The elastic and fatigue behaviors of primary and secondary bovine compact bone were examined by in vitro mechanical testing, and analyzed morphologically to determine the structural correlates of these mechanical behaviors. Specimens were cyclically loaded in uniaxial tension at a strain range of 1200 (mu)(epsilon) and strain rates of either 0.01 s^-1 or 0.03 s^-1;​ long-term specimens were loaded 15 million or more cycles, short-term specimens for 1 million cycles. After loading, specimen microstructure was examined quantitatively to determine porosity, percent osteonal bone and osteon density;​ wet density and ash content were determined as well. Numerical and surface densities for microdamage were measured in short-term and nonloaded control specimens.

Primary bone was approximately 28% stiffer than osteonal bone, but strain rates, within the physiological range used here, did not affect elastic modulus. Elastic modulus decreased nonlinearly with increasing porosity, such that E('(PROPORTIONAL)) P^0.55. Increased osteonal remodeling was negatively correlated with specimen modulus, independent of any specimen density effects. Ash content was positively correlated with elastic modulus and negatively correlated with percent osteonal bone. The data suggest that decreased elastic modulus observed with increased amounts of osteonal bone are, in part, related to remodeling related porosity increases and mineralization decreases. Changes in microscopic organization may contribute to decreased elastic modulus as well.

Long-term fatigue studies indicated that compact bone fatigue, evidenced by a stiffness decrease (1-10%), occurred during the first 1-3 million loading cycles and did not progress to fatigue failure. Short-term experiments indicated that high strain rate loading is more fatiguing and damaging to bone than is low strain rate loading;​ stiffness loss was greater in both primary and secondary bone specimens loaded at the high experimental strain rate. Analysis of microdamage in primary bone specimens suggests that more and larger microcracking occurred in specimens loaded at the high strain rate.