Radial bulging of the annulus fibrosus during compression of the intervertebral disc.

An analysis of the bulging of the annulus fibrosus as a function of the height of the intervertebral disc during compression is presented. The theory was compared with the results of experimental tests on rabbit lumbar discs. When viewed in profile the ventral bulge closely resembled an arc of a circle; at each stage of compression it appeared to extend from the cranial to the caudal extremities of the annulus fibrosus. The radius of curvature of the arc was measured as a function of disc height and the results compared with theoretical predictions. Good agreement was obtained if it was assumed that about half the volume change swept out by the approach of the cranial and caudal end-plates was accommodated by the annular bulge. The lengths of the collagenous fibres reinforcing the outer lamella of the ventral annulus were estimated from the dimensions of the ventral bulge; no appreciable length increase appeared to accompany compression.