The L3-L4 spinal motion segment is located in the middle of the lumbar spine and consists of the L3 and L4 vertebrae and the anatomical structures connecting them.
This segment provides flexibility to the trunk and low back, supports the upper body, and protects the cauda equina and nerve pathways. The L3 and L4 vertebrae are connected in the back by a pair of facet joints that allow limited forward, backward, and twisting motions.
Articular cartilage enables the facet joints to move smoothly, while muscles, tendons, and ligaments hold the vertebrae together. A strain or tear to any of these tissues can cause back pain and stiffness. Between the vertebrae is an intervertebral disc that provides flexibility and shock absorption.
The disc is made up of a tough outer ring called the annulus fibrosus, which encapsulates a gel like interior called the nucleus pulposus. A thin vertebral endplate is situated between the vertebral body and the intervertebral disc. It is made up of two layers a bony layer which attaches to the vertebral body, and a cartilaginous layer which attaches to the intervertebral disc. The end plate facilitates the transport of nutrients to the discs, and protects the vertebrae and discs from compressive loads on the spine.
The L3-L4 motion segment provides a bony enclosure for the cauda equina nerves that continue down from the spinal cord. The L3 spinal nerve branches out from the spinal cord and exits on each side through the intervertebral foramen. It receives sensory information from the skin on parts of the low back, thigh, and leg. The motor component of the L3 spinal nerve sends signals to various muscles in the pelvis, thigh, and leg.
Spinal conditions affecting the L3 L4 motion segment include spinal stenosis, vertebrogenic pain, disc herniation, facet joint osteoarthritis, degenerative disc disease, and degenerative spondylolisthesis.
The signs and symptoms from these conditions may be localized to the low back, or may radiate to the thigh and leg if the L3 nerve becomes inflamed or compressed. If the cauda equina is compressed at the L3 spinal level, it may cause severe pain, neurological deficits in the legs, and loss of bowel and bladder function.
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