Relocation of the bending axis during flexion-extension of lumbar intervertebral discs and its implications for prolapse.

All the lumbar discs were dissected from three freshly killed, adult rabbits. Ligaments and facet joints were removed but each disc remained attached to bone. The discs were securely mounted in specially constructed bending apparatus. Ventral disc height and bending angle were measured simultaneously allowing the bending axis position to be calculated. Except for lumbosacral discs, the axis shifted ventrally during the transition from flexion to extension. The existence of separate flexion and extension axes has been reported for human lumbar discs; it is argued that similarity of disc structure leads to similar mechanical behaviour in rabbit and human discs. The positions of the two bending axes are such that flexion, but not extension, could damage the annulus fibrosus of the intervertebral disc.