Influence of Insertion Techniques for Resin Cement and Mechanical Cycling on the Bond Strength Between Fiber Posts and Root Dentin Ana Carolina de Oliveira Souzaa / Fernanda de Cássia Papaiz Gonçalvesa / Lilian Costa Anamib / Renata Marques de Meloc / Marco Antonio Bottinod / Luis Felipe Valandroe
Purpose: To evaluate the effect of the insertion technique for resin cement and mechanical cycling on the bond strength between fiber posts and root dentin. Materials and Methods: Sixty-four single-rooted bovine teeth were endodontically prepared to receive glass-fiber posts. The insertion of cement into the root canal was performed using one of the following techniques: POS, insertion with the post; LEN, the use of a lentulo-type drill; EXP, insertion with a straight-tip explorer; or CEN, the use of a Centrix syringe. Half of the specimens were mechanically cycled. All specimens were sectioned into slices of 1.8 mm for the push-out test and 0.5 mm for analysis of the cement layer quality. Results: The insertion technique affected the interaction between factors (bond strength and mechanical cycling; p < 0.0001). Insertion of the Centrix syringe after mechanical cycling showed the highest bond values (13.6 ± 3.2 MPa). Group-to-group comparisons for baseline and cycled conditions indicated that mechanical cycling significantly influenced the bond strength (p < 0.0001) of the POS and CEN groups. The quality of the cement layer did not differ between the techniques when evaluated in the middle (p = 0.0612) and cervical (p = 0.1119) regions, but did differ in the apical region (p = 0.0097), where the CEN group had better layer quality for the two conditions tested (baseline and cycled). Conclusion: The use of the Centrix syringe improved the homogeneity of the cement layer, reducing the defects in the layer and increasing adhesive strength values to dentin, even after mechanical cycling. Keywords: fiber post, cement layer, interface, adhesion, retention, mechanical loading, fatigue. J Adhes Dent 2015; 17: 175–180. doi: 10.3290/j.jad.a33993
a
PhD Student, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Experimental design, performed the experiments in partial fulfillment of requirements for a degree, wrote the manuscript.
b
PhD Student, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Consulted on and performed statistical evaluation, contributed substantially to discussion, proofread the manuscript.
c
Researcher, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Idea, hypothesis, experimental design, contributed substantially to discussion, proofread the manuscript.
d
Chairman, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Idea, proofread the manuscript.
e
Associate Professor, Department of Restorative Dentistry, School of Dentistry, Federal University of Santa Maria, Santa Maria, RS, Brazil. Idea, hypothesis, experimental design, proofread the manuscript.
Correspondence: Dr. Renata Marques de Melo, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, Av. Eng. Fco. José Longo, 777, São José dos Campos/SP, Brazil 12245-000. Tel: +55-12-3947-9032. Fax: +55-12-3047-9000. e-mail: [email protected]
Vol 17, No 2, 2015
Submitted for publication: 02.12.14; accepted for publication: 19.03.15
G
enerally, when an endodontically treated tooth requires reconstruction and has little tooth structure remaining, the root canal is used for setting intracanal retainers, with consequent core retention improvement.18,20,31,35 Glass-reinforced fiber posts have been utilized for intracanal anchorage, since they cause less damage to intraradicular tissues, do not corrode, have a modulus of elasticity close to that of dentin (16–40 GPa),2,23 have reparable failures,5,24,25 and are more easily removed compared with cast posts and cores.7,8 However, clinical trials15,16 have reported that fiber post debonding is the main failure of this technique, indicating a possible weakness at the dentin/cement interface, as a consequence of the higher C-factor38 and difficult adhesion procedures19 (root canal cleansing, bond agent application, cement insertion). For retention improvement, the cementation of glassfiber posts must be performed by a bonding technique and is appreciably influenced by the technique used.17,39 175
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Table 1
Chemical composition of adhesive and cement
Material
Chemical composition*
Ambar adhesive (FGM)
Active ingredients: methacrylic monomers, photoinitiators, co-initiators, stabilizer. Inactive ingredients: inert filler (silica nanoparticles) and solvent (ethanol).
Allcem cement (FGM)
Bisphenol-A-diglycidyl ether dimethacrylate (bis-GMA), bisphenol-A-diglycidyl ether dimethacrylate ethoxylate (bis-EMA), triethylene glycol dimethacrylate (TEG-DMA), co-initiators, initiators (camphorquinone and dibenzoyl peroxide) and stabilizers. Fillers: barium-alumino silicate glass microparticles and silicon dioxide nanoparticles.
*Manufacturers’ information.
The application technique of resin cements can be an inherent limitation of the fiber post cementation, as bubbles (voids) may form within the cement and at the adhesive interface.11,41,42 These imperfections may reduce the ability of the cement to retain the glass-fiber post in the root canal and therefore influence the longevity of the restoration.41 Several techniques are used for insertion of the resin cement into the root canal, with the most common being the use of a particular syringe, a lentulo-type drill, a straight-edge explorer, and the insertion of the cement with the post itself. The insertion of the resin cement with syringe-type applicators significantly reduces the area of bubbles within the cement compared with microbrush insertion.41 Another study noted the good quality of the cement layer obtained by insertion with a Centrix syringe and a lentulo drill.29 The differences between the cement patterns from insertion techniques deserve attention, because the more homogeneous the cement, the greater the intracanal post retention.41 However, there are no data in the literature about the influence of the combined effect of mechanical cycling and different insertion techniques. It is known that the deterioration of the cement by loads resulting from mechanical loading5,30,43 and hydrolytic degradation of the bond between dentin, adhesive, and cement over time37 can lead to the displacement of fiber posts. Given that the major cause of failure of glass-fiber posts is lack of adhesion, it is pertinent to evaluate the effects of different techniques of cement insertion and mechanical cycling on the bond strength of glass-fiber posts along the root canal and the effects of these techniques on the quality of the resin cement layer. The null hypotheses tested here were that (1) the tested techniques would not affect the bond strength between fiber post and root dentin and (2) mechanical cycling would not decrease the bond strength results.
MATERIALS AND METHODS Sixty-four uniradicular bovine teeth were cleaned with periodontal curettes, stored in 1.23% chlorhexidine for 2 h for disinfection, and kept in distilled water at 37°C (temperature of the oral cavity) until use. A mark was made 16 mm from the root apex and the crown was sectioned 176
by means of a diamond disk under water cooling. Roots were prepared by means of preparation burs (White Post, FGM; Joinville, Brazil) to #3 at a length of 12 mm and were embedded in acrylic resin cylinders placed in PVC tubes of 25 mm diameter, so that 2 mm of the cervical third of the root was exposed. To prevent the inclusion of bias, such as remaining filling material that could further affect the bond strength, the roots were not endodontically filled prior to post preparation. The roots were randomly divided into 4 groups (n = 16) according to the method of insertion of the resin cement for setting the glass-fiber post (post, lentulo drill, explorer, and Centrix). Fiber Post Cementation, Studied Groups All roots received glass-fiber posts (White Post DC, FGM) cemented with resin cement (Allcem, FGM; Table 1) as indicated by the manufacturer. The dentin was conditioned by 37% phosphoric acid for 15 s, and then the root canal was completely washed with water with the aid of irrigation and suction tips (NaviTip Tips and Capillary Tips, Ultradent; South Jordan, UT, USA) for 60 s. The root canals were dried with #80 absorbent paper tips, and Ambar adhesive (FGM; Table 1) was applied with a brush (Cavibrush, FGM). The cement was manipulated and inserted into the root canals according to the techniques described for each group: y POS (post): with a help of a clinic clamp, the pin was surrounded by cement along its entire length and inserted into the root canal; y LEN (lentulo drill): cement was inserted into the canal with a #40 lentulo drill (Dentsply Maillefer; Ballaigues, Switzerland) of 21 mm length mounted at low speed and kept in the channel for 3 s; y EXP (explorer): the active tip of a straight-tip #5 explorer was encased in cement and inserted from the most apical to the cervical region through circular movements; y CEN (Centrix): cement was inserted into the applicator tip, the tip was positioned in a Centrix syringe (DFL; Rio de Janeiro, Brazil), the cement was placed in the most apical area, and the application was performed in the apical-cervical direction. After insertion of the fiber post, the cement was light cured from the coronal portion for 40 s with the XL3000 curing light (3M ESPE; St Paul, MN, USA) at 600 mW/cm2 The Journal of Adhesive Dentistry
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of intensity, and the samples were stored in distilled water at 37°C for 24 h. After the posts had been cemented, a core was built with a composite resin (shade DA2, Opallis, FGM). The resin was initially applied to the remaining cervical dentin on the posts in layers of approximately 1 to 2 mm and polymerized. Then, the composite resin was applied to an acetate matrix with standardized format, positioned over the post and dentin, resulting in standardized reconstructions. Mechanical Cycling Half of the samples (n = 8) from each cement insertion technique were subjected to cyclic loading. The samples were positioned in the base plate at a 45-degree angle to the mechanical fatigue simulator (RE 11,000 Plus, Erios; Sao Paulo, Brazil) and received 1,200,000 pulses from 10 to 100 N at a 4 Hz frequency with a stainless steel applicator of 1.6 mm diameter. Fatigue was achieved with water irrigation at 37°C, and then the samples were stored for the same time as for mechanical cycling in distilled water at 37°C. Push-out Test and Evaluation of Cement Layer Quality All specimens were then sectioned perpendicular to the long axis of the root by means of a diamond disk under water cooling (LabCut 1010, Extec; Enfield, CT, USA). The first cervical slice (approx. 1 mm) was discarded. Six slices of each sample were obtained, 3 with 1.8 mm thickness for the push-out test and 3 (cervical, medial and apical) with 0.5 mm thickness for analysis of the cement layer. The slices for the push-out test were placed in a metal device with a central hole (Ø = 3 mm), with the larger diameter of the canal facing the base. A cylindrical stainless steel tip (Ø = 0.85 mm) was positioned at the fiber post area, and the load was applied perpendicular to the long axis of the root. The test was performed in a universal testing machine (EMIC DL 1000, EMIC; Sao Jose dos Pinhais, Brazil) with a 50-kg-cell load at a speed of 1 mm/min. The evaluation of cement layer quality was performed by stereomicroscopy (Discovery V20, Zeiss; Göttingen, Germany) with a standardized magnification of 100X and classified according to the quantity and diameter of the bubbles (voids) (Table 2). Cutting, testing, and analysis steps were blinded for the operator. Data Analysis Cohesive failures verified in push-out tests were discarded and not included in data analysis, since they did not represent the interfacial bond strength. After verification of the data distribution, the results of the push-out test were subjected to parametric analysis by two-way ANOVA, and differences were detected by a simple-effects method (insertion technique or aging conditions) with Tukey’s or Sidak’s test (α = 0.05). The cement layer quality scores were analyzed by the nonparametric Kruskal-Wallis test followed by Dunn’s test (p < 0.05). Vol 17, No 2, 2015
Table 2 Scores for the cement layer quality according to the numbers and sizes of bubbles (voids) Score
Numbers and sizes of bubbles (Ø)
1
No bubbles
2
Up to 10 small bubbles (Ø ≤ 0.1 mm)
3
10 to 20 small bubbles (Ø ≤ 0.1 mm)
4
Over 20 small bubbles (Ø ≤ 0.1 mm)
5
Up to 10 small bubbles (Ø ≤ 0.1 mm) and 1 or more larger bubbles (0.11 ≤ Ø ≤ 0.5 mm)
6
11 to 20 small bubbles (Ø ≤ 0.1 mm) and 1 or more larger bubbles (0.11 ≤ Ø ≤ 0.5 mm)
7
Over 20 small bubbles (Ø ≤ 0.1 mm) and 1 or more larger bubbles (0.11 ≤ Ø ≤ 0.5 mm)
8
Large bubbles (0.51 ≤ Ø ≤ 0.6 mm)
9
Large bubbles (0.61 ≤ Ø ≤ 0.7 mm)
10
Large bubbles (Ø > 0.7 mm)
RESULTS ANOVA showed that the insertion methods for resin cement were significantly influenced by the interaction of factors (insertion techniques and aging; p < 0.0001). Mechanical cycling influenced the bond strength between fiber post and root canal for POS and CEN groups (p < 0.0001). For the baseline condition, there was no significant difference among groups, while CEN showed the highest bond strength values among mechanically cycled groups (13.6 ± 3.2 MPa, Table 3). The median scores for the quality of the cement layer (Fig 1) varied between 10 and 2.5. No statistically significant differences were found for the middle (H = 13.48, df = 7, p = 0.0612) and cervical (H = 11.67, df = 7, p = 0.1119) areas, but the quality of cement at the apical area (H = 18.54, df = 7, p = 0.0097) was influenced by the tested techniques (Fig 2).
DISCUSSION As a result of the technique used for application of the cement and polymerization shrinkage itself, gaps, voids and bubbles can be observed within the cementation interface.6 Due to the reduced root canal geometry, any controlled application of the various agents of an adhesive cementation system is difficult.33,40,41 Accordingly, the aim of this study was to investigate the quality of the cement layer, the presence of voids and their possible relationship with bond strength. For simulation of the loads and masticatory forces to which an in vivo restoration would be subjected, the mechanical cycling test was performed with 106 cycles in some of the specimens, simulating 6 months in function,22 177
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Table 3 Bond strength data in MPa as means and standard deviations (SD), number of tested specimens, and cohesive failures (in %) Cycled Mean ± SD POS
2.8 ±
Tested specimens
1.71,C
Baseline % of cohesive failure
26
15.4
Mean ± SD 9.5 ±
Tested specimens
% of cohesive failure
28
9
3.42,a
LEN
9 ± 4.3B
25
16
6.3 ± 1.9a
28
3.6
EXP
4.2 ± 1.8C
30
3.3
8 ± 4.7a
30
10
CEN
13.6 ± 3.23,A
29
3.4
9.4 ± 4.24,a
28
5
Different superscript numbers mean statistically significance differences between cycled and baseline conditions for a same insertion technique. Different superscript upper-case letters represent statistically significant differences among cycled groups. Same superscript lower-case letters mean no statistical significance difference among baseline groups. PS: post; LEN: lentulo drill; EXP: explorer; CEN: Centrix syringe.
a
b
0,5 mm
c
0,5 mm
0,5 mm
Fig 1 Representative images of scores for the cement layer quality obtained by stereomicroscopy (magnification 100X). a) score 1; b) score 5; c) score 10.
10
AB A AB
AB B APICAL
SCORE
8 6
MIDDLE
AB AB
CERVICAL
B
4 2
PO S
ba PO sel in LE S cy e N cl ba ed LE sel N ine EX cy P cle ba d EX seli CE P cy ne N cl ba ed CE sel N ine cy cle PO d Sb as PO el in LE S cy e N cl ba ed LE sel N ine EX cy P cle ba d EX seli CE P cy ne N cl ba ed CE sel N ine cy cle PO d Sb as PO el in LE S cy e N cl ba ed LE sel N ine EX cy P cle ba d EX seli CE P cy ne N cl ba ed CE sel N ine cy cle d
0
Fig 2 Boxplot of scores for the quality of the cement layer with 10th and 90th percentiles for tested groups. Different capital letters mean statistically significant differences for the apical areas among groups (p = 0.0097). Middle and cervical areas showed no statistically significant differences. For group abbreviations, see Table 3.
and no post detachment (pre-mechanical test failure) was observed after cycling. The results of this study show that there were no statistically significant differences (baseline) in values of immediate bond strength (non-aging) with different techniques, as supported by other studies.11,29 However, when the specimens were subjected to cyclic loading, some differences were observed. In this context, 178
mechanical cycling had a statistically significant detrimental impact on the POS technique (lower values after aging). For the CEN group, mechanical cycling improved the bond strength between fiber post and root dentin. Therefore, the second null hypothesis was partially accepted. It is probable that the “post” technique promoted the highest adhesive failures, due to poor insertion of cement The Journal of Adhesive Dentistry
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and the consequent formation of bubbles (voids) along the root (especially in the apical region), generating the lowest bond strength values (Table 3). Imperfections caused by these techniques reduced the ability of the cement to retain the fiber post in the root canal, which might influence the longevity of the restoration. Thus, restorations treated with these techniques appeared to suffer greater deleterious effects of mechanical stress than those treated with other techniques. Regarding the use of a lentulo drill or explorer, it can be observed that the mechanical cycling did not affect these groups. However, when compared to the other cycled groups, LEN showed lower results than CEN; EXP had even lower bond strength results and was similar to POS. Therefore, the first null hypothesis was rejected. Fakiha et al13 also observed better results of retention for the lentulo drill compared to the post technique, and D’Arcangelo et al12 stated that syringe and lentulo techniques are preferable when bonding posts. The use of an explorer is common practice in clinics, and the lentulo drill has been reported as a tool that enables a favorable distribution of the resin cement into the root canal, leading to a uniform cement layer.1,12,35,36 However, when using the lentulo drill, caution is advised to avoid premature polymerization, since this can make it impossible to achieve proper insertion of the post.1 Since various insertion techniques led to different irregularities in the cement layer, mechanical cycling had a distinct effect on the tested groups. Specimens cemented with the use of the Centrix syringe (CEN) showed a cement layer with few voids and imperfections, verified by low scores for baseline and cycling conditions, probably optimizing the bond strength values found in the study. The use of the syringe-type applicator may explain the presence of a layer relatively free of voids due to the homogeneous dispersion of the material, promoting superior adhesion of the glass-fiber post to the root canal41 and greater stability in function. In the specific case of the CEN group, which showed better scores for quality of the cement layer, the bond strength values did not suffer a decrease after mechanical cycling. This would suggest that in a bettercontrolled situation where the cement layer is more homogeneous, an increase in the cement’s degree of conversion would occur in the long term. In fact, the degree of conversion28 and hardness32 of the resin cement can increase after several months in the root canal environment. In contrast, the reduced bond strength after aging for the POS group suggests that the deleterious effect of mechanical cycling is apparent under more challenging conditions (higher score values for the quality of cement). In this case, lower bonding values after cycling can be associated with the formation of cracks in the cement layer or loss of adhesion between cement and dentin or post6 as a consequence of degradation of this layer. It must be emphasized that the indication for a particular cementation method should be based not solely on bond strength, but in conjunction with data on microleakage,27 curing depth,21 and aging.26 Vol 17, No 2, 2015
The apical region was more sensitive to cementation techniques, being the only one to present statistically different results for cement layer quality according to the insertion methods (baseline and cycled CEN groups showed superior cement quality compared with the cycled POS group; the other groups were similar). In this region, the hybrid layer formed is not homogeneous and technique sensitivity greater, due to the difficulty of etching and adhesive system application as well as to the morphological differences in apical root dentin compared with that in the middle third and cervical dentin.9 Thus, the choice of insertion method is essential not only from a mechanical standpoint, but also for an adequate apical seal of the root canal.11,12,36 The in vitro analysis of clinical situations can lead to slight procedural modifications to standardize study parameters. The use of bovine teeth has been reported in the literature in push-out studies.4,10,14,26,29 According to Schilke et al,34 the number and diameter of dentin tubules in bovine central incisors and human third molars were statistically similar. This suggests that, when standardized preparations are made, coronal dentin of bovine incisors is an adequate substitute for human dentin in adhesion studies.34
CONCLUSION Within the limitations of this study, it can be concluded that the use of the Centrix syringe improved the quality of the cement layer in all areas of the root canal, reducing the number of imperfections and promoting the stability of bond strength between the fiber post and intracanal dentin.
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Clinical relevance: The insertion technique for the resin cement can affect the quality of the cement layer and the retention of the glass-fiber post in the root canal. The use of a special syringe for resin cement insertion inside the canal root might optimize fiber post retention.
The Journal of Adhesive Dentistry
Purpose: To evaluate the effect of the insertion technique for resin cement and mechanical cycling on the bond strength between fiber posts and root dentin. Materials and Methods: Sixty-four single-rooted bovine teeth were endodontically prepared to receive glass-fiber posts. The insertion of cement into the root canal was performed using one of the following techniques: POS, insertion with the post; LEN, the use of a lentulo-type drill; EXP, insertion with a straight-tip explorer; or CEN, the use of a Centrix syringe. Half of the specimens were mechanically cycled. All specimens were sectioned into slices of 1.8 mm for the push-out test and 0.5 mm for analysis of the cement layer quality. Results: The insertion technique affected the interaction between factors (bond strength and mechanical cycling; p < 0.0001). Insertion of the Centrix syringe after mechanical cycling showed the highest bond values (13.6 ± 3.2 MPa). Group-to-group comparisons for baseline and cycled conditions indicated that mechanical cycling significantly influenced the bond strength (p < 0.0001) of the POS and CEN groups. The quality of the cement layer did not differ between the techniques when evaluated in the middle (p = 0.0612) and cervical (p = 0.1119) regions, but did differ in the apical region (p = 0.0097), where the CEN group had better layer quality for the two conditions tested (baseline and cycled). Conclusion: The use of the Centrix syringe improved the homogeneity of the cement layer, reducing the defects in the layer and increasing adhesive strength values to dentin, even after mechanical cycling. Keywords: fiber post, cement layer, interface, adhesion, retention, mechanical loading, fatigue. J Adhes Dent 2015; 17: 175–180. doi: 10.3290/j.jad.a33993
a
PhD Student, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Experimental design, performed the experiments in partial fulfillment of requirements for a degree, wrote the manuscript.
b
PhD Student, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Consulted on and performed statistical evaluation, contributed substantially to discussion, proofread the manuscript.
c
Researcher, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Idea, hypothesis, experimental design, contributed substantially to discussion, proofread the manuscript.
d
Chairman, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, SP, Brazil. Idea, proofread the manuscript.
e
Associate Professor, Department of Restorative Dentistry, School of Dentistry, Federal University of Santa Maria, Santa Maria, RS, Brazil. Idea, hypothesis, experimental design, proofread the manuscript.
Correspondence: Dr. Renata Marques de Melo, Institute of Science and Technology, UNESP, Universidad Estadual Paulista, São José dos Campos, Av. Eng. Fco. José Longo, 777, São José dos Campos/SP, Brazil 12245-000. Tel: +55-12-3947-9032. Fax: +55-12-3047-9000. e-mail: [email protected]
Vol 17, No 2, 2015
Submitted for publication: 02.12.14; accepted for publication: 19.03.15
G
enerally, when an endodontically treated tooth requires reconstruction and has little tooth structure remaining, the root canal is used for setting intracanal retainers, with consequent core retention improvement.18,20,31,35 Glass-reinforced fiber posts have been utilized for intracanal anchorage, since they cause less damage to intraradicular tissues, do not corrode, have a modulus of elasticity close to that of dentin (16–40 GPa),2,23 have reparable failures,5,24,25 and are more easily removed compared with cast posts and cores.7,8 However, clinical trials15,16 have reported that fiber post debonding is the main failure of this technique, indicating a possible weakness at the dentin/cement interface, as a consequence of the higher C-factor38 and difficult adhesion procedures19 (root canal cleansing, bond agent application, cement insertion). For retention improvement, the cementation of glassfiber posts must be performed by a bonding technique and is appreciably influenced by the technique used.17,39 175
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Table 1
Chemical composition of adhesive and cement
Material
Chemical composition*
Ambar adhesive (FGM)
Active ingredients: methacrylic monomers, photoinitiators, co-initiators, stabilizer. Inactive ingredients: inert filler (silica nanoparticles) and solvent (ethanol).
Allcem cement (FGM)
Bisphenol-A-diglycidyl ether dimethacrylate (bis-GMA), bisphenol-A-diglycidyl ether dimethacrylate ethoxylate (bis-EMA), triethylene glycol dimethacrylate (TEG-DMA), co-initiators, initiators (camphorquinone and dibenzoyl peroxide) and stabilizers. Fillers: barium-alumino silicate glass microparticles and silicon dioxide nanoparticles.
*Manufacturers’ information.
The application technique of resin cements can be an inherent limitation of the fiber post cementation, as bubbles (voids) may form within the cement and at the adhesive interface.11,41,42 These imperfections may reduce the ability of the cement to retain the glass-fiber post in the root canal and therefore influence the longevity of the restoration.41 Several techniques are used for insertion of the resin cement into the root canal, with the most common being the use of a particular syringe, a lentulo-type drill, a straight-edge explorer, and the insertion of the cement with the post itself. The insertion of the resin cement with syringe-type applicators significantly reduces the area of bubbles within the cement compared with microbrush insertion.41 Another study noted the good quality of the cement layer obtained by insertion with a Centrix syringe and a lentulo drill.29 The differences between the cement patterns from insertion techniques deserve attention, because the more homogeneous the cement, the greater the intracanal post retention.41 However, there are no data in the literature about the influence of the combined effect of mechanical cycling and different insertion techniques. It is known that the deterioration of the cement by loads resulting from mechanical loading5,30,43 and hydrolytic degradation of the bond between dentin, adhesive, and cement over time37 can lead to the displacement of fiber posts. Given that the major cause of failure of glass-fiber posts is lack of adhesion, it is pertinent to evaluate the effects of different techniques of cement insertion and mechanical cycling on the bond strength of glass-fiber posts along the root canal and the effects of these techniques on the quality of the resin cement layer. The null hypotheses tested here were that (1) the tested techniques would not affect the bond strength between fiber post and root dentin and (2) mechanical cycling would not decrease the bond strength results.
MATERIALS AND METHODS Sixty-four uniradicular bovine teeth were cleaned with periodontal curettes, stored in 1.23% chlorhexidine for 2 h for disinfection, and kept in distilled water at 37°C (temperature of the oral cavity) until use. A mark was made 16 mm from the root apex and the crown was sectioned 176
by means of a diamond disk under water cooling. Roots were prepared by means of preparation burs (White Post, FGM; Joinville, Brazil) to #3 at a length of 12 mm and were embedded in acrylic resin cylinders placed in PVC tubes of 25 mm diameter, so that 2 mm of the cervical third of the root was exposed. To prevent the inclusion of bias, such as remaining filling material that could further affect the bond strength, the roots were not endodontically filled prior to post preparation. The roots were randomly divided into 4 groups (n = 16) according to the method of insertion of the resin cement for setting the glass-fiber post (post, lentulo drill, explorer, and Centrix). Fiber Post Cementation, Studied Groups All roots received glass-fiber posts (White Post DC, FGM) cemented with resin cement (Allcem, FGM; Table 1) as indicated by the manufacturer. The dentin was conditioned by 37% phosphoric acid for 15 s, and then the root canal was completely washed with water with the aid of irrigation and suction tips (NaviTip Tips and Capillary Tips, Ultradent; South Jordan, UT, USA) for 60 s. The root canals were dried with #80 absorbent paper tips, and Ambar adhesive (FGM; Table 1) was applied with a brush (Cavibrush, FGM). The cement was manipulated and inserted into the root canals according to the techniques described for each group: y POS (post): with a help of a clinic clamp, the pin was surrounded by cement along its entire length and inserted into the root canal; y LEN (lentulo drill): cement was inserted into the canal with a #40 lentulo drill (Dentsply Maillefer; Ballaigues, Switzerland) of 21 mm length mounted at low speed and kept in the channel for 3 s; y EXP (explorer): the active tip of a straight-tip #5 explorer was encased in cement and inserted from the most apical to the cervical region through circular movements; y CEN (Centrix): cement was inserted into the applicator tip, the tip was positioned in a Centrix syringe (DFL; Rio de Janeiro, Brazil), the cement was placed in the most apical area, and the application was performed in the apical-cervical direction. After insertion of the fiber post, the cement was light cured from the coronal portion for 40 s with the XL3000 curing light (3M ESPE; St Paul, MN, USA) at 600 mW/cm2 The Journal of Adhesive Dentistry
Souza et al
of intensity, and the samples were stored in distilled water at 37°C for 24 h. After the posts had been cemented, a core was built with a composite resin (shade DA2, Opallis, FGM). The resin was initially applied to the remaining cervical dentin on the posts in layers of approximately 1 to 2 mm and polymerized. Then, the composite resin was applied to an acetate matrix with standardized format, positioned over the post and dentin, resulting in standardized reconstructions. Mechanical Cycling Half of the samples (n = 8) from each cement insertion technique were subjected to cyclic loading. The samples were positioned in the base plate at a 45-degree angle to the mechanical fatigue simulator (RE 11,000 Plus, Erios; Sao Paulo, Brazil) and received 1,200,000 pulses from 10 to 100 N at a 4 Hz frequency with a stainless steel applicator of 1.6 mm diameter. Fatigue was achieved with water irrigation at 37°C, and then the samples were stored for the same time as for mechanical cycling in distilled water at 37°C. Push-out Test and Evaluation of Cement Layer Quality All specimens were then sectioned perpendicular to the long axis of the root by means of a diamond disk under water cooling (LabCut 1010, Extec; Enfield, CT, USA). The first cervical slice (approx. 1 mm) was discarded. Six slices of each sample were obtained, 3 with 1.8 mm thickness for the push-out test and 3 (cervical, medial and apical) with 0.5 mm thickness for analysis of the cement layer. The slices for the push-out test were placed in a metal device with a central hole (Ø = 3 mm), with the larger diameter of the canal facing the base. A cylindrical stainless steel tip (Ø = 0.85 mm) was positioned at the fiber post area, and the load was applied perpendicular to the long axis of the root. The test was performed in a universal testing machine (EMIC DL 1000, EMIC; Sao Jose dos Pinhais, Brazil) with a 50-kg-cell load at a speed of 1 mm/min. The evaluation of cement layer quality was performed by stereomicroscopy (Discovery V20, Zeiss; Göttingen, Germany) with a standardized magnification of 100X and classified according to the quantity and diameter of the bubbles (voids) (Table 2). Cutting, testing, and analysis steps were blinded for the operator. Data Analysis Cohesive failures verified in push-out tests were discarded and not included in data analysis, since they did not represent the interfacial bond strength. After verification of the data distribution, the results of the push-out test were subjected to parametric analysis by two-way ANOVA, and differences were detected by a simple-effects method (insertion technique or aging conditions) with Tukey’s or Sidak’s test (α = 0.05). The cement layer quality scores were analyzed by the nonparametric Kruskal-Wallis test followed by Dunn’s test (p < 0.05). Vol 17, No 2, 2015
Table 2 Scores for the cement layer quality according to the numbers and sizes of bubbles (voids) Score
Numbers and sizes of bubbles (Ø)
1
No bubbles
2
Up to 10 small bubbles (Ø ≤ 0.1 mm)
3
10 to 20 small bubbles (Ø ≤ 0.1 mm)
4
Over 20 small bubbles (Ø ≤ 0.1 mm)
5
Up to 10 small bubbles (Ø ≤ 0.1 mm) and 1 or more larger bubbles (0.11 ≤ Ø ≤ 0.5 mm)
6
11 to 20 small bubbles (Ø ≤ 0.1 mm) and 1 or more larger bubbles (0.11 ≤ Ø ≤ 0.5 mm)
7
Over 20 small bubbles (Ø ≤ 0.1 mm) and 1 or more larger bubbles (0.11 ≤ Ø ≤ 0.5 mm)
8
Large bubbles (0.51 ≤ Ø ≤ 0.6 mm)
9
Large bubbles (0.61 ≤ Ø ≤ 0.7 mm)
10
Large bubbles (Ø > 0.7 mm)
RESULTS ANOVA showed that the insertion methods for resin cement were significantly influenced by the interaction of factors (insertion techniques and aging; p < 0.0001). Mechanical cycling influenced the bond strength between fiber post and root canal for POS and CEN groups (p < 0.0001). For the baseline condition, there was no significant difference among groups, while CEN showed the highest bond strength values among mechanically cycled groups (13.6 ± 3.2 MPa, Table 3). The median scores for the quality of the cement layer (Fig 1) varied between 10 and 2.5. No statistically significant differences were found for the middle (H = 13.48, df = 7, p = 0.0612) and cervical (H = 11.67, df = 7, p = 0.1119) areas, but the quality of cement at the apical area (H = 18.54, df = 7, p = 0.0097) was influenced by the tested techniques (Fig 2).
DISCUSSION As a result of the technique used for application of the cement and polymerization shrinkage itself, gaps, voids and bubbles can be observed within the cementation interface.6 Due to the reduced root canal geometry, any controlled application of the various agents of an adhesive cementation system is difficult.33,40,41 Accordingly, the aim of this study was to investigate the quality of the cement layer, the presence of voids and their possible relationship with bond strength. For simulation of the loads and masticatory forces to which an in vivo restoration would be subjected, the mechanical cycling test was performed with 106 cycles in some of the specimens, simulating 6 months in function,22 177
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Table 3 Bond strength data in MPa as means and standard deviations (SD), number of tested specimens, and cohesive failures (in %) Cycled Mean ± SD POS
2.8 ±
Tested specimens
1.71,C
Baseline % of cohesive failure
26
15.4
Mean ± SD 9.5 ±
Tested specimens
% of cohesive failure
28
9
3.42,a
LEN
9 ± 4.3B
25
16
6.3 ± 1.9a
28
3.6
EXP
4.2 ± 1.8C
30
3.3
8 ± 4.7a
30
10
CEN
13.6 ± 3.23,A
29
3.4
9.4 ± 4.24,a
28
5
Different superscript numbers mean statistically significance differences between cycled and baseline conditions for a same insertion technique. Different superscript upper-case letters represent statistically significant differences among cycled groups. Same superscript lower-case letters mean no statistical significance difference among baseline groups. PS: post; LEN: lentulo drill; EXP: explorer; CEN: Centrix syringe.
a
b
0,5 mm
c
0,5 mm
0,5 mm
Fig 1 Representative images of scores for the cement layer quality obtained by stereomicroscopy (magnification 100X). a) score 1; b) score 5; c) score 10.
10
AB A AB
AB B APICAL
SCORE
8 6
MIDDLE
AB AB
CERVICAL
B
4 2
PO S
ba PO sel in LE S cy e N cl ba ed LE sel N ine EX cy P cle ba d EX seli CE P cy ne N cl ba ed CE sel N ine cy cle PO d Sb as PO el in LE S cy e N cl ba ed LE sel N ine EX cy P cle ba d EX seli CE P cy ne N cl ba ed CE sel N ine cy cle PO d Sb as PO el in LE S cy e N cl ba ed LE sel N ine EX cy P cle ba d EX seli CE P cy ne N cl ba ed CE sel N ine cy cle d
0
Fig 2 Boxplot of scores for the quality of the cement layer with 10th and 90th percentiles for tested groups. Different capital letters mean statistically significant differences for the apical areas among groups (p = 0.0097). Middle and cervical areas showed no statistically significant differences. For group abbreviations, see Table 3.
and no post detachment (pre-mechanical test failure) was observed after cycling. The results of this study show that there were no statistically significant differences (baseline) in values of immediate bond strength (non-aging) with different techniques, as supported by other studies.11,29 However, when the specimens were subjected to cyclic loading, some differences were observed. In this context, 178
mechanical cycling had a statistically significant detrimental impact on the POS technique (lower values after aging). For the CEN group, mechanical cycling improved the bond strength between fiber post and root dentin. Therefore, the second null hypothesis was partially accepted. It is probable that the “post” technique promoted the highest adhesive failures, due to poor insertion of cement The Journal of Adhesive Dentistry
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and the consequent formation of bubbles (voids) along the root (especially in the apical region), generating the lowest bond strength values (Table 3). Imperfections caused by these techniques reduced the ability of the cement to retain the fiber post in the root canal, which might influence the longevity of the restoration. Thus, restorations treated with these techniques appeared to suffer greater deleterious effects of mechanical stress than those treated with other techniques. Regarding the use of a lentulo drill or explorer, it can be observed that the mechanical cycling did not affect these groups. However, when compared to the other cycled groups, LEN showed lower results than CEN; EXP had even lower bond strength results and was similar to POS. Therefore, the first null hypothesis was rejected. Fakiha et al13 also observed better results of retention for the lentulo drill compared to the post technique, and D’Arcangelo et al12 stated that syringe and lentulo techniques are preferable when bonding posts. The use of an explorer is common practice in clinics, and the lentulo drill has been reported as a tool that enables a favorable distribution of the resin cement into the root canal, leading to a uniform cement layer.1,12,35,36 However, when using the lentulo drill, caution is advised to avoid premature polymerization, since this can make it impossible to achieve proper insertion of the post.1 Since various insertion techniques led to different irregularities in the cement layer, mechanical cycling had a distinct effect on the tested groups. Specimens cemented with the use of the Centrix syringe (CEN) showed a cement layer with few voids and imperfections, verified by low scores for baseline and cycling conditions, probably optimizing the bond strength values found in the study. The use of the syringe-type applicator may explain the presence of a layer relatively free of voids due to the homogeneous dispersion of the material, promoting superior adhesion of the glass-fiber post to the root canal41 and greater stability in function. In the specific case of the CEN group, which showed better scores for quality of the cement layer, the bond strength values did not suffer a decrease after mechanical cycling. This would suggest that in a bettercontrolled situation where the cement layer is more homogeneous, an increase in the cement’s degree of conversion would occur in the long term. In fact, the degree of conversion28 and hardness32 of the resin cement can increase after several months in the root canal environment. In contrast, the reduced bond strength after aging for the POS group suggests that the deleterious effect of mechanical cycling is apparent under more challenging conditions (higher score values for the quality of cement). In this case, lower bonding values after cycling can be associated with the formation of cracks in the cement layer or loss of adhesion between cement and dentin or post6 as a consequence of degradation of this layer. It must be emphasized that the indication for a particular cementation method should be based not solely on bond strength, but in conjunction with data on microleakage,27 curing depth,21 and aging.26 Vol 17, No 2, 2015
The apical region was more sensitive to cementation techniques, being the only one to present statistically different results for cement layer quality according to the insertion methods (baseline and cycled CEN groups showed superior cement quality compared with the cycled POS group; the other groups were similar). In this region, the hybrid layer formed is not homogeneous and technique sensitivity greater, due to the difficulty of etching and adhesive system application as well as to the morphological differences in apical root dentin compared with that in the middle third and cervical dentin.9 Thus, the choice of insertion method is essential not only from a mechanical standpoint, but also for an adequate apical seal of the root canal.11,12,36 The in vitro analysis of clinical situations can lead to slight procedural modifications to standardize study parameters. The use of bovine teeth has been reported in the literature in push-out studies.4,10,14,26,29 According to Schilke et al,34 the number and diameter of dentin tubules in bovine central incisors and human third molars were statistically similar. This suggests that, when standardized preparations are made, coronal dentin of bovine incisors is an adequate substitute for human dentin in adhesion studies.34
CONCLUSION Within the limitations of this study, it can be concluded that the use of the Centrix syringe improved the quality of the cement layer in all areas of the root canal, reducing the number of imperfections and promoting the stability of bond strength between the fiber post and intracanal dentin.
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Clinical relevance: The insertion technique for the resin cement can affect the quality of the cement layer and the retention of the glass-fiber post in the root canal. The use of a special syringe for resin cement insertion inside the canal root might optimize fiber post retention.
The Journal of Adhesive Dentistry
