Lumbar disc degeneration is an almost universal phenomena that is a function of age. There is no evidence that it is either painful or dysfunctional. The major changes are biochemical. There is a decrease in proteoglycans in the nucleus pulposis so that about half are lost by the age of 65, and an increase in collagen in the nucleus and the anulus. In addition, the elastic collagen in the anulus decreases with age to reach about 8% at 62 from about 13% at 26 years. Gradually, the collagen in the anulus comes to resemble the collagen in the nucleus.

These two biochemical changes have profound biomechanical and structural effects on the disc. There is a decrease in the hydrophilic properties of the disc so that water cannot be imbibed or retained as readily. The disc essentially dries up and becomes more fibrous, less flexible and more compressible. The disc becomes stiffer, less deformable and recovers from creep less easily. Range of motion is therefore lost between vertebrae.

The pulposis and anulus become less distinct and as a result of the increased compressibility and deformability of the nucleus the nucleus is less and less able to transmit vertical loads and shares this chore with the anulus. The effect of this loading on the anulus is to make the lamellae separate with fissuring and clefting occurring. Over time, the adult disc height increases by about 10% while its anteroposterior dimension increases by about 10% in women and 2% in men. Disc thinning does not appear to be part of degeneration. The disc height increase is a result of vertebral body height as the horizontal trabeculae are selectively resorbed. This reduces the cross bracing support of the vertical trabeculae which then collapse. The collapse is more evident over the center of the body causing the end plate to become more concave on its discal face. The center of the disc fills in this space and becomes more biconvex and so taller.

Part 2 will describe lumbar disc degradation and compare degeneration with degradation.