Mechanotransduction in bone: do bone cells act as sensors of fluid flow?

When compact bone is subjected to bending loads, interstitial fluid in the bone matrix flows away from regions of high compressive stress. The amount of interstitial fluid flow is strongly influenced by the loading rate in a dose-dependent fashion. We hypothesize that interstitial fluid flow affects bone formation, and we tested this hypothesis indirectly by measuring the effect of different loading frequencies on bone formation rate in vivo. The right tibiae of adult female rats were subjected to applied bending at frequencies of 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 Hz for a 2-wk period. The rats were then killed and histomorphometric measurements of bone formation were made of the midshaft of the tibia. Bending of the tibia increased bone formation rate in the higher-frequency (0.5 to 2.0 Hz) loading groups as much as fourfold, yet no increase in bone formation rate was observed for loading frequencies below 0.5 Hz. In a separate experiment, we found stress-generated potentials (SGP) in the rat tibia to increase monotonically with increasing loading frequency. The dose-response relationship between loading frequency and the bone formation response closely resembles the relationship between loading frequency and SGP within bone. The qualitative similarity between these two relationships suggests that increased bone formation is associated with increased SGP, which are caused by interstitial fluid flow. Bone cells are known to be sensitive to electric fields and may respond directly to SGP. Also, fluid shear forces have been shown to stimulate bone cells in culture, so it is possible that increased interstitial fluid flow directly affects bone formation.