Posteruptive Changes in Dental Enamel

In the life of the enamel, there is at eruption an abrupt change in its local environment. The tooth, first bathed in tissue fluid, comes in contact with a fluid of totally different composition saliva-and with bacteria and food debris. Therefore, there is every reason to believe that at and shortly after eruption, changes take place in the enamel to adapt the surface layers to the new environment (posteruptive maturation). Brudevold and others' summarized a number of facts that point to these changes in the enamel surface. There is a considerable difference in the uptake of radioisotopes in the enamel of erupting and functioning teeth of rhesus monkeys.2 In animals, it often has been shown that caries susceptibility is high just after eruption and decreases afterwards. ' This effect, however, must be explained partly by the absence of a cariogenic flora.4 The stainability of rat enamel decreases with increasing posteruptive age' and is influenced by external conditions.6 Newly erupted surfaces that are freely accessible to the fluids of the mouth are well protected by fluoridated drinking water, whereas teeth that have erupted 3 or more years before water fluoridation started show progressively less protection (Fig. 1). Although we are well aware of differences in the enamel surfaces of newly erupted teeth and "older" teeth, the nature of the processes involved after eruption is only partly known. The acquisition and exchange of ions in the surface enamel after eruption is clearly demonstrated by the studies of Brudevold and co-workers. ' 2 There are, in spatial respect, still opportunities for a further acquisition of ions.7 The composition of the saliva and, perhaps far more important, of the surface coating of the tooth (dental plaque in the broadest sense) is influential in this maturation. In a positive way, the plaque protects the enamel surface by its buffering capacity and further acts as reservoir for minerals. In a negative way, the plaque acts as a place of accumulation of substrate for acid production by bacteria. The salivary mucoid coating or any other surface layer with a low isoelectric point could, bv creating a membrane phenomenon as described by Donnan, significantly promote a precipitation of ions in the enamel.8 In regard to the local environment of the enamel, there are important differences between the various surfaces (fissures, surfaces lining the interproximal area, and the free smooth surfaces). Early after and sometimes during eruption, a new process starts-dental caries. Between the enamel and its environment there must be a constant exchange of anorganic material. Depending on the environment, the flow of ions into the enamel will be greater or smaller than, or equal to, the flow of ions leaving the enamel. These processes have been demonstrated in vitro. '-" In case the first change leading to caries is solely a partial withdrawal of anorganic ions without an injury of the tooth surface, restitutio ad integrum should be possible under favorable conditions. Remineralization of white spots by the frequent application of Andresen's remineralization powder was shown in vitro and in vivo by Ehrensberger in 1930. ' Mannerberg12, 13 demonstrated in vivo the disappearance, in part, of scratches in the enamel surface. He explained this disappearance by the precipitation of anorganic salts in the scratches and by surface abrasion. Lenz and Miihlemann14 showed that the repair of scratches could be simulated also by a salivary pellicle. The histologic study of the carious lesion usually means that the tooth studied is destroyed, so in one tooth only one stage of the process can be seen. In this clinical study, it was possible to examine a surface longitudinally and changes in individual surfaces could be studied.