Effect of deep marginal elevation with different intermediate materials on the fracture resistance of direct and...

To compare an injectable hybrid composite for deep marginal elevation with a resin-modified glass ionomer and a flowable composite, and to assess their effects on the fracture resistance of direct restorations and indirect resin composite inlays at different depths. Ninety non-carious maxillary premolar teeth were selected to receive standardized mesio-occluso-distal (MOD) cavity preparations. The specimens were systematically classified into subgroups and classes as follows: Groups (Restorative Material): Specimens were divided into two main groups (N = 45) based on the nano-ceramic resin composite used (Spectra ST): direct restorations or indirect inlay restorations. Subgroups (Intermediate Material): Each group was further divided into three equal subgroups (N = 15) based on the intermediate material (B) used: resin-modified glass ionomer (B1:RMGI), flowable resin composite (B2), and injectable hybrid composite (B3).Classes (Gingival Box Depth): Finally, each subgroup was subdivided into three classes (N = 5) based on the depth of the gingival box (D) relative to the cementoenamel junction (CEJ): at the CEJ (D1), 2 mm above the CEJ (D2), and 2 mm below the CEJ (D3). Fracture resistance was tested using a universal testing machine. Fracture types were assessed under a stereomicroscope. Selected samples underwent scanning electron microscopy analysis. Data were statistically analyzed using Fisher's exact test, z-tests, Shapiro-Wilk tests, Levene's tests, and a three-way ANOVA. direct restorations and indirect inlay restorations did not significantly affect fracture resistance when considered as a standalone factor (p = 0.686). For the injectable hybrid composite, the indirect composite inlay exhibited higher fracture resistance at and above the CEJ (p < 0.001*). The flowable composite showed a significant difference across depths when used as a direct composite restoration (p = 0.025*). For RMGI, the indirect composite inlay was significantly superior only below the CEJ (p < 0.001*). The box depth below the CEJ generally reduced fracture resistance. While a comparison between direct restorations vs. indirect composite inlay restorations alone does not dictate fracture resistance, material choice and cavity depth are critical. Indirect composite inlay restorations using injectable hybrid composites provide superior strength at/above the Cementum-Enamel Junction (CEJ), while RMGI excels only below it; greater depth reduces overall resistance.