1990 Volvo Award in Experimental Studies: Anulus Tears and Intervertebral Disc Degeneration

Abstract

An animal model was developed to test the hypothesis that discrete peripheral tears within the anulus lead to secondary degenerative changes in other disc components. In 21 adult sheep, a cut was made in the left anterolateral anulus of three randomly selected lumbar discs. The cut was parallel and adjacent to the inferior end-plate, and had a controlled depth of 5 mm. This left the inner third of the anulus and the nucleus pulposus intact and closely reproduced the rim Lear lesion described by Schmorl. Animals were randomly allocated to different groups in relation to the length of time interval between operation and death, varying from 1 to 18 months. At death, the lumbar spine was cut into individual joint units and each disc sectioned into six parasagittal slabs. After observation of the slabs under the dissecting microscope, two of the six slabs, the one containing the anulus lesion and a contralateral, were processed for histology. The results of this study suggest that, despite the great care taken at operation to ensure that the inner anulus was left intact, progressive failure of the inner anulus was seen in all sheep and occurred in the majority of discs between 4 and 12 months after the operation. Although the outermost anulus showed the ability to heal, the defect induced by the cut led initially to deformation and bulging of the collagen bundles, and eventually to inner extension of the tear and complete failure. These findings suggest that discrete tears of the outer anulus may have a role in the formation of concentric clefts and in accelerating the development of radiating clefts. Peripheral tears of the anulus fibrosus therefore may play an important role in the degeneration of the intervertebral joint complex.