Stress-generated potentials in bone: effects of bone fluid composition and kinetics.

It is now generally accepted that stress-generated potentials (SGPs) at low frequencies are due to an electrokinetic phenomenon in the small interior surfaces of bone and are directly proportional to the zeta potential, a property of the poorly characterized bone surface-bone fluid interface. We hypothesized that this interface would be labile and might explain the controversy over whether or not the polarity of SGPs can invert under certain conditions. In this paper, the effects of alterations in the steeping fluid on SGPs for 87 samples from 15 animals were examined in four-point bending for steeping times of < or = 65 h. Calcium and fluoride in distilled-deionized water and constant ionic strength solutions produced concentration-dependent inversions in the SGP sign. A new steady state was reached in approximately 18 h. The effects of the fluoride anion (unlike the calcium cation) apparently were reversible. The results strongly suggest that the zeta potential at the labile bone surface-bone fluid interface can undergo dramatic changes, not only in magnitude but also in sign. The results further suggest that the preparation of bone samples is critical to the understanding of this interface in vivo, and they support the hypothesis that SGPs have a role in bone remodeling.