Some new developments in the rheology of bone.

Bone has been shown to be a viscoelastic material which is thermorheologically complex. This implies that multiple mechanisms determine the viscoelastic response of bone. New calculations of the contribution of fluid flow to the viscoelastic behavior of bone have been made. The calculation of the relaxation time was based on Biot's model for the relaxation of fluid flow in porous materials while the relaxation strength was estimated from simple composite models. These models together with recent measurements of the permeability of bone predict that part of the relaxation due to fluid flow will occur at fairly high frequencies, perhaps above 10 kHz for in vitro samples. Measurements of ultrasonic wave propagation and attenuation in wet bone have been performed from 0.5 MHz to 15 MHz and the mechanical damping was measured from 100 Hz to 1 kHz with fluid viscosities ranging from 10(-3) to 5 X 10(-2) (MKS). However, a complete understanding of the rheological properties of bone will require additional experiments to bridge the gap between the low frequency measurements and the ultrasonic measurements.