Spinal Stenosis occurs when the space around the spinal cord narrows. This tends to increase the pressure on the spinal cord and the spinal nerve roots, and will result in pain, numbness and weakness in the legs.
Spinal stenosis is most commonly caused due to arthritis. Arthritis is nothing but the degeneration of any joint in the body. Arthritis results in the degeneration of the intervertebral disc by loss of water content. In the early years, these discs tends to have a lot of water content which tends to dry out and weaken with age. When the discs loose integrity they tend to cause problems with settling or collapse leading to loss of disc space and loss of disc space height.
The picture below shows the normal discs when young ( left ) and the aged discs that have lost integrity and collapsed ( right).
When the spine settles, it usually results in two things – weight gets transferred to the facet joints and the tunnels that the nerves exit through, tends to get smaller.
With the weight being transferred to the facet joints, the experience increased pressure, thereby resulting in degeneration which causes arthritis. The cartilage that tends to protect the joints wears away. When they wear away completely, the bones start rubbing against each other. Sometimes, to makeup for the lost cartilage, the body might end up responding by growing bone spurs to support the vertebrae. Over a period of times, these bone spurs which are nothing but bone overgrowth may narrow down the space that the nerves pass through.
Sometimes, the body might also respond by increasing the size of the ligaments around the joints instead of bonespurs. This also narrows down the space that the nerves pass through. When this narrows down to such an extent that it starts irritating the adjacent nerves, symptoms get extremely painful.