Mechanical properties of the collagen network in human articular cartilage as measured by osmotic stress technique.

We have used an isotropic osmotic stress technique to assess the swelling pressures of human articular cartilage over a wide range of hydrations in order to determine from these measurements, for the first time, the tensile stress in the collagen network, Pc, as a function of hydration. Osmotic stress was applied by means of calibrated solutions of polyethylene glycol. Calculations of osmotic stress were based on the balance, at equilibrium, between the applied stress, the collagen stress, and the proteoglycan osmotic pressure, piPG, acting within the extrafibrillar matrix compartment. Pc vs hydration was determined for several normal human samples, both native and trypsin-treated, and for cartilage from one osteoarthritic (OA) joint. We found that for normal cartilage the collagen network does not become "limp" until the volume of cartilage has decreased by 20-25% of its initial value and that its contribution to the balance of forces in cartilage therefore must be taken into account over a much wider range of hydrations than was previously thought. For normal cartilage, the Pc vs hydration curves exhibit a steep increase with increasing hydration; trypsin treatment does not change their slope, showing that PG concentration does not influence the inherent stiffness of the collagen network. By contrast, the curves for OA specimens are considerably shallower and displaced to higher hydrations. Our findings thus highlight the role of the stiffness of the collagen network in limiting hydration in normal cartilage and ensuring a high PG concentration in the matrix, which is essential for effective load-bearing and is lost in OA.