Effects of tissue age on bone tissue material composition and nanomechanical properties in the rat cortex

Although osteoporosis is known to alter bone tissue composition, the effects of such compositional changes on tissue material properties have not yet been examined. The natural gradient in tissue mineral content arising from skeletal appositional growth provides a basic model for investigation of relationships between tissue composition and mechanical properties. The purpose of this study was to examine the effects of tissue age on bone tissue composition and nanomechanical properties. The nanomechanical properties and composition of regions of differing tissue age were characterized in the femoral cortices of growing rats using nanoindentation and Raman spectroscopy. In addition, spatial maps of the properties of periosteal tissue were examined to investigate in detail the spatial gradients in the properties of newly formed tissue. Newly formed tissue (0-4 days) was 84% less stiff and had 79% lower mineral:matrix ratio than older intracortical (15-70 days) tissue. Tissue modulus, hardness, mineral:matrix ratio, and carbonate:phosphate ratio increased sharply with distance from the periosteum and attained the properties of intracortical tissue within 4 days of formation. The mineral: matrix ratio explained 54% and 62% of the variation in tissue indentation modulus and hardness, respectively. Our data demonstrate significant variations in tissue mechanical properties with tissue age and relate mechanical properties to composition at the microscale.