Changes in the surface chemistry of enamel exposed to acid. A surface study of caries and erosion

Dental caries is a result of acid production by plaque bacteria and manifests with mineral loss and hard tissue dissolution. Dental erosion is the irreversible loss of dental hard tissue caused by acidic agents without bacterial involvement. Numerous techniques have been described for the early detection of dental caries and erosion. However, they all rely on detecting already lost tissue, suggesting that the disease process has been operating for some time. Recent research suggests that uptake of protons from bacterial or dietary acids precedes hard tissue dissolution.

The aim of this research project was to examine whether protonation of sound and carious sterile and non-sterile human primary tooth surfaces which have been subjected to acids of known pH occurs and can be detected in vitro and in situ with a simple and clinically applicable technique.

A total of 219 human primary tooth surfaces were investigated in vitro. Of these, 110 sound and carious sterile and non-sterile tooth surfaces were examined at baseline and 109 after an acidic challenge. Universal pH indicator solution was used to assess the protonation state of the tooth surfaces. Significantly more tooth surfaces were found protonated after the acidic challenge compared to baseline for all the groups of teeth (p<0.01) with the exception of non-sterile tooth surfaces with caries into dentine (p=0.467). This was probably because the tooth surfaces were already protonated at baseline as a result of the carious process.

The in situ study involved five participants and thirty sound enamel slabs. The enamel slabs were obtained from human primary teeth and were incorporated into mandibular removable appliances. The appliances were placed intra-orally for 48 hours to allow plaque accumulation. After removal from the mouth, the enamel slabs were subjected to
10% w/v sucrose rinse. The protonation state of the tooth surface was assessed with universal pH indicator solution. The proportion of protonated tooth surfaces was significantly higher compared to the proportion of non-protonated tooth surfaces (p<0.001).

The results of these studies suggest that protonation of human primary tooth surfaces after an acidic challenge occurs and can be detected with a clinically applicable technique both in vitro and in situ. Measuring the degree of protonation can be a novel means to detect tooth surfaces that are at increased risk of caries and erosion at a very early stage before any irreversible change of the hard tissue structure.