Injury of the annulus fibrosus and disc protrusions. An in vitro investigation on human lumbar discs.

Discs of 25 specimens of human lumbar motion segments were subjected to an internal division of the annulus fibrosus, sparing only a peripheral layer 1 mm thick. Thus an attempt was made to simulate an internal disruption of the annulus caused by a traumatic episode or a degenerative process. The disc bulge that developed at the site of the injury was observed under axial loads in the physiologic load range, after axial compression fracture and after intradiscal injection. Under 1,000-N load, the bulge amounted to less than 0.5 mm; typically it increased to less than 1.0 mm after fracture. An extrusion of disc material at the site of the annulus injury was never observed. The results suggest that a radial division of the annulus is not sufficient to produce a clinically relevant disc herniation; further prerequisites are a fragmentation of the disc material and a separation from the endplates.