Effect of hydrostatic pressure on regional bond strengths of compomers to dentine

Journal of Dentistry Journal of Dentistry 28 (2000) 501–508 www.elsevier.com/locate/jdent

Effect of hydrostatic pressure on regional bond strengths of compomers to dentine L. Zheng*, P.N.R. Pereira, P. Somphone, T. Nikaido, J. Tagami Department of Operative Dentistry, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113 8549, Japan

Abstract Objectives: The aim of this study was to evaluate the effect of hydrostatic pressure on the regional bond strengths of compomers to dentine. Methods: Thirty freshly extracted molars were ground flat to expose the dentine and randomly divided into two groups for bonding: no hydrostatic pressure and hydrostatic pressure of 15 cm H2O. Xeno CF, Dyract AP and F 2000 were applied to dentine surfaces pretreated by the respective bonding systems following the manufactures’ instructions, and then restored with Clearfil AP-X. After 24 h storage in water, the teeth were sectioned into 0.7-mm thick slabs and visually divided into three regional subgroups: the region communicating with the pulp through dentinal tubules (pulp horn); the region between the pulp horns (center); and the region between the pulp horn and DEJ (periphery). The specimens were trimmed to a cross-sectional area of 1 mm 2 and subjected to the micro-tensile bond test. The data were analyzed by oneand three-way ANOVA, and Fisher’s PLSD …p ⬍ 0:05†: Results: There were no significant regional differences of bond strengths for all the compomers tested …p ⬎ 0:05†: However, hydrostatic pressure significantly decreased the bond strength of F 2000 to all regions … p ⬍ 0:05†; while the bond strength of Dyract AP significantly decreased only at the pulp horn region …p ⬍ 0:05†: On the other hand, the bond strengths of Xeno CF seemed not to be affected by hydrostatic pressure … p ⬎ 0:05†: For Dyract AP and F 2000, the fracture modes were affected by hydrostatic pressure, while, for Xeno CF, there were no significant differences between the fracture modes with non- or positive hydrostatic pressure. Significance: Simulated pulpal pressure of 15 cm H2O had a greater influence on the bond strengths of compomers to dentine than did dentine regions. Therefore, when measuring the bond strengths of compomers to dentine under the simulated in vivo conditions, the wetness of the dentine surface, as well as the intrinsic properties of each material should be seriously considered. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Compomer; Tensile bond strength; Hydrostatic pressure

1. Introduction Compomers are materials that incorporate both resin composite and glass-ionomer technologies [1]. The compomer can be mainly polymerized with visible-light curing; however, the acid–base reaction subsequently is initiated by water uptake from the oral environment [2]. These materials appear to have bonding and margin-sealing ability equivalent to composite resins [3,4], although fluoride release has been shown to be less than from resin-modified glass-ionomers and un-modified glass ionomers [5–7]. The compomers are packaged with a one-component dental adhesive system to enhance the bond strengths and integrity of the marginal seal of the restoration [8]. The bonding mechanism of the material, therefore, depends mainly upon the hybri-

* Corresponding author. Tel.: ⫹ 81-3-5803-5483; fax: ⫹ 81-3-58030195. E-mail address: [email protected] (L. Zheng).

dization by diffusion of the monomer into treated demineralized dentine and polymerization in situ [9]. Recently, a new method for testing bond strengths has been developed, permitting the measurement of small bonding areas as small as 0.5 mm 2 [10]. The micro-tensile bond test allows testing various regions of dentine, such as cariesaffected dentine [11], sclerotic dentine, wedge-shaped defects [12], flat dentine surface [13], and cavity floor [14,15]. It can also be used to test regional bond strengths to dentine with different permeability and different degrees of wetness [16]. Dentine moisture, as well as regional structural differences, are important factors that can affect the bond strengths to dentine [16,17]. In deep dentine close to the pulp chamber, the permeability and wetness are greater than that in superficial dentine [18]. Thus, pulpal pressure may influence the degree of intrinsic wetness on the dentine surface [19–23]. It recently has been reported that phosphoric acid etching significantly decreased bond strengths at the pulp horn region, whereas self-etching

0300-5712/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0300-571 2(00)00025-7

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Fig. 1. Schematic representation of the apparatus used to produce a pressure of 15 cm H2O at the dentine surface for bonding.

primers created relatively uniform bond strengths to normal dentine; therefore, regional differences may also be material dependent [16]. The purpose of this study was to measure the regional bond strengths of single-step bonding systems for compomers restoration, with or without hydrostatic pressure, by means of the micro-tensile bond test.

2. Materials and methods Thirty caries-free molars stored frozen were selected for this study. After thawing, the teeth were divided randomly into two groups of 15 teeth each for bonding: Group I (no pulpal pressure), Group II (pulpal pressure of 15 cm H2O) [24]. The specimens were prepared according to the previous study [16] as follows: In Group I, the occlusal enamel was removed using an Isomet Saw (Buehler Ltd., IL, USA) with water cooling, and the dentine surface was ground flat with a 600-grit silicon carbide paper under running water. The pulp tissue was preserved. In Group II, the occlusal enamel was removed, and then the crown segments were also prepared by removing the roots below the CEJ using a bur. The pulp tissue was removed with a broach, taking care to avoid touching the walls of the pulp chamber. The dentine surfaces were ground flat with 600-grit SiC paper. The segments were glued with cyanoacrylate glue (Zapit, DVA, Anaheim, CA, USA) to a 2 × 2 × 0:7 cm3 Plexiglas block through which an 18-gauge × 2-cm long stainless steel tube had been inserted. This tube permitted communication between the pulp chamber and a 5-ml plastic syringe barrel, and ensured that the pulp chamber and dentine were always full of distilled water. The other end of the 18-gauge tube was then connected to a plastic tube which led to the syringe barrel full of water in

order to produce a pressure of 15 cmH2O at the dentine surface of the crown segment during bonding (Fig. 1). The three kinds of compomers and adhesive systems that were used are listed in Table 1. These materials were Xeno CF/XenoBond (Sankin Kogyo Co., Tokyo, Japan); Dyract AP/Prime&Bond (Dentsply DeTray, Konstanz, Germany); and F 2000 Compomer/F 2000 Primer/Adhesive (3M Dental Products, St. Paul, MN, USA). They were applied on the dentine surface according to the manufacturers’ instructions. Xeno Bond and F 2000 Primer/Adhesive contain two components for mixing before application. On the other hand, Prime&Bond is a one-bottle adhesive system. These adhesive systems, which had been applied on the dentine surface for 20 s (Xeno CF, Dyract AP) and 30 s (F 2000), respectively, were gently air-dried, and then lightcured for 10 s. One coating of the Primer/Adhesive was performed for F 2000, whereas two coatings were performed for Xeno Bond and Prime&Bond according to the manufacturers’ indications. Following this, a thin layer of approximately 2 mm of each compomer was placed on the dentine surface, then light-cured for 30 s (Xeno CF and Dyract AP) and 40 s (F 2000), respectively. To ensure that the compomers would not break cohesively during testing, the resin composite Clearfil AP-X (Kuraray Co., Osaka, Japan) was then applied, building up a block of resin composite to a height of approximately 3–5 mm. All bonded specimens were stored in tap water at 37⬚C for 24 h with the pulp chambers wet with water but without pulpal pressure. The specimens were sectioned into approximately 0.7mm thick slabs along the long axis of the tooth. The slabs were divided into three regional subgroups according to the following criteria: peripheral region, which is located between the pulp horns and DEJ (peri.); pulp horn region, which communicates with the pulp horn through dentineal tubules (ph); and central region, which is located between

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Table 1 Adhesive systems and compomers used for bonding Brand name

Components

Composition

pH

Procedure

Xeno CF

Xeno Bond CF

Pyrophosphate monomer, UDMA, PEM-F, microsilicafiller, photoinitiators, HEMA, water, ethanol FAS glass, UDMA, HEMA, 2.6E, PMDM, photoinitiators

1.0–1.3

5 s application, gentle air drying, 10 s light cure, two coats Apply paste, 30 s light cure

UDMA, PENTA, TEGDMA, photoinitiators, stabilizers, acetone Polymerisable resins, TCB resin, SFS glass, strontium fluoride, photoinitiators, stabilizers

2.0–2.3

5 s application, gentle air drying, 10 s light cure, two coats Apply paste, 30 s light cure

HEMA, Maleic acid, Vitrebond, copolymer, water, ethanol, photoinitiators FAS glass, colloidal silica, CDMA, GDMA, hydrophilic polymer, CPQ/amine

1.1–1.5

5 s application, gentle air drying, 10 s light cure Apply paste, 40 s light cure

Paste Dyract AP

Primer&Bond Paste

F 2000

Prime/Adhesive Paste

UDMA, urethane dimethacrylate; PEM-F, pentafluorocyclophosphazene; HEMA, 2-hydroxyethy1-methacrylate; FAS glass, fluoroaluminosilicate glass; PMDM, pyromellitic dimethacrylate; PENTA, phosphorate penta-acrylate ester; TEGDMA, triethylene glycol dimethacrylate; SFS glass, strontium-fluorosilicate glass; CDMA, methacrylated polycarboxylic acid; GDMA, glycery1 dimethacrylate; CPQ/amine, camphoroquinone.

the pulp horns (center). The slabs were then trimmed with super-fine diamond burs (c-16 ff, GC Ltd., Tokyo, Japan) for micro-tensile bond test (mTBS), with the narrowest region of approximately 1 mm 2 located at the respective bonded interface to be tested. The slabs were kept moist during preparation for the micro-tensile bond test (Fig. 2). These specimens were then affixed with Zapit to the BencoreMulti-T (Danville Engineering Co., Danville, CA, USA), which was placed in a universal testing machine (EZ-Test, Shimadzu Co., Kyoto, Japan) and subjected to tensile force at a crosshead speed of 1 mm/min. The load at failure and the surface area for each specimen was used to calculate the bond strength in MPa. The data were analyzed by three-way analysis of variance (ANOVA). The three factors analyzed were material, region, and pressure or no pressure. After that, one-way ANOVA and Fisher’s PLSD test at the confidence level of 95% were performed …p ⬍ 0:05†: In order to observe the fracture modes, the debonded

specimens which were fixed for at least 8 h in 10% neutral buffered formalin were trimmed, placed on stubs followed by room desiccation, gold sputter-coated, and observed with SEM (JXA-840 JEOL, Tokyo, Japan). The fracture modes were classified into one of three categories: I, partial or complete failure at the interface; B, partial or complete cohesive failure in bonding agent; or C, partial or complete cohesive failure in compomer.

3. Results The regional micro-tensile bond strengths of the compomer/adhesive systems to dentine are summarized in Table 2. Three-way ANOVA (Table 3) revealed that the bond strengths were influenced by material …F ˆ 8:34; p ⬍ 0:005† and pressure …F ˆ 37:97; p ⬍ 0:0001†; but were not affected by dentine region …F ˆ 0:17; p ˆ 0:84:†:

Fig. 2. Specimen preparation for micro-tensile bond test.

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Table 2 Regional micro-tensile bond strengths of compomers to dentine Materials

Pupal pressure

Xeno CF

Periphery

Horn

Center

Pˆ0 P ˆ 15

a

29:4 ^ 9:6 (12) 26:1 ^ 9:5 (11) a

25:3 ^ 10:0 (10) 21.8 (4.9) (11) a

Dyract AP

Pˆ0 P ˆ 15

26:9 ^ 7:8 (14) a 22:1 ^ 9:2 (12) a

27:9 ^ 8:8 (12) a 19:8 ^ 4:7 (13) b

28:4 ^ 8:9 (11) a 22:1 ^ 7:1 (12) a

F 2000

Pˆ0 P ˆ 15

22:8 ^ 3:3 (10) a 13:8 ^ 5:3 (13) b

24:1 ^ 5:2 (10) a 17:8 ^ 8:2 (11) b

26:6 ^ 3:7 (10) a 15:8 ^ 3:7 (12) b

a

25:2 ^ 8:4 (13) a 21:2 ^ 6:6 (10) a

MPa ^ SD (n); groups identified with similar superscript letters are not significantly different … p ⬎ 0:05†; groups with different letters are significantly different … p ⬍ 0:05†.

The micro-tensile bond strength of each material with/without hydrostatic pressure was then analyzed by one-way ANOVA and Fisher’s PLSD test at the confidence level of 95%. For Xeno CF, there was no significant regional difference with or without hydrostatic pressure …p ⬎ 0:05†; and bond strengths ranged from 21 to 29 MPa. However, for Dyract AP, hydrostatic pressure decreased the bond strengths significantly at the pulp horn region only (no pulpal pressure: pulp horn ˆ 27.9 ^ 8.8; positive pulpal pressure: pulp hornˆ 19:8 ^ 4:8; p ⬍ 0:05†: For F 2000, the bond strengths to all regions reduced significantly with positive hydrostatic pressure (no pulpal pressure: periphery ˆ 22.8 ^ 3.1 MPa, pulp horn ˆ 23.5 ^ 5.1 MPa, center ˆ 26.6 ^ 3.7 MPa; positive pulpal pressure: periphery ˆ 13.8 ^ 5.3 MPa, pulp horn ˆ 17.8 ^ 8.2 MPa, center ˆ 15.9 MPa; p ⬍ 0:05†: The fracture modes of all the specimens after the microtensile bond test are summarized in Table 4. For Xeno CF, most failures were mixed, either partially cohesive in compomer, bonding agent and interface, or partially cohesive in bonding agent and compomer. There were no significant differences for fracture modes between non- or positive hydrostatic pressure. Low magnification SEM micrographs of representative specimens (Fig. 3a and b) disclose a similar fracture mode. The hybrid layer seemed to break cohesively in both specimens; the scratches indicate the top of the hybrid layer. Fig. 3c and d represents higher magnifications of the compomer and dentine sides, respectively. The hybrid layer failed cohesively, disclosing the resin-infiltrated top of the hybrid layer, and a less infiltrated bottom of the hybrid layer. For Dyract AP in the absence of hydrostatic pressure, the fracture modes were classified mainly as a mixed cohesive failure in compomer, bonding agent and interface; or a mixed cohesive failure in compomer and bonding agent. SEM micrographs (Fig. 4a and b) of representative specimens reveal the failure pattern of both compomer and dentine sides. Fig. 4a shows that the hybrid layer was torn from the dentine surface together with the bonding agent. Fig. 4b discloses a mixture of bonding/interface failures. When pulpal pressure was applied, however, the fracture modes significantly changed with an increase in failures at

the interface. Fig. 4c and d show typical SEM images of the interface after debonding. A very mild removal of the smear layer and demineralization of the dentine surface can be observed, which may have led to the significant decrease in bond strengths at the pulpal horn region due to water perfusion. For F 2000, without hydrostatic pressure, the fractures of the specimens at all regions were either a mixture of cohesive failures in compomer, bonding agent, and interface (Fig. 5a) or completely cohesive failures in the compomer. However, when pulpal pressure was applied, there was a significant increase of cohesive failures in the bonding agent. Fig. 5b and c represent low and high magnification of SEM images at the pulp horn region when hydrostatic pressure was applied. Fig. 5b discloses the fracture pattern of the bonding agent, compomer and interface. Higher magnification of the same specimen reveals failure of the hybrid layer as well as the bonding agent (Fig. 5c). 4. Discussion The measurement of tensile bond strength in a simulated oral environment is an experimental technique that can help estimate how the bonding systems might behave in vivo. When temperature and relative humidity were altered, the variations of the tensile bond strengths of bonding systems were found [25]. Moreover, it has been reported that a wet dentin surface produced bond strength variability [26–28]. The effect of pulpal fluid under a simulated pulpal pressure has also been reported to influence bond strengths [19–24]. Sano et al. [10] introduced a novel technique, the microtensile bond test, which permits the evaluation of small Table 3 Analysis of the factors Three-way ANOVA

F value

p value

Pressure Material Material ⴱ pressure Region Material ⴱ region Pressure ⴱ region

37.971 8.34 2.063 0.174 1.844 0.222

⬍ 0.0001 0.0003 0.13 0.8402 0.1221 0.8008

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Table 4 Distribution of fracture modes according to the different regions and bonding condition Brand name

Pulpal pressure

Pulp horn

Center

Periphery

Xeno CF

Pˆ0 P ˆ 15

C/B/I(40%), B/C(50%), B(10%) C/B/I(58%), B/C(42%)

C/B/I(50%), B/C(50%) C/B/I(55%), B/C(45%)

C/B/I(58%), B/C(42%) C/B/I(18%), B/C(72%), B(10%)

Dyract AP

Pˆ0 P ˆ 15

C/B/I(50%), B/C (50%) C/B/I(59%), B/C (8%), I(33%)

C/B/I(75%), B/C(36%) C/B/I(80%), I(20%)

C/B/I(40%), B/C(50%), C(10%) C/B/I(72%), B/C(20%), I(8%)

F 2000

Pˆ0 P ˆ 15

C/B/I (44%), C(56%) B(64%), C/B/I (9%), C(27%)

C/B/I(44%), C(56%) B(73%), C/B/I (18%), C(9%)

C/B/I(58%), C (42%) B(23%), C/B/I(54%), C(23%)

Abbreviations: C, complete or partial cohesive failure in compomer; B, complete or partial cohesive failure in bonding agent; I, complete or partial failure at interface.

surface areas (ca. 1 mm 2) for bonding. This new testing method has enabled the determination of bond strengths to various clinically relevant substrates and regions, such as caries-effected dentine [11] and sclerotically altered dentine of cervical or abrasion legions [12]. The bonding procedures of commonly used dentine adhesive systems for adhesive restorations require several steps to achieve good adhesion to dentine. Conventional bonding systems for direct resin composites are usually composed of a three-step bonding procedure, such as conditioning, priming, and bonding. Recently, two-step-bonding systems have been developed, which has simplified the bonding procedure. One of the two-step systems is a self-etching bonding system, which combines the etching and priming procedures into one step by a self-etching primer [29,30]. An acidic

monomer in the primer can mildly demineralize the smear layer and the underlying dentine. Another system is a onebottle system, which combines the priming and bonding procedures into one step. Phosphoric acid is used to remove the smear layer and demineralize the superficial dentine. A moist dentine surface should be required to maintain a noncollapsed demineralized collagen network mediated by hydrogen bonding [26,27]. Usually, one-bottle adhesive systems contain ethanol or acetone as a solvent allowing application to the moist dentine surface. As described above, the instructions of the bonding protocols vary according to each system, which may confuse practitioners and cause technical errors during bonding procedures [31]. Thus, simplified bonding systems should be developed in order to reduce the technical sensitivity of

Fig. 3. Scanning electron micrographs of the Xeno CF debonded specimens after micro-tensile bond test: (a) Dentine side of Xeno CF restored under absence of pulpal pressure. Failure occurred within the hybrid layer. Note that scratches (white star) of the SiC paper still remain at the top (T) of the hybrid layer; (b) Xeno CF restored under positive pulpal pressure. A mixed failure occurred within the bonding resin (B) and the interface (I); (c) Higher magnification of the hybrid layer which remained attached to the compomer after debonding. Note that the hybrid layer broke cohesively disclosing the top (T) and bottom (B) of the hybrid layer; and (d) Dentine side of the same specimen as in (c), disclosing the top (note the scratches, white star) and the bottom of the hybrid layer.

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Fig. 4. Scanning electron micrographs of Dyract AP debonded specimens: (a) Compomer side of the fractured specimen, disclosing mixed failure within adhesive and interface. Note that parts of the hybrid layer (H) remained attached to the adhesive (A); (b) Dentine side of a specimen that showed mixed adhesive (A) interface (I) failure; (c) Dentine side of a pulp horn specimen restored under positive pulpal pressure. Note the SiC scratches (white star) and little demineralization and occluded dentine tubules; and (d) Compomer side of the same specimen, disclosing probable remnants of smear layer and smear plugs.

Fig. 5. Scanning electron micrographs of the debonded specimens of F 2000: (a) Compomer side of a no-pulpal pressure specimen cohesively failed within the compomer (C) and interface (I); (b) Dentine side of a specimen restored under positive pulpal pressure. Note the fracture pattern of the bonding resin (B); and (c) Higher magnification of the previous micrograph, disclosing bonding resin (B) and the underlying demineralized dentine (D).

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the bonding system. Recently, a single-step bonding system has been developed, which has shown the potential for good bonding of both compomer and resin composite to enamel and dentine [9]. The single-step bonding systems which are recommended for compomers (Xeno Bond, Prime&Bond, and F 2000 Prime/Adhesive) include one-bottle or twobottle adhesives. These are acidic agents with a low pH, because of the acidic methacrylates incorporated in the adhesives [9,32]. These adhesives also contain dimethacrylates, photoinitiators, and solvents, which can be water, ethanol, or acetone. It is believed that the single-step adhesive functions as a self-etching, priming and bonding agent in only a single application to dentine. The present study indicated that the bond strengths of three compomers to dentine were influenced by material and pulpal pressure, but not affected by region. This might be explained by the fact that all the systems studied did not contain a separate acid-etching agent, which requires rinsing after application. For Xeno CF, bond strengths ranged from 21 to 29 MPa, and there were no significant differences in bond strengths between the non- and positive pressure groups. It was reported previously that Xeno Bond functioned as a mild conditioner to enamel and dentine, because the pyro-phosphate monomer was included in the adhesive [9]. The acidity of the primer was relatively stronger (pH 1.1–1.3) than those of other adhesives in commercially available compomers [32]. A hybrid layer was formed at the interface between Xeno Bond and dentine, which strongly supported the micromechanical interlocking between the single-step bonding system for compomer and dentine [9]. Most failures were mixed, either partially cohesive in compomer, bonding agent and interface, or partially cohesive in bonding agent and compomer. Higher magnification of the SEM image (Fig. 3c and d) disclosed that the hybrid layer also failed cohesively during debonding, allowing the upper half of the hybrid layer to remain attached to the compomer, leaving the lower part of the hybrid layer on the dentine side of the specimen. For Dyract AP, hydrostatic pressure significantly lowered the bond strengths at the pulp horn region, whereas there were no differences at the periphery and the center regions. Prime&Bond contains an acidic monomer of PENTA; however, the pH of the adhesive was relatively high (2.0– 2.3). The adhesive could remove the smear layer on the intertubular dentine without completely removing the smear plugs [32]. Therefore, it may have been difficult for the adhesive to penetrate into the dentine tubules, whereas HEMA and PENTA in the adhesive played a role promoting the penetration of the monomers into the dentine. The SEM observation of the debonded specimens of the no-pulpal pressure group (Fig. 4a and b) indicated that the mechanical properties of the adhesive may be weak because of the HEMA, acetone, and water components in the adhesive. On the other hand, at the pulp horn region under positive hydrostatic pressure, fracture occurred at the interface (Fig. 4c and d). The pulp horn was reported to have the highest

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permeability in dentine surface [18,23]. This suggests that water contamination on the dentine surface through the dentinal tubules would prevent the priming capacity of the primer by rapidly reducing the acidity, which is necessary to demineralize the underlying mineralized dentine. Consequently, it would not permit adequate penetration and polymerization of the adhesive at the interface. For F 2000, positive hydrostatic pressure significantly reduced the bond strengths in all regions. The acidity of F 2000 Prime/Adhesive is considered to be low (pH 1.1–1.5) because of the maleic acid component. By applying the F 2000 Prime/Adhesive, the smear layer is removed, increasing dentine permeability. Therefore, the perfusion of water through the tubules under positive pressure may have produced an over-wet condition on the dentine surface, resulting in a decrease of the bond strengths. A hybrid layer of 3–4-mm thick has been observed for F 2000 [33], confirming that the bonding mechanism occurs by micromechanical interlocking. The fracture modes of F 2000 specimens that were restored under non-pulpal pressure mainly occurred cohesively within the compomer, possibly because interfacial strength may have exceeded the cohesive strength of the F 2000 compomer (Fig. 5a). Under positive hydrostatic pressure, however, the presence of excessive water on the dentine surface decreased the bond strengths between the F 2000 compomer and the dentine, leading to an increased percentage of failure within the bonding resin (Fig. 5b and c). Presumably, the water interfered with the polymerization of the bonding resin, due to the dilute HEMA concentration prior to polymerization. 5. Conclusion The simulated pulpal pressure of 15 cmH2O had greater influence on bond strengths of compomers to dentine than did dentine regions. Therefore, when measuring bond strengths of compomers to dentine under the simulated in vivo conditions, the wetness of the dentine surface as well as the intrinsic properties of each material should be seriously considered. References [1] DeTrey Dentsply, Technical brochure for Dyract, 1993; quoted in Ref. [2]. [2] DeTrey Dentsply, Dyract product information version, 1994. [3] Yap AU, Lim CC, Neo JC. Marginal sealing ability of three cervical restorative systems. Quintessence International 1995;26:817–20. [4] Sjodin L, Uusitalo M, Van DJ, et al. Resin modified glass ionomer cement. In vitro microleakage in direct class V and class II sandwich restorations. Swedish Dental Journal 1996;20:77–86. [5] Torabzadeh H, Aboush YEY, Lee AR, et al. Comparative assessment of long-term fluoride release from light-curing glass-ionomer cements. Journal of Dental Research 1994;73:853 (Abstract 531). [6] Wilson AD, Groffman DM, Kuhn AT, et al. The release of fluoride and other chemical species from a glass-ionomer cement. Biomaterials 1985;6:431–3.

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