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C. A. L. Bassett 



are generated in bone by piezo-electric phenomena, but also to determine the electrical 

 properties of osseous tissue. Extensive studies of these properties have led Becker 

 (Becker and Brown, 1965) to the conclusion that bone possesses most of the known 

 solidstate or semiconductor properties of p-n junctions diodes. Non-living bone demon- 

 strates rectification, photoconductivity and photovoltaic effects suggesting that the crys- 

 tals of calcium hydroxyapatite have p-type characteristics and the crystals of collagen 

 have n-type. The action spectrum, measured by emission spectroscopy and recombi- 

 nation radiation, is composed of three distinct peaks which suggests further that two 

 different semiconductors are associated. This being the case, bone possesses broader 

 electric capabilities than it would if it were composed only of crystals with simple, 

 classic piezo-electric properties, such as quartz. Under such circumstances, substances 

 such as vitamins, hormones and trace elements, active in minute quantities, may 

 function by changing the electrical properties of bone at the interface between the 

 semiconductors and their bathing electrolytes, as has been demonstrated for cupric 

 ion and germanium crystals (Boddy and Brattain, 1962). These substances also 

 might accomplish similar modifications in behavior by being incorporated into crystal 

 lattices as impurities (doping agents) or by providing for charge transfer (Becker 

 et al., 1964). Furthermore, it is conceivable that byproducts of aberrant metabolism 

 may alter the electric properties or environment of bone and result in pathologic 

 changes in mass or histologic appearance. 



Returning to the concept that Wolff's law is an example of a negative feedback 

 system, it is possible, on the basis of the foregoing, to identify four of the five 

 steps involved in the mechanism (Fig. 1). The initial, environmental signal is a de- 

 forming force. It activates multitudi- 

 nous, piezo-electric transducers, lo- 

 cated in thn extracellular osseous ma- 

 trix, which generate electric potentials 

 proportional to the applied force. The 

 potentials then stimulate a second 

 transducer mechanism to alter osseous 

 architecture, in a manner which will 

 best resist the applied force. If the 

 force is directed along the axes of pre- 

 existing bone structures, the alter- 

 ations may involve only an increase 

 in bone mass, while, if the force pro- 

 ducers shear, the modifications will 

 involve realignment. 



Before presenting details of data 

 tures indicating that the second transducer 



involves the response of cells and their 

 byproducts to electric fields, additional factors relating to the generation of the 

 electric potentials should be considered. At the present time, two different con- 

 cepts about the identity of the electric generator have developed. On the one 

 hand, Shamos and Lavine (1964) believe that the demonstration of piezo-electric 

 properties in tendon by Fukada et al. (1959), coupled with earlier observations 

 on bone (Fukuda and Yasuda, 1957; Shamos et al., 1963) indicates that collagen 



Sfrucrarul change 

 appropr/afe to 

 reduce stress 



Ce//u/ar activity 

 col/ager) alignment 



Fig. 1. Representation o 

 system controlling the ( 



Electrical current 

 proportional 

 to stress 



sed negative feedback com 

 ion and mass of bone str 



