Abstract
Objectives
This study aimed to investigate the durability of four adhesive luting systems bonded to dentin with and without simulated hydrostatic pulpal pressure (PP).
Methods
Composite blocks were bonded to dentin with four adhesive systems: Multilink Automix (MA), Multilink Sprint (MS), Clearfil Esthetic cement (CE) and RelyX ARC (RAC) under either a PP of 0 or 15 cm H 2 O. After 3 d water storage at 37 °C or thermal cycling (TC), of 30 d with 5000 TC or 90 d with 15,000 TC micro-tensile bond strength (μTBS) was tested. Failure analysis of the bonding interface was performed using a scanning electron microscope (SEM).
Results
Independent of PP application groups MA and RAC showed significantly higher μTBS than CE and MS ( P ≤ 0.05). A significant decrease in μTBS was found for RAC and MS when subjected to PP ( P ≤ 0.05), whereas CE and MA showed no significant difference ( P > 0.05). TC had no significant influence on the μTBS in RAC, MA and CE without PP application ( P > 0.05), whereas CE with PP and MS showed a significant decrease in μTBS ( P ≤ 0.05) when subjected to TC.
Significance
Based on these results, there were significant differences between materials. Pulpal pressure and artificial aging also seem to have an effect on in-vitro evaluation of bonding durability. If considered relevant to the materials’ service performance then these conditions should be applied in the materials’ testing.
1
Introduction
Dentistry has matured from Black’s “extension for prevention” principles to a minimal intervention approach . Nevertheless prosthetic preparations often require the removal of a large amount of enamel resulting in exposed dentin surfaces. Successful bonding of luting agents to tooth structures is imperative for retention, marginal adaptation and durability of indirect tooth colored restorations . Although the bond strength to enamel proves to be stable the bonding durability of luting resins to vital dentin is compromised by two main factors: adverse chemical interactions (e.g. resin hydrolysis, bonding hydrophobic cement to hydrophilic dentin) and an outward flow from dentinal tubules, which is driven by a pulpal pressure (PP) estimated to be approximately 15 cm H 2 O . The dentin tubule fluid movement may permeate polymerized adhesive interfaces and hinder the subsequent bonding of the cement .
Conventional resin cements use etching and washing of the dentin, which increases its permeability . This surface wetness due to pulpal pressure may cause reduction in bonding. In contrast self-etch adhesives and self-adhesive cements use simplified adhesive procedures which just alter, but do not remove the dentinal smear layer . Smear layer covered dentin exhibits less permeability after the application of bonding agents , and this might be expected to allow drier surfaces and therefore stronger bonding. This has not been the case, with self-etch adhesives showing inferior micro-tensile bond strength (μTBS) values in comparison to etch-and-rinse adhesives in several studies . Perhaps the remaining altered smeared layer may reduce the wetting ability and adhesive resin penetration , possibly explaining the lower bond strengths. The latest adhesive systems contain water and organic solvents to try to improve resin permeation into the underlying dentin surface .
Some in-vitro and in-vivo investigations have attempted to evaluate the durability and the influence of PP on resin–dentin bonding . Additionally, morphologic evidence has been published of hydrolytic or enzymatic degradation of collagen fibrils over time which might limit the durability of resin–dentin bonding . The enzymes are thought to be bacterial or host-derived matrix metalloproteinases. However, none of these studies combined the application of PP together with accelerated aging procedures (e.g. thermal cycling).
Little information is available about the durability of contemporary self-etching systems and their bonding mechanisms under PP and thermal cycling. Therefore, the purpose of this study was to evaluate the bond strength to dentin of four latest-generation luting systems with or without pulpal pressure during bonding, and with no, with 5000 or with 15,000 thermal cycles aging over 3 d, 30 d or 90 d, respectively. Storage temperature was 37 °C in all cases. Additionally the bond failure modes were investigated with a scanning electron microscope (SEM).
The null hypotheses to be tested were: (1) the μTBS of the four luting systems does not decrease with the application of PP or TC, and (2) there are no differences in bond strength to dentin between the four luting systems.
2
Materials and methods
2.1
Tooth preparation
Seventy-two intact non-carious human third molars were extracted and stored in 1% thymol-saturated isotonic saline solution at 4 °C. The teeth were collected after the patients’ informed consents were obtained under a protocol approved by the Faculty of Dentistry’s Ethics Committee at the University of Zhejiang, China. All the teeth were used within 1 month after extraction.
Surfaces for dentin bonding were prepared by cutting away the occlusal enamel and dentin perpendicular to each tooth’s long axis about 1 mm below the dentin-enamel junction using a low speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA, Fig. 1 ). Dentin surfaces were then wet polished with 600 grit SiC paper to create a standard surface roughness and smear layer. All specimens were cleaned in distilled water in an ultrasonic bath for 20 min and then stored in distilled water at 4 °C.
All teeth had their pulp chambers exposed by removing the roots below the cemento-dentinal junction using the diamond saw mentioned above. Pulp tissue was removed carefully leaving the predentinal surfaces and the dentinal tubules intact. The occlusal and pulpal dentin surfaces of all specimens were checked with a stereomicroscope (10× magnification, Wild Makroskop M420, Heerbrugg, Switzerland) for the absence of enamel and pulp tissue. The resulting dentin surface for adhesion was defined as superficial dentin. For all specimens the dentin was kept wet during all preparations by placing a moist cotton pallet in the pulp cavity, storing them in distilled water or placing them on a wet dish. A group size of 18 teeth per material was made.
Each pulp chamber was sealed with PMMA resin (Palapress Vario, Heraeus Kulzer, 5 Hanau, Germany) incorporating an 18-gauge stainless steel tube. This tube connected the pulp chamber to a 30 ml plastic container filled with distilled water. The container was held so that its water level was 15 cm above the dentin surface to be bonded. This created a 15 cm of water PP ( Fig. 1 ) . Without the application of PP the plastic container was held at an equal level to the dentin surface and therefore created a PP of 0 cm water. All 72 teeth were used within 48 h after preparation.
2.2
Bonding procedures and micro-tensile bond strength (μTBS) testing
Composite blocks were fabricated by filling rectangular silicone molds (10 mm × 10 mm × 20 mm) with flowable composite (Multicore Flow, Ivoclar Vivadent, Table 1 ) which was then light-cured for 80 s (550 mW/cm 2 , Optilux 500, Kerr, Danbury, USA). Prior to cementing procedures, 10 mm × 10 mm × 4 mm parallel-sided composite blocks were cut with a low-speed diamond saw (Isomet, Buehler). The bonding surface of each composite block was ground with 600-grit SiC paper, cleaned in an ultrasonic bath for 20 min, stored in distilled water at 4 °C and used within 48 h. In order to remove the contaminations on the bonding surface of composite blocks, the bonding surface was cleaned with silica-free 32% phosphoric acid and dried with water- and oil-free air.
| Material (manufacturer) | Composition | Batch no |
|---|---|---|
| Experimental cements | ||
| Clearfil Esthetic cement (Kuraray Medical Inc, Tokyo, Japan) | ED primer A: HEMA, MDP, water, accelerator. ED primer B: Methacrylate monomer, water, initiator accelerator. Paste A: Bis-GMA, TEGDMA, other methacrylate monomers, silanated glass fillers, colloidal silica. Paste B: Bis-GMA, TEGDMA, other metharcylate monomers silanated glass filler, colloidal silica, benzoyl peroxide, di-camphorquinone, pigments. | 243AA, 121AA |
| Mutilink Automix (Ivoclar Vivadent, Schaan, Liechtenstein) | Primer A: an aqueous solution of initiators. Primer B: HEMA and phosphonic acid and acrylic, acidic monomers. Cement: Dimethacrylate and HEMA, barium glass, ytterbium trifluorid, initiators, stabilisators, spheroid dioxides. | K17908 |
| Multilink Sprint (Ivoclar Vivadent, Schaan, Liechtenstein) | Dimethacylates and acidic monomers, barium, glass ytterbium trifluorid and silicon dioxide. | K08951 |
| Control cement | ||
| RelyX ARC (3M ESPE, Seefeld, Germany) | Adper Scotchbond 1 XT primer: acrylates, HEMA, Bis-GMA, metacrylated modified polycarboxylic acid. Cement: Bis-GMA, TEGDMA, Dimetacrylate polymer | 20070530 |
| Composite material | K07368 | |
| Multicore flow (Ivoclar Vivadent, Schaan, Liechtenstein) | The monomer matrix consists of Bis-GMA, urethane dimethacrylate and triethylene glycol dimethacrylate (28.5 wt%). The inorganic fillers are barium glass, ytterbiumtrifluoride. Ba-Al-fluuorosilicate glass and highly dispersed silicon dioxide (71.0 wt%). Additional contents are catalysts, stabilizers and pigments (0.5 wt%) | |
Before starting the bonding procedure a bonding agent (Heliobond, Ivoclar Vivadent) was applied and was light-cured for 30 s (550 mW/cm2, Optilux 500, Kerr, Danbury, USA) to activate the bonding surface. Four latest-generation dentin luting systems were selected for this investigation: a one-step self-etching system—Multilink Sprint (MS, Ivoclar Vivadent, Schaan, Liechtenstein), two two-step self-etching systems—Multilink Automix (MA, Ivoclar Vivadent) and Clearfil Esthetic cement (CE, Kuraray, Tokyo, Japan), and a conventional three-step etch-and-rinse system—RelyXARC (RAC, 3 M ESPE, Seefeld, Germany).
Each luting system was applied to 18 teeth. The luting systems and their chemical compositions are listed in Table 1 and their respective adhesive application procedures are shown in Table 2 . All adhesive luting agents and cements were used according to the manufacturers’ instructions. The primers were applied on the dentin surfaces in all the specimens with distilled water in their respective pulpal chambers with applied pulpal pressure simulation for 9 of the 18 teeth in each material’s group. The composite blocks were placed immediately afterwards under a constant load of 7.5 N for 10 min. Following the light curing of the luting cements and the removal of the seating load, the hydrostatic PP remained for 10 min during the additional period of auto-polymerization.
| Product name (code) | Dentin pretreatment | Luting agent mixing |
|---|---|---|
| Clearfil Esthetic cement (CE) | Dispense an equal amount of ED Primer II A and B, mix 30 s, apply on dentin for 30 s, mild air drying | Squeeze paste A and B from the automix dispenser syringe, apply on surface, lute to resin block, light curing 40 s from each side, apply Oxyguard II for 3 min |
| Multilink Automix (MA) | Dispense an equal amount MA primer A and B, mix 30 s, apply on dentin for 15 s, air dry | Apply the desired quantity directly on the surface with the automix syringe, lute to resin block, light cure 40 s from each side, apply Oxyguard II for 3 min |
| Multilink Sprint (MS) | No pretreatment | Apply the desired quantity directly on the surface from the automix syringe, lute to resin block, light cure for 40 s from each side |
| RelyX ARC (RAC) | Apply Scotchbond etchant (37% phosphoric acid) on dentin surface for 15 s, water rinse, gently air dry, apply Adper Scotchbond 1 XT adhesive 15 s, light cure 10 s | Mix base and catalyst paste for 10 s, lute to resin block, light cure for 40 s from each side |
After completing the adhesive procedures the specimens were stored in 37 °C water for 3 d, 30 d or 90 d with additional 5000 (30 d) or 15,000 thermal cycles (90 d) from 5 to 55 °C with a dwell time of 30 s. Thermal cycling was intended to accelerate the degradation of resin–dentin bond, as a form of artificial aging. Following the different aging conditions the teeth were sectioned according to the non-trimming technique for μTBS testing into 1.0 mm × 1.0 mm × 8 mm slabs perpendicular to the adhesive-tooth interface, using a low-speed diamond saw (Isomet, Buehler). Each tooth provided approximately six beams with the peripheral beams discarded to reduce variation. From each of the 24 subgroups (defined by material/PP/TC, see Table 3 ), 12 beams were selected and used for bond strength evaluation. Each of the 288 beams were glued with cyanoacrylate adhesive to the sliding part of the universal testing machine (Zwick BZ 010/TNZA, Zwick, Ulm, Germany) and the μTBS was measured at a cross-head speed of 1 mm/min.
| Groups | Pulpal pressure dentin ( n = 12) | No pulpal pressure dentin (N = 12) | ||||
|---|---|---|---|---|---|---|
| 3 d | 30 d | 90 d | 3 d | 30 d | 90 d | |
| RAC | 15.3(5.9) B β a | 26.0(13.8)3 B β a | 26.6(11.9) A β a | 44.9(10.6) A α b | 42.8(16.1) A α b | 45.7(18.3) A α b |
| MA | 38.0(17.7) A α a | 41.21(13.3) A α a | 33.4(11.1) A α a | 43.8(10.4) A α a | 42.2(15.0) A α a | 34.5(11.6) A α a |
| CE | 21.3(10.3) B α a | 29.7(14.4) B α a | 2.2(2.3) B β b | 27.7(12.7) B α a | 26.9(11.7) B α a | 24.8(10.1) B α a |
| MS | 0 (0) C β a | 0 (0) C β a | 0 (0) B α a | 22.0(11.0) B α a | 11.0(5.5) C α b | 3.5(5.1) C α c |
2.3
Statistical methods
Although a sample size of n = 12 per group should have been sufficient to provide a parametric distribution, not all experimental groups showed a normal distribution (Kolmogorov–Smirnov and Shapiro–Wilks test). Therefore the μTBS of the four luting resins were statistically analyzed using Kruskal-Wallis H multiple comparison tests followed by non-parametric Mann-Whitney-U tests for pairwise comparisons of groups at a confidence level of 95%. Specimens which failed prematurely during artificial aging were included in the statistical calculation as “zero bond strength” values.
2.4
SEM examination and failure mode analysis
After μTBS testing, the debonded dentin specimens were observed with a scanning electron microscope (SEM, Philips XL 30 CP, Philips, Eindhoven, The Netherlands) to evaluate the bond failure modes. The specimens were air-dried for 24 h, gold sputtered and observed at a tube voltage of 10–25 kV. As other failure modes did not occur, failure modes were classified into one of the following five modes ( Fig. 3 ): A: cohesive failure located in the dentin; B: adhesive failure at the resin–dentin interface; C: mixed adhesive and cohesive failure; D: cohesive failure in the luting resin and E: adhesive failure at the resin–composite interface. The portion of each failure mode on the debonded dentin surfaces was determined from the SEM micrographs with scale paper and expressed as a percentage of the total bonded surface area for each test group.
2
Materials and methods
2.1
Tooth preparation
Seventy-two intact non-carious human third molars were extracted and stored in 1% thymol-saturated isotonic saline solution at 4 °C. The teeth were collected after the patients’ informed consents were obtained under a protocol approved by the Faculty of Dentistry’s Ethics Committee at the University of Zhejiang, China. All the teeth were used within 1 month after extraction.
Surfaces for dentin bonding were prepared by cutting away the occlusal enamel and dentin perpendicular to each tooth’s long axis about 1 mm below the dentin-enamel junction using a low speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA, Fig. 1 ). Dentin surfaces were then wet polished with 600 grit SiC paper to create a standard surface roughness and smear layer. All specimens were cleaned in distilled water in an ultrasonic bath for 20 min and then stored in distilled water at 4 °C.
All teeth had their pulp chambers exposed by removing the roots below the cemento-dentinal junction using the diamond saw mentioned above. Pulp tissue was removed carefully leaving the predentinal surfaces and the dentinal tubules intact. The occlusal and pulpal dentin surfaces of all specimens were checked with a stereomicroscope (10× magnification, Wild Makroskop M420, Heerbrugg, Switzerland) for the absence of enamel and pulp tissue. The resulting dentin surface for adhesion was defined as superficial dentin. For all specimens the dentin was kept wet during all preparations by placing a moist cotton pallet in the pulp cavity, storing them in distilled water or placing them on a wet dish. A group size of 18 teeth per material was made.
Each pulp chamber was sealed with PMMA resin (Palapress Vario, Heraeus Kulzer, 5 Hanau, Germany) incorporating an 18-gauge stainless steel tube. This tube connected the pulp chamber to a 30 ml plastic container filled with distilled water. The container was held so that its water level was 15 cm above the dentin surface to be bonded. This created a 15 cm of water PP ( Fig. 1 ) . Without the application of PP the plastic container was held at an equal level to the dentin surface and therefore created a PP of 0 cm water. All 72 teeth were used within 48 h after preparation.
2.2
Bonding procedures and micro-tensile bond strength (μTBS) testing
Composite blocks were fabricated by filling rectangular silicone molds (10 mm × 10 mm × 20 mm) with flowable composite (Multicore Flow, Ivoclar Vivadent, Table 1 ) which was then light-cured for 80 s (550 mW/cm 2 , Optilux 500, Kerr, Danbury, USA). Prior to cementing procedures, 10 mm × 10 mm × 4 mm parallel-sided composite blocks were cut with a low-speed diamond saw (Isomet, Buehler). The bonding surface of each composite block was ground with 600-grit SiC paper, cleaned in an ultrasonic bath for 20 min, stored in distilled water at 4 °C and used within 48 h. In order to remove the contaminations on the bonding surface of composite blocks, the bonding surface was cleaned with silica-free 32% phosphoric acid and dried with water- and oil-free air.
