- Email: [email protected]
Single-step adhesives are permeable membranes
Journal of Dentistry 30 (2002) 371–382 www.elsevier.com/locate/jdent
Single-step adhesives are permeable membranes Franklin R. Taya,*, David H. Pashleyb, Byoung I. Suhc, Ricardo M. Carvalhod, Anut Itthagaruna a
Paediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong, SAR, People’s Republic of China b Department of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129, USA c Bisco, Inc, 1100 West Irving Park Road, Schaumburg, IL 60193, USA d Department of Operative Dentistry, Bauru School of Dentistry, University of Sa˜o Paulo, Sa˜o Paulo, Brazil Revised 5 October 2002; accepted 17 October 2002
Abstract Objectives. This study tested the hypotheses that micro-tensile bond strengths of all currently available single-step adhesives to dentine are adversely affected by delayed activation of a light-cured composite, and that such a phenomenon only occurs in the presence of water from the substrate side of the bonded interface. Methods. In experiment I, a control three-step adhesive (All-Bond 2, Bisco) and six single-step adhesives (One-Up Bond F, Tokuyama; Etch&Prime 3.0, Degussa; Xeno CF Bond, Sankin; AQ Bond, Sun Medical; Reactmer Bond, Shofu and Prompt L-Pop, 3M ESPE) were bonded to sound, hydrated dentine. A microfilled composite was placed over the cured adhesive and was either light-activated immediately, or after leaving the composite in the dark for 20 min. In experiment II, three single-step adhesives (Etch&Prime 3.0, Xeno CF Bond and AQ Bond) were similarly bonded to completely dehydrated dentine using the same delayed light-activation protocol. In experiment III, a piece of processed composite was used as the bonding substrate for the same three single-step adhesives. The microfilled composite was applied to the cured adhesives using the same immediate and delayed light-activation protocols. Bonded specimens were sectioned for micro-tensile bond strength evaluation. Fractographic analysis of the specimens was performed using SEM. Stained, undemineralised sections of unstressed, bonded specimens were also examined by TEM. Results. When bonded to hydrated dentine, delayed light-activation had no effect on the control three-step adhesive, but significantly lowered the bond strengths of all the single-step adhesives ( p , 0.05). This adverse effect of delayed light-activation was not observed in the three single-step adhesives that were bonded to either dehydrated dentine or processed composite. Morphological manifestations of delayed light-activation of composite in the hydrated dentine bonding substrate were exclusively located along the composite – adhesive interface, and were present as large voids, resin globules and honeycomb structures that formed partitions around a myriad of small blisters along the fractured interfaces. Conclusion. These features resembled the ‘overwet phenomenon’ that was previously reported along the dentine – adhesive interfaces of some acetone-based three-step adhesives. The cured adhesive layer in single-step adhesives may act as semi-permeable membranes that allow water diffusion from the bonded hydrated dentine to the intermixed zone between the adhesive and the uncured composite. Osmotic blistering of water droplets along the surface of the cured adhesive layer and emulsion polymerisation of immiscible resin components probably account for the compromised bond strength in single-step adhesives after delayed activation of light-cured composites. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Single-step adhesive; Delayed light-activation; Light-cured composite; Semi-permeable membrane; Osmotic blistering
1. Introduction Dentine adhesives are currently available as three-step, two-step and single-step systems depending on how the three cardinal steps of etching, priming and bonding to tooth * Corresponding author. Tel.: þ 86-852-28590251; fax: þ 86-85223933201. E-mail address: [email protected] (F.R. Tay).
substrates are accomplished or simplified [1]. Two-step systems are sub-divided into the self-priming adhesives that require a separate etching step, and the self-etching primers that require an additional bonding step [2]. The recently introduced all-in-one adhesives further combined these three bonding procedures into a single-step application. Irrespective of their packaging designs, these systems are supplied as two-component assemblies to maintain adequate shelflives. They are mixed together immediately before use, and
0300-5712/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 6 4 - 7
372
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
the mixture of hydrophilic and hydrophobic resin components is then applied to the tooth substrate. Of the six commercially available all-in-one adhesives, Prompt L-Pop (3M ESPE, Seefeld, Germany), Etch&Prime 3.0 (Degussa AG, Hanau, Germany), AQ Bond (Sun Medical, Kyoto, Japan) are unfilled versions, whereas One-Up Bond F (Tokuyama, Tokyo, Japan), Reactmer Bond (Shofu, Kyoto, Japan), and Xeno CF Bond (Sankin, Tokyo, Japan) are filled versions that contain fluoride-releasing glass fillers or monomers. For a long time, dentists have assumed that resin composites bond well to dentine adhesives, and that the weak links occur between the adhesives and dentine [3 – 9]. However, this may not be entirely true with the advent of contemporary two-step, and single-step adhesives. We previously showed that some two-step, self-priming adhesives are incompatible with chemical-cured composites, and that the decreases in bond strength are inversely proportional to the acidity of these single-bottle systems [10] Single-step adhesives are even more acidic in nature by virtue of their self-etching capabilities. There is evidence to suggest that they may not be compatible even with hybrid light-cured hybrid composites that are placed on top of these cured adhesives for too long before light-activation [11] In that study, micro-tensile bond strengths of two single-step adhesives were found to decrease exponentially with the time-delay in light-activation of the composite. The objectives of this study were to further examine the hypotheses that all currently available single-step adhesives to dentine are adversely affected by delayed activation of a light-cured, microfilled composite, and that such a phenomenon only occurs in the presence of water from the substrate side of the bonded interface. Specimens bonded to hydrated and dehydrated dentine, as well as processed composites using these adhesives were evaluated using the microtensile bond testing technique. Specimens that were stressed to failure were examined using scanning electron microscopy (SEM). In addition, unstressed, intact resin – dentine interfaces that were bonded with delayed lightactivation of the resin composite were examined with transmission electron microscopy (TEM). The null hypotheses that were tested are: (1) prolonged contact of lightcured resin composite to cured single-step adhesives before light-activation does not result in compromised bond strengths to sound, hydrated dentine, and (2) the presence or absence of water on the substrate side of the bonded interface of single-step adhesives does not affect the results of delayed activation of a light-cured composite.
2. Materials and methods 2.1. Tooth preparation for bond strength evaluation Thirty-four caries-free, human third molars were stored in 0.5% chloramine T at 4 8C, and used within 1 month
following extraction. The occlusal enamel of each tooth was first removed using a slow-speed saw equipped with a diamond-impregnated disk (Isomet, Buehler Ltd, Lake Bluff, IL, USA) under water lubrication. A 180-grit silicon carbide paper was used under running water to create a smear layer on the dentine surface. Bonding was subsequently performed on the occlusal surfaces of deep, coronal dentine. 2.1.1. Experiment I: effect of delayed activation of a light-cured composite on the mTBSs of single-step adhesives to hydrated dentine Twenty-eight teeth were used in this part of the study. They were randomly divided into seven groups of four teeth each. All the teeth in Experiment I were bonded in their normal hydrated status, as they were retrieved from the storage medium. All-Bond 2 (Bisco Inc, Schamburg, IL, USA; Group AB), a three-step adhesive system, was used as the control group. The six light-cured, single-step adhesives, One-Up Bond F (Group OU), Etch&Prime 3.0 (Group EP), Xeno CF Bond (Group CF), AQ Bond (Group AQ; also marketed in North America as Touch&Bond by Parkell Inc, Farmingdale, NY, USA), Reactmer Bond (Group RB) and Prompt LPop (Group PL) constituted the other six groups. A microfilled resin composite that contains pre-polymerised, TMPT organic fillers (Metafil CX, Sun Medical Co. Ltd, Shiga, Japan; also marketed in North America as Epic – TMPT by Parkell, Inc.) was used as the bonding composite in all groups. This microfilled composite does not contain inorganic glass fillers and facilitates subsequent TEM preparation of bonded, unstressed specimens. The general composition and batch numbers of these adhesives and resin composite are listed in Table 1. The application techniques of these adhesives are also outlined in Table 2. During the application of the primer mixture of the control three-step adhesive and the single-step adhesive mixtures, care was taken to ensure that the dentine surfaces were adequately covered by resin after evaporation of the solvents. In the event that matte dentine was encountered, additional coats were applied to produce shiny surfaces prior to lightactivation of the adhesives. Each adhesive group was further divided into two subgroups of two teeth each, based upon the time of contact of the resin composite with the cured adhesive layer prior to light-activation. In subgroup X-0 (where X ¼ adhesive group designation), a 1 mm layer of the microfilled composite was applied to each bonded dentine surface and light-activated immediately for 40 s, using a halogen light-curing unit (Variable Intensity Polymerizer, Bisco) with the curing intensity set at 500 mW cm22. Additional composite layers were subsequently added to the first composite layer, in 1 mm increments and light-activated separately, until a 5 mm thick core buildup was achieved. In subgroup X-20, the first layer of composite that was in contact with
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
373
Table 1 Composition of the adhesives and composite used in the study Material
Three-step adhesive
Single-step adhesive
Resin composite
Brand name
Type
Composition
Lot number
All-bond 2
Unfilled
Etchant. 32% phosphoric acid gel, xanthum gum thickener Primer A. NTG-GMA, acetone, ethanol, water Primer B. BPDM, photoinitiator, acetone D/E bonding resin. Bis-GMA, UDMA, HEMA Water, MMA, HEMA, coumarin dye, methacryloyloxyalkyl acid phosphate, methacryloxyundecane dicarboxylic acid (MAC-10), multifuctional methacrylic monomer, fluoroaluminosilicate glass, photoinitiator (aryl borate catalyst) Universal: water, ethanol, HEMA, stabilisers
0100001442
(Bisco Inc. Schaumburg, IL, USA)
One-up bond F (Tokuyama Corp. Tokyo, Japan)
Filled
Etch&Prime 3.0 (Degussa AG Hanau, Germany)
Unfilled
Xeno CF Bond (Sankin Tokyo, Japan)
Filled
AQ Bonda (Sun Medical Co. Ltd Shiga, Japan)
Unfilled
Reactmer bond (Shofu Inc. Kyoto, Japan)
Filled
Prompt L-Pop (3M ESPE Seefeld, Germany)
Unfilled
Metafil CXb (Sun Medical Co. Ltd Japan)
Microfilled
Catalyst: pyrophosphate, HEMA, photoinitiators, stabilisers Water, ethanol, HEMA, methacryloxyethylpyrophosphate, fluoride-releasing phosphazene monomer, UDMA, micro-filler, photoinitiator Base liquid. Water, acetone, 4-META, HEMA, MMA, UDMA, photoinitiator AQ sponge. Polyurethane foam, Initiator ( p-TSNa) Reactmer Bond A. Water, acetone, F-PRG fillers, FASG fillers, Initiators (TMBA, p-TSNa) Reactmer bond B. 4-AET, 4-AETA, HEMA, UDMA, photoinitiator Water, stabiliser, parabenes, methacrylated phosphoric acid esters, fluoride complex, photoinitiator (BAPO) Dimethacrylates such as UDMA (34 wt% organic TMPT filler (40 wt%) micro silica (26 wt%) photoinitiator (with aromatic tertiary amine) pigments, stabiliser
0000011967 0000011968 0000007876 23074510902
204
Universal: 350-04
Catalyst: 350-33 VK5
040002
FW0062849
TL2
Abbreviations: 4-AET, 4-acryloxyethyltrimellitic acid; 4-AETA, 4-acryloxyethyltrimellitic anhydride; 4-META, 4-methacryloxyethyltrimellitic anhydride; BAPO, bis-acyl phosphine oxide; Bis-GMA, bisphenol A diglycidyl ether dimethacrylate; BPDM, biphenyl dimethacrylate; F-PRG, full-reaction type pre-reacted glass ionomer filler; FASG, fluoroaluminosilicate glass; HEMA, 2-hydroxylethyl methacrylate; MMA, methyl methacrylate; NTG-GMA, Ntolylglycine-glycidyl methacrylate; p-TSNa, p-toluenesulfinic acid sodium salt; TMBA, trimethyl barbituric acid; TMPT, trimethylolpropane-trimethacrylate; UDMA, urethane dimethacrylate. a Also marketed in North America as Touch&Bond by Parkell, Inc., Farmingdale, NY, USA. b Also marketed in North America as Epic-TMPT by Parkell, Inc.
the cured adhesive layer was left in the dark for 20 min before light-activation. Subsequent core buildup followed the procedures described in the previous subgroup. The choice of 20 min was based on our previous study that such a period of delayed light-activation could result in null bond strength in some single-step adhesives [11]. We realised that such a lengthy delayed period was far removed from clinical practice, as clinicians are unlikely to leave a composite unactivated for more than 2 –3 min. However, it was the purpose of this study to investigate the effects of prolonged contact of unpolymerised composites on the surfaces of single-step adhesives that were bonded to hydrated dentine.
2.1.2. Experiment II: effect of delayed activation of a light-cured composite on the mTBSs of single-step adhesives to completely dehydrated dentine Three of six single-step adhesives (Etch&Prime 3.0, Xeno CF Bond and AQ Bond) were randomly selected for this experiment. The two teeth that were used for each adhesive were first dehydrated in an ascending ethanol series (70, 80, 95%, three changes in 100%) for 2 h each, following the TEM preparation protocol by Tay et al. [12]. After the third change, the teeth were left to completely dehydrate in absolute ethanol for an additional 48 h. This produced a bonding dentine substrate that is completely devoid of moisture, but
374
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Table 2 Application techniques of the control three-step and the single-step adhesives Adhesive systems
Dentine conditioning
Priming
Bonding
All-Bond 2
Uni-etch for 15 s, rinse and kept slightly moist
Mixed All-Bond 2 primer A and B, air-drylight-activate for 20 s
Apply D/E bonding resin,gently thinned down,light-activate for 10 s
One-Up Bond F
Mix equal droplets of the bonding agents A (clear liquid) and B (bright yellow liquid) for until a pink, homogenous liquid mixture was obtained Apply to tooth substrates and left undisturbed for 20 s, no rinsing Without further air-drying, light-activate for 10 s Mix equal amount of the universal and catalyst Apply to tooth substrates and allow acting for 30 s, no rinsing Briefly air-dry, light-activate for 10 s Re-apply adhesive mixture, briefly air-dried and light-activate for 10 s Mix equal amount of the universal and catalyst Apply to tooth substrates and allow acting for at least 20 s, no rinsing Briefly air-dry, light-activate for 10 s Re-apply adhesive mixture, briefly air-dried and light-activate for 10 s Dispense one drop of AQ liquid into dispensing well containing one piece of AQ sponge Apply the adhesive-coated AQ sponge to dentine substrates for 20 s, no rinsing Air-dry 3– 5 s, re-apply adhesive, air-dry 5–10 s, light-activate for 10 s using only halogen light-curing unit Mix equal droplets of reactmer bond bonding agent A (white liquid) and the B (amber liquid) for 5 s Apply to tooth substrates and allow acting for 20 s, no rinsing Briefly air-dry, light-activate for 20 s Activate blister pack by emptying the liquid out of the red blister into the yellow blister The activated mixture was applied to tooth substrates with agitation for 15 s, no rinsing Briefly air-dry, light-activate for 10 s using only halogen light-curing unit
Etch&Prime 3.0
Xeno CF Bond
AQ Bond
Reactmer Bond
Prompt L-Pop
without reducing its buffering capacity for the acidic single-step adhesives. In subgroup D-X-20 (where X ¼ adhesive group designation), the completely dehydrated dentine bonding surfaces were similarly treated with one of the three single-step adhesives. The delayed light-activation protocol of the resin composite followed that previously described for subgroup X-20. 2.1.3. Experiment III: effect of delayed activation of a light-cured composite on the mTBSs of single-step adhesives to processed composites To verify that any deterioration in mTBS during delayed light-activation is caused solely by the diffusion of water from the bonded substrate through the cured adhesive layer, Experiment I was repeated for the three single-step adhesives, Etch&Prime 3.0, Xeno CF Bond and AQ Bond, using light- and heat-processed Metafil CX as the bonding substrate. Five-millimeter thick layers of Metafil CX were dispensed into 4 £ 2 cm flat Teflon moulds (Electron Microscopy Sciences, Fort Washington, PA, USA). The moulds containing the uncured composite were placed
inside an experimental composite inlay processing chamber (Nitro-Therma-Lite; Bisco, Inc.) and light-activated under a pressurised nitrogen atmosphere maintained at 551.6 kPa (i.e. 80 psi) for one complete cycle at 125 8C for 20 min. The bonding surface of each processed composite block was ground with 180-grit SiC paper and further sandblasted with 50 mm alumina for 10 s. These blocks were sonicated in distilled water, air-dried, and bonded using the three single-step adhesives in the manner previously described. In the immediate light-activation subgroup C-X-0 (where C ¼ adhesive group designation), Metafil CX was applied and light-activated incrementally as in subgroup X-0. Similarly, the delayed light-activation subgroup C-X-20, the first 1 mm layer of Metafil CX was placed over the cured adhesive and left in the dark for 20 min before lightactivation. 2.2. mTBS evaluation After storage in distilled water at 37 8C for 24 h, bonded teeth from Experiment I and II were sectioned occluso-gingivally into serial slabs using an Isomet saw
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
under water lubrication. One slab from each bonded tooth was not subjected to tensile testing and reserved for TEM examination. The other slabs tooth were sectioned into 0.9 £ 0.9 mm2 composite – dentine beams, according to the technique for the ‘non-trimming’ version of the micro-tensile test reported by Shono et al. [13]. Each group of bonded teeth yielded 20– 22 beams for bond strength evaluation. Beams with premature failure during sectioning were assigned a null bond strength value and were included in the compilation of the mean tensile bond strength. The bonded composite blocks from Experiment III were similarly aged in distilled water at 37 8C for 24 h and then sectioned into 0.9 mm slabs. Due to the high incidence of cohesive failures in resin composites initially observed using the Shono’s technique, each slab was hand-trimmed into 0.9 £ 0.9 mm2 dumbbell-shaped specimens according to the version of micro-tensile bond testing reported by Sano et al. [14]. Fifteen specimens were produced for each of the six experimental subgroups. Specimens were fixed to a Bencor Multi-T device (Danville Engineering, San Ramon, CA, USA), using Zapit cyanoacrylate (Dental Ventures of America, Corona, CA, USA) and tested to failure under tension in a universal testing machine (Model 4440; Instron Inc, Canton, MA, USA) at a crosshead speed of 1 mm/min. 2.3. Statistical analysis For each adhesive, the bond strength data obtained for the corresponding subgroups were statistically analysed with either Mann – Whitney Rank Sum tests or Kruskal – Wallis one-way ANOVA on ranks, using SigmaStat Version 2.03 (SPSS, Chicago, IL, USA). Statistical significance was set in advance at the 0.05 probability level. In adhesive groups that involved additional subgroups from Experiments II and III, multiple comparisons were done with Dunn’s test at a ¼ 0.05. 2.4. SEM fractographic analysis Six fractured composite – dentine beams from Experiment I and II of each adhesive that were representative of the mean bond strengths of the corresponding subgroups were prepared for SEM examination. Both the dentine and composite sides of the fractured beams were air-dried. They were not dehydrated using methods that involve passing the specimens through organic solvents [15], to avoid the possibility of extracting uncured monomers or partially polymerised oligomers from the fractured interfaces. Completely air-dried specimens were secured to brass stubs using Zapit. They were sputter-coated with gold/ palladium and examined using a scanning electron microscope (Cambridge Stereoscan 440, Cambridge, United Kingdom) operating at 12 kV. Failures were classified as: (a) adhesive failure, if the fracture site was maintained
375
entirely within the adhesive; (b) mixed failure, if the fracture site continued from the adhesive into either the composite or dentine; and (c) substrate failure, if the fracture occurred exclusively within the composite or dentine. In addition, abnormalities along the fractured interfaces that were associated with delayed light-activation of the resin composites were also recorded. 2.5. TEM examination of unstressed, bonded dentine –composite interfaces TEM was only performed to provide additional information on the abnormal features associated with delayed light-activation of resin composites on adhesive-coated, hydrated dentine, and to validate that the previous SEM observations were not artefacts produced during tensile stress, or by contamination of the fractured interfaces. The two remaining composite –dentine slabs from subgroup X20 (Experiment I) of each adhesive were used for TEM examination. Undemineralised sections of the bonded specimens, 90 – 120 nm thick, were prepared according to the TEM protocol described in Tay et al. [12]. These undemineralised sections were collected using single slot, carbon- and formvar-coated copper grids (Electron Microscopy Sciences, Fort Washington, PA, USA). They were double-stained with uranyl acetate for 10 min and Reynold’s lead citrate for an additional 5 min. Examination was performed using a TEM (Philips EM208S, Eindhoven, The Netherlands) operating at 100 kV. Digitised images were recorded using the charge couple device (CCD) camera (Bioscan, Model 792, Gatan Inc, Pleasanton, CA, USA) attached to the microscope.
3. Results mTBS data of the three-step, control adhesive and the six single-step adhesives bonded to hydrated dentine (Experiment I) are summarised in Table 3 and also graphically shown in Fig. 1. Delayed light-activation has no effect on the control adhesive ( p . 0.05). In contrast, delayed light-activation significantly reduced the mTBS of all the single-step adhesives ( p , 0.05). For the three single-step adhesives tested in Experiments II and III, delayed light-activation has no effect when these adhesives were bonded to completely dehydrated dentine ( p . 0.05; Table 3). Similarly, no deterioration in mTBSs was observed in these single-step adhesives when delayed light-activation was used with the processed composite as the bonding substrate ( p . 0.05; Table 3). Fig. 2A shows a representative mixed failure mode that was observed in AB-0 and AB-20 subgroups. No abnormalities were detected along the fractured interfaces. Similarly, mixed failure was the predominant failure mode in the corresponding immediate light-activation subgroups
c
Control three-step adhesive. Values are means ^ standard deviation in MPa. For each adhesive (i.e. across a row), subgroups labelled with the same lower case superscript are not statistically significant ( p . 0.05). Number of beams tested that were without premature failure. Beams with premature failure during specimen preparation were included as zero bond strengths in the calculation of mean bond strength. a
b
NA NA 38.8 ^ 7.0a[15/15] 47.3 ^ 15.3a[15/15] 32.4 ^ 9.6a[15/15] NA NA NA NA 35.0 ^ 9.5a[15/15] 48.3 ^ 11.9a[15/15] 38.2 ^ 10.9a[15/15] NA NA All-Bond 2 (AB)a One-Up Bond F(OU) Etch&Prime 3.0(EP) Xeno CF Bond(CF) AQ Bond(AQ) Reactmer bond(RB) Prompt L-Pop(PL)
48.4 ^ 11.7ab[22/22]c 42.9 ^ 7.9a[22/22] 37.0 ^ 9.3a[20/20] 34.5 ^ 8.3a[21/21] 26.8 ^ 6.1a[23/23] 25.8 ^ 3.6a[21/21] 24.1 ^ 5.2a[22/22]
50.9 ^ 9.6a[20/20] 10.1 ^ 4.4b[20/20] 0.6 ^ 1.7b[3/21] 10.9 ^ 5.1b[22/22] 9.3 ^ 3.2b[22/22] 9.7 ^ 5.0a[21/21] 2.2 ^ 2.7b[13/22]
NA NA 37.4 ^ 6.5a[20/20] 35.2 ^ 5.7a[21/21] 26.7 ^ 5.3a[20/20] NA NA
Delayedlight-activation (C-X-20) Immediatelight-activation (C-X-0) Delayed light-activation on completely dehydrateddentine (D-X-20) Immediate light-activation onhydrateddentin (X-0)
Delayed light-activation on hydrated dentine (X-20)
Experiment II Experiment I
Deep coronal dentine Group designations(X)
Table 3 mTBS of the control three-step adhesive and the single-step adhesives examined in the study
Experiment III
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Processed composite
376
(i.e. X-0) of each single-step adhesive (Fig. 2B). Pure adhesive failures were rare in these subgroups. SEM examination of fractured specimens and TEM examination of unstressed specimens from the delayed light-activation subgroups of the six single-step adhesives bonded to hydrated dentine (i.e. X-20) revealed that the dentine –adhesive interfaces were intact and that abnormal morphological features occurred solely along the composite – adhesive interfaces. They could be classified into voids (Fig. 3), resin globules (Fig. 4) and honeycomb structures that formed partitions around a myriad of small blisters along the fractured interfaces (Figs. 5 and 6). All unstressed TEM specimens that appeared intact by visual inspection after laboratory processing were found to have separated on TEM examination, with the spaces infiltrated by the embedding epoxy resin.
4. Discussion As delayed light-activation of resin composite only adversely affected the bond strengths of hydrated dentine bonded with single-step adhesives but not the control threestep adhesive (Experiment I), we have to reject the first null hypothesis. This abnormal phenomenon occurred in all the single-step adhesives available, irrespective of their resin composition and initiator systems (Table 1). The phenomenon also occurred regardless of whether a hybrid composite with ion-leachable glass fillers [11] or a microfilled composite was used for the restoration. Based on the results of Experiment I alone, it was tempting to attribute the phenomenon to some kind of adverse interactions that occurred between uncured resin monomers from the adhesive and the resin composite (i.e. the intermixed zone
Fig. 1. Delayed activation of light-cured composites adversely affects the bonding of all commercially available single-step adhesives to sound dentine. In contrast, the micro-tensile bond strength of a three-step adhesive system is not affected by delayed light-activation.
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 2. Control specimens: (A) low magnification SEM micrograph of the dentine side of a fractured beam in the All-Bond 2 delayed light-activation subgroup (AB-20). A mixed failure mode was observed involving dentine (D), adhesive primer (P), bonding resin (B) and resin composite (C). The fracture interfaces were free of voids; (B) SEM micrograph of a mixed failure mode that was representative of those that occurred in the immediate light-activation subgroups of the six single-step adhesives. The one shown here was taken from the fractured composite side of a fractured beam in the Etch&Prime 3.0 immediate light-activation subgroup (EP-0). Minimal voids could be found within the fractured composite (C). A: fractured adhesive; D: fractured dentine. Asterisk: area in which collagen fibrils could be identified at high magnification.
(IZ)). Reaction of isocyanate groups of UDMA-containing composites with water, for example, could generate carbon dioxide that might contribute to the honeycomb structure. Likewise, condensation polymerisation involving dicarboxylic acids and diols could result in the liberation of water
377
Fig. 3. Adverse effect of delayed light-activation of a resin composite that was applied over cured single-step adhesives. I. Voids (arrows): (A) SEM micrograph of the dentine side of a fractured beam in the Reactmer Bond delayed light-activation subgroup (RB-20). Numerous large voids up to 50 mm in diameter could be observed along the interface between the fractured composite (C) and fractured adhesive (A). D: exposed dentine; (B) SEM micrograph from the dentine side of a fractured beam in the AQ Bond delayed light-activation subgroup (AQ-20). An intermediate zone along the fractured composite adhesive interface could be clearly observed, in which the bases of numerous large voids were present. D: exposed dentine.
as a by-product, which in turn could be trapped within the adhesive – composite interfaces. However, these chemical reactions could not be expected at the ambient temperature in which our experiments were conducted. The observed results that adverse reactions did not occur when delayed light-activation was performed using processed composite as bonding substrates (Experiment III) corroborated
378
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 4. Adverse effect of delayed light-activation. II. Resin globules (arrowheads): (A) SEM micrograph of the region labelled with an asterisk in Fig. 3A (RB20). Numerous resin globules were observed within the fractured adhesive resin (A): (B) SEM micrograph of the region labelled with an asterisk in Fig. 3B (AQ-20). Numerous resin globules could be seen on the surface of the adhesive layer (A); (C) TEM micrograph of the fractured dentine side of a One-Up Bond F (OU-20) specimen. Failure occurred along the adhesive–composite interface, which was subsequently infiltrated by epoxy resin (E). Numerous electronlucent resin globules were observed along the surface of an IZ. A: adhesive containing fluoroaluminosilicate glass fillers (G). H: hybrid layer; U: undemineralized dentine; (D). TEM micrograph of the fractured composite side of a Prompt L-Pop (PL-20) specimen, showing the presence of numerous resin globules within the microfilled composite. P: pre-polymerised TMPT fillers; M: composite resin matrix. Partitions (pointer) within the resin globules were probably contributed by the Prompt L-Pop adhesive. E: epoxy resin.
that the phenomenon observed was not caused by inherent resin–initiator incompatibility that was previously reported to occur between acidic resin monomers and chemical-cured composites that utilise binary redox initiator systems [10, 16–18]. As water is an indispensable component in single-step adhesives (Table 1), the results further suggested that the phenomenon was not caused by retention of incompletely evaporated water within the oxygen inhibition layer of the adhesives [19,20]. The results also focused our attention to
the fact that the underlying cause of this abnormal phenomenon is likely to be environmental in nature. Our conjecture was eventually substantiated by the results of Experiment II, affirming that water derived from the bonding substrate is responsible for the decline in bond strength of the single-step adhesives after delayed light-activation of resin composites. We thus have to reject the second null hypothesis. The morphological manifestation of resin globules and blisters along the composite –adhesive interface in delayed
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
379
Fig. 5. Adverse effect of delayed light-activation. III. SEM of the honeycomb structures: (A) Fractured composite side of a specimen beam in the Reactmer Bond delayed light-activation subgroups (RB-20); (B) High magnification of B (RB-20) showing the presence of a honeycomb resin structure that formed the partitions of a myriad of blisters around along the fractured interface. Bar ¼ 1 mm; (C) High magnification of the honeycomb structure from the dentine side of a fractured beam in the delayed light-activation subgroup of an unfilled single-step adhesive, AQ Bond (AQ-20).
light-activation is reminiscent of the characteristic features of the ‘overwet phenomenon’ that was observed along the adhesive – dentine interface of some acetone-based adhesives when a wet bonding technique was used [21, 22]. The ‘overwet phenomenon’ that was seen in total-etch, three-step and two-step adhesives may also be caused by transudation of dentinal fluid, from tubules that are rendered patent after acid-conditioning, in vital teeth that exhibit positive pulpal pressures [23]. However, the single-step adhesives employed in this study are self-etching in nature and do not require the removal of smear plugs. Moreover, the experiments on hydrated dentine in this study were not
performed under dentine perfusion [24,25], and none of the previously reported TEM features of the overwet phenomenon could be seen along the dentine – adhesive interfaces, which remained intact in all specimens examined. Thus, the only explanation for the overwet phenomenon along the adhesive – composite interface that we can think of is that the cured single-step adhesive layers act as permeable membranes that permit water to diffuse from the substrate side to the IZ. It has been shown that water is capable of passing through diffusion barriers provided by organic resin coatings [26,27]. Most single-step adhesives contain
380
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 6. Adverse effect of delayed light-activation. III. TEM of the honeycomb structures (pointers): (A) AQ Bond subgroup (AQ-20). Finger-like projections (pointers) corresponded with the honeycomb structure that formed partitions around blisters along an IZ between the composite (C) and the adhesive (A). P: TMPT fillers in the composite; H: hybrid layer; U: undemineralised dentine. E: epoxy resin; (B) the fractured honeycomb structure from the composite side of the Reactmer Bond subgroup (RB-20). The IZ contained fluoroaluminosilicate glass fillers (G) from the filled adhesive. Numerous globular resin phases (arrowheads) were identified. C: composite; E: epoxy resin; (C) The dentine side of the same specimen in Fig. 5B (subgroup RB-20). A: filled adhesive layer that contained fluoroaluminosilicate glass fillers (G) and fumed silica (arrow). IZ: intermixed zone. Arrowhead: resin globular phases within the IZ. E: epoxy resin.
hydroxyethyl methacrylate (HEMA) which can polymerise in the presence of water to form ‘microporous’ hydrogel with pore sizes ranging from 10 to 100 nm [28]. Differential water movement across the cured adhesive layer may occur in the presence of increased concentrations of dissolved inorganic ions, uncured, water-soluble, hydrophilic resin monomers, or dissolved collagen/proteoglycans components within the oxygen inhibition layer of the cured adhesive. This concentration difference may establish an osmotic pressure gradient,
causing water movement from a region of low solute concentration (i.e. dentinal tubules in hydrated dentine substrate) to a region of high solute concentration (i.e. IZ along the adhesive – composite interface) [29]. This, in turn, may result in osmotic blistering of microscopic water droplets along the uncured IZ [29,30]. Blister initiation and growth via osmosis is common in resin-coated metal systems, being one of the first signs for corrosion that eventually causes delamination of the coating. In conventional osmotic blistering, interfacial contamination
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
in the form of trapped ions between the metal and resin surface results in the coating acting as a semi-permeable membrane, allowing water transport from the environment into the interface. Blisters are initiated in weak spots along the adhesive interface. With time, blisters may enlarge to a point where adjacent blisters coalesce. The initial osmotic pressure buildup within the blisters is high, but gradually diminishes as the concentration of the impurities is being diluted [30,31]. What we observed in delayed lightactivation of resin composites may be comparable to conventional osmotic blistering. However, unlike conventional osmotic blistering, the osmotic gradient results in water movement from the dentine substrate into an unpolymerised interface, producing an immiscible blend of hydrophobic and hydrophilic monomers. Emulsion polymerisation of the hydrophobic resins in water (oil-in-water type emulsion) [32] and the diluted hydrophilic adhesive resins within the hydrophobic composite (water-in-oil type emulsion) [32] eventually resulted in the formation of different types of resin globules within the interface of a single specimen (Fig. 4). Resin polymerisation around the osmotic blisters produces in the honeycomb resin structures observed in Figs. 5 and 6. Similar to our previous use of a hydrophobic impression material to take impressions of dentinal fluid transudates from acid-etched dentine, [23] the honeycomb structures that were formed as the relatively hydrophobic resin composite polymerised are essentially negative impressions of the water droplets that extruded out of cured adhesive interfaces. The larger voids observed in Fig. 3 may also be accounted for by the coalescence of the smaller water blisters. The high osmotic pressure build up within these blisters may also explain why most of the TEM specimens were spontaneously fractured during laboratory processing. We previously reported that micro-tensile bond strengths of single-step adhesives decreased exponentially with time [11]. Using two mathematical models based on Fickian diffusion and osmotic theory to predict blister growth in resin coating systems, Pommersheim and Nguyen showed that irrespective of whether impurities were uniformly distributed or present over concentrated spots along the interface, the changes in blister radii, surface areas, heights, volumes and osmotic pressures all varied directly as a fractional power of time [33]. The implication of these mathematical models in the present context is that the overwet phenomenon caused by osmotic blistering along the uncured adhesive – composite interface is negligible in immediate light-activation, wherein the adhesive oxygen from the adhesive inhibition layer is quickly absorbed into the composite layer [34], trapping these ‘impurities’ at least semi-permanently within the interface as the resin polymerises. This probably explained why delayed light-activation has no adverse effect on the micro-tensile bond strength of the control three-step adhesive, or when a cured single-step adhesives were covered with a layer of lightcured bonding resin prior to the application of the composite [11]. It must be remembered, however, that even under these
381
circumstances, the adhesive layer still function as a semipermeable membrane. Inherent weaknesses such as airvoids within the resin – dentine interfaces cannot be totally eliminated even with the utmost care during adhesive application [35,36]. This implies that with time, water may accumulate within these inherent weak spots via osmosis, with the buildup of osmotic pressure helping to enlarge these flaws so that blisters can form. If the composite is light-activated immediately, such weak spots may no longer reside solely along the adhesive –composite interface. This provides an alternative explanation to the leaching of hydrophilic resin components in interpreting why bond strength decreased on ageing of adhesives that contain polymerised hydrophilic resin components [6,7]. This hypothesis has to be further substantiated. In conclusion, clinicians should be aware of the potential drop in bond strength on prolonged contact of single-step adhesives with light-cure composites before light-activation. Although clinicians are unlikely to leave a lightcured composite unactivated for more than 2 – 3 min, multiple direct or indirect restorations should be lightactivated individually and as soon as the composite is applied. The use of these acidic adhesives to treat dentin surfaces before luting indirect restorations or posts with dual-cure resin composite cements should be avoided. Although the light-activated reaction can be initiated soon after luting, some clinicians may take longer to remove excess cement before light-activation. Moreover, the chemical reaction lasts over hours and may be significantly compromised. It is possible also that the recently introduced light-activation techniques that attempt to reduce the effect of polymerisation shrinkage stress may lead to compromised results with the use of these single-step adhesives [37]. This is an immediate concern that requires further research, as dentists and researchers have usually assumed that bond failures occur between adhesives and dentine and not between adhesives and composites. Permeability of single-step adhesives to water may also hasten the rate of water sorption and leaching of resin components [38], challenging the durability of resin – dentine bonds produced by these adhesives. This is a subject of considerable concern and should be investigated further.
Acknowledgements We thank Amy Wong and W.S. Lee of the Electron Microscopy Unit, the University of Hong Kong for technical assistance. The AQ Bond and Metafil CX used in this study were supplied by Sun Medical Co. Ltd. The One-Up Bond F was supplied by Tokyyama Corp. The Reactmer Bond was also sponsored by Shofu Inc. This study was supported, in part, by CRC grant 1020 3278 24993 08004 323 01 from The University of Hong Kong, Hong Kong SAR, China, by grant CNPq #300481/95-0 from The University of Sa˜o Paulo, Brazil, and by grant DE 06427 from the National
382
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Institute of Dental and Craniofacial Research, USA. The authors are grateful to Michelle Barnes for secretarial support.
[19]
[20]
References [1] Kugel G, Ferrari M. The science of bonding: from first to sixth generation. The Journal of the American Dental Association 2000; 131(Suppl.):20S–5S. [2] Tanumiharja M, Burrow MF, Tyas MJ. Microtensile bond strengths of seven dentin adhesive systems. Dental Materials 2000;16:180–7. [3] Kiyomura M. Bonding strength to bovine dentin with 4-META/ MMA-TBB resin—long-term stability and influence of water. Journal of the Japanese Society for Dental Materials and Devices 1987;6: 860–72. [4] Wang T, Nakabayashi N. Effect of 2-(methacryloxy)-ethyl phenyl hydrogen phosphate on adhesion to dentin. Journal of Dental Research 1991;70:59 –66. [5] Eick JD, Robinson SJ, Byerley TJ, Chappell RP, Spencer P, Chappelow CC. Scanning transmission electron microscopy/energydispersive spectroscopy analysis of the dentin adhesive interface using a labeled 2-hydroxyethylmethacrylate analogue. Journal of Dental Research 1995;74:1246– 52. [6] Sano H, Yoshikawa T, Pereira PN, Kanemura N, Mongami M, Tagami J, Pashley DH. Long-term durability of dentin bonds made with a self-etching primer, in vivo. Journal of Dental Research 1999; 78:906–11. [7] Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. In vivo degradation of resin–dentin bonds in humans over 1 to 3 years. Journal of Dental Research 2000;79:1385–91. [8] Spencer P, Wang Y, Walker MP, Wielicka DM, Swafford JR. Interfacial chemistry of the dentin/adhesive bond. Journal of Dental Research 2000;79:1458– 63. [9] Walker MP, Wang Y, Swafford J, Evans A, Spencer P. Influence of additional acid etch treatment on resin cement dentin infiltration. Journal of Prosthodontics 2000;9:77–81. [10] Sanares AME, Itthagarun A, King NM, Tay FR, Pashley DH. Adverse surface interactions between one-bottle light-cured adhesives and chemical-cured composites. Dental Materials 2001;17:542–56. [11] Tay FR, King NM, Suh BI, Pashley DH. Effect of delayed activation of light-cured resin composites on bonding of all-in-one adhesives. Journal of Adhesive Dentistry 2001;3:207–25. [12] Tay FR, Moulding KM, Pashley DH. Distribution of nanofillers from a simplified-step adhesive in acid-conditioned dentin. Journal of Adhesive Dentistry 1999;2:103–17. [13] Shono Y, Ogawa T, Terashita M, Carvalho RM, Pashley EL, Pashley DH. Regional measurement of resin– dentin bonding as an array. Journal of Dental Research 1999;78:699–705. [14] Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho R, Pashley DH. Relationship between surface area for adhesion and tensile bond strength—evaluation of a micro-tensile bond test. Dental Materials 1994;10:236–40. [15] Perdiga˜o J, Lambrechts P, Van Meerbeek B, Vanherle G, Lopes AL. Field emission SEM comparison of four postfixation drying techniques for human dentin. Journal of Biomedical Materials Research 1995;29:1111– 20. [16] Nakamura M. Adhesive self-curing acrylic resin. Composition of 4META bonding agent. Japanese Journal of Dental Materials 1985;4: 672–91. [17] Yamauchi J, Yamada K, Shibatani K. (inventors). Kuraray Company Limited, assignee. Adhesive compositions for the hard tissues of the human body. United States Patent 4,182,035; January 8 1990. [18] Ikemura K, Endo T. Effect on adhesion of new polymerization initiator systems comprising 5-monosubstituted barbituric acids, aromatic
[21]
[22]
[23] [24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32] [33]
[34]
[35]
[36]
[37]
[38]
sulphonate amides, and tert-butyl peroxymaleic acid in dental adhesive resin. Journal of Applied Polymer Science 1999;72:1655 –68. Pashley EL, Zhang Y, Lockwood PE, Rueggeberg FA, Pashley DH. Effects of HEMA on water evaporation from water–HEMA mixtures. Dental Materials 1998;14:6–10. Paul SJ, Leach M, Rueggeberg FA, Pashley DH. Effect of water content on the physical properties of model dentine primer and bonding resins. Journal of Dentistry 1999;27:209 –14. Tay FR, Gwinnett AJ, Wei SHY. The overwet phenomenon: a scanning electron microscopic study of surface moisture in the acidconditioned, resin–dentin interface. American Journal of Dentistry 1996;9:109–14. Tay FR, Gwinnett AJ, Wei SHY. Micromorphological spectrum from overdrying to overwetting acid-conditioned dentin in water-free, acetone-based, single-bottle primer/adhesives. Dental Materials 1996; 12:236–44. Itthagarun A, Tay FR. Self-contamination of deep dentin by dentin fluid. American Journal of Dentistry 2000;13:195 –200. Pereira PNR, Sano H, Ogata M, Zheng L, Nakajima M, Tagami J, Pashley DH. Effect of region and dentine perfusion on bond strengths of resin-modified glass ionomer cements. Journal of Dentistry 2000; 28:347–54. Escribano N, Del-Nero O, de la Macorra JC. Sealing and dentin bond strength of adhesive systems in selected areas of perfused teeth. Dental Materials 2001;17:149–55. Funke W. Toward a unified view of the mechanism responsible for paint defects by metallic corrosion. Industrial and Engineering Chemistry Products Research and Development 1984;24:343–7. Nguyen T, Bentz D, Byrd E. Method for measuring water diffusion in a coating applied to a substrate. Journal of Coating Technology 1995; 67:37–46. Chirila TV, Chen YC, Griffin BJ, Constable IJ. Hydrophilic sponges based on 2 hydroxyethyl methacrylate I: effect of monomer mixture composition on the pore size. Polymer International 1993;32:221 –32. van der Meer-Lerk LA, Heertjes PM. Blistering of varnish films on substrates induced by salts. Journal of Oil and Colour Chemists Association 1975;58:79–84. van der Meer-Lerk LA, Heertjes PM. Mathematical model of growth of blisters in varnish films on different substrates. Journal of Oil and Colour Chemists Association 1979;62:256–63. Nguyen T, Hubbard J, Pomersheim JM. Unified model for the degradation of organic coatings on steel in a neutral electrolyte. Journal of Coating Technology 1996;68:45 –56. Myers D. Surfactant science and technology, 2nd ed. New York: VCH Publishers, Inc; 1992. p. 209–45. Pomersheim JM, Nguyen T. Prediction of blistering in coating systems. In: Bierwagen GP, editor. Organic coatings for corrosion control. Proceedings of the American Chemical Society Symposium Series No. 689, Washington, DC: American Chemical Society publisher; 1998. p. 137 –50. Rueggeberg FA, Margeson DH. The effect of oxygen inhibition on an unfilled/filled composite system. Journal of Dental Research 1990;69: 1652–8. Pashley DH, Sano H, Cuicchi B, Yoshiyama M, Carvalho RM. Adhesion testing of dentin bonding agents: a review. Dental Materials 1995;11:117–25. Eick JD, Gwinnett AJ, Pashley DH, Robinson SJ. Current concepts on adhesion to dentin. Critical Reviews in Oral Biology and Medicine 1997; 8:306–35. Kanca J, Suh BI. Pulse activation: reducing resin-based composite contraction stresses at the enamel cavosurface margins. American Journal of Dentistry 1999;12:107–12. Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. Resin– tooth adhesive interfaces after long-term function. American Journal of Dentistry 2001;14:211–25.
Single-step adhesives are permeable membranes Franklin R. Taya,*, David H. Pashleyb, Byoung I. Suhc, Ricardo M. Carvalhod, Anut Itthagaruna a
Paediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong, SAR, People’s Republic of China b Department of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129, USA c Bisco, Inc, 1100 West Irving Park Road, Schaumburg, IL 60193, USA d Department of Operative Dentistry, Bauru School of Dentistry, University of Sa˜o Paulo, Sa˜o Paulo, Brazil Revised 5 October 2002; accepted 17 October 2002
Abstract Objectives. This study tested the hypotheses that micro-tensile bond strengths of all currently available single-step adhesives to dentine are adversely affected by delayed activation of a light-cured composite, and that such a phenomenon only occurs in the presence of water from the substrate side of the bonded interface. Methods. In experiment I, a control three-step adhesive (All-Bond 2, Bisco) and six single-step adhesives (One-Up Bond F, Tokuyama; Etch&Prime 3.0, Degussa; Xeno CF Bond, Sankin; AQ Bond, Sun Medical; Reactmer Bond, Shofu and Prompt L-Pop, 3M ESPE) were bonded to sound, hydrated dentine. A microfilled composite was placed over the cured adhesive and was either light-activated immediately, or after leaving the composite in the dark for 20 min. In experiment II, three single-step adhesives (Etch&Prime 3.0, Xeno CF Bond and AQ Bond) were similarly bonded to completely dehydrated dentine using the same delayed light-activation protocol. In experiment III, a piece of processed composite was used as the bonding substrate for the same three single-step adhesives. The microfilled composite was applied to the cured adhesives using the same immediate and delayed light-activation protocols. Bonded specimens were sectioned for micro-tensile bond strength evaluation. Fractographic analysis of the specimens was performed using SEM. Stained, undemineralised sections of unstressed, bonded specimens were also examined by TEM. Results. When bonded to hydrated dentine, delayed light-activation had no effect on the control three-step adhesive, but significantly lowered the bond strengths of all the single-step adhesives ( p , 0.05). This adverse effect of delayed light-activation was not observed in the three single-step adhesives that were bonded to either dehydrated dentine or processed composite. Morphological manifestations of delayed light-activation of composite in the hydrated dentine bonding substrate were exclusively located along the composite – adhesive interface, and were present as large voids, resin globules and honeycomb structures that formed partitions around a myriad of small blisters along the fractured interfaces. Conclusion. These features resembled the ‘overwet phenomenon’ that was previously reported along the dentine – adhesive interfaces of some acetone-based three-step adhesives. The cured adhesive layer in single-step adhesives may act as semi-permeable membranes that allow water diffusion from the bonded hydrated dentine to the intermixed zone between the adhesive and the uncured composite. Osmotic blistering of water droplets along the surface of the cured adhesive layer and emulsion polymerisation of immiscible resin components probably account for the compromised bond strength in single-step adhesives after delayed activation of light-cured composites. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Single-step adhesive; Delayed light-activation; Light-cured composite; Semi-permeable membrane; Osmotic blistering
1. Introduction Dentine adhesives are currently available as three-step, two-step and single-step systems depending on how the three cardinal steps of etching, priming and bonding to tooth * Corresponding author. Tel.: þ 86-852-28590251; fax: þ 86-85223933201. E-mail address: [email protected] (F.R. Tay).
substrates are accomplished or simplified [1]. Two-step systems are sub-divided into the self-priming adhesives that require a separate etching step, and the self-etching primers that require an additional bonding step [2]. The recently introduced all-in-one adhesives further combined these three bonding procedures into a single-step application. Irrespective of their packaging designs, these systems are supplied as two-component assemblies to maintain adequate shelflives. They are mixed together immediately before use, and
0300-5712/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 6 4 - 7
372
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
the mixture of hydrophilic and hydrophobic resin components is then applied to the tooth substrate. Of the six commercially available all-in-one adhesives, Prompt L-Pop (3M ESPE, Seefeld, Germany), Etch&Prime 3.0 (Degussa AG, Hanau, Germany), AQ Bond (Sun Medical, Kyoto, Japan) are unfilled versions, whereas One-Up Bond F (Tokuyama, Tokyo, Japan), Reactmer Bond (Shofu, Kyoto, Japan), and Xeno CF Bond (Sankin, Tokyo, Japan) are filled versions that contain fluoride-releasing glass fillers or monomers. For a long time, dentists have assumed that resin composites bond well to dentine adhesives, and that the weak links occur between the adhesives and dentine [3 – 9]. However, this may not be entirely true with the advent of contemporary two-step, and single-step adhesives. We previously showed that some two-step, self-priming adhesives are incompatible with chemical-cured composites, and that the decreases in bond strength are inversely proportional to the acidity of these single-bottle systems [10] Single-step adhesives are even more acidic in nature by virtue of their self-etching capabilities. There is evidence to suggest that they may not be compatible even with hybrid light-cured hybrid composites that are placed on top of these cured adhesives for too long before light-activation [11] In that study, micro-tensile bond strengths of two single-step adhesives were found to decrease exponentially with the time-delay in light-activation of the composite. The objectives of this study were to further examine the hypotheses that all currently available single-step adhesives to dentine are adversely affected by delayed activation of a light-cured, microfilled composite, and that such a phenomenon only occurs in the presence of water from the substrate side of the bonded interface. Specimens bonded to hydrated and dehydrated dentine, as well as processed composites using these adhesives were evaluated using the microtensile bond testing technique. Specimens that were stressed to failure were examined using scanning electron microscopy (SEM). In addition, unstressed, intact resin – dentine interfaces that were bonded with delayed lightactivation of the resin composite were examined with transmission electron microscopy (TEM). The null hypotheses that were tested are: (1) prolonged contact of lightcured resin composite to cured single-step adhesives before light-activation does not result in compromised bond strengths to sound, hydrated dentine, and (2) the presence or absence of water on the substrate side of the bonded interface of single-step adhesives does not affect the results of delayed activation of a light-cured composite.
2. Materials and methods 2.1. Tooth preparation for bond strength evaluation Thirty-four caries-free, human third molars were stored in 0.5% chloramine T at 4 8C, and used within 1 month
following extraction. The occlusal enamel of each tooth was first removed using a slow-speed saw equipped with a diamond-impregnated disk (Isomet, Buehler Ltd, Lake Bluff, IL, USA) under water lubrication. A 180-grit silicon carbide paper was used under running water to create a smear layer on the dentine surface. Bonding was subsequently performed on the occlusal surfaces of deep, coronal dentine. 2.1.1. Experiment I: effect of delayed activation of a light-cured composite on the mTBSs of single-step adhesives to hydrated dentine Twenty-eight teeth were used in this part of the study. They were randomly divided into seven groups of four teeth each. All the teeth in Experiment I were bonded in their normal hydrated status, as they were retrieved from the storage medium. All-Bond 2 (Bisco Inc, Schamburg, IL, USA; Group AB), a three-step adhesive system, was used as the control group. The six light-cured, single-step adhesives, One-Up Bond F (Group OU), Etch&Prime 3.0 (Group EP), Xeno CF Bond (Group CF), AQ Bond (Group AQ; also marketed in North America as Touch&Bond by Parkell Inc, Farmingdale, NY, USA), Reactmer Bond (Group RB) and Prompt LPop (Group PL) constituted the other six groups. A microfilled resin composite that contains pre-polymerised, TMPT organic fillers (Metafil CX, Sun Medical Co. Ltd, Shiga, Japan; also marketed in North America as Epic – TMPT by Parkell, Inc.) was used as the bonding composite in all groups. This microfilled composite does not contain inorganic glass fillers and facilitates subsequent TEM preparation of bonded, unstressed specimens. The general composition and batch numbers of these adhesives and resin composite are listed in Table 1. The application techniques of these adhesives are also outlined in Table 2. During the application of the primer mixture of the control three-step adhesive and the single-step adhesive mixtures, care was taken to ensure that the dentine surfaces were adequately covered by resin after evaporation of the solvents. In the event that matte dentine was encountered, additional coats were applied to produce shiny surfaces prior to lightactivation of the adhesives. Each adhesive group was further divided into two subgroups of two teeth each, based upon the time of contact of the resin composite with the cured adhesive layer prior to light-activation. In subgroup X-0 (where X ¼ adhesive group designation), a 1 mm layer of the microfilled composite was applied to each bonded dentine surface and light-activated immediately for 40 s, using a halogen light-curing unit (Variable Intensity Polymerizer, Bisco) with the curing intensity set at 500 mW cm22. Additional composite layers were subsequently added to the first composite layer, in 1 mm increments and light-activated separately, until a 5 mm thick core buildup was achieved. In subgroup X-20, the first layer of composite that was in contact with
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
373
Table 1 Composition of the adhesives and composite used in the study Material
Three-step adhesive
Single-step adhesive
Resin composite
Brand name
Type
Composition
Lot number
All-bond 2
Unfilled
Etchant. 32% phosphoric acid gel, xanthum gum thickener Primer A. NTG-GMA, acetone, ethanol, water Primer B. BPDM, photoinitiator, acetone D/E bonding resin. Bis-GMA, UDMA, HEMA Water, MMA, HEMA, coumarin dye, methacryloyloxyalkyl acid phosphate, methacryloxyundecane dicarboxylic acid (MAC-10), multifuctional methacrylic monomer, fluoroaluminosilicate glass, photoinitiator (aryl borate catalyst) Universal: water, ethanol, HEMA, stabilisers
0100001442
(Bisco Inc. Schaumburg, IL, USA)
One-up bond F (Tokuyama Corp. Tokyo, Japan)
Filled
Etch&Prime 3.0 (Degussa AG Hanau, Germany)
Unfilled
Xeno CF Bond (Sankin Tokyo, Japan)
Filled
AQ Bonda (Sun Medical Co. Ltd Shiga, Japan)
Unfilled
Reactmer bond (Shofu Inc. Kyoto, Japan)
Filled
Prompt L-Pop (3M ESPE Seefeld, Germany)
Unfilled
Metafil CXb (Sun Medical Co. Ltd Japan)
Microfilled
Catalyst: pyrophosphate, HEMA, photoinitiators, stabilisers Water, ethanol, HEMA, methacryloxyethylpyrophosphate, fluoride-releasing phosphazene monomer, UDMA, micro-filler, photoinitiator Base liquid. Water, acetone, 4-META, HEMA, MMA, UDMA, photoinitiator AQ sponge. Polyurethane foam, Initiator ( p-TSNa) Reactmer Bond A. Water, acetone, F-PRG fillers, FASG fillers, Initiators (TMBA, p-TSNa) Reactmer bond B. 4-AET, 4-AETA, HEMA, UDMA, photoinitiator Water, stabiliser, parabenes, methacrylated phosphoric acid esters, fluoride complex, photoinitiator (BAPO) Dimethacrylates such as UDMA (34 wt% organic TMPT filler (40 wt%) micro silica (26 wt%) photoinitiator (with aromatic tertiary amine) pigments, stabiliser
0000011967 0000011968 0000007876 23074510902
204
Universal: 350-04
Catalyst: 350-33 VK5
040002
FW0062849
TL2
Abbreviations: 4-AET, 4-acryloxyethyltrimellitic acid; 4-AETA, 4-acryloxyethyltrimellitic anhydride; 4-META, 4-methacryloxyethyltrimellitic anhydride; BAPO, bis-acyl phosphine oxide; Bis-GMA, bisphenol A diglycidyl ether dimethacrylate; BPDM, biphenyl dimethacrylate; F-PRG, full-reaction type pre-reacted glass ionomer filler; FASG, fluoroaluminosilicate glass; HEMA, 2-hydroxylethyl methacrylate; MMA, methyl methacrylate; NTG-GMA, Ntolylglycine-glycidyl methacrylate; p-TSNa, p-toluenesulfinic acid sodium salt; TMBA, trimethyl barbituric acid; TMPT, trimethylolpropane-trimethacrylate; UDMA, urethane dimethacrylate. a Also marketed in North America as Touch&Bond by Parkell, Inc., Farmingdale, NY, USA. b Also marketed in North America as Epic-TMPT by Parkell, Inc.
the cured adhesive layer was left in the dark for 20 min before light-activation. Subsequent core buildup followed the procedures described in the previous subgroup. The choice of 20 min was based on our previous study that such a period of delayed light-activation could result in null bond strength in some single-step adhesives [11]. We realised that such a lengthy delayed period was far removed from clinical practice, as clinicians are unlikely to leave a composite unactivated for more than 2 –3 min. However, it was the purpose of this study to investigate the effects of prolonged contact of unpolymerised composites on the surfaces of single-step adhesives that were bonded to hydrated dentine.
2.1.2. Experiment II: effect of delayed activation of a light-cured composite on the mTBSs of single-step adhesives to completely dehydrated dentine Three of six single-step adhesives (Etch&Prime 3.0, Xeno CF Bond and AQ Bond) were randomly selected for this experiment. The two teeth that were used for each adhesive were first dehydrated in an ascending ethanol series (70, 80, 95%, three changes in 100%) for 2 h each, following the TEM preparation protocol by Tay et al. [12]. After the third change, the teeth were left to completely dehydrate in absolute ethanol for an additional 48 h. This produced a bonding dentine substrate that is completely devoid of moisture, but
374
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Table 2 Application techniques of the control three-step and the single-step adhesives Adhesive systems
Dentine conditioning
Priming
Bonding
All-Bond 2
Uni-etch for 15 s, rinse and kept slightly moist
Mixed All-Bond 2 primer A and B, air-drylight-activate for 20 s
Apply D/E bonding resin,gently thinned down,light-activate for 10 s
One-Up Bond F
Mix equal droplets of the bonding agents A (clear liquid) and B (bright yellow liquid) for until a pink, homogenous liquid mixture was obtained Apply to tooth substrates and left undisturbed for 20 s, no rinsing Without further air-drying, light-activate for 10 s Mix equal amount of the universal and catalyst Apply to tooth substrates and allow acting for 30 s, no rinsing Briefly air-dry, light-activate for 10 s Re-apply adhesive mixture, briefly air-dried and light-activate for 10 s Mix equal amount of the universal and catalyst Apply to tooth substrates and allow acting for at least 20 s, no rinsing Briefly air-dry, light-activate for 10 s Re-apply adhesive mixture, briefly air-dried and light-activate for 10 s Dispense one drop of AQ liquid into dispensing well containing one piece of AQ sponge Apply the adhesive-coated AQ sponge to dentine substrates for 20 s, no rinsing Air-dry 3– 5 s, re-apply adhesive, air-dry 5–10 s, light-activate for 10 s using only halogen light-curing unit Mix equal droplets of reactmer bond bonding agent A (white liquid) and the B (amber liquid) for 5 s Apply to tooth substrates and allow acting for 20 s, no rinsing Briefly air-dry, light-activate for 20 s Activate blister pack by emptying the liquid out of the red blister into the yellow blister The activated mixture was applied to tooth substrates with agitation for 15 s, no rinsing Briefly air-dry, light-activate for 10 s using only halogen light-curing unit
Etch&Prime 3.0
Xeno CF Bond
AQ Bond
Reactmer Bond
Prompt L-Pop
without reducing its buffering capacity for the acidic single-step adhesives. In subgroup D-X-20 (where X ¼ adhesive group designation), the completely dehydrated dentine bonding surfaces were similarly treated with one of the three single-step adhesives. The delayed light-activation protocol of the resin composite followed that previously described for subgroup X-20. 2.1.3. Experiment III: effect of delayed activation of a light-cured composite on the mTBSs of single-step adhesives to processed composites To verify that any deterioration in mTBS during delayed light-activation is caused solely by the diffusion of water from the bonded substrate through the cured adhesive layer, Experiment I was repeated for the three single-step adhesives, Etch&Prime 3.0, Xeno CF Bond and AQ Bond, using light- and heat-processed Metafil CX as the bonding substrate. Five-millimeter thick layers of Metafil CX were dispensed into 4 £ 2 cm flat Teflon moulds (Electron Microscopy Sciences, Fort Washington, PA, USA). The moulds containing the uncured composite were placed
inside an experimental composite inlay processing chamber (Nitro-Therma-Lite; Bisco, Inc.) and light-activated under a pressurised nitrogen atmosphere maintained at 551.6 kPa (i.e. 80 psi) for one complete cycle at 125 8C for 20 min. The bonding surface of each processed composite block was ground with 180-grit SiC paper and further sandblasted with 50 mm alumina for 10 s. These blocks were sonicated in distilled water, air-dried, and bonded using the three single-step adhesives in the manner previously described. In the immediate light-activation subgroup C-X-0 (where C ¼ adhesive group designation), Metafil CX was applied and light-activated incrementally as in subgroup X-0. Similarly, the delayed light-activation subgroup C-X-20, the first 1 mm layer of Metafil CX was placed over the cured adhesive and left in the dark for 20 min before lightactivation. 2.2. mTBS evaluation After storage in distilled water at 37 8C for 24 h, bonded teeth from Experiment I and II were sectioned occluso-gingivally into serial slabs using an Isomet saw
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
under water lubrication. One slab from each bonded tooth was not subjected to tensile testing and reserved for TEM examination. The other slabs tooth were sectioned into 0.9 £ 0.9 mm2 composite – dentine beams, according to the technique for the ‘non-trimming’ version of the micro-tensile test reported by Shono et al. [13]. Each group of bonded teeth yielded 20– 22 beams for bond strength evaluation. Beams with premature failure during sectioning were assigned a null bond strength value and were included in the compilation of the mean tensile bond strength. The bonded composite blocks from Experiment III were similarly aged in distilled water at 37 8C for 24 h and then sectioned into 0.9 mm slabs. Due to the high incidence of cohesive failures in resin composites initially observed using the Shono’s technique, each slab was hand-trimmed into 0.9 £ 0.9 mm2 dumbbell-shaped specimens according to the version of micro-tensile bond testing reported by Sano et al. [14]. Fifteen specimens were produced for each of the six experimental subgroups. Specimens were fixed to a Bencor Multi-T device (Danville Engineering, San Ramon, CA, USA), using Zapit cyanoacrylate (Dental Ventures of America, Corona, CA, USA) and tested to failure under tension in a universal testing machine (Model 4440; Instron Inc, Canton, MA, USA) at a crosshead speed of 1 mm/min. 2.3. Statistical analysis For each adhesive, the bond strength data obtained for the corresponding subgroups were statistically analysed with either Mann – Whitney Rank Sum tests or Kruskal – Wallis one-way ANOVA on ranks, using SigmaStat Version 2.03 (SPSS, Chicago, IL, USA). Statistical significance was set in advance at the 0.05 probability level. In adhesive groups that involved additional subgroups from Experiments II and III, multiple comparisons were done with Dunn’s test at a ¼ 0.05. 2.4. SEM fractographic analysis Six fractured composite – dentine beams from Experiment I and II of each adhesive that were representative of the mean bond strengths of the corresponding subgroups were prepared for SEM examination. Both the dentine and composite sides of the fractured beams were air-dried. They were not dehydrated using methods that involve passing the specimens through organic solvents [15], to avoid the possibility of extracting uncured monomers or partially polymerised oligomers from the fractured interfaces. Completely air-dried specimens were secured to brass stubs using Zapit. They were sputter-coated with gold/ palladium and examined using a scanning electron microscope (Cambridge Stereoscan 440, Cambridge, United Kingdom) operating at 12 kV. Failures were classified as: (a) adhesive failure, if the fracture site was maintained
375
entirely within the adhesive; (b) mixed failure, if the fracture site continued from the adhesive into either the composite or dentine; and (c) substrate failure, if the fracture occurred exclusively within the composite or dentine. In addition, abnormalities along the fractured interfaces that were associated with delayed light-activation of the resin composites were also recorded. 2.5. TEM examination of unstressed, bonded dentine –composite interfaces TEM was only performed to provide additional information on the abnormal features associated with delayed light-activation of resin composites on adhesive-coated, hydrated dentine, and to validate that the previous SEM observations were not artefacts produced during tensile stress, or by contamination of the fractured interfaces. The two remaining composite –dentine slabs from subgroup X20 (Experiment I) of each adhesive were used for TEM examination. Undemineralised sections of the bonded specimens, 90 – 120 nm thick, were prepared according to the TEM protocol described in Tay et al. [12]. These undemineralised sections were collected using single slot, carbon- and formvar-coated copper grids (Electron Microscopy Sciences, Fort Washington, PA, USA). They were double-stained with uranyl acetate for 10 min and Reynold’s lead citrate for an additional 5 min. Examination was performed using a TEM (Philips EM208S, Eindhoven, The Netherlands) operating at 100 kV. Digitised images were recorded using the charge couple device (CCD) camera (Bioscan, Model 792, Gatan Inc, Pleasanton, CA, USA) attached to the microscope.
3. Results mTBS data of the three-step, control adhesive and the six single-step adhesives bonded to hydrated dentine (Experiment I) are summarised in Table 3 and also graphically shown in Fig. 1. Delayed light-activation has no effect on the control adhesive ( p . 0.05). In contrast, delayed light-activation significantly reduced the mTBS of all the single-step adhesives ( p , 0.05). For the three single-step adhesives tested in Experiments II and III, delayed light-activation has no effect when these adhesives were bonded to completely dehydrated dentine ( p . 0.05; Table 3). Similarly, no deterioration in mTBSs was observed in these single-step adhesives when delayed light-activation was used with the processed composite as the bonding substrate ( p . 0.05; Table 3). Fig. 2A shows a representative mixed failure mode that was observed in AB-0 and AB-20 subgroups. No abnormalities were detected along the fractured interfaces. Similarly, mixed failure was the predominant failure mode in the corresponding immediate light-activation subgroups
c
Control three-step adhesive. Values are means ^ standard deviation in MPa. For each adhesive (i.e. across a row), subgroups labelled with the same lower case superscript are not statistically significant ( p . 0.05). Number of beams tested that were without premature failure. Beams with premature failure during specimen preparation were included as zero bond strengths in the calculation of mean bond strength. a
b
NA NA 38.8 ^ 7.0a[15/15] 47.3 ^ 15.3a[15/15] 32.4 ^ 9.6a[15/15] NA NA NA NA 35.0 ^ 9.5a[15/15] 48.3 ^ 11.9a[15/15] 38.2 ^ 10.9a[15/15] NA NA All-Bond 2 (AB)a One-Up Bond F(OU) Etch&Prime 3.0(EP) Xeno CF Bond(CF) AQ Bond(AQ) Reactmer bond(RB) Prompt L-Pop(PL)
48.4 ^ 11.7ab[22/22]c 42.9 ^ 7.9a[22/22] 37.0 ^ 9.3a[20/20] 34.5 ^ 8.3a[21/21] 26.8 ^ 6.1a[23/23] 25.8 ^ 3.6a[21/21] 24.1 ^ 5.2a[22/22]
50.9 ^ 9.6a[20/20] 10.1 ^ 4.4b[20/20] 0.6 ^ 1.7b[3/21] 10.9 ^ 5.1b[22/22] 9.3 ^ 3.2b[22/22] 9.7 ^ 5.0a[21/21] 2.2 ^ 2.7b[13/22]
NA NA 37.4 ^ 6.5a[20/20] 35.2 ^ 5.7a[21/21] 26.7 ^ 5.3a[20/20] NA NA
Delayedlight-activation (C-X-20) Immediatelight-activation (C-X-0) Delayed light-activation on completely dehydrateddentine (D-X-20) Immediate light-activation onhydrateddentin (X-0)
Delayed light-activation on hydrated dentine (X-20)
Experiment II Experiment I
Deep coronal dentine Group designations(X)
Table 3 mTBS of the control three-step adhesive and the single-step adhesives examined in the study
Experiment III
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Processed composite
376
(i.e. X-0) of each single-step adhesive (Fig. 2B). Pure adhesive failures were rare in these subgroups. SEM examination of fractured specimens and TEM examination of unstressed specimens from the delayed light-activation subgroups of the six single-step adhesives bonded to hydrated dentine (i.e. X-20) revealed that the dentine –adhesive interfaces were intact and that abnormal morphological features occurred solely along the composite – adhesive interfaces. They could be classified into voids (Fig. 3), resin globules (Fig. 4) and honeycomb structures that formed partitions around a myriad of small blisters along the fractured interfaces (Figs. 5 and 6). All unstressed TEM specimens that appeared intact by visual inspection after laboratory processing were found to have separated on TEM examination, with the spaces infiltrated by the embedding epoxy resin.
4. Discussion As delayed light-activation of resin composite only adversely affected the bond strengths of hydrated dentine bonded with single-step adhesives but not the control threestep adhesive (Experiment I), we have to reject the first null hypothesis. This abnormal phenomenon occurred in all the single-step adhesives available, irrespective of their resin composition and initiator systems (Table 1). The phenomenon also occurred regardless of whether a hybrid composite with ion-leachable glass fillers [11] or a microfilled composite was used for the restoration. Based on the results of Experiment I alone, it was tempting to attribute the phenomenon to some kind of adverse interactions that occurred between uncured resin monomers from the adhesive and the resin composite (i.e. the intermixed zone
Fig. 1. Delayed activation of light-cured composites adversely affects the bonding of all commercially available single-step adhesives to sound dentine. In contrast, the micro-tensile bond strength of a three-step adhesive system is not affected by delayed light-activation.
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 2. Control specimens: (A) low magnification SEM micrograph of the dentine side of a fractured beam in the All-Bond 2 delayed light-activation subgroup (AB-20). A mixed failure mode was observed involving dentine (D), adhesive primer (P), bonding resin (B) and resin composite (C). The fracture interfaces were free of voids; (B) SEM micrograph of a mixed failure mode that was representative of those that occurred in the immediate light-activation subgroups of the six single-step adhesives. The one shown here was taken from the fractured composite side of a fractured beam in the Etch&Prime 3.0 immediate light-activation subgroup (EP-0). Minimal voids could be found within the fractured composite (C). A: fractured adhesive; D: fractured dentine. Asterisk: area in which collagen fibrils could be identified at high magnification.
(IZ)). Reaction of isocyanate groups of UDMA-containing composites with water, for example, could generate carbon dioxide that might contribute to the honeycomb structure. Likewise, condensation polymerisation involving dicarboxylic acids and diols could result in the liberation of water
377
Fig. 3. Adverse effect of delayed light-activation of a resin composite that was applied over cured single-step adhesives. I. Voids (arrows): (A) SEM micrograph of the dentine side of a fractured beam in the Reactmer Bond delayed light-activation subgroup (RB-20). Numerous large voids up to 50 mm in diameter could be observed along the interface between the fractured composite (C) and fractured adhesive (A). D: exposed dentine; (B) SEM micrograph from the dentine side of a fractured beam in the AQ Bond delayed light-activation subgroup (AQ-20). An intermediate zone along the fractured composite adhesive interface could be clearly observed, in which the bases of numerous large voids were present. D: exposed dentine.
as a by-product, which in turn could be trapped within the adhesive – composite interfaces. However, these chemical reactions could not be expected at the ambient temperature in which our experiments were conducted. The observed results that adverse reactions did not occur when delayed light-activation was performed using processed composite as bonding substrates (Experiment III) corroborated
378
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 4. Adverse effect of delayed light-activation. II. Resin globules (arrowheads): (A) SEM micrograph of the region labelled with an asterisk in Fig. 3A (RB20). Numerous resin globules were observed within the fractured adhesive resin (A): (B) SEM micrograph of the region labelled with an asterisk in Fig. 3B (AQ-20). Numerous resin globules could be seen on the surface of the adhesive layer (A); (C) TEM micrograph of the fractured dentine side of a One-Up Bond F (OU-20) specimen. Failure occurred along the adhesive–composite interface, which was subsequently infiltrated by epoxy resin (E). Numerous electronlucent resin globules were observed along the surface of an IZ. A: adhesive containing fluoroaluminosilicate glass fillers (G). H: hybrid layer; U: undemineralized dentine; (D). TEM micrograph of the fractured composite side of a Prompt L-Pop (PL-20) specimen, showing the presence of numerous resin globules within the microfilled composite. P: pre-polymerised TMPT fillers; M: composite resin matrix. Partitions (pointer) within the resin globules were probably contributed by the Prompt L-Pop adhesive. E: epoxy resin.
that the phenomenon observed was not caused by inherent resin–initiator incompatibility that was previously reported to occur between acidic resin monomers and chemical-cured composites that utilise binary redox initiator systems [10, 16–18]. As water is an indispensable component in single-step adhesives (Table 1), the results further suggested that the phenomenon was not caused by retention of incompletely evaporated water within the oxygen inhibition layer of the adhesives [19,20]. The results also focused our attention to
the fact that the underlying cause of this abnormal phenomenon is likely to be environmental in nature. Our conjecture was eventually substantiated by the results of Experiment II, affirming that water derived from the bonding substrate is responsible for the decline in bond strength of the single-step adhesives after delayed light-activation of resin composites. We thus have to reject the second null hypothesis. The morphological manifestation of resin globules and blisters along the composite –adhesive interface in delayed
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
379
Fig. 5. Adverse effect of delayed light-activation. III. SEM of the honeycomb structures: (A) Fractured composite side of a specimen beam in the Reactmer Bond delayed light-activation subgroups (RB-20); (B) High magnification of B (RB-20) showing the presence of a honeycomb resin structure that formed the partitions of a myriad of blisters around along the fractured interface. Bar ¼ 1 mm; (C) High magnification of the honeycomb structure from the dentine side of a fractured beam in the delayed light-activation subgroup of an unfilled single-step adhesive, AQ Bond (AQ-20).
light-activation is reminiscent of the characteristic features of the ‘overwet phenomenon’ that was observed along the adhesive – dentine interface of some acetone-based adhesives when a wet bonding technique was used [21, 22]. The ‘overwet phenomenon’ that was seen in total-etch, three-step and two-step adhesives may also be caused by transudation of dentinal fluid, from tubules that are rendered patent after acid-conditioning, in vital teeth that exhibit positive pulpal pressures [23]. However, the single-step adhesives employed in this study are self-etching in nature and do not require the removal of smear plugs. Moreover, the experiments on hydrated dentine in this study were not
performed under dentine perfusion [24,25], and none of the previously reported TEM features of the overwet phenomenon could be seen along the dentine – adhesive interfaces, which remained intact in all specimens examined. Thus, the only explanation for the overwet phenomenon along the adhesive – composite interface that we can think of is that the cured single-step adhesive layers act as permeable membranes that permit water to diffuse from the substrate side to the IZ. It has been shown that water is capable of passing through diffusion barriers provided by organic resin coatings [26,27]. Most single-step adhesives contain
380
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Fig. 6. Adverse effect of delayed light-activation. III. TEM of the honeycomb structures (pointers): (A) AQ Bond subgroup (AQ-20). Finger-like projections (pointers) corresponded with the honeycomb structure that formed partitions around blisters along an IZ between the composite (C) and the adhesive (A). P: TMPT fillers in the composite; H: hybrid layer; U: undemineralised dentine. E: epoxy resin; (B) the fractured honeycomb structure from the composite side of the Reactmer Bond subgroup (RB-20). The IZ contained fluoroaluminosilicate glass fillers (G) from the filled adhesive. Numerous globular resin phases (arrowheads) were identified. C: composite; E: epoxy resin; (C) The dentine side of the same specimen in Fig. 5B (subgroup RB-20). A: filled adhesive layer that contained fluoroaluminosilicate glass fillers (G) and fumed silica (arrow). IZ: intermixed zone. Arrowhead: resin globular phases within the IZ. E: epoxy resin.
hydroxyethyl methacrylate (HEMA) which can polymerise in the presence of water to form ‘microporous’ hydrogel with pore sizes ranging from 10 to 100 nm [28]. Differential water movement across the cured adhesive layer may occur in the presence of increased concentrations of dissolved inorganic ions, uncured, water-soluble, hydrophilic resin monomers, or dissolved collagen/proteoglycans components within the oxygen inhibition layer of the cured adhesive. This concentration difference may establish an osmotic pressure gradient,
causing water movement from a region of low solute concentration (i.e. dentinal tubules in hydrated dentine substrate) to a region of high solute concentration (i.e. IZ along the adhesive – composite interface) [29]. This, in turn, may result in osmotic blistering of microscopic water droplets along the uncured IZ [29,30]. Blister initiation and growth via osmosis is common in resin-coated metal systems, being one of the first signs for corrosion that eventually causes delamination of the coating. In conventional osmotic blistering, interfacial contamination
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
in the form of trapped ions between the metal and resin surface results in the coating acting as a semi-permeable membrane, allowing water transport from the environment into the interface. Blisters are initiated in weak spots along the adhesive interface. With time, blisters may enlarge to a point where adjacent blisters coalesce. The initial osmotic pressure buildup within the blisters is high, but gradually diminishes as the concentration of the impurities is being diluted [30,31]. What we observed in delayed lightactivation of resin composites may be comparable to conventional osmotic blistering. However, unlike conventional osmotic blistering, the osmotic gradient results in water movement from the dentine substrate into an unpolymerised interface, producing an immiscible blend of hydrophobic and hydrophilic monomers. Emulsion polymerisation of the hydrophobic resins in water (oil-in-water type emulsion) [32] and the diluted hydrophilic adhesive resins within the hydrophobic composite (water-in-oil type emulsion) [32] eventually resulted in the formation of different types of resin globules within the interface of a single specimen (Fig. 4). Resin polymerisation around the osmotic blisters produces in the honeycomb resin structures observed in Figs. 5 and 6. Similar to our previous use of a hydrophobic impression material to take impressions of dentinal fluid transudates from acid-etched dentine, [23] the honeycomb structures that were formed as the relatively hydrophobic resin composite polymerised are essentially negative impressions of the water droplets that extruded out of cured adhesive interfaces. The larger voids observed in Fig. 3 may also be accounted for by the coalescence of the smaller water blisters. The high osmotic pressure build up within these blisters may also explain why most of the TEM specimens were spontaneously fractured during laboratory processing. We previously reported that micro-tensile bond strengths of single-step adhesives decreased exponentially with time [11]. Using two mathematical models based on Fickian diffusion and osmotic theory to predict blister growth in resin coating systems, Pommersheim and Nguyen showed that irrespective of whether impurities were uniformly distributed or present over concentrated spots along the interface, the changes in blister radii, surface areas, heights, volumes and osmotic pressures all varied directly as a fractional power of time [33]. The implication of these mathematical models in the present context is that the overwet phenomenon caused by osmotic blistering along the uncured adhesive – composite interface is negligible in immediate light-activation, wherein the adhesive oxygen from the adhesive inhibition layer is quickly absorbed into the composite layer [34], trapping these ‘impurities’ at least semi-permanently within the interface as the resin polymerises. This probably explained why delayed light-activation has no adverse effect on the micro-tensile bond strength of the control three-step adhesive, or when a cured single-step adhesives were covered with a layer of lightcured bonding resin prior to the application of the composite [11]. It must be remembered, however, that even under these
381
circumstances, the adhesive layer still function as a semipermeable membrane. Inherent weaknesses such as airvoids within the resin – dentine interfaces cannot be totally eliminated even with the utmost care during adhesive application [35,36]. This implies that with time, water may accumulate within these inherent weak spots via osmosis, with the buildup of osmotic pressure helping to enlarge these flaws so that blisters can form. If the composite is light-activated immediately, such weak spots may no longer reside solely along the adhesive –composite interface. This provides an alternative explanation to the leaching of hydrophilic resin components in interpreting why bond strength decreased on ageing of adhesives that contain polymerised hydrophilic resin components [6,7]. This hypothesis has to be further substantiated. In conclusion, clinicians should be aware of the potential drop in bond strength on prolonged contact of single-step adhesives with light-cure composites before light-activation. Although clinicians are unlikely to leave a lightcured composite unactivated for more than 2 – 3 min, multiple direct or indirect restorations should be lightactivated individually and as soon as the composite is applied. The use of these acidic adhesives to treat dentin surfaces before luting indirect restorations or posts with dual-cure resin composite cements should be avoided. Although the light-activated reaction can be initiated soon after luting, some clinicians may take longer to remove excess cement before light-activation. Moreover, the chemical reaction lasts over hours and may be significantly compromised. It is possible also that the recently introduced light-activation techniques that attempt to reduce the effect of polymerisation shrinkage stress may lead to compromised results with the use of these single-step adhesives [37]. This is an immediate concern that requires further research, as dentists and researchers have usually assumed that bond failures occur between adhesives and dentine and not between adhesives and composites. Permeability of single-step adhesives to water may also hasten the rate of water sorption and leaching of resin components [38], challenging the durability of resin – dentine bonds produced by these adhesives. This is a subject of considerable concern and should be investigated further.
Acknowledgements We thank Amy Wong and W.S. Lee of the Electron Microscopy Unit, the University of Hong Kong for technical assistance. The AQ Bond and Metafil CX used in this study were supplied by Sun Medical Co. Ltd. The One-Up Bond F was supplied by Tokyyama Corp. The Reactmer Bond was also sponsored by Shofu Inc. This study was supported, in part, by CRC grant 1020 3278 24993 08004 323 01 from The University of Hong Kong, Hong Kong SAR, China, by grant CNPq #300481/95-0 from The University of Sa˜o Paulo, Brazil, and by grant DE 06427 from the National
382
F.R. Tay et al. / Journal of Dentistry 30 (2002) 371–382
Institute of Dental and Craniofacial Research, USA. The authors are grateful to Michelle Barnes for secretarial support.
[19]
[20]
References [1] Kugel G, Ferrari M. The science of bonding: from first to sixth generation. The Journal of the American Dental Association 2000; 131(Suppl.):20S–5S. [2] Tanumiharja M, Burrow MF, Tyas MJ. Microtensile bond strengths of seven dentin adhesive systems. Dental Materials 2000;16:180–7. [3] Kiyomura M. Bonding strength to bovine dentin with 4-META/ MMA-TBB resin—long-term stability and influence of water. Journal of the Japanese Society for Dental Materials and Devices 1987;6: 860–72. [4] Wang T, Nakabayashi N. Effect of 2-(methacryloxy)-ethyl phenyl hydrogen phosphate on adhesion to dentin. Journal of Dental Research 1991;70:59 –66. [5] Eick JD, Robinson SJ, Byerley TJ, Chappell RP, Spencer P, Chappelow CC. Scanning transmission electron microscopy/energydispersive spectroscopy analysis of the dentin adhesive interface using a labeled 2-hydroxyethylmethacrylate analogue. Journal of Dental Research 1995;74:1246– 52. [6] Sano H, Yoshikawa T, Pereira PN, Kanemura N, Mongami M, Tagami J, Pashley DH. Long-term durability of dentin bonds made with a self-etching primer, in vivo. Journal of Dental Research 1999; 78:906–11. [7] Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. In vivo degradation of resin–dentin bonds in humans over 1 to 3 years. Journal of Dental Research 2000;79:1385–91. [8] Spencer P, Wang Y, Walker MP, Wielicka DM, Swafford JR. Interfacial chemistry of the dentin/adhesive bond. Journal of Dental Research 2000;79:1458– 63. [9] Walker MP, Wang Y, Swafford J, Evans A, Spencer P. Influence of additional acid etch treatment on resin cement dentin infiltration. Journal of Prosthodontics 2000;9:77–81. [10] Sanares AME, Itthagarun A, King NM, Tay FR, Pashley DH. Adverse surface interactions between one-bottle light-cured adhesives and chemical-cured composites. Dental Materials 2001;17:542–56. [11] Tay FR, King NM, Suh BI, Pashley DH. Effect of delayed activation of light-cured resin composites on bonding of all-in-one adhesives. Journal of Adhesive Dentistry 2001;3:207–25. [12] Tay FR, Moulding KM, Pashley DH. Distribution of nanofillers from a simplified-step adhesive in acid-conditioned dentin. Journal of Adhesive Dentistry 1999;2:103–17. [13] Shono Y, Ogawa T, Terashita M, Carvalho RM, Pashley EL, Pashley DH. Regional measurement of resin– dentin bonding as an array. Journal of Dental Research 1999;78:699–705. [14] Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho R, Pashley DH. Relationship between surface area for adhesion and tensile bond strength—evaluation of a micro-tensile bond test. Dental Materials 1994;10:236–40. [15] Perdiga˜o J, Lambrechts P, Van Meerbeek B, Vanherle G, Lopes AL. Field emission SEM comparison of four postfixation drying techniques for human dentin. Journal of Biomedical Materials Research 1995;29:1111– 20. [16] Nakamura M. Adhesive self-curing acrylic resin. Composition of 4META bonding agent. Japanese Journal of Dental Materials 1985;4: 672–91. [17] Yamauchi J, Yamada K, Shibatani K. (inventors). Kuraray Company Limited, assignee. Adhesive compositions for the hard tissues of the human body. United States Patent 4,182,035; January 8 1990. [18] Ikemura K, Endo T. Effect on adhesion of new polymerization initiator systems comprising 5-monosubstituted barbituric acids, aromatic
[21]
[22]
[23] [24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32] [33]
[34]
[35]
[36]
[37]
[38]
sulphonate amides, and tert-butyl peroxymaleic acid in dental adhesive resin. Journal of Applied Polymer Science 1999;72:1655 –68. Pashley EL, Zhang Y, Lockwood PE, Rueggeberg FA, Pashley DH. Effects of HEMA on water evaporation from water–HEMA mixtures. Dental Materials 1998;14:6–10. Paul SJ, Leach M, Rueggeberg FA, Pashley DH. Effect of water content on the physical properties of model dentine primer and bonding resins. Journal of Dentistry 1999;27:209 –14. Tay FR, Gwinnett AJ, Wei SHY. The overwet phenomenon: a scanning electron microscopic study of surface moisture in the acidconditioned, resin–dentin interface. American Journal of Dentistry 1996;9:109–14. Tay FR, Gwinnett AJ, Wei SHY. Micromorphological spectrum from overdrying to overwetting acid-conditioned dentin in water-free, acetone-based, single-bottle primer/adhesives. Dental Materials 1996; 12:236–44. Itthagarun A, Tay FR. Self-contamination of deep dentin by dentin fluid. American Journal of Dentistry 2000;13:195 –200. Pereira PNR, Sano H, Ogata M, Zheng L, Nakajima M, Tagami J, Pashley DH. Effect of region and dentine perfusion on bond strengths of resin-modified glass ionomer cements. Journal of Dentistry 2000; 28:347–54. Escribano N, Del-Nero O, de la Macorra JC. Sealing and dentin bond strength of adhesive systems in selected areas of perfused teeth. Dental Materials 2001;17:149–55. Funke W. Toward a unified view of the mechanism responsible for paint defects by metallic corrosion. Industrial and Engineering Chemistry Products Research and Development 1984;24:343–7. Nguyen T, Bentz D, Byrd E. Method for measuring water diffusion in a coating applied to a substrate. Journal of Coating Technology 1995; 67:37–46. Chirila TV, Chen YC, Griffin BJ, Constable IJ. Hydrophilic sponges based on 2 hydroxyethyl methacrylate I: effect of monomer mixture composition on the pore size. Polymer International 1993;32:221 –32. van der Meer-Lerk LA, Heertjes PM. Blistering of varnish films on substrates induced by salts. Journal of Oil and Colour Chemists Association 1975;58:79–84. van der Meer-Lerk LA, Heertjes PM. Mathematical model of growth of blisters in varnish films on different substrates. Journal of Oil and Colour Chemists Association 1979;62:256–63. Nguyen T, Hubbard J, Pomersheim JM. Unified model for the degradation of organic coatings on steel in a neutral electrolyte. Journal of Coating Technology 1996;68:45 –56. Myers D. Surfactant science and technology, 2nd ed. New York: VCH Publishers, Inc; 1992. p. 209–45. Pomersheim JM, Nguyen T. Prediction of blistering in coating systems. In: Bierwagen GP, editor. Organic coatings for corrosion control. Proceedings of the American Chemical Society Symposium Series No. 689, Washington, DC: American Chemical Society publisher; 1998. p. 137 –50. Rueggeberg FA, Margeson DH. The effect of oxygen inhibition on an unfilled/filled composite system. Journal of Dental Research 1990;69: 1652–8. Pashley DH, Sano H, Cuicchi B, Yoshiyama M, Carvalho RM. Adhesion testing of dentin bonding agents: a review. Dental Materials 1995;11:117–25. Eick JD, Gwinnett AJ, Pashley DH, Robinson SJ. Current concepts on adhesion to dentin. Critical Reviews in Oral Biology and Medicine 1997; 8:306–35. Kanca J, Suh BI. Pulse activation: reducing resin-based composite contraction stresses at the enamel cavosurface margins. American Journal of Dentistry 1999;12:107–12. Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. Resin– tooth adhesive interfaces after long-term function. American Journal of Dentistry 2001;14:211–25.