OBJECTIVE: Success of resin infiltration for the masking of MIH-lesions remains difficult to predict, prompting suggestions to adjust the treatment protocol. This exploratory in vivo study aims to evaluate whether monitoring the resin infiltration process using transmitted light, compared to ambient light, enables a better estimation of when the infiltration process is completed, and to assess how the treatment success, i.e. masking effect and infiltration proportion, is influenced accordingly. METHODS: 15 patients with 19 MIH-lesions, diagnosed according to EAPD diagnostic criteria, were enrolled. MIH-lesions were randomly assigned to two treatment groups. In one group resin infiltration progression was monitored under transmitted light, whereas in the second group ambient light was used, representing the standard procedure. Pictures were taken before infiltration and when progression became evident until infiltration was judged to be completed. Infiltration proportion and color difference between the opacity and surrounding sound enamel were calculated and analyzed using the independent-sample t-test with a level of significance of p <
0.05. RESULTS: Compared to the ambient light group, the transmitted light group showed a significantly higher infiltration time (17.7 ± 8.2 min vs. 9.3 ± 1.6 min)
a significantly higher mean infiltration proportion (97.6 ± 2.8% vs. 83.9 ± 9.7%) and a significantly better masking effect (i.e. lower mean color difference (∆E) between effected and sound enamel: 4.5 ± 2.4 vs. 7.5 ± 3.3). CONCLUSION: Transillumination-guided resin infiltration required prolonged infiltration time but led to an increased infiltration proportion and improved masking effects. CLINICAL SIGNIFICANCE: Transillumination guided resin infiltration enables a more accurate judgement as to when the infiltration process has been completed, which leads, through extension of the infiltration time, to a significantly higher mean infiltration proportion and provides favorable esthetic outcomes.