One-year clinical and micromorphological performance of injectable and conventional resin composites in Class I restorations: a randomized split-mouth clinical trial

This study aimed to compare the one-year clinical performance of a highly filled injectable resin composite and a conventional resin composite in posterior Class I restorations and to support the clinical findings with scanning electron microscopy assessments. Thirty-eight participants were included in this randomized split-mouth clinical trial. A total of 120 posterior Class I restorations were placed using G-ænial Universal Injectable ( n = 60) or G-ænial Posterior ( n = 60). The restorations were evaluated at one week, six months, and 12 months according to the esthetic, functional, and biological criteria of the FDI evaluation system. For micromorphological assessment, 64 restorations in 20 randomly selected patients were digitally scanned. Three-dimensional resin replicas were produced and examined by scanning electron microscopy. Outcomes were summarized by material and assessment time. SEM negative marginal steps were analysed using an exploratory longitudinal GEE with participant-level clustering and assessment time in the model. A participant-level bootstrap was additionally used to estimate the 95% confidence interval for the pooled absolute difference in observed proportions. At six months, 116 restorations in 37 participants were evaluated; 102 restorations in 33 participants were available at 12 months. All restorations assessed at 12 months received clinically acceptable FDI scores. The FDI scores were nearly uniform, precluding reliable criterion-specific estimates of between-material differences. Micromorphological assessment included 64 restorations at one week, 60 at six months, and 56 at 12 months. Across the three assessment periods, negative marginal steps were recorded in 15 of 90 G-ænial Posterior observations and 19 of 90 G-ænial Universal Injectable observations. The estimated absolute difference was 4.4% points (95% participant-bootstrap confidence interval, − 11.0 to 20.8% points). For SEM negative marginal steps, an exploratory participant-clustered longitudinal analysis yielded a material odds ratio of 1.35 (95% confidence interval, 0.45–4.08). Other SEM findings were too sparse for reliable comparative models and were summarized descriptively. Both restorative materials showed high clinical acceptability in conservative posterior Class I restorations over 12 months, with only minor clinical and micromorphological changes observed during follow-up. However, because this study was not designed as an equivalence or non-inferiority trial, the findings should not be interpreted as demonstrating equivalence or interchangeability between the materials. Longer-term studies with larger and more diverse samples are required. ClinicalTrials.gov, NCT07806578. Registered on 2 September 2026. Retrospectively registered.