Disc height loss is a common finding.
The discs (Intervertebral Disc) are the specialized cushions that reside between each of the spinal vertebrae. They act to resist compression and to allow movement. With time and overuse of compression, x-ray or other imaging techniques will very often find disc height loss. It is so common that often radiologists do not report on them when they read the images. Fractions of disc height loss can be the difference between symptoms and no symptoms. The discs are perfectly designed although they can only withstand so much force. Their construction materials include two main types. And to get an understanding of the structure, let us use a tire as an analogous example.
A tire has several plys or layers to improve structural support as the air presses from the inside out. Tires that have 6 layers up to 10 layers. The more layers (plys) the stronger the tire. The disc is similar to this but instead of 6-10 plys, it has 25-35 ply.
Above is a sky view of a disc. The black rings are to indicate the many annular fibres to improve strength. But instead of the center being air (like that of a tire), the disc middle is made up of a clear gelatinous structure. As compression is elicited, the gelatinous material presses outward and on the inner walls of the many fibres. And just like low tire pressure, the walls of the disc bulge and in turn, lose height. This height is measured by the distance between the vertebrae. To much bulging can also cause irritation of nerves and also herniation.