Pathogenesis of tears of the anulus investigated by multiple-level transaxial analysis of the T12-L1 disc.

Abstract

Three transaxial slices, dividing each disc into four layers of equal thickness, were made in each of 19 T12-L1 discs. Naked-eye and stereoscopic examination was used to record abnormalities of the T12 (superior) surface of the upper slice, opposing surfaces of the central slice, and the L1 (inferior) surface of the lower slice. To characterize and quantify structural abnormalities to determine their incidence and three-dimensional arrangement, and to test the hypotheses 1) that the frequency and location of tears of the anulus are related to age and nucleus condition; and 2) that rim lesions initiate the development of concentric tears. Most previous studies of disc disease have been based on the examination of single sagittal slices, some on single transaxial slices, and a few have used both. This single-slice approach underrecords abnormalities that have not involved the disc center, and may inhibit the interpretation of magnetic resonance imaging and computed tomography images. Spines from 19 human cadavers (mean age, 47.4 years; range, 20-79 years) were used. An initial transaxial slice through the center of the T12-L1 disc was followed by cranial and caudal transaxial slices midway between the center and endplate. Soft tissues were then removed to allow examination of the endplate. Abnormalities recorded at each stage were summated for all disc levels. The incidence of the abnormalities in each disc sector was analyzed using the Spearman-Rank correlation coefficient and the Bonferroni correction. With the exception of radiating tears, which most commonly affected the posterior disc, the right anterior quadrant tended to show abnormalities more frequently than the other quadrants. Although concentric tears (in 74%), rim lesions (in 47%), and radiating tears (in 47%) were frequent, no correlations were found between these three types of anulus tear. Concentric tears were present after approximately 10% of the anulus had undergone some delamination. Rim lesions correlated with focal thickening of anulus lamellae. One fifth of radiating tears extended to involve the outer anulus zone. Neither hypothesis was substantiated. Because lesions of the nucleus and anulus lack uniform shape and are three-dimensionally complex, it is inappropriate to interpret cadaver disc disease on single, mid-disc slices. The three different types of anulus tears appear to evolve independent of age and each other.