Influence of Delivered Radiant Exposure Values on Bonding of Fiber Posts to Root Canals Anna Szesza / Johanna Cuadros-Sánchezb / Viviane Hassb / Gerson Kniphoff da Cruzc / Cesar A.G. Arraisd / Alessandra Reise / Alessandro Dourado Loguerciof

Purpose: To evaluate the effect of different radiant exposure values delivered to two simplified etch-and-rinse adhesive systems on push-out bond strength (PBS) of fiber posts to root canal, as well as nanoleakage (NL) and in situ degree of conversion (DC) within the hybrid layer. Materials and Methods: The roots of human premolars were endodontically prepared and divided into 6 groups according to the combination of the main factors adhesive/resin cement (2 commercial brands) and radiant exposure (4, 16, 48, and 288 J/cm2). The posts were cemented and the PBS tested at 0.5 mm/min (n = 7). The NL (n = 3) was evaluated using SEM after immersion of specimens in 50% silver nitrate. Micro-Raman spectroscopy was performed to determine the in situ DC (n = 2). Data were analyzed by three-way repeated measures ANOVA and Tukey’s post-hoc test (5%). Results: PBS (MPa) showed a significant difference only for the middle third when an increase in radiant exposure from 4 to 16 J/cm2 or higher was used (p < 0.05). The NL (%) decreased significantly with increasing radiant exposure from 48 to 288 J/cm2 in the middle and apical thirds when compared to lower radiant exposure (p < 0.05). The radiant exposure of 288 J/cm2 significantly increased the DC (%) in the middle and apical thirds, compared the other radiant exposure values (p < 0.05). Conclusion: The increase in radiant exposure delivered to the cervical third of root canals during post cementation improved the adhesive performance of simplified etch-and-rinse adhesive systems in the apical and middle thirds. Keywords: push-out bond strength, silver nitrate, degree of conversion, fiber posts, root dentin. J Adhes Dent 2015; 17: 181–188.. doi: 10.3290/j.jad.a34057

Submitted for publication: 09.10.14; accepted for publication: 31.03.15

T

he use of fiber posts cemented with simplified adhesive systems and resin cements has become clinicians’ preferred choice to restore endodontically treated teeth, as the use of such posts does not compromise esthetics like metal posts do. Some evidence indicates that their use reduces the incidence of root fracture in comparison to the use of metal

posts, 11,18,31,44 although this benefit has not been confirmed.28 However, a recent review of clinical studies showed that loss of retention due to poor polymerization of resin cements and bonding agents inside the root canal is the major failure mode of fiber posts luted in root canals.7,11 This is usually attributed to the limited transmission of curing light into post spaces at deeper

a

PhD Student, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Grossa, Paraná, Brazil. Idea, hypothesis, experimental design, performed the experiments, wrote the manuscript, contributed substantially to discussion.

e

Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Idea, hypothesis, experimental design, consulted on and performed statistical evaluation, contributed substantially to discussion.

b

PhD Student, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Performed the experiments, contributed substantially to discussion.

f

c

Adjunct Professor, Department of Physics, School of Physics, Ponta Grossa State University, Ponta Grossa, Brazil. Performed the experiments, contributed substantially to discussion.

Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Ponta Grossa State University, Ponta Grossa, Brazil. Experimental design, consulted on and performed statistical evaluation, wrote the manuscript, contributed substantially to discussion.

d

Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, State University of Ponta Grossa, Ponta Grossa, Paraná, Brazil. Performed the experiments, contributed substantially to discussion.

Vol 17, No 2, 2015

Correspondence: Professor Alessandro D. Loguercio, Department of Restorative Dentistry, Ponta Grossa State University, Rua Carlos Cavalcanti, 4748, Bloco M, Sala 64A, Uvaranas, Ponta Grossa, Paraná, Brazil 84030-900. Tel:+55-429-902-9903, Fax: +55-42-3220-3741. e-mail: [email protected]

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vs more superficial regions.30,34,43 As a consequence, even in the presence of translucent posts, the amount of light reaching the apical third of the post space might not be sufficient30,43 to effectively cure the cement at that level, leading to poor mechanical and bonding properties.30,34,43 For this reason, when self-adhesive resin cements are not the clinician’s choice, dual-curing resin cements in combination with dual-curing simplified adhesive systems are generally recommended to bond light-transmitting or translucent fiber posts to the root canal wall.9,30,39,43,49,54 Dual-curing adhesive materials were developed to provide the benefits of light-curing materials in the cervical area of a post space and those of chemically curing materials in the apical area.17,31 However, the actual bonding performance of dual-curing bonding agents in a post space does not appear better than that using light-curing adhesives.33 This is because proper bonding to the apical portion might be severely compromised by the adverse interactions between simplified adhesive and resin cement due to a lack of light exposure. Without light activation, dual-curing resin cements work exclusively as self-curing cements. In this mode, the cement takes longer to cure, and this allows more time for the adverse reaction and transudation of water from dentin to occur.37 Although purportedly able to polymerize even in the complete absence of light, dual-curing resins develop better mechanical properties when they are exposed to curing light.37,42 Therefore, exposure to curing light has been suggested even when dual-curing cements are used.9,12,30,58 Some simplified adhesives were found to be incompatible with chemically cured resin cements.46 Adverse chemical interaction between unpolymerized acidic adhesive resin monomers and the basic tertiary amine catalyst in the chemically cured composites was thought to be responsible for the observed incompatibility.35 The second problem is that simplified adhesive systems are highly hydrophilic and act as permeable membranes. As they lack solvent-free resin coatings,46 they behave as permeable membranes,56 permitting rapid water movement across the polymerized adhesive.55 This phenomenon of water permeation has also been observed in endodontically treated teeth.14 To overcome these problems and improve the performance of the adhesive system/resin cement interface,9,33,49 clinicians have been advised to use a high-intensity light-curing unit (LCU) and/or prolong the light exposure time of the adhesive system/resin cement, as these procedures may improve the adhesion of the adhesive/resin cement to root canal dentin,1,2,38 although the results are controversial. In other words, some studies confirmed that prolonged exposure to curing light improved the adhesion of adhesive/resin cement to root canal dentin, while others found no differences in bond strength given longer exposure to curing light.1,2,38 Most importantly, adequate polymerization of composite resins strongly correlates with the radiant exposure delivered to the restoration.20 The radiant exposure (J/ cm2) received by the resin is the mathematical product of radiant emittance (mW/cm2) and exposure time. Thus, after a 10-s exposure, a commercial curing light delivering 182

1000 mW/cm2 would deliver a radiant exposure of 10 J/ cm2. Resin manufacturers provide minimum curing times and irradiance levels and thus define the minimum radiant exposure requirements for their resins. Depending on the brand and shade, these have been reported to range from 6 to 24 J/cm2 for a 2-mm-thick increment of dental resin.36,50,53 No previous study has tried to establish a correlation between different radiant exposure and adhesive properties in the apical third. Thus, the aim of the present study was to compare the push-out bond strength (PBS), nanolekage (NL), and in situ degree of conversion (DC) of simplified etch-and-rinse adhesives/resin cements light activated with different radiant exposure values before fiber post cementation, and to correlate the different properties evaluated (PBS, NL, and DC) with the delivered radiant exposure. The null hypotheses were that 1. higher radiant exposure delivered to resin cement and the hybrid layer and 2. the use of a dual-curing adhesive system instead of a light-curing system results in increased DC and reduced nanoleakage within the hybrid layer; 3. there is no significant difference in bond strength among root thirds as a result of improved DC and nanoleakage pattern with increasing radiant exposure values.

MATERIAL AND METHODS This study was approved by the ethics committee of the local university (protocol # 270.010). Ninety-six extracted human mandibular premolars with a root length of 14 mm measured from the cementoenamel junction (CEJ) were used. The teeth were stored in distilled water at 4°C and used within 6 months after extraction. The teeth were sectioned transversely immediately below the CEJ using a low-speed diamond saw (Isomet 1000, Buehler; Lake Bluff, IL, USA). After endodontic access, the working length was determined by inserting a #10 Flexofile (Dentsply Maillefe; Ballaigues, Switzerland) into each canal until it was visible at the apical foramen and subtracting 1 mm from this length. The crown-down technique was used for instrumentation with Gates Glidden drills #2 to #4 (Dentsply Maillefer) with apical enlargement to size 40 and 06 taper. After every instrument change, the canal was alternately irrigated with 1 ml of 1% NaOCl and 17% EDTA solutions. The roots were dried with paper points (Dentsply Maillefer) and filled with AH Plus (Denstply DeTrey; Konstanz, Germany); tapered gutta-percha points were used with the vertical warm condensation technique. The root access was temporarily filled with a chemically polymerizing glass-ionomer cement (Maxxion R, FGM; Joinville, SC, Brazil) and the specimens were stored at 37°C in 100% humidity. After one week, the gutta-percha was removed using Gates Glidden burs, leaving 4 mm of the apical seal, and the post space was prepared with a low-speed bur provided by the post manufacturer (FGM) to a fixed depth of 10 mm from the CEJ. The root canals were irrigated with 10 ml of distilled water and dried with paper points. At The Journal of Adhesive Dentistry

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Table 1

Brand, composition, and batch number of the bonding agents and resin cements used

Product (manufacturer)

Composition (manufacturer’s information)

Batch number

Ambar* (FGM)

Adhesive resin: urethane dimethacrylate resin, HEMA, methacrylate acidic monomers, methacrylate hydrophilic monomers, silanated silicon dioxide, camphorquinone, ethyl 4-dimethylaminobenzoate, ethanol

121212

Excite DSC* (Ivoclar Vivadent)

Adhesive resin: alcohol, phosphonic acid acrylate, HEMA, SiO2, initiators and stabilizers, dimethacrylates Activator: aromatic sodium sulfinate salt (self-curing initiator)

M69564

Variolink II** (Ivoclar Vivadent)

Paste of dimethacrylates, inorganic fillers, ytterbium trifluoride, initiators, stabilizers and pigments; bis-GMA, triethylene glycol dimethacrylate, urethane dimethacrylate, benzoylperoxide

R50396

All Cem** (FGM)

Bis-GMA, triethylene glycol dimethacrylate, bis-EMA, benzoylperoxide

300113

HEMA: 2-hydroxyethyl methacrylate; bis-GMA: bisphenol A diglycidyl ether methacrylate; bis-EMA: ethoxylated bisphenol A glycol dimethacrylate. * Bonding agent; ** resin cement.

this point, the teeth were randomly divided into 8 groups (n = 12) formed by the combination of the main factors adhesive system/resin-cement (Ambar/All Cem [FGM] and Excite DSC /Varilonk II [Ivoclar Vivadent; Schaan, Liechnstein]) and radiant exposure (4, 16, 48, and 288 J/ cm2). The product compositions are displayed in Table 1. The root canal walls were etched with 35% phosphoric acid (Condac, FGM) for 15 s, rinsed with water, and then gently dried with paper points. Subsequently, adhesives were applied inside the root canals with microbrushes (Vigodent; Rio de Janeiro, RJ, Brazil) strictly according to the manufacturer’s instructions, removing adhesive excess with a paper point. The adhesive layer was exposed to light from a high radiant-emittance LED (Radii Cal, SDI; Baywater, Australia; emittance: 1200 mW/cm2, wavelength range: 440 nm to 480 nm; peak wavelength range: 460 nm) for 20 s. The resin cement of each manufacturer was inserted into the root space with a Centrix syringe (DFL; São Paulo, SP, Brazil), the posts (White Post DC 2, FGM) were inserted immediately and exposed to the curing light (Radii Cal, SDI) set to different intensities and exposure periods to result in the following radiant exposure values: 10  s × 400  mW/cm2 (4 J/cm2); 40 s × 400  mW/ cm2 (16 J/cm2); 40  s × 1200 mW/cm2 (48 J/cm2) and 120 s × 1200 mW/cm2 (288 J/cm2). For the purpose of changing radiant emittance, a device of black rubber base was made, so that ambient light was completely blocked, with a diameter equal to the tip of the curing light used. This device was attached to the light-curing–unit tip, which decreased the radiant emittance of 1200 mW/cm2 to 400 mW/cm2 over a distance of approximately 1.5 cm between the tip of the curing light and the root. After storage in water at 37°C for one week, the specimens were sectioned perpendicular to the long axis into six 1-mm serial slices under water cooling (Isomet 1000, Buehler). Both sides of each slice were photographed with an optical microscope (Olympus, model BX 51, Olympus, Tokyo, Japan) at 40X magnification to measure the coronal and apical diameters of the posts in order to calVol 17, No 2, 2015

culate the individual bonding areas (UTHSCSA ImageTool 3.0 software; University of Texas Health Science Center, San Antonio, TX, USA). The push-out bond strength test (PBS; n = 7 teeth) was performed in a universal testing machine (Instron; Canton, MA, USA) at 0.5 mm/min, and the maximum failure load was calculated in MPa.13,29 The failure mode was also evaluated by light microscopy and classified as adhesive fracture between dentin and cement, cohesive fracture in dentin or post, and mixed fractures.13,29 For nanoleakage evaluation (NL; n = 3 teeth), the slices were immersed in 50 wt% ammoniacal silver nitrate solution for 48 h and photodeveloped to allow deposition of silver ions as metallic silver grains within voids along the bonded interface.6,57 After polishing with up to 2500-grit SiC paper, each slab was cleaned ultrasonically, air dried, mounted on stubs, and sputter coated with gold (MED 010, Balzers Union; Balzers, Liechtenstein). The resin/ dentin interfaces were analyzed by scanning electron microscopy in back-scattered mode (SSX-550, Shimadzu; Tokyo, Japan). The relative percentage of NL at the bonded interface was measured in four regions (5 μm × 5 μm) of the bonded slab (medial, distal, vestibular, and lingual) as described elsewhere.15 The in situ degree of conversion (DC, n = 2 teeth) was measured with a micro-Raman spectrometer (Senterra, BrukerOptik; Ettlingen, Germany). After polishing and cleaning, each slice was placed on the microscope of the spectrometer. The micro-Raman spectrometer was calibrated to zero out the instrument, and then calibrated for the coefficient values using a silicon sample. The following micro-Raman parameters were used: 20 mW neon laser with 532 nm wavelength, spatial resolution of ca 3 μm, spectral resolution ca 5 cm-1, accumulation time of 30 s with 5 co-additions, and 100X magnification (Olympus Model BK 51; London, UK) to a beam diameter of approximately 1 μm.32 In each slice, the spectra were taken on four areas of the intertubular hybrid layer, as described for NL evaluation. For reference, the spectra of the unpolymerized adhesives were taken and the in situ DC 183

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4 J/cm2

16 J/cm2

48 J/cm2

b

a

AMBAR/ All Cem

c

d

CP

CP

CP

HL

HL

HL

D

D

D

e

g

f

EXCITE/ Variolink II

288 J/cm2

CP

CP

HL

HL

D

D

CP HL D

h CP HL

D

CP HL

D

Fig 1 Representative backscattered SEM images of the post/cement/adhesive/root interfaces bonded with Ambar/All Cem and Excite DSC/Variolink II. Silver nitrate uptake (arrows) occurred largely within the hybrid layer (HL) for all radiant exposure values used. When higher radiant exposure (c/d/g/h) was applied, the amount of silver nitrate uptake decreased mainly in the apical third (CP = resin cement/post interface; D = dentin).

was calculated according to Wu et al.59 Post-processing of spectra was performed using the dedicated OpusSpectroscopy Software version 6.5 (BrukerOptik). The ratio of double-bond content of monomer to polymer in the adhesive was calculated according to the following formula: DC(%) =

1–R(cured) R(uncured)

× 100

where “R” is the ratio of aliphatic and aromatic peak intensities at 1639 cm-1 and 1609 cm-1, respectively, in cured and uncured adhesives. In addition, the most intense peaks observed for all materials and the corresponding chemical bonding were recorded. The mean PBS, NL, and DC of all slices from the same tooth were averaged for statistical purposes. The data from each test were evaluated by three-way repeated measures ANOVA (adhesive vs radiant exposure vs root third) and Tukey’s test (α = 5%).

RESULTS Three-way repeated measures ANOVA detected statistically significant differences in all evaluated properties (Tables 2 to 4; p = 0.001). For push-out bond strength, both combinations of adhesive/cement promoted the lowest values in the apical third, regardless of the delivered radiant exposure (Table 2; p = 0.001). Despite the apparent increase in PBS values when the delivered radiant exposure ranged from 16 to 288 J/cm2, the differences in PBS were only significant in the middle third (Table 2; p = 0.01). No difference in PBS values between adhesive/cement was observed, independent of the type of adhesive system applied (only light cur184

ing or dual curing) (p > 0.05). Regarding the fracture pattern, only a few cohesive failures were observed in dentin, whereas the predominant pattern of failure was adhesive between dentin/cement, followed by mixed. No adhesive failure between the cement and the post was observed (Table 2). For nanoleakage evaluation, the apical third generally showed the highest NL, regardless of the adhesive system, mainly when 4 and 16 J/cm2 were delivered (Table 3 and Fig 1; p = 0.001). However, these differences in NL between thirds decreased or disappeared when higher radiant exposure (48 to 288 J/cm2) was applied (p = 0.001). The NL pattern was very similar for both adhesive systems, regardless of the type of adhesive system applied (Fig 1; p < 0.05). For the degree of conversion within the hybrid layer, the apical third showed the lowest DC, irrespective of the adhesive system, mainly applying 4 J/cm2 (Table 4; p = 0.001). The cervical third showed the highest and the middle third an intermediate DC (Table 4; p = 0.0001). When a higher radiant exposure was used, an increase in DC was observed in the middle and apical thirds of both adhesives (Table 4; p = 0.0001), which was statistically significant in most conditions. However, only the highest radiant exposure (288 J/cm2) promoted the highest DC values (p = 0.0001), which in turn were as high as some values observed in the cervical third.

DISCUSSION Overall, the delivery of higher radiant exposure increased DC and reduced NL within the hybrid layer, regardless of the adhesive system and resin cement used. However, this apparent relationship between The Journal of Adhesive Dentistry

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Table 2

Means and standard deviations of push-out bond strength, failure mode for the experimental groups

Experimental conditions*

Push-out bond strength (MPa)

Radiant exposure (J/cm2) Ambar/ All Cem

4

16

48

288

4

16

48

288

Cervical

12.3 ± 4.1A,B

14.4 ± 1.9A

13.7 ± 3.4A,B

15.1 ± 1.6A

10/3/1

7/5/2

5/8/1

8/6/0

Medium

7.1 ± 3.5C

9.6 ± 4.0B

12.4 ± 5.5A,B

14.2 ± 5.2A

9/4/1

9/4/1

10/1/3

9/3/2

5.2 ± 2.7C,D

4.2 ± 1.3D

6.8 ± 2.7C,D

8.1 ± 1.8B,C

12/2/0

7/6/1

7/5/2

6/5/3

3.7B

3.3A

3.8A,B

16.7 ±

3.0A

8/5/1

7/6/1

11/3/0

7/4/3

11.5 ±

4.7B

11/3/0

8/5/1

10/3/1

7/6/1

8.0 ± 2.8B,C

13/1/0

9/4/1

10/2/2

8/2/4

Apical Excite DSC/ Variolink II

Failure mode**

Cervical

11.3 ±

3.3B,C

Medium

8.1 ±

Apical

6.2 ± 2.3C,D

14.8 ± 8.4 ±

2.3B,C

5.3 ± 0.6C,D

13.5 ±

11.1 ±

1.7B

8.4 ± 2.2B,C

* Different superscript letters mean statistically significant difference (p < 0.05) when three-way repeated measures ANOVA (adhesive vs radiant exposure vs root third) and Tukey’s test (α = 5%) were used; ** Adhesive fracture between dentin and cement / cohesive fracture in dentin or post / mixed fracture.

Table 3 Means and standard deviations of nanoleakage for the experimental groups Experimental conditions * Radiant exposure (J/cm2) Ambar/All Cem

Excite DSC/ Varilonk II

Nanoleakage (%) 4

16

48

288

Cervical

9.6 ± 2.8A,B

10.2 ± 2.3A,B

7.9 ± 3.7A

7.2 ± 2.7A

Medium

16.4 ± 5.6C

12.1 ± 5.1B

11.3 ± 2.9B

7.0 ± 3.5A

Apical

38.1 ± 7.3E

26.4 ± 3.8D

12.9 ± 3.3B

9.7 ± 2.3A,B

Cervical

12.0 ± 3.8B

9.4 ± 2.7A

7.2 ± 2.5A

7.0 ± 2.2A

Medium

18.3 ± 4.5C

14.0 ± 4.5B,C

7.1 ± 3.4A

6.8 ± 2.1A

Apical

43.2 ± 8.3E

30.3 ± 6.8D

8.1 ± 4.1A

6.9 ± 3.1A

*Different superscript letters mean statistically significant difference (p < 0.05) when three-way repeated measures ANOVA (adhesive vs radiant exposure vs root third) and Tukey’s test (α = 5%) were used.

Table 4

Means and standard deviations of in situ degree of conversion for the experimental groups

Experimental conditions * Radiant exposure (J/cm2) Ambar/All Cem

Excite DSC/ Varilonk II

In situ degree of conversion 4

16

48

288

Cervical

63.5 ± 2.8 e

70.5 ± 4.3 c,d

88.3 ± 4.1 a

90.8±2.6 a

Medium

50.5 ± 2.2 g

61.3 ± 3.1 e,f

75.2 ± 3.6 b,c

77.2±2.8 b

Apical

43.2 ± 3.5 h

46.3 ± 3.8 h

50.2 ± 3.0 g

67.2±5.9 d,e

Cervical

62.6 ± 2.9 e

73.9 ± 6.3 c

73.5 ± 4.6 c

84.5±5.2 a,b

Medium

29.4 ± 3.3 i

56.0 ± 6.1 f,g

55.4 ± 5.1 f,g

75.9±4.8 b,c

Apical

15.1 ± 4.9 j

43.6 ± 5.4 h

48.5 ± 6.1 g,h

72.4±5.9 c

*Different superscript letters means statistically significant difference (p < 0.05) when three-way repeated measures ANOVA (adhesive vs radiant exposure vs root third) and Tukey’s test (α = 5%) were used.

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radiant exposure and DC was only noted at the apical and middle thirds. Thus, the first hypothesis regarding the DC values was partially accepted. As expected, the bonding agents in the cervical third received the greatest amount of radiant exposure, and therefore the highest DC values were observed in that region. As curing light penetrates the root canal and passes through the fiber post, a considerable reduction in irradiance is observed.19,30,34,43 Particularly for the specific post evaluated in this study, only approximately 8.37% and 19.06% of the total amount of light emitted by the LCU reached the resin cement in the apical and middle thirds, respectively.18 In other words, while 4, 16, 48, and 288 J/cm2 were delivered to the resin cement and bonding agent in the cervical third, the resin cement layer in the apical third received approximately 0.33, 1.34, 4.02, and 24 J/cm 2, respectively. This helps explain why only the delivery of 288 J/cm2 in the cervical third was able to promote DC values in the apical third as high as those observed in the top third. As a matter of fact, this also explains why only a slight and sometimes not significant increase in DC was observed in the middle and apical thirds, even when the radiant exposure delivered in the cervical third increased 4 or even 6 times. Higher radiant exposure delivered to the middle and apical thirds also resulted in lower NL within the hybrid layer, so the first hypothesis regarding the NL was accepted. This finding might be attributed to the increase in DC values observed when higher radiant exposure was delivered to those root thirds and confirms the relationship between DC and NL previously observed by other authors.10,32 Furthermore, based on the evidence that the bonding agents within the hybrid layer achieved only approximately 50% of their potential cure (Table 4) when low radiant exposure was delivered to the middle and apical thirds in comparison to the polymerization extent observed in the cervical third, it is reasonable to assume that previous exposure of resin adhesive layer to curing light resulted in very poorly polymerized bonding agent within the hybrid layer. This is in agreement with the results observed by other authors,3,25 who attributed such poor polymerization of the adhesive layer to the lower irradiance due to the limited light transmission through the root canal and the parallel light-exposure direction to the adhesive surface. As a result, higher radiant exposure delivered to the resin cement was crucial not only to increase the DC values of the resin cement layer but also to promote further polymerization within the hybrid layer in all root thirds. The increase in DC in the middle and apical thirds increases the packing and density of the polymer network within the hybrid layer and resin cement layer,22,45 which in turn becomes less prone to hydrolytic degradation.22,45 In other words, a better polymerized bonding agent and resin cement layer ensure more stable long-term bonding in those thirds when compared to poorly polymerized adhesive and cement layers. Although the post surface length bonded to the root canal is not related to root resistance to fracture,47,48 a greater length of the post surface bonded to the root walls increases post 186

resistance to dislodgment,7,8,11 which is the main reason for clinical failure of posts cemented to root canals.7,11 In an attempt to compensate for the reduced irradiance of light reaching the adhesive systems and resin cements in middle and apical thirds, a dual-curing adhesive system (Excite DSC) was used in this study. However, lower radiant exposure values delivered to middle and apical thirds reduced the DC values and increased the NL similarly for both adhesive systems. Indeed, the detrimental effects of the lowest radiant exposure (4 J/cm2) delivered to the apical third on the DC values were more pronounced in Excite DSC/Variolink II than in Ambar/Al Cem. Thus, the second hypothesis was rejected. Such low DC values observed within the hybrid layer in the apical third when that adhesive system was used might be attributed to lack of chemical initiators, such as benzoyl peroxide, although it is called a dual-curing adhesive system. Instead, such products contain an aromatic sodium sulfinate salt added to reduce the incompatibility between the acidic monomers in the adhesive resin and the self-curing component in the dual-curing resin cement.5,46 Based on this finding, it is reasonable to assume that the sodium sulfinate salts did not contribute to the polymerization initiated solely by the curing light, because the adhesive system (which contains sodium sulfinate) was cured prior to resin cement application. Despite the increase in DC values with increasing radiant exposure in middle and apical thirds, the PBS strength in those thirds did not improved as expected. Therefore, the third hypothesis was rejected. This finding is in agreement with other authors 1,21,34 and implies that other factors besides DC and NL may impair the PBS values in those regions. For instance, differences in regional dentin morphology, such as lower density of dentinal tubules in the apical third in comparison to that in the cervical third,16,27 more difficult access to different root thirds during bonding procedures,26 a greater amount of smear layer, residual root canal sealer and remaining gutta-percha usually found in the apical third in comparison to the other root thirds,1,27 can all impair the PBS values in those regions. Moreover, it is well known that a prepared root canal presents a high C-factor (close to 200),54 so the increase in the DC values of resin cement within the root canal as a consequence of higher radiant exposure delivered by the LCU may result in higher shrinkage stress generated during the resin cement polymerization.24,51 It should be mentioned, however, that these results were obtained when all exposures to curing light were performed right after the post was inserted, so there was no interval between post cementation and exposure to curing light. Therefore, the current results should not be extrapolated to the clinical situation where a delay occurs between post cementation and exposure to light, as such a delay may result in higher monomer conversion even when low radiant exposure values are delivered.40 In this in vitro study, only one dual-curing adhesive system was evaluated, so the results cannot be extrapolated to other products containing self-curing chemical activators such as benzoyl peroxide. In this regard, only conventional dual-curing resin cements were tested, so these results should not be expected when other types of resin The Journal of Adhesive Dentistry

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cements (eg, self-adhesive) are used. Due to their more complex curing mechanism and bonding to tooth substrate,23 it is possible that varying delivered radiant exposure may lead to different results from those observed in the current study. Besides, all bonding procedures were performed at room temperature, which is different from that observed in the teeth (ranging from 33°C to 35°C).52 Because of the increased temperature, higher monomer conversion values of the bonding agent and resin cement layers might be expected,4,41 as well as increased shrinkage stress as a consequence.51 However, only further studies can clarify the influence of root canal temperature on the bond strength of posts to dentin.

6.

7.

8.

9. 10.

11. 12.

CONCLUSION 13.

Within the limitations of this study, it is possible to conclude that: y Higher radiant exposure values delivered to the cervical third during post cementation in the root canal increased degree of conversion and reduced nanoleakage within the hybrid layer. y The use of a dual-curing adhesive system containing sodium sulfinate salts did not improve the evaluated properties when the bonding agent was cured prior to resin cement application. y Bond strength of adhesive systems to root canal increased with higher radiant exposure values, but the values were not as high as those observed at the cervical third.

14.

15.

16.

17.

18.

19.

ACKNOWLEDGMENTS This study was performed by Anna Szesz in partial fulfillment of her MS degree at the State University of Ponta Grossa (UEPG), Brazil. This study was partially supported by CAPES and the National Council for Scientific and Technological Development (CNPq) under grants 305075/2006-3 and 303933/2007-0. The authors are very grateful to FGM Dental Products, 3M ESPE (Brazil), and Dentsply (Brazil) for the generous donation of the adhesives and resin cements used in the present study, and Prof. Dr. Giovana Mongruel Gomes for assistance in the beginning of this study.

20.

21.

22. 23.

24.

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Clinical relevance: It is important to use high radiant exposure values (radiant emittance, eg, 1200 mW/ cm2 and prolonged polymerization time [120 s]) in clinical practice because it can influence the adhesive properties of root restorations with translucent posts, mainly in the apical third.

The Journal of Adhesive Dentistry