Your sciatic nerve runs from your spinal cord to your buttock and hip area and down the back of each leg. This nerve controls many of the muscles in your lower legs and provides feeling to your thighs, legs and feet.
Sciatica frequently occurs when a nerve root is compressed in your lower (lumbar) spine — often as a result of a herniated disk in your lower back. Disks are pads of cartilage that separate the bones (vertebrae) in your spine. They keep your spine flexible and act as shock absorbers to cushion the vertebrae when you move.
But as you grow older, the disks may start to deteriorate, becoming drier, flatter and more brittle. Eventually, the tough, fibrous outer covering of the disk may develop tiny tears, causing the jelly-like substance in the disk's center to seep out (herniation or rupture). The herniated disk may then press on a nerve root, causing pain in your back, leg or both. If the damaged disk is in the middle or lower part of your back, you may also experience numbness, tingling or weakness in your buttock, leg or foot.
Although a herniated disk is a common cause of sciatic nerve pain, other conditions also can put pressure on your sciatic nerve, including:
• Lumbar spinal stenosis. Your spinal cord is a bundle of nerves that extends the length of your spine. It's housed inside a channel (spinal canal) within the vertebrae. Nerves branch off the spinal cord, providing communication between your brain and the rest of your body. In spinal stenosis, one or more areas in the spinal canal narrow, putting pressure on the spinal cord or on the roots of the branching nerves. When the narrowing occurs in the lower spine, the lumbar and sacral nerve roots may be affected.
• Spondylolisthesis. This condition, often the result of degenerative disk disease, occurs when one vertebra slips slightly forward over another vertebra. The displaced bone may pinch the sciatic nerve where it leaves your spine.
• Piriformis syndrome. Running directly above the sciatic nerve, the piriformis muscle starts at your lower spine and connects to each thighbone (femur). Piriformis syndrome occurs when the muscle becomes tight or goes into spasms, putting pressure on the sciatic nerve. The pain may radiate down the back of your thigh but doesn't extend below the knee. Prolonged sitting, car accidents and falls also can contribute to piriformis syndrome.
• Spinal tumors. In the spine, tumors can occur inside the spinal cord, within the membranes (meninges) that cover the spinal cord, or in the space between the spinal cord and the vertebrae. As it grows, a tumor compresses the cord itself or the nerve roots.
• Trauma. A car accident, fall or blow to your spine can injure the lumbar or sacral nerve roots.
• Sciatic nerve tumor or injury. Sometimes, the sciatic nerve itself may be affected by a tumor or injury.
• Other causes. In some cases, your doctor may not be able to find a cause for your sciatica. A number of problems can affect your bones, joints and muscles, all of which could potentially result in sciatic pain.