Anatomy Function allows spinal motion and provides stability links adjacent vertebral bodies together responsible for 25% of spinal column height Composition annulus fibrosus outer structure that encases the nucleus pulposus composed of type I collagen that is obliquely oriented, water, and proteoglycans characterized by high tensile strength and its ability to prevent intervertebral distraction remains flexible enough to allow for motion high collagen / low proteoglycan ratio (low % dry weight of proteoglycans) fibroblast-like cells responsible for producing type I collagen and proteoglycans nucleus pulposus central portion of the intervertebral disc that is surrounded by the annulus fibrosis composed of type II collagen, water, and proteoglycans approximately 88% water hydrophilic matrix is responsible for height of the intervertebral disc characterized by compressibility a hydrated gel due to high polysaccharide content and high water content (88%) proteoglycans interact with water and resist compression Aggrecan is a proteoglycan primarily responsible for maintaining water content of the disc viscoelastic matrix distributes the forces smoothly to the annulus and the end plates low collagen / high proteoglycan ratio (high % dry weight of proteoglycans) chondrocyte-like cells responsible for producing type II collagen and proteoglycans survive in hypoxic conditions Blood Supply the disk is avascular with capillaries terminating at the end plates nutrition reaches nucleus pulposus through diffusion through pores in the endplates annulus is not porous enough to allow diffusion Innervation the dorsal root ganglion gives rise to the sinuvertebral nerve which innervates the superficial fibers of annulus no nerve fibers extend beyond the superficial fibers neuropeptides thought to participate in sensory transmission include substance P calcitonin VIP CPON Fixation attached to vertebral bodies by hyaline cartilage Disc Biomechanics Disc viscoelastic characteristics demonstrates creep which allows for deformity over time demonstrates hysteresis which allows for energy absorption with repetitive axial compression this property decreases with time Stresses annulus fibrosus highest tensile stresses nucleus pulposus highest compressive stress intradiscal pressure is position dependent pressure is lowest when lying supine pressure is intermediate when standing pressure is highest when sitting and flexed forward with weights in the hands when carrying weight, the closer the object is to the body the lower the pressure Stability following subtotal discectomy, extension is most stable loading mode Pathoanatomy Disc Herniation herniated disks are associated with a spontaneous increase in the production of osteoprotegrin (OPG) interleukin-1 beta receptor activator of nuclear factor-kB ligand (RANKL) parathyroid hormone (PTH) Disc aging leads to an overall loss of water content and conversion to fibrocartilage. Specifically there is a decrease in nutritional transport water content absolute number of viable cells proteoglycans pH increase in an increase keratin sulfate to chondroitin sulfate ratio lactate degradative enzyme activity density of fibroblast-like cells fibroblast-like cells reside in the annulus fibrosus only no change in absolute quantity of collagen