Assessing shear bond strength of various surface treatments of 3D-printed provisional material with bis-acryl relining material
A provisional crown, also known as the interim or temporary crown, is usually delivered to improve esthetics, mastication, and phonetics for a limited duration; then, it will be replaced by a definitive dental restoration . The provisional crowns help prevent tooth sensitivity, protect the pulp, preserve periodontal health, maintain the integrity of the arch, withstand the occlusal loads, decrease the risk of tooth migration, and enhance the esthetic . Provisional crowns are fabricated using several materials and techniques , such as custom-made provisional restoration made from polymethyl methacrylate or bis-acryl relining material . The advancement of technology led to the rapid adoption of digitally designed and manufactured . The 3D-printed provisional restorations, an additive technique, provide adequate wear resistance with a low complication of residual monomer, including exothermic reaction and polymerization shrinkage, compared to the conventional provisional restorations . In addition, additive technology could minimize the waste produced and reduce dimension configuration compared with subtractive technology . The 3D-printed restorations are also subject to various post-curing processes, leading to improved polymerization and high physical and mechanical properties . These characteristics are essential for the materials used as therapeutic devices, such as provisional restorations for fixed prostheses, implant-supported treatments, or oral rehabilitations. The 3D printing uses digital modeling techniques to produce geometrically complex products . The high resolution of direct light processing (DLP)-based 3D printing offers significant advantages in printing resolution, efficiency, and working conditions . Prefabricated provisional prostheses may require additional customization or repair to ensure their occlusal functions and provide a marginal seal on the abutment, essential to avoid periodontal complications . Chen et al., suggested that relining 3D-printed material should be conducted with comparable materials. The more similar chemical structures, the higher bond strength could be achieved . Therefore, bis-acryl resin composite may be the candidate of choice since it contains cross-link monomers that similar to the 3D-printed provisional material . In addition, relining with bis-acryl resin composite could reduce the polymerization shrinkage, decrease the dimensional alteration, and exothermic release . A concern with 3D-printed resin-based resin composites is that the post-curing process via additional vacuum-light curing, or high-temperature oven may reduce the remaining unreacted monomers . This may decrease the possibility of co-polymerization of relining resin-based materials with the provisional restoration, which may affect the bond strength between the materials . Various methods were suggested to enhance bond strength between 3D-printed and repaired resin-based composites . The first method was the use of solvents such as ethanol or acetone. This process was expected to remove residual monomers and optimize the surface adaptation with the new resin composite, resulting in an enhanced bond strength . The second method was applying mechanical cleaning with airborne-particle abrasion that could enhance the surface contact area and surface texture, eliminate impurities, and increase the surface’s wettability . Airborne-particle abrasion significantly enhanced the shear bond strength for the repaired resin composite from 1.9 MPa to 3.6 MPa . However, this approach requires costly equipment such as air compressors and may raise health concerns due to the risk of inhaling particles, leading to complications in the respiratory system . The third method was applying the universal adhesive on the surface of the 3D-printed provisional material. Universal adhesive promoted the diffusion of the adhesive monomer into the micro-retentions, enhancing bond strength . Moreover, the 10-MDP-containing adhesive had a high affinity to inorganic filler particles, which may form a covalent bond to the unreacted methacrylate groups on the matrix or the inorganic compound, resulting in a high bond strength . Proper seating and marginal adaptation of the provisional restoration often require relining of the restorations. Understanding the effect of various surface treatments on the bond strength of 3D-printed provisional restorations and bis-acryl resin composite relining materials is critical for achieving successful and reliable restorations. Thus, the purpose of the current study was to examine the shear bond strength of 3D-printed methacrylate-based provisional material with bis acryl-based relining material using various surface treatment protocols. The null hypothesis was that there would be no significant difference in the shear bond strength between 3D-printed methacrylate-based provisional material and bis-acryl resin composites under different surface treatment protocols. 

Preparation of specimens The materials and chemicals used in the current study are provided in Table . The CAD software (Meshmixer, Version 3.5, Autodesk Inc, San Francisco, CA, USA) was used to digitally design the acrylic resin provisional specimens (P Pro crown and bridge, Straumann, Basel, Switzerland) into squares (9 × 9 × 2 mm 3 ). The file format was exported as Standard Tessellation Language (STL) and loaded into the CAM software for a 3D printing process. The specimens ( n = 15 per group) were 3D-printed and positioned vertically with supports. The 3D printing was conducted using a DLP-based 3D printer (P30 RapidShape, Straumann, Basel, Switzerland). The 3D-printed specimens were post-processed according to the manufacturer’s recommendations, which involved cleaning in the automatic cleaning unit (P-wash, Straumann, Basel, Switzerland) with isopropanol. Then, the specimens were polymerized in an automatic curing unit (P-cure, Straumann, Basel, Switzerland). All specimens were embedded with self-curing epoxy resin in a polyvinyl chloride tube and left to polymerize at room temperature for 24 h. Then, the surface of each specimen was polished using 220-grit silicon carbide paper, followed by 500, 800, and 1000-grit silicon carbide paper to standardize the roughness of the surfaces. The polishing process was conducted with the grinding machine (Tegamin-25, Struers, Cleveland, OH, USA) for 15 s at 200 rpm under running water . After polishing, the specimens were ultrasonically cleaned for 10 min to eliminate any remaining silicon carbide paper residues. The experimental design of the current study is illustrated in Fig. . The polished specimens were randomly allocated to 5 groups with different surface treatments as follows (Fig. ). Group 1 (CT): No surface treatment as a control. Group 2 (ET): Applied 95% ethanol for 3 min by micro brush and then air-dried for 10 s . Group 3 (UA): Applied universal adhesive (CLEARFIL Universal Bond Quick, Kuraray Noritake Dental, Tokyo, Japan) using an applicator in a rubbing motion for 10 s, dried with blowing mild air until the adhesive did not move. Then, light-cured polymerized was performed for 10 s using a light curing unit (Elitedent Q-4 LED, Rolence, Taoyuan City, Taiwan ROC) with a light intensity of 1400 mW/cm 2 and a wavelength of 440–480 nm . Group 4 (AA): Airborne-particle abrasion with 50 μm aluminum oxide (Al 2 O 3 ) particles at 1.5 bar pressure for 10 s with a distance of 10 mm from the specimen’s surface. Then, the specimens were ultrasonically cleaned for 10 min to remove remnants of aluminum oxide particles. Group 5 (AA-UA): The specimens were treated with airborne-particle abrasion similar to group 4, followed by the application of universal adhesive similar to that of group 3. The specimens in each group were then bonded with the bis-acryl-based relining material (Protemp 4, 3 M ESPE, St. Paul, MN, USA). In brief, polyethylene tubes (Tygon 3350 tubing, Saint Gobain Co, Courbevoie, France) with an internal diameter of 5 mm and 4 mm in height were placed in the center of each surface-treated specimen. Bis-acryl-based relining material was inserted into the cylindrical tubes using dispenser gun and impression mixing tip. All specimens were left at room temperature for 24 h, allowing the complete polymerization of the relining material. Then, the cylindrical tubes were removed by a No.11 surgical blade to prevent pre-stress. Shear bond strength (SBS) testing After 24 h of relining material application, the macro shear bond strength was measured using a universal testing machine (AGS-X, SHIMADZU, Kyoto, Japan). The specimens were placed in a shear bond strength testing jig (knife-edge blade), and the load was applied to the interface between the relining material and the 3D-printed provisional restoration. The test was performed using a 50 N load cell and a crosshead speed of 1 mm/min. The load at failure (F, Newton) was recorded and the shear bond strength (SBS, MPa) was calculated using Eq. . 1 [12pt]{minimal}
				
				 
				 
				 
				
				
				
				
				$$\:SBS=\:F/A$$ Where F is the failure load (N) and A is the bonded area (m 2 ). Failure mode analysis The fracture surfaces of the tested specimens were examined under a stereomicroscope (Leica Zoom 2000, Leica Microsystems, Wetzlar, Germany) at 20x magnification. The failure modes of each specimen were classified based on the following criteria . Adhesive failure: >75% of the failure area occurred between 3D-printing and relining materials. Cohesive failure: >75% of the failure area occurred within 3D-printing or relining materials. Mixed failure: mixed with adhesive failure, cohesive failure between 25% and 75% of the failure area. Additionally, the representative specimens were examined under a scanning electron microscope (SEM; JSM-IT500HR, JEOL, Tokyo, Japan). The specimens were coated with Au (Gold sputtering unit, JEOL, Akishima, Japan). The test was performed using the accelerated voltage of 20 kV with a working distance of approximately 10 mm and magnification of 50×. Statistical analysis Values in the study were reported as mean and standard deviation. Data were analyzed using GraphPad Prism for MacOS version 10.3.0 (GraphPad Software, San Diego, CA, USA). The data were determined for normality using the Shapiro-Wilk test. Then, the shear bond strength between groups was compared using one-way ANOVA followed by the Tukey HSD test. For failure mode analysis, the results were analyzed using the Chi-Square test. A p -value of less than 0.05 was considered statistically significant. The sample size was calculated using G*Power version 3.1.9.6 (University of Düsseldorf, Düsseldorf, Germany)  based on the results from a pilot test. The result indicated that the sample size ( n = 15 per group) for the SBS test should provide power greater than 0.95 at alpha = 0.05 for one-way ANOVA. 

The materials and chemicals used in the current study are provided in Table . The CAD software (Meshmixer, Version 3.5, Autodesk Inc, San Francisco, CA, USA) was used to digitally design the acrylic resin provisional specimens (P Pro crown and bridge, Straumann, Basel, Switzerland) into squares (9 × 9 × 2 mm 3 ). The file format was exported as Standard Tessellation Language (STL) and loaded into the CAM software for a 3D printing process. The specimens ( n = 15 per group) were 3D-printed and positioned vertically with supports. The 3D printing was conducted using a DLP-based 3D printer (P30 RapidShape, Straumann, Basel, Switzerland). The 3D-printed specimens were post-processed according to the manufacturer’s recommendations, which involved cleaning in the automatic cleaning unit (P-wash, Straumann, Basel, Switzerland) with isopropanol. Then, the specimens were polymerized in an automatic curing unit (P-cure, Straumann, Basel, Switzerland). All specimens were embedded with self-curing epoxy resin in a polyvinyl chloride tube and left to polymerize at room temperature for 24 h. Then, the surface of each specimen was polished using 220-grit silicon carbide paper, followed by 500, 800, and 1000-grit silicon carbide paper to standardize the roughness of the surfaces. The polishing process was conducted with the grinding machine (Tegamin-25, Struers, Cleveland, OH, USA) for 15 s at 200 rpm under running water . After polishing, the specimens were ultrasonically cleaned for 10 min to eliminate any remaining silicon carbide paper residues. The experimental design of the current study is illustrated in Fig. . The polished specimens were randomly allocated to 5 groups with different surface treatments as follows (Fig. ). Group 1 (CT): No surface treatment as a control. Group 2 (ET): Applied 95% ethanol for 3 min by micro brush and then air-dried for 10 s . Group 3 (UA): Applied universal adhesive (CLEARFIL Universal Bond Quick, Kuraray Noritake Dental, Tokyo, Japan) using an applicator in a rubbing motion for 10 s, dried with blowing mild air until the adhesive did not move. Then, light-cured polymerized was performed for 10 s using a light curing unit (Elitedent Q-4 LED, Rolence, Taoyuan City, Taiwan ROC) with a light intensity of 1400 mW/cm 2 and a wavelength of 440–480 nm . Group 4 (AA): Airborne-particle abrasion with 50 μm aluminum oxide (Al 2 O 3 ) particles at 1.5 bar pressure for 10 s with a distance of 10 mm from the specimen’s surface. Then, the specimens were ultrasonically cleaned for 10 min to remove remnants of aluminum oxide particles. Group 5 (AA-UA): The specimens were treated with airborne-particle abrasion similar to group 4, followed by the application of universal adhesive similar to that of group 3. The specimens in each group were then bonded with the bis-acryl-based relining material (Protemp 4, 3 M ESPE, St. Paul, MN, USA). In brief, polyethylene tubes (Tygon 3350 tubing, Saint Gobain Co, Courbevoie, France) with an internal diameter of 5 mm and 4 mm in height were placed in the center of each surface-treated specimen. Bis-acryl-based relining material was inserted into the cylindrical tubes using dispenser gun and impression mixing tip. All specimens were left at room temperature for 24 h, allowing the complete polymerization of the relining material. Then, the cylindrical tubes were removed by a No.11 surgical blade to prevent pre-stress. 

After 24 h of relining material application, the macro shear bond strength was measured using a universal testing machine (AGS-X, SHIMADZU, Kyoto, Japan). The specimens were placed in a shear bond strength testing jig (knife-edge blade), and the load was applied to the interface between the relining material and the 3D-printed provisional restoration. The test was performed using a 50 N load cell and a crosshead speed of 1 mm/min. The load at failure (F, Newton) was recorded and the shear bond strength (SBS, MPa) was calculated using Eq. . 1 [12pt]{minimal}
				
				 
				 
				 
				
				
				
				
				$$\:SBS=\:F/A$$ Where F is the failure load (N) and A is the bonded area (m 2 ). 

The fracture surfaces of the tested specimens were examined under a stereomicroscope (Leica Zoom 2000, Leica Microsystems, Wetzlar, Germany) at 20x magnification. The failure modes of each specimen were classified based on the following criteria . Adhesive failure: >75% of the failure area occurred between 3D-printing and relining materials. Cohesive failure: >75% of the failure area occurred within 3D-printing or relining materials. Mixed failure: mixed with adhesive failure, cohesive failure between 25% and 75% of the failure area. Additionally, the representative specimens were examined under a scanning electron microscope (SEM; JSM-IT500HR, JEOL, Tokyo, Japan). The specimens were coated with Au (Gold sputtering unit, JEOL, Akishima, Japan). The test was performed using the accelerated voltage of 20 kV with a working distance of approximately 10 mm and magnification of 50×. 

Values in the study were reported as mean and standard deviation. Data were analyzed using GraphPad Prism for MacOS version 10.3.0 (GraphPad Software, San Diego, CA, USA). The data were determined for normality using the Shapiro-Wilk test. Then, the shear bond strength between groups was compared using one-way ANOVA followed by the Tukey HSD test. For failure mode analysis, the results were analyzed using the Chi-Square test. A p -value of less than 0.05 was considered statistically significant. The sample size was calculated using G*Power version 3.1.9.6 (University of Düsseldorf, Düsseldorf, Germany)  based on the results from a pilot test. The result indicated that the sample size ( n = 15 per group) for the SBS test should provide power greater than 0.95 at alpha = 0.05 for one-way ANOVA. 

The shear bond strength between 3D-printed materials and relining materials is displayed in Fig. . The shear bond strength was a statistically significant difference amongst the experimental groups ( p < 0.001). AA-UA group exhibited statistically significant highest bond strength (14.39 ± 1.07 MPa), followed by AA group (6.39 ± 0.59 MPa), UA group (3.34 ± 0.21 MPa), and ET group (2.11 ± 0.23 MPa), respectively. However, no statistically significant difference was observed between the ET and CT groups ( p = 0.8963). The mode of failure is displayed in Fig. . Optical and SEM images are displayed in Fig. and Fig. . The frequency of failure distribution varied significantly across the experimental groups ( p < 0.01). Adhesive failures were dominantly observed in the CT group (93.33%), ET group (86.67%), and UA group (86.67%). However, cohesive failures were predominantly observed in the AA group (66.67%). The AA-UA group showed cohesive failures (53.33%) and mixed failures (46.67%). SEM images of the fracture surfaces observed from the representation specimens demonstrated smooth fracture surfaces (Fig. ). Groups CT and ET showed relatively smooth surface of the 3D-printed material, indicating the adhesive failures where the relining material was completely dislodged. The SEM image of group UA represented a smooth specimen surface with distinctive areas, which could be the universal adhesive. In addition, the irregular surfaces with remnants of adhesive or relining materials were detected on the surface of the group AA, and AA-UA, indicating mixed failures. 

In this study, a significant different of the shear bond strength between 3D-printed provisional and bis-acryl based relining materials were observed. Therefore, the null hypothesis was rejected. However, it should be noted that the current study was an in vitro study, and the clinical relevance should be interpreted cautiously. No surface treatment or using only ethanol to clean the surface of the 3D-printed provisional material when relined with bis-acryl-based material showed the lowest bond strengths. These low bond strengths were reflected in the high frequency of adhesive failure at the bonding interfaces. This is in agreement with previous studies that demonstrated a significant correlation between the low shear bond strength and adhesive failure at the bonding interfaces . Theoretically, the post-polymerization treatment of 3D-printing provisional restoration (cleaning with ethanol) was conducted to remove unreacted toxic methacrylate monomers . The post-curing process might reduce the uncured monomers, lowering the availability of vinyl groups (C = C) at the surface to bond with the relined resin-based materials . Therefore, we hypothesized that after the post-curing process, only the remaining unreacted monomers might promote co-polymerization between 3D-printing provisional restoration and the relining bis-acryl-based materials. This might partially lead to the low level of bond strength observed in ET and CT groups compared with other groups. Universal adhesive could enhance the bond strength up to 74% compared to the control group. The bond strength of universal adhesive applied on the surface of 3D-printed provisional material when relining with bis-acryl based material might derived from HEMA, Bis-GMA, and 10-MDP that polymerized with methacrylate-based monomers in 3D-printed provisional material . However, the bond strength was still lower than that of the AA and AA-UA groups, in which the mechanical retentions were included. The highest shear bond strength was detected in the AA-UA group, followed by groups AA, UA, and ET. This trend was within the range that was reported from a scoping review that compared the methods for repairing resin composite . Zimmer et al. suggested that the most common methods to improve the bond strength in resin composite repair were blasted with air abrasion and etched with 37% phosphoric acid, followed by the application of a hydrophobic bonding agent or universal adhesive . Therefore, in this study, the bond strength was approximately 4.3 times increased upon the use of universal adhesive combined with airborne-particle abrasion when compared to that of only the application of universal adhesive. The possible mechanism could be that the low-viscosity adhesive could flow and adapt on the irregular sandblasted surface of the 3D printed materials, thus enhancing the wetting and mechanical interlocking of the material. Although the bis-acryl-based relining material had high viscosity, the bond strength can be improved by the airborne-particle abrasion process. It might be possible that the increased irregularities on the surface provided by the alumina particles (50 μm in diameter) also facilitated the adaptation of the relined materials and promoted mechanical locking with the surface . The enhanced bond strength when using the universal adhesive could be additionally due to the chemical bond provided by the phosphate monomers of 10-MDP with the alumina oxide particles that could remain embedded in the resin matrix of the 3D-printing provisional materials . A limitation of the current study was that the specimens were not subjected to an accelerated aging process, which could degrade the bond strength and reflect the long-term performance of the restorations . Therefore, future work should include the use of thermocycling or prolonged immersion time to examine the long-term performance. Another factor that may affect the actual clinical outcome is saliva contamination, which should be examined in future work . In addition, the current study focused on using 3D-printed provisional material to be relined with bis-acryl relining material, which may not represent various materials available on the dental market. Thus, future work should include the other commercially available provisional and relining materials. 

The different interfacial treatments affected the shear bond strength of the relining bis-acryl-based material to the 3D-printed provisional restorations. The bond strength could be improved by using universal adhesive or airborne-particle abrasion. However, the highest shear bond strength was achieved when the surface of 3D-printed provisional material was airborne-particle abraded, followed by the application of universal adhesive. This practical protocol may assist clinicians in repairing the provisional restorations for fixed prosthodontic treatments, providing them with a clear path for improving bond strength in their practice. 

