Causes
Pressure on the sciatic nerve can have many causes, some serious and some more minor. Sciatica may be the result of something as simple as bad posture, muscle strain or spasm, pregnancy, being overweight, wearing high heels, or sleeping on a too-soft mattress. More seriously, it can result from pressure exerted on the sciatic nerve or its nerve roots by a slipped disc in the back. In some cases, sciatica is due to nerve inflammation caused by a form of arthritis, or is caused by the squeezing of the sciatic nerve by a tumor wrapped around the spinal cord in the lower back (the lumbar spine).
The spinal cord is a soft bundle of nerves that starts at the base of the brain and runs down the middle of the back. It's the main pathway for communication between the brain and the rest of the body. The cord is covered by the protective spinal column (the vertebrae). Nerves enter and exit the spinal cord all the way down its length, passing through small openings between each vertebra. Nerve roots also stick out through these openings, exchanging nerve signals with the rest of the body.
The vertebrae are separated from each other by discs made of cartilage, a substance like bone but softer. Each disc has a tough outside layer and a soft inside part that acts as a shock absorber, cushioning the vertebrae when the body moves around. If the disc is damaged by injury or through aging, the inner part can bulge or slip through the outer layer. The "extruded" or "herniated" inner part of the disc may then compress or inflame a nerve root sticking out between two adjacent vertebrae. The disc may even injure the nerve root. If this happens in the lower back area, it leads to pain traveling down the sciatic nerve in the leg.
Degenerative arthritis (osteoarthritis) is another common source of nerve root damage that may cause sciatica. Osteoarthritis is characterized by the formation of bone spurs - bits of protruding bone - that may press on the nerve roots. Sometimes, older people may also develop a condition called spinal stenosis, where the space around the spinal cord narrows and squeezes nerves. More rarely, infections like meningitis or spinal cord tumors can affect the nerves. It's also possible for the sciatic nerve to be compressed further outside the spinal column in the pelvis or buttocks.