There are quite a few causes that can lead to onset sciatica.  The most common causes have are spine related disorders.

Below is a list of causes of sciatica.  Even though they are different, they all effectively generate compression on the sciatic nerve, resulting in sciatica.

• Herniated Disc – This is the most common cause.  A herniated disc is when a disc that is separating the vertebrae in the spine breaks or ruptures.  There is a gel like substance inside the disc that acts as cushions between veterbrae.  A broken disc at the lumbar spinal region can result in the gel flowing out and pressing down on the sciatic nerve.
• Lumbar Spinal Stenosis – Spinal stenosis is the narrowing of the spinal canal.  The reduction in space for the spinal cord can essentially pinch the nerves.  In the lumbar region, pinching of the sciatic nerve can cause back and leg pain.
• Piriformis syndrome – The muscle located between the spine and to the top of the thigh bone is known as the piriformis muscle (in the groin muscle area.)  Problems with this muscle can lead to spasms, which can press down on the sciatic nerve.  Piriformis syndrome can occur from sitting down too long or direct injury.
• Spondylolisthesis – Spondylolisthesis is when a vertebra of the spine slips forward and onto the bone of another vertebra.  This displacement can lead to the onset of sciatica by obstructing the sciatic nerve.
• Bone spurs – Osteophytes may start to grow as the body’s response to the degenerating discs of the vertebrae.  These growths may affect the sciatic nerve if located on the lumbar spinal region.
• Spinal tumors – Spinal tumors can grow on the inside of the spinal cord.  As they grow bigger, they will push harder on the spinal cord and the nerve roots
• Sciatic nerve tumors – Tumors can also grow directly on the sciatic nerve.  Neurofibromatosis is the abnormal cell growth on nerve tissue that can cause schwannomas, which is another name for sciatic nerve tumors.  This is a potentially life threatening condition if it is cancer.
• Injury – An injury sustained directly to the lower spine or below the buttocks can cause inflammation or irritation of the sciatic nerve or the tissue around it, compressing the nerve and causing pain.

The only way to know for sure what disorder may be causing sciatic pain is to get a CT or MRI scan from a doctor.  Fortunately, many of the above symptoms can be treated easily with sciatic stretches and exercise.