Correlation of mechanical viscoelastic properties to microstructure of equine cortical bone tissue

Abstract

Dynamic Mechanical Analysis (DMA) has long been used as a method of determining viscoelastic mechanical properties of polymeric materials. More recently, DMA has been used for characterizing the fiber/matrix interface in composite materials. The present work has adopted this approach to determine the viscoelastic properties of cortical bone in torsion. Very little work has been done in the area of determining the storage and loss moduli of compact bone, with the notable exception of Lakes el al. (J. Biomechanics, Vol. 12, pp. 689-698, 1979). Moreover, no studies have correlated the viscoelastic properties of bone to microstructural parameters, such as wet density, dry density, porosity, and percent mineralization. Specimens of cortical bone were taken from equine third metacarpal bones and machined into rectangular slabs approximately 46 mm long, 10 mm wide, and 2 mm thick. After initial testing to determine the linear viscoelastic region of equine compact bone, the specimens were tested in a Rheometrics RDS-11 machine. The specimens were tested moist at a constant strain amplitude of 0.015% in frequency sweeps over three decades (0, I-I 00 rad/s). A slice from the nu'd-section of each specimen was then microradiographed and the image analyzed using image analysis software on a PC. The porosity, percent secondary Haversian area, and extent of cement lines were measured. Wet density, dry density, and ash weight measurements were made from additional samples cut from the specimen mid-section. Effects of the grease used to seal in moisture content prohibited conclusive results for the correlation of mechanical moduli to the densities for the entire set of test specimens. There was, however, a significant positive correlation between the mechanical moduli and the wet and dry density for each independent set of tests. The porosity generally reduced the mechanical moduli of cortical bone tissue. However, this relationship was only n-u'ld in strength. The effect of cement lines on material damping did not produce conclusive results. This is most likely the result of a large number of specimens dominated by primary bone. This disparity of mineralization probably did not isolate the mechanical behavior of the cement line enough to show a significant trend.