The lumbar spine is a unique bony and ligamentous structure. It can withstand excessive loads while simultaneously protecting neurologic function and providing flexibility and stability.
This intricate interplay between the different anatomic components of the lumbar spine provides efficient motion and function that enables us so much mobility.
The five vertebral bodies and intervertebral disks of the lumbar spine withstand significant physiologic loads. The intervertebral segment of the lumbar spine consists of a three-articulation complex, the disk-vertebral body and two posterior apophyseal (facet) joints, to resist these high loads and stresses.
The vertebral bodies are a cylindrical mass of cancellous bone with a cortical shell. In between are the discs consisting of the anulus fibrosus, the nucleus pulposus, and the cartilaginous and bony end plates of the vertebral bodies.
The vertebral bodies and discs from the anterior column of the spine, which is responsible for resisting approximately 80% of axial compressive loads and maintaining spinal rigidity and alignment.
The intervertebral disc maintains separation of the vertebral bodies.
The anterior part of the disk is thicker than the posterior part and is responsible for most of the lumbar lordosis because the vertebral bodies are near uniform inn shape.
Nearly two thirds of lumbar lordosis is localized between the L4 and S1 segments.
The posterior column of the spine consists of
- Spinous processes
- Lamina
- Transverse processes
- Facet joints
These structures control movement and resist forces.
The facet joints have hyaline cartilaginous surfaces articulations formed by the superior and inferior articular processes of subjacent vertebrae with a fibrous joint capsule.
The superior articular process faces posterior and medially, and the inferior articular process faces lateral and anteriorly.
The joint surface is oblique to the sagittal plane and facilitates slight flexion and extension or sagittal plane rotation as each articular process slides on the other.
The alignment of the facet joints relative to the sagittal plane ranges from 120 degree to 150 degree. This orientation resists anterior or posterior translation in normal anatomic alignment and axial rotation in the horizontal plane.
This complex bony anatomy serves as a load-bearing structure, a passive restraint to torsional stain and excessive tensioning of anulus fibrosus, and as a means of protection against disk injury.
The anterior column also has ligamentous structures and muscles.
The functional ligaments of the lumbar spine are
- Anterior and posterior longitudinal ligaments,
- Interspinous ligaments
- Ligamentum flavum
The iliolumbar ligaments attach to the ilium and L5 vertebra transverse processes act to limit anterior translation and rotation making the lumbosacral junction a very stable articulation.
The parapspinal muscles in the lumbar spine originate and insert on the spinous, transverse, and mammillary process near the facet joints. They serve as an active restraint to flexion and possibly to provide proprioceptive sensation during motion.
The posterior spinal musculature provides minimal restraint to rotation, which is principally compensated for by the oblique musculature.
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