82 C. A. L. Bassett 



is assured, although there may be a limited range of deformation, under normal, 

 physiologic conditinns. 



This argument has not been advanced to reject the concept that unmineralized 

 collagen may develop electric potentials in response to mechanical stress. In fact, it 



Fig. 3. Diagram of proposed electro-generator unit in bone, most probably the apatite-collagen (C + A) junction, 

 rather than collagen (C) or apatite (A) alone. Whole bone at left, osteone in center, mineralized-collagen 



at right 



seems reasonable to assume that this long-chain, cross-linked, crystalline polymer may 

 have piezo-electric properties, too, or may develop displacement potentials of the type 

 described by Christiansen et al. (1961). Once this assumption has been made, 

 however, it is important to consider again the fact that the rigid mineral component 

 of bone limits the deformation of the elastic collagen and, thereby, limits also the 

 capacity of collagen to produce stress generated potentials by itself. On the other 

 hand, unsupported collagen in soft tissues, such as tendon, periodontal membrane and 

 Sharpey's fibers, generally is subjected to tensile deformation which is concentrated 

 and may approach the elastic limits, resulting in significant stress. It is possible, 

 therefore, that these and similar collagenous structures may generate electricity. 

 Bone is not the only connective tissue that adapts its alignment and mass to meet 

 functional demands! 



Before leaving the subject of electro-mechanical properties of bone, Tischendorf's 

 studies (1951) should be considered briefly. This investigator demonstrated that the 

 major microscopic units of bone, such as osteones and lamellae, slid on one another, 

 in response to a deforming force. "When the force was removed, the structures 

 returned to their normal relationships, probably through elastic recoil. More recently. 

 Smith and Walmsley (1959) concluded that the deformation of bone under stress 

 varied with stress duration. From these studies and others (Cochran et al., un- 

 published), it is reasonable to assume that, although bone may exhibit visco-elastic 

 flow or creep, the rate at which stress is developed in the skeleton in vivo probably 

 is rapid enough to deform individual collagen-apatite junctions. 



Thus far, a proposed negative-feedback control system and the nature of the hrst 

 transducer which converts mechanical signals to electrical potentials have been con- 



