PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 215 



other organic constituents of plaque. A critique of axailable experi- 

 mental evidence supports this view (Jenkins, 1S61). On the other 

 hand, hydrogen ion, in reality an extremely strong complexing agent 

 for phosphate and hydroxyl groups, is adequately available. Should 

 other agents be found important, however, they would also have to 

 complv with the physical-chemical principles set forth earlier. 



Morphology^ and Formation of Incipient Carious Lesions: 

 A Brief Review 



Carious Lesion Morphology 



The morphology of an incipient carious lesion has been well de- 

 scribed by Nishimura (1926), Darhng (1956), Gustafson (1957), 

 and Schmidt and Keil ( 1958 ) , and is reviewed by Darling in chapter 

 6 of this volume. An example of an incipient carious lesion as de- 

 scribed in these works is illustrated with photomicrographs ( Figs. 1, 

 2, 3a, 3b, and 3c) taken of a section through a natural white spot. 

 The typical early lesion consists first of an apparently sound layer 

 of enamel at the surface. Beneath this surface layer is a region of 

 decalcification where enamel substance has been removed in quan- 

 titv. These regions can be observed by light microscopy (Fig. 1), 

 but the proof of relative degrees of demineralization is provided by 

 microradiography (Fig. 2). It is clearly apparent that there is a 

 reduced mineral content in the decalcified subsurface region and a 

 high mineral content in the relatively sound outer layer. Some loss 

 of enamel substance from this relatively sound outer layer has been 

 demonstrated bv comparison with the outer layer of normal enamel, 

 using precise microradiographic measurements ( Soni and Brudevold, 

 1959). 



The incipient carious lesion can be separated further into zones by 

 observation with polarized light using a full-wave plate, first order 

 red (Gustafson, 1957). A lesion showing these zones of birefringence 

 ( Fig. 3a, the same specimen as in Fig. 1 ) can be compared with the 

 diagrammatic representation of Gustafson (Fig. 3b). In darkfield 

 illumination, the lesion has a characteristic lighter appearance ( Fig. 

 3c). 



