MICROSTRUCTURAL CHANGES IN EARLY CARIES 173 



followed by the body of the lesion, and the intermediate stages occur 

 in their appropriate places. 



The most direct method of assessing demineralization is by micro- 

 radiography of ground sections. This technique as used for dental 

 purposes was pioneered by Applebaum ( 1932 ) and has since been 

 used very successfully by Bergman and Engfeldt (1954), Guzman 

 et al (1957), Gustafson (1961), Darling (1956, 1958), and others. 



By this technique it can be shown that the area of radiolucence 

 or demineralization extends to the superficial border of the dark 

 zone or possibly just into this zone (Fig. 2). It stops short of the 

 enamel surface in most cases, leaving a zone of relatively unaffected 

 enamel about 30 microns in width at the enamel surface. With care, 

 greater detail can be shown within the lesion (Miller, 1958; Berg- 

 man and Engfeldt, 1954; Darhng, 1958; Gustafson, 1961). All seem 

 to agree that the earliest radiographic change which can be seen is 

 related to the cross striations of the prisms, which are the incre- 

 mental lines of growth of these structures. This is followed by the 

 development of lines of radiolucence running parallel to the prisms, 

 which are at first intersected by fine lines of radiopacity, again re- 

 lated to the cross striations (Fig. 3). These are later lost. In radio- 

 graphs of sections transverse to the prisms (Fig. 4) the pattern of 

 radiopaque material strongly suggests the outlines of the prisms with 

 their "cores" demineralized, but no one has yet identified them as 

 such. In sections longitudinal to the prisms the striae of Retzius or 

 incremental striae of the enamel are grossly demineralized and fonn 

 a dominant feature of many lesions. They appear to form the ad- 

 vancing edge of demineralization at the cervical margin of most 

 interstitial lesions, giving a "sawtooth" effect at this edge (Fig. 2). 

 Alternating with these striae are lines of relatively undemineralized 

 enamel which some say (Darling, 1958) lie immediately beneath 

 the striae of Retzius. In late lesions just before surface breakdown, 

 radiolucent lines parallel to the striae of Retzius and continuous with 

 those in the body of the lesion can be seen passing through the sur- 

 face zone (Fig. 5), which until this stage is wholly radiopaque. 

 Guzman et al. (1957) have described lines of radiolucence parallel 

 to the prisms in the surface zone. They believe that the lines cor- 

 respond to the prism cores, but they seem to occur in a relatively 



