Mechanical and pathologic consequences of induced concentric anular tears in an ovine model

Study Design. Relations between induced concentric tears in the sheep disc and the mechanics of the intervertebral joint and vertebral body bone were analyzed. Objective. To examine the effect of concentric disc tears on the mechanics of the spine. Summary of Background Data. Degeneration of the intervertebral disc results in changes to the mechanics and morphology of the spine, but the effect of concentric disc tears is unknown. Methods. In this study, 48 merino wethers were subjected to surgery, and discs were randomly selected for either a needlestick injury or induction of a concentric tear in the anterior and left anterolateral anulus. Sheep were randomly assigned to groups for killing at 0, 1, 3, 6, 12, and 18 months. From each sheep, two spine segments were mechanically tested: one with a needlestick injury and one with a concentric tear. Macroscopic disc morphology was assessed by three axial slices of the disc. Sagittal bone slices were taken from cranial and caudal vertebral bodies for histologic analysis. Results. Induced concentric tears decrease the stiffness of intact spine segments in left bending and the disc alone in flexion. In all other mechanical tests, the needlestick injury had the same effect as the induced concentric tear. In the isolated disc, the disc stiffness at 6 months was increased for right bending, as compared with the response at 1 month. This was associated with increased anterior lamellar thickening and increased vertebral body bone volume fraction. Conclusions. Concentric tears and needlestick injury in the anterior anulus lead to mechanical changes in the disc and both anular lamellar thickness and vertebral body bone volume fraction. A needlestick injury through the anulus parallel to the lamellae produces progressive damage. [Key words: concentric disc tears, intervertebral disc, lumbar spine, mechanics, vertebral body]