ABSTRACT

In the normal vertebral spine, the healthy disk is perfectly homogeneous and very hydrophilic. When it is loaded axially, its height decreases (Fig. 12.1) and the annulus bulges at the periphery like a tire under an excessive load. The decrease in the height of the disk space can be 1.5 mm for the lumbar disk of an individual carrying 100 kg on the shoulders. In fact, one must first add to this 100-kg weight, the weight of the trunk, and of the head, which is about 40 kg for a man weighing 80 kg, and then the intrinsic compressive force produced by muscular contraction that increases compression on the disk. This means that the disk will have to support a load of more than 150 kg when the individual is upright and the disk is in normal position. If the individual bends forward, the pressure on the nucleus pulposus will be transmitted most forcefully to the posterior annular fibers — the weakest of the fibers that make up the annulus fibrosus.