Kenneth T. Taylor

THE DISTRIBUTION OF MATERIAL PROPERTIES IN THE EQUINE THIRD METACARPAL BONE SERVES TO ENHANCE SAGITTAL BENDING

The distribution of material properties within the equine third metacarpal bone (MC3), and its possible effect on the mechanics of the structure, was quantitatively evaluated using single-load-to-failure compressive materials testing of specimens from ten horses. Bone samples from six regions within five proximodistal levels of MC3 were milled into right cylinders and compressed at a strain rate of 0.01 s-1. Diaphyseal MC3 bone material was stiffer, stronger, deformed less to yield and failure, and absorbed more energy to yield, than metaphyseal cortical bone material. Lateral and medial MC3 cortical bone material was stiffer and deformed less to yield and failure, than dorsal and palmar material. This distribution of material properties appears to increase the structural compliance in the sagittal plane, and may serve to enhance the predictability of the strain distribution during normal locomotion, as is provided in other bones by a sagittal curvature.

Stiff and strong compressive properties are associated with brittle post-yield behavior in equine compact bone material

Our hypothesis was that post‐yield mechanical behavior of compact bone material in compression, defined as the stress, strain, or energy absorbed between 0.2% strain‐offset and the point of maximum stress, is correlated with material density, modulus, strength, histomorphometric evidence of remodeling, and post‐failure gross specimen morphology. Post‐yield behavior of compact bone material from the third metacarpal bone of 10 horses, ages 5 months to 20 years, was investigated using single‐load compression‐to‐failure. The post‐yield stress, strain, and absorbed energy were compared with the compressive elastic modulus, yield stress, ash density, post‐failure macroscopic appearance of the specimen, and histologic evidence of remodeling. High values of elastic modulus, yield stress, and ash density were associated with low values of post‐yield mechanical properties (stress, strain, and absorbed energy).

Development and validation of a series of three-dimensional finite element models of the equine metacarpus

Three-dimensional finite element (FE) models of the left metacarpi of five adult horses were developed from quantitative computed tomography data, using the algorithms of Keyak et al. (1990, J Biomed. Engng 12, 389-397). The metacarpi were then equipped with 12 rosette strain gauges and loaded non-destructively in a mechanical testing machine. The bones and the models were loaded in axial compression, with the load evenly distributed across the distal row of carpal bones, and with a point load placed mediad to the sagittal midline, to a load equivalent to three times body weight (-15 kN); and in sagittal four-point bending to -2 kN. Maximum and minimum principal strains from the models were compared with those at the strain gauge rosettes. There were significant (p < 0.001) and strong (0.69 < r < 0.90) correlations between predicted and observed surface principal strains, most often resolving as second- or third-order polynomial relationships. In most cases, particularly at extreme strain magnitudes, the models tended to overestimate the observed strain magnitudes. These data suggest that the models are robust and accurate predictors of surface strains. Validation of these models lends further support for the use of this method of automated three-dimensional FE modeling, with its emphasis on accurate, individualized portrayal of structural geometry and material distribution, in research applications, and specifically for the use of these models in the study of the biology and mechanics of the equine metacarpus.