Layering dissimilar restorative materials is a common technique in dentistry. Calcium silicate cements (CSCs) are increasingly preferred as base materials due to their biocompatibility, bioactivity, and ability to promote pulp repair. This study focuses on two CSCs: MTA Angelus (faster setting time) and Bio-C Repair putty (ready-to-use). The interaction between CSCs and overlying restorations (Cention Forte, a novel glass ionomer cement with high flexural strength and Equia Forte, a reinforced glass ionomer cement) is crucial for restoration success. The timing of restoration placement over the CSC (immediate vs. delayed) also influences the interface and mechanical properties. This study aimed to evaluate the interfacial characteristics and microhardness of these layered materials at different time intervals.

Traditional base materials like zinc oxide-eugenol cement and calcium hydroxide are less frequently used now, with CSCs taking their place. MTA Angelus offers a shorter setting time (15 min) compared to ProRoot MTA (2-3 h), which is beneficial clinically. Bio-C Repair is another CSC presented as a ready-to-use putty, improving handling. Several factors influence the success of layering restorations with CSCs: the interaction between the materials and the time elapsed between CSC placement and final restoration application. Recently introduced restorative materials like Equia Forte and Cention Forte offer improved mechanical properties and esthetics. Cention Forte's alkaline filler releases hydroxide ions, preventing demineralization, while Equia Forte's ultrafine glass particles enhance mechanical characteristics. However, the interaction of these materials with CSCs and the effect of timing on their interfacial properties require further investigation.

This randomized in-vitro study used 72 extracted primary molars. Coronal cavities were prepared, and teeth were randomly assigned to six groups (n=12 per group): Cention Forte with/without primer and Equia Forte, each combined with either MTA Angelus powder or Bio-C Repair putty. Within each group, restorations were applied immediately (subgroup A) or after 2 hours (subgroup B) to allow complete CSC setting. Samples were incubated for one week at 37°C and 100% humidity. Longitudinal sections were prepared for evaluation. Scanning electron microscopy (SEM) at low magnification examined interfacial gaps. Energy-dispersive X-ray spectroscopy (EDX) at the interface and at 200µm and 400µm distances measured element migration. Microhardness was measured using a Vickers microhardness tester with a 50g load and three indentations per sample at the exposed dentine surface. Statistical analysis (Paired t-test, independent t-test, One-Way ANOVA, Tukey's Post hoc test) was used to compare groups.

SEM analysis showed no significant gaps between CSCs and restorations in any group at either time interval, except for minor insignificant gaps in two samples in subgroup A of group EQ/P. EDX analysis revealed the elemental composition of each material (Tables 1 and 2). Microhardness analysis at 200µm showed no significant difference between subgroups A and B except in Cention Forte groups (subgroup A significantly lower than subgroup B; p=0.002, 0.03), Cx/M subgroup A was significantly higher than subgroup B (p=0.003), and EQ/P subgroup A was significantly higher than subgroup B (p<0.0001). At 400µm, Cx/M subgroup A was significantly higher than subgroup B (p=0.003) and EQ/P subgroup A was significantly higher than subgroup B (p<0.0001).

The absence of interfacial gaps suggests good adhesion between the CSCs and restorations. The variations in microhardness likely reflect the interplay between the setting reactions of the different materials and the time allowed for these reactions to complete. Delayed application of Cention Forte seems to improve microhardness, possibly due to complete CSC setting and ionic exchange between the materials. The superior performance of Cention Forte without primer in immediate application might result from better interlocking between the materials. Equia Forte's behavior suggests that its delayed application with MTA Angelus powder is optimal. The higher microhardness values observed at 200µm compared to 400µm in some cases indicate a possible diffusion of elements from deeper layers, affecting the surface hardness.

Delayed application of Cention Forte over both types of CSCs is recommended for optimal microhardness and potentially improved longevity. Immediate application of Cention Forte without primer is suitable if a primer is not applied to the CSC. Delayed application of Equia Forte over MTA Angelus powder shows advantages. Further research is needed to assess the clinical implications of these findings and investigate other CSCs and restorative materials.

This study is limited to an in-vitro model, which may not fully replicate the complex clinical conditions. The sample size, while calculated based on a previous study, could be increased for enhanced statistical power. Additional long-term studies would be valuable to assess the durability and clinical performance of these layering techniques.