Impact of Demineralization Time on Enamel Microhardness Reduction and Lesion Depth: An In Vitro Study

The duration of acid exposure is a critical factor in determining an individual's risk of developing caries. The objective of this study was to investigate the correlation between demineralization time and two key parameters: microhardness and depth of an enamel caries lesion.

Sixty recently extracted human teeth were mounted in acrylic resin and randomly divided into five groups (n = 12). First, 400 grit and then 1,000 grit sandpaper were applied to the buccal surfaces for five minutes each. Other surfaces, except buccal surfaces, were coated with an acid-resistant varnish. The groups were stored in the same demineralization solution (pH 4.35-4.65), respectively, for a duration of 60, 72, 84, 96, and 108 hours. Before and after exposure to acidic solutions, the surface microhardness of samples (SMH) was evaluated using a Vickers pyramid diamond tip with a 100 gram load applied for 15 seconds. Three measurements were taken from each sample surface, and the average sample value was obtained by averaging the measurements. The depth of the lesions in three specimens from each group was evaluated from five different demineralized pits under a stereomicroscope. Data were analyzed by one-way ANOVA and post-hoc Tukey's honestly significant difference (HSD) tests.

After acid exposure, SMH values significantly decreased in all groups (p = 0.001; p < 0.01) Statistically significant differences were shown between all groups in terms of Δhardness (p < 0.05). After 72 hours of demineralization, the microhardness values tend to decrease more as the exposure time increases (p = 0.001; p < 0.01). The mean lesion depths were varied from 4.01 ± 0.71 µm to 13.7 ± 1.17 µm.

Our findings show that there is a positive correlation between the duration of demineralization and lesion depth and a negative correlation between the duration of demineralization and microhardness. We assume that initial lesions may deepen quickly, especially after 72 hours.