PATHOGENESIS OF TEARS OF THE ANULUS INVESTIGATED BY MULTIPLE-LEVEL TRANSAXIAL ANALYSIS OF THE T12-L1 DISC

Study Design. Three transaxial slices, dividing each disc into four la yers of equal thickness, were made in each of 19 T12-L1 discs. Naked-e ye and stereoscopic examination was used to record abnormalities of th e T12 (superior) surface of the upper slice, opposing surfaces of the central slice, and the L1 (inferior) surface of the lower slice. Objec tives. To characterize and quantify structural abnormalities to determ ine 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 conditions; and 2) that rim lesions ini tiate the development of concentric tears. Summary of background data. Most previous studies of disc disease have been based on the examinat ion of single sagittal slices, some on single transaxial slices, and a few have used both. This single-slice approach underrecords abnormali ties that have not involved the disc center, and may inhibit the inter pretation of magnetic resonance imaging and computed tomography images . Methods. Spines from 19 human cadavers (mean age, 47.4 years; range, 20-79 years) were used. An initial transaxial slice through the cente r of the T12-L1 disc was followed by cranial and caudal transaxial sli ces midway between the center and endplate. Soft tissues were then rem oved to allow examination of the endplate. Abnormalities recorded at e ach stage were summated for all disc levels. The incidence of the abno rmalities in each disc sector was analyzed using the Spearman-Rank cor relation coefficient and the Bonferroni correction. Results. With the exception of radiating tears, which most commonly affected the posteri or 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. C oncentric tears were present after approximately 10% of the anulus had undergone some delamination. Rime lesions correlated with focal thick ening of anulus lamellae. One fifth of radiating tears extended to inv olve the outer anulus zone. Conclusions. Neither hypothesis was substa ntiated. 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 t ypes of anulus tears appear to evolve independent of age and each othe r.