Andre Gisele Tadeu Paulillo

(1)

Effect of the composite photoactivation mode

on microtensile bond strength and Knoop

microhardness

Andre

´ L.F. e Silva

a

, Gisele D.S. Pereira

b

, Carlos T.S. Dias

c

,

Luı´s Alexandre Maffei Sartini Paulillo

a,

*

a

Department of Restorative Dentistry, Piracicaba School of Dentistry, University of Campinas, Piracicaba, SP, Brazil

b

School of Dentistry, Gama Filho University, Rio de Janeiro, RJ, Brazil

c

ESALQ, University of Sa˜o Paulo, Piracicaba, SP, Brazil

Received 22 July 2004; received in revised form 5 February 2005; accepted 4 April 2005

KEYWORDS Photoactivation mode; Polymerization shrinkage; Adhesive system; Microtensile; Knoop microhardness

Summary Objective. This study evaluated the effect of the composite photo-activation mode on microtensile bond strength and Knoop microhardness.

Methods. Standard class I cavities (3!4!3 mm) were restored with two adhesives systems, Single Bond (SB) and Clearfil SE Bond (CE), and the TPH composite. The photoactivation of the composite was carried out using three modes: Conventional (CO: 400 mW/cm2!40 s), Soft-Start (SS: 100 mW/cm2!10 sC600 mW/cm2!30 s) and Pulse-Delay (PD: 100 mW/cm2_{!3 sC3 min waitC600 mW/cm}2_{!37 s). For the}

microtensile test, beams obtained from the buccal wall bond interface were tested under tension at 0.5 mm/min crosshead speed until failure. For the microhardness test, the restorations were sectioned in the mesio-distal direction and indentations were made on the internal composite surface of each half at three different depths. Data of two tests were analyzed using two-way ANOVA and LSMeans (aZ0.05). Results. In the microtensile test, SS presented the highest values. PD presented intermediate values without differing significantly from the other modes. For adhesives, SB presented the highest values. In the microhardness test, PD presented the highest values, differing significantly from SS. CO presented intermediate values but without any statistical difference from the others. The SS–CE interaction presented the lowest values with statistical differences from all the others. Significance. By the SS technique, the highest bond strength was obtained. However, this technique made it possible for the adhesive system to intervene with the hardness of the composite.

www.intl.elsevierhealth.com/journals/dema

* Corresponding author. Address: Av. Limeira 901 Vila Areia˜o, Faculdade de Odontologia de Piracicaba, Unicamp, Piracicaba, SP CEP 13414-903, Brazil. Tel.: C55 19 3412 5340; fax: C55 19 3412 5223.

(2)

Introduction

Developments in resin chemistry and filler materials have led to the production of dental composites with improved physical and mechanical properties, which may be considered suitable for the restor-ation of posterior teeth[1,2]. Although the proper-ties of modern composite resins have been improved, polymerization shrinkage still remains a clinically significant problem[3].

The polymerization shrinkage of a resin compo-site can create contraction forces that may disrupt the bond to cavity walls [4]. This competition between the mechanical stress in polymerizing resin composites and the bonding of adhesive resins to the cavity walls is one of the main causes of marginal failure and subsequent microleakage observed with composite restorations [5]. Other-wise, if the interface remains intact, residual forces create stresses to surrounding tooth structure, and may result in overt tooth strain or fracture[6].

The magnitude of the curing stress has been found to be dependent on the degree of cure (DC), the ratio of the bonded to unbonded surface area of the specimen (C factor), and on the material composition [7]. Silikas et al. [8] observed that higher DC may lead to superior physical and mechanical properties. However, according to these authors, there is a linear relationship between the polymerization shrinkage strain and DC. Therefore, the maintenance of the integrity of the restoration margins without the loss of the ultimate physical and mechanical properties can be obtained through the relaxation of the stress through the composite flow[9,10].

Bonded walls are areas limiting shrinkage, while free walls are areas that allow the material to flow, relaxing the shrinkage stress[11]. The free surface will move under contraction, with lower stress onto the bonded surface. Cavities with a higher number of free surfaces, low C-factor, are favorable to the maintenance of the bond[12,13]. In cavities with a high C-factor, the increase of the free surfaces can be obtained by insertion of the composite in increments that increase the area of free surfaces

[14]. Yoshikawa et al. [13] demonstrated that incremental filling was unable to improve the bond strengths to a cavity floor of box-like cavities compared with a bulk filling method.

Furthermore, the viscoelastic nature of the polymerizing composite must also be considered. During polymerization there is an irreversible increase in the elastic modulus of the composite. The moment that it occurs the loss of the flowing capacity of the material is called gel-point. Prior to

the gel-point, polymerization contraction will not create stress at the restoration margins or within the material, as it is compensated by flow. However, rigid contraction after the gel-point has received much attention and has been responsible for the induction of stress in the bonding interface

[15,16]. Kinomoto et al.[17]demonstrated that the speed of the polymerization reaction has a great effect in the generation of stress. When the polymerization reaction proceeds more slowly, as in the case of self-curing composites, the gel-point is reached later, and more time is available for flow to compensate for polymerization contraction. In photocuring composites, the speed of the polym-erization reaction can be controlled by the light intensity at which the reaction is started. With lower intensities less photosensitizer molecules are actived, at the same time producing less radicals and conversion of double bonds[18].

Uno and Asmussem [19] suggested that pre-polymerization with low light intensity, followed by final polymerization at high intensity, could reduce the stress generated by shrinkage. Accord-ing to the authors this photoactivation mode (soft-start) results in slower contraction and more flow time to compensate for shrinkage strain while maintaining the degree of cure. Mehl et al. [20]

through the soft-start mode, obtained a better marginal adaptation than with the conventional technique, without the loss of the physical and mechanical properties of the composites.

To allow more time for composite flow, the mode of pulse-delay was proposed. According to this technique, polymerization is started with a short irradiation of 3 s at low intensity. A 3 min of delay is allowed for stress relaxation by flow, before curing is completed with a second irradiation at high intensity, increasing the degree of cure[7,21].

The aim of this study was to evaluate the effect of the composite photoactivation mode: (1) on the microtensile bond strength of the interface between restoration and dentin in the vestibular wall of class I cavities and (2) on the Knoop microhardness of composite. It was hypothesized that composite light-curing by soft-start and pulse-delay modes would increase the microtensile bond strength without affecting the hardness, when compared to CO mode.

Materials and methods

Freshly extracted sound third molars (stored in 0.05% thymol) were used in this study. The teeth had the apical third of their roots embedded in

(3)

polystyrene resin cylinders to facilitate handling. The occlusal enamel of each tooth was removed, using a slow-speed saw (Extec Corp., Enfield, CT, USA) and the exposed dentin was abraded with 320 and 600-grit silicon carbide (SiC) abrasive papers to obtain a flat dentin surface. Standardized uniform box-shaped class I cavities were prepared with a precision cavity preparation device. The prep-arations were outlined with coarse diamond burs (2143 (KG Sorensen, Barueri, SP, Brazil) operated with a high-speed hand-piece using copious air– water spray. The cavities had the following dimensions: mesio-distal width of 4 mm, bucco-lingual width of 3 mm and depth of 3 mm. A new bur was used for every five preparations. The C-factor of the cavity was approximately 4.5.

To conduct this study, a hybrid composite resin (Spectrum TPH, shade A 3.5) and two adhesive systems, a one-bottle and a self-etching primer, were used. Materials, manufacturers, composition and batch numbers are listed in Table 1. A commercial light-cure unit that allowed indepen-dent command over time and intensity (VIP, BISCO, Inc., Schaumburg, IL) was selected for this study. For composite photoactivation three irradiation modes were utilized. The conventional (CO) involves irradiation at 400 mW/cm2 for 40 s. The soft-start (SS) initially uses low intensity (100 mW/ cm2 for 10 s) followed by a final cure at high intensity (600 mW/cm2 for 30 s). The pulse-delay (PD) mode employs an initial low intensity exposure (100 mW/cm2 for 3 s) followed by 3 min waiting time and a final cure at high intensity (600 mW/cm2 for 37 s). The light exposure period was kept constant (40 s) for all photoactivation modes.

The teeth were randomly assigned to six experimental groups of 10 teeth each as follows:

Group 1. Single Bond (SB) adhesive system and CO irradiation mode.

Group 2. SB adhesive system and SS irradiation mode.

Group 3. SB adhesive system and PD irradiation mode.

Group 4. Clearfil SE Bond (CE) adhesive system and CO irradiation mode.

Group 5. CE adhesive system and SS irradiation mode.

Group 6. CE adhesive system and PD irradiation mode.

For SB groups, a 35% phosphoric acid gel (3M Scotchbond Etchant) was applied to the entire cavity for 15 s. The acid was rinsed off with water for 15 s and the excess water was removed with a small damp cotton pellet[22]. SB adhesive system was applied according to the manufacturer’s instructions to all cavity walls, which were checked for a shiny surface. The adhesive layer was thinned with a directed low-pressure air stream and light-cured for 20 s. For CE groups, the self-etching primer was applied to the cavities, left undisturbed for 20 s and evaporated with an air-syringe. The adhesive was then applied, spread gently with an air-syringe and light-cured for 10 s. Cavities were restored with Spectrum TPH hybrid composite resin in three oblique increments (less than 2 mm), which were photocured according to each experimental group.

After storage in water at 37 8C for 24 h, five specimens of each group were prepared for the microtensile bond test. Using a slow-speed saw, the specimens were serially sectioned in the buccallingual direction, parallel to the occlusal surface, to obtain two 0.9 mm slabs. Each slab was further sectioned into two 0.9!0.9 mm2 buccal

Table 1 Materials used.

Materials Composition Batch number Manufacturer

3M Scotchbond Etchant

35% Phosphoric acid, colloidal silica 1 WR 3M ESPE, St Paul, MN, USA

Single Bond HEMA, Bis-GMA, PAA, CQ, Ethanol and water 16 B 3M ESPE, St Paul, MN, USA

Clearfil SE Bond Primer: 10-MDP,HEMA, hydrophilic dimethacrylate, N,N-diethanol p-toluidine, water

00326 B0 Kuraray Co. Ltd, Osaka, Japan Bonding. 10-MDP,Bis-GMA, HEMA, hydrophobic

dimethacrylate, CQ, N,N-diethanol p-toluidine, silanated colloidal silica

0422 A

Spectrum TPH Urethane modified Bis-GMA, silanated Ba-Al-B-silicate glass, CQ, EDAB

10960/03 Dentsply/De

Trey, Konstanz, Germany

10-MDP, 10-methacryloyloxydecyl dihydrogen phosphate; HEMA, 2-hydroxyethyl methacrylate; PAA, polyalkenoic acid copolymer; Bis-GMA, bisphenol-glycidyl methacrylate; CQ, dl-canforquinone.

(4)

dentin-composite beams (Fig. 1). The beams were attached to the flat grips of a microtensile testing device with cyanoacrylate glue (Super Bonder gel, Loctite, Henkel, Brazil) and tested under tension in a Universal Testing Machine (Model 4411, Instron Corp., Canton, MA, USA) with a crosshead speed of 0.5 mm/min until failure. After testing, the speci-mens were carefully removed from the fixtures with a scalpel blade and the cross-sectional area at the fracture site was measured to the nearest 0.01 mm with a digital caliper (Starret 727-6/150, Starret, SP, Brazil) to calculate the ultimate tensile bond strength and express the results in MPa. Differences in microtensile bond strengths were evaluated for statistical significance using a two-way ANOVA (factors: adhesive system and photoactivation mode) and LSMeans at aZ0.05 significance level.

The five remaining specimens per group were stored dry for 24 h at 37 8C and prepared for microhardness measurements. The crown was sec-tioned parallel to the occlusal surface at the cervical region using a slow-speed saw, and the root was discarded. The specimen was then mesio-distally sectioned, parallel to the long axis of the tooth, resulting in two halves (Fig. 1), which were

embedded in polystyrene resin cylinders to facilitate handling. The included restorations were finished with a 1000-grit SiC paper under water and then polished with 6, 3, 1 and 0.5 mm diamond paste (Arotec Ind. Com., Sa˜o Paulo, Brazil) using a polishing cloth. Microhardness was measured by means of a Knoop indenter under 25 g load and 20 s dwell time (HMV-2000, Shimadzu, Japan). Indentations were made at nine positions on each specimen (Fig. 2) and the hardness readings were obtained in Knoop Hardness (KHN). The data obtained in the micro-hardness test were submitted to two-way ANOVA (factors: adhesive system, photoactivation mode and depth), and LSMeans at aZ0.05 significance level.

Results

Microtensile bond strength test

The results are shown in Table 2. The analysis showed that there was no significant effect for

A

B

Figure 1 Schematic representation of specimen preparation for microtensile (A) and Knoop microhardness (B) tests.

100 µm 1500 µm 2900 µm 100 µm 1500 µm 3900 µm

Figure 2 Schematic representation of sites evaluated for KHN.

Table 2 Results of microtensile bond strength test. Adhesive system SB 19.05(5.58)a CE 12.40(3.83)b Photoactivation mode SS 18.01(4.3)a PD 15.41(5.31)ab CO 13.76(7.14)b

Photoactivation mode!adhesive system

SS–SB 22.23(6.65)a PD–SB 18.89(4.56)ab CO–SB 16.02(3.80)abc SS–CE 13.78(4.51)bc PD–CE 11.93(3.26)c CO–CE 11.49(3.61)c

Means in MPa (SD). Same superscript letters indicate no statistical difference (P!0.05; LSMeans).

(5)

the double interaction: adhesive system!photo-activation mode (PZ0.5568). However, there was a statistically significant effect for the factors photoactivation mode (PZ0.0230) and adhesive system (P!0.0001).

The SB showed the highest mean microtensile strength with a significant statistical difference from the CE. The SS presented the highest mean microtensile strength with a significant statistical difference from the CO. The PD presented an intermediate value without a statistical difference from the other two modes. When the same adhesive system was used, there was no significant differ-ence between the photoactivation modes.

Microhardness measurements

The results are shown in Table 3. The analysis showed that there was no statistically significant effect for the triple interaction photoactivation mode!Adhesive!Depth (PZ0.0865), for the double interaction Photoactivation mode!Depth (PZ0.6044) and for the factor Depth (PZ0.8378). However, there was a statistically significant effect for the factors Photoactivation mode (P!0.0001), Adhesive (P!0.0001) and for the double interaction Photoactivation mode!Adhesive (P!0.0001).

PD presented the highest microhardness with a significant statistical difference from SS. CO presented an intermediate value and without a statistical difference to the other two modes. SB presented a higher microhardness than CE. How-ever, there was a significant statistical difference between the adhesives only for the SS mode.

Discussion

Polymerization shrinkage during photoactivation of a composite by CO is not uniform. It occurs very rapidly during the first 20 s and it slows down in the final 20 s[21]. Reduction of this initial speed can be obtained through the use of low intensity units that produce few free radicals [23]. This lower speed gives the composite more time for molecular rearrangement, reducing the stress caused by polymerization shrinkage[24]. The SS reduced the initial shrinkage, enabling the material to flow in this period. This resulted in a reduction of stress at the bond interface, demonstrated through the higher microtensile bond strength values obtained by this technique. These findings are in agreement with other studies that had obtained better sealing of the restoration margins and thus, less marginal microleakeage through composite photoactivation using the SS mode[25,26].

In PD, one seeks to reduce the polymerization reaction speed and, therefore, to relieve part of the stress generated by polymerization shrinkage. In this study, however, this effect was not obtained and the bond values for this technique did not differ from the ones achieved with CO. Yap et al. [21]

through the PD in which the initial activation was carried out for 3 s at an intensity of 100 mW/cm2, like in the one used in this study, did not detect any shrinkage of the composite during photoactivation and the waiting period. The authors believe that the energy density of the initial photoactivation (the intensity multiplied by the exposure period) was not enough to effectively initiate a polymerization reaction and all the shrinkage the composite underwent, was compensated for by the flow of the material. The reaction only became effective when the second high intensity exposure occurred. Thus, the reaction occurred under high intensity with a similar reaction speed to that of the CO.

In the microtensile tests a significant difference occurred among the bond systems used. SB presented superior bond strength values to the CE. SB is a one-bottle system, in which the adhesive and primer are in the same solution and are used after conditioning with phosphoric acid. The CE system is a self-etching primer that demineralizes and infiltrates the dentin simultaneously. Labora-tory bond strength tests frequently use SiC sandpa-per to standardize smear layer thickness [27]. However, clinically, cavities are prepared with steel or diamonds burs. Van Meerbeek et al. [28]

found no difference in the bond strength between a system with self-etching primer and another with prior acid conditioning when SiC sandpapers were

Table 3 Results of Knoop microhardness test. Adhesive system SB 86.48(5.42)a CE 79.33(6.29)b Photoactivation mode PD 85.60(4.89)a CO 81.67(5.67)ab SS 81.37(8.76)b

Photoactivation mode!adhesive system

SS–SB 88.90(3.90)a PD–SB 88.62(3.41)ab PD–CE 82.75(4.41)ab CO–SB 81.92(5.68)ab CO–CE 81.42(5.74)b SS–CE 73.85(4.75)c

Means in KHN (SD). Same superscript letters indicate no statistical difference (P!0.05; LSMeans).

(6)

used. However, when diamond burs were used, the self-etching primer system presented lower bond strength. On the other hand, Ogata et al.[29]found no difference in the bond strength between the two systems when diamond burs were used. In order to obtain a good adhesive bond to dentin, it is necessary for the dentin to be demineralized, to display the collagen fibers that will later be infiltrated by monomers [30]. During cavity prep-aration the formation of a smear layer occurs; it adheres weakly to the dentin structure and needs to be removed so that the adhesive joins with the dentin. Diamond burs at high speed induce fric-tional stress, which creates a thicker and coarser smear layer [29]. These smear layers cannot be completely removed by self-etching primers due to their weak acidity. Thus, demineralization of the underlying dentin and further penetration of the bonding resin into the demineralized dentin could have been insufficient for optimal bond strength

[31]. This could have contributed to the lower bond strength achieved by CE system.

In addition to polymerization dynamics, the degree of conversion also has an influence on the stress developed during polymerization shrinkage

[32]. A higher degree of conversion leads to more shrinkage and, consequently, to greater stress development [8]. The degree of conversion of a lightcuring composite is directly related to the energy density received during photoactivation. Koran et al.[33]found that the increase in density led to an increase in the hardness values until photoactivation with 17 J/cm2, after which there was no significant alteration in hardness values. In this study only the SS resulted in lower energy density (16 J/cm2). But as the increment of the composite resin used here was small, one expects that there were no statistical differences among the hardness values for the photoactivation techniques. However, PD presented the highest hardness values (85.69 KHN), which were significantly different from those of the SS mode (81.37 KHN). The CO presented intermediate hardness values (81.67 KHN), although this was without presenting statistically significant differences from the other two techniques.

Another important parameter to be emphasized is that, in addition to the photoactivation mode. The interaction with the adhesive system also has a significant influence on the hardness values. With SB system, the photoactivation modes presented hardness values compatible with energy density. The SS and PD had hardness values close to and higher than the conventional mode. However, there were no statistical differences among the three modes. The behavior of the hardness changed when the CE system was used. For this adhesive system SS

presented lower hardness values, differing signifi-cantly from the other two modes, but there was no difference between them. The influence of the adhesive on the hardness becomes evident when the same photoactivation mode is used, with only the adhesive system varying. For CO and PD the hardness values for SB and CE were similar, demonstrating that in these modes, the adhesive system did not have any influence on hardness. However, in the SS the adhesive system there was a great influence on the hardness values. SB system presented a mean hardness of 88.90 KHN with a statistically significant difference to the hardness that was presented for the CE-73.84 KHN.

DC and crosslink density are factors that influence final polymer properties. The SS mode employed a low intensity (100 mW/cm2) for 10 s followed by an increased intensity (600 mW/cm2) in the final 30 s [34]. According to Asmussen and Peustzfeldt [35], a pre-cure at low energy density will start polymerization by the formation of oligomers, building up foci of polymerized material in discontinuous microgel regions. The formation of the oligomer results in raising the viscosity of the composite. Thus, these partially polymerized chains have their mobility decreased and a second irradiation could not have been effective to ‘complete’ the polymerization reaction. However, at lower pre-cure energy density, like the PD mode, the formation of oligomer is limited and does not influence the final degree of cure. In addition, the SS mode can also interfere with crosslink density. Light-cure with low initial intensity is associated with few polymer growth centers, resulting in a more linear structure with relatively few crosslinks

[36]. These polymer structure differences and composite behavior during the polymerization reaction when light-curing was done by SS mode, could have enabled the adhesive system to inter-vene in the final properties of the material.

Loguercio et al. [37] demonstrated that the boundary conditions affect the polymerization shrinkage vectors as well as the percentage of linear polymerization. When the polymerizing composite is constrained by adhesion to cavity walls, their contraction is limited, which results in more stress developing. Choi et al.[38] showed a relationship between lower mechanical properties and composites polymerized in more highly con-strained cavities, presumably due to the presence of higher residual stresses. These changes could be seen by the reduction in composite hardness. Therefore, it is hypothesized that boundary con-ditions and the characteristics of the different adhesive systems would intervene in different ways on the composite strains during polymerization.

(7)

This results in polymers with distinct mechanical properties.

According to Ausiello et al. [39], the adhesive layer serves as a stress absorber during the polymerization shrinkage of the composite, by virtue of its low elasticity modulus that allows its increased deformation as the composite shrinks. This effect is dependent upon the composite establishing a strong bond to the adhesive layer

[39]. The results of this present study suggest that the SB system was more effective in attenuating the composite polymerization stress. This can be confirmed by the highest bond strength being reached by SB in the microtensile test. Thus, there was more residual stress when CB was used, causing a detrimental effect on the mechanical properties of the composite. However, within the limitations of this study, further studies are necessary to confirm these findings.

The hypothesis of this study was partially confirmed. The SS mode presented higher mean microtensile strength than the CO mode and the hardness means between these two modes were similar. But, the SS mode presented the lowest hardness when CB was used. However, for the PD mode, the microtensile bond strength as well as the hardness did not present a significant difference from the CO mode.

Acknowledgements

This study was supported by FAPESP (grant no. 03/04209-0).

References

[1] Hickel R, Manhart J, Garcia-Godoy F. Clinical results and new developments of direct posterior restorations. Am J Dent 2000;13 (Spec No):41D–554.

[2] Kugel G. Direct and indirect adhesive restorative materials: a review. Am J Dent 2000;13 (Spec No):35D–440.

[3] De Goes MF, Garcia-Godoy F, Cardenas L. Linear and volumetric polymerization shrinkage of restorative materials. J Dent Res 1999;78:484 [Abst. No 3025]. [4] Yap AU, Wang HB, Siow KS, Gan LM. Polymerization

shrinkage of visible-light-cured composites. Oper Dent 2000;25:98–103.

[5] Ferracane JL, Condon JR, Pham B, Mitchem JC. Relating composite contraction stress to leakage in classs V cavities. J Dent Res 1999;78:482 [Abst. No 3016].

[6] Meredith N, Setchell DJ. In vitro measurement of cuspal strain and displacement in composite restored teeth. J Dent 1997;25:331–7.

[7] Suh BI. Contolling and understanding the polymerization shrinkage-induced stress in light-cured composites. Com-pend Contin Educ 1999;20:S34–S41.

[8] Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000;16:292–6.

[9] Ferracane JL, Mitchem JC, Condon JR, Todd R. Wear and marginal breackdown of composites with various degrees of cure. J Dent Res 1997;76:1508–16.

[10] Watts DC. Elastic modului and visco-elastic relaxation. J Dent 1994;22:154–8.

[11] Davidson CL, De Gee AJ. Relaxation of polymerization contraction stresses by flow in dental composite. J Dent Res 1984;63:146–8.

[12] Feilzer AJ, De Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987;66:1636–9.

[13] Yoshikawa T, Burrow MF, Tagami J. A light curing method for improving marginal sealing and cavity wall adaptation of resin composite restorations. Dent Mater 2001;17:359–66. [14] Opdam NJ, Feilzer AJ, Roeters JJ, Smale I. Class I occlusal

composite resin restorations: in vivo post-operative sensi-tivity, wall-adaptation and microleakage. Am J Dent 1998; 11:229–34.

[15] Davidson CL, De Gee AJ, Feilzer A. The competition between the composite-dentin bond strenght and the polymerization contraction stress. J Dent Res 1984;63:1396–9.

[16] Sakaguchi RL, Berge HX. Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites. J Dent 1998;26:695–700. [17] Kinomoto Y, Torii M, Takeshige F, Ebisu S. Comparison of

polymerization contraction stresses between self-and light-curing composites. J Dent 1999;27:383–9.

[18] Feilzer AJ, Dooren LH, De Gee AJ, Davidson CL. Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface. Eur J Oral Sci 1995;103:322–6. [19] Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991; 99:440–4.

[20] Mehl A, Hickel R, Kunzelmann KH. Physical properties and gap formation of light-cured composites with and without ‘soft-start-polymerization’. J Dent 1997;25:321–30. [21] Yap AU, Soh MS, Siow KS. Post-gel shrinkage with pulse

activation and sof-start polymerization. Oper Dent 2002; 27:81–7.

[22] Pereira GDS, Paulillo L, De Goes MF, Dias CTS. How wet should dentin be? Comparison of methods to remove excess water during moist bonding J Adhes Dent 2001;3:257–64. [23] Asmussen E, Peutzfeldt A. Influence of pulse-delay curing

on softening of polymer S structures. J Dent Res 2001;80: 1570–3.

[24] Lim BS, Ferracane JL, Sakagushi RL, Condon JR. Reduction of polymerization contraction stress for dental composites by two-step light-activation. Dent Mater 2002;18:436–44. [25] Yoshikawa T, Sano H, Burrow MF, Tagami J, Pashley DH.

Effects of dentin depth and cavity configuration on bond strength. J Dent Res 1999;78:898–905.

[26] Barros GK, Aguiar FH, Santos AJ, Lovadino JR. Effect of different intensity light curing modes on microleakage of two resin composite restorations. Oper Dent 2003;28:642–6. [27] Pashley DH, Sano H, Ciucchi B, Yoshiyama M, Carvalho RM.

Adhesion testing of dentin bonding agents: a review. Dent Mater 1995;11:117–25.

[28] Van Meerbeek B, De Munck J, Mattar D, Van Landuyt K, Lambrechts P. Microtensile bond strengths of an etch&rinse and self-etch adhesive to enamel and dentin as a function of surface treatment. Oper Dent 2003;28:647–60.

[29] Ogata M, Harada N, Yamaguchi M, Tagami J. Effect of self-etching primer vs phosphoric acid etchant on bonding to bur-prepared dentin. Oper Dent 2002;27:447–54.

(8)

[30] Nakabayashi N, Pashley DH. Hibridization of dental hard tissues. Chicago: Quintessence; 1998.

[31] Ogata M, Harada N, Yamaguchi S, Nakajima M, Pereira PN, Tagami J. Effects of different burs on dentin bond strengths of self-etching primer bonding systems. Oper Dent 2001;26: 375–82.

[32] Sakaguchi RL, Wiltbank BD, Murchison CF. Contraction force rate of polymer composites is linearly correlated with irradiance. Dent Mater 2004;20:402–7.

[33] Koran P, Kurschner R. Effect of sequential versus continuous irradiation of a light-cured resin composite on shrinkage, viscosity, adhesion, and degree of polymerization. Am J Dent 1998;11:17–22.

[34] Lovell LG, Lu H, Elliott JE, Stansbury JW, Bowman CN. The effect of cure rate on the mechanical properties of dental resins. Dent Mater 2001;17:504–11.

[35] Asmussen E, Peutzfeldt A. Two-step curing: influence on conversion and softening of a dental polymer. Dent Mater 2003;19:466–70.

[36] Soh MS, Yap ADJ. Influence of curing modes on crosslink density in polymer structures. J Dent 2004; 32:321–6.

[37] Loguercio AD, Reis A, Schroeder M, Balducci I, Versluis A, Ballester RY. Polymerization shrinkage: effects of boundary conditions and filling technique of resin composite restor-ations. J Dent 2004;32:459–70.

[38] Choi KK, Ferracane JL, Ryu GJ, Choi SM, Lee MJ, Park SJ. Effects Of Cavity Configuration on Composite Restoration. Oper Dent 2004;29:462–9.

[39] Ausiello P, Apicella A, Davidson CL. Effect of adhesive layer properties on stress distribution in composite-a 3D finite element analysis. Dent Mater 2002;18:295–303.