Disk Degeneration

      One of the earliest signs of disk degeneration is loss of water content or desiccation, most noticeable in the nucleus pulposus. MR can detect early disk degeneration because, as the disks lose water, the MR signal decreases on gradient-echo and T2-weighted images. With more advanced degeneration, the disk collapses and gas may form within the disk. Calcification is not uncommon in chronic degenerative disk disease.

      As a consequence of intervertebral disk degeneration, normal axial loading on the spine stretches and lengthens the anular fibers, resulting in rounded, symmetric bulging of the disk beyond the margins of the vertebral body. A bulging disk encroaches on the ventral spinal canal and inferior portions of the neuroforamina but does not displace or impinge the nerve roots. The combination of sagittal and axial views provides excellent visualization of the relationships of the disk to the spinal canal and neural foramina. When there is a generalized paucity of epidural fat, producing an MR "myelogram" with gradient-echo or T2-weighted images is helpful to show the relationship of the disk with the thecal sac.


      In an anatomic and MR study of cadaveric spines, Yu and colleagues Endnote found three types of anular tears in degenerated disks. Concentric tears (Type I) are caused by rupture of the short transverse fibers connecting the lamellae of the anulus, and were seen as crescentic or oval spaces filled with fluid or mucoid material. In radial tears (Type II) the longitudinal fibers are disrupted through all layers of the anulus, from the surface of the anulus to the nucleus. Transverse tears (Type III) result from rupture of Sharpey's fibers near their attachments with the ring apophysis, and are imaged as irregular fluid-filled cavities at the periphery of the anulus.

      Anular tears are depicted on MR scans as small focal areas of hyperintensity on sagittal T2- weighted images. Endnote Transverse tears are located at the periphery of the anulus adjacent to the vertebral margins. Radial tears tend to be more irregular and obliquely oriented. High-signal- intensity zones on T2-weighted MR images are commonly seen along the posterior margin of degenerated disks in asymptomatic patients. The high-signal-intensity does not imply acute disk disruption, and no association with trauma has been proven. Endnote They probably represent small transverse or concentric tears in the outer annular fibers

      Complete disruption of the anulus exposes the nuclear material to the epidural tissues, inducing a focal inflammatory reaction. Vascular granulation tissue forms and grows into the disk through the anular tear. Enhanced MR images will detect more anular tears than T1 or T2-weighted images - mostly radial tears, but also a few transverse tears. Endnote

      Degeneration of the intervertebral disk has secondary effects on the adjacent vertebral end plates and bone marrow. As discussed earlier in the section on pathophysiology, fissures develop in the cartilaginous end plates in concert with disk degeneration. Vascular granulation tissue grows into the fissures and induces an edematous reaction and vascular congestion in the adjacent bone marrow. Modic's group Endnote has classified the bone marrow changes according to the signal intensity on MR images. This first reaction of bone marrow edema and vascular congestion, called Type 1 change, is hypointense on T1 and hyperintense on T2-weighted images. Type 1 change routinely enhances with gadolinium and can simulate osteomyelitis. With time, the bone marrow converts to a predominantly fatty marrow (Type 2 change). Longitudinal studies have shown this fatty marrow replacement to be stable over a 2-3 year period. Type 2 change is hyperintense on T1 and isointense to hypointense on T2-weighted images, the exact signal intensity dependent on the degree of T2-weighting. Chronic disk disease leads to dense sclerosis of the vertebral end plates and adjacent vertebral bodies (Type 3 change). Conversion from Type 1 to Type 3 change generally requires a few years time. Type 3 change is reflected on the MR images as hypointensity on both T1 and T2-weighted images.  

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