The sciatic nerve is made up of five spinal nerve roots – L4, L5, S1, S2 and S3.  When either the sciatic nerve or its nerve roots are compressed or irritated, sciatica can arise causing a sensation of acute leg pain, numbness or even weakness of the leg.

Sciatica is commonly caused by a prolapsed lumbar disc, pressing on the nerve root just prior to it emerging from the lumbar spine. Sometimes there is also an arthritic process of wear and tear that has narrowed the spinal canal – either the main central nerve channel or the exit foramen (hole at the side of the spine), - and sometimes these changes are treated with surgery on the spine to remove the prolapse, (an endoscopic or microdiscectomy, or to widen the canal, (a central or nerve root decompression).

These prolapses and narrowings in addition to pain can give rise to a loss of function of the nerve root causing numbness or weakness affecting the leg.  The most common functions to be affected are ankle movements and sensation around the foot, calf and ankle. 

Pain felt in the central lower back comes from the discs, facet joints and spinal muscles directly. Pain felt in the leg from the knee downwards is definitely coming from the nerve roots being pressed on. Pain felt to one side of the spine, in the buttock or down the leg to above the thigh is in ‘everyman’s land’ and may reflect poor localisation of the spinal pain or limited referral of the nerve pain.