Bending and torsional stiffness measurements of equine radii and tibiae

Abstract

This study was a re-evaluation of static mechanical testing of equine long bones as part of a larger study on impact failure modes and strength of the same bones. Supplementary to other morphological and mechanical tests, static stiffness properties of these long bones were non-destructively determined. The goal was to quantify stiffness properties of the diaphysis of equine radii and tibiae in quasi-static bending as well as when loaded in torsion and to check for possible age and gender effects on these properties. Fifty-six equine bones (tibiae and radii) from fifteen horses were first tested in a torsion machine and subsequently fifty-five of them investigated using a 3-point-bending test-setup. A maximum torque of 150 N∗m and a maximum bending moment of 920 N∗m were applied in steps. Loading and unloading was performed in order to check for hysteresis effects. The outcome for both type of bones (tibiae and radii) was described statistically in relation to age (young, middle aged, and old) and gender (geldings and mares). Additional information on the side (left and right), breed and use of the horse (competition versus 'other', such as pleasure) was excluded from statistical modelling after preliminary analysis. While the loading-unloading cycles in bending showed some hysteresis due to localized deformation, the unloading curve followed the loading curve in torsional loading. Bending stiffness of tibiae is on average 6'813 N/mm and of radii 6'130 N/mm. Torsional stiffness of tibiae is 2.36∗ 106 N∗m(rad/mm), and of radii 1.90∗106 N∗m(rad/mm). Tibiae were clearly stiffer than radii. A trend of higher bone stiffness for geldings compared with mares and for younger horses could be found, although not statistically significant.