What causes sciatica? Common triggers and risk factors

The lumbar discs — the shock-absorbing pads between the vertebrae — have a tough outer ring (annulus fibrosus) and a gel-like interior (nucleus pulposus). When the outer ring develops a tear or weakness, the inner material can bulge or herniate outward, pressing directly on the nerve root that forms part of the sciatic nerve. This is the most common structural cause of true sciatica, and it most frequently occurs at the L4-L5 or L5-S1 disc levels.

Disc herniation can happen suddenly (lifting something heavy, a sudden twisting movement) or gradually through accumulated wear and tear. The pain is typically sharp and clearly radiating — following a specific path down the leg corresponding to the affected nerve root.

The piriformis is a small muscle in the deep gluteal region that runs from the sacrum to the outer hip. The sciatic nerve passes directly beneath it — and in some people, through it. When the piriformis is tight, inflamed, or in spasm, it can compress the sciatic nerve, producing symptoms that are often indistinguishable from disc-related sciatica.

Piriformis syndrome is frequently underdiagnosed because it doesn’t show up on MRI (which images spinal structures, not soft tissue in the hip). It is particularly common in people who sit for long periods, runners, and those with hip asymmetry or SI joint dysfunction. It responds very well to targeted treatment because the compression is muscular — and muscles can be released.

Spinal stenosis describes narrowing of the spinal canal — the channel through which the spinal cord and nerve roots travel. This narrowing is usually caused by degenerative changes: bone spurs, thickened ligaments, or arthritic changes at the facet joints. As the canal narrows, the nerve roots become chronically compressed, producing symptoms that are typically more diffuse and bilateral than disc-related sciatica.

Stenosis-related sciatica is often described as aching, heaviness, or cramping in the legs that worsens with walking or standing and relieves with sitting or bending forward — a pattern called neurogenic claudication. It tends to affect older patients and often involves both legs rather than just one.

The sacroiliac joints connect the sacrum (base of the spine) to the iliac bones of the pelvis. When these joints are irritated or hypermobile, they can produce pain that radiates into the buttock and down the leg in a pattern that closely mimics sciatica. This is sometimes called pseudo-sciatica — it follows a similar distribution but has a different origin.

SI joint dysfunction is particularly common following pregnancy, falls onto the tailbone, prolonged asymmetric sitting, or significant changes in physical activity. It often produces pain specifically with rolling over in bed, climbing stairs, or transitioning from sitting to standing.

As lumbar discs age, they naturally lose hydration and height. This narrowing of the disc space reduces the cushioning between vertebrae and can change the mechanical loading on surrounding structures — including the foramina (the openings through which nerve roots exit the spine). As disc height decreases, the foramen narrows, potentially compressing the nerve root.

Degenerative disc changes rarely cause acute severe sciatica on their own — they are more commonly a background vulnerability that makes other triggers more likely to produce nerve symptoms. Many people with significant degenerative changes on imaging have no symptoms; others with modest changes have significant ones. The degree of structural change does not reliably predict symptom severity.