outer annular tear) in 48% of patients with sudden-onset non-specific lower back pain. When the enhancing disc was injected with local anaesthetic, 73% of these patients experienced pain reduction.[19]

Degenerative disc disease: a long-standing 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 tumour necrosis factor (TNF)-alpha.[22][23]​

Levels of immune-inflammatory and hypothalamic-pituitary-adrenal axis biomarkers may differ in people with non-specific 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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