In vitro study on the wear behaviour of human tooth enamel in citric acid solution

With a worldwide increase in the ingestion of acidic soft drinks, the wear of human teeth in acidic oral environment is becoming normal, especially in young people. Previous studies mostly focused on the friction and wear behaviours of human teeth in the artificial saliva, and wear behaviours in acidic environment are still far from being well understood. In this paper, the wear behaviour of human tooth enamel against titanium alloy in 0.001 M citric acid solution (pH 3.20) has been investigated using a reciprocating wear machine. Three normal loads, 10 N, 20 N, and 40 N, were used. Results showed that in the citric acid solution, a honeycomb-like structure appeared on the surface of enamel as a result of severe dissolution of the enamel rods, and the surface hardness decreased. Under a low normal loading level, the surface softening of enamel caused by erosion dissolution played a significant role in its wear behaviour, and the wear mechanism of enamel was dominant by adhesion delamination. Hence, enamel wear in the citric acid solution was significantly higher than in the artificial saliva. With the load increasing, brittle fracture by the loading force aggravated, and obvious flake delamination happened. Enamel wear tended to be characterized mainly by mechanical removal. As a result, both the wear morphology and wear loss of enamel in the citric acid solution were similar to those in the artificial saliva at a high normal load of 40 N. These results indicated that there existed a competitive mechanism between the mechanical action and the chemical action when enamel wear occurred in the citric acid solution. The effect of erosion on the wear behaviour of enamel was more obvious under a low load than under a high load. The results would help extend understanding of the tooth wear process in acidic environment and provide a more rational explanation for the mechanism of tooth wear in the mouth.