THE EFFECT OF FLUIDS ON STRESS GENERATED POTENTIALS IN BONE

The ability of bone to generate an electrical polarization in response to an applied stress may be involved in the growth and repair of bone. However, the origin of these stress generated potentials (SGP) in wet bone remains uncertain. This work studied the relationship between the properties of the fluid in bone and the amplitude and the time dependence of the SGPs in order to determine whether streaming potentials have a role in this phenomenon.

Human and bovine compact bone samples were stressed in four point bending using step and one hertz sinusoidal loading. These samples were soaked in aqueous solutions containing NaCl or sucrose (C(,12)H(,22)O(,11)) in varying amounts. Samples were tested while fully wet using macroscopic and microscopic electrodes. The SGP was measured as a function of ion concentration, solution electrical conductivity, and solution viscosity. Other experiments were also performed.

As the concentration of NaCl in the soaking solution increased, SGPs decreased in magnitude and changed polarity. The relaxation time of the SGP response to step loading exhibited little or no relationship to the solution electrical conductivity.

SGPs decreased, relaxation times increased, and bi-phasic responses to step loading were observed as the soaking solution viscosity increased. These results are consistent with the streaming potential hypothesis.

A model is proposed to explain SGPs on microscopic and macroscopic levels. This model considers that fluid flow in bone is caused by an initial deformation and a subsequent viscoelastic relaxation. SGPs result because such fluid flow generates an electrical current via a streaming potential mechanism. This model predicts the magnitude, time, and spatial relationships of SGPs in wet bone as a function of the loading and the electrical, fluid, and materials properties of wet bone. It is consistent with the results of this and other studies.

The observed dependence of SGPs upon ion concentration and fluid viscosity are consistent with the predictions of classical streaming potential theory. This work supports the suggestion that SGPs in wet bone are chiefly due to streaming potential effects.