Density of trabecular framework and osteogenic activity in the spongiosa of long bones subjected to drastic changes in mechanical loading in the dog.

In 2 groups of dogs aged from 4 months to 3 years, we estimated the density of the spongiosa and the distribution of osteogenic activity in transverse sections of the metaphyses and epiphyses of the experimentally overloaded or unloaded right radii, together with those of the contralateral control bones. Overloading elicited an increase in the density of the spongiosa framework to a higher degree within the peripheral than the deeper metaphyseal spongiosa or within the epiphyses. Some time after operation, the osteogenic activity appeared reduced through the spongiosa in every district although to a greater extent in the regions in which its more pronounced condensation had occurred, i.e. within the metaphyseal subcortical spongiosa. In the unloaded bones, the density of the spongiosa was markedly reduced compared to the control. The differences in the architectural pattern between subcortical and deeper spongiosa were attenuated. Conversely, osteogenic activity was enhanced in every district, being evenly distributed within the spongiosa framework both in the metaphysis and the epiphysis. From comparison between the results obtained by the 2 sets of experiments it seems reasonable to infer that mechanical stress appreciably stimulates bone mass increase and the ensuing remodelling of the spongiosa architecture; at the same time, the more heavily loaded portions of the framework would be subjected to a lesser degree of structural renewal than that occurring in the spongiosa regions less essential to mechanical resistance. The latter, therefore, would rather perform the function of labile stores of mineral salts.