A, the anatomical structure of the intervertebral disc intervertebral disc consists of cartilage, fibrous ring and nucleus pulposus three parts. 1, cartilage end plate. Cartilage plate, like articular cartilage, can withstand pressure, prevent the vertebrae from overload pressure, protect the vertebral body, the main cartilage end plate remains intact, the vertebral body will not occur due to pressure resorption phenomenon. 2.Fiber ring. Fibrous ring is divided into three layers: outer, middle and inner. Between the layers there are bonding-like material to make each other firmly together, and no clear demarcation between the nucleus pulposus. The entire fiber ring is arranged in almost concentric circles, the peripheral fibers are more vertical, and the more to the center, the greater the inclination. The ring of fibers is very strong, closely attached to the cartilage end plate, maintaining the stability of the spine. 3. Nucleus pulposus. Depending on the age, the water content of the nucleus pulposus can account for 75-90% of the total nucleus pulposus. The various components in the nucleus pulposus are combined to form a three-dimensional reticular gel-like structure. Changes in height in normal individuals are associated with changes in the water content of the nucleus pulposus. The nucleus pulposus is plastic and becomes flattened under pressure, allowing pressure to be transmitted in all directions. In the activity of adjacent vertebrae, the nucleus pulposus acts as a fulcrum, like a ball, moving forward or backward with the flexion and extension of the spine. Second, the relationship between lumbar intervertebral discs, intervertebral foramina and nerve roots Nerve roots are most vulnerable to pressure in the intervertebral foramen, the upper and lower diameter of the intervertebral foramen is larger than the anterior and posterior diameter, when a herniated disc, small synovial swelling, osteophytes, etc. can narrow the intervertebral foramen, which is smaller than the diameter of the nerve root, thus compressing the nerve root and causing nerve root compression symptoms. In general, herniated discs of lumbar 3 and 4 compress the nerve root of lumbar 4; herniated discs of lumbar 4 and 5 compress the nerve root of lumbar 5; and herniated discs of lumbar 5 sacral 1 compress the nerve root of sacral 1. Third, the key mechanical characteristics of the intervertebral disc In flexion, the disc is compressed anteriorly and the posterior part of the nucleus pulposus is stretched. Flexion results in posterior displacement of the nucleus pulposus. Movement causes elongation of the spinal canal, which puts tension on the spinal cord and peripheral nervous system. In full flexion, the intradiscal pressure measured at the nucleus pulposus can increase by as much as 80%. In extension, the disc is compressed from behind and the anterior portion of the nucleus pulposus is stretched. The amplitude of motion is influenced by the spinous process or the inferior articular process on the vertebral plate below. The load may be concentrated in the isthmus. Stretching causes anterior displacement of the nucleus pulposus. Extension reduces the volume of the spinal canal and intervertebral foramen. Nucleus pulposus pressure may decrease by up to 35% during extension. IV. Clinical significance of diurnal changes in the intervertebral disc When no weight is applied at night, the osmotic pressure of proteoglycan causes water absorption; when weight is applied during the day, water is pressed out of the disc causing a 10% decrease in disc height; there is a 1-2% change in height throughout the day; the range of motion increases during the day; the stiffness of flexion movement in the morning is 300% of that in the evening of the previous day. The pathophysiology of disc herniation Spengler divided lumbar disc herniation into three types: protruded, extruded, and sequestered. central herniation. The pathological changes of each type are also proposed. 1. Postero-lateral herniation, the weakest posterior part of the annulus fibrosus is on both sides of the midline of the intervertebral disc. Here, type II collagen increases due to disc degeneration, and the annulus fibrosus itself is weak, while the strong central fibers of the posterior longitudinal ligament are not reinforced. Therefore, it is the most common site of lumbar disc herniation. According to the size of the herniated nucleus pulposus tissue and the degree of intradiscal pressure, it can be under the posterior longitudinal ligament. In this case, the herniation appears as a hard and smooth bulge. It also separates the posterior longitudinal ligament from the vertebral body. As the size increases, the ligament separates further from the vertebral body so that the herniated nucleus pulposus tissue can move in either direction, usually medially or laterally, in line with the direction of the nerve root, parallel to it, or protruding into the intervertebral foramen. The nucleus pulposus may be completely free or may remain attached to the fibrous tissue within the nucleus pulposus. This is the most common type. The nucleus pulposus may contact the nerve root at any point in the segment between the dural sac and the intervertebral foramen. In most cases, the nucleus pulposus protrudes directly underneath or on the inner and outer sides of the nerve root, causing the nerve root to pull and tense. Large protrusions can not only cause nerve root tension, but can also compress the nerve root, pushing it up against the bony lamina or overfolded ligamentum flavum. When spinal stenosis (developmental or acquired) is present, there is a significantly greater chance of nerve root compression. Some authors believe that excessive folding of the ligamentum flavum does not generally cause compression. However, in the opinion of Depalma and Rothman, this is the more common compression factor. Dalyan et al. found that in anterior flexion of the lumbar spinal canal, the ligamentum flavum was tense and the volume of the spinal canal increased. In hyperextension, the ligamentum flavum folds, the volume becomes smaller, and the nerve roots can be impinged. When the lumbosacral angle increases, especially near the horizontal position, the lumbar lamina can imbed the dural sac and form an annular compression. In this case, the nerve roots are mainly compressed, but not tense. Therefore, the complaint is sensory-motor impairment rather than pain. 2. Intervertebral foramen protrusion, the intervertebral disc may protrude backward into the spinal canal through the posterior fibrous ring and posterior longitudinal ligament, or into the intervertebral foramen, and the ruptured protrusion may gradually pass under the posterior longitudinal ligament to the intervertebral foramen. In the intervertebral foramen, the herniation may compress the nerve roots causing them to become tense. The straight leg raise test or supine jerk test, etc. can produce severe radiating pain in the lower extremities. stephens et al. observed the lumbar intervertebral foramen in 20 isolated spinal specimens and found that in the case of lumbar disc abnormalities, the elliptical intervertebral foramen can be significantly altered and the foramen deformed, which can compress the nerve roots and produce symptoms and signs 3. central type herniation, a true central type of disc herniation, is a nucleus pulposus material The true central disc herniation is a protrusion of the nucleus pulposus material through the middle of the posterior portion of the annulus fibrosus and reaches the posterior longitudinal ligament. The posterior longitudinal ligament thickens in the middle fibers, strengthening the posterior annulus fibrosus, so that complete rupture of the outer layer of the annulus fibrosus is rare. The posterior longitudinal ligament can rupture during extreme flexion of the spine and cause the nucleus pulposus to pass into the spinal canal. In central lumbar disc herniation, prolonged standing causes compression of the nerve roots and local formation of fibrous connective tissue surrounding the nerve roots, which also causes abnormal position of the nerve roots, edema, and the formation of cords for compression. The nucleus pulposus under the posterior longitudinal ligament may also constitute compression of the nerve root and produce symptoms.