Determining the effect of dietary-acid type, concentration and pH on the erosion of human enamel.
Method:
Solutions of citric (1% and 6% w/v) and phosphoric-acid (0.02 w/v%) were used as typical dietary-acids, either un-buffered or buffered to pH 3.8 with 3M NaOH. To simulate drinking a sequential model was developed involving immersing third molar enamel specimens (n=18) in alternate solutions of either one of the candidate acid solution for 30 s and phosphate buffered saline (PBS) for 30 until four cycles of acid immersion had occurred. A control group (n=3) that was immersed only in PBS for four cycles was also analysed.
Erosion was monitored in terms of hardness and reduced modulus; measured using a nano-indenter (Hysitron, USA). Specimens were measured before and after acid exposure, with 16 measurements taken, in air, over a 30x30um area.
Result:
Both parameters proved to be useful measures of early erosion of enamel. In general, pH had the greatest effect (table 1). Un-buffered 6% citric acid produced with greatest reduction in both H and Er (P<0.01). Buffering the acids to the same pH reduced the overall reduction in either parameter but 6% citric-acid still produced the largest effect. Interestingly, acid type was also important, with un-buffered phosphoric acid only producing a significant reduction in H, while in buffered phosphoric-acid there was only a significant reduction in Er. No significant changes were found for the PBS specimens.
Conclusion:
We chose acids at dietary-relevant concentration and discovered that pH has the greatest effect on erosion. However, when acids are buffered to the same pH concentration becomes important. Importantly, by monitoring both Er and H differences in the behaviour elicited by individual acids could be identified.
Acid |
pH |
Reduced Modulus [GPa] |
Hardness [GPa] |
||||
t=0 |
t=120s |
Change [%] |
t=0 |
t=120s |
Change [%] |
||
Citric 6% |
1.88 |
54(2) |
19(7) |
65 ** |
2.2(0.1) |
0. (0.1) |
91** |
Citric 1% |
2.28 |
59(3) |
34(7) |
42 ** |
2.4(0.2) |
0.4(0.1) |
83** |
Phosphoric 0.02% |
2.54 |
50(3) |
50(4) |
0 |
2.1(0.1) |
1.3(0.2) |
38* |
Citric 6% |
3.8 |
43(3) |
28(3) |
35 * |
1.1(.2) |
0.3(0.1) |
73* |
Citric 1% |
3.8 |
45(3) |
34(3) |
24 * |
1.2(0.2) |
0.6(0.1) |
50* |
Phosphoric 0.02% |
3.8 |
45(2) |
39(2) |
13 * |
1.4(0.1) |
1.4(0.1) |
0 |
PBS |
7.4 |
61(3) |
63(3) |
-3 |
2.3(0.2) |
2.6(0.2) |
-13 |
Table 1 summarising results data: Data are mean(SD)
Significant differences shown in cells, where * P<0.05, ** P< 0.01 (two-way-ANOVA)