Characterizing bone strain distributions in vivo using three triple rosette strain gages.

Three triple-element rosette strain gages were attached to the equine third metacarpal midshaft to record site-specific strains engendered by locomotion. The distribution of strains acting upon the midshaft cross section were characterized using a combined beam theory and finite element model analysis that did not presume the manner by which the bone was inertially loaded. A medium-speed trot (3.6 ms-1) was chosen as a representative speed and gait, with normal and shear strains, and strain energy density (SED) distributions determined throughout the stance and subsequent swing phase. Importantly, the sites of maximum compression (-2400 mu epsilon), tension (810 mu epsilon), shear (1500 mu epsilon), and SED (54 kPa) were not located at any of the gage attachment sites, emphasizing that a minimum of three rosette gages are necessary to resolve the peaks and locations of functionally induced normal and shear strains. Considering the nonuniform strain distributions across the cortex, we conclude that the third metacarpal is subject to a complex loading milieu comprised of bending, axial compression, end shear, and torsion. As this complex manner of loading was consistent through the entire stance phase, it would appear that, at least during the trot, specific sites within the same cross section are subject to vastly different magnitudes of strain stimulus.