The spinal disk changes throughout life (be- ginning soon after birth), but these changes accelerate after skeletal maturity, at approximately age 20 years.
Once skeletal maturity is achieved, all intervertebral disks undergo progressive alterations in volume, shape, mi- crostructure, and composition. As such, these changes can decrease motion, adverse- ly affect the mechanics of the spine, and lead to the two most common clinical disorders of the axial skeleton: herniated nucleus pulposus and degenerative diseases of the spine.
The most extensive changes occur in the nucleus pulposus, where there is a sharp de- cline in the number of viable cells and con- centrations of proteoglycans that bind water. The aggregate proteoglycans also become fragmented. Accompanying these changes are increases in the concentration of col- lagens and noncollagen proteins as dense granular material accumulates throughout the extracellular matrix. As a result, the nucleus pulposus becomes firm and white rather than soft and translucent.
With aging, the size of the outer anulus fibrosus remains constant, while the inner anulus fibrosus increases in size. This growth comes at the expense of the nucleus pulposus, which becomes more fibrotic. Myxomatous degeneration also occurs in parts of the anulus fibrosus. Fissures and cracks appear, some extending from the periphery to the central portion of the disk.
Over time, the outer lamellae of the outer anulus fibrosus become stiff fibrocartilage. The height of the disk may decline, and prominent fissures and clefts form in the center. These changes affect mobility, alter alignment, and change the loads applied to the facet joints, paraspinous muscles, and the spinal ligaments. In sum, age-related changes following skeletal maturity decrease structural integrity and contribute to changes in disk volume and shape, which, in turn, in- crease the probability of mechanical failure leading to disk herniation.