POLK, J.D.; LIEBERMAN, D.E.; DEMES, B.; Harvard University; Harvard University; Stony Brook University: Predicting long bone loading from cross-sectional geometry

Long bone loading histories are commonly evaluated using midshaft cross-sectional properties. Functional inferences are made by assuming that long bones are deformed like long, straight beams in pure bending, with neutral axes that run through the area centroids of their cross sections. This assumption is tested in vivo using sheep exercised on a treadmill at 1.5 m?s-1 in which rosette strain gauges were mounted at three locations around the midshaft of the tibia and metatarsal. Calculation of normal strain distributions at the midshaft indicate that the neutral axis of bending (NA) does not run through the area centroid. In addition, orientation of the centroidal axes around which maximum second moments of areas (Imax) are calculated are unrelated to the planes in which the bones experience bending. Because SMAs are fourth-power functions, calculations of cross-sectional properties based on the assumption that the NA runs through the area centroid have substantial errors in magnitude (up to 100%) compared to cross-sectional properties calculated around experimentally-determined neutral axes. The polar moment of area, J, is least subject to error. We conclude that comparisons of cross-sectional properties are problematic across species and between bones for which there are no data on how they are loaded.