INTERNAL REMODELING OF COMPACT BONE 381 



as it matures during the mineralization of osteons. The terms labile 

 and stable bone mineral have been in use for some time, but there 

 has not been a chemical definition of these terms. There is, however, 

 particularly in the work of Dallemagne et al. (1961), a suggestion 

 that as bone mineral matures it may change in chemical composi- 

 tion, and hence in reactivity. This possibilitv may be explored, al- 

 though any tentative conclusions must be regarded as speculative. 



The basic structure of the bone mineral, as now generally ac- 

 cepted, is that of hijdroxyapatite, Caio(P04)6(OH)2. Dallemagne, 

 however, has for some years supported the view that the mineral, 

 as it exists in living bone, is a hydrated tricalciuni phosphate, 

 Ca9(P04)6H2(OH)2 (alpha tricalciuni phosphate), and has believed 

 that when this mineral is isolated from its organic matrix, it assumes 

 the more stable form of hydroxy apatite. Posner and Perloff ( 1957 ) , 

 however, have proposed the concept of a calcium-deficient apatite, 

 and regard Dallemagne's hydrated calcium phosphate as a special 

 case of such a mineral. Posner et al. (1960) propose that such a cal- 

 cium-deficient apatite can be accounted for by hydrogen bonding. 

 One example may be Ca-, ( PO4 ) 4 ( OH ) ( 0.,P0— H— OPOs ) . 



The hypothesis is here suggested that the highly reactive form 

 of the bone mineral, subject to rapid ion exchange at the surfaces of 

 the crystals, may be regarded as a calcium-deficient apatite, with 

 hydrogen bonding accounting for its crystal structure, and that as 

 this mineral matures with aging it is transformed into the more 

 stable hydroxyapatite. 



Summary 



Internal remodeling of compact bone continues throughout the 

 life of the individual. By providing a continuing supply of reactive 

 bone mineral, from which calcium lost from the blood in a rapid 

 turnover of this element is replaced, this performs a function essen- 

 tial to life. 



Internal remodeling is accomplished by formation of resorption 

 cavities, or tunnels, in the bone, as a result of osteoclastic resorption; 

 an analogy with rock boring by biologic organisms, as described 

 elsewhere in this volume, is suggested. 



