The spinal cord branches out into individual nerves that exit the spine and run out to all parts of the body. The part of the nerve that connects to the spinal cord is called a nerve root. If one of these roots is injured or pinched, pain, weakness, numbness, or tingling may be felt in the part of the body fed by that nerve. Common causes of nerve root injury include a herniated disc, spinal stenosis, or degenerative disc disease. A herniated disc occurs when a spinal disc ruptures and presses against the nerve root [Figure 1]. Spinal stenosis occurs when the bones that form the spinal canal grow inward, pinching the nerve root [Figure 2]. Degenerative disc disease occurs when a spinal disc weakens, allowing vertebral bones above and below the disc to shift out of position. The bones can touch, pinching nearby nerve roots. Bony spurs may form and these may also press against the nerves.
Figure 1: A cross sectional view showing a lumbar disc herniation (Red, solid arrow). The herniation is pushing against the nerve root as it exits the spine (Blue, hollow arrow).
Figure 2: A cross sectional view of the lumbar spine showing a normal amount of space for the spinal canal (Right) compared to a spine with stenosis and a narrowed space for the spinal canal (left).
The nerve roots leaving the lower back innervate the legs. When a lumbar root is injured, pain, weakness, numbness, or tingling may be felt in the buttocks, legs, or feet. This pain is usually called sciatica [Figure 3]. Damage at the L2 level can create thigh pain and hip weakness. Damage at the L3 level can create thigh pain and knee and thigh weakness. Damage at the L4 level can create pain from the lower back to the foot and foot weakness. Damage at the L5 level can create pain from the outer leg to the top of the foot and foot weakness. Damage at the S1 level can create pain from the calf to the outer foot and foot weakness.
Figure 3: Diagram of the human body showing the pathways nerves commonly take after exiting the spine.