Cortical elasticity in aging rats with and without growth hormone treatments

This study quantified the orthotropic elastic changes in cortical bone due to aging as well as determined any elastic changes after acute treatments of growth hormone (GH). Three groups of twenty rats represented three age groups of young adult (9 months), middle age (20 months), and old (31 months) rats. During a ten day period, half of the rats in each age group were given twice-daily doses of recombinant human GH while the remaining half were injected with a vehicle control (saline). The effects of aging and GH on the elastic characteristics of cortical bone were quantified via ultrasonic wave propagation. Propagation velocities of longitudinal and shear waves were measured through cubic cortical specimens from the posterior femoral diaphysis. Density was measured by Archimedes' technique. The normalized, orthotropic elastic properties of Young's moduli (E(ii)), shear moduli (G(ij)) and Poisson's ratios (v(ij)) were calculated and used to compare the groups (where i and j = 1, 2, or 3 reference the radial, circumferential, and longitudinal axes, respectively). Cortical elastic moduli consistently increased with age with the strongest effects demonstrated in radial dependent properties such as E(11) (+25.3% from 9 to 31 months, p = 0.0004) and G(12) (+12.6% from 20 to 31 months, p = 0.0419). The ratio of transverse to axial displacement (Poisson's ratio) typically decreased with age (9 to 31 months) as seen in nu(31) (-24.95%, p = 0.0134) and nu(32) (-20.7%, p = 0.0015). Overall, a ten day treatment with GH produced no global statistical change in elastic properties (p > 0.05). However, GH did minimize the age related differences that were measured for E(22), E(33), and nu(32) between the 9 and 31 month old groups essentially returning old bone to its youthful elastic state. These finding add orthotropic detail to the current understanding of changing cortical elastic properties during aging as well as providing a reference for further studies of GH.