In this study, the most up-to-date 3D permanent resin was used together with SLA technology. It can be assumed that this technology will be used more in the future, due to the ease of use and reasonable cost. However, as for every material, there is a risk that it may break in the post-treatment period. A material that has the advantages of ease of use in respect of speed of production, form, and cost should be a reasonable form of repair. From this starting point, the effect was evaluated of different intraoral surface treatments on the bonding of the material with composite resin in the repair of small fractures which do not require a complete change of the restoration. Micro tensile bond strength values were not significantly different among the surface treatments (p ≥ 0.05). Therefore, the hypothesis was not rejected.
A strong bond between the restoration material and composite increases the repair capacity of the material. By increasing the contact area on the material surface, the surface procedures are applied to increase the surface energy and wettability [23, 24], and thus a strong micromechanical locking is created between the composite and bonding agent. There are many studies in the literature about the repair and adhesion of CAD-CAM blocks, which have come into current routine clinical practice. To increase adhesion, the focus point is that mechanical and chemical bonding is increased.
Information in literature has proven that roughening with a burr, etching with aluminum oxide and silica-coated particles, laser conditioning, and conditioning with acid, are methods to increase the mechanical bond which will show the best performance in vitro. In addition, for support and chemical bonding, silanization, primer use, and bond use are the chemicals that must be used for substrate conditioning [25, 26].
All the surface processes applied in this study were observed to be at an acceptable level, and except for the negative control group, no significant difference was determined between the groups. The bonding was seen to be significantly increased in the groups applied with sanding and laser compared to the negative control group to which no process was applied. Although the values obtained from the negative control group showed no statistically significant difference compared to the positive control group values, the values of the positive control group, to which primer was applied, were higher.
The roughness of the surface causes the adhesive to flow, creating longer tags and stronger micromechanical locking. The mechanical surface treatments with hydrofluoric acid, phosphoric acid, sandblasting, and Er,Cr;YSGG laser in the current study increased the bond strength.
In a previous study related to the repair of CAD-CAM blocks, the bonding values of groups with no pre-procedure applied to the substrate surface of a conventional methacrylate-based composite were seen to be below the acceptable limit of bonding values [25]. Schwenter et al. found that in the bond between polymer infiltrated ceramic block (VITA Enamic) and 3 different resin composites, there was a significant increase in shear resistance provided by micromechanical bonding with the use of hydrofluoric acid. It was also seen in that study that additional silane application to the surfaces treated with acid increased the bonding. This increase was reported to be due to the chemical bond between silicate and silane [27].
In another study, the use of silane in resin-infused hybrid ceramic block (VITA Enamic) and nanoceramic (Lava Ultimate) CAD-CAM blocks was seen to increase micro tensile bond strength [28]. Ceramic and nanoceramic CAD-CAM blocks were applied with two different primers and/or sanding in another study, and the bond strength of the cement to resin was seen to be significantly greater in the samples of both blocks which had been sandblasted and applied with primer compared to the control group [29]. Barutçigil et al. evaluated other surface roughening methods, and in a study in which the surface of hybrid ceramic blocks (VITA Enamic) was roughened with Er,Cr:YSGG laser, the bonding values obtained from CoJet sandblasting, 50 µm Al2O3 sandblasting, and 10% hydrofluoric acid etching was reported to be similar to those of the laser process [30].
In another study in the repair of different hybrid ceramic CAD-CAM blocks (IPS e.max CAD, Vita Suprinity, Vita Enamic, Lava Ultimate) with different ceramic repair kits, 9% hydrofluoric acid and airborne particle abrasion with 50µm aluminum oxide surface roughening processes were applied to the IPS e.max CAD and Vita Suprinity groups before the application of the repair kits. After the use of the repair kits, the bonding values in all the groups showed no statistically significant difference, and all were found to be successful [10]. The results of these studies reporting that mechanical surface treatment increases the bond strength are similar to the results of the current study.
Na-Kyung examined the repair of 3D temporary resin with traditional bis-acrylic composite and methylmethacrylate monomer by applying different surface roughening processes. The group repaired with bis-acrylic composite without any additional surface treatment showed the highest micro-shear value and the difference from all the other groups was statistically significant. It was also shown that additional mechanical or chemical surface treatments did not increase the shear bond strength in the repair groups of the two different materials [31]. These results aligned with the results of the current study. Different surface roughening treatments in permanent resin does not significantly increase the bond strength when the repair is made with composites containing Bis-GMA.
Chemical surface treatment can also enhance the repair bond strength [32, 33]. The chemical bond between the monomer and filler in composite materials is obtained by an organic silanizing agent called gamma-methacryloxypropyltrimethoxysilane (γ-MPTS) [34]. Conventional silane agents are formed from γ-MPTS, acetic acid, ethanol, and water in one or two-bottle systems. Alkoxy (-OR) groups of γ-MPTS molecules hydrolise to silanol (Si-OH) groups in an acidic environment with water, then by entering a reaction with hydroxyl groups on the silica-based ceramic surface, oxane (Si-O-Si) bonds are formed [32]. By adding 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) co-polymerized with resin monomers through the hydrophobic methacrylate terminal to both the resin cement and the bonding agent and into metal primers, a chemical bond is constructed between the resin cement and ceramic [35]. In studies by Okutan et al., the bonding strength between previously sandblasted monolithic zirconia ceramics and self-adhesive resin cement was examined by applying and not applying primer containing MDP, and greater bond strength was obtained in the samples applied with primer agent containing MDP [36]. Consistent with the results of the present study, it has been observed that primer with silane and MDP and a separate bond enhances the bond strength.
Taking into consideration that luting cement also has ceramic content, multipurpose primer and bond containing MDP, MDTP, and γ-MPTS were applied to all the experimental groups except the negative control group. The positive control group was defined as the group applied with only primer and bond and no surface treatment. Although the bond strength value obtained by the positive control group was numerically lower than those of the surface treatment applied groups, the difference was not statistically significant. The application of primer (with silane and MDP content) and bond showed sufficient bond strength in repairs.
Güngör et al. examined the effect of surface treatments applied to Lava Ultimate, Vita Enamic, and GC Cerasmart hybrid blocks on bond strength before and after 10,000 thermocycles. It was reported that aging decreased bond strength, and in the group not applied with aging, the highest bond strength was in the group applied with silane following burr abrasion and acid etching [13].
There are yet few studies in literature related to the use of 3D printer technology in the field of dentistry, and the majority of those studies have been related to the testing of the mechanical and physical properties of different types of resin [37].
Adhesive failure occurs when the adhesive separates sooner than the adherents fracture. In the current study, 100% adhesive breakage in the adhesive joint was observed in all the groups. As this is the weakest point of connection between the two materials, this result was expected.
This was an in vitro study, and even though a direct correlation may not be able to be shown with application in a clinical environment, this study can be considered of guide for future studies.