The bones of the spinal column, or vertebrae, run down the back connecting the skull to the pelvis. These bones protect nerves as they exit the brain and travel down the back and then to the entire body. The spinal column is divided into several segments -- the cervical spine (the neck), the thoracic spine (the part of the back behind the chest), the lumbar spine (lower back), and sacral spine (the part connected to the pelvis that does not move). The spinal vertebrae are separated by cartilage disks filled with a gelatinous substance, which provide cushioning to the spinal column. These disks may herniate (move out of place) or rupture from trauma or strain, especially if degenerative changes have occurred in the disk.
Radiculopathy refers to any disease affecting the spinal nerve roots. A herniated disk is one cause (but not the only cause) of radiculopathy (sciatica).
Most herniation takes place in the lumbar area of the spine. Lumbar disk herniation occurs 15 times more often than cervical (neck) disk herniation, and it is one of the most common causes of lower back pain. The cervical disks are affected 8% of the time and the upper-to-mid-back (thoracic) disks only 1 to 2% of the time.
Nerve roots (large nerves that branch out from the spinal cord) may become compressed resulting in neurological symptoms, such as sensory or motor changes.
Disk herniation occurs more frequently in middle aged and older men, especially those involved in strenuous physical activity. Other risk factors include any congenital conditions that affect the size of the lumbar spinal canal.
| 
