Structural and Material Mechanical Quality of Femoral Shafts in Rats Exposed to Simulated High Altitude from Infancy to Adulthood.

The growth of the body and bone mass and the mechanical properties of appendicular bone are impaired in immature rats exposed to different simulated high altitudes (SHA) (1850-5450 m) between the 32nd and the 74th days of postnatal life. Now, we report the effects of exposure to 4100 m on the above cited variables in female rats from infancy (age: 1 month) to adulthood (age: 8 months) to define the occurrence of catch up and to establish whether the effects of altitude are transient or permanent. The ex vivo right femur was mechanically tested in three-point bending. Body weight and length, and structural (loads at yielding and fracture, and stiffness) and architectural (diaphyseal cross-sectional area, cortical area, and cross-sectional moment of inertia) properties were measured at 2, 4, 6, and 8 months of exposure to SHA. The negative influence of hypoxia on all variables was similar at different ages or, in other words, the difference among ages was maintained at any extent of hypoxia. Hypoxia did not affect the elastic modulus, thus suggesting that the mechanical properties of the bone tissue were maintained. Catch up did not occur. The resulting osteopenic bone remained appropriate to its mechanical function during the entire exposure to SHA.

Mechanical testing at the whole-bone level of the femur in immature rats stunted by cornstarch consumption.

Both body weight and somatic muscle forces are the main "mechanical factors" in the determination of bone strength in the "weight-bearing bones". However, other "non-mechanical factors", such as dietary proteins, also exist, which modulate bone physiology. This study was designed to explore the mechanical behavior of the femur in post-weaning female rats stunted by feeding on cornstarch. Forty female rats aged 30 days were fed freely with one of the two diets: control (CD) and experimental (ED). CD was the standard rat laboratory diet, whereas ED was cornstarch supplemented with vitamins and minerals. Control (C) and experimental (E) animals were divided into 4 groups: C40 and E40 rats were given CD and ED, respectively, for 40 days; C105 were fed the CD for 105 days; and E40-105 were given the ED for 40 days and then the CD for the remaining experimental period (65 days). Growth of rats was assessed following Parks' model. The biomechanical structural properties of the right femur middiaphysis were estimated using a 3-point bending test. The geometric properties of both the entire bone and the cross-section were determined. The left femur was ashed and both the Ca mass and the Ca concentration were determined. Rats fed the ED failed to achieve normal weight gain. Complete catch-up was observed at the end of a 65 day period of nutritional rehabilitation. The femoral weight and length were negatively affected by the ED, as were the mid-diaphyseal cross-sectional area, the mineralized cortical area, and the cross-sectional moment of inertia. All of these parameters showed incomplete catch-up. The structural bone mechanical properties indicative of strength and stiffness were seriously negatively affected. Intrinsic material bone properties, as assessed by the modulus of elasticity and the maximal elastic stress, were within normal values. In summary, the experimental bone was weaker than the control and structurally incompetent. The considered bone was smaller than the control one, showing a significant reduction in the cross-sectional area and the moment of inertia. However, material properties as well as the ash fraction and Ca concentration were similar in E and C bones. Therefore, E bone is weaker than the C one because of its smaller bone mass, which appears to have been negatively influenced by the ED in relation to its effects on overall body mass.

Biomechanical properties of the mid-shaft femur in middle-aged hypophysectomized rats as assessed by bending test.

Both stiffness and strength of bones are thought to be controlled by the "bone mechanostat". Its natural stimuli would be the strains of bone tissue (sensed by osteocytes) that are induced by both gravitational forces (body weight) and contraction of regional muscles. Body weight and muscle mass increase with age. Biomechanical performance of load-bearing bones must adapt to these growth-induced changes. Hypophysectomy in the rat slows the rate of body growth. With time, a great difference in body size is established between a hypophysectomized rat and its age-matched control, which makes it difficult to establish the real effect of pituitary ablation on bone biomechanics. The purpose of the present investigation was to compare mid-shaft femoral mechanical properties between hypophysectomized and weight-matched normal rats, which will show similar sizes and thus will be exposed to similar habitual loads. Two groups of 10 female rats each (H and C) were established. H rats were 12-month-old that had been hypophysectomized 11 months before. C rats were 2.5-month-old normals. Right femur mechanical properties were tested in 3-point bending. Structural (load-bearing capacity and stiffness), geometric (cross-sectional area, cortical sectional area, and moment of inertia), and material (modulus of elasticity and maximum elastic stress) properties were evaluated. The left femur was ashed for calcium content. Comparisons between parameters were performed by the Student's t test. Average body weight, body length, femur weight, femur length, and gastrocnemius weight were not significantly different between H and C rats. Calcium content in ashes was significantly higher in H than in C rats. Cross-sectional area, medullary area, and cross-sectional moment of inertia were higher in C rats, whereas cortical area did not differ between groups. Structural properties (diaphyseal stiffness, elastic limit, and load at fracture) were about four times higher in hypophysectomized rats, as were the bone material stiffness or Young's modulus and the maximal elastic stress (about 7×). The femur obtained from a middle-aged H rat was stronger and stiffer than the femur obtained from a young-adult C rat, both specimens showing similar size and bone mass and almost equal geometric properties. The higher than normal structural properties shown by the hypophysectomized femur were entirely due to changes in the intrinsic properties of the bone; it was thus stronger at the tissue level. The change of the femoral bone tissue was associated with a high mineral content and an unusual high modulus of elasticity and was probably due to a diminished bone and collagen turnover.