Sciatica Pathophysiology and Clinical Implications

Primary Mechanisms

The American College of Radiology suggests that mechanical compression typically occurs at the L4-L5 and L5-S1 levels, where the nerve roots exit the spinal canal and form the sciatic nerve distribution 2
Spinal stenosis, narrowing of the spinal canal from degenerative changes, disc herniations, and malalignment, can also cause nerve root compression leading to sciatica, as reported by the American College of Radiology 3

The American College of Radiology notes that mechanical compression without inflammation often remains asymptomatic, which explains why many patients have disc herniations on MRI but no pain, with 57% of symptomatic patients with low back pain and 65% with radiculopathy showing disc herniation, compared to 20-28% of asymptomatic individuals 2
Epidural pathology, including abscess or hematoma, can compress nerve roots and cause sciatica, according to the American College of Radiology 3
Primary or metastatic tumors in the extradural or intradural extramedullary spaces can encroach upon the spinal canal, as stated by the American College of Radiology 3

Sciatica develops through a dual mechanism requiring both mechanical nerve root compression and chemical inflammation, most commonly from lumbar disc herniation at the L4-L5 or L5-S1 levels, as summarized by Praxis Medical Insights 1