Effect of Deproteinization on the Clinical Success of Composite Restorations in Molars Affected by Molar–Incisor Hypomineralization

Background/Objectives: Molar–incisor hypomineralization (MIH) is associated with impaired enamel structure and increased risk of restoration failure. Sodium hypochlorite (NaOCl) deproteinization has been suggested to improve adhesive bonding to MIH-affected enamel; however, evidence regarding its clinical effectiveness remains limited. This study evaluated the effect of NaOCl deproteinization on the clinical success and survival of composite restorations in MIH-affected first permanent molars. Methods: This non-randomized split-mouth observational clinical study included 42 MIH-affected first permanent molars from 21 children aged 6–12 years. Teeth were allocated into two groups according to the restorative protocol applied: conventional adhesive restoration or NaOCl deproteinization before adhesive restoration. Restorations were evaluated at 6, 12, and 18 months using modified USPHS criteria. McNemar’s test was used for paired comparisons of categorical outcomes. Restoration survival was evaluated using a Cox proportional hazards regression model with patient-level robust cluster variance estimation to account for the split-mouth design. Results: Favorable clinical outcomes were observed in both groups at the 6-, 12-, and 18-month follow-up evaluations. No statistically significant differences were found between the NaOCl-treated and untreated groups for any modified USPHS parameter throughout the follow-up period (p > 0.05). Although higher failure rates for secondary caries and retention were observed in the NaOCl-treated group at 18 months, these differences were not statistically significant (p = 0.625 and p = 0.250, respectively). Survival analysis using a cluster-adjusted Cox regression model also demonstrated no statistically significant association between NaOCl application and restoration failure (HR = 1.22, 95% CI: 0.25–5.89; p = 0.808). Conclusions: In this exploratory study, NaOCl deproteinization did not demonstrate a statistically significant influence on the clinical success or survival of composite restorations in MIH-affected molars during the 18-month follow-up period. However, the limited statistical power of the study does not allow definitive exclusion of a clinically important difference between the treatment protocols. Therefore, these findings should be interpreted with caution. Additional long-term investigations with larger cohorts are warranted.