The spinal degeneration associated with acquired stenosis usually results from aging. Most patients are men and women age 50 or older. As people age, the ligaments of their spine thicken and calcify. This loss of elasticity may shorten the spine, narrowing the spinal canal and compressing the nerve roots.

Aging also contributes to deterioration of the intervertebral discs, which separate vertebrae and act as “shock absorbers.” Wear and tear on the discs, such as from degenerative disc disease, can cause one lumbar vertebra to slip over another, a condition known as spondylolisthesis. This compresses the spinal nerve, leading to numbness, tingling and weakness in the legs when a person walks or stands for long periods of time.

As the intervertebral discs become flatter and more brittle with age, the interior jelly-like substance may poke through the tough, fibrous coating, a condition known as a herniated disc. This may put pressure on the nerves or spinal cord, resulting in spinal stenosis, which may cause excruciating pain.

In addition, aging can cause bones and joints to become larger, and a form of arthritis known as osteoarthritis may leave the body even more vulnerable to spinal stenosis. Osteoarthritis affects the cartilage that cushions the ends of the bones in the joints. Over time, the cartilage deteriorates and the smooth surface becomes rough. If bone rubs against bone, pain results.

 Other sources of narrowing of the spine include:

• Spinal tumor. A mass of abnormal cells can occur inside the spinal cord, within the membranes (meninges) that cover the spinal cord or in the space between the spinal cord and vertebrae. In addition, cancer can spread (metastasize) to the spine from other parts of the body, such as the prostate gland or breasts. These tumors may compress the spinal cord or nerve roots, causing severe back pain that may extend down through the hips, legs or feet. Some patients may also experience muscle weakness and loss of sensation, difficulty walking or paralysis, and loss of bladder or bowel function.
  
  
• Injury. Car accidents or other trauma can dislocate the spine or spinal canal, pressuring the spinal cord and the lower motor neurons. In some cases the spinal cord is partly or completely severed. Spinal fractures can cause fragments of bone to penetrate the spinal canal. Swelling of tissue after back surgery can also put pressure on the spinal cord or nerves.

• Paget’s disease. Although bone seems tough and inanimate, it is a living tissue in which old bone is constantly removed and replaced with new bone. Patients with Paget’s disease have bone that is generated at a faster-than-normal rate. This can create abnormally large bones in the spine that may compress the spinal cord or nerve roots, resulting in excruciating pain that may radiate down into the legs. Some patients also experience numbness, tingling or weakness in the legs, or double vision.
  
  
• Scoliosis. An abnormal sideways curvature of the spine, scoliosis can cause arthritis and structural changes such as stenosis.

• Achondroplasia. This genetic disorder slows the rate of bone formation during fetal development and in early childhood. Patients with this condition have a shorter stature, including a smaller spinal canal that pressures the spinal cord. This can cause severe back and leg pain and paralysis of the legs. Babies and children with achondroplasia may die suddenly from compression of the upper end of the spinal cord that interferes with breathing.
  
  
• Ossification of the posterior longitudinal ligament. This condition occurs when calcium deposits turn fibrous tissue into bone on the ligament running up and down the spine inside the spinal canal. These deposits may cause pain if they press on the nerves of the spinal canal. 
  
  
• Skeletal fluorosis. This disease is caused by high levels of fluoride in the body, which can cause immobilization of the spine and joints. This makes any kind of movement painful. Skeletal fluorosis usually results from long-term inhalation of industrial dusts or gases contaminated with fluorides, long-term consumption of water containing large amounts of fluoride and accidental ingestion of insecticides that contain fluoride. It is rare in the United States.