The lumbar spine refers to the lower back, where the spine curves inward toward the abdomen. It starts about five or six inches below the shoulder blades, and connects with the thoracic spine at the top and extends downward to the sacral spine.
"Lumbar" is derived from the Latin word "lumbus," meaning lion, and the lumbar spine earns its name. It is built for both power and flexibility - lifting, twisting and bending.
The lumbar spine has several distinguishing characteristics:
- The lower the vertebra is in the spinal column, the more weight it must bear. The five vertebrae of the lumbar spine (L1-L5) are the biggest unfused vertebrae in the spinal column, enabling them to support the weight of the entire torso.
- The lumbar spine's lowest two spinal segments, L4- L5 and L5-S1, which include the vertebrae and discs, bear the most weight and are therefore the most prone to degradation and injury.
- The lumbar spine meets the sacrum at the lumbosacral joint (L5-S1). This joint allows for considerable rotation, so that the pelvis and hips may swing when walking and running.
- The spinal cord travels from the base of the skull through the spinal column and ends at about T12-L1 - where the thoracic spine meets the lumbar spine. At that point numerous nerve roots from the spinal cord continue down and branch out, forming the "cauda equina," named for its resemblance to a horse's tail. These nerves extend to the lower extremities (buttocks, legs and feet). Because the spinal cord does not run through the lumbar spine, it is quite rare that a lower back problem would result in spinal cord damage or paralysis.
- The lower spine curves slightly inward, toward the abdomen. This inward curve of the spine is called lordosis.
Lumbar Intervertebral Segment
Physicians usually explain a patient's pathology by focusing on one intervertebral segment, or spinal segment. The lumbar spine has 5 intervertebral segments, termed lumbar segment 1 through 5 (e.g. L1, L2, L3, L4 and L5).
Each lumbar spine segment is comprised of:
- Two vertebrae, such as L4-L5, stacked vertically with an intervertebral disc between them. A healthy disc is cushiony, with a lot of water, and has a sponge-like substance. It acts like a shock absorber in the spine, allowing flexibility and providing protection from jarring movements.
- The two adjacent vertebrae are connected in the back of the spine by two small joints called facet joints. The facet joints of the lumbar spine allow movement to bend and twist the low back in all directions.
- There are nerves that branch off from the spinal column at each level of the spine. They pass through small holes in the back of the lower spine. They then connect together to form the sciatic nerve, which travels into the legs down the back of each thigh and into the calves and feet.
Doctors usually talk about a patient’s lumbar disc problem, or nerve or other lower back problem, as the level that includes two vertebrae and the disc between them, such as L3-L4 or L4-L5. If the disc at the very bottom of the spine is affected, that segment is called the lumbosacral joint L5-S1 (the S stands for sacral, which are the segments below the lumbar spine).