Impact of fluoride, milk and water rinsing on surface rehardening of acid softened enamel. An in situ study

Abstract

Purpose: To evaluate the impact of fluoride, milk and water rinsing on surface rehardening of acid softened enamel in situ. Methods: Ten subjects performed six tests of 4 hours each. In each test, three softened enamel samples were attached to intraoral appliances. For softening, samples were immersed extraorally in an acidic beverage for 120 seconds. Subsequently, specimens were worn intraorally for 5 minutes (Tests 1-3) or 30 minutes (Tests 4-6). Thereafter, the volunteers rinsed with a 250 ppm SnF2/Olaflur solution (Tests 1 and 4), milk (Tests 2 and 5) or noncarbonated mineral water (Tests 3 and 6) for 60 seconds. At each test, one sample was covered with tape during intraoral rinsing and thus, served as control. After rinsing, both test and the control samples were exposed to the oral cavity for up to 4 hours after demineralization. Surface microhardness (SMH) of the specimens was assessed at baseline, immediately after softening and 4 hours after softening. For each subject, the secretion rate of resting and stimulated saliva, buffering capacity and pH-value as well as calcium and phosphate concentration of saliva were measured. Statistical analysis was performed by ANCOVA followed by stratified analyses with Bonferroni correction. Results: Baseline Knoop Hardness (mean +/- S.D.) amounted to 403.1 +/- 39.4. Immediately after softening, mean SMH was reduced to 214.4 +/- 24.1 KHN. Rinsing with 250 ppm fluoride, milk or water after 5 minutes or 30 minutes intraoral exposure of softened samples had a significant effect on rehardening. The increase of SMH (AKHN) was highest after rinsing with fluoride (5 minutes: 95.0 +/- 18.3; 30 minutes: 94.2 +/- 24.3) followed by milk (5 minutes: 77.1 +/- 14.1; 30 minutes: 80.3 +/- 18.7) and water (5 minutes: 49.0 +/- 9.9; 30 minutes: 47.0 +/- 14.1), but did not achieve baseline values. It is concluded that a single rinse with a 250 ppm SnF2/Olaflur solution, milk or water increases rehardening of previously acid softened enamel.