electrically stimulated osteogenesis
electrically stimulated osteogenesis
Etymology: Gk, elektron, amber; L, stimulare, to incite; Gk, osteon, bone, genein, to produce
a bone regeneration process induced by surgically implanted electrodes conveying electric current, especially at nonunion fracture sites. The process is effective because of the different electric potentials within bone tissue. Viable nonstressed bone is electronegative in the metaphyseal regions and over a fracture callus and electropositive in the diaphyses and other less active regions. Electric stimulation of fractures can accelerate osteogenesis, forming bone more quickly in the area of a surgically inserted negative electrode. The precise mechanisms by which electricity induces osteogenesis are not understood, but research shows that when cathodes are implanted at a fracture site and an electric potential of less than 1 volt is applied, oxygen is consumed at the cathode, and hydroxyl ions are produced, decreasing the oxygen tension of the local tissue and increasing the alkalinity. Low tissue oxygen tension encourages bone formation, which follows a predominantly anaerobic metabolic pathway. Studies of bone-forming junctions demonstrate that an alkaline pH exists in the zone of hypertrophic cells of the bone growth plate when calcification starts. Electrically stimulated osteogenesis can be achieved with a device that stimulates the fracture site electrically by means of several surgically implanted cathodes. Cathode pins are connected to an external power supply that delivers 20 μA to each pin. The cathodes are inserted and positioned in the fracture space with the aid of image intensification or other radiographic techniques. Other methods for applying electrical current to fractured bone involve open surgical procedures and implantation of electrodes. The percutaneous technique involving the insertion of cathode pins is performed with a local anesthetic and usually involves less postoperative pain than open surgery methods. The number and the position of the cathodes in the percutaneous technique vary, depending on the bone involved. Generally two cathodes are used for nonunion fractures of small bones, such as the medial malleolus or the carpal navicular. Three or more cathodes are used in the clavicle and the bones of the forearm. Four cathodes are used in the treatment of large bones, such as the tibia, the femur, and the humerus. Cathodes are generally inserted from opposite directions into the nonunion site. If four cathodes are used, two are placed above, and two below the fracture site. The exposed tips of the cathodes rest directly in the nonunion space. Patients who receive such treatment are routinely released from the hospital the day after the procedure, and the stimulation of their fractures by a portable power supply strapped to the skin over the fracture site continues during the healing period. The osteogenesis is radiographically monitored, and after about 12 weeks the cathode pins are removed and the affected portion of the limb involved is placed in a weight-bearing cast. Use of the cathode-pin method of electrically stimulated osteogenesis is contraindicated in the treatment of pathological fractures associated with benign or malignant tumors and in the treatment of congenital conditions such as congenital pseudarthrosis and osteogenesis imperfecta. The cathode-pin method is also contraindicated in the presence of active systemic infections, clinically active osteomyelitis, proven patient sensitivity to the nickel or chromium from which the pins are made, or synovial pseudarthrosis, unless the fluid-filled cavity at the nonunion site is excised before the cathode pins are inserted. The success rate of treatment with the percutaneous method of electrically stimulated osteogenesis is significantly reduced in nonunions in which the gap is wider than one half the diameter of the bone involved. Compare oedipus complex. See also phallic stage.