Water and electrolyte content of human intervertebral discs under variable load.

The human intervertebral disc acts as an osmotic system. Water, salt, and other low-molecular substances penetrate the cartilage plates and annulus fibrosus. The content of water, sodium, potassium, and ashes in different regions of 69 human lumbar intervertebral discs was examined before and after being loaded with certain weights. Under load, the disc loses water (annulus 11%, nucleus 8%) and gains sodium and potassium. The higher concentration of electrolytes in the disc after a long period of loading increases its osmotic absorption force and enables the disc to hold back the remaining water, even against a considerable pressure. After reduction of the pressure, water is quickly reabsorbed and the disc gains height and volume. The pumping mechanism maintains the nutrition and biomechanical function of the intervertebral disc.