Biphasic dose-response curves of cortisol effects on rat diaphyseal bone biomechanics

Doses of 8, 16 (low), 32, 48, 64 (medium), and 150 (high) mg/kg/day of cortisol were administered to groups of 8 growing rats each during 16 days, and their femurs were then submitted to 3-point bending tests at low strain rate. Low doses had no effect. Medium doses, previously shown to improve calcium (Ca) balance and weight gain in the species, augmented diaphyseal elastic and ultimate strength, stiffness, and plastic-to-elastic deformation ratio with respect to untreated controls. This effect was achieved either by enhancing bone mass (volume, sectional moment of inertia, wall/lumen ration) without changes in material quality parameters (32 mg/kg/day) or, conversely, by increasing bone tissue mechanical properties (stress, modulus of elasticity) not affecting bone geometry (48 and 64 mg/kg/day). The highest dose, known to depress Ca balance and weight gain, impaired diaphyseal mechanical performance in controls by substantially reducing bone mass without major variation in bone material properties, that is, developing a true osteopenic state in mechanical terms. The energy elastically absorbed per unit volume (proportional to the risk of comminute fractures) was greater with the highest dose because of enhanced deformability and diminished bone mass. The biphasic dose-response curves obtained, grossly parallel to those previously demonstrated for metabolic actions of cortisol in the same species, showed that biomechanical repercussion of this treatment on bone depends on different, dose-dependent effects which vary independently in temporal course, intensity, and sign.