136 R. r. SOGNNAES 



enamel, following topical application of the acid, were greater than 

 could be predicted from inspection and replicas of the enamel sur- 

 face. 



The microradiographic technique was, therefore, applied to teeth 

 which had been etched with 5 per cent nitric acid and then left un- 

 attended in the mouth of these normal animals for various lengths 

 of time. The results of the longest experiment done to date are shown 

 in Fig. 27A, B, and C. The first illustration (Fig. 27A) indicates the 

 depth of demineralization immediately following the application of 

 acid. Seven and a half months later, when comparable teeth were 

 extracted, the chalkiness was no longer apparent. Furthermore, upon 

 sectioning of these teeth, it was noted that the demineralized zones 

 along the Retzius lines seen in the microradiographs of control teeth 

 had disappeared. Although an occasional defect still remained in the 

 surface ( Fig. 27B ) , there was actually a somewhat increased radio- 

 density in the superficial layers of the enamel ( Fig. 27B and C ) . 



Next it became of interest to determine the rapidity of these 

 changes. In other animals, teeth were, therefore, extracted 2 days to 

 a few weeks following demineralization. Grossly, we already ob- 

 served differences 2 days after application of the acid. The cor- 

 responding microscopic appearance of the enamel is shown in Fig. 

 27D in transmitted light, and the microradiographic appearance in 

 Fig. 27E, 2 days after application of the acid. Already it appears 

 that the microdensity is somewhat increased. The enamel of teeth 

 remaining in the mouth for 1 week after application of the acid is 

 shown in Fig. 27F. At this point, there is a further increase in the 

 microdensity of the enamel surface and disappearance of the pre- 

 viously pronounced Retzius lines. In these and similar tests in other 

 animals, the additional observation was made that the enamel, even 

 though it had regained an elevated microdensity, still manifested a 

 greater stainability than intact enamel of the control teeth. 



In view of the clinical use of orthophosphoric acid in the dental 

 cement employed under orthodontic bands, it was considered of in- 

 terest to observe the behavior of the enamel following application of 

 this acid. Again, as shown in Fig. 27G, this acid also had a tendency 

 to produce subsurface changes along the Retzius lines, even though 

 the gross appearance of the enamel after short applications did not 



