PORE VOLUME OCCLUDED BY RESIN (INFILTRANT) IN HUMAN FLUOROTIC ENAMEL

PORE VOLUME OCCLUDED BY RESIN (INFILTRANT) IN HUMAN FLUOROTIC ENAMEL

   Polarizing microscopy helps in the quantification of the volume of hypomineralized enamel pores occluded by infiltrant (Icon). For the first time in the literature, the volume of hypomineralized enamel pores occluded by resin (infiltrant) is quantified taking into account all enamel component volumes (mineral, organic and water volumes) (Sousa et al., Arch Oral Biol, 2017; https://doi.org/10.1016/j.archoralbio.2017.06.017). Previous data on enamel occluded pore volume neglected organic and water volumes (Robinson et al., J Dent Res, 55:812-8, 1976; Robinson et al., Caries Res, 35:136-41, 2001). It is important to note the data reported by the later studies represented the proportion of the pore volume occluded by resin, NOT the actual occluded pore volume. The total pore volume measured by the procedure that neglects water and organic volumes is lower than the actual total pore volume. The proportion of the pore volume occluded by resin is the ratio of the occluded pore volume by the total pore volume. When the total pore volume is underestimated, the proportion of the pore volume occluded by resin is overestimated.

PROPORTION OF PORE VOLUME OCCLUDED BY RESIN

Figure 1 shows the total pore volume, the pore volume occluded by resin infiltrate (orange), and corresponding values of the proportions of the occluded pore volume. This a model based on the neglection of the organic and water pore volumes.

Figure 1. Proportion of the pore volume occluded by resin (infiltrant) in a model without organic and water volumes.

DIFFERENT MODELS ON THE PORE VOLUME COMPONENTS

            Figure 2 shows the different models on the pore volume components. When organic and water volumes are neglected (Fig 2A-C), the experimental (model 1) total pore volume (blue outline) is smaller than the actual total pore volume (dotted outline). The use of model 1 total pore volume increases the proportion of the pore volume occluded by resin by considering in the denominator a total pore volume lower than the actual one. Because the actual total pore volume is not measured, the actual occluded pore volume cannot be measured. When all component pore volumes are considered (model 2), both the occluded pore volume and the proportion of the pore volume occluded by resin can be measured. Figure 2B and 2E show similar occluded pore volumes, but the corresponding proportion of occluded pore volumes largely disagree.

  

Figure 2. Two models of enamel pore volume components. A-C, without organic and water pore volumes. Dotted lines and continuous lines represent the actual and measured total pore volumes, respectively. D-E, including organic matter (red), total water, firmly bound water, loosely bound water (in the pore center), the volume of resin (orange), and the air volume.

            After dehydration, loosely bound water is replaced by air, and the total pore volume is comprised by the organic, firmly bound water, and air volumes. After resin penetration, the air volume is partially replaced by the resin volume. The pore volume after infiltration comprises four component volumes: organic, firmly bound water, resin, and air.

WHAT ARE THE NEW DATA?

            Sousa et al. (Arch Oral Biol, 2017) report both the actual occluded pore volume and the proportion of the pore volume occluded by resinous infiltrant (Icon) in fluorotic enamel. Interpretation of enamel birefringence in combination with microradiography made it possible to quantify all enamel component volumes. The focus was on the proportion of the pore volume occluded by resin because it can be compared to previous data.

Fluorotic enamel component volumes after resin infiltration

   In a point at 100 mm from the enamel surface, the following values were found:

Mineral volume	74.45%
Organic volume	15.04%
Firmly bound water volume	6.92%
Air volume	1.05%
Resin volume	2.54%
Total	100%

The occluded pore volume was 2.54%, and the proportion of the pore volume occluded by resin was 9.94%. Neglecting the organic and water volumes, the proportion of the pore volume occluded by resin is 70.75%.

The mean proportion of the pore volume occluded by resin was 10% (± 4%), ranging from 1.8 to 17.7%.  The proportion of the pore volume filled by air (before infiltration) correlate strongly with the proportion of the occluded volume, being consistent with the theory of capillarity transport. The proportion of the occluded volume accounted for ~ 70% of the air volume available for infiltration. This is consistent with previous report that neglected organic and water volumes. But the total pore volume is much larger than previously reported.

            Infiltration reduces the permeability of enamel by reducing the amount of the water volume. Changes in the air volume over time are important. Reduction causes an increase in enamel permeability and enhances enamel translucency (improve esthetic outcome). Increase causes the opposite. The close contact of the resin with the water pore volume raises the question of whether the resin is degraded over time or not. The firmly bound water volume provides pathway for cariogenic acid, making it possible caries progression in infiltrated enamel.