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 tr eatments of growth hormone (GH). Three groups of twenty rats represent ed 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 i n 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 cortic al bone were quantified via ultrasonic wave propagation. Propagation v elocities of longitudinal and shear waves were measured through cubic cortical specimens from the posterior femoral diaphysis. Density was m easured 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 (wher e i and j = 1, 2, or 3 reference the radial, circumferential, and long itudinal axes, respectively). Cortical elastic moduli consistently inc reased with age with the strongest effects demonstrated in radial depe ndent 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 tr ansverse to axial displacement (Poisson's ratio) typically decreased w ith age (9 to 31 months) as seen in nu(31) (-24.95%, p = 0.0134) and n u(32) (-20.7%, p = 0.0015). Overall, a ten day treatment with GH produ ced no global statistical change in elastic properties (p > 0.05). How ever, GH did minimize the age related differences that were measured f or E(22), E(33), and nu(32) between the 9 and 31 month old groups esse ntially returning old bone to its youthful elastic state. These findin g add orthotropic detail to the current understanding of changing cort ical elastic properties during aging as well as providing a reference for further studies of GH.