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Microtensile bond strength of resin cements to caries-affected dentin
Microtensile bond strength of resin cements to caries-affected dentin Thaís Y. U. Suzuki, DDS, MS,a André G. L. Godas, DDS,b Ana P. A. Guedes, DDS, MS,c Anderson Catelan, DDS, MS,d Sabrina Pavan, DDS, MS, PhD,e André L. F. Briso, DDS, MS, PhD,f and Paulo H. dos Santos, DDS, MS, PhDg Araçatuba School of Dentistry, Sao Paulo State University (UNESP), Araçatuba, Brazil; Piracicaba Dental School (UNICAMP), Piracicaba, Brazil; Adamantina School of Dentistry (FAI), Adamantina, Brazil, Brazil Statement of problem. The bonding of resin materials to caries-affected dentin, especially self-adhesive cements, remains a challenge in dentistry. Purpose. The purpose of this study was to evaluate the bond strength of different resin cements to sound or cariesaffected dentin at 24 hours and 6 months after the bonding procedure. Material and methods. Thirty-six human molars were used, 18 sound and 18 affected by caries. Indirect composite resin blocks (Tescera) were bonded to dentin by using 3 different resin cements: RelyX ARC, Panavia F, and RelyX Unicem. A universal testing machine was used to measure the microtensile bond strength 24 hours and 6 months after the bonding procedure. Representative specimens were analyzed with a scanning electron microscopy. The results were submitted to 3-way analysis of variance and the Fisher test (α=.05). Results. The highest values of microtensile bond strength were found with RelyX ARC for both tooth conditions (P<.001). There was no difference between RelyX Unicem and Panavia F (P>.05). There was no difference between caries-affected and sound dentin (P=.89). Conclusions. Caries did not influence the bonding strength of resin cements to dentin. (J Prosthet Dent 2013;110:47-55)
Clinical Implications
An improved understanding of the bonding process of resin cements to caries-affected dentin could enhance the durability of the cementation of indirect restorations. The search for improved bonding between indirect restorations and dental tissues has been the focus of
many studies.1,2 Acid-base dental cements, such as zinc phosphate and zinc polycarboxylate,3,4 have been
used for many years.5,6 The acid-base reaction is responsible for the set of these materials. However, these ce-
Supported by grant No. 2009/17472-7 from the Sao Paulo Research Foundation (FAPESP). Postgraduate student, Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry. Postgraduate student, Department of Restorative Dentistry, Araçatuba School of Dentistry. c Postgraduate student, Department of Restorative Dentistry, Araçatuba School of Dentistry. d Postgraduate student, Department of Restorative Dentistry, Piracicaba Dental School. e Assistant Professor, Adamantina School of Dentistry. f Associate Professor, Department of Restorative Dentistry, Araçatuba School of Dentistry. g Assistant Professor, Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry. a
b
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Volume 110 Issue 1 ments exhibit solubility in the oral cavity, with microleakage decreasing their mechanical properties.3,5 With the advent of composite resin for direct use and the improvement of their mechanical properties, resin cements were developed. These materials contain a monomeric system such as bisphenol-A glycidyl methacrylate (Bis-GMA) or urethane dimethacrylate (UDMA) in combination with other lower molecular weights such as triethylene glycol-dimethacrylate (TEGDMA),7 which provide low viscosity and fluidity to the material.8 These cements have many advantages over the water-based cements, such as improved bond strength, less microleakage, acceptable biocompatibility,9 lower solubility,9 higher mechanical properties, and satisfactory esthetics.10-13 The conventional resin luting technique depends on prior conditioning of the tooth structure and subsequent penetration of the adhesive system as well as the surface of the restoration.14-16 The efficiency of the adhesive system is dependent on its capacity to penetrate the demineralized dentin,17 forming an interdiffusion zone with the exposed collagen fibrils known as the hybrid layer, which is responsible for both retention and marginal seal.13,18-20 However, if the exposed collagen fibrils are not completely penetrated by the adhesive, the bond durability can be compromised by the hydrolysis of these fibrils, resulting in a process known as nanoleakage.21 This nanoporosity in the hybrid layer is responsible for failures in the bonding interface, which would allow solutions such as oral fluids, bacterial products, and proteolytic enzymes to penetrate these areas and therefore interfere with the success of the bonding.21-24 This degradation could result in the development of secondary caries, pulp inflammation, and dentin sensitivity.23,24 In order to overcome some of the limitations associated with the etchand-rinse technique, self-adhesive resin cements have been developed. This approach has reintroduced the
concept of incorporation of the smear layer as a bonding substrate, but with new formulations of monomers that penetrate the smear layer and underlying dentin.18 This resin cement requires no pre-acid etching of the dental surface; the application is performed in one step,16,25,26 similar to conventional cements.5 The bonding mechanisms of self-adhesive resin cements are based on acidic monomers that demineralize the smear layer and underlying dentin, resulting in micromechanical retention and infiltration of the adhesive monomer in the tooth substrate.27 Although the basic mechanism of adhesion is similar for all the selfadhesive cements, there is still little information about the application on modified substrates such as caries-affected dentin.28 In general, the adhesion to noncarious cervical sclerotic dentin would be prejudiced, since this surface has a hypermineralized superficial layer29,30 and sclerotic areas are acid resistant and filled with whitlockite crystals that obliterate the tubules,31,32 making it difficult to form resin tags.33,34 Although these modified substrates are different from normal dentin tissue, they are often considered as the main substrates for restorative procedures.35 In this manner, it would be clinically relevant to understand the bonding mechanisms of resin cements to caries-affected dentin. Therefore, the purpose of this study was to measure the microtensile bond strength of self-adhesive and conventional resin cements to sound or caries-affected dentin at 24 hours and 6 months after the bonding procedure. The null hypotheses tested were: (1) the type of dentin substrate would not affect the values of microtensile bond strength; (2) there would be no difference between the values of microtensile bond strength when comparing the periods of 24 hours and 6 months; and (3) there would be no difference in the values of microtensile bond strength between the conventional and self-adhesive resin cements.
The Journal of Prosthetic Dentistry
MATERIAL AND METHODS Thirty-six human molars, 18 sound and 18 caries-affected, were used in this study. No power analysis was performed to determine adequate sample size, which was based on previous studies that used the same methodology adopted in the present study.26,36 The project was approved by the Research and Ethics Committee of the Araçatuba School of Dentistry, Sao Paulo State University (protocol # 2614/09). The teeth were thawed, cleaned, and then kept frozen (-20°C) for 2 to 3 weeks before the start of the study. The occlusal surfaces of all teeth were ground flat with #180, 320, and 600 grit silicon carbide abrasive paper (Extec Corp, Enfield, Conn) under running water in an automatic polishing machine (APL-4; Arotec, Sao Paulo, Brazil) to remove enamel and expose a flat dentin surface (Fig. 1A). For the teeth affected by caries, a caries detector solution (Vide Carie; Inodon, Porto Alegre, Brazil) was applied on the dentin surface to identify the caries-infected tissue, which was removed during preparation of the specimens. The teeth were ground until the bright pigmentation of the caries-infected dentin was removed and the low, diffuse, softer color of the caries-affected dentin remained. The teeth were divided into 3 groups (n=6), according to the luting procedures described below. The teeth in the ARC Group were etched with 37% phosphoric acid (3M ESPE, St Paul, Minn) for 15 seconds, washed, and dried with paper towels without dehydrating the dentin. Adper Single Bond 2 bonding system (3M ESPE) was applied in 2 consecutive layers to the moist dentin by gently agitating the brush saturated with material on the surface of the dentin for 15 seconds. Then the teeth were gently air-dried for solvent evaporation and photopolymerized (Ultraled; Dabi Atlante, Ribeirão Preto, Brazil) for 20 seconds. Blocks of a prepolymerized laboratory resin-based
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July 2013 A
B
D
C
E
1 Specimen preparation. A, Occlusal surfaces of teeth were ground flat to expose dentin. B, Blocks of Tescera composite resin were luted. C, Teeth were sectioned in long axis. D, Half of teeth were stored for 6 months in distilled water at 37°C. E, Specimens were sectioned perpendicularly to adhesive interface to produce beams with adhesive area of approximately 1.0 mm2 for microtensile test. composite (Tescera; Bisco Inc, Schaumburg, Ill) measuring 11 mm in diameter and 4 mm thick were luted on the prepared dentin with RelyX ARC resin cement (3M ESPE). The excess cement was removed from the margins with disposable brushes and the cement was photopolymerized for 40 seconds each side. The blocks of Tescera were previously made by inserting the material in aluminum molds in an oxygen-free environment and photopolymerizing with a light source under water, pressure, and heat (Tescera ATL; Bisco Inc). They were then airborne particle abraded throughout with 50 μm aluminum oxide for 5 seconds (0.52 MPa air pressure; 10 mm from the tip), washed with distilled water, and dried with compressed air. To standardize the luting procedure, a load of 1 N was applied to the top of the resin cylinders after luting the composite resin to the dentin. The teeth in the PAN group were etched with a self-etching adhesive system (Ed Primer; Kuraray Medical Inc, Tokyo, Japan). One drop of Liquid A and one drop of Liquid B were mixed
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and actively applied for 3 minutes over the dentin surface with a brush tip and left in contact with the dentin for 60 seconds. Then, using a sponge or paper point, the excess primer was eliminated and the surface dried thoroughly with a gentle air stream. Blocks of Tescera composite resin were luted (Panavia F; Kuraray Medical Inc). Disposable brushes were used to remove excess cement from the margins and the cement was photopolymerized for 40 seconds on each side. In the UNI group, the Tescera resin blocks were luted directly to dentin with RelyX Unicem self-adhesive resin cement (3M ESPE). The excess cement was removed from the margins with disposable brushes and the cement was photopolymerized for 40 seconds on each side. No previous treatment was done to the dentin before the resin cement application. After the bonding process (Fig. 1B), the teeth were stored in distilled water37,38 at 37°C for 24 hours. After this period, the teeth were sectioned perpendicular to the adhesive-tooth interface (Fig. 1C) with a low speed
diamond saw (Isomet 2000; Buehler Ltd, Lake Bluff, Ill) under water cooling. One half of the specimens were used to evaluate the microtensile bond strength 24 hours after the bonding process, while the other half were stored for 6 months in distilled water at 37°C (Fig. 1D), changed weekly. For the microtensile test,39-41 the bonded specimens were sectioned perpendicular to the adhesive interface with the low-speed diamond saw (Isomet 2000, Buehler) under water cooling to produce beams with an adhesive area of approximately 1.0 mm2 (Fig. 1E). The ends of the stick-like specimens were fixed with a cyanoacrylate adhesive (Super Bonder gel; Henkel Corp, Rocky Hill, Conn) to a testing apparatus and subjected individually to microtensile testing in a universal testing machine (EMIC model DL3000; São José dos Pinhais, Paraná, Brazil) at a crosshead speed of 0.5 mm/min to evaluate the microtensile bond strength (MPa). The results were analyzed by 3-way analysis of variance (ANOVA)-dentin condition (sound or caries-affected), resin cement (RelyX
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Table I. Values of microtensile bond strength (MPa) of resin cements to sound and caries-affected dentin RelyX Unicem
RelyX ARC
Panavia F
Sound dentin
17.22 ±6.45ab
22.56 ±9.44 a
9.97 ±5.08b
Caries-affected dentin
14.78 ±8.43a
25.71 ±11.72 a
16.90 ±6.95a*
Sound dentin
14.56 ±3.83ab
21.73 ±9.08 a
11.41 ±2.66b
Caries-affected dentin
13.86 ±3.20
22.40 ±14.70 a
6.83 ±2.80b*
24 hours 6 months
ab
Different superscripted lowercase letters in row are significantly different (P<.05). *Statistically significant difference between 24 hours and 6 months.
Table II. Analysis of variance for microtensile bond df
Sum of Squares
Mean Square
F
P
Lambda
Power
Cement
2
1602
801
13.55
<.001
27.092
.999
Type of dentin
1
4.24
4.24
0.07
.790
0.072
.058
Time
1
125.14
125.14
2.12
.151
2.116
.282
Cement × Dentin
2
37.67
18.83
0.32
.727
0.637
.097
Group × Time
2
22.25
11.12
0.19
.829
0.376
.077
Tooth × Time
1
70.36
70.36
1.19
.280
1.190
.178
1.14
.328
2.272
.232
Group × Tooth × Time
2
134.34
67.17
Residual
56
3311.23
59.13
ARC, RelyX Unicem, or Panavia F), and assessment time (24 hours or 6 months)-and the protected least significant difference (PLSD) Fisher test (α=.05). The fractured beams of debonded specimens were analyzed with a stereomicroscope at magnifications of ×6 and ×66 for analysis of the failure mode. Failure modes were classified into 4 groups: (1) adhesive failure at the hybrid layer; (2) cohesive failure in dentin; (3) cohesive failure in adhesive resin; and (4) mixed failure. Representative specimens of experimental groups were fixed and coated with gold (BAL-TEC SCD 050; Balzers AG, Balzers, Liechtenstein) and observed under scanning electron microscopy (SEM JSM5600LV; JEOL, Tokyo, Japan) to illustrate the patterns of fracture.
RESULTS The highest microtensile bond strength values were found for RelyX
ARC in all experimental conditions (Table I). However, there was no significant difference between the RelyX ARC and RelyX Unicem (P>.05). Panavia F showed the lowest microtensile bond strength at 24 hours for sound dentin (9.97 ±8.5 MPa), which was significantly lower than that of RelyX ARC (22.56 ±9.44 MPa) (P=.009). After 6 months, Panavia F showed lower microtensile bond strength compared to RelyX ARC for both sound (11.41 ±2.66 MPa and 21.73 ±9.08 MPa, respectively) and caries-affected dentin (6.83 ±2.80 MPa and 22.40 ±14.70 MPa, respectively) (P=.009). RelyX Unicem showed intermediate values of microtensile bond strength in all experimental conditions, with no significant difference for the other cements. In the comparison between 24 hours and 6 months of evaluation in caries-affected dentin, the values of microtensile bond strength for Panavia F after 24 hours were significantly higher (16.90 ±6.95 MPa) than the
The Journal of Prosthetic Dentistry
values obtained after 6 months (6.83 ±2.80 MPa) (P=.008). For the other cements, there was no difference in bond strengths between the periods of evaluation. The condition of the dentin (normal or caries-affected) presented a statistically significant influence only for Panavia F when analyzed 6 months after bonding to dentin. The sound dentin showed higher values of microtensile bond strength for Panavia F after 6 months of evaluation (11.41 ±2.66 MPa) compared to caries-affected dentin (6.83 ±2.80 MPa) (P=.016). In the other experimental conditions, no significant difference between the 2 substrates was found. The results of analysis of variance (Table II) showed that among the 3 factors studied, there was a statistically significant difference for the resin cement factor (P<.001). For the other factors, as well as the interaction between them, the difference was not statistically significant.
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July 2013 100% 90%
Mixed failure
80%
Adhesive resin failure
70%
Dentin failure
60%
Hybrid layer failure
50% 40% 30% 20% 10% 0%
RelyX ARC 24 hours
RelyX ARC 6 months
Panavia F 24 hours
Panavia F 6 months
RelyX Unicem RelyX Unicem 24 hours 6 months
2 Incidence (%) of failure patterns of sound dentin. 100% 90%
Mixed failure
80%
Adhesive resin failure
70%
Dentin failure
60%
Hybrid layer failure
50% 40% 30% 20% 10% 0%
RelyX ARC 24 hours
RelyX ARC 6 months
Panavia F 24 hours
Panavia F 6 months
RelyX Unicem RelyX Unicem 24 hours 6 months
3 Incidence (%) of failure patterns of caries-affected dentin. There was a predominance of adhesive failure, independent of the substrate or the period of time analyzed. Only RelyX ARC showed some variation in the fracture mode, with a significant number of dentin failures and mixed failures, in addition to adhesive failures (Figs. 2, 3).
DISCUSSION Based on the results of this study, only the third null hypothesis (type of resin cement) was rejected. In this study, 3 independent variables were studied: the type of substrate (sound and caries-affected dentin), the type of resin cement used (RelyX ARC, RelyX Unicem, and Panavia F), and the storage time of specimens (24 hours and 6 months). Regarding the type of substrate, for most experimental groups no difference was found be-
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tween sound or caries-affected dentin, results which are similar to previous studies.28,30 The only exception was for Panavia F bonded to caries-affected dentin after 24 hours, which showed higher microtensile bond strength, leading to the rejection of the first null hypothesis. Probably, the specimens classified as caries-affected dentin, may have contained variable amounts of normal dentin. This phenomenon could be responsible for more variation in the results when the bonding was done in the caries-affected dentin. Some authors have reported differences in the microtensile bond strength between sound and caries-affected dentin.29,30,32 Considering the limitation of the methodology employed in the present study, it is difficult to measure the degree of mineralization of the caries-affected dentin and ensure a complete uniformity of the substrate,
which could explain the lack of significant difference in microtensile bond strengths between the sound or cariesaffected dentin. According to Marshall et al,42 the intertubular dentin of cariesaffected dentin showed reduced or unchanged nanomechanical properties as compared with normal intertubular dentin. The mechanical properties of intertubular dentin are the major determinants of dentin characteristics as a whole.43 Due to increased porosity in the intertubular dentin, a thicker hybrid layer would be expected in the caries-affected dentin, facilitating the diffusion of acidic conditioners and adhesive monomers.44 However, the lack of resin tag formation due to the presence of acid-resistant intratubular mineral deposits, associated with a decrease in the cohesive strength of dentin in caries-affected tissue could interfere with the bond strength values.44
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4 Representative specimen of RelyX ARC after 6 months bonded to sound dentin. There are areas of exposure of dentin tubules in bottom of hybrid layer (A), while in other areas there appeared to be evidence of adhesive covering dentin in middle of hybrid layer (B) (original magnification ×500). The second null hypothesis was rejected because there was no significant difference between the values obtained at 24 hours and 6 months, except for Panavia F when bonded to caries-affected dentin. This could be explained by the storage method used in this study. The specimens were sectioned after the bonding process, and half of the tooth was used for evaluation after 24 hours and the other half for evaluation after 6 months. Thus, for the evaluation after 6 months, the beams were obtained immediately before the mechanical test. In some previous studies, the bonded teeth were reduced to small beams before the aging process39,41 to accelerate aging. Goodis et al37 and Da Fonte Porto Carreiro et al38 reported that storage in distilled water showed a lower variation in bond strength values compared to other methods of aging. However, Tezvergil-Mutluay et al45 reported that aging media should contain calcium and zinc ions to permit the endogenous proteases in dentin to function. The authors concluded that incubation in water underestimates degradation of bond strengths. In this study, RelyX ARC resin cement showed higher values of microtensile bond strength when compared to Panavia F and RelyX Unicem,
5 Representative specimen of RelyX ARC after 6 months bonded to caries-affected dentin. There are regions where there appears to be adhesive within dentinal tubules in bottom of hybrid layer (A), and hybrid layer was retained and adhesive separated from surface of hybrid layer (B) (original magnification ×500).
rejecting the third null hypothesis which confirms previous studies.12,26 This result could be explained by the bonding mechanisms of etch-andrinse adhesive such as the Adper Single Bond adhesive system that was used with RelyX ARC. The 37% phosphoric acid removes the smear layer, demineralizes the dentin surface, and opens the dentinal tubules. This exposes the collagen fibers, creating an adequate substrate for penetration of the hydrophilic monomer of the adhesive system that involves the collagen network, establishing micromechanical retention.14 According to Braga et al,15 RelyX ARC dual cement associated with Adper Single Bond 2 adhesive system with the wet bonding technique creates a thick hybrid layer. This thicker hybrid layer could explain the higher values of microtensile bond strength when compared to the other resin cements. Higher variations were found in the type of failure with the presence of many mixed and cohesive failures in dentin combined with the higher values of microtensile bond strength. Figure 4 represents a type of mixed failure in a representative specimen of sound dentin luted with RelyX ARC, which reveals that the adhesive occupies much of the dentin surface. In the area labeled A, dentin seems to be exposed in some areas within the
The Journal of Prosthetic Dentistry
bond perimeter, as evidenced by the presence of open dentinal tubules in the bottom of the hybrid layer. In the area labeled B, in the middle of the hybrid layer, the resin tags were probably more retentive, forcing the advancing crack to shift to the adhesive that infiltrated the hybrid layer. Even in area B, there were some open tubules where resin tags had been pulled out. There were adjacent areas where the resin tags sheared off, leaving residual tags in the tubules. The same parameter can be observed in the specimens of caries-affected dentin also luted with RelyX ARC (Fig. 5). By 6 months, the stress concentrations shifted due to water sorption plasticizing the resins. At 6 months, most of the failures in A occurred at the bottom of the hybrid layer. About half the tubules pulled out with the adhesive/composite resin, and about half fractured but remained in the dentin. In area B, the hybrid layer was retained and the adhesive separated from the surface of the hybrid layer (Fig. 5). The results showed no significant difference between RelyX Unicem and Panavia F. The manufacturer of Panavia F recommends treating the substrate with a self-etching adhesive system (ED Primer) This system is thought to eliminate the need for prior acid etching, as well as wash-
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July 2013
6 Representative specimen of Panavia F after 6 months of bonding to sound dentin. Smear layer was observed on almost all surfaces, with few dentinal tubules exposed. There was no evidence of resin tags (original magnification ×500).
7 Representative specimen of Panavia F after 6 months of bonding to caries-affected dentin. Similar to Fig. 6, dentin surface was covered by smear layer, and in some areas there appeared to be little evidence of coverage of resin cement on smear layer (*) (original magnification ×500).
8 Representative specimen of the RelyX Unicem after 24 hours of bonding to sound dentin. In this case, smear layer seems to be covering dentin surface where there was no evidence of exposure of dentinal tubules (original magnification ×500).
9 Representative specimen of the RelyX Unicem after 24 hours of bonding to caries-affected dentin. Similar to sound substrate, in previous figure, dentin surface appeared to be completely covered by smear layer, not exposing dentinal tubules (original magnification ×500).
ing and drying of the substrate. The acidic primer is able to demineralize the dentin by modifying the smear layer without removing it completely, maintaining the smear plug, while promoting the infiltration of adhesive monomers and the formation of the hybrid layer.17 According to Hikita et al25 and Monticelli et al17 there was a similarity in the formulations of the primer of the self-etch adhesive of Panavia F and self-adhesive resin cements. Both products contain multifunctional methacrylate phosphate, which the manufacturer claims reacts with the hydroxyapatite of tooth tis-
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sue, which could explain the similarity in the results. In the Panavia F groups, the formation of a minimum hybrid layer was observed, which could compromise a good bond strength when evaluated immediately after the bonding process.17,18 Through scanning electron microscopy, a resin-infiltrated smear layer coverage on the dentin surface can be observed with minimal exposure of dentinal tubules (Fig. 6). The smear layer is not removed before etching and there is no evidence of resin tags. Probably the adhesives adhere to the surface of the hybrid layer, which then fails cohesively. The
same pattern was noted for Panavia F bonded to caries-affected dentin, in which small islands of residual luting material can be observed (Fig. 7). In self-adhesive cements, the initial acidity of these materials has a short-term effect, with a rapid increase in the pH resulting in a single interaction with the superficial dentin without formation of resin tags.16,25,27 According to Pavan et al,9 the acidity present in this cement is not enough to demineralize the dentin and expose the dentinal tubules. This could explain the lowest bond strength found in this study. Figure 8 presents
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Volume 110 Issue 1 a sound dentin surface almost completely covered by the smear layer, the same as a representative specimen of caries-affected dentin (Fig. 9). Although this study used a disclosing dye (Vide Carie; Inodon) for the exact location of the caries-affected dentin, there are some limiting factors that must be considered, such as the exact location of the caries-affected dentin when sectioning the teeth to obtain the beams for the microtensile test. Also, the technique of flattening the dentin surface by grinding the crowns tends to remove excessive caries-affected dentin. In this situation, additional unaffected dentin could be exposed compared with the clinical situation. In this study, teeth were stored in distilled water at 37°C, not aged with thermal or pH cycling. Another potential limitation of the study is the lack of power analysis to determine adequate sample size, especially for the interactions among the variables (Table II). Additional studies are needed to complement the discussion about the luting procedure in sound and caries-affected dentin with new formulations of resin cements.
CONCLUSIONS The results of this study showed the mean bond strengths of selfetching cements to caries-affected dentin were not different to the values to sound dentin. No difference was found between 24 hours and 6 months after bonding, except for Panavia F, which showed higher values of bond strength after 24 hours compared with 6 months. Furthermore, in the comparison among the resin cements, RelyX ARC, which depends on a total-etch adhesive system to bond to the dentin, obtained the highest values of microtensile bond strength.
REFERENCES 1. Blatz MB, Dent M, Sadan A, Kern A. Resinceramic bonding: a review of the literature. J Prosthet Dent 2003;89:268-74. 2. Lührs AK, Guhr S, Günay H, Geurtsen W. Shear bond strength of self-adhesive resins compared to resin cements with etch and rinse adhesives to enamel and dentin in vitro. Clin Oral Investig 2010;14:193-9. 3. Knibbs PJ, Walls AW. A laboratory and clinical evaluation of three dental luting cements. J Oral Rehab 1989;16:467-73. 4. Sen D, Poyrazoglu E, Tuncelli B. The retentive effects of pre-fabricated posts by luting cements. J Oral Rehab 2004;31:585-9. 5. Diaz-Arnold AM, Vargas MA, Haselton DR. Current status of luting agents for fixed prosthodontics. J Prosthet Dent 1999;81:135-41. 6. Nicholson JW, Czarnecka B. The kinetics of water loss from zinc phosphate and zinc polycarboxylate dental cements. J Mater Sci Mater Med 2008;19:1719-22. 7. Borges MA, Matos IC, Mendes LC, Gomes AS, Miranda MS. Degradation of polymeric restorative materials subjected to a high caries challenge. Dent Mater 2011;27:244-52. 8. Itota T, Nakatsuka T, Tanaka K, Tashiro Y, McCabe JF, Yoshiyama M. Neutralizing effect by resin-based materials containing silane-coated glass fillers. Dent Mater J 2010;29:362-8. 9. Pavan S, dos Santos PH, Berger S, BedranRusso AK. The effect of dentin pretreatment on the microtensile bond strength of self-adhesive resin cements. J Prosthet Dent 2010;104:258-64. 10.Cheung GSP. Reducing marginal leakage of posterior composite resin restorations: a review of clinical techniques. J Prosthet Dent 1990;63:286-8. 11.Sanares AME, Itthagarun A, King NM, Tay FR, Pashley DH. Adverse surface interactions between one-bottle light-cured adhesives and chemical-cured composites. Dent Mater 2001;17:542-56. 12.Mak YF, Lai SCN, Cheung GS, Chan AW, Tay FR, Pashley DH. Micro-tensile bond testing of resin cements to dentin and an indirect resin composite. Dent Mater 2002;18:609-21. 13.Yang B, Ludwig K, Adelung R, Kern M. Micro-tensile bond strength of three luting resins to human regional dentin. Dent Mater 2006;22:45-56. 14.Van Meerbeek B, Peumans M, Verschueren M, Gladys S, Braem M, Lambrechts P, et al. Clinical status of ten dentin adhesive systems. J Dent Res 1994;73:1690-702. 15.Braga RR, Bellester RY, Daronch M. Influence of time and adhesive system on the extrusion shear strength between feldspathic porcelain and bovine dentin. Dent Mater 2000;16:303-10. 16.Aguiar TR, Di Francescantonio M, Ambrosano GM, Giannini M. Effect of curing mode on Bond sthength of self-adhesive resin luting cements to dentin. J Biomed Mater Res B Appl Biomater 2010;93:122-7. 17.Monticelli F, Osorio R, Mazzitelli C, Ferrari M, Toledano M. Limited descalcification/ diffusion of self-adhesive cements into dentin. J Dent Res 2008;87:974-9.
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18.Reis A, Grandi V, Carlotto L, Bertoli G, Patzlaff R, Rodrigues Accorinte Mde L, et al. Effect of smear layer thickness and acidity of self-etching solutions on early and long-term bond strength to dentin. J Dent 2005;33:549-59. 19.Nikaido T, Inoue G, Takagaki T, Waidyasekera K, Iida Y, Shinohara MS, et al. New strategy to create “Super Dentin” using adhesive technology: Reinforcement of adhesive-dentin interface and protection of tooth structures. Japan Dent Sci Rev 2010;47:31-42. 20.Vaidyanathan TK, Vaidyanathan J. Recent advances in the theory and mechanism of adhesive resin bonding to dentin: a critical Review. J Biomed Mater Res B Appl Biomater 2009;88:558-78. 21.Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Carvalho R, et al. Relationship between surface área for adhesion and tensile Bond strength--evaluation of a micro-tensile bond test. Dent Mater 1994;10:236-40. 22.Burrow M, Satoh M, Tagami J. Dentin bond durability after three years using a dentin bonding agent with and without priming. Dent Mater 1996;12:302-7. 23.Mjör I, Toffenetti F. Secondary caries: A literature review with case reports. Quintessence Int 2000;31:165-79. 24.Pashley DH, Pashley EL, Carvalho RM, Tay FR. The effects of dentin permeability on restorative dentistry. Dent Clin North Am 2002;46:211-45. 25.Hikita K, Van Meerbeek B, De Munck J, Ikeda T, Van Landuyt K, Maida T, et al. Bonding effectiveness of adhesive luting agents to enamel and dentin. Dent Mater 2007;23:71-80. 26.Viotti RG, Kasak A, Pena CE, Alexandre RS, Arrais CA, Reis AF. Microtensile bond strength of new self-adhesive luting agents and conventional multistep systems. J Prosthet Dent 2009;102:306-12. 27.De Munck J, Vargas M, Landuyt KV, Hikita K, Lambrechts P, Van Meerbek B. Bonding of an auto-adhesive luting material to enamel and dentin. Dent Mater 2004;20:963-71. 28.Sonoda H, Banerjee A, Sherriff M, Tagami J, Watson TF. An in vitro investigation of microtensile bond strengths of two dentine adhesives to caries-affected dentin. J Dent 2005;33:335-42. 29.Nakajima M, Kitasako Y, Okuda M, Foxton RM, Tagami J. Elemental distributions and microtensile bond strength of the adhesive interface to normal and caries-affected dentin. J Biomed Mater Res B Appl Biomater 2005;72:268-75. 30.Pereira PNR, Nunes MF, Miguez PA, Swift EJ Jr. Bond strengths of a 1-step self-etching system to caries-affected and normal dentin. Oper Dent 2006;31:677-81. 31.Tay FR, Kwong SM, Itthagarun A, King NM, Yip HK, Moulding KM, et al. Bonding of a self-etching primer to non-carious cervical scleroticdentin: interfacial ultra structure and microtensile bond strength evaluation. J Adhes Dent 2000;2:9-28. 32.Tay F, Pashley DH. Resin bond to cervical sclerotic dentin: a review. J Dent 2004;32:173-96.
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July 2013 33.Kwong SM, Cheung GS, Kei LH, Itthagarun A, Smales RJ, Tay FR, et al. Micro-tensile bond strengths to sclerotic dentin using a self-etching and a total-etching technique. Dent Mater 2002;18:359-69. 34.Ceballos L, Camejo DG, Fuentes MV, Osorio R, Toledano M, Carvalho RM, et al. Microtensile bond strength of total-etch and self-etching adhesives to caries-affected dentin. J Dent 2003;31:469-77. 35.Marshall GW, Marshall SJ, Kinney JH, Balooch M. The dentin substrate: structure and properties related to bonding. J Dent 1997;6:441-58. 36.Sheikh H, Heymann HO, Swift EJ Jr, Siemiecki TL, Ritter AV. Effect of saliva contamination and cleansing solutions on the bond strengths of self-etch adhesives to dentin. J Esthet Restor Dent 2010;22:402-10. 37.Goodis HE, Marshall Jr GW, White JM, Gee L, Hornberger B, Marshall SJ. Storage effects on dentin permeability and shear bond strengths. Dent Mater 1993;9:79-84. 38.Da Fonte Porto Carreiro A, Dos Santos Cruz CA, Vergani CE. Hardness and compressive strength of indirect composite resins: effects of immersion in distilled water. J Oral Rehabil 2004;31:1085-9.
39.Mendonça JS, Souza MH Jr, Carvalho RM. Effect of storage time on microtensile strength of polyacid-modified resin composites. Dent Mater 2003;19:308-12 40.Scherrer SS, Cesar PF, Swain MV. Direct comparison of the bond strength results of the different test methods: a critical literature review. Dent Mater 2010;26:e78-e93. 41.Skovron L, Kogeo D, Gordillo LA, Meier MM, Gomes OM, Reis A, et al. Effects of immersion time and frequency of water exchange on durability of etch-and-rinse adhesive. J Biomed Mater Res B Appl Biomater 2010;95:339-46. 42.Marshall GW, Habelitz S, Gallagher R, Balooch M, Balooch G, Marshall SJ. Nanomechanical properties of hydrated carious human dentin. J Dent Res 2001;80:1768-71. 43.Kinney JH, Balooch M, Marshall GW, Marshall SJ. A micromechanics model of the elastic properties of human dentine. Arch Oral Biol 1999;44:813-22.
44.Yoshiyama M, Tay FR, Doi J, Nishitani Y, Yamada T, Itou K, et al. Bonding of selfetch and total-etch adhesives to carious dentin. J Dent Res 2002;81:556-60. 45.Tezvergil-Mutluay A, Agee KA, Hoshika T, Carrilho M, Breschi L, Tjäderhane L, et al. The requirement of zinc and calcium ions for functional MMP activity in demineralized dentin matrices. Dent Mater 2010;26:1059-67. Corresponding author: Dr Paulo Henrique dos Santos Department of Dental Materials and Prosthodontics Araçatuba School of Dentistry - UNESP Rua José Bonifácio, 1193 16015-050 Araçatuba, SP BRAZIL Fax: +551836362802 E-mail: [email protected] Copyright © 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Influences of implant neck design and implant-abutment joint type on peri-implant bone stress and abutment micromovement: three-dimensional finite element analysis Yamanishi Y, Yamaguchi S, Imazato S, Nakano T, Yatani H. Dent Mater 2012;28:1126-33. Objectives: Occlusal overloading is one of the causes of peri-implant bone resorption, and many studies on stress distribution in the peri-implant bone by three-dimensional finite element analysis (3D FEA) have been performed. However, the FEA models previously reported were simplified and far from representing what occurs in clinical situations. In this study, 3D FEA was conducted with simulation of the complex structure of dental implants, and the influences of neck design and connections with an abutment on peri-implant bone stress and abutment micromovement were investigated. Methods: Three types of two-piece implant CAD models were designed: external joint with a conical tapered neck (EJ), internal joint with a straight neck (IJ), and conical joint with a reverse conical neck (CJ). 3D FEA was performed with the setting of a “contact” condition at the component interface, and stress distribution in the periimplant bone and abutment micromovement were analyzed. Results: The shear stress was concentrated on the mesiodistal side of the cortical bone for EJ. EJ had the largest amount of abutment micromovement. While the von Mises and shear stresses around the implant neck were concentrated on the labial bone for IJ, they were distributed on the mesiodistal side of the cortical bone for CJ. CJ had the least amount of abutment micromovement. Significance: Implants with a conical joint with an abutment and reverse conical neck design may effectively control occlusal overloading on the labial bone and abutment micromovement. Reprinted with permission of the Academy of Dental Materials.
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Clinical Implications
An improved understanding of the bonding process of resin cements to caries-affected dentin could enhance the durability of the cementation of indirect restorations. The search for improved bonding between indirect restorations and dental tissues has been the focus of
many studies.1,2 Acid-base dental cements, such as zinc phosphate and zinc polycarboxylate,3,4 have been
used for many years.5,6 The acid-base reaction is responsible for the set of these materials. However, these ce-
Supported by grant No. 2009/17472-7 from the Sao Paulo Research Foundation (FAPESP). Postgraduate student, Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry. Postgraduate student, Department of Restorative Dentistry, Araçatuba School of Dentistry. c Postgraduate student, Department of Restorative Dentistry, Araçatuba School of Dentistry. d Postgraduate student, Department of Restorative Dentistry, Piracicaba Dental School. e Assistant Professor, Adamantina School of Dentistry. f Associate Professor, Department of Restorative Dentistry, Araçatuba School of Dentistry. g Assistant Professor, Department of Dental Materials and Prosthodontics, Araçatuba School of Dentistry. a
b
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Volume 110 Issue 1 ments exhibit solubility in the oral cavity, with microleakage decreasing their mechanical properties.3,5 With the advent of composite resin for direct use and the improvement of their mechanical properties, resin cements were developed. These materials contain a monomeric system such as bisphenol-A glycidyl methacrylate (Bis-GMA) or urethane dimethacrylate (UDMA) in combination with other lower molecular weights such as triethylene glycol-dimethacrylate (TEGDMA),7 which provide low viscosity and fluidity to the material.8 These cements have many advantages over the water-based cements, such as improved bond strength, less microleakage, acceptable biocompatibility,9 lower solubility,9 higher mechanical properties, and satisfactory esthetics.10-13 The conventional resin luting technique depends on prior conditioning of the tooth structure and subsequent penetration of the adhesive system as well as the surface of the restoration.14-16 The efficiency of the adhesive system is dependent on its capacity to penetrate the demineralized dentin,17 forming an interdiffusion zone with the exposed collagen fibrils known as the hybrid layer, which is responsible for both retention and marginal seal.13,18-20 However, if the exposed collagen fibrils are not completely penetrated by the adhesive, the bond durability can be compromised by the hydrolysis of these fibrils, resulting in a process known as nanoleakage.21 This nanoporosity in the hybrid layer is responsible for failures in the bonding interface, which would allow solutions such as oral fluids, bacterial products, and proteolytic enzymes to penetrate these areas and therefore interfere with the success of the bonding.21-24 This degradation could result in the development of secondary caries, pulp inflammation, and dentin sensitivity.23,24 In order to overcome some of the limitations associated with the etchand-rinse technique, self-adhesive resin cements have been developed. This approach has reintroduced the
concept of incorporation of the smear layer as a bonding substrate, but with new formulations of monomers that penetrate the smear layer and underlying dentin.18 This resin cement requires no pre-acid etching of the dental surface; the application is performed in one step,16,25,26 similar to conventional cements.5 The bonding mechanisms of self-adhesive resin cements are based on acidic monomers that demineralize the smear layer and underlying dentin, resulting in micromechanical retention and infiltration of the adhesive monomer in the tooth substrate.27 Although the basic mechanism of adhesion is similar for all the selfadhesive cements, there is still little information about the application on modified substrates such as caries-affected dentin.28 In general, the adhesion to noncarious cervical sclerotic dentin would be prejudiced, since this surface has a hypermineralized superficial layer29,30 and sclerotic areas are acid resistant and filled with whitlockite crystals that obliterate the tubules,31,32 making it difficult to form resin tags.33,34 Although these modified substrates are different from normal dentin tissue, they are often considered as the main substrates for restorative procedures.35 In this manner, it would be clinically relevant to understand the bonding mechanisms of resin cements to caries-affected dentin. Therefore, the purpose of this study was to measure the microtensile bond strength of self-adhesive and conventional resin cements to sound or caries-affected dentin at 24 hours and 6 months after the bonding procedure. The null hypotheses tested were: (1) the type of dentin substrate would not affect the values of microtensile bond strength; (2) there would be no difference between the values of microtensile bond strength when comparing the periods of 24 hours and 6 months; and (3) there would be no difference in the values of microtensile bond strength between the conventional and self-adhesive resin cements.
The Journal of Prosthetic Dentistry
MATERIAL AND METHODS Thirty-six human molars, 18 sound and 18 caries-affected, were used in this study. No power analysis was performed to determine adequate sample size, which was based on previous studies that used the same methodology adopted in the present study.26,36 The project was approved by the Research and Ethics Committee of the Araçatuba School of Dentistry, Sao Paulo State University (protocol # 2614/09). The teeth were thawed, cleaned, and then kept frozen (-20°C) for 2 to 3 weeks before the start of the study. The occlusal surfaces of all teeth were ground flat with #180, 320, and 600 grit silicon carbide abrasive paper (Extec Corp, Enfield, Conn) under running water in an automatic polishing machine (APL-4; Arotec, Sao Paulo, Brazil) to remove enamel and expose a flat dentin surface (Fig. 1A). For the teeth affected by caries, a caries detector solution (Vide Carie; Inodon, Porto Alegre, Brazil) was applied on the dentin surface to identify the caries-infected tissue, which was removed during preparation of the specimens. The teeth were ground until the bright pigmentation of the caries-infected dentin was removed and the low, diffuse, softer color of the caries-affected dentin remained. The teeth were divided into 3 groups (n=6), according to the luting procedures described below. The teeth in the ARC Group were etched with 37% phosphoric acid (3M ESPE, St Paul, Minn) for 15 seconds, washed, and dried with paper towels without dehydrating the dentin. Adper Single Bond 2 bonding system (3M ESPE) was applied in 2 consecutive layers to the moist dentin by gently agitating the brush saturated with material on the surface of the dentin for 15 seconds. Then the teeth were gently air-dried for solvent evaporation and photopolymerized (Ultraled; Dabi Atlante, Ribeirão Preto, Brazil) for 20 seconds. Blocks of a prepolymerized laboratory resin-based
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July 2013 A
B
D
C
E
1 Specimen preparation. A, Occlusal surfaces of teeth were ground flat to expose dentin. B, Blocks of Tescera composite resin were luted. C, Teeth were sectioned in long axis. D, Half of teeth were stored for 6 months in distilled water at 37°C. E, Specimens were sectioned perpendicularly to adhesive interface to produce beams with adhesive area of approximately 1.0 mm2 for microtensile test. composite (Tescera; Bisco Inc, Schaumburg, Ill) measuring 11 mm in diameter and 4 mm thick were luted on the prepared dentin with RelyX ARC resin cement (3M ESPE). The excess cement was removed from the margins with disposable brushes and the cement was photopolymerized for 40 seconds each side. The blocks of Tescera were previously made by inserting the material in aluminum molds in an oxygen-free environment and photopolymerizing with a light source under water, pressure, and heat (Tescera ATL; Bisco Inc). They were then airborne particle abraded throughout with 50 μm aluminum oxide for 5 seconds (0.52 MPa air pressure; 10 mm from the tip), washed with distilled water, and dried with compressed air. To standardize the luting procedure, a load of 1 N was applied to the top of the resin cylinders after luting the composite resin to the dentin. The teeth in the PAN group were etched with a self-etching adhesive system (Ed Primer; Kuraray Medical Inc, Tokyo, Japan). One drop of Liquid A and one drop of Liquid B were mixed
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and actively applied for 3 minutes over the dentin surface with a brush tip and left in contact with the dentin for 60 seconds. Then, using a sponge or paper point, the excess primer was eliminated and the surface dried thoroughly with a gentle air stream. Blocks of Tescera composite resin were luted (Panavia F; Kuraray Medical Inc). Disposable brushes were used to remove excess cement from the margins and the cement was photopolymerized for 40 seconds on each side. In the UNI group, the Tescera resin blocks were luted directly to dentin with RelyX Unicem self-adhesive resin cement (3M ESPE). The excess cement was removed from the margins with disposable brushes and the cement was photopolymerized for 40 seconds on each side. No previous treatment was done to the dentin before the resin cement application. After the bonding process (Fig. 1B), the teeth were stored in distilled water37,38 at 37°C for 24 hours. After this period, the teeth were sectioned perpendicular to the adhesive-tooth interface (Fig. 1C) with a low speed
diamond saw (Isomet 2000; Buehler Ltd, Lake Bluff, Ill) under water cooling. One half of the specimens were used to evaluate the microtensile bond strength 24 hours after the bonding process, while the other half were stored for 6 months in distilled water at 37°C (Fig. 1D), changed weekly. For the microtensile test,39-41 the bonded specimens were sectioned perpendicular to the adhesive interface with the low-speed diamond saw (Isomet 2000, Buehler) under water cooling to produce beams with an adhesive area of approximately 1.0 mm2 (Fig. 1E). The ends of the stick-like specimens were fixed with a cyanoacrylate adhesive (Super Bonder gel; Henkel Corp, Rocky Hill, Conn) to a testing apparatus and subjected individually to microtensile testing in a universal testing machine (EMIC model DL3000; São José dos Pinhais, Paraná, Brazil) at a crosshead speed of 0.5 mm/min to evaluate the microtensile bond strength (MPa). The results were analyzed by 3-way analysis of variance (ANOVA)-dentin condition (sound or caries-affected), resin cement (RelyX
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Table I. Values of microtensile bond strength (MPa) of resin cements to sound and caries-affected dentin RelyX Unicem
RelyX ARC
Panavia F
Sound dentin
17.22 ±6.45ab
22.56 ±9.44 a
9.97 ±5.08b
Caries-affected dentin
14.78 ±8.43a
25.71 ±11.72 a
16.90 ±6.95a*
Sound dentin
14.56 ±3.83ab
21.73 ±9.08 a
11.41 ±2.66b
Caries-affected dentin
13.86 ±3.20
22.40 ±14.70 a
6.83 ±2.80b*
24 hours 6 months
ab
Different superscripted lowercase letters in row are significantly different (P<.05). *Statistically significant difference between 24 hours and 6 months.
Table II. Analysis of variance for microtensile bond df
Sum of Squares
Mean Square
F
P
Lambda
Power
Cement
2
1602
801
13.55
<.001
27.092
.999
Type of dentin
1
4.24
4.24
0.07
.790
0.072
.058
Time
1
125.14
125.14
2.12
.151
2.116
.282
Cement × Dentin
2
37.67
18.83
0.32
.727
0.637
.097
Group × Time
2
22.25
11.12
0.19
.829
0.376
.077
Tooth × Time
1
70.36
70.36
1.19
.280
1.190
.178
1.14
.328
2.272
.232
Group × Tooth × Time
2
134.34
67.17
Residual
56
3311.23
59.13
ARC, RelyX Unicem, or Panavia F), and assessment time (24 hours or 6 months)-and the protected least significant difference (PLSD) Fisher test (α=.05). The fractured beams of debonded specimens were analyzed with a stereomicroscope at magnifications of ×6 and ×66 for analysis of the failure mode. Failure modes were classified into 4 groups: (1) adhesive failure at the hybrid layer; (2) cohesive failure in dentin; (3) cohesive failure in adhesive resin; and (4) mixed failure. Representative specimens of experimental groups were fixed and coated with gold (BAL-TEC SCD 050; Balzers AG, Balzers, Liechtenstein) and observed under scanning electron microscopy (SEM JSM5600LV; JEOL, Tokyo, Japan) to illustrate the patterns of fracture.
RESULTS The highest microtensile bond strength values were found for RelyX
ARC in all experimental conditions (Table I). However, there was no significant difference between the RelyX ARC and RelyX Unicem (P>.05). Panavia F showed the lowest microtensile bond strength at 24 hours for sound dentin (9.97 ±8.5 MPa), which was significantly lower than that of RelyX ARC (22.56 ±9.44 MPa) (P=.009). After 6 months, Panavia F showed lower microtensile bond strength compared to RelyX ARC for both sound (11.41 ±2.66 MPa and 21.73 ±9.08 MPa, respectively) and caries-affected dentin (6.83 ±2.80 MPa and 22.40 ±14.70 MPa, respectively) (P=.009). RelyX Unicem showed intermediate values of microtensile bond strength in all experimental conditions, with no significant difference for the other cements. In the comparison between 24 hours and 6 months of evaluation in caries-affected dentin, the values of microtensile bond strength for Panavia F after 24 hours were significantly higher (16.90 ±6.95 MPa) than the
The Journal of Prosthetic Dentistry
values obtained after 6 months (6.83 ±2.80 MPa) (P=.008). For the other cements, there was no difference in bond strengths between the periods of evaluation. The condition of the dentin (normal or caries-affected) presented a statistically significant influence only for Panavia F when analyzed 6 months after bonding to dentin. The sound dentin showed higher values of microtensile bond strength for Panavia F after 6 months of evaluation (11.41 ±2.66 MPa) compared to caries-affected dentin (6.83 ±2.80 MPa) (P=.016). In the other experimental conditions, no significant difference between the 2 substrates was found. The results of analysis of variance (Table II) showed that among the 3 factors studied, there was a statistically significant difference for the resin cement factor (P<.001). For the other factors, as well as the interaction between them, the difference was not statistically significant.
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July 2013 100% 90%
Mixed failure
80%
Adhesive resin failure
70%
Dentin failure
60%
Hybrid layer failure
50% 40% 30% 20% 10% 0%
RelyX ARC 24 hours
RelyX ARC 6 months
Panavia F 24 hours
Panavia F 6 months
RelyX Unicem RelyX Unicem 24 hours 6 months
2 Incidence (%) of failure patterns of sound dentin. 100% 90%
Mixed failure
80%
Adhesive resin failure
70%
Dentin failure
60%
Hybrid layer failure
50% 40% 30% 20% 10% 0%
RelyX ARC 24 hours
RelyX ARC 6 months
Panavia F 24 hours
Panavia F 6 months
RelyX Unicem RelyX Unicem 24 hours 6 months
3 Incidence (%) of failure patterns of caries-affected dentin. There was a predominance of adhesive failure, independent of the substrate or the period of time analyzed. Only RelyX ARC showed some variation in the fracture mode, with a significant number of dentin failures and mixed failures, in addition to adhesive failures (Figs. 2, 3).
DISCUSSION Based on the results of this study, only the third null hypothesis (type of resin cement) was rejected. In this study, 3 independent variables were studied: the type of substrate (sound and caries-affected dentin), the type of resin cement used (RelyX ARC, RelyX Unicem, and Panavia F), and the storage time of specimens (24 hours and 6 months). Regarding the type of substrate, for most experimental groups no difference was found be-
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tween sound or caries-affected dentin, results which are similar to previous studies.28,30 The only exception was for Panavia F bonded to caries-affected dentin after 24 hours, which showed higher microtensile bond strength, leading to the rejection of the first null hypothesis. Probably, the specimens classified as caries-affected dentin, may have contained variable amounts of normal dentin. This phenomenon could be responsible for more variation in the results when the bonding was done in the caries-affected dentin. Some authors have reported differences in the microtensile bond strength between sound and caries-affected dentin.29,30,32 Considering the limitation of the methodology employed in the present study, it is difficult to measure the degree of mineralization of the caries-affected dentin and ensure a complete uniformity of the substrate,
which could explain the lack of significant difference in microtensile bond strengths between the sound or cariesaffected dentin. According to Marshall et al,42 the intertubular dentin of cariesaffected dentin showed reduced or unchanged nanomechanical properties as compared with normal intertubular dentin. The mechanical properties of intertubular dentin are the major determinants of dentin characteristics as a whole.43 Due to increased porosity in the intertubular dentin, a thicker hybrid layer would be expected in the caries-affected dentin, facilitating the diffusion of acidic conditioners and adhesive monomers.44 However, the lack of resin tag formation due to the presence of acid-resistant intratubular mineral deposits, associated with a decrease in the cohesive strength of dentin in caries-affected tissue could interfere with the bond strength values.44
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4 Representative specimen of RelyX ARC after 6 months bonded to sound dentin. There are areas of exposure of dentin tubules in bottom of hybrid layer (A), while in other areas there appeared to be evidence of adhesive covering dentin in middle of hybrid layer (B) (original magnification ×500). The second null hypothesis was rejected because there was no significant difference between the values obtained at 24 hours and 6 months, except for Panavia F when bonded to caries-affected dentin. This could be explained by the storage method used in this study. The specimens were sectioned after the bonding process, and half of the tooth was used for evaluation after 24 hours and the other half for evaluation after 6 months. Thus, for the evaluation after 6 months, the beams were obtained immediately before the mechanical test. In some previous studies, the bonded teeth were reduced to small beams before the aging process39,41 to accelerate aging. Goodis et al37 and Da Fonte Porto Carreiro et al38 reported that storage in distilled water showed a lower variation in bond strength values compared to other methods of aging. However, Tezvergil-Mutluay et al45 reported that aging media should contain calcium and zinc ions to permit the endogenous proteases in dentin to function. The authors concluded that incubation in water underestimates degradation of bond strengths. In this study, RelyX ARC resin cement showed higher values of microtensile bond strength when compared to Panavia F and RelyX Unicem,
5 Representative specimen of RelyX ARC after 6 months bonded to caries-affected dentin. There are regions where there appears to be adhesive within dentinal tubules in bottom of hybrid layer (A), and hybrid layer was retained and adhesive separated from surface of hybrid layer (B) (original magnification ×500).
rejecting the third null hypothesis which confirms previous studies.12,26 This result could be explained by the bonding mechanisms of etch-andrinse adhesive such as the Adper Single Bond adhesive system that was used with RelyX ARC. The 37% phosphoric acid removes the smear layer, demineralizes the dentin surface, and opens the dentinal tubules. This exposes the collagen fibers, creating an adequate substrate for penetration of the hydrophilic monomer of the adhesive system that involves the collagen network, establishing micromechanical retention.14 According to Braga et al,15 RelyX ARC dual cement associated with Adper Single Bond 2 adhesive system with the wet bonding technique creates a thick hybrid layer. This thicker hybrid layer could explain the higher values of microtensile bond strength when compared to the other resin cements. Higher variations were found in the type of failure with the presence of many mixed and cohesive failures in dentin combined with the higher values of microtensile bond strength. Figure 4 represents a type of mixed failure in a representative specimen of sound dentin luted with RelyX ARC, which reveals that the adhesive occupies much of the dentin surface. In the area labeled A, dentin seems to be exposed in some areas within the
The Journal of Prosthetic Dentistry
bond perimeter, as evidenced by the presence of open dentinal tubules in the bottom of the hybrid layer. In the area labeled B, in the middle of the hybrid layer, the resin tags were probably more retentive, forcing the advancing crack to shift to the adhesive that infiltrated the hybrid layer. Even in area B, there were some open tubules where resin tags had been pulled out. There were adjacent areas where the resin tags sheared off, leaving residual tags in the tubules. The same parameter can be observed in the specimens of caries-affected dentin also luted with RelyX ARC (Fig. 5). By 6 months, the stress concentrations shifted due to water sorption plasticizing the resins. At 6 months, most of the failures in A occurred at the bottom of the hybrid layer. About half the tubules pulled out with the adhesive/composite resin, and about half fractured but remained in the dentin. In area B, the hybrid layer was retained and the adhesive separated from the surface of the hybrid layer (Fig. 5). The results showed no significant difference between RelyX Unicem and Panavia F. The manufacturer of Panavia F recommends treating the substrate with a self-etching adhesive system (ED Primer) This system is thought to eliminate the need for prior acid etching, as well as wash-
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6 Representative specimen of Panavia F after 6 months of bonding to sound dentin. Smear layer was observed on almost all surfaces, with few dentinal tubules exposed. There was no evidence of resin tags (original magnification ×500).
7 Representative specimen of Panavia F after 6 months of bonding to caries-affected dentin. Similar to Fig. 6, dentin surface was covered by smear layer, and in some areas there appeared to be little evidence of coverage of resin cement on smear layer (*) (original magnification ×500).
8 Representative specimen of the RelyX Unicem after 24 hours of bonding to sound dentin. In this case, smear layer seems to be covering dentin surface where there was no evidence of exposure of dentinal tubules (original magnification ×500).
9 Representative specimen of the RelyX Unicem after 24 hours of bonding to caries-affected dentin. Similar to sound substrate, in previous figure, dentin surface appeared to be completely covered by smear layer, not exposing dentinal tubules (original magnification ×500).
ing and drying of the substrate. The acidic primer is able to demineralize the dentin by modifying the smear layer without removing it completely, maintaining the smear plug, while promoting the infiltration of adhesive monomers and the formation of the hybrid layer.17 According to Hikita et al25 and Monticelli et al17 there was a similarity in the formulations of the primer of the self-etch adhesive of Panavia F and self-adhesive resin cements. Both products contain multifunctional methacrylate phosphate, which the manufacturer claims reacts with the hydroxyapatite of tooth tis-
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sue, which could explain the similarity in the results. In the Panavia F groups, the formation of a minimum hybrid layer was observed, which could compromise a good bond strength when evaluated immediately after the bonding process.17,18 Through scanning electron microscopy, a resin-infiltrated smear layer coverage on the dentin surface can be observed with minimal exposure of dentinal tubules (Fig. 6). The smear layer is not removed before etching and there is no evidence of resin tags. Probably the adhesives adhere to the surface of the hybrid layer, which then fails cohesively. The
same pattern was noted for Panavia F bonded to caries-affected dentin, in which small islands of residual luting material can be observed (Fig. 7). In self-adhesive cements, the initial acidity of these materials has a short-term effect, with a rapid increase in the pH resulting in a single interaction with the superficial dentin without formation of resin tags.16,25,27 According to Pavan et al,9 the acidity present in this cement is not enough to demineralize the dentin and expose the dentinal tubules. This could explain the lowest bond strength found in this study. Figure 8 presents
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Volume 110 Issue 1 a sound dentin surface almost completely covered by the smear layer, the same as a representative specimen of caries-affected dentin (Fig. 9). Although this study used a disclosing dye (Vide Carie; Inodon) for the exact location of the caries-affected dentin, there are some limiting factors that must be considered, such as the exact location of the caries-affected dentin when sectioning the teeth to obtain the beams for the microtensile test. Also, the technique of flattening the dentin surface by grinding the crowns tends to remove excessive caries-affected dentin. In this situation, additional unaffected dentin could be exposed compared with the clinical situation. In this study, teeth were stored in distilled water at 37°C, not aged with thermal or pH cycling. Another potential limitation of the study is the lack of power analysis to determine adequate sample size, especially for the interactions among the variables (Table II). Additional studies are needed to complement the discussion about the luting procedure in sound and caries-affected dentin with new formulations of resin cements.
CONCLUSIONS The results of this study showed the mean bond strengths of selfetching cements to caries-affected dentin were not different to the values to sound dentin. No difference was found between 24 hours and 6 months after bonding, except for Panavia F, which showed higher values of bond strength after 24 hours compared with 6 months. Furthermore, in the comparison among the resin cements, RelyX ARC, which depends on a total-etch adhesive system to bond to the dentin, obtained the highest values of microtensile bond strength.
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39.Mendonça JS, Souza MH Jr, Carvalho RM. Effect of storage time on microtensile strength of polyacid-modified resin composites. Dent Mater 2003;19:308-12 40.Scherrer SS, Cesar PF, Swain MV. Direct comparison of the bond strength results of the different test methods: a critical literature review. Dent Mater 2010;26:e78-e93. 41.Skovron L, Kogeo D, Gordillo LA, Meier MM, Gomes OM, Reis A, et al. Effects of immersion time and frequency of water exchange on durability of etch-and-rinse adhesive. J Biomed Mater Res B Appl Biomater 2010;95:339-46. 42.Marshall GW, Habelitz S, Gallagher R, Balooch M, Balooch G, Marshall SJ. Nanomechanical properties of hydrated carious human dentin. J Dent Res 2001;80:1768-71. 43.Kinney JH, Balooch M, Marshall GW, Marshall SJ. A micromechanics model of the elastic properties of human dentine. Arch Oral Biol 1999;44:813-22.
44.Yoshiyama M, Tay FR, Doi J, Nishitani Y, Yamada T, Itou K, et al. Bonding of selfetch and total-etch adhesives to carious dentin. J Dent Res 2002;81:556-60. 45.Tezvergil-Mutluay A, Agee KA, Hoshika T, Carrilho M, Breschi L, Tjäderhane L, et al. The requirement of zinc and calcium ions for functional MMP activity in demineralized dentin matrices. Dent Mater 2010;26:1059-67. Corresponding author: Dr Paulo Henrique dos Santos Department of Dental Materials and Prosthodontics Araçatuba School of Dentistry - UNESP Rua José Bonifácio, 1193 16015-050 Araçatuba, SP BRAZIL Fax: +551836362802 E-mail: [email protected] Copyright © 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Influences of implant neck design and implant-abutment joint type on peri-implant bone stress and abutment micromovement: three-dimensional finite element analysis Yamanishi Y, Yamaguchi S, Imazato S, Nakano T, Yatani H. Dent Mater 2012;28:1126-33. Objectives: Occlusal overloading is one of the causes of peri-implant bone resorption, and many studies on stress distribution in the peri-implant bone by three-dimensional finite element analysis (3D FEA) have been performed. However, the FEA models previously reported were simplified and far from representing what occurs in clinical situations. In this study, 3D FEA was conducted with simulation of the complex structure of dental implants, and the influences of neck design and connections with an abutment on peri-implant bone stress and abutment micromovement were investigated. Methods: Three types of two-piece implant CAD models were designed: external joint with a conical tapered neck (EJ), internal joint with a straight neck (IJ), and conical joint with a reverse conical neck (CJ). 3D FEA was performed with the setting of a “contact” condition at the component interface, and stress distribution in the periimplant bone and abutment micromovement were analyzed. Results: The shear stress was concentrated on the mesiodistal side of the cortical bone for EJ. EJ had the largest amount of abutment micromovement. While the von Mises and shear stresses around the implant neck were concentrated on the labial bone for IJ, they were distributed on the mesiodistal side of the cortical bone for CJ. CJ had the least amount of abutment micromovement. Significance: Implants with a conical joint with an abutment and reverse conical neck design may effectively control occlusal overloading on the labial bone and abutment micromovement. Reprinted with permission of the Academy of Dental Materials.
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