## 4 Discussion
All experimental factors (\"material\", \"surface treatment\" and \"storage time\") had significant effect of immediate repair \(\upmu\)SBS. When \(X\)-tra fil was photo-polymerized against polyester strip, it yielded significantly higher repair bond strength compared to when the material was photo-polymerized in contact with air (Table 2). Although the presence of matrix prevents the formation of the OIL [31], the OIL did not guarantee a higher bond strength between layers of resin composites [32]. The degree of conversion of methacrylate-based bulk fill resin composites after 24 h exhibited significantly higher values in contrast to their immediate values [33]. This meant that there were still reactive monomer sites for attachment, to which the repair material could adhere successfully. In methacrylate-based resin composite, the high degree of polymerization could allow optimal adhesion between resin composite layers, regardless of the presence or absence of the oxygen-inhibited layer [32]. The presence of matrix provides a highly smooth flat repair surface that could help in the intimate adaptation of repair material over the substrate. The findings of this study were in agreement with that of a previous study [20] investigating the effect of the presence of the oxygen-inhibition layer on bond strength of methacrylate-based resin composites. On the other hand, after six months of storage (Table 2), a significant reduction in bond strength was detected when the material was light cured against the polyester strip, compared to the presence of the oxygen inhibition layer. It could be hypothesized that the low height (0.7 mm) of the composite micro-cylinders had allowed the passage of light during photo-polymerization to reach the oxygen inhibition layer on the substrate surface. Additional photo-polymerization might have played a role in enhancing the formation of covalent and secondary bonds and creation of an \"interpenetrating network\", which improved the \"mechanical interlocking\" [34] between the substrate layer and the overlying repair material. On the contrary, in case of polyester strip, the surface was not covered with an oxygen inhibition layer, accordingly further improvement in the adhesion through development of covalent bonds between the two layers (substrate and repair material) might not be expected, leading to a significant reduction in bond strength. On the other hand, there was no statistically significant difference in \(\upmu\)SBS values between oxygen inhibition and matrix application for Admira Fusion X-tra (Table 3). Although armocer resin do not contain a traditional Bis-GMA monomer, its manufacturer (VOCO GmbH) that it exhibits an oxygen inhibition layer when the material is photo-polymerized in air, similar to methacrylate-based resin composite. It was reported previously that the presence of oxygen inhibition layer was not essential to improve the immediate \(\upmu\)SBS of armocer-based resin composite [21], though the latter was conducted on Admira Fusion resin composite, which is considered as the non-bulk version of Admira Fusion X-tra. In addition, the results of six-month storage showed also insignificant results between oxygen inhibition and polyester strip. This might prove that the presence of an oxygen inhibition layer was not essential to bond the different layers of the armocer-based resin composite. Both resin composites in this study could be repaired successfully using the ceramic repair system (CRS), either with or without the use of the SiC grinding bur. Previous studies showed that the use of ceramic repair system to repair immediate [21] and aged [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35] resin composites improved the repair bond strength. According to manufacturer's instructions, SiC bur is used to condition the ceramic surface following preparation with abrasive bur. In case of resin composite, the step of using bur could be omitted.