The lumbar spine contains 5 vertebrae, labeled L1 to L5, which progressively increase in size going down the lower back. The vertebrae are connected with joints at the back to enable bending and twisting movements of the spine. They protect the spinal cord and the cauda equina in the lower back by enclosing these tissues within a bony canal.
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Structure of the Lumbar Vertebra
The lumbar vertebrae are almost similar in structure, with some exceptions. The important parts and their differences are described below.
- L1 and L2 have a smaller height in front compared to the back
- L3 has an equal height in the front and at the back
- L4 and L5 have a taller height in front compared to the back
The vertebral bone is resistant to bending or buckling and contains cavities that allow blood vessels to grow into the vertebral body for nourishment.
A pedicle is a short section of thick and rounded bone that connects the vertebral body to the vertebral arch at the back. The pedicles also help transfer loads from the vertebral body to the vertebral arch.3
The back of the vertebra contains a bony vertebral arch with an open central space. The vertebral arch has the following components3:
- Laminae. The pedicles continue behind the vertebral body to form the laminae, the major portion of the vertebral arch. The laminae decrease in height from L1 to L5.
- Spinous process. At the mid-point of the vertebral arch, a bony protrusion called the spinous process projects backward and downward. This process can be felt while touching the lower back and serves as an attachment for various muscles of the spine.
- Transverse process. On either side of the spinous process, there are bony protrusions called the transverse processes. Similar to the spinous process, these also serve as attachment points for spinal muscles.
- Vertebral foramen. The hollow space inside the vertebral arch is the vertebral foramen. When the vertebrae are stacked, this space forms the spinal canal.
The lumbar spinal canal houses and protects the spinal cord (down to the L2 vertebra) and cauda equina (downward after L2). The vertebrae are connected to each other through facet joints and a network of ligaments.
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Anatomy of the Facet Joints
At the junction of the pedicle and lamina on the right and left sides, bony protrusions project upward and downward, called the superior and inferior articular process. These processes form facet joints (zygapophysial joints) with the adjacent upper and lower vertebrae. The articulating surfaces of these processes are covered with 1 to 2 mm thick hyaline cartilage.3
While the facet joints allow movements between adjacent vertebrae, their main function is to determine the direction of movement and limit excessive movement. The facets also help carry loads, particularly during backward bending and twisting movements of the spine.3
Role of the Intervertebral Foramen
A pair of intervertebral foramina (bony openings) are present between the pedicles of adjacent vertebrae. These foramina provide a passage for the spinal nerve roots as they branch off the spinal cord or cauda equina and exit the vertebral column. Other nerves, lymph vessels, and arteries also pass through these foramina.3
The lumbar vertebrae are subject to a high degree of mechanical loads and may fracture, slip (spondylolisthesis), dislocate, and/or degenerate. These conditions may cause lower back pain, which may radiate to the legs (sciatica) if a spinal nerve is involved.
Read more about Causes of Lower Back Pain