ADAPTIVE MODELING IN A MAMMALIAN SKELETAL MODEL SYSTEM

Juvenile BALB/c mice were used as a model system to test the effects o f various loading and exercise regimens on the growth and development of femora. Six treatments and three controls were used to document cha nges in geometric, mechanical, and material properties of the femora a ssociated with strength. In each age-matched experiment, body weight a nd the strength, length, anterior and posterior diameters, cross-secti onal area, moments of inertia in the anteroposterior and lateromedial directions, cortical wall thickness, and mineral content of the femora were assessed and found to vary significantly among treatment groups. An adaptive interpretation of these data was provided by calculating Pearson correlation coefficients between moment at failure (one measur e of strength) and each geometric, mechanical and material property of the femora that contributes to strength. We make the assumption that at the termination of the experiment the greater the coordination betw een changes in strength and changes in the parameters that contribute to strength (the greater the number of correlations), the more adaptiv ely modeled the femora are. Adaptive modeling here refers to the manne r in which the femora grow and develop (adapt) under a given treatment regimen. Absolute strength of whole femora was reflected by our measu re of adaptive modeling in all groups with one exception. In each expe riment, the voluntary exercise controls were the most adaptively model ed. The least adaptively modeled groups also showed a general retardat ion of growth. It appears that juvenile mouse femora demonstrate a wid e range of responses to different conditions of loading and exercise a nd that some of these changes are likely permanent. Moreover, at least two major variables-1) mechanical loading and 2) glucocorticoid media ted psychological stress-appear to contribute to the differences seen between the treatment groups.