The purpose of this study was to develop criteria for distinguishing normal and degenerated lumbar intervertebral disks. Cryomicrotome sections from and magnetic resonance images of 31 cadavers were reviewed. The immature nucleus pulposus, found up to age 2 years; the transitional nucleus pulposus, found in teenagers; and the adult nucleus pulposus were associated with an intact anulus fibrosus without tears except for small concentric or transverse tears in the periphery of the anulus fibrosus. Discoloration of the nucleus pulposus, diminishing disk height, and diminishing signal intensity were associated consistently with a radial tear of the anulus fibrosus. The severely degenerated nucleus pulposus was associated also with a radial tear or complete disruption of the anulus fibrosus. The results suggest that intervertebral disk normally progresses from the immature to the transitional to the adult nucleus pulposus. When a radial tear develops in the anulus there is shrinkage and disorganization of fibrocartilage in the nucleus pulposus and replacement of the disk by dense fibrous tissue and cystic spaces.
Magnetic resonance (MR) imaging, correlated with anatomic sections, was used to characterize the progressive and regressive changes in the nucleus pulposus in neonates. The spines of five fetuses and five full-term infants between 16 and 40 weeks old were studied. In anatomic sections, the nucleus pulposus was sharply demarcated from the anulus fibrosus, Sharpey fibers were conspicuous, and a plate of primitive notochord was evident in the equator of the disk. On long repetition time (TR)/long echo time (TE) or long TR/short TE MR images, Sharpey fibers (low signal intensity) and notochord (low signal intensity) could be differentiated from the high-signal-intensity nucleus pulposus and anulus fibrosus. The major differences between the fetal and infant spines were the amount of notochord in the disk and ossification in the vertebral body.
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