Adhesive Performance Assessment of Universal Adhesives and Universal Adhesive/Composite Cement Combinations

Abstract

Purpose: To investigate the bonding performance of three universal adhesives (UAs) to dentin and the effect of different curing modes and hydrofluoric-acid (HF) etching of lithium-disilicate glass-ceramic on the adhesive performance of two UA/composite cement (CC) combinations.

Materials and methods: In the first project part, the immediate and aged (25k and 50k thermocycles) microtensile bond strength (µTBS) of the two light-curing UAs G2-Bond Universal (G2B; GC) and Scotchbond Universal Plus (SBUp; 3M Oral Care), and the self-curing UA Tokuyama Universal Bond II (TUBII; Tokuyama) to flat dentin was measured, when applied in both E&R and SE bonding mode using a split-tooth design (n = 10). The resultant adhesive-dentin interfaces were characterized using TEM. In the second project part, CAD/CAM composite blocks were luted to flat dentin with either Scotchbond Universal Plus/RelyX Universal (SBUp/RxU; 3M Oral Care) or Tokuyama Universal Bond II/Estecem II Plus (TUBII/ECIIp; Tokuyama Dental) using different curing modes (AA mode: auto-curing of both adhesive and cement; AL mode: auto-curing of adhesive and light-curing of cement), upon which their immediate and aged (25k and 50k thermocycles) µTBS was measured. In the third project part, the same UA/CC combinations were luted to CAD/CAM glass-ceramic to measure their immediate and aged (6-month water storage) shear bond strength (SBS).

Results: In E&R bonding mode, the performance of G2B, SBUp and TUBII was not significantly different in terms of µTBS, while G2B and SBUp significantly outperformed TUBII in SE bonding mode. No significant difference in µTBS was found between the SBUp/RxU and TUBII/ECIIp UA/CC combinations, regardless of bonding mode, aging time, or curing mode. The cement-curing mode did not significantly influence µTBS, while a significantly higher µTBS was recorded for the UA/CC combinations applied in E&R bonding mode. HF significantly improved the SBS of the UA/CC combinations to glass-ceramic.

Conclusion: The self-curing adhesive performed better when applied in E&R than in SE bonding mode. The curing mode did not influence the adhesive performance of the composite cements, while an E&R bonding mode rendered more favorable adhesion in a self-curing luting protocol. When bonding to glass-ceramic, the adhesive performance of the universal adhesive/composite cement combinations benefited from HF etching.

Keywords: TEM; adhesion; bond strength; bonding; light curing; self-curing.

Fig 1

Box-and-whisker plots (Project Part 1a) of the 0kTC, 25kTC-aged and 50kTC-aged μTBS of the universal adhesives (UAs) G2B, SBUp and TUBII to flat dentin when applied in E&R and SE bonding mode. The thick horizontal line within each box represents the median μTBS. The black dot within each box represents the mean μTBS. The red square within each box represents the LME fitted mean. The horizontal lines in each box represent, from top to bottom, the maximum μTBS, the upper quartile, the median μTBS, the lower quartile and the minimum μTBS measured for each experimental group (excluding possible outliers). Statistically significant differences in μTBS with the respective reference are indicated using distinct characters.

Fig 2

Failure-mode distribution (Project Part 1a) of the G2B, SBUp and TUBII μ-specimens upon μTBS testing when bonded to flat dentin in E&R and SE bonding mode. Dentin: cohesive failure in dentin; Interface: adhesive interfacial failure; composite: cohesive failure in composite; mixed: mixed failure.

Fig 3

Representative SEM photomicrographs (Project Part 1a) of μ-specimens (dentin side) illustrating the failure mode (1) when G2B was bonded to flat dentin in E&R bonding mode in (a1), revealing a mixed failure enlarged in (a2), and when applied in SE mode in (b1), revealing a mixed failure enlarged in (b2), (2) when SBUp was bonded to flat dentin in E&R bonding mode in (c1), revealing a mixed failure enlarged in (c2), and when applied in SE mode in (d1), revealing a mixed failure enlarged in (d2) and some interfacial voids (hand pointer), and (3) when TUBII was bonded to flat dentin in E&R bonding mode in (e1), revealing a mixed failure enlarged in (e2), and when applied in SE mode in (f1), revealing voids nearly across the whole surface as enlarged in (f2). Ad: adhesive resin; Co: composite; De: dentin.

Fig 4

Representative TEM photomicrographs (Project Part 1b) illustrating the ultra-structure of the adhesive-dentin interface produced by G2B in (a1-a4), SBUp in (b1-b4) and TUBII in (c1-c4), when bonded to flat dentin following an E&R and SE bonding mode. a1: non-demineralized, non-stained section of the adhesive-dentin interface produced by G2B applied in E&R bonding mode, revealing a tight bond consisting of a hybrid layer of about 4-5 µm with an abrupt transition to the underlying unaffected dentin. a2: demineralized, stained section of the adhesive-dentin interface produced by G2B applied in E&R mode, revealing a particle-filled resin tag surrounded by an hybridized tubule wall within an opened dentinal tubule. a3: non-demineralized, non-stained section of the adhesive-dentin interface produced by G2B applied in SE mode, disclosing a thin HAp-rich hybrid layer of about 0.5 µm with a gradual transition to unaffected dentin. Significant nano-layering was detected (white arrows) in the adhesive resin above the interface, as magnified in the black-bordered insert. a4: demineralized, stained section of the adhesive-dentin interface produced by G2B applied in SE mode, revealing a clearly defined hybrid layer of about 0.5 µm. b1: non-demineralized, non-stained section of the adhesive-dentin interface produced by SBUp applied in E&R mode, revealing a tight bond consisting of an HAp-free, collagen-rich hybrid layer of about 3-4 µm with an abrupt transition to the underlying unaffected dentin. b2: demineralized, stained section of the adhesive-dentin interface produced by SBUp applied in E&R mode, revealing a well-defined hybrid layer with a resin tag formed within an opened dentinal tubule. b3: non-demineralized, non-stained section of the adhesive-dentin interface produced by SBUp applied in SE mode, disclosing a partially demineralized hybrid layer of 0.4-0.8 µm with a gradual transition to unaffected dentin. b4: demineralized, stained section of the adhesive-dentin interface produced by SBUp applied in SE mode, revealing a distinct 0.6-µm hybrid layer. c1: non-demineralized, non-stained section of the adhesive-dentin interface produced by TUBII applied in E&R mode, revealing a completely demineralized hybrid layer of around 5.0 µm with an abrupt transition to the underlying unaffected dentin. c2: demineralized, stained section of the adhesive-dentin interface produced by TUBII applied in E&R mode, showing an homogeneously stained hybrid layer and densely silica-filled adhesive resin. c3: non-demineralized, non-stained section of the adhesive-dentin interface produced by TUBII applied in SE mode, disclosing a thin, HAp-rich hybrid layer of about 0.4 µm with a gradual transition to unaffected dentin. c4: demineralized, stained section of the adhesive-dentin interface produced by TUBII applied in SE mode, revealing a distinct and homogeneously stained hybrid layer. AR: adhesive resin; D: dentin; HL: hybrid layer; RT: resin tag.

Fig 5

Box-and-whisker plots (Project Part 2) of the 0kTC, 25kTC-aged and 50kTC-aged μTBS of the universal adhesives/ composite cement combinations (UA/CCs) SBUp/RxU and TUBII/ECIIp adhesively luted onto flat dentin, when applied in E&R and SE bonding mode and either following an AA or AL curing mode. The thick horizontal line within each box represents the median μTBS. The black dot within each box represents the mean μTBS. The red square within each box represents the LME fitted mean. The horizontal lines in each box represent, from top to bottom, the maximum μTBS, the upper quartile, the median μTBS, the lower quartile and the minimum μTBS measured for each experimental group (excluding possible outliers). Statistically significant differences in μTBS with the respective reference are indicated using distinct characters.

Fig 6

Failure-mode distribution (Project Part 2) of SBUp/RxU and TUBII/ECIIp μ-specimens upon μTBS testing when bonded to flat dentin in E&R and SE bonding mode and AA and AL curing mode. Dentin: cohesive failure in dentin; cement-dentin: adhesive failure at the cement-dentin interface; cement: cohesive failure in cement; cement-composite: adhesive failure at the cement-composite interface; composite: cohesive failure in composite; mixed: mixed failure.

Fig 7

Representative SEM photomicrographs (Project Part 2) of μ-specimens (dentin side) illustrating the failure mode (1) for SBUp/RxU luted in AA curing mode in (a-d) and AL mode in (e-h), and (2) for TUBII/ECIIp when applied in AA curing mode in (A-D) and AL mode in (E-H). (a) SBUp/RxU μ-specimen luted in AA curing mode and E&R bonding mode, revealing a cohesive failure in cement enlarged in (b); (c) SBUp/RxU μ-specimen luted in AA curing mode and SE bonding mode, revealing a primarily adhesive failure at the cement-dentin interface enlarged in (d); (e) SBUp/RxU μ-specimen luted in AL curing mode and E&R bonding mode, revealing a mixed failure enlarged in (f); (g) SBUp/RxU μ-specimen luted in AL curing mode and SE bonding mode, revealing an adhesive failure at the cement-dentin interface enlarged in (h). (A) TUBII/ECIIp μ-specimen applied in AA curing mode and E&R bonding mode, revealing a mixed failure enlarged in (B); (C) TUBII/ECIIp μ-specimen applied in AA curing mode and SE bonding mode, revealing an adhesive failure at the cement-dentin interface enlarged in (D); (E): TUBII/ECIIp μ-specimen applied in AL curing mode and E&R bonding mode, revealing a mixed failure enlarged in (F); (G): TUBII/ECIIp μ-specimen applied in AL curing mode and SE bonding mode, revealing a mixed failure enlarged in (H). Ad: adhesive resin; Ce: cement; Co: composite; De: dentin; HL: hybrid layer.

Fig 8

Representative SEM photomicrographs (Project Part 2) showing a large number of irregular interfacial voids (hand pointers) detected at the surface of fractured specimens (composite side) of (1) SBUp/RxU applied in AA curing mode and E&R bonding mode in (a,b) and in SE bonding mode in (c,d), and in AL mode and E&R bonding mode in (e,f) and in SE bonding mode in (g,h), and (2) TUBII/ECIIp applied in AA curing mode and E&R bonding mode in (A,B) and in SE bonding mode in (C,D), and in AL mode and E&R bonding mode in (E,F) and in SE bonding mode in (G,H). Ad: adhesive resin; Ce: cement; De: dentin.

Fig 9

Bar graph (Project Part 3) in (a) presenting the immediate and aged SBS of the universal adhesives/composite cement combinations (UA/CCs) SBUp/RxU and TUBII/ECIIp adhesively luted to lithium-disilicate glass-ceramic (IPS e.max CAD, Ivoclar), with the glass-ceramic surface either etched with hydrofluoric acid (HF) or not (pre-)treated (NT). Groups with the same capital or lowercase letters indicate no significant difference in SBS among the experimental groups when SBS was measured immediately or upon 6-month aging, respectively. Asterisks indicate significant difference between the immediate and aged SBS of the same UA/CC combinations applied following the same procedure. Failure-mode distribution (Project Part 3) in (b) of SBUp/RxU and TUBII/ECIIp specimens upon SBS testing when bonded to glass-ceramic CAD-CAM blocks upon HF etching (HF) or when not (pre-)treated (NT). Ceramic: cohesive failure in glass-ceramic; interface: adhesive failure at the cement-ceramic interface; cement: cohesive failure in cement; mix: mixed failure.