Novel effects on material's properties and pre- and post-yield behavior of rat bones - V. Chronic effects of aluminum accumulation on cortical bone

GUSTAVO ROBERTO COINTRY; ARMANDO LUIS NEGRI; JOSÉ LUIS FERRETTI

Calcidica (Grecia)

Congreso; IV International Workshop on Musculoskeletal and Neuronal Interactions; 2004

In order to analyze the effects of Al accumulation on pre- and post-yield behavior of cortical bone, 14 rats aged 90 days received ip doses of 27 mg/d of elemental Al as Al(OH)3 during 26 weeks while other 14 remained as controls. Their femur diaphyses were studied tomographically (pQCT) and tested in bending. The load/deformation curves obtained showed the successive, linearly elastic (Hookean, pre-yield) and nonlinear, "plastic" (non-Hookean, post-yield) deformation periods of bones, separated by the yield point. No effects on body weight were observed. Aluminemia and bone histological and ash data confirmed Al accumulation. Treatment reduced cortical bone mineralization (volumetric cortical BMD, p<0.01) with a negative impact on the intrinsic stiffness of cortical tissue (Young’s elastic modulus, p<0.05). Despite the absence of any cortical mass increase (cross-sectional area), an improvement of the spatial distribution of the available cortical tissue (cross-sectional moment of inertia, MI, p<0.05) occurred through a directional modulation of the modeling drifts during growth. Up to the yield point, neither the strength, strain, or structural stiffness (load/deformation ratio) of the diaphyses were affected by treatment. However, Al intoxication reduced significantly the ultimate load, Wmax and the “post-yield” fraction  Wp  of that load (an estimation of bone "toughness", p<0.01). A positive correlation between Wmax and Wp for all the studied animals as a whole was observed. The presumably adaptive response of bone modeling (as assessed by the MI) to the induced impairment of the intrinsic stiffness of bone tissue should have resulted adequate for maintaining a normal structural stiffness (load/deformation ratio) of femur diaphyses according to the bone “mechanostat” theory, but not so to provide a complete neutralization of the impaired diaphyseal strength (Wmax). Although a relative inhibition of bone formation could not be discarded, an Al-induced impairment of bone "toughness" (Wp) should have caused the striking disruption observed between effects on bone stiffness and strength. In addition to describe an unusual finding, these results suggest that the microstructural elements affecting the post-yield behavior of cortical bone in these conditions ("creeping factors") ought to be further investigated as a promising field in skeletal research.