Immobilization distorts allometry of rat femur: implications for disuse osteoporosis

It is inherent to mammalian limb bones that they tend to maintain the same relative proportions of bone mass with changing body mass. This assumption seemed not to comply with experimental data on bone mineral content (BMC) and areal density (BMD) we have previously observed with immobilized rat femora. Using allometric scaling (BMC = aWEIGHTb), we showed in this study that a 3-week period of unilateral immobilization of the left hindlimb of 27 Spraque-Dawley male rats at the age of 13 weeks resulted in disproportionate scaling between femoral BMC and body weight (range: 430-937 g) during subsequent 32 weeks. The allometric exponents (b) were 0.76 (95% confidence interval: 0.64-0.88) for the immobilized left femur and 0.62 (0.51-0.73) for the right intact femur, both of which were in significant contrast to the value of 0.99 (0.90-1.08) observed in 36 free-living control rats (410-910 g). For BMD, the corresponding exponent values were 0.52 (0.44-0.60), 0.44 (0.36-0.52), and 0.67 (0. 61-0.73), the values being significantly different from each other also. We conclude that a short period of immobilization significantly distorts the normal allometric scaling relationships between the body weight and femoral BMC and BMD in growing male rats. The obvious adverse effect on peak bone mass suggests that an immobilization period during growth may condemn the given bone to a lifetime of relative fragility. Therefore, further consideration of this issue is warranted.