An in vivo assessment of muscular activity and the importance of electrical phenomena in bone remodelling.

Modified orthopaedic pins were placed close to the medial and distal epiphyses of the tibia in 4 anaesthetised rabbits, in order to allow the application of controlled external loading cycles. Rosette strain gauges were placed at midshaft level, where the greatest compressive and tensile strains were expected during loading. Two weeks later, following stabilisation of the pins by bone healing, the animals were anaesthetised again and silver-silver chloride electrodes were attached close to the strain gauges in order to measure the changes in electrical potential difference. A sinusoidal load cycle was exerted between the pins with peak levels of 100 N or 250 N depending on the age of the animal. A fluctuation in potential difference, in synchrony with the strain recorded by the strain gauges, was recorded. The maximum potential difference was 2.2 mV, and it was not possible to exceed this with increased bone strain. After demonstration of the piezoelectric effect, repeated stimuli were applied to the sciatic nerve, producing a twitch in the muscles adjacent to the tibia. The electric potential difference from the muscles completely overwhelmed the local potential difference at the bone surface. A further two animals were prepared as previously described, and one electrode was placed on the endosteal surface. The electrical events mirror the changes found across the limb. The stimulus to bone remodeling, as distinct from growth, is usually loading in association with muscular activity. The fact that the changes in electrical fields at the bone surface are predominantly those originating in the muscles indicates that local electrical phenomena generated by bone strain cannot be the factors initiating the cellular response that is responsible for bone remodeling.