Therapeutic Applications
As we described in chapters 2 and 4, after bone was found to be piezoelectric, much effort was devoted to the study of the possible physiological role of the piezoelectric voltages. In one approach, in the early 1960's, working with C. Andrew Bassett of Columbia University, we studied bone's physiological response to an external voltage and found that it stimulated the growth of bone in the canine tibia (1). Many investigators subsequently confirmed the phenomenon of electrical osteogenesis using various animal models (2 -7). The general observation has been that currents of 5-50 µamp promote bone growth; 0.1-5 µamp produce little or no effect, and currents above about 50 µamp (depending on the animal model and the electrical circuitry) result in necrosis and gross tissue destruction.
Commencing in the early 1970's, various investigators began applying electromagnetic energy to patients suffering from orthopedic diseases characterized by the failure of bone to grow normally. The most frequently treated condition has been the nonunion of the long bone-a condition in which, following a fracture, the bone fails to heal spontaneously (8). Three principal methods of application of electromagnetic energy to bone subsequently emerged. In the coil method, a noninvasive technique, Helmholtz coils were fitted to the exterior of the site to be treated, and electrically driven so that the time-varying magnetic field induced a bulk electric field with the tissue (9). In the second method, four stainless steel pins were inserted percutaneously to the treatment site, and a cathodic current was applied; this treatment was reported to stimulate the healing of fractures (10), and nonunions (11). The third method, developed in our laboratory, also used a DC cathodic electrode to stimulate healing, but it employed a silver wire and significantly less current. We used the silver-wire method to successfully treat nonunions (12) and, with some modification, infected nonunions (13). Some important aspects of the three principal methods are summarized in table 11.1. Further details, and other techniques of electrical osteogenesis, are described elsewhere (8).
Table 11.1 The three principal method of clinical osteogenesis. *Peak current density in tissue.
Despite the seemingly established clinical success of electrical osteogenesis, important questions remain unanswered. One of the most important is that of the mechanism of action of the applied electromagnetic energy. The idea which initially led to the electrical bone-growth studies in animals involved the idea of simulating the natural piezoelectric currents. We now know that these currents are in the order of 10-13 amp, which is approximately 6-8 orders of magnitude below typical clinical osteogenetic currents. It seems clear, therefore, that the present clinical use of electrical treatments to stimulate bone growth do not involve the piezoelectric effect (14).