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MMA-TBB resin to enamel and dentine: Primary vs permanent teeth
journal of dentistry 42 (2014) 425–431
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Bonding ability of 4-META self-etching primer used with 4-META/MMA-TBB resin to enamel and dentine: Primary vs permanent teeth Yumiko Hosoya a,*, Franklin R. Tay b a Department of Pediatric Dentistry, Course of Medical and Dental Science, Nagasaki University Graduate School of Biomedical Sciences, 1-7-1, Sakamoto, Nagasaki 852-8588, Japan b Department of Endodontics, College of Dental Medicine, Georgia Regents University, Augusta, GA 30912-1129, USA
article info
abstract
Article history:
Objectives: The present study compared the efficacies of the self-etching Teeth Primer (TP:
Received 19 August 2013
4-META), and the etchants Red Activator (RA; 65% phosphoric acid) and Green Activator (GA;
Received in revised form
10% citric acid with 3% ferric chloride), for bonding to enamel and dentine of human primary
12 January 2014
and permanent teeth, when used with 4-META/MMA-TBB resin (Bondfill SB).
Accepted 13 January 2014
Methods: Forty-eight non-carious primary canines and third molars were used. Eight groups were prepared: Group 1 (primary enamel with RA), Group 2 (permanent enamel with RA), Group 3 (primary enamel with TP), Group 4 (permanent enamel with TP), Group 5 (primary
Keywords:
dentine with GA), Group 6 (permanent dentine with GA), Group 7 (primary dentine with TP)
4-META self-etching primer
and Group 8 (permanent dentine with TP). Micro-tensile bond strengths (MTBS) were
4-META/MMA-TBB resin
measured and analyzed statistically using ANOVAs and Tukey HSD tests at a = 0.05. Efficacy
Micro-tensile bond strength
of etching/priming and the morphology of bonded interfaces were observed with SEM.
Primary teeth
Results: Etching/priming efficacy of TP on enamel was low. The MTBS of Group 2 was
Permanent teeth
significantly higher than the other groups (Groups 1, 3 and 4). For dentine, significant
Scanning electron microscopy
differences in MTBS were observed, in the order of Groups 6 > 8 > 7 = 5 ( p < 0.05). The MTBSs of permanent dentine were significantly higher than primary dentine. For primary teeth, there was no significant difference in the MTBSs between enamel and dentine, irrespective of primer or etchant ( p > 0.05). Conclusion: TP primer/Bondfill SB may be used as an alternative to other adhesive/resin composite systems for bonding to enamel and dentine of primary teeth. # 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Attrition and wear of enamel and dentine, gingival recession and formation of wedge-shaped defects occur throughout life due to clenching, grinding, mastication, tooth brushing and other factors. For restoration of these defects, it is beneficial for the wear resistance of dental materials to approximate those of tooth substrates, particularly for areas that are in
constant occlusal contact. An adhesive resin chemically initiated with tri-n-butylborane (TBB) has been used for bonding between tooth structure and restorative materials. One of the problems associated with the TBB-initiated resin system is its insufficient wear resistance, due to the absence of inorganic fillers.1,2 A modified TBB resin containing pre-polymerized organic filler particles (Bondfill SB, Sun Medical Co., Ltd., Moriyama, Japan) was recently developed for filling, luting, or sealing
* Corresponding author. Tel.: +81 95 819 7673; fax: +81 95 819 7676. E-mail address: [email protected] (Y. Hosoya). 0300-5712/$ – see front matter # 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jdent.2014.01.007
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journal of dentistry 42 (2014) 425–431
procedures employed in preventive and restorative dentistry.3 This modified resin demonstrated better wear characteristics when compared with conventional unfilled TBB-initiated resins.4 Bondfill SB is a resin composite that utilizes the 4methacryloyloxyethyl trimellitate anhydride/methyl methacrylate-TBB (4-META/MMA-TBB) system for adhesion to tooth substrates. For practitioners who are using the etch-and-rinse bonding technique, the manufacturer has originally developed an etchant (65% phosphoric acid; Red Activator) for bonding to enamel, and a conditioner consisting of 10% citric acid with 3% ferric chloride (Green Activator) for bonding to dentine. Because enamel and dentine in primary teeth (especially for dentine) are more susceptible to primers and etchants than permanent teeth,5–9 over-etching may occur when the same etching time employed for permanent teeth is adopted for etching primary teeth. To alleviate this problem, a 4-META containing selfetching primer (Teeth Primer, Sun Medical Co., Ltd.) was developed and included in Bondfill SB for practitioners who prefer the self-etching approach, and for bonding to tooth substrates in primary teeth. As the pH of Teeth Primer is 3.0, the bonding technique has been termed the ultra-mild self-etching approach.10,11 Although the use of mild self-etching primers/ adhesives is not ideal for bonding to permanent tooth enamel,12,13 limited information is available for their bonding efficacy to primary tooth enamel. Thus, the objective of the present study was to compare the use of the self-etching Teeth Primer (TP) in Bondfill SB, with the 65% phosphoric acidcontaining Red Activator (RA) for bonding to primary and permanent human enamel, and with the Green Activator (GA) for bonding to primary and permanent human dentine. The null hypotheses tested were: (1) there is no difference in the microtensile bond strength of Bondfill SB to enamel or dentine treated with TP, RD or GA; and (2) there is no difference in the microtensile bond strength of TP-primed specimens between primary teeth and permanent teeth for either enamel or dentine.
2.
to the regulations of ethics committees of Nagasaki University. The extracted teeth were frozen in physiologic saline within 10 min after extraction. Table 1 shows the components and protocol of the materials used in the present study. The primary or permanent teeth were each divided into 4 groups according to whether enamel or dentine was bonded, and the primer or activator employed for substrate treatment. Group 1 (primary tooth enamel etched with Red Activator: RA); Group 2 (permanent tooth enamel etched with RA); Group 3 (primary tooth enamel primed with Teeth Primer: TP); Group 4 (permanent tooth enamel primed with TP); Group 5 (primary tooth dentine etched with Green Activator: GA); Group 6 (permanent tooth dentine etched with GA); Group 7 (primary tooth dentine primed with TP); and Group 8 (permanent tooth dentine primed with TP).
2.1.
Flat enamel and dentine surfaces were prepared using silicon carbide papers under copious water-cooling on the labial surfaces of primary canines and the occlusal surfaces of third molars (N = 12 for each of 8 groups). Final polishing was achieved using 1200-grit silicon carbide paper. In Groups 1 and 2, 65% phosphoric acid (RA) was applied on enamel for 30 s, rinsed with water for 15 s and dried with an air-water syringe. In Groups 5 and 6, 10% citric acid with 3% ferric chloride (GA) was applied on dentine for 10 s, rinsed for 15 s and dried with an air–water syringe. For Groups 3, 4, 7 and 8, Teeth Primer (TP) was applied on enamel or dentine for 20 s and air-dried for 20 s. For Groups 3, 4, 7 and 8, the specimens were immersed in acetone for 10 s to eliminate resinous material from the primed tooth substrates. All specimens were dried in a desiccator and sputter-coated with platinum (Superfine Coater ESC-101; Elionix Co., Tokyo, Japan) prior to SEM observation (JSM-5610LV, JOEL, Tokyo, Japan).
2.2.
Materials and methods
Forty-eight non-carious primary canines that were extracted to expedite eruption of the succedaneous teeth or for orthodontic reasons were used as primary tooth specimens. Forty-eight extracted non-carious upper third molars were used as permanent tooth specimens. Informed consent for tooth collection was obtained from parents and subjects according
Scanning electron microscopy (SEM)
Micro-tensile bond strength (MTBS) evaluation
Flat enamel or dentine surfaces were prepared in the manner previously described using 180-grit silicon carbide paper for final polishing. Enamel or dentine surfaces were etched or primed in the same manner as for the SEM observation. After etching or priming, a mixture of the polymer and monomercatalyst of Bondfill SB was applied and built up using the brush-on technique. For primary teeth, Bondfill SB was applied
Table 1 – Components of the materials and protocols utilised in the preset study. Material (Batch no.)
Components
Protocol
Teeth Primer: TP (0056) Red Activator: RA (TE1)
4-META, water, acetone, reducing agent (pH 3.0) 65% phosphoric acid (pH 0.3)
Green Activator: GA (SF2)
10% citric acid with 3% ferric chloride (pH 0.6)
Apply primer to enamel and dentine for 20 sec and air dry Apply activator to enamel for 30 sec, wash with water and air dry Apply activator to dentine for 10 sec, wash with water and air dry
Bondfill SB Liquid (10F0629) Powder (10K03173) Catalyst V (TT1)
4-META, MMA, poly-functional methacrylate PolyMMA, TMPT pre-polymerized filler, pigment TBB, TBB-O, hydrocarbon
Apply mixture of the polymer powder and monomer catalyst, using the brush-on technique to the bonded tooth surface
Abbreviations: 4-META: 4-methacryloxyethyl trimellitate anhydride; MMA: methyl methacrylate; TMPT: trimethylopropane trimethacrylate; TBB: tri-n-butylborane; TBB-O: partially oxidized tri-n-butylborane.
journal of dentistry 42 (2014) 425–431
and built up on both the labial and lingual tooth surfaces to obtain specimens with adequate length for MTBS evaluation. Prior to applying Bondfill SB, RA and Super Bond C & S (Sun Medical Co., Ltd.) was applied on the lingual enamel surface. For both primary and permanent teeth, organic material in pulp chamber was removed and filled with Super Bond C & S after GA etching. The bonded teeth were left to dry at room temperature for 30 min and then soaked in water at 37 8C for 24 h. Teeth were sectioned using a low-speed circular diamond saw (Isomet; Buehler Ltd., Lake Bluff, IL, USA) under copious water-cooling. The slices were each 0.7 mm thick, prepared perpendicular to the bonded surface buccolingually. Three to seven tooth slices were obtained from each tooth. To avoid the effect of anisotropic features such as enamel prism and dentinal tubule orientation as confounding factors on MTBS, three slices obtained from most mesiodistally centerd area of each tooth were used for examination. Thus, the MTBS obtained for each group (primary or permanent enamel or dentine) represents the mean result derived from 12 teeth 3 slices = 36 specimens. Each bonded slice was trimmed into a dumbbell shape using a superfine diamond point (SF-319, ISO 014; Shofu Inc., Kyoto, Japan) under copious water-cooling, with the remaining bonded surface being approximately 1.0 mm2. The exact cross-sectional area was calculated after measuring the width of the remaining bonded area with a pair of digital callipers (500–151 CD-15C; Mitutoyo, Tokyo, Japan). The dumbbellshaped specimens were fixed on a jig with cyanoacrylate glue (Model Repair Blue; Dentsply Sankin Co., Ltd., Tokyo, Japan), and subjected to a tensile load in a universal testing machine (EZ-Test; Shimazu Corp., Kyoto, Japan) at a crosshead speed of 1.0 mm/min. Micro-tensile bond strength (Unit: MPa) was calculated using the peak load at failure divided by the original cross-sectional area. The fractured specimens were dried in a desiccator after MTBS testing. The fractured surfaces of these specimens were sputter-coated with platinum for SEM examination. The modes of fracture were classified as: (a) adhesive fracture; if 100% of the bonded interface failed between enamel/dentine and the resin; (b) cohesive resin fracture, if 100% of the failure was in resin; or (c) mixed fracture, if the failure was partially adhesive and partially cohesive resin fracture.
2.3 Data analyses The mean of the bond strength data derived from the three slices of each tooth was used to generate results for statistical analysis; each tooth was used as the statistical unit. Statistical analysis was performed using a software package (PASW Statistics 18.0; SPSS Inc., Chicago, IL, USA). Three-factor analysis of variances could not be employed to examine the effects of tooth type (permanent vs primary), tooth substrates (enamel vs dentine) and treatment methods (TP vs RA vs GA) on MTBS even when the normality and equal variance assumptions of the data were met. This is due to the mutual exclusivity of the use of RA on enamel and GA on dentine. Accordingly, a one-factor analysis of variance (ANOVAs) was first employed to compare the results of treatment on enamel and dentine for primary and permanent teeth (8 groups). Post hoc multiple comparisons for each statistical evaluation were
427
performed with the Tukey test. Then, 3 two-factor ANOVAs were used to examine: (i) the effects of enamel treatment method (TP-priming vs RA-etching) and tooth type (primary vs permanent teeth), and the interaction of these two factors on MTBS for enamel; (ii) the effects of dentine treatment method (TP-priming vs GA-etching) and tooth type (primary vs permanent teeth), and the interaction of these two factors on MTBS for dentine; and (iii) the effects of tooth substrate (enamel vs dentine) and tooth type (primary teeth vs permanent teeth), and the interaction of these two factors on MTBS for TP-primed specimens. For all tests, statistical significances were preset at a = 0.05.
3 Results 3.1 Etching/priming efficacy on enamel and dentine Fig. 1 compared the etching/priming efficacy of each primer or etchant on enamel and dentine in primary (Fig. 1(1, 3, 5 and 7)) and permanent teeth (Fig. 1(2, 4, 6 and 8)). For both the primary and permanent teeth, application of RA to enamel (Fig. 1(1 and 2)) and GA to dentine (Fig. 1(5 and 6)) resulted in removal of the smear layer and exposure of partially-dematerialized enamel prisms or patent dentinal tubule orifices. However, the etching efficacy of TP on primary and permanent tooth enamel was poor. The smear layer was retained and enamel prismatic structure could not be identified (Fig. 1(3 and 4)). The smear layer was not completely removed for both primary and permanent tooth dentine primed with TP and dentinal tubules were only partially-exposed (Fig. 1(7 and 8)).
3.2 Micro-tensile bond strength and fracture modes Significant differences were observed for the MBTSs among the 8 treatment groups ( p < 0.05). Comparison of MTBSs and fracture modes are shown in Table 2. For enamel, the MBTS of Group 2 (use of RA on permanent tooth enamel) was significantly higher than the other three enamel groups ( p < 0.05). There was no significant difference among the other three enamel groups (Groups 1, 3 and 4). For dentine, significant differences in MTBS were observed, in the order: Group 6 (GA on permanent dentine) > Group 8 (TP on permanent dentine) > Group 5 (on primary dentine) = Group 7 (TP on primary dentine) ( p < 0.05). The results of Tukey HSD test are shown in Table 3. The MTBS of dentine was significantly higher than that of enamel for permanent teeth ( p < 0.0001); however, no significant difference was identified between the MTBS of dentine and the MTBS of enamel for primary teeth ( p = 0.636). The MTBS of permanent teeth was significantly higher than that of primary teeth for dentine ( p < 0.0001); however, no significant difference between permanent and primary teeth was identified for enamel ( p = 0.067). For TP primer, the MTBS of dentine was significantly higher than that of enamel ( p = 0.020). The results of two-factor ANOVA are shown in Table 4. Two-factor ANOVA for enamel indicated that the MTBSs of specimens etched with RA were significantly higher than specimens treated with TP ( p < 0.001). The MTBSs of specimens bonded to permanent teeth were also significantly
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journal of dentistry 42 (2014) 425–431
Fig. 1 – Scanning electron microscopy images illustrating the etching/priming efficacy on enamel and dentine in the 8 groups. In enamel of primary (1) and permanent tooth (2) etched with RA, a distinctive etching pattern could be identified from the exposed enamel prisms. In enamel of primary (3) and permanent tooth (4) primed with TP, the enamel surface was covered with a smear layer and no prismatic structures were evident. In dentine of primary (5) and permanent tooth (6) etched with GA, smear layer was removed and dentinal tubule orifices could be seen. In dentine of primary (7) and permanent tooth (8) primed with TP, smear layer was removed. Both open and occluded dentinal tubules were observed in primary teeth (7), while partially-occluded dentinal tubules were seen in permanent teeth (8).
higher than those bonded to primary teeth ( p < 0.001). The interaction of these two factors was significantly different ( p < 0.001). Two-factor ANOVA for dentine indicated that MTBSs of specimens etched with GA were significantly higher than specimens treated with TP ( p = 0.007). The MTBSs of specimens bonded to permanent teeth were also significantly higher than those bonded to primary teeth ( p < 0.001). The interaction of these two factors was significantly different ( p = 0.004). Two-factor ANOVA for TP-primed specimens
indicated that the MTBSs of specimens bonded to dentine were significantly higher than specimens bonded to enamel ( p < 0.001). The MTBSs of specimens bonded to permanent teeth were significantly higher than those bonded to primary tooth ( p = 0.031). The interaction of these two factors was significantly different ( p = 0.024). The predominant failure mode was cohesive resin fracture except for Group 1 (RA on primary enamel), in which the major failure mode was mixed fracture (Table 2). Representative
Table 2 – Microtensile bond strengths among the 8 treatment groups that were classified based on primary/permanent tooth, enamel or dentine, and primer or activator. Group
Primary/ Permanent tooth
Primer/Activator
1 2 3 4 5 6 7 8
Primary Enamel Permanent Enamel Primary Enamel Permanent Enamel Primary Dentine Permanent Dentine Primary Dentine Permanent Dentine
Red Activator Red Activator Teeth Primer Teeth Primer Green Activator Green Activator Teeth Primer Teeth Primer
Microtensile bond strengths Mean (SD) Unit: MPa 22.6 29.3 19.5 19.4 21.6 39.1 22.0 28.6
(7.5) (7.4) (3.5) (4.9) (6.1) (9.7) (7.5) (9.3)
a b a a a c a b
Failure modes (%) Adhesive
5.3
Mixed
Cohesive (resin)
61.1 25.0 5.6 3.6 36.8 3.7 5.9 41.9
38.9 75.0 94.4 96.4 57.9 96.3 94.1 58.1
Values are means (standard deviations), in MPa. Values with the same superscript indicate no significant difference among the 8 groups (p < 0.05). Values with same superscript letters indicate no significant difference among 8 groups at p < 0.05. Failure modes; Adhesive: adhesive fracture, Mix: mixed with adhesive, cohesive resin and enamel or dentine fracture, Cohesive (Re): cohesive resin fracture.
Table 3 – Result ( p value) of Tukey HSD comparison among microtensile bond strength (MTBS) of different tooth substrate, enamel/dentine, and Tooth Primer (TP). Primary/Permanent
Enamel vs Dentine
Enamel/ Dentine
Primary vs Permanent
Primer/ Activator
Enamel vs Dentine
Primary tooth Permanent tooth
p = 0.636 p < 0.0001
Enamel Dentine
p = 0.067 p < 0.0001
TP
p = 0.020
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journal of dentistry 42 (2014) 425–431
Table 4 – Two-factor analysis of microtensile bond strength (MTBS) of each enamel and dentine, and Teeth Primer (TP primer; TP) for the two main factors and interaction. MTBS ( p value)
MTBS ( p value)
Enamel/ Dentine
TP primer vs RA or GA Activator
Primary tooth vs Permanent tooth
Interaction
Primer/ Activator
Enamel/ Dentine
TP Primer vs RA or GA Activator
Interaction
Enamel Dentine
p < 0.001 p = 0.007
p < 0.001 p < 0.001
P < 0.001 p = 0.004
TP RA GA
p = 0.001 – –
p = 0.031 – –
p < 0.024 – –
fractured interfaces after MTBS testing for the 8 groups are shown in Fig. 2. For all groups, a layer of resinous material was observed on the enamel or dentine. For enamel, many fractured resin tags were observed for permanent teeth etched with RA in Group 2 (Fig. 2(2)). For dentine, resin tags were present in the dentinal tubules in all groups, especially for permanent teeth etched with GA in Group 6 (Fig. 2(6)).
4 Discussion Since there were significant differences in the MTBSs for enamel treated with RA or TP, and for dentine treated with TP or GA (Table 4), the first null hypothesis that there is no difference in the MTBS of Bondfill SB to enamel or dentine treated with TP, RD or GA has to be rejected. Because significant differences were found for the MTBSs of TP-primed specimens between primary and permanent teeth, for either enamel or dentine (Tables 2–4), the second null hypothesis also has to be rejected. Teeth Primer has a pH value of 3.0 and is classified as an ‘‘ultra-mild’’ self-etching primer (pH > 2.5).11 It has been reported that an etch-and-rinse approach with phosphoric acid remains the gold standard for bonding to enamel, since this bonding approach produces the most durable bond to enamel, and protects the more vulnerable bond to dentine against degradation.12–14 The use of ‘‘mild’’ self-etch adhesives is considered inadequate by many researchers for
bonding to enamel, especially for unground or aprismatic enamel.11,12 For permanent teeth, clinical research has clearly shown that marginal defects at the enamel margins of a resin composite restoration develop rather rapidly, whereas the dentine margins appear to maintain their marginal integrity much longer.15,16 For bonding to permanent teeth in the present study (Tables 4 and 4), MTBSs for enamel etched with 65% phosphoric acid (RA) and dentine etched with 10% citric acid and 3% ferric chloride (GA) were significantly higher than those primed with the 4-METAcontaining Teeth Primer (TP). However, for primary teeth, there was no significant difference in the MTBSs among the 4 groups, regardless of etchant/primer employed and the type of bonding substrate (enamel/dentine) examined (one-factor ANOVA and Tukey test in Tables 2 and 3). Unlike permanent teeth, enamel and dentine of primary teeth are more susceptible to the etching effects of primer and etchants.5–9 A previous study showed that etching primary tooth enamel with 10% phosphoric acid significantly improved resin adhesion regardless of the method of surface treatment; none of the other enamel surface treatment methods was comparable to phosphoric acid etching.16 However, overetching may result when primary tooth enamel is etched with RA for 30 s. Indeed, results from the present study showed that the same range of MTBS may be achieved by etching primary tooth enamel with TP. Thus, a shorter etching time may be recommended when RA is used on primary enamel without compromising the bonding efficacy of the resin
Fig. 2 – Scanning electron microscopy images of representative fractured interfaces after MTBS testing. In all groups, a fractured layer of resinous material could be seen along the surface of enamel or dentine (open arrows). For enamel, many resin tags could be observed in Group 2 (2). For dentine, resin infiltrated the dentinal tubules (5–8), especially for Group 6 (6), in which long resin tags (asterisk) were found.
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journal of dentistry 42 (2014) 425–431
composite. Conversely, TP was less effective than RA for bonding to permanent enamel. The use of partially-oxidized TBB (i.e. TBBO) for the initiation of polymerization of 4-META/MMA resin monomers produced high bond strength to permanent tooth dentine when the latter was first treated with GA.17 The 4-META resin monomer, which contained both hydrophobic and hydrophilic functional groups, increased the molecular weight of the polymer formed and promoted adhesion to tooth structures.18 However, primary tooth dentine etched with GA for the same etching time as permanent tooth dentine exhibited morphologic features of over-etching.7 Previous studies comparing the bond strengths of resin composites between primary and permanent tooth dentine produced dichotomous results.6,8,9,19–21 In the present study, pre-treatment of dentine with GA and TP in permanent dentine (Groups 6 and 8; Table 2) resulted in significantly higher MTBS, when compared with corresponding treatments in primary dentine (Groups 5 and 7; Table 2). Post hoc Tukey HSD test also showed significantly higher MTBSs in permanent dentine than in primary dentine (Table 3). Differences in the chemical composition and microstructure between primary and permanent tooth dentine are generally considered as the main factors accountable for the inferior performance of adhesive systems in primary dentine.9,19,20 The lower mineral content of primary dentine may result in a lower buffering capacity when it comes into contact with acidic substrates.5,22 The tensile strength of dentine per se may also influence the results of MTBS, due to cohesive facture of dentine during tensile testing. However, very limited information is available on the tensile properties of primary dentine.23 Likewise, no information is available that compares the tensile properties between primary and permanent dentine. Compared to GA, TP should have a milder etching potential on dentine due to its high pH. The demineralization necessary for adhesive monomers to infiltrate dentine usually occurs at a pH of 3.0 (Hosoya, unpublished results). Thus, application of TP should result in less damage to dentine. Teeth Primer should also produce optimal polymerization of the adhesive resin monomers in the presence of moisture derived from dentine, due to use of the TBBO polymerization initiator.24 Most of the adhesives and resin composites currently available are suitable for restoration of teeth that require removal of dental caries. However, in deep cavities, unstable adhesion may occur because of the lack of penetration by the curing light. In contrast, TBBO, which is triggered by the presence of moisture, enables Bondfill SB to be hardened from the adhesive interface without resorting to the use of light-curing. Thus, clinical application of Bondfill SB as a self-adhesive filling material may be indicated for restorations when conventional methods of restoration are difficult to perform. In the present study, the major failure mode was cohesive resin fracture except for Group 1 (primary tooth enamel etched with RA), in which the predominant failure mode was mixed fracture. For the other groups, a layer of resin was observed along the surface of enamel or dentine after MTBS testing (Fig. 2). Cohesive resin fracture not only indicates strong adhesion between resinous materials and the tooth substrates, but also speculates the low mechanical properties of Bondfill SB. The latter contains trimethylol propane
trimethacrylate (TMPT) pre-polymerized fillers, and is harder than conventional 4-META/MMA resin cement. However, it is dubious if the mechanical properties of Bondfill SB are adequate to enable this resin composite to be used for restoring lost tooth structures. Bondfill SB is a chemical-cured resin and takes over 3 min to harden. Compared with lightcured resin composites, the long curing time is a disadvantage. Future studies are required to examine the properties of this material from a clinical perspective.
5 Conclusions Within the limits of the present in vitro study, it may be concluded that the use of TP primer with Bondfill SB may be effective for restoring enamel and dentine of primary teeth. However, bond strength of primary dentine associated with the use of Teeth Primer and Bondfill SB is significantly lower than that of permanent teeth.
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Bonding ability of 4-META self-etching primer used with 4-META/MMA-TBB resin to enamel and dentine: Primary vs permanent teeth Yumiko Hosoya a,*, Franklin R. Tay b a Department of Pediatric Dentistry, Course of Medical and Dental Science, Nagasaki University Graduate School of Biomedical Sciences, 1-7-1, Sakamoto, Nagasaki 852-8588, Japan b Department of Endodontics, College of Dental Medicine, Georgia Regents University, Augusta, GA 30912-1129, USA
article info
abstract
Article history:
Objectives: The present study compared the efficacies of the self-etching Teeth Primer (TP:
Received 19 August 2013
4-META), and the etchants Red Activator (RA; 65% phosphoric acid) and Green Activator (GA;
Received in revised form
10% citric acid with 3% ferric chloride), for bonding to enamel and dentine of human primary
12 January 2014
and permanent teeth, when used with 4-META/MMA-TBB resin (Bondfill SB).
Accepted 13 January 2014
Methods: Forty-eight non-carious primary canines and third molars were used. Eight groups were prepared: Group 1 (primary enamel with RA), Group 2 (permanent enamel with RA), Group 3 (primary enamel with TP), Group 4 (permanent enamel with TP), Group 5 (primary
Keywords:
dentine with GA), Group 6 (permanent dentine with GA), Group 7 (primary dentine with TP)
4-META self-etching primer
and Group 8 (permanent dentine with TP). Micro-tensile bond strengths (MTBS) were
4-META/MMA-TBB resin
measured and analyzed statistically using ANOVAs and Tukey HSD tests at a = 0.05. Efficacy
Micro-tensile bond strength
of etching/priming and the morphology of bonded interfaces were observed with SEM.
Primary teeth
Results: Etching/priming efficacy of TP on enamel was low. The MTBS of Group 2 was
Permanent teeth
significantly higher than the other groups (Groups 1, 3 and 4). For dentine, significant
Scanning electron microscopy
differences in MTBS were observed, in the order of Groups 6 > 8 > 7 = 5 ( p < 0.05). The MTBSs of permanent dentine were significantly higher than primary dentine. For primary teeth, there was no significant difference in the MTBSs between enamel and dentine, irrespective of primer or etchant ( p > 0.05). Conclusion: TP primer/Bondfill SB may be used as an alternative to other adhesive/resin composite systems for bonding to enamel and dentine of primary teeth. # 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Attrition and wear of enamel and dentine, gingival recession and formation of wedge-shaped defects occur throughout life due to clenching, grinding, mastication, tooth brushing and other factors. For restoration of these defects, it is beneficial for the wear resistance of dental materials to approximate those of tooth substrates, particularly for areas that are in
constant occlusal contact. An adhesive resin chemically initiated with tri-n-butylborane (TBB) has been used for bonding between tooth structure and restorative materials. One of the problems associated with the TBB-initiated resin system is its insufficient wear resistance, due to the absence of inorganic fillers.1,2 A modified TBB resin containing pre-polymerized organic filler particles (Bondfill SB, Sun Medical Co., Ltd., Moriyama, Japan) was recently developed for filling, luting, or sealing
* Corresponding author. Tel.: +81 95 819 7673; fax: +81 95 819 7676. E-mail address: [email protected] (Y. Hosoya). 0300-5712/$ – see front matter # 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jdent.2014.01.007
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procedures employed in preventive and restorative dentistry.3 This modified resin demonstrated better wear characteristics when compared with conventional unfilled TBB-initiated resins.4 Bondfill SB is a resin composite that utilizes the 4methacryloyloxyethyl trimellitate anhydride/methyl methacrylate-TBB (4-META/MMA-TBB) system for adhesion to tooth substrates. For practitioners who are using the etch-and-rinse bonding technique, the manufacturer has originally developed an etchant (65% phosphoric acid; Red Activator) for bonding to enamel, and a conditioner consisting of 10% citric acid with 3% ferric chloride (Green Activator) for bonding to dentine. Because enamel and dentine in primary teeth (especially for dentine) are more susceptible to primers and etchants than permanent teeth,5–9 over-etching may occur when the same etching time employed for permanent teeth is adopted for etching primary teeth. To alleviate this problem, a 4-META containing selfetching primer (Teeth Primer, Sun Medical Co., Ltd.) was developed and included in Bondfill SB for practitioners who prefer the self-etching approach, and for bonding to tooth substrates in primary teeth. As the pH of Teeth Primer is 3.0, the bonding technique has been termed the ultra-mild self-etching approach.10,11 Although the use of mild self-etching primers/ adhesives is not ideal for bonding to permanent tooth enamel,12,13 limited information is available for their bonding efficacy to primary tooth enamel. Thus, the objective of the present study was to compare the use of the self-etching Teeth Primer (TP) in Bondfill SB, with the 65% phosphoric acidcontaining Red Activator (RA) for bonding to primary and permanent human enamel, and with the Green Activator (GA) for bonding to primary and permanent human dentine. The null hypotheses tested were: (1) there is no difference in the microtensile bond strength of Bondfill SB to enamel or dentine treated with TP, RD or GA; and (2) there is no difference in the microtensile bond strength of TP-primed specimens between primary teeth and permanent teeth for either enamel or dentine.
2.
to the regulations of ethics committees of Nagasaki University. The extracted teeth were frozen in physiologic saline within 10 min after extraction. Table 1 shows the components and protocol of the materials used in the present study. The primary or permanent teeth were each divided into 4 groups according to whether enamel or dentine was bonded, and the primer or activator employed for substrate treatment. Group 1 (primary tooth enamel etched with Red Activator: RA); Group 2 (permanent tooth enamel etched with RA); Group 3 (primary tooth enamel primed with Teeth Primer: TP); Group 4 (permanent tooth enamel primed with TP); Group 5 (primary tooth dentine etched with Green Activator: GA); Group 6 (permanent tooth dentine etched with GA); Group 7 (primary tooth dentine primed with TP); and Group 8 (permanent tooth dentine primed with TP).
2.1.
Flat enamel and dentine surfaces were prepared using silicon carbide papers under copious water-cooling on the labial surfaces of primary canines and the occlusal surfaces of third molars (N = 12 for each of 8 groups). Final polishing was achieved using 1200-grit silicon carbide paper. In Groups 1 and 2, 65% phosphoric acid (RA) was applied on enamel for 30 s, rinsed with water for 15 s and dried with an air-water syringe. In Groups 5 and 6, 10% citric acid with 3% ferric chloride (GA) was applied on dentine for 10 s, rinsed for 15 s and dried with an air–water syringe. For Groups 3, 4, 7 and 8, Teeth Primer (TP) was applied on enamel or dentine for 20 s and air-dried for 20 s. For Groups 3, 4, 7 and 8, the specimens were immersed in acetone for 10 s to eliminate resinous material from the primed tooth substrates. All specimens were dried in a desiccator and sputter-coated with platinum (Superfine Coater ESC-101; Elionix Co., Tokyo, Japan) prior to SEM observation (JSM-5610LV, JOEL, Tokyo, Japan).
2.2.
Materials and methods
Forty-eight non-carious primary canines that were extracted to expedite eruption of the succedaneous teeth or for orthodontic reasons were used as primary tooth specimens. Forty-eight extracted non-carious upper third molars were used as permanent tooth specimens. Informed consent for tooth collection was obtained from parents and subjects according
Scanning electron microscopy (SEM)
Micro-tensile bond strength (MTBS) evaluation
Flat enamel or dentine surfaces were prepared in the manner previously described using 180-grit silicon carbide paper for final polishing. Enamel or dentine surfaces were etched or primed in the same manner as for the SEM observation. After etching or priming, a mixture of the polymer and monomercatalyst of Bondfill SB was applied and built up using the brush-on technique. For primary teeth, Bondfill SB was applied
Table 1 – Components of the materials and protocols utilised in the preset study. Material (Batch no.)
Components
Protocol
Teeth Primer: TP (0056) Red Activator: RA (TE1)
4-META, water, acetone, reducing agent (pH 3.0) 65% phosphoric acid (pH 0.3)
Green Activator: GA (SF2)
10% citric acid with 3% ferric chloride (pH 0.6)
Apply primer to enamel and dentine for 20 sec and air dry Apply activator to enamel for 30 sec, wash with water and air dry Apply activator to dentine for 10 sec, wash with water and air dry
Bondfill SB Liquid (10F0629) Powder (10K03173) Catalyst V (TT1)
4-META, MMA, poly-functional methacrylate PolyMMA, TMPT pre-polymerized filler, pigment TBB, TBB-O, hydrocarbon
Apply mixture of the polymer powder and monomer catalyst, using the brush-on technique to the bonded tooth surface
Abbreviations: 4-META: 4-methacryloxyethyl trimellitate anhydride; MMA: methyl methacrylate; TMPT: trimethylopropane trimethacrylate; TBB: tri-n-butylborane; TBB-O: partially oxidized tri-n-butylborane.
journal of dentistry 42 (2014) 425–431
and built up on both the labial and lingual tooth surfaces to obtain specimens with adequate length for MTBS evaluation. Prior to applying Bondfill SB, RA and Super Bond C & S (Sun Medical Co., Ltd.) was applied on the lingual enamel surface. For both primary and permanent teeth, organic material in pulp chamber was removed and filled with Super Bond C & S after GA etching. The bonded teeth were left to dry at room temperature for 30 min and then soaked in water at 37 8C for 24 h. Teeth were sectioned using a low-speed circular diamond saw (Isomet; Buehler Ltd., Lake Bluff, IL, USA) under copious water-cooling. The slices were each 0.7 mm thick, prepared perpendicular to the bonded surface buccolingually. Three to seven tooth slices were obtained from each tooth. To avoid the effect of anisotropic features such as enamel prism and dentinal tubule orientation as confounding factors on MTBS, three slices obtained from most mesiodistally centerd area of each tooth were used for examination. Thus, the MTBS obtained for each group (primary or permanent enamel or dentine) represents the mean result derived from 12 teeth 3 slices = 36 specimens. Each bonded slice was trimmed into a dumbbell shape using a superfine diamond point (SF-319, ISO 014; Shofu Inc., Kyoto, Japan) under copious water-cooling, with the remaining bonded surface being approximately 1.0 mm2. The exact cross-sectional area was calculated after measuring the width of the remaining bonded area with a pair of digital callipers (500–151 CD-15C; Mitutoyo, Tokyo, Japan). The dumbbellshaped specimens were fixed on a jig with cyanoacrylate glue (Model Repair Blue; Dentsply Sankin Co., Ltd., Tokyo, Japan), and subjected to a tensile load in a universal testing machine (EZ-Test; Shimazu Corp., Kyoto, Japan) at a crosshead speed of 1.0 mm/min. Micro-tensile bond strength (Unit: MPa) was calculated using the peak load at failure divided by the original cross-sectional area. The fractured specimens were dried in a desiccator after MTBS testing. The fractured surfaces of these specimens were sputter-coated with platinum for SEM examination. The modes of fracture were classified as: (a) adhesive fracture; if 100% of the bonded interface failed between enamel/dentine and the resin; (b) cohesive resin fracture, if 100% of the failure was in resin; or (c) mixed fracture, if the failure was partially adhesive and partially cohesive resin fracture.
2.3 Data analyses The mean of the bond strength data derived from the three slices of each tooth was used to generate results for statistical analysis; each tooth was used as the statistical unit. Statistical analysis was performed using a software package (PASW Statistics 18.0; SPSS Inc., Chicago, IL, USA). Three-factor analysis of variances could not be employed to examine the effects of tooth type (permanent vs primary), tooth substrates (enamel vs dentine) and treatment methods (TP vs RA vs GA) on MTBS even when the normality and equal variance assumptions of the data were met. This is due to the mutual exclusivity of the use of RA on enamel and GA on dentine. Accordingly, a one-factor analysis of variance (ANOVAs) was first employed to compare the results of treatment on enamel and dentine for primary and permanent teeth (8 groups). Post hoc multiple comparisons for each statistical evaluation were
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performed with the Tukey test. Then, 3 two-factor ANOVAs were used to examine: (i) the effects of enamel treatment method (TP-priming vs RA-etching) and tooth type (primary vs permanent teeth), and the interaction of these two factors on MTBS for enamel; (ii) the effects of dentine treatment method (TP-priming vs GA-etching) and tooth type (primary vs permanent teeth), and the interaction of these two factors on MTBS for dentine; and (iii) the effects of tooth substrate (enamel vs dentine) and tooth type (primary teeth vs permanent teeth), and the interaction of these two factors on MTBS for TP-primed specimens. For all tests, statistical significances were preset at a = 0.05.
3 Results 3.1 Etching/priming efficacy on enamel and dentine Fig. 1 compared the etching/priming efficacy of each primer or etchant on enamel and dentine in primary (Fig. 1(1, 3, 5 and 7)) and permanent teeth (Fig. 1(2, 4, 6 and 8)). For both the primary and permanent teeth, application of RA to enamel (Fig. 1(1 and 2)) and GA to dentine (Fig. 1(5 and 6)) resulted in removal of the smear layer and exposure of partially-dematerialized enamel prisms or patent dentinal tubule orifices. However, the etching efficacy of TP on primary and permanent tooth enamel was poor. The smear layer was retained and enamel prismatic structure could not be identified (Fig. 1(3 and 4)). The smear layer was not completely removed for both primary and permanent tooth dentine primed with TP and dentinal tubules were only partially-exposed (Fig. 1(7 and 8)).
3.2 Micro-tensile bond strength and fracture modes Significant differences were observed for the MBTSs among the 8 treatment groups ( p < 0.05). Comparison of MTBSs and fracture modes are shown in Table 2. For enamel, the MBTS of Group 2 (use of RA on permanent tooth enamel) was significantly higher than the other three enamel groups ( p < 0.05). There was no significant difference among the other three enamel groups (Groups 1, 3 and 4). For dentine, significant differences in MTBS were observed, in the order: Group 6 (GA on permanent dentine) > Group 8 (TP on permanent dentine) > Group 5 (on primary dentine) = Group 7 (TP on primary dentine) ( p < 0.05). The results of Tukey HSD test are shown in Table 3. The MTBS of dentine was significantly higher than that of enamel for permanent teeth ( p < 0.0001); however, no significant difference was identified between the MTBS of dentine and the MTBS of enamel for primary teeth ( p = 0.636). The MTBS of permanent teeth was significantly higher than that of primary teeth for dentine ( p < 0.0001); however, no significant difference between permanent and primary teeth was identified for enamel ( p = 0.067). For TP primer, the MTBS of dentine was significantly higher than that of enamel ( p = 0.020). The results of two-factor ANOVA are shown in Table 4. Two-factor ANOVA for enamel indicated that the MTBSs of specimens etched with RA were significantly higher than specimens treated with TP ( p < 0.001). The MTBSs of specimens bonded to permanent teeth were also significantly
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Fig. 1 – Scanning electron microscopy images illustrating the etching/priming efficacy on enamel and dentine in the 8 groups. In enamel of primary (1) and permanent tooth (2) etched with RA, a distinctive etching pattern could be identified from the exposed enamel prisms. In enamel of primary (3) and permanent tooth (4) primed with TP, the enamel surface was covered with a smear layer and no prismatic structures were evident. In dentine of primary (5) and permanent tooth (6) etched with GA, smear layer was removed and dentinal tubule orifices could be seen. In dentine of primary (7) and permanent tooth (8) primed with TP, smear layer was removed. Both open and occluded dentinal tubules were observed in primary teeth (7), while partially-occluded dentinal tubules were seen in permanent teeth (8).
higher than those bonded to primary teeth ( p < 0.001). The interaction of these two factors was significantly different ( p < 0.001). Two-factor ANOVA for dentine indicated that MTBSs of specimens etched with GA were significantly higher than specimens treated with TP ( p = 0.007). The MTBSs of specimens bonded to permanent teeth were also significantly higher than those bonded to primary teeth ( p < 0.001). The interaction of these two factors was significantly different ( p = 0.004). Two-factor ANOVA for TP-primed specimens
indicated that the MTBSs of specimens bonded to dentine were significantly higher than specimens bonded to enamel ( p < 0.001). The MTBSs of specimens bonded to permanent teeth were significantly higher than those bonded to primary tooth ( p = 0.031). The interaction of these two factors was significantly different ( p = 0.024). The predominant failure mode was cohesive resin fracture except for Group 1 (RA on primary enamel), in which the major failure mode was mixed fracture (Table 2). Representative
Table 2 – Microtensile bond strengths among the 8 treatment groups that were classified based on primary/permanent tooth, enamel or dentine, and primer or activator. Group
Primary/ Permanent tooth
Primer/Activator
1 2 3 4 5 6 7 8
Primary Enamel Permanent Enamel Primary Enamel Permanent Enamel Primary Dentine Permanent Dentine Primary Dentine Permanent Dentine
Red Activator Red Activator Teeth Primer Teeth Primer Green Activator Green Activator Teeth Primer Teeth Primer
Microtensile bond strengths Mean (SD) Unit: MPa 22.6 29.3 19.5 19.4 21.6 39.1 22.0 28.6
(7.5) (7.4) (3.5) (4.9) (6.1) (9.7) (7.5) (9.3)
a b a a a c a b
Failure modes (%) Adhesive
5.3
Mixed
Cohesive (resin)
61.1 25.0 5.6 3.6 36.8 3.7 5.9 41.9
38.9 75.0 94.4 96.4 57.9 96.3 94.1 58.1
Values are means (standard deviations), in MPa. Values with the same superscript indicate no significant difference among the 8 groups (p < 0.05). Values with same superscript letters indicate no significant difference among 8 groups at p < 0.05. Failure modes; Adhesive: adhesive fracture, Mix: mixed with adhesive, cohesive resin and enamel or dentine fracture, Cohesive (Re): cohesive resin fracture.
Table 3 – Result ( p value) of Tukey HSD comparison among microtensile bond strength (MTBS) of different tooth substrate, enamel/dentine, and Tooth Primer (TP). Primary/Permanent
Enamel vs Dentine
Enamel/ Dentine
Primary vs Permanent
Primer/ Activator
Enamel vs Dentine
Primary tooth Permanent tooth
p = 0.636 p < 0.0001
Enamel Dentine
p = 0.067 p < 0.0001
TP
p = 0.020
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Table 4 – Two-factor analysis of microtensile bond strength (MTBS) of each enamel and dentine, and Teeth Primer (TP primer; TP) for the two main factors and interaction. MTBS ( p value)
MTBS ( p value)
Enamel/ Dentine
TP primer vs RA or GA Activator
Primary tooth vs Permanent tooth
Interaction
Primer/ Activator
Enamel/ Dentine
TP Primer vs RA or GA Activator
Interaction
Enamel Dentine
p < 0.001 p = 0.007
p < 0.001 p < 0.001
P < 0.001 p = 0.004
TP RA GA
p = 0.001 – –
p = 0.031 – –
p < 0.024 – –
fractured interfaces after MTBS testing for the 8 groups are shown in Fig. 2. For all groups, a layer of resinous material was observed on the enamel or dentine. For enamel, many fractured resin tags were observed for permanent teeth etched with RA in Group 2 (Fig. 2(2)). For dentine, resin tags were present in the dentinal tubules in all groups, especially for permanent teeth etched with GA in Group 6 (Fig. 2(6)).
4 Discussion Since there were significant differences in the MTBSs for enamel treated with RA or TP, and for dentine treated with TP or GA (Table 4), the first null hypothesis that there is no difference in the MTBS of Bondfill SB to enamel or dentine treated with TP, RD or GA has to be rejected. Because significant differences were found for the MTBSs of TP-primed specimens between primary and permanent teeth, for either enamel or dentine (Tables 2–4), the second null hypothesis also has to be rejected. Teeth Primer has a pH value of 3.0 and is classified as an ‘‘ultra-mild’’ self-etching primer (pH > 2.5).11 It has been reported that an etch-and-rinse approach with phosphoric acid remains the gold standard for bonding to enamel, since this bonding approach produces the most durable bond to enamel, and protects the more vulnerable bond to dentine against degradation.12–14 The use of ‘‘mild’’ self-etch adhesives is considered inadequate by many researchers for
bonding to enamel, especially for unground or aprismatic enamel.11,12 For permanent teeth, clinical research has clearly shown that marginal defects at the enamel margins of a resin composite restoration develop rather rapidly, whereas the dentine margins appear to maintain their marginal integrity much longer.15,16 For bonding to permanent teeth in the present study (Tables 4 and 4), MTBSs for enamel etched with 65% phosphoric acid (RA) and dentine etched with 10% citric acid and 3% ferric chloride (GA) were significantly higher than those primed with the 4-METAcontaining Teeth Primer (TP). However, for primary teeth, there was no significant difference in the MTBSs among the 4 groups, regardless of etchant/primer employed and the type of bonding substrate (enamel/dentine) examined (one-factor ANOVA and Tukey test in Tables 2 and 3). Unlike permanent teeth, enamel and dentine of primary teeth are more susceptible to the etching effects of primer and etchants.5–9 A previous study showed that etching primary tooth enamel with 10% phosphoric acid significantly improved resin adhesion regardless of the method of surface treatment; none of the other enamel surface treatment methods was comparable to phosphoric acid etching.16 However, overetching may result when primary tooth enamel is etched with RA for 30 s. Indeed, results from the present study showed that the same range of MTBS may be achieved by etching primary tooth enamel with TP. Thus, a shorter etching time may be recommended when RA is used on primary enamel without compromising the bonding efficacy of the resin
Fig. 2 – Scanning electron microscopy images of representative fractured interfaces after MTBS testing. In all groups, a fractured layer of resinous material could be seen along the surface of enamel or dentine (open arrows). For enamel, many resin tags could be observed in Group 2 (2). For dentine, resin infiltrated the dentinal tubules (5–8), especially for Group 6 (6), in which long resin tags (asterisk) were found.
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journal of dentistry 42 (2014) 425–431
composite. Conversely, TP was less effective than RA for bonding to permanent enamel. The use of partially-oxidized TBB (i.e. TBBO) for the initiation of polymerization of 4-META/MMA resin monomers produced high bond strength to permanent tooth dentine when the latter was first treated with GA.17 The 4-META resin monomer, which contained both hydrophobic and hydrophilic functional groups, increased the molecular weight of the polymer formed and promoted adhesion to tooth structures.18 However, primary tooth dentine etched with GA for the same etching time as permanent tooth dentine exhibited morphologic features of over-etching.7 Previous studies comparing the bond strengths of resin composites between primary and permanent tooth dentine produced dichotomous results.6,8,9,19–21 In the present study, pre-treatment of dentine with GA and TP in permanent dentine (Groups 6 and 8; Table 2) resulted in significantly higher MTBS, when compared with corresponding treatments in primary dentine (Groups 5 and 7; Table 2). Post hoc Tukey HSD test also showed significantly higher MTBSs in permanent dentine than in primary dentine (Table 3). Differences in the chemical composition and microstructure between primary and permanent tooth dentine are generally considered as the main factors accountable for the inferior performance of adhesive systems in primary dentine.9,19,20 The lower mineral content of primary dentine may result in a lower buffering capacity when it comes into contact with acidic substrates.5,22 The tensile strength of dentine per se may also influence the results of MTBS, due to cohesive facture of dentine during tensile testing. However, very limited information is available on the tensile properties of primary dentine.23 Likewise, no information is available that compares the tensile properties between primary and permanent dentine. Compared to GA, TP should have a milder etching potential on dentine due to its high pH. The demineralization necessary for adhesive monomers to infiltrate dentine usually occurs at a pH of 3.0 (Hosoya, unpublished results). Thus, application of TP should result in less damage to dentine. Teeth Primer should also produce optimal polymerization of the adhesive resin monomers in the presence of moisture derived from dentine, due to use of the TBBO polymerization initiator.24 Most of the adhesives and resin composites currently available are suitable for restoration of teeth that require removal of dental caries. However, in deep cavities, unstable adhesion may occur because of the lack of penetration by the curing light. In contrast, TBBO, which is triggered by the presence of moisture, enables Bondfill SB to be hardened from the adhesive interface without resorting to the use of light-curing. Thus, clinical application of Bondfill SB as a self-adhesive filling material may be indicated for restorations when conventional methods of restoration are difficult to perform. In the present study, the major failure mode was cohesive resin fracture except for Group 1 (primary tooth enamel etched with RA), in which the predominant failure mode was mixed fracture. For the other groups, a layer of resin was observed along the surface of enamel or dentine after MTBS testing (Fig. 2). Cohesive resin fracture not only indicates strong adhesion between resinous materials and the tooth substrates, but also speculates the low mechanical properties of Bondfill SB. The latter contains trimethylol propane
trimethacrylate (TMPT) pre-polymerized fillers, and is harder than conventional 4-META/MMA resin cement. However, it is dubious if the mechanical properties of Bondfill SB are adequate to enable this resin composite to be used for restoring lost tooth structures. Bondfill SB is a chemical-cured resin and takes over 3 min to harden. Compared with lightcured resin composites, the long curing time is a disadvantage. Future studies are required to examine the properties of this material from a clinical perspective.
5 Conclusions Within the limits of the present in vitro study, it may be concluded that the use of TP primer with Bondfill SB may be effective for restoring enamel and dentine of primary teeth. However, bond strength of primary dentine associated with the use of Teeth Primer and Bondfill SB is significantly lower than that of permanent teeth.
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