Comparative softening of human and bovine enamel under erosive conditions

Objectives: Bovine enamel is increasingly used as a substitute for human enamel in in vitro demineralization studies. Previous comparative studies have predominantly investigated conditions representative of caries; those investigating dental erosion have been somewhat inconclusive. In this study, atomic force microscopy nanoindentation was used to compare acid-mediated softening of human and bovine enamel after very short acid exposures representative of dental erosion. The null hypothesis was that there was no difference in softening between human and bovine enamel.

Methods: 56 human and bovine enamel samples prepared from 21 permanent human molars and 10 bovine incisors were embedded in epoxy resin and polished. Specimens were exposed to citric acid solution (14.4 mmol.L-1, pH 3.20) for 0,2,5,10,20, or 30s (n=8 per group) at an equivalent linear speed of 0.25 m.s-1, rinsed in deionised water and allowed to dry in air. Nanoindentation was performed using a Hysitron Triboscope™. Hardness was calculated using the Oliver and Pharr method, and the mean of 5 values from each sample was used for analysis.

Results: Bovine enamel had a lower baseline hardness than human enamel (4.10 and 4.42 GPa respectively), but the difference was not statistically significant. Softening of human and bovine enamel was evident after very short exposure times, and hardness decreased as a function of exposure time. The two tissues behaved very similarly; hardness of human enamel at each exposure time was not statistically significantly different from that of bovine enamel at the same exposure time.

Conclusions: Human and bovine enamel exhibited a similar response in terms of reduction in surface hardness to acid exposure at very short timescales under conditions representative of dental erosion. The null hypothesis was accepted. Within the limitations of this study, the results support the use of bovine enamel as a substitute for human enamel in in vitro erosion experiments.