Modeling and remodeling in a developing artiodactyl calcaneus: A model forevaluating Frost's Mechanostat hypothesis and its corollaries

The artiodactyl (mule deer) calcaneus was examined for structural and mater ial features that represent regional differences in cortical bone modeling and remodeling activities. Cortical thickness, resorption and formation sur faces, mineral content (percent ash), and microstructure were quantified be tween and within skeletally immature and mature bones. These features were examined to see if they are consistent with predictions of Frost's Mechanos tat paradigm of mechanically induced bone adaptation in a maturing "tension /compression" bone (Frost, 1990a,b, Anat Rec 226:403-413, 414-422). Consist ent with Frost's hypothesis that surface modeling activities differ between the "compression" (cranial) and "tension" (caudal) cortices, the elliptica l cross-section of the calcaneal diaphysis becomes more elongated in the di rection of bending as a result of preferential (> 95%) increase in thicknes s of the compression cortex. Regional differences in mineral content and po pulation densities of new remodeling events (NREs = resorption spaces plus newly forming secondary osteons) support Frost's hypothesis that intracorti cal remodeling activities differ between the opposing cortices: 1.) in imma ture and mature bones, the compression cortex had attained a level of miner alization averaging 8.9 and 6.8% greater (P < 0.001), respectively, than th at of the tension cortex, and 2.) there are on average 350 to 400% greater population densities of NREs in the tension cortices of both age groups (P < 0.0003). No significant differences in cortical thickness, mineral conten t, porosity, or NREs were found between medial and lateral cortices of the skeletally mature bones, suggesting that no modeling or remodeling differen ces exist along a theoretical neutral axis. However, in mature bones these cortices differed considerably in secondary osteon cross-sectional area and population density. Consistent with Frost's hypothesis, remodeling in the compression cortex produced bone with microstructural organization that dif fers from the tension cortex. However, the increased remodeling activity of the tension cortex does not appear to be related to a postulated low-strai n environment. Although most findings are consistent with predictions of Fr ost's Mechanostat paradigm, there are several notable inconsistencies. Addi tional studies are needed to elucidate the nature of the mechanisms that go vern the modeling and remodeling activities that produce and maintain norma l bone. It is proposed that the artiodactyl calcaneus will provide a useful experimental model for these studies. Anat Rec 263:167-185, 2001. (C) 2001 Wiley-Liss, Inc.