Adhesive Performance Assessment of Universal Adhesives and Universal Adhesive/Composite Cement Combinations
Bonding Efficiency/Durability of UAs to Flat Dentin (Project Part 1a) Based on the informed consent approved by the Commission of Medical Ethics of KU Leuven with file number S64350, thirty non-carious human third molars (n = 10/experimental group) were collected and stored in distilled water at 4°C for at least 1 week and up to 1 month. The bonding efficacy/durability of the two-component self-curing 1-step UA Tokuyama Universal Bond II (TUBII, Tokuyama; Tokyo, Japan), light-curing 1-step UA Scotchbond Universal Plus (SBUp, 3M Oral Care; Seefeld, Germany), and light-curing 2-step UA G2-Bond Universal (G2B, GC; Tokyo, Japan) was investigated. The collected teeth were randomly allocated into three experimental groups with 10 teeth per group. After cutting the occlusal third of tooth crowns at the level of mid-coronal dentin using a slow-speed diamond saw (Micracut 151, Metkon; Bursa, Turkey), a shallow 300-µm groove was cut at the center of the exposed dentin surface to divide the surface into two equal parts. A standardized smear layer was next prepared using a high-speed medium-grit diamond bur (882.314.014, Komet; Lemgo, Germany), mounted in the MicroSpecimen Former (University of Iowa; Iowa City, IA, USA). Any tooth with remaining enamel or exposure of pulp tissue was excluded upon checking the exposed dentin surface using a stereomicroscope (Stemi 2000-CS, Zeiss; Oberkochen, Germany). Before having applied the dental adhesives, the teeth were warmed in 100% humidity at 37°C for at least 30 min. The split-tooth design was employed by placing a thin razor blade into the groove to separate the dentin surface into halves. Half of the dentin surface was etched with phosphoric acid (Tokuyama Etching Gel HV, Tokuyama) for 15 s and rinsed with water for 15 s; this was the E&R half. The other half was not treated with phosphoric acid and served as the SE half. Upon removal of the blade, the whole dentin surface was rinsed and gently dried. The adhesives investigated were next applied on the exposed dentin surface according to the respective manufacturer’s instructions . Next, the restorative resin composite Estelite Asteria (Tokuyama) was applied in 3 layers up to a height of 5 mm, each layer was cured for 20 s, followed by additional light curing for 20 s from each lateral side and the top side. All light curing was done using the LED light-curing unit SmartLite Pro (Dentsply Sirona; Konstanz, Germany), having a light output of at least 1200 mW/cm 2 , as confirmed regularly using a Marc Resin Calibrator (BlueLight Analytics; Halifax, Canada). The specimens were next immediately stored in 100% humidity at 37°C for 24 h, before being transferred to pre-warmed 37°C distilled water for 6 days. Before the actual test, each tooth was sectioned using a diamond saw (Accutom-50, Struers; Ballerup, Denmark) to obtain micro(μ)-specimens with a base area of 1x1 mm and a height of 8–10 mm. The central six μ-specimens from each half were checked using a stereomicroscope to ensure that the interface did not include enamel. One-third of the μ-specimens were subjected to μTBS testing immediately without thermocycling (0kTC), one-third of μ-specimens were tested after aging by 25,000 thermocycles (25kTC), while the remaining μ-specimens were tested after undergoing aging by 50,000 thermocycles (50kTC). After measuring the dimensions with a digital caliper (Holex, Hoffmann Group; Munich, Germany), each μ-specimen was glued with cyanoacrylate glue (Model Repair II Blue Dentsply-Sankin; Ohtawara, Japan) to BIOMAT jigs and stressed using an LRX testing device (Lloyd; Hampshire, UK) equipped with a load cell of 100 N and a crosshead speed of 1 mm/min until failure to record the force in N. All μ-specimens that failed before testing were regarded as pre-test failures (ptfs) and were included in the mean calculation as 0 MPa. The μTBS-testing protocol strictly followed the Academy of Dental Materials guidelines. The fractured halves of each μ-specimen were collected after μTBS testing, upon which the failure mode was determined using a stereomicroscope as one of four different types: adhesive interfacial failure, cohesive failure in dentin, cohesive failure in composite, and mixed failure (when multiple failure patterns were observed on the failed surface without any single failure pattern accounting for more than 85% of the whole area). Following conventional processing of SEM specimens, including fixation (2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer), gradual dehydration in ethanol (25%, 50%, 75%, 95%, 100%), and drying with hexamethyldisilazane (HMDS), specimens representing each experimental group were selected and gold-sputter coated (JFC-1300, Jeol; Tokyo, Japan) before being examined using scanning electron microscopy (SEM, JSM-6610LV, Jeol). Transmission Electron Microscopic (TEM) Adhesive-Dentin Interfacial Characterization (Project Part 1b) Flat dentin surfaces, originating from two teeth per UA, were prepared as detailed above except for the smear layer, which was produced in a standardized way by wet-polishing using P600-grit SiC-paper (WS Flex 18C, Hermes Schleifmittel; Hamburg, Germany) in a grinding/polishing machine (Buehler Beta Grinder-polisher, Buehler; Lake Bluff, IL, USA). The UAs G2B, SBUp and TUBII were again applied to the exposed dentin surface in E&R and SE bonding mode according to the respective manufacturer’s instructions . One layer of the adhesive resin Clearfil SE Bond 2 (Kuraray Noritake; Tokyo, Japan) approximately 1 mm thick was applied on top of the UA and light cured for 20 s. The teeth were next stored in distilled water at 37°C for 1 week before being sectioned into 0.6- to 0.8-mm-thick slabs. Non-demineralized and demineralized specimens were processed for TEM according to a procedure described in detail elsewhere, including 38-h immersion in acidic Gooding & Stewart fluid (Prosan; Gent, Belgium) for demineralized specimens, fixation in 2.5% glutaraldehyde in 0.1 M sodium cacodylate for at least 12 h, rinsing in 0.1 M sodium cacodylate buffer for 1 min with 3 changes, dehydration in ascending grades of ethanol (25%, 50%, 75%, 95%, 100%) 2 times each and 10 min per treatment, immersion in 99% propylene oxide 3 times with 10 min per treatment, and embedding in epoxy embedding medium (Sigma-Aldrich; St Louis, MO, USA). Ultra-thin sections of 70-90 nm, originating from at least 2 slabs and 2 teeth, were prepared using an ultramicrotome (Ultracut UCT, Leica; Vienna, Austria) equipped with a 45-degree diamond knife (Diatome; Nidau, Switzerland). These were then examined with TEM (JEM-1400 Flash, Jeol), unstained (for non-demineralized sections) or positively stained (for demineralized sections) with UranyLess (Electron Microscopy Sciences; Hatfield, PA, USA) for 8 min and lead citrate (Electron Microscopy Sciences) for 3 min. Adhesive Luting Efficacy of UA/CC Combinations to Flat Dentin (Project Part 2) This project part investigated the adhesive luting efficacy of the two-component self-curing 1-step UA TUBII (Tokuyama) in combination with the dual-curing CC Estecem II Plus (ECIIp; Tokuyama) and the light-curing 1-step UA SBUp (3M Oral Care) in combination with the dual-curing CC RelyX Universal (RxU; 3M Oral Care). Anticipating a worse-case scenario considering the abovementioned light-curing benefits for CC and the above-discussed debate on the need for separate light curing of the adhesive prior to luting, the UA/CC combinations were applied on dentin following not only an AA curing mode, indicating that both UA and CC were allowed to auto-cure and were not additionally light cured, but also an AL curing mode, indicating that UA was allowed to auto-cure while CC was additionally light cured upon luting the composite block. Forty non-carious human third molars (n = 10/experimental group) were collected and prepared as detailed above, except for the smear layer that was produced in a standardized way by wet-polishing using P600-grit SiC-paper in the grinding/polishing machine. P600-grit SiC paper corresponds to the use of a red-banded (46-grit) diamond, typically used clinically as the final bur when finishing tooth preparation for semi-direct/indirect tooth restorations. CAD-CAM composite blocks (Estelite Block II, Tokuyama) were sectioned using the high-speed diamond saw to produce 8x8x5 mm blocks. The block surface to be bonded was additionally ground using P600-grit SiC-paper, followed by sandblasting (Cojet Prep, 3M Oral Care) using Cojet Sand (30-µm silica-coated aluminum oxide sand; 3M Oral Care) with 0.2-MPa pressure for 15 s while keeping a 1-cm distance, and then cleaned in distilled water for 10 min using an ultrasonic bath (Branson 5800, Branson; Danbury, CT, USA). Before luting, the composite-block surface to be luted was coated with the respective UA without light curing, while kept shielded from light. This procedure was recommended by the application instructions of both UAs. The dental adhesive was next applied on the exposed dentin surface according to the manufacturer’s instructions , without separate light curing following both the AA and AL curing mode. Automix composite cement was deposited on the pretreated composite block, upon which the block was positioned on dentin; once seated, it was loaded with 1 kg for 1 min using a BIOMAT custom-made loading device to ensure the load was applied at the block’s center. Only a small but sufficient amount of CC was applied to limit cement excess, which did not require removal of cement excess. Immediately upon loading, glycerine air-block gel (Aquasonic 100, Parker Laboratories; Fairfield, CT, USA) was applied to the cement to reduce polymerization inhibition by oxygen. After luting, the specimens were kept in the dark following the AA curing mode (8 min for TUBII/ECIIp and 6 min for SBUp/RxU, as per setting time indicated by the respective manufacturer), or – following the AL curing mode – light cured for 10 s from each lateral aspect as well as the top (50 s in total) using the LED light-curing unit SmartLite Pro (Dentsply Sirona). After removing the air-block gel, the specimens were stored in 100% humidity at 37°C for 24 h, before being placed in pre-warmed 37°C water for 6 days. The specimens were subsequently sectioned and tested using the same μTBS testing protocol, as detailed above. The fractured halves of each μ-specimen were collected after μTBS testing, and the failure mode was determined as one of 6 types using the stereomicroscope: cohesive failure in dentin, adhesive failure at the cement-dentin interface, cohesive failure in cement, adhesive failure at the cement-composite interface, cohesive failure in composite, and mixed failure. Representative SEM specimens from all experimental groups were prepared and examined following the same protocol as described above. Adhesive Luting Efficiency of UA/CC Combinations to Lithium-Disilicate Glass-Ceramic (Project Part 3) This project part investigated the adhesive luting efficacy of the same two UA/CC combinations (TUBII/ECIIp, SBUp/RxU) to IPS e.max CAD (Ivoclar; Schaan, Liechtenstein) blocks when treated with or without hydrofluoric acid (HF). Cross-sectioned glass-ceramic blocks, embedded in acrylic resin, were ground for 1 min in the grinding/polishing machine using P600-grit SiC paper. All surfaces were carefully verified for homogeneous surface treatment, upon which they were randomly divided into 8 experimental groups (n = 10/experimental group). Prior to the adhesive luting procedures, the IPS e.max CAD (Ivoclar) blocks were warmed to 37°C in an incubator at 100% humidity for at least 30 min. A hole with a 3-mm diameter was punched in a piece of double-sided tape prior to sticking the tape to the prepared glass-ceramic surface, with the hole centered in respect to the exposed area. The glass-ceramic blocks were either left without further pre-treatment (NT: no treatment) or were etched with Porcelain Etch (HF: hydro-fluoric-acid etchant, Ultradent; South Jordan, UT, USA) for 30 s. For the HF groups, HF-etching gel was applied on the surface delimited by the hole in the tape for 30 s, after which the sample was thoroughly rinsed under a stream of tap water for 1 min. The specimens were next dried with clean and oil-free air. The respective UA was then applied on the etched/non-etched glass-ceramic surface according to the manufacturer’s instructions . The UAs were not separately light cured. A small portion of automix cement was then applied to the specimen area delimited by the hole in the tape using the automix syringe; SBS testing was performed in a standardized Ultradent jig (Ultradent). Upon placement, the cement (along with the UA) was light cured for 20 s using the LED light-curing unit SmartLite Pro (Dentsply Sirona). Cement excess was carefully removed after light curing using a razor blade. After preparation, all specimens were stored in 100% humidity at 37°C for 1 h, after which the specimens were transferred into pre-warmed water (37°C) for 1 week. After 1 week, all immediate specimens were subjected to a shear bond-strength (SBS) test using a universal testing machine (Instron 5848 MicroTester; Norwood, MA, USA), when applying a crosshead speed of 1 mm/min until failure occurred. For the actual SBS test, the specimens were positioned in the Ultradent specimen holder according to the ISO 29022 standard. All fractured specimens were collected and observed using the stereomicroscope to determine the failure modes (one of four types): cohesive failure in glass-ceramic, adhesive failure at the cement-ceramic interface, cohesive failure in cement, and mixed failure. Statistical Analysis Two different linear mixed-effects models (LME; R-4.0.3; R Foundation for Statistical Computing, Vienna, Austria) were built to statistically analyze the outcome of the two μTBS studies at a significance level of α = 0.05: LME model A (LME-A) in Project Part 1a (μTBS of UAs), involving the three variables ‘adhesive’, ‘aging’ and ‘bonding mode’ as fixed factors, and the variable ‘tooth’ as a random factor; LME model B (LME-B) for Project Part 2 (μTBS of UA/CC combinations), involving the four variables ‘adhesive’, ‘aging’, ‘bonding mode’ and ‘curing’ as fixed factors, and the variable ‘tooth’ as a random factor. For Project Part 3, the SBS data were statistically analyzed using the Kruskal-Wallis test followed by post-hoc Dunn’s test with Bonferroni correction and the Mann-Whitney U-test at a significance level of α=0.05.   Based on the informed consent approved by the Commission of Medical Ethics of KU Leuven with file number S64350, thirty non-carious human third molars (n = 10/experimental group) were collected and stored in distilled water at 4°C for at least 1 week and up to 1 month. The bonding efficacy/durability of the two-component self-curing 1-step UA Tokuyama Universal Bond II (TUBII, Tokuyama; Tokyo, Japan), light-curing 1-step UA Scotchbond Universal Plus (SBUp, 3M Oral Care; Seefeld, Germany), and light-curing 2-step UA G2-Bond Universal (G2B, GC; Tokyo, Japan) was investigated. The collected teeth were randomly allocated into three experimental groups with 10 teeth per group. After cutting the occlusal third of tooth crowns at the level of mid-coronal dentin using a slow-speed diamond saw (Micracut 151, Metkon; Bursa, Turkey), a shallow 300-µm groove was cut at the center of the exposed dentin surface to divide the surface into two equal parts. A standardized smear layer was next prepared using a high-speed medium-grit diamond bur (882.314.014, Komet; Lemgo, Germany), mounted in the MicroSpecimen Former (University of Iowa; Iowa City, IA, USA). Any tooth with remaining enamel or exposure of pulp tissue was excluded upon checking the exposed dentin surface using a stereomicroscope (Stemi 2000-CS, Zeiss; Oberkochen, Germany). Before having applied the dental adhesives, the teeth were warmed in 100% humidity at 37°C for at least 30 min. The split-tooth design was employed by placing a thin razor blade into the groove to separate the dentin surface into halves. Half of the dentin surface was etched with phosphoric acid (Tokuyama Etching Gel HV, Tokuyama) for 15 s and rinsed with water for 15 s; this was the E&R half. The other half was not treated with phosphoric acid and served as the SE half. Upon removal of the blade, the whole dentin surface was rinsed and gently dried. The adhesives investigated were next applied on the exposed dentin surface according to the respective manufacturer’s instructions . Next, the restorative resin composite Estelite Asteria (Tokuyama) was applied in 3 layers up to a height of 5 mm, each layer was cured for 20 s, followed by additional light curing for 20 s from each lateral side and the top side. All light curing was done using the LED light-curing unit SmartLite Pro (Dentsply Sirona; Konstanz, Germany), having a light output of at least 1200 mW/cm 2 , as confirmed regularly using a Marc Resin Calibrator (BlueLight Analytics; Halifax, Canada). The specimens were next immediately stored in 100% humidity at 37°C for 24 h, before being transferred to pre-warmed 37°C distilled water for 6 days. Before the actual test, each tooth was sectioned using a diamond saw (Accutom-50, Struers; Ballerup, Denmark) to obtain micro(μ)-specimens with a base area of 1x1 mm and a height of 8–10 mm. The central six μ-specimens from each half were checked using a stereomicroscope to ensure that the interface did not include enamel. One-third of the μ-specimens were subjected to μTBS testing immediately without thermocycling (0kTC), one-third of μ-specimens were tested after aging by 25,000 thermocycles (25kTC), while the remaining μ-specimens were tested after undergoing aging by 50,000 thermocycles (50kTC). After measuring the dimensions with a digital caliper (Holex, Hoffmann Group; Munich, Germany), each μ-specimen was glued with cyanoacrylate glue (Model Repair II Blue Dentsply-Sankin; Ohtawara, Japan) to BIOMAT jigs and stressed using an LRX testing device (Lloyd; Hampshire, UK) equipped with a load cell of 100 N and a crosshead speed of 1 mm/min until failure to record the force in N. All μ-specimens that failed before testing were regarded as pre-test failures (ptfs) and were included in the mean calculation as 0 MPa. The μTBS-testing protocol strictly followed the Academy of Dental Materials guidelines. The fractured halves of each μ-specimen were collected after μTBS testing, upon which the failure mode was determined using a stereomicroscope as one of four different types: adhesive interfacial failure, cohesive failure in dentin, cohesive failure in composite, and mixed failure (when multiple failure patterns were observed on the failed surface without any single failure pattern accounting for more than 85% of the whole area). Following conventional processing of SEM specimens, including fixation (2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer), gradual dehydration in ethanol (25%, 50%, 75%, 95%, 100%), and drying with hexamethyldisilazane (HMDS), specimens representing each experimental group were selected and gold-sputter coated (JFC-1300, Jeol; Tokyo, Japan) before being examined using scanning electron microscopy (SEM, JSM-6610LV, Jeol).   Flat dentin surfaces, originating from two teeth per UA, were prepared as detailed above except for the smear layer, which was produced in a standardized way by wet-polishing using P600-grit SiC-paper (WS Flex 18C, Hermes Schleifmittel; Hamburg, Germany) in a grinding/polishing machine (Buehler Beta Grinder-polisher, Buehler; Lake Bluff, IL, USA). The UAs G2B, SBUp and TUBII were again applied to the exposed dentin surface in E&R and SE bonding mode according to the respective manufacturer’s instructions . One layer of the adhesive resin Clearfil SE Bond 2 (Kuraray Noritake; Tokyo, Japan) approximately 1 mm thick was applied on top of the UA and light cured for 20 s. The teeth were next stored in distilled water at 37°C for 1 week before being sectioned into 0.6- to 0.8-mm-thick slabs. Non-demineralized and demineralized specimens were processed for TEM according to a procedure described in detail elsewhere, including 38-h immersion in acidic Gooding & Stewart fluid (Prosan; Gent, Belgium) for demineralized specimens, fixation in 2.5% glutaraldehyde in 0.1 M sodium cacodylate for at least 12 h, rinsing in 0.1 M sodium cacodylate buffer for 1 min with 3 changes, dehydration in ascending grades of ethanol (25%, 50%, 75%, 95%, 100%) 2 times each and 10 min per treatment, immersion in 99% propylene oxide 3 times with 10 min per treatment, and embedding in epoxy embedding medium (Sigma-Aldrich; St Louis, MO, USA). Ultra-thin sections of 70-90 nm, originating from at least 2 slabs and 2 teeth, were prepared using an ultramicrotome (Ultracut UCT, Leica; Vienna, Austria) equipped with a 45-degree diamond knife (Diatome; Nidau, Switzerland). These were then examined with TEM (JEM-1400 Flash, Jeol), unstained (for non-demineralized sections) or positively stained (for demineralized sections) with UranyLess (Electron Microscopy Sciences; Hatfield, PA, USA) for 8 min and lead citrate (Electron Microscopy Sciences) for 3 min.   This project part investigated the adhesive luting efficacy of the two-component self-curing 1-step UA TUBII (Tokuyama) in combination with the dual-curing CC Estecem II Plus (ECIIp; Tokuyama) and the light-curing 1-step UA SBUp (3M Oral Care) in combination with the dual-curing CC RelyX Universal (RxU; 3M Oral Care). Anticipating a worse-case scenario considering the abovementioned light-curing benefits for CC and the above-discussed debate on the need for separate light curing of the adhesive prior to luting, the UA/CC combinations were applied on dentin following not only an AA curing mode, indicating that both UA and CC were allowed to auto-cure and were not additionally light cured, but also an AL curing mode, indicating that UA was allowed to auto-cure while CC was additionally light cured upon luting the composite block. Forty non-carious human third molars (n = 10/experimental group) were collected and prepared as detailed above, except for the smear layer that was produced in a standardized way by wet-polishing using P600-grit SiC-paper in the grinding/polishing machine. P600-grit SiC paper corresponds to the use of a red-banded (46-grit) diamond, typically used clinically as the final bur when finishing tooth preparation for semi-direct/indirect tooth restorations. CAD-CAM composite blocks (Estelite Block II, Tokuyama) were sectioned using the high-speed diamond saw to produce 8x8x5 mm blocks. The block surface to be bonded was additionally ground using P600-grit SiC-paper, followed by sandblasting (Cojet Prep, 3M Oral Care) using Cojet Sand (30-µm silica-coated aluminum oxide sand; 3M Oral Care) with 0.2-MPa pressure for 15 s while keeping a 1-cm distance, and then cleaned in distilled water for 10 min using an ultrasonic bath (Branson 5800, Branson; Danbury, CT, USA). Before luting, the composite-block surface to be luted was coated with the respective UA without light curing, while kept shielded from light. This procedure was recommended by the application instructions of both UAs. The dental adhesive was next applied on the exposed dentin surface according to the manufacturer’s instructions , without separate light curing following both the AA and AL curing mode. Automix composite cement was deposited on the pretreated composite block, upon which the block was positioned on dentin; once seated, it was loaded with 1 kg for 1 min using a BIOMAT custom-made loading device to ensure the load was applied at the block’s center. Only a small but sufficient amount of CC was applied to limit cement excess, which did not require removal of cement excess. Immediately upon loading, glycerine air-block gel (Aquasonic 100, Parker Laboratories; Fairfield, CT, USA) was applied to the cement to reduce polymerization inhibition by oxygen. After luting, the specimens were kept in the dark following the AA curing mode (8 min for TUBII/ECIIp and 6 min for SBUp/RxU, as per setting time indicated by the respective manufacturer), or – following the AL curing mode – light cured for 10 s from each lateral aspect as well as the top (50 s in total) using the LED light-curing unit SmartLite Pro (Dentsply Sirona). After removing the air-block gel, the specimens were stored in 100% humidity at 37°C for 24 h, before being placed in pre-warmed 37°C water for 6 days. The specimens were subsequently sectioned and tested using the same μTBS testing protocol, as detailed above. The fractured halves of each μ-specimen were collected after μTBS testing, and the failure mode was determined as one of 6 types using the stereomicroscope: cohesive failure in dentin, adhesive failure at the cement-dentin interface, cohesive failure in cement, adhesive failure at the cement-composite interface, cohesive failure in composite, and mixed failure. Representative SEM specimens from all experimental groups were prepared and examined following the same protocol as described above.   This project part investigated the adhesive luting efficacy of the same two UA/CC combinations (TUBII/ECIIp, SBUp/RxU) to IPS e.max CAD (Ivoclar; Schaan, Liechtenstein) blocks when treated with or without hydrofluoric acid (HF). Cross-sectioned glass-ceramic blocks, embedded in acrylic resin, were ground for 1 min in the grinding/polishing machine using P600-grit SiC paper. All surfaces were carefully verified for homogeneous surface treatment, upon which they were randomly divided into 8 experimental groups (n = 10/experimental group). Prior to the adhesive luting procedures, the IPS e.max CAD (Ivoclar) blocks were warmed to 37°C in an incubator at 100% humidity for at least 30 min. A hole with a 3-mm diameter was punched in a piece of double-sided tape prior to sticking the tape to the prepared glass-ceramic surface, with the hole centered in respect to the exposed area. The glass-ceramic blocks were either left without further pre-treatment (NT: no treatment) or were etched with Porcelain Etch (HF: hydro-fluoric-acid etchant, Ultradent; South Jordan, UT, USA) for 30 s. For the HF groups, HF-etching gel was applied on the surface delimited by the hole in the tape for 30 s, after which the sample was thoroughly rinsed under a stream of tap water for 1 min. The specimens were next dried with clean and oil-free air. The respective UA was then applied on the etched/non-etched glass-ceramic surface according to the manufacturer’s instructions . The UAs were not separately light cured. A small portion of automix cement was then applied to the specimen area delimited by the hole in the tape using the automix syringe; SBS testing was performed in a standardized Ultradent jig (Ultradent). Upon placement, the cement (along with the UA) was light cured for 20 s using the LED light-curing unit SmartLite Pro (Dentsply Sirona). Cement excess was carefully removed after light curing using a razor blade. After preparation, all specimens were stored in 100% humidity at 37°C for 1 h, after which the specimens were transferred into pre-warmed water (37°C) for 1 week. After 1 week, all immediate specimens were subjected to a shear bond-strength (SBS) test using a universal testing machine (Instron 5848 MicroTester; Norwood, MA, USA), when applying a crosshead speed of 1 mm/min until failure occurred. For the actual SBS test, the specimens were positioned in the Ultradent specimen holder according to the ISO 29022 standard. All fractured specimens were collected and observed using the stereomicroscope to determine the failure modes (one of four types): cohesive failure in glass-ceramic, adhesive failure at the cement-ceramic interface, cohesive failure in cement, and mixed failure.   Two different linear mixed-effects models (LME; R-4.0.3; R Foundation for Statistical Computing, Vienna, Austria) were built to statistically analyze the outcome of the two μTBS studies at a significance level of α = 0.05: LME model A (LME-A) in Project Part 1a (μTBS of UAs), involving the three variables ‘adhesive’, ‘aging’ and ‘bonding mode’ as fixed factors, and the variable ‘tooth’ as a random factor; LME model B (LME-B) for Project Part 2 (μTBS of UA/CC combinations), involving the four variables ‘adhesive’, ‘aging’, ‘bonding mode’ and ‘curing’ as fixed factors, and the variable ‘tooth’ as a random factor. For Project Part 3, the SBS data were statistically analyzed using the Kruskal-Wallis test followed by post-hoc Dunn’s test with Bonferroni correction and the Mann-Whitney U-test at a significance level of α=0.05.   Bonding Efficacy/Durability of UAs to Flat Dentin (Project Part 1a) All μTBS data are numerically detailed in and graphically presented as boxplots in . As indicated in , all three variables significantly contributed to the statistical model. For the interactions, only the adhesive x bonding mode interaction was significant, meaning that the bonding mode had a different impact on the μTBS of the different adhesives, and vice versa. No difference in μTBS was recorded when the three UAs were applied in E&R bonding mode. When applied in SE bonding mode, the μTBS of TUBII was significantly lower than that of G2B and SBUp, this for all three 0kTC (control), 25kTC-aged or 50kTC-aged groups. A significant decrease in μTBS upon aging was only recorded for G2B when applied in SE bonding mode and after 50kTC aging. Comparing the E&R versus SE bonding modes, the μTBS of G2B and TUBII was significantly higher when applied in E&R than in SE bonding mode, regardless of the aging period. For SBUp, only upon 50k TC aging, the μTBS was significantly higher when applied in E&R than in SE bonding mode. The failure mode distribution examined by stereomicroscopy for the adhesives bonded to flat dentin is shown in . The failure patterns of G2B, SBUp and TUBII applied in E&R bonding mode to dentin were relatively similar, except that the majority of G2B and SBUp failures were categorized as adhesive interfacial failure, and for TUBII as cohesive failure in composite (except for 0kTC). A less balanced failure mode distribution was recorded when the three adhesives were applied in SE bonding mode, as the predominant failure pattern was adhesive interfacial failure, this even 100% for TUBII. Representative SEM photomicrographs of fractured μ-specimens (dentin side) are presented in , mostly illustrating mixed failure modes. While few interfacial voids were observed along the failed dentin surface when SBUp was applied in SE Bonding mode, the failed surface representing TUBII applied in SE bonding mode disclosed interfacial voids along the entire fractured interface. TEM Adhesive-Dentin Interfacial Characterization (Project Part 1b) Representative TEM photomicrographs of the adhesive-dentin interfacial ultra-structure of G2B, SBUp and TUBII bonded to flat dentin are illustrated in . All adhesives revealed tightly bonded interfaces without interfacial debonding during preparation and imaging of the specimens/sections. A thicker (3-5 μm) hybrid layer was produced by the UAs applied in E&R bonding mode than when they were applied in SE bonding mode (0.3-0.8 μm). The E&R hybrid layer of the UAs was typically HAp-free with an abrupt transition to the underlying unaffected dentin. Collagen fibrils, sporadically exhibiting cross-banding, could be observed within the E&R hybrid layers on images of stained, demineralized sections. The SE hybrid layer produced by the UAs was only partially demineralized, with abundant HAp still surrounding collagen fibrils within the submicron hybrid layer. The transition to unaffected dentin was more gradual. Overall, no major differences in interfacial ultra-structure were observed by TEM for the three UAs investigated when applied either in E&R or SE bonding mode. Adhesive Luting Efficacy of UA/CC Combinations to Flat Dentin (Project Part 2) All μTBS data are numerically detailed in and graphically presented as boxplots in . As indicated in , among the four main variables, only the variable ‘bonding mode’ significantly contributed to the statistical model. For the interactions, only the adhesive x bonding mode x aging interaction was significant, meaning the combination of adhesive and bonding mode varied for the three aging periods. The μTBS of SBUp/RxU and TUBII/ECIIp adhesively luted to flat dentin was not significantly different, whether the adhesive was applied in E&R or SE bonding mode, this when tested immediately (0kTC) or upon aging (25kTC and 50kTC). The curing mode did not have a significant impact on the μTBS, considering no significant difference was found for the same UA/CC combinations applied in either AA or AL curing mode. Aging by 25kTC and 50kTC did not significantly decrease the μTBS of any of the UA/CC combinations. Being the only significant difference recorded, UA/CC combinations applied in E&R bonding mode performed significantly better than when they were applied in SE bonding mode, regardless of UA/CC combination. The failure mode distribution examined by stereomicroscopy for the UA/CC combinations adhesively luted to flat dentin is shown in . Overall, a balanced failure mode distribution was recorded when the UA/CC combinations were adhesively luted in E&R bonding mode, while adhesive failure at the cement-dentin interface was the predominant failure mode recorded when the UA/CC combinations were applied in SE mode. When applied in E&R bonding mode, TUBII/ECIIp presented more adhesive failures at the cement-dentin interface, while SBUp/RxU revealed more adhesive failures at the cement-composite interface. When applied in SE bonding mode, the failure patterns of SBUp/RxU and TUBII/ECIIp were similar. No distinct difference in failure mode was observed when the UA/CC combinations were adhesively luted in either AL or AA curing mode. Representative SEM photomicrographs of fractured μ-specimens are shown in (dentin side) and (composite side). Most striking are the interfacial voids observed nearly along the entire fractured surface for UA/CC specimens applied in SE bonding mode, regardless of UA/CC combination and curing mode. When applied in E&R bonding mode, interfacial voids were less intensively observed . Adhesive Luting Efficacy of UA/CC Combinations to Lithium-Disilicate Glass-Ceramic (Project Part 3) The SBS data are detailed in and graphically presented in , with the failure-mode distribution presented in . In general, SBS of the UA/CC combinations with HF treatment was significantly higher than that of the non-etched/treated UA/CCs, whether for SBUp/RxU or TUBII/ECIIp and when measured immediately and upon 6-month aging. No significant difference was found between the UA/CC combinations. Aging upon 6-month water storage significantly decreased SBS for all UA/CC experimental groups except for TUBII/ECIIp_HF. The most predominant failure pattern was adhesive failure at the cement/ceramic interface, in particular when a low SBS was measured.   All μTBS data are numerically detailed in and graphically presented as boxplots in . As indicated in , all three variables significantly contributed to the statistical model. For the interactions, only the adhesive x bonding mode interaction was significant, meaning that the bonding mode had a different impact on the μTBS of the different adhesives, and vice versa. No difference in μTBS was recorded when the three UAs were applied in E&R bonding mode. When applied in SE bonding mode, the μTBS of TUBII was significantly lower than that of G2B and SBUp, this for all three 0kTC (control), 25kTC-aged or 50kTC-aged groups. A significant decrease in μTBS upon aging was only recorded for G2B when applied in SE bonding mode and after 50kTC aging. Comparing the E&R versus SE bonding modes, the μTBS of G2B and TUBII was significantly higher when applied in E&R than in SE bonding mode, regardless of the aging period. For SBUp, only upon 50k TC aging, the μTBS was significantly higher when applied in E&R than in SE bonding mode. The failure mode distribution examined by stereomicroscopy for the adhesives bonded to flat dentin is shown in . The failure patterns of G2B, SBUp and TUBII applied in E&R bonding mode to dentin were relatively similar, except that the majority of G2B and SBUp failures were categorized as adhesive interfacial failure, and for TUBII as cohesive failure in composite (except for 0kTC). A less balanced failure mode distribution was recorded when the three adhesives were applied in SE bonding mode, as the predominant failure pattern was adhesive interfacial failure, this even 100% for TUBII. Representative SEM photomicrographs of fractured μ-specimens (dentin side) are presented in , mostly illustrating mixed failure modes. While few interfacial voids were observed along the failed dentin surface when SBUp was applied in SE Bonding mode, the failed surface representing TUBII applied in SE bonding mode disclosed interfacial voids along the entire fractured interface.   Representative TEM photomicrographs of the adhesive-dentin interfacial ultra-structure of G2B, SBUp and TUBII bonded to flat dentin are illustrated in . All adhesives revealed tightly bonded interfaces without interfacial debonding during preparation and imaging of the specimens/sections. A thicker (3-5 μm) hybrid layer was produced by the UAs applied in E&R bonding mode than when they were applied in SE bonding mode (0.3-0.8 μm). The E&R hybrid layer of the UAs was typically HAp-free with an abrupt transition to the underlying unaffected dentin. Collagen fibrils, sporadically exhibiting cross-banding, could be observed within the E&R hybrid layers on images of stained, demineralized sections. The SE hybrid layer produced by the UAs was only partially demineralized, with abundant HAp still surrounding collagen fibrils within the submicron hybrid layer. The transition to unaffected dentin was more gradual. Overall, no major differences in interfacial ultra-structure were observed by TEM for the three UAs investigated when applied either in E&R or SE bonding mode.   All μTBS data are numerically detailed in and graphically presented as boxplots in . As indicated in , among the four main variables, only the variable ‘bonding mode’ significantly contributed to the statistical model. For the interactions, only the adhesive x bonding mode x aging interaction was significant, meaning the combination of adhesive and bonding mode varied for the three aging periods. The μTBS of SBUp/RxU and TUBII/ECIIp adhesively luted to flat dentin was not significantly different, whether the adhesive was applied in E&R or SE bonding mode, this when tested immediately (0kTC) or upon aging (25kTC and 50kTC). The curing mode did not have a significant impact on the μTBS, considering no significant difference was found for the same UA/CC combinations applied in either AA or AL curing mode. Aging by 25kTC and 50kTC did not significantly decrease the μTBS of any of the UA/CC combinations. Being the only significant difference recorded, UA/CC combinations applied in E&R bonding mode performed significantly better than when they were applied in SE bonding mode, regardless of UA/CC combination. The failure mode distribution examined by stereomicroscopy for the UA/CC combinations adhesively luted to flat dentin is shown in . Overall, a balanced failure mode distribution was recorded when the UA/CC combinations were adhesively luted in E&R bonding mode, while adhesive failure at the cement-dentin interface was the predominant failure mode recorded when the UA/CC combinations were applied in SE mode. When applied in E&R bonding mode, TUBII/ECIIp presented more adhesive failures at the cement-dentin interface, while SBUp/RxU revealed more adhesive failures at the cement-composite interface. When applied in SE bonding mode, the failure patterns of SBUp/RxU and TUBII/ECIIp were similar. No distinct difference in failure mode was observed when the UA/CC combinations were adhesively luted in either AL or AA curing mode. Representative SEM photomicrographs of fractured μ-specimens are shown in (dentin side) and (composite side). Most striking are the interfacial voids observed nearly along the entire fractured surface for UA/CC specimens applied in SE bonding mode, regardless of UA/CC combination and curing mode. When applied in E&R bonding mode, interfacial voids were less intensively observed .   The SBS data are detailed in and graphically presented in , with the failure-mode distribution presented in . In general, SBS of the UA/CC combinations with HF treatment was significantly higher than that of the non-etched/treated UA/CCs, whether for SBUp/RxU or TUBII/ECIIp and when measured immediately and upon 6-month aging. No significant difference was found between the UA/CC combinations. Aging upon 6-month water storage significantly decreased SBS for all UA/CC experimental groups except for TUBII/ECIIp_HF. The most predominant failure pattern was adhesive failure at the cement/ceramic interface, in particular when a low SBS was measured.   In Project Part 1, the bonding efficacy and interfacial interaction of three UAs (1-step light-curing UA Scotchbond Universal Plus [SBUp, 3M Oral Care], 2-step light-curing UA G2-Bond Universal [G2B, GC], and 1-step self-curing UA Tokuyama Universal Bond II [TUBII, Tokuyama]) were investigated when bonded to flat dentin, in order to determine their best performance when applied in optimal (laboratory) conditions. UAs, like SBUp, were originally designed and developed as simplified one-bottle adhesives that combine the function of the primer and adhesive resin into a single solution with a balanced hydrophilicity/hydrophobicity, to be applied in a simplified 1-step application (2 steps in E&R bonding mode or when selectively etching enamel, a combined E&R-on-enamel/SE-on-dentin bonding mode). Aiming to better hydrophobically seal the adhesive interface at dentin, 2-step two-bottle UAs, like G2B, were more recently developed and involve the separate application of an acidic hydrophilic primer followed by a solvent-free hydrophobic adhesive resin. , Aiming to further simplify the UA application procedure by omitting a separate light-curing step, the self-curing UA TUBII provides a two-bottle UA to be mixed prior to the application without light curing. Besides specific functional monomers, like 4-methacryloxyethyl trimellitic acid (4-MET) for G2B and 3D-SR for TUBII , all three UAs investigated in this study contain 10-MDP, to date the most effective functional monomer. Nevertheless, no detailed technical information regarding the 10-MDP concentration (percentage) or purity (quality) is known, although the latter two parameters on their own (besides other factors) have been reported to substantially influence bonding performance. , Methacrylate-based dental materials set via free-radical addition polymerization. Dental adhesives are generally light curable, containing photo-initiators sensitive to blue light in the 400-515 nm range. Although not always listed by the manufacturer, the most commonly used photo-initiator system combines camphorquinone (CQ) with a tertiary amine, as in SBUp. As the most common chemical polymerization initiator, benzoylperoxide is combined with a tertiary amine, probably as in TUBII, although this information is not unambiguously discernable from the manufacturer’s information . No significant difference in μTBS was detected among the three UAs when they were applied in E&R bonding mode. However, when applied in SE bonding mode, G2B and SBUp significantly outperformed TUBII, for which low μTBS was recorded along with a substantially higher number of ptfs. The first hypothesis, that the bonding efficacy of the self-curing UA would not underperform that of the light-curing adhesives, was hence accepted when applied in E&R bonding mode, but rejected when applied in SE bonding mode. In general, aging did not significantly decrease all of the UAs’ bonding performance (except for G2B applied in SE mode and upon 50k TC). Apart from compositional differences that could result in lower SE bonding effectiveness, the significantly lower bonding effectiveness recorded for TUBII could also be related to its self-curing. Although it is difficult to explain why self-curing would affect the SE bonding mode more than the E&R bonding mode, the much more superficial surface interaction resulting from SE bonding with limited micromechanical interlocking potential (submicron hybrid layer) and even possible smear-layer interference may be a plausible explanation. E&R bonding generates significantly more micromechanical interlocking and completely dissolves/removes all surface smear. Confirming the lower bonding performance of the self-curing UA TUBII applied in SE bonding mode, SEM fractographic analysis revealed not only substantially more adhesive interfacial failures but also disclosed interfacial voids at the adhesive interface that must have weakened the bond and led to bond failure at lower loading . The occurrence of such interfacial voids should be ascribed to entrapment of water absorbed from the underlying dentin through osmosis. , Due to their hydrophilic nature, 1-step adhesives – including 1-step/bottle UAs – act as semi-permeable membranes in an enhanced manner not only prior to polymerization but also even after polymerization, as has been documented previously. In this study, young third molars (originating from young adolescents) were used; their dentin is highly permeable, which may have promoted interfacial water uptake through osmosis. Such interfacial void production has repeatedly been reported before in the case of adhesive luting, when the adhesive is not separately light cured and solely co-(light) cured with the CC upon seating of the restoration and partial/full cement-excess removal. Delayed curing of the adhesive interface gives water time to be absorbed (see below regarding the Project Part 2 discussion). According to the manufacturer’s instructions, TUBII is advised to be applied within 3 min after mixing the two components, confirming its relatively slow self-curing. Therefore, compared with G2B and SBUp, which were immediately light cured, TUBII was delay-cured, by which the adhesive interface was not immediately stabilized, allowing substantial water droplets to be incorporated at the interface. According to the manufacturer, remaining solvents within TUBII will delay its self-curing, for which reason solvents should be removed as much as possible without over-thinning the adhesive-resin layer. Voids were also observed at the adhesive interface of SBUp with dentin, especially when SBUp was applied in SE bonding mode. This can probably be ascribed to its hydrophilic combined primer/adhesive resin one-bottle design, while much fewer interfacial voids were detected, most likely prevented by immediate light curing. No interfacial voids were observed by TEM, as G2B and SBUp were immediately light cured upon the respective manufacturer’s instructions, while also TUBII must have cured relatively quickly, at least faster when compared to its application in the μTBS study, since a 1-mm-thick adhesive-resin layer of Clearfil SE Bond 2 (Kuraray Noritake) was applied on top of TUBII and immediately light cured for 20 s. The latter may even have resulted in co-curing of TUBII. Nevertheless, when TUBII was applied in E&R bonding mode, fewer or even no osmosis-induced interfacial voids were observed on the fractured surface of μ-specimens or were only found in a small region. While TUBII applied in E&R bonding mode was also allowed to self-cure with a time delay, favorable bonding effectiveness and bond durability were recorded, comparable to that of the two other light-curing UAs. As very well known and once more confirmed by TEM in Project Part 1b, a thick (3-5 µm) E&R hybrid layer vs a submicron SE hybrid layer was formed, depending on whether the UA was applied following an E&R or an SE bonding mode, respectively. The thicker E&R hybrid layer may have slowed down water absorption from the underlying dentin to reach the adhesive interface, potentially being a plausible reason for the higher μTBS of TUBII recorded when applied in E&R instead of SE bonding mode. It is noteworthy that pulpal pressure in vital teeth can accelerate fluid flow towards the adhesive interface, , while it may not have relevant unfavorable interfacial effect in case of relatively impermeable sclerotic/old dentin; hardly any water sorption will occur in non-vital teeth. Further experiments should be conducted to assess whether pulpal pressure can possibly over-wet the dentin surface and impair the adhesion of TUBII even when used in E&R bonding mode. The lower bond strength of TUBII applied in SE bonding mode, as compared to that of G2B and SBUp, may also be associated with compositional differences between the UAs investigated. All three UAs contain 10-MDP, a highly effective monomer that forms insoluble 10-MDP/Ca salts and therefore is today widespread in recent adhesives, particularly UAs. , Previous research reported that 10-MDP forms stable chemical/ionic bonds to HAp, which is important in the wet oral environment to achieve durable adhesion. The functional monomer 10-MDP is more effective than other functional monomers, e.g., 4-MET, as also contained in G2B, and 2-methacryloxyethyl phenyl hydrogen phosphate (phenyl-P). Compared with SBUp and TUBII, in this study, TEM revealed more prominent nano-layering for G2B, which might be ascribed to different 10-MDP concentrations contained in the respective UAs investigated. Elaborating further on the speculative explanation that G2B might contain more 10-MDP, SBUp nevertheless did not present significantly lower SE bonding effectiveness, despite an assumed lower concentration of 10-MDP. This may indicate that SBUp was sufficiently optimized for the functional monomer concentration, while TUBII was much less; again, this is a speculative explanation, as the UA’s detailed composition is not fully revealed. Self-curing restorative materials were used in dentistry before the introduction of polymer-based dental materials and light curing. With the development of photo-initiators and more recent, more efficient LED light-curing units, the setting time of light-curing dental materials has dramatically decreased and their curing efficacy has substantially improved. Most resin-based dental materials provide this advantageous ‘on-command’ (light-)curing, except for specific indications when dual-curing provides additional clinical benefits, such as in the case of adhesively luting semi-direct and indirect composite or ceramic (often CAD/CAM) restorations using CC. Indeed, light transmission can be difficult and lead to poorer polymerization when curing must be performed through the restoration, which is highly dependent on the restoration thickness and translucency. It should be noted that even the most opaque dental zirconia ceramics are evolving towards more translucent formulations that better enable light curing through the restorative material. , Recent results demonstrate a lack of significantly reduced mechanical properties under such circumstances, for which until recently trade-offs in optical vs mechanical properties would have been necessary. , UAs are called universal, as they can also be used to adhesively lute semi-direct/indirect restorations when 1-step UAs are combined with CCs. Representing the light-curing and self-curing 1-step UA, respectively, the μTBS of SBUp/RxU and TUBII/ECIIp was not found to significantly differ when applied in the same curing mode and bonding mode, either immediately (0kTC) or upon 25kTC- and 50kTC-aging. The bonding efficacy of the two UA/CC combinations, applied either in E&R or SE bonding mode, was comparable irrespective of curing mode. Hence, the second hypothesis that the two UA/CC combinations would not differ in bonding performance, was accepted, as was the third hypothesis, that the curing regimes would not affect the luting efficacy of the CC. This result was in accordance with the findings of previously conducted studies. , Combining photo-initiators and chemical initiators, polymerization of dual-curing CCs is initiated first chemically upon mixing of the base and catalysator paste, and second by light upon restoration luting. Reduced light-curing efficacy, due to light attenuation through the restoration, is expected to be compensated by chemical (self-)curing which is initiated sooner but is generally less effective. Light-curing irradiance decreases significantly with increased restoration thickness, up to more than 80% and 95% in 1.5-mm and 3-mm ceramic disks, respectively; this is obviously highly dependent on the type and translucency of the ceramic. Likewise, light irradiation within root canals, such as when adhesively luting fiber posts, can be inefficient, as light will have difficultly reaching the deeper root areas. Concern has also been raised regarding incompatibility between acidic water-based 1-step adhesives, which includes UAs, and self-curing cements, because the oxygen-inhibition layer of the adhesive may contain sufficient acidic monomers to interact with the amine-based chemical catalytic components of dual-curing CCs and so reduce their polymerization. , Insufficient polymerization self-evidently also reduces bond strength. RxU was bonded to dentin with assistance from the light-curing UA SBUp, while ECIIp was assisted by the self-curing UA TUBII. Nevertheless, SBUp was also not separately light cured as part of the adhesive luting protocol recommended by its manufacturer. SBUp is claimed to be compatible with all dual- and self-curing composite materials because the transition metal-salt added can act as a dual-curing accelerator by catalyzing the decomposition of the peroxide component. The E&R μTBS of both UA/CC combinations was significantly higher than their SE μTBS. While this was recorded for TUBII when bonded directly to dentin in Project Part 1a, in Project Part 2, this was also documented for SBUp when combined with RxU as part of an adhesive luting protocol. Confirming the lower adhesive luting performance in SE bonding mode, almost 100% adhesive failures at the cement-dentin interface were observed, along with substantially more ptfs, the latter especially upon TC aging and for TUBII/ECIIp. SEM failure analysis provides a clear explanation for the lower SE luting performance, since interfacial voids were detected along almost the entire composite cement-dentin interface, regardless of UA/CC combinations and curing mode. Interfacial voids were more frequently observed when the UA/CC combination was applied in SE than E&R bonding mode. Although previous studies demonstrated that the adhesive should be separately light cured as part of adhesive luting, the adhesives tested were only applied in SE bonding mode. , This benefit of separately light curing adhesives is not (or less) obvious when the UA is applied in E&R bonding mode. Further studies should be conducted to verify whether the interfacial void production could be reduced or even eliminated in terms of treating sclerotic impermeable dentin (often the case when luting indirect/semi-direct restorations) or non-vital teeth. Regarding SBS to lithium-disilicate glass-ceramic, overall, both UA/CC combinations yielded SBS that did not differ significantly, but bonded significantly better to HF-etched than to non-etched glass-ceramic. Hence, the Project Part 3 hypothesis that the adhesive luting efficacy of the UA/CC combinations to lithium-disilicate glass-ceramic would not be affected when the glass-ceramic was not HF etched, was rejected. Although UAs contain acidic functional monomers, such as 10-MDP, they ineffectively (self-)etch glass-ceramic surfaces. Both UAs investigated in this study incorporated γ-MPTES, which has been documented to be more effective than conventional silane coupling agents. Silane added into one-bottle UAs appeared less effective because silane is insufficiently stable in an aqueous acidic solution. TUBII separated silane from the acidic functional monomers in a two-bottle configuration, which most likely explains why the SBS of TUBII/ECIIp_HF did not significantly decrease after 6-month water aging in contrast to that recorded for SBUp/RxU_HF.   For direct bonding, the self-curing 1-step UA performed unsatisfactorily when applied in SE but not in E&R bonding mode, while no difference in bonding effectiveness and durability between SE and E&R bonding was recorded for the light-curing 1-step and 2-step/bottle UAs. When the UA was not separately light cured as part of an adhesive luting protocol, SE did not perform as well as E&R adhesive luting. Adhesively luting glass-ceramic restorations still requires prior HF etching of the glass-ceramic surface, while silane is more effective when separated from the acidic monomer components in UAs.   