Correlation of swelling pressure and intrafibrillar water in young and aged human intervertebral discs.

Fluid balance in the intervertebral disc under applied load is determined primarily by its swelling pressure, that is, the external pressure at which it neither loses nor gains water. This depends on the composition of the tissue, in particular on its proteoglycan concentration. Proteoglycans develop a high osmotic pressure due to their fixed negatively charged groups. Because of their size, proteoglycans are excluded from the collagen's intrafibrillar volume; hence their osmotic activity is determined only by the extrafibrillar water. Here, we show that in order to evaluate correctly the swelling pressure in the annuli fibrosi of human intervertebral disc, it is essential to evaluate its proportion of intrafibrillar water. We used low-angle X-ray scattering and osmotic stress techniques to determine the lateral packing of the collagen molecules in the fibrils of the annuli fibrosi (ages: 25-77). It was found that the lateral packing and, hence, the intrafibrillar water content depends on age, external osmotic pressure, and location in the tissue. Subtracting intrafibrillar water from total hydration yields the amount of extrafibrillar water, from which the true fixed charge density of the tissue could be estimated. From a force balance, it would appear that collagen tension plays only a minor role in the equilibrium of the human intervertebral disc under load, in contrast to articular cartilage, where collagen tension is important for load bearing.