PHYSICAL CHEMISTRY OF P:NAMEL DISSOLUTION 'io t 



of the formation of incipient carious lesions are: the positive iden- 

 tification of the organic fractions in the lesion; the events leading to 

 the relatively sound outer layer of enamel, especially with a view to 

 separating the effects of by-passing and reprecipitation; diffusion 

 measurements through enamel, particularlv relating to transport of 

 acid and reaction products; and the role of acidic forms of phosphate 

 as well as the various weak complexes of calcium with phosphate, 

 bicarbonate, and lactate in the transport of the reaction products. 



Summary 



Incipient carious lesions of enamel, tvpical of those formed in vivo, 

 have been produced bv purely chemical svstems. These svstems were 

 used to investigate the variables affecting incipient carious lesion 

 formation and elucidate the mechanisms involved. 



The essential factors necessary for the formation of incipient cari- 

 ous lesions are (1) a reactant (acid) to dissolve the hydroxyapatite 

 and (2) an organic polymer to protect the external surface of the 

 enamel. The inclusion of the reaction products — calcium and phos- 

 phate — has mediating effects ranging from complete inhibition of 

 dissolution to subtle alterations in subsurface decalcification, per- 

 mitting closer approach to duplication of the natural lesions. 



Incipient carious lesion formation can be explained for the general 

 case by the dynamics of the heterogeneous system consisting of an 

 acidic solution and a calcium phosphate solid. This involves chemical 

 phase transformation, equilibria, and kinetics. The rate of enamel 

 dissolution during incipient carious lesion formation is directly 

 proportional to total acid concentration (dissociated plus undis- 

 sociated), a "first order" diffusion-controlled reaction. This reaction 

 can be inhibited or brought to equilibrium by the presence of ions 

 which form protective deposits on apatite surfaces as salts of the 

 reaction products, such as dicalcium phosphate or calcium fluoride. 



The existence of dicalcium phosphate on the surface of enamel, 

 bone, and synthetic hydroxyapatite, and the existence of calcium 

 fluoride on the surface of fluorapatite, in equilibrium with acid solu- 

 tions, has been established. 



