Electrochemical Aspects of Bone Remodeling and Fracture Repair

5 ELECTROCHEMICAL ASPECTS OF BONE REMODELING AND FRACTURE REPAIR 

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

The capacity of hard tissue such as bone to generate potentials in response to mechanical stress has been known from the beginning of the nineteenth century. A piezoelectric theory to account for the electric potential observed in dry bone on deformation was proposed by Fukada and Yasuda in 1957 and subsequently explored by many others in the 1950s and 1960s as well as by Friedenberg et al. in 1971. 

FIGURE 24.1 Surface potentials generated by bending of bone. 

However, there are a significant number of cases, sometimes estimated as 19% of fractures, where repair does not occur in a reasonable amount of time. The problems are associated primarily with severe injury, infection, arthritis, or biochemical abnormalities. A very common cause, known as the compartment syndrome, is related to severe swelling pressure on the blood vessels that limits blood access to the muscles. In many of these cases, electrical stimulation has been shown to be effective in accelerating repair. 

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