When the disc is degenerated, the pressure becomes lower in the NP, the disc height is narrowed and tears may be seen in the annulus (Fig. 3). Three main kinds of tears—concentric,
FIGURE 22 Computed tomography discography. Opacification of a radial tear and of the outer annulus that is more than 30° of the disc circumference: grade 4 lesion, according to the modified Dallas classification.
radial, and transverse—have been described (9,28,41-44). Concentric tears are due to delamination of the annulus lamellae. They are usually considered as a result of the aging process and to be not clinically significant. Radial tears originate from the NP and extend radially toward the periphery of the disc. They are theoretically always present in case of disc herniation. Transverse tears are due to the avulsion of the peripheral fibers at the insertion of the annulus onto the ring apophysis. A fourth kind of tear described as small and peripheral has also been detected, and probably corresponds to the initial stage of the previous lesions (45).
Some classifications of these tear patterns have been proposed. The modified Dallas classification (46) is the most often used (9). According to this classification, five grades are defined. In grade 0, the contrast media remains within the NP. Grade 1 is defined by the extension of contrast media in a radial fissure of the inner-third limited to the annulus. In grade 2, the contrast reaches the middle-third of the annulus. Extension of the contrast medium in the outer-third of the annulus, focally or radially, which involves less than 30° of the disc's circumference, corresponds to a grade 3 lesion. In grade 4, the contrast media occupies more than 30° of the outer annulus (Fig. 22) (46).
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