Lumbar spine surgery is done in the lumbar section of the spinal cord. The lumbar spine or low back is the third major region of the spine. Most people have five bones or vertebrae in the lumbar spine, although it is not unusual to have six. Each vertebra is stacked on top of the other and between each vertebra is a gel-like cushion called a disc (inter vertebral disc). The discs help to absorb pressure, distribute stress, and keep the vertebrae from grinding against each other.
The lumbar vertebral canal is roughly triangular in shape and is narrowest in its anteroposterior diameter in the axial plane. The average anteroposterior diameter of the lumbar canal in adults, as determined by anatomic and radio graphic studies, ranges from 15 to 23 mm.4 The canal is bounded anteriorly by the posterior edge of the vertebral body including the posterior longitudinal ligament, which is closely apposed to the posterior vertebral body surface, laterally by the pedicles, posterolaterally by the facet joints and articular capsules, and posteriorly by the lamina and ligaments flava (yellow ligaments). Entrapment of the cauda equina roots, which pass within the dural sac, can occur as a result of progressive hypertrophy of any of the osseocartilaginous and ligamentous elements surrounding the spinal canal. Moreover, the intervertebral disc, which is composed of a gelatinous, centrally located nucleus pulposus and a peripherally located annulus fibrosus, is prone to rupture or herniate posteriorly or posterolaterally as a result of degenerative changes or trauma, producing neural element compromise.
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The vertebral body is a thin ring of dense cortical bone. The vertebral body is shaped like an hourglass, thinner in the center with thicker ends. Outer cortical bone extends above and below the superior and inferior ends of the vertebrae to form rims. The superior and inferior endplates are contained within these rims of bone.
The pedicles are two short rounded processes that extend posteriorly from the lateral margin of the dorsal surface of the vertebral body. They are made of thick cortical bone. The laminae are two flattened plates of bone extending medially from the pedicles to form the posterior wall of the vertebral foramen. The Pars Interarticularis is a special region of the lamina between the superior and inferior articular processes. A fracture or congenital anomaly of the pars may result in a spondylolisthesis.
Intervertebral discs are found between each vertebra. The discs are flat, round structures about a quarter to three quarters of an inch thick with tough outer rings of tissue called the annulus fibrosis that contain a soft, white, jelly-like center called the nucleus pulposus. Flat, circular plates of cartilage connect to the vertebrae above and below each disc. Intervertebral discs separate the vertebrae, but they act as shock absorbers for the spine. They compress when weight is put on them and spring back when the weight is removed.
Intervertebral discs make up about one-third of the length of the spine and constitute the largest organ in the body without its own blood supply. The discs receive their blood supply through movement as they soak up nutrients. The discs expand while at rest allowing them to soak up nutrient rich fluid. When this process is inhibited through repetitive movement, injury or poor posture, the discs become thinner and more prone to injury. This may be a cause of the gradual degeneration of the structure and function of the disc over time.