BIOELECTRIC POTENTIALS IN BONE
Bioelectric potentials are measured from the surface of nonstressed bone (see Plate 2-38). In the intact tibia, the metaphyseal and epiphyseal regions are net electronegative, whereas the diaphyseal, or midshaft, region is relatively neutral in net charge. When a fracture occurs in the diaphysis, the entire tibial surface becomes electronegative, with a large peak of electronegativity occurring over the fracture site and persisting well after the fracture heals. A second peak of electronegativity occurs over the farthest growth plate. This latter finding is fascinating because a fractured extremity in a child frequently exhibits overgrowth not at the fracture site but in the growth plate near the end of the bone. The nature of the signal directing the growth plate to accelerate growth has never been identified, but the increased electronegativity over the growth plate/metaphyseal area that accompanies a midshaft fracture may be involved.
To determine the source of action potentials in nonstressed bone, a series of experiments were performed on rabbit tibiae indicating that viable cells and tissues were involved. When the vascular (or nerve) supply of the leg was interrupted, the electric potential over the proximal tibia did not change even 30 minutes after ligation (or denervation). The introduction of cytotoxic drugs (or other necrosis-inducing agents) led to a significant drop in electronegative potential to the bone, suggesting that cell viability was necessary for bioelectric potentials. In particular, localized necrosis led to a significant drop in bioelectric potential just at the nonviable site.
Potentials arising from nonstressed bone are called bioelectric potentials, meaning that they arise from living bone. Such potentials are dependent on cell viability and not on stress. Active areas of growth and repair are electronegative, and less active areas are electrically neutral or electropositive. Studies have also shown that the application of small electric currents to bone stimulates osteogenesis at the site of the negative electrode (cathode).
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