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Effect of two fiber post types and two luting cement systems on regional post retention using the push-out test
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 372–377
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of two fiber post types and two luting cement systems on regional post retention using the push-out test Vivian J.-J. Wang a , Ya-Ming Chen a,∗ , Kevin H.-K. Yip b,∗∗ , Roger J. Smales c , Qing-Fei Meng a , Lijuan Chen a a
College of Stomatology, Nanjing Medical University, Nanjing, PR China Discipline of Family Dentistry and Endodontics, Faculty of Dentistry, Prince Philip Dental Hospital, The University of Hong Kong, 34 Hospital Road, Hong Kong, PR China c School of Dentistry, The University of Adelaide, Adelaide, South Australia, Australia b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate regional root canal push-out bond strengths for two fiber-reinforced
Received 16 November 2006
post types using two adhesive systems.
Accepted 31 May 2007
Methods. The crowns of 24 recently extracted sound maxillary central incisors were sectioned transversely 2 mm coronal to the labial cemento-enamel junction, and the roots treated endodontically. Following standardized post space preparations, fiber-reinforced
Keywords:
posts (C-POST; AESTHETI-PLUS) were placed using two adhesive systems (acid-etch ONE-
Fiber post
STEP PLUS/C&B CEMENT; self-adhesive RelyX Unicem), in four equal groups. Push-out bond
Post retention
strength tests were performed at four sites in each root. Results were analyzed using split-
Luting cement
plot ANOVA, with a = 0.05 for statistical significance.
Push-out test
Results. AESTHETI-PLUS quartz fiber-reinforced posts showed significantly higher push-out strengths than C-POST carbon fiber-reinforced posts (P < 0.0001). The separate acid-etch adhesive system resulted in significantly higher bond strengths than the self-etch selfadhesive system (P < 0.0001). Bond strengths decreased significantly from coronal to apical root canal regions (P < 0.0001). Significance. The quartz fiber-reinforced post placed using the separate acid-etch adhesive system provided significantly better post retention than the carbon fiber-reinforced post placed using the self-etch self-adhesive system. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Previously, an endodontically treated tooth was usually restored when required with a custom-fabricated cast alloy post and core or a prefabricated alloy post and resin composite core. More recently, fiber-reinforced polymer posts are advocated due to their advantages of corrosion resis-
tance, non-hypersensitivity, esthetic appeal, easier removal for endodontic re-treatment, and single visit office placement [1]. Fiber posts also more closely match the modulus of elasticity of sound root dentin, and numerous in vitro studies have shown that the posts distributed occlusal stresses more evenly in the root dentin, usually leading to fewer and more favorable root fractures, which were often reparable [2–4].
∗ Corresponding author at: Department of Prosthetic Dentistry, College of Stomatology, Nanjing Medical University, Nanjing, PR China. Tel.: +86 25 85031831; fax: +86 25 86516414. ∗∗ Corresponding author at: Discipline of Family Dentistry and Endodontics, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR, PR China. Tel.: +862 2859 0286; fax: +862 2547 0164. E-mail addresses: yaming [email protected] (Y.-M. Chen), [email protected] (K.H.-K. Yip). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.05.012
373
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 372–377
A review of fiber-based post systems placed in permanent teeth found very few published clinical studies, mainly involving one type of carbon fiber post [2]. More recent clinical studies also have involved very few types of glass and quartz fiber posts [5–8]. Most of the studies were short-term and lacked metal alloy posts as controls. Failures usually occurred from debonding or, less often, fracture of the fiber posts. Three types of tests have been used for assessing bonding to root canal dentin: the microtensile bond strength test, the push-out test, and the push-in test (which is no longer favored). The microtensile bond strength test can be used with large numbers of small beam-shaped specimens [9], but standard deviation values and premature failure rates may be high for both trimmed and non-trimmed specimens [10]. The pushout test was first advocated by Roydhouse [11], and measures the shear bond strength of relatively thick cross-sectional specimens. An early use of the push-out test in prepared root canals was to assess the shear bond strengths of a direct and indirect placement resin composite to the root canal walls [12]. ‘All-in-one’ resin-based adhesives are widely promoted to simplify the dentin bonding procedure and to reduce the time taken to bond resins to dentin. When used to bond a glass fiber-reinforced post, a simplified dual-cure selfadhesive resin luting cement requiring no pre-treatment of the root canals showed significantly lower push-out bond strengths than a total-etch adhesive system used with a dualcure resin luting cement [10]. It remains to be determined whether a similar result would be obtained when using the same self-adhesive resin luting cement with other types of fiber-reinforced posts. The objective of this in vitro study was to investigate the push-out bond strengths, in coronal and apical root canal dentin sites of endodontically treated maxillary central incisor teeth, for two fiber-reinforced post types cemented using two adhesive systems. The null hypothesis tested was that the push-out bond strengths would be unaffected by post type, adhesive system and root canal dentin site.
2.
Materials and methods
2.1.
Specimen preparation
Twenty-four sound maxillary central incisors with fully developed apices were extracted for periodontal reasons. The teeth were obtained from Chinese adults who lived in the same locality, which was without water fluoridation. The teeth were
cleaned and examined stereoscopically at 10× magnifications to verify the absence of caries and cracks, before being stored in 0.9% saline solution at 4 ◦ C for no longer than 2 weeks. Diamond disks were used to section transversely the natural crowns 2 mm coronal to the labial cemento-enamel junction. Standardized root canal preparations were completed with size four drills (Gates-Glidden, Dentsply Maillefer SA, Ballaigues, Switzerland), before placing laterally condensed gutta percha points (Dentsply International, Inc., York, PA, USA) coated with root canal sealer (AH 26, Dentsply International, Inc.) to obturate the canals. The endodontically treated roots were stored at 37 ◦ C in 0.9% saline solution for 1 week. Post-hole spaces or channels, 10 mm deep, were completed with #2 pre-shaping and finishing parallel-sided post drills (Bisco, Inc., Schaumberg, IL, USA) in a low speed handpiece, creating double-tapered cylindrical post preparations having two different sized diameters. Radiographs confirmed the removal of gutta percha apart from the presence of 3 mm remaining at the apical region of the roots. The post-holes were rinsed with 5% NaOCl and dried with paper points, until the last paper point drawn out was dry. The 24 prepared roots were assigned to four equal groups (A1, A2, B1, and B2) according to a table of random numbers. The root length, and the mesial–distal and labial–palatal diameters of each root at the root face, was measured to 0.02 mm with a vernier caliper (Vernier Caliper Model 93218-0654, Harbin Measuring and Cutting Tool Group Co. Ltd., Harbin, PR China). There were no significant differences in these dimensions among the four groups (Table 1). Prefabricated carbon fiber-reinforced posts (#2 C-POST, Bisco, Inc.) were placed in group A, and prefabricated quartz fiber-reinforced posts (#2 AESTHETI-PLUS, Bisco, Inc.) were placed in group B. In groups A1 and B1 (acid-etch), the walls of the prepared post-holes were first etched for 15 s using 32% phosphoric acid gel (UNI-ETCH, Bisco, Inc.), rinsed thoroughly with water and dried lightly with paper points. Two thin layers of lightly filled resin-based adhesive (ONE-STEP PLUS, Bisco, Inc.) were applied to the walls of the post-holes, dried carefully with paper points and oil-free air for 10 s, and light cured (Variable Intensity Polymerizer Junior, Bisco, Inc.) from a coronal direction for 20 s at 600 mW/cm2 . The fiber posts also were covered with a thin layer of the light cured adhesive. Hand-mixed self-cured resin-based luting cement (C&B CEMENT, Bisco, Inc.) was then injected into the post spaces before placement of the posts, according to the manufacturer’s instructions. In groups A2 and B2 (self-etch), the walls of the prepared post-
Table 1 – Mean dimensions (mm) of randomly assigned maxillary central incisor roots in each group Post type (N = 24)
Adhesive system group
Mesial–distal root face width
Labial–palatal root face width
Root length
Carbon fiber
A1: acid-etch A2: self-etch
7.27 (0.28)a 7.07 (0.37)
6.28 (0.24) 6.27 (0.24)
13.37 (0.86) 13.37 (0.87)
Quartz fiber
B1: acid-etch B2: self-etch
7.27 (0.35) 7.09 (0.35)
6.26 (0.50) 6.10 (0.47)
13.09 (0.51) 13.35 (1.23)
0.62
0.82
P-value (one-way ANOVA) a
Mean (standard deviation).
0.93
374
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 372–377
Fig. 2 – Effect of post type, etch procedure/cement, and root segment site on push-out bond strengths. A-E = acid-etch, S-E = self-etch.
Fig. 1 – Diagrammatic representation of the push-out test set-up.
dividing N by the area of the bonded interface: holes were rinsed thoroughly with water and dried lightly with paper points, without desiccating the dentin. Encapsulated dual-cured resin-based self-adhesive luting cement (RelyX Unicem, 3M ESPE, St. Paul, MN, USA) was mixed mechanically for 15 s and the fiber posts then coated and placed with slight vibration in the post spaces, according to the manufacturer’s instructions. All prepared roots were stored in 0.9% saline solution at 37 ◦ C for 1 week.
2.2.
MPa =
N rh, 2
where = 3.14, r is the radius of post segment (mm), and h is the thickness of post segment (mm). Each dimension was measured to 0.02 mm before testing using the vernier caliper (Model 93218-0654, Harbin Measuring and Cutting Tool Group Co. Ltd.).
2.3.
Bond strength assessment
Statistical analysis
The push-out bond strengths at the four root dentin segment sites were analyzed using a statistical software package (SPSS v11.0, SPSS Corp., Chicago, IL, USA). One-way ANOVA with Tukey’s post hoc multiple comparison tests and split-plot ANOVA were employed, with the probability level set at ˛ = 0.05 for statistical significance.
A paralleling jig was used to ensure vertical alignment of each tooth root while being embedded in a cylinder of clear self-cured acrylic resin (Shanghai Dental Materials Manufacturing Co., Shanghai, PR China). A water-cooled diamond blade (Isomet Low Speed Saw, Buehler Ltd., Lake Bluff, IL, USA) was used to section each root transversely into 4.2 mm-thick segments located coronal to the apical 3 mm of retained root canal filling. The segments were labeled S1–S4 from coronal to apical. The flat coronal dentin surface of each 2 mm-thick segment was secured face down with cyanoacrylate adhesive (Zapit, DVA, Anaheim, CA, USA) to a horizontal stainless steel platform having a central circular perforation (Fig. 1). A vertical static load was applied via a metal rod to the post section in each segment, using a universal load testing machine (MTS Synergie 100, MTS, Eden Prairie, MN, USA) at a crosshead speed of 0.5 mm/min. The maximum push-out force (N) for bond failure was recorded. The bond strength (MPa) was calculated by
3.
Results
The mean bond strengths (MPa) achieved on extrusion of the posts from the four root segment sites in each group are showed in Table 2, and illustrated in Fig. 2. Using splitplot ANOVA, irrespective of the etch procedure/cement, the AESTHETI-PLUS quartz fiber-reinforced post was significantly more retentive than the C-POST carbon fiber-reinforced post (P < 0.0001). Irrespective of the type of fiber-reinforced post, the resin-based luting cement employing a separate acid-
Table 2 – Mean bond strengths (MPa) achieved on extrusion of the posts from each root segment site in each group Post type (N = 24)
Adhesive system group
Root segment site (N = 96) S1
S2
S3
S4
Carbon fiber
A1: acid-etch A2: self-etch
12.41 (0.64) a 6.34 (0.23)
11.96 (0.77) b 5.35 (0.28)
10.40 (0.65) 4.47 (0.32)
9.80 (0.61) 4.02 (0.25)
Quartz fiber
B1: acid-etch B2: self-etch
13.05 (0.47) a 7.72 (0.49)
12.60 (0.71) b 7.43 (0.65)
11.16 (0.38) 6.24 (0.46)
10.48 (0.34) 5.51 (0.22)
<0.0001
<0.0001
<0.0001
<0.0001
P-value (one-way ANOVA)
Mean (standard deviation). Results with the same letters (a and b) are not significantly different.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 372–377
etch was significantly more retentive than the resin-based luting cement that did not employ a separate acid-etch (P < 0.0001). There was a significant correlation between post type and etch procedure/cement (P = 0.0032). Push-out bond strengths decreased from coronal (S1, S2) to apical (S3, S4) sites (P < 0.0001). This reduction in strengths was not dependent on the etching technique used.
4.
Discussion
4.1.
Tooth root selection and push-out test
The maxillary central incisor teeth were obtained from adults who lived in the same locality, and each randomized group of post space prepared tooth roots had very similar root forms and measured dimensions (Table 1). An advantage of using the push-out test method was the small standard deviation of the mean obtained for each root canal site (Table 2), with the absence of significantly different variances (Bartlett’s test), allowing small sample sizes and parametric ANOVA [10]. Though 2 mm-thick test specimens were used in the present study, 1 mm-thick test specimens have been advocated to obtain a more uniform stress distribution [10].
4.2.
Bond strength assessments
Shear bond strengths depend on the degree and stability of interfacial micromechanical interlocking and chemical adhesion between the root canal dentin, dentin bonding agent/resin-based luting cement/silane coupling agent, and fiber post. For many reasons, the initial and long-term bond strengths achieved in clinical practice may be significantly less than optimal. Different root regions show different distributions and densities of dentinal tubules. The densities decreased significantly from coronal to apical root regions [13,14], and were directly related to the bond strengths achieved in the present study and in most other [15–20], but not all [21–23], in vitro studies. Apical root dentin is a less favorable bonding substrate because of areas devoid of tubules, irregular secondary dentin, cementum-like tissue on the root canal wall and numerous accessory canals [24]. The thickness of the hybrid layer was found to be dependent on tubule density [25], and teeth restored with carbon fiber posts extracted after 6 months to 6 years of clinical service showed reduced resin infiltration of dentin in the apical root canal region [26]. A separate phosphoric acid-etch removes the thick surface smear layer on root canal dentin and the smear plugs in dentinal tubules formed during post space preparation, to allow more effective micromechanical retention of resin-based cements [17,27,28]. In the present study, the UNI-ETCH/ONE-STEP PLUS/ C&B CEMENT procedure achieved significantly higher mean bond strengths than the resin-based self-adhesive RelyX Unicem, irrespective of the type of fiber post used (Table 2). These findings support those reported previously for RelyX Unicem [10]. It is probable that the methacrylated phosphoric esters in the adhesive cannot penetrate adequately through the retained partly dissolved smear layer on the root canal walls, resulting in interfacial gaps and
375
lower bond strengths in vitro [10,28]. Reduced resin tag and hybrid layer formation have been shown in vivo for several ‘one-bottle’ adhesive systems, especially at the apical root canal region [29]. Difficulties also may be encountered with conveying sufficient primer-adhesive solution to the apical region of long narrow canals [30], together with other manipulation problems arising from inadequate root canal access and the materials involved [9,18,27]. Very high configuration factor (C-factor) stresses may be generated within long narrow post spaces because of rapid shrinkage with reduced flow relief during the polymerization of resin-based cements, leading to lower dentin bond strengths [9]. The C-factor (ratio of bonded to non-bonded surfaces) associated with the posts may exceed 200 [9], and lead to numerous interfacial gaps [27,31]. The C-factor in root canals is much higher and more complex than that in cavity preparations for resin composite restorations [32]. To overcome the reduced bond strengths to root canal dentin caused by the rapid polymerization shrinkage of thin layers of lightcure resin composite cements in particular, the use of self-cure resin cements and glass-ionomer cements has been proposed [9,33,34]. The retention of fiber posts to root canal dentin may depend largely on frictional sliding resistance to dislodgement, rather than to the relatively low micromechanical and chemical adhesion achieved by resin-based dentin bonding agents [9,33]. Bond strengths of resin composites to fiber posts are low because of the absence of chemical union between methacrylate-based resins and the epoxy resins of the posts [35]. The composition of the resin polymer matrix of the post affects the ability of resin adhesives to penetrate between the fibers and achieve adequate micromechanical adhesion [36]. The type of resin composite core material used for bonding to the fiber post also affects the microtensile bond strengths obtained, with dual-cure being preferable to light-cure composites [37], and highly filled being preferable to less-filled composites [38]. The use of silanes to improve significantly, resin bonding to fiber posts is a controversial subject [17]. For example, in one in vitro study the placement of a silane coupling agent gave the highest and most uniform bond strengths for a translucent quartz fiber post [39]. But, this procedure did not improve the bond strengths at any region of the same translucent quartz fiber post in another study [17]. However, etching the epoxy resin surfaces of this same post with hydrogen peroxide, followed by silanization of the exposed quartz fibers with the same silane and infiltration with a low-viscosity flowable resin composite, significantly enhanced the interfacial micromechanical bond strength [35]. Airborne-particle-abraded glass fiber posts were shown, after aging for 30 days and stressing, to have significantly higher retention compared with non-abraded posts in one other in vitro study [40].
5.
Conclusions
The null hypothesis proposed was not accepted. The quartz fiber-reinforced post was significantly more retentive than the carbon fiber-reinforced post; employing a separate acidetch procedure was significantly more retentive than using
376
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 372–377
a self-etch procedure; and the coronal root canal region was significantly more retentive than the apical root canal region (P < 0.0001).
Acknowledgements This study was financially supported (Nanjing Medical University A/c 1347) and undertaken by the Dental Material Research Laboratory of Dr. Ya-Ming Chen, Department of Prosthodontics, School of Stomatology, Nanjing Medical University, PR China. The unrestricted use of a financial donation from Bisco, Inc., and the financial assistance received by Dr. K.H.-K. Yip from The China Travel Fund, The University of Hong Kong, are gratefully acknowledged. We greatly appreciate the statistical advice received from Prof. Feng Chen, Department of Statistics, School of Public Health, Nanjing Medical University, and ˜ Department the assistance and advice of Prof. Jorge Perdigao, of Restorative Sciences, University of Minnesota, Minneapolis, MN, USA.
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available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of two fiber post types and two luting cement systems on regional post retention using the push-out test Vivian J.-J. Wang a , Ya-Ming Chen a,∗ , Kevin H.-K. Yip b,∗∗ , Roger J. Smales c , Qing-Fei Meng a , Lijuan Chen a a
College of Stomatology, Nanjing Medical University, Nanjing, PR China Discipline of Family Dentistry and Endodontics, Faculty of Dentistry, Prince Philip Dental Hospital, The University of Hong Kong, 34 Hospital Road, Hong Kong, PR China c School of Dentistry, The University of Adelaide, Adelaide, South Australia, Australia b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate regional root canal push-out bond strengths for two fiber-reinforced
Received 16 November 2006
post types using two adhesive systems.
Accepted 31 May 2007
Methods. The crowns of 24 recently extracted sound maxillary central incisors were sectioned transversely 2 mm coronal to the labial cemento-enamel junction, and the roots treated endodontically. Following standardized post space preparations, fiber-reinforced
Keywords:
posts (C-POST; AESTHETI-PLUS) were placed using two adhesive systems (acid-etch ONE-
Fiber post
STEP PLUS/C&B CEMENT; self-adhesive RelyX Unicem), in four equal groups. Push-out bond
Post retention
strength tests were performed at four sites in each root. Results were analyzed using split-
Luting cement
plot ANOVA, with a = 0.05 for statistical significance.
Push-out test
Results. AESTHETI-PLUS quartz fiber-reinforced posts showed significantly higher push-out strengths than C-POST carbon fiber-reinforced posts (P < 0.0001). The separate acid-etch adhesive system resulted in significantly higher bond strengths than the self-etch selfadhesive system (P < 0.0001). Bond strengths decreased significantly from coronal to apical root canal regions (P < 0.0001). Significance. The quartz fiber-reinforced post placed using the separate acid-etch adhesive system provided significantly better post retention than the carbon fiber-reinforced post placed using the self-etch self-adhesive system. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Previously, an endodontically treated tooth was usually restored when required with a custom-fabricated cast alloy post and core or a prefabricated alloy post and resin composite core. More recently, fiber-reinforced polymer posts are advocated due to their advantages of corrosion resis-
tance, non-hypersensitivity, esthetic appeal, easier removal for endodontic re-treatment, and single visit office placement [1]. Fiber posts also more closely match the modulus of elasticity of sound root dentin, and numerous in vitro studies have shown that the posts distributed occlusal stresses more evenly in the root dentin, usually leading to fewer and more favorable root fractures, which were often reparable [2–4].
∗ Corresponding author at: Department of Prosthetic Dentistry, College of Stomatology, Nanjing Medical University, Nanjing, PR China. Tel.: +86 25 85031831; fax: +86 25 86516414. ∗∗ Corresponding author at: Discipline of Family Dentistry and Endodontics, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR, PR China. Tel.: +862 2859 0286; fax: +862 2547 0164. E-mail addresses: yaming [email protected] (Y.-M. Chen), [email protected] (K.H.-K. Yip). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.05.012
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A review of fiber-based post systems placed in permanent teeth found very few published clinical studies, mainly involving one type of carbon fiber post [2]. More recent clinical studies also have involved very few types of glass and quartz fiber posts [5–8]. Most of the studies were short-term and lacked metal alloy posts as controls. Failures usually occurred from debonding or, less often, fracture of the fiber posts. Three types of tests have been used for assessing bonding to root canal dentin: the microtensile bond strength test, the push-out test, and the push-in test (which is no longer favored). The microtensile bond strength test can be used with large numbers of small beam-shaped specimens [9], but standard deviation values and premature failure rates may be high for both trimmed and non-trimmed specimens [10]. The pushout test was first advocated by Roydhouse [11], and measures the shear bond strength of relatively thick cross-sectional specimens. An early use of the push-out test in prepared root canals was to assess the shear bond strengths of a direct and indirect placement resin composite to the root canal walls [12]. ‘All-in-one’ resin-based adhesives are widely promoted to simplify the dentin bonding procedure and to reduce the time taken to bond resins to dentin. When used to bond a glass fiber-reinforced post, a simplified dual-cure selfadhesive resin luting cement requiring no pre-treatment of the root canals showed significantly lower push-out bond strengths than a total-etch adhesive system used with a dualcure resin luting cement [10]. It remains to be determined whether a similar result would be obtained when using the same self-adhesive resin luting cement with other types of fiber-reinforced posts. The objective of this in vitro study was to investigate the push-out bond strengths, in coronal and apical root canal dentin sites of endodontically treated maxillary central incisor teeth, for two fiber-reinforced post types cemented using two adhesive systems. The null hypothesis tested was that the push-out bond strengths would be unaffected by post type, adhesive system and root canal dentin site.
2.
Materials and methods
2.1.
Specimen preparation
Twenty-four sound maxillary central incisors with fully developed apices were extracted for periodontal reasons. The teeth were obtained from Chinese adults who lived in the same locality, which was without water fluoridation. The teeth were
cleaned and examined stereoscopically at 10× magnifications to verify the absence of caries and cracks, before being stored in 0.9% saline solution at 4 ◦ C for no longer than 2 weeks. Diamond disks were used to section transversely the natural crowns 2 mm coronal to the labial cemento-enamel junction. Standardized root canal preparations were completed with size four drills (Gates-Glidden, Dentsply Maillefer SA, Ballaigues, Switzerland), before placing laterally condensed gutta percha points (Dentsply International, Inc., York, PA, USA) coated with root canal sealer (AH 26, Dentsply International, Inc.) to obturate the canals. The endodontically treated roots were stored at 37 ◦ C in 0.9% saline solution for 1 week. Post-hole spaces or channels, 10 mm deep, were completed with #2 pre-shaping and finishing parallel-sided post drills (Bisco, Inc., Schaumberg, IL, USA) in a low speed handpiece, creating double-tapered cylindrical post preparations having two different sized diameters. Radiographs confirmed the removal of gutta percha apart from the presence of 3 mm remaining at the apical region of the roots. The post-holes were rinsed with 5% NaOCl and dried with paper points, until the last paper point drawn out was dry. The 24 prepared roots were assigned to four equal groups (A1, A2, B1, and B2) according to a table of random numbers. The root length, and the mesial–distal and labial–palatal diameters of each root at the root face, was measured to 0.02 mm with a vernier caliper (Vernier Caliper Model 93218-0654, Harbin Measuring and Cutting Tool Group Co. Ltd., Harbin, PR China). There were no significant differences in these dimensions among the four groups (Table 1). Prefabricated carbon fiber-reinforced posts (#2 C-POST, Bisco, Inc.) were placed in group A, and prefabricated quartz fiber-reinforced posts (#2 AESTHETI-PLUS, Bisco, Inc.) were placed in group B. In groups A1 and B1 (acid-etch), the walls of the prepared post-holes were first etched for 15 s using 32% phosphoric acid gel (UNI-ETCH, Bisco, Inc.), rinsed thoroughly with water and dried lightly with paper points. Two thin layers of lightly filled resin-based adhesive (ONE-STEP PLUS, Bisco, Inc.) were applied to the walls of the post-holes, dried carefully with paper points and oil-free air for 10 s, and light cured (Variable Intensity Polymerizer Junior, Bisco, Inc.) from a coronal direction for 20 s at 600 mW/cm2 . The fiber posts also were covered with a thin layer of the light cured adhesive. Hand-mixed self-cured resin-based luting cement (C&B CEMENT, Bisco, Inc.) was then injected into the post spaces before placement of the posts, according to the manufacturer’s instructions. In groups A2 and B2 (self-etch), the walls of the prepared post-
Table 1 – Mean dimensions (mm) of randomly assigned maxillary central incisor roots in each group Post type (N = 24)
Adhesive system group
Mesial–distal root face width
Labial–palatal root face width
Root length
Carbon fiber
A1: acid-etch A2: self-etch
7.27 (0.28)a 7.07 (0.37)
6.28 (0.24) 6.27 (0.24)
13.37 (0.86) 13.37 (0.87)
Quartz fiber
B1: acid-etch B2: self-etch
7.27 (0.35) 7.09 (0.35)
6.26 (0.50) 6.10 (0.47)
13.09 (0.51) 13.35 (1.23)
0.62
0.82
P-value (one-way ANOVA) a
Mean (standard deviation).
0.93
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Fig. 2 – Effect of post type, etch procedure/cement, and root segment site on push-out bond strengths. A-E = acid-etch, S-E = self-etch.
Fig. 1 – Diagrammatic representation of the push-out test set-up.
dividing N by the area of the bonded interface: holes were rinsed thoroughly with water and dried lightly with paper points, without desiccating the dentin. Encapsulated dual-cured resin-based self-adhesive luting cement (RelyX Unicem, 3M ESPE, St. Paul, MN, USA) was mixed mechanically for 15 s and the fiber posts then coated and placed with slight vibration in the post spaces, according to the manufacturer’s instructions. All prepared roots were stored in 0.9% saline solution at 37 ◦ C for 1 week.
2.2.
MPa =
N rh, 2
where = 3.14, r is the radius of post segment (mm), and h is the thickness of post segment (mm). Each dimension was measured to 0.02 mm before testing using the vernier caliper (Model 93218-0654, Harbin Measuring and Cutting Tool Group Co. Ltd.).
2.3.
Bond strength assessment
Statistical analysis
The push-out bond strengths at the four root dentin segment sites were analyzed using a statistical software package (SPSS v11.0, SPSS Corp., Chicago, IL, USA). One-way ANOVA with Tukey’s post hoc multiple comparison tests and split-plot ANOVA were employed, with the probability level set at ˛ = 0.05 for statistical significance.
A paralleling jig was used to ensure vertical alignment of each tooth root while being embedded in a cylinder of clear self-cured acrylic resin (Shanghai Dental Materials Manufacturing Co., Shanghai, PR China). A water-cooled diamond blade (Isomet Low Speed Saw, Buehler Ltd., Lake Bluff, IL, USA) was used to section each root transversely into 4.2 mm-thick segments located coronal to the apical 3 mm of retained root canal filling. The segments were labeled S1–S4 from coronal to apical. The flat coronal dentin surface of each 2 mm-thick segment was secured face down with cyanoacrylate adhesive (Zapit, DVA, Anaheim, CA, USA) to a horizontal stainless steel platform having a central circular perforation (Fig. 1). A vertical static load was applied via a metal rod to the post section in each segment, using a universal load testing machine (MTS Synergie 100, MTS, Eden Prairie, MN, USA) at a crosshead speed of 0.5 mm/min. The maximum push-out force (N) for bond failure was recorded. The bond strength (MPa) was calculated by
3.
Results
The mean bond strengths (MPa) achieved on extrusion of the posts from the four root segment sites in each group are showed in Table 2, and illustrated in Fig. 2. Using splitplot ANOVA, irrespective of the etch procedure/cement, the AESTHETI-PLUS quartz fiber-reinforced post was significantly more retentive than the C-POST carbon fiber-reinforced post (P < 0.0001). Irrespective of the type of fiber-reinforced post, the resin-based luting cement employing a separate acid-
Table 2 – Mean bond strengths (MPa) achieved on extrusion of the posts from each root segment site in each group Post type (N = 24)
Adhesive system group
Root segment site (N = 96) S1
S2
S3
S4
Carbon fiber
A1: acid-etch A2: self-etch
12.41 (0.64) a 6.34 (0.23)
11.96 (0.77) b 5.35 (0.28)
10.40 (0.65) 4.47 (0.32)
9.80 (0.61) 4.02 (0.25)
Quartz fiber
B1: acid-etch B2: self-etch
13.05 (0.47) a 7.72 (0.49)
12.60 (0.71) b 7.43 (0.65)
11.16 (0.38) 6.24 (0.46)
10.48 (0.34) 5.51 (0.22)
<0.0001
<0.0001
<0.0001
<0.0001
P-value (one-way ANOVA)
Mean (standard deviation). Results with the same letters (a and b) are not significantly different.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 372–377
etch was significantly more retentive than the resin-based luting cement that did not employ a separate acid-etch (P < 0.0001). There was a significant correlation between post type and etch procedure/cement (P = 0.0032). Push-out bond strengths decreased from coronal (S1, S2) to apical (S3, S4) sites (P < 0.0001). This reduction in strengths was not dependent on the etching technique used.
4.
Discussion
4.1.
Tooth root selection and push-out test
The maxillary central incisor teeth were obtained from adults who lived in the same locality, and each randomized group of post space prepared tooth roots had very similar root forms and measured dimensions (Table 1). An advantage of using the push-out test method was the small standard deviation of the mean obtained for each root canal site (Table 2), with the absence of significantly different variances (Bartlett’s test), allowing small sample sizes and parametric ANOVA [10]. Though 2 mm-thick test specimens were used in the present study, 1 mm-thick test specimens have been advocated to obtain a more uniform stress distribution [10].
4.2.
Bond strength assessments
Shear bond strengths depend on the degree and stability of interfacial micromechanical interlocking and chemical adhesion between the root canal dentin, dentin bonding agent/resin-based luting cement/silane coupling agent, and fiber post. For many reasons, the initial and long-term bond strengths achieved in clinical practice may be significantly less than optimal. Different root regions show different distributions and densities of dentinal tubules. The densities decreased significantly from coronal to apical root regions [13,14], and were directly related to the bond strengths achieved in the present study and in most other [15–20], but not all [21–23], in vitro studies. Apical root dentin is a less favorable bonding substrate because of areas devoid of tubules, irregular secondary dentin, cementum-like tissue on the root canal wall and numerous accessory canals [24]. The thickness of the hybrid layer was found to be dependent on tubule density [25], and teeth restored with carbon fiber posts extracted after 6 months to 6 years of clinical service showed reduced resin infiltration of dentin in the apical root canal region [26]. A separate phosphoric acid-etch removes the thick surface smear layer on root canal dentin and the smear plugs in dentinal tubules formed during post space preparation, to allow more effective micromechanical retention of resin-based cements [17,27,28]. In the present study, the UNI-ETCH/ONE-STEP PLUS/ C&B CEMENT procedure achieved significantly higher mean bond strengths than the resin-based self-adhesive RelyX Unicem, irrespective of the type of fiber post used (Table 2). These findings support those reported previously for RelyX Unicem [10]. It is probable that the methacrylated phosphoric esters in the adhesive cannot penetrate adequately through the retained partly dissolved smear layer on the root canal walls, resulting in interfacial gaps and
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lower bond strengths in vitro [10,28]. Reduced resin tag and hybrid layer formation have been shown in vivo for several ‘one-bottle’ adhesive systems, especially at the apical root canal region [29]. Difficulties also may be encountered with conveying sufficient primer-adhesive solution to the apical region of long narrow canals [30], together with other manipulation problems arising from inadequate root canal access and the materials involved [9,18,27]. Very high configuration factor (C-factor) stresses may be generated within long narrow post spaces because of rapid shrinkage with reduced flow relief during the polymerization of resin-based cements, leading to lower dentin bond strengths [9]. The C-factor (ratio of bonded to non-bonded surfaces) associated with the posts may exceed 200 [9], and lead to numerous interfacial gaps [27,31]. The C-factor in root canals is much higher and more complex than that in cavity preparations for resin composite restorations [32]. To overcome the reduced bond strengths to root canal dentin caused by the rapid polymerization shrinkage of thin layers of lightcure resin composite cements in particular, the use of self-cure resin cements and glass-ionomer cements has been proposed [9,33,34]. The retention of fiber posts to root canal dentin may depend largely on frictional sliding resistance to dislodgement, rather than to the relatively low micromechanical and chemical adhesion achieved by resin-based dentin bonding agents [9,33]. Bond strengths of resin composites to fiber posts are low because of the absence of chemical union between methacrylate-based resins and the epoxy resins of the posts [35]. The composition of the resin polymer matrix of the post affects the ability of resin adhesives to penetrate between the fibers and achieve adequate micromechanical adhesion [36]. The type of resin composite core material used for bonding to the fiber post also affects the microtensile bond strengths obtained, with dual-cure being preferable to light-cure composites [37], and highly filled being preferable to less-filled composites [38]. The use of silanes to improve significantly, resin bonding to fiber posts is a controversial subject [17]. For example, in one in vitro study the placement of a silane coupling agent gave the highest and most uniform bond strengths for a translucent quartz fiber post [39]. But, this procedure did not improve the bond strengths at any region of the same translucent quartz fiber post in another study [17]. However, etching the epoxy resin surfaces of this same post with hydrogen peroxide, followed by silanization of the exposed quartz fibers with the same silane and infiltration with a low-viscosity flowable resin composite, significantly enhanced the interfacial micromechanical bond strength [35]. Airborne-particle-abraded glass fiber posts were shown, after aging for 30 days and stressing, to have significantly higher retention compared with non-abraded posts in one other in vitro study [40].
5.
Conclusions
The null hypothesis proposed was not accepted. The quartz fiber-reinforced post was significantly more retentive than the carbon fiber-reinforced post; employing a separate acidetch procedure was significantly more retentive than using
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a self-etch procedure; and the coronal root canal region was significantly more retentive than the apical root canal region (P < 0.0001).
Acknowledgements This study was financially supported (Nanjing Medical University A/c 1347) and undertaken by the Dental Material Research Laboratory of Dr. Ya-Ming Chen, Department of Prosthodontics, School of Stomatology, Nanjing Medical University, PR China. The unrestricted use of a financial donation from Bisco, Inc., and the financial assistance received by Dr. K.H.-K. Yip from The China Travel Fund, The University of Hong Kong, are gratefully acknowledged. We greatly appreciate the statistical advice received from Prof. Feng Chen, Department of Statistics, School of Public Health, Nanjing Medical University, and ˜ Department the assistance and advice of Prof. Jorge Perdigao, of Restorative Sciences, University of Minnesota, Minneapolis, MN, USA.
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