Within the lower back or the lumbar region, there are five individual cylindrical bones. These vertebrae, labeled L1 to L5, enable many of the bending and twisting movements of the spine and carry almost all of the upper body’s weight. Like all vertebrae, they also protect the spinal cord and act as attachment points for muscles.
Each lumbar vertebrae has a vertebral body and a vertebral arch . The body is large and wider from side to side than from front to back. L1 and L2 have a smaller height in the front than the back, while L4 and L5 are taller in the back than in the front. L3 is roughly equal height on both sides.
A vertebra’s arch consists of a pair of pedicles and laminae and supports several processes. A pedicle is a small section of round, thick bone that connects the arch to the body. Pedicles also help shift loads from the vertebral body to the arch. In the lumbar region, the pedicles change shape from the upper vertebrae to the lower ones, becoming shorter and broader.
Both pedicles extend behind the body and become the laminae, which form the lower-rear section of the arch. Laminae are short, broad, and strong. Like the pedicles, the laminae change from the upper lumbar region to the lower. They begin taller than they are wide and become wider and shorter in the lower vertebrae.
Seven processes — knob-like projections — emerge from the arch at various places and serve as attachment points for muscles. These include the
Upward-pointing articular processes are superior articular processes, while downward-pointing ones are inferior. Along with the adjacent vertebrae, these processes form the facet joints that allow movement between vertebrae, though their main purpose is to maintain vertebral alignment and control the range of motion. Facet joints also support loads, particularly during movements like bending backward or twisting.
Each vertebra has a vertebral foramen, a triangular hole within the arch. Together, the vertebral foramen house the spinal cavity that contains the spinal cord and its meninges. Adjacent vertebrae have similar openings — intervertebral foramina — that create a passage for the spinal nerve roots. Other nerves, arteries, and lymph vessels also travel through an intervertebral foramen in some locations. Each intervertebral foramen decreases in size from L1 to L5. They also become smaller when a person bends backward and larger when they bend forward.
It is extremely difficult to measure a single vertebrae’s range of motion. This is partially because of variations specific to each person, but age and sex also appear to play a role. Additionally, the lumbar vertebrae must be able to transmit forces and backward movements to the sacrum. This requires complex interactions between the muscles, spine, and various neural components.
Because the lumbar region must support such a heavy load and provide for an impressive range of motion, it is also prone to many injuries and conditions. Issues like spondylolysis and spondylolisthesis affect L5 more than any other vertebra. Disk herniation is common, and age can dramatically increase the risk. Lumbarization is when the first segment of the sacrum is too mobile; it may be classified as an extra lumbar vertebrae, L6.
The first lumbar vertebra is level with the transpyloric plane. Many important structures lay in this plane, including the pylorus of the stomach, the ninth rib, the termination of the spinal cord, renal vessels, the fundus of the gallbladder, the hila of the kidneys, and many more. Understanding this landmark helps visualize key structures within the body.
Unlike the early lumbar vertebra, L5 is much shorter in the front than the rear. It features a smaller spinous process, wider spaces between the inferior articular processes, and a thicker transverse processes. While L5 is usually the last lumbar vertebra, some people have one extra or one fewer. Lumbar disorders that normally affect L5 will typically affect L4 or L6 in these individuals.
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