Dental Traumatology 2014; 30: 270–279; doi: 10.1111/edt.12089

Restoration of endodontically treated teeth with major hard tissue loss – influence of post surface design on pull-out bond strength of fiber-reinforced composite posts Andreas Thomas Alfred Koch1, Stefanie Martina Binus1, Barbara Holzschuh1, Anselm Petschelt1, John M. Powers2, Christine Berthold1,3 1 Dental Clinic 1 – Operative Dentistry and Periodontology, Friedrich-Alexander-University, Erlangen, Germany; 2Houston Center for Biomaterials and Biomimetics, University of Texas School of Dentistry at Houston, Houston, TX, USA; 3Faculty of Dentistry, Division of Endodontics, University of British Columbia, Vancouver, BC, Canada

Key words: adhesion; surface design; failure analysis; endodontically treated teeth; bovine teeth Correspondence to: Christine Berthold, Faculty of Dentistry, Division of Endodontics, University of British Columbia, 2199 Wesbrook Mall, V6N 1Z3 Vancouver, BC, Canada Tel.: +1 (778) 990 1003 Fax: +1 (604) 822 3562 e-mails: [email protected]; [email protected] Accepted 6 November, 2013

Abstract – Aim: The aim was to evaluate the influence of post surface design and luting system on bond strength of quartz-fiber-reinforced composite posts (QFRCPs) luted to root canal dentin. Materials and methods: Single-rooted bovine teeth (n = 650) were randomly assigned (13 groups, n = 50), sectioned, endodontically treated, filled, and post space (length 8 mm) prepared. Custom-made plain-surfaced fiber posts (PSXRO) and (both RTD) macroretentive Macro-Lock Post Illusion X-RO (MLXRO) were inserted into the post spaces using six luting systems: Ketac Cem (KC), Fuji Plus (FP), RelyX Unicem, Multilink Primer_Multilink, Sealbond Ultima_CoreCem, and LuxaBond_LuxaCore Z. As control, a titanium post was cemented with KC. After water storage (24 h, 37°C), pull-out test was performed, followed by failure mode assessment. Bond strength was calculated in MPa and analyzed using ANOVA, Dunnett-T3test, and Student’s t-test with Bonferroni correction. Results: Post design and luting system significantly influenced the bond strength [MPa] (P < 0.05). Compared with the control 4.3 (1.5), all test groups exhibited higher bond strengths (P < 0.05), except for group PSXRO/KC 4.2 (1.0). The remaining bond strengths were PSXRO: FP 8.6 (1.5), RelyX Unicem 10.4 (3.4), Multilink Primer_Multilink 12.7 (3.0), SealBond Ultima_CoreCem 12.7 (3.0), LuxaBond_LuxaCore Z 15.7 (2.5), and MLXRO: KC 7.2 (2.2), FP 13.4 (2.5), RelyX Unicem 9.2 (2.9), Multilink Primer_Multilink 12.5 (4.5), SealBond Ultima_CoreCem 13.7 (4.6), LuxaBond_LuxaCore Z 20.6 (2.2). The bond strengths of MLXRO were higher than those of PSXRO when luted with KC, FP, and LuxaBond_LuxaCore Z (P < 0.05). Conclusion: The post surface design and luting system selection influenced the bond strength of conventionally and adhesively luted QFRCPs to bovine root canal dentin.

Dentoalveolar injuries often result in irreversible damage to the pulp (1–3), in combination with fracture and loss of considerable amounts of coronal hard tissue. After endodontic treatment, these teeth can be restored, either directly with resin composite buildups or indirectly with veneers or crowns (4, 5). In cases of extensive coronal hard tissue loss, additional retention for the restoration is desirable to ensure long-term success (5, 6). The insertion of a post into the root canal increases the retention area for the core buildup material. In addition, it can reinforce the thin root canal walls of immature teeth to reduce the risk of cervical root fractures, by ideally creating a monobloc between 270

the post, the root canal dentin, the luting system, and the core buildup material (5, 7–9). In the past, prefabricated or individually cast metal posts, luted with conventional zinc phosphate or glass-ionomer cement, were the gold standard for postendodontic core buildup restorations. Clinical studies demonstrated reasonable long-term success rates (10). However, aside from debonding (11), the occurrence of unfavorable vertical or horizontal root fractures over time was the main failure mode when metal or ceramic posts were used (12). The significantly different modulus of elasticity of the rigid metal (53–109 GPa) or ceramic posts (90–120 GPa) in comparison with the © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Postendodontic restoration with FRC-posts more flexible dentin (18 GPa) was discussed as a cofactor causing these fractures (13, 14). Fiber-reinforced composite posts (FRCPs), introduced approximately 20 years ago, are characterized by advantages such as esthetic appearance, ease of removal, and lack of risk for corrosion or allergies (7, 15). In addition, the need of deep and wide cavity preparation to establish primary post stabilization by friction is reduced owing to the use of adhesive luting systems for post insertion (7, 15, 16). The similar modulus of elasticity of FRCPs and dentin reduces the concentration of stress along the root dentin (17), which results in significantly reduced root fracture rates compared with metal posts (12, 18). The failure mode for teeth restored with FRCPs was more favorable (fracture line predominantly located within the core or post) than for metal posts (fractures predominantly located within the root, 19). Clinical studies of endodontically treated teeth, restored with FRCPs, found failure rates between 3.2% and 9.8% over 1–7 years (7, 20). However, long-term results (as are available for metal posts) are still needed. Conventional (zinc phosphate or glass-ionomer cement) and adhesive luting (adhesive system in combination with resin cement) can be used for post insertion. Adhesive luting systems are usually more technique sensitive than conventional luting cements (21). However, inserting posts with an adhesive luting system results in greater retention, less microleakage, and higher resistance against root fracture (7). The selection of the luting system can influence the bonding properties of FRCPs to the root canal dentin (22). However, the bond strengths obtained within the root canal are significantly lower than those obtained with coronal dentin. Depending on the location within the root canal, bond strength can vary because of differences in tubule density, width, and orientation (23). Clinical studies revealed that retention loss between the post and the tooth was the main reason for restoration failure (24). When analyzing the failure mode of adhesively luted FRCPs in vitro, the adhesion between the post and the luting system was predominantly identified as the ‘weakest link in the chain.’ (25) To improve the adhesion between the post and the luting system, different post surface pretreatments were discussed. Bond strength increased significantly when chemical pretreatment (silicatization, silanization) or mechanical pretreatment (sandblasting or etching) was used to enhance the surface (26). The introduction of macroretentive post designs is another approach to improve the retention between the post surface and the luting system (25). The aim of this study was to evaluate the bond strength of adhesively and conventionally luted FRCPs with two different surface designs and a conventionally luted titanium post. The following hypotheses were tested: (1) the post surface design does not influence the bond strength, (2) the selection of the luting system does not influence the bond strength, and (3) the bond strengths of the FRCPs do not differ with respect to the titanium post (control). © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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Materials and methods Specimen preparation and endodontic treatment

A total of 650 freshly extracted bovine deciduous front teeth (Di3) with fully developed apices were selected. External debris was removed using a hand scaler (Cattoni #107, HuFriedy, Rotterdam, the Netherlands), and radiographs were taken to eliminate specimens with irregular or oval-shaped root canals. The teeth were first stored in 0.5% Chloramine-T solution (1 week, 8°C) and then in deionized water (4 weeks maximum, 8°C). The crowns were removed at the cemento–enamel junction (root length 17 mm) using a diamond disk (947D, Hager & Meisinger, Neuss, Germany) with water spray cooling. The endodontic treatment and sectional root canal filling were carried out according to the protocol previously described by our group (5). Post insertion

Depending on the luting system (Table 1) and post type (Table 2), the specimens (n = 650) were randomly assigned (Fig. 1) to six main test groups (n = 100) according to the luting systems [KC, Fuji Plus (FP), RelyX Unicem (RXU), MLP_ML, SBU_CC, LB_LCZ] and then divided into two subgroups each (n = 50) according to the fiber post design [Plane surface XRO (PSXRO), Macro-Lock Post Illusion X-RO (MLXRO)]. Fifty teeth were assigned to the control group (Fig. 1) KC_co/TiP. The post spaces were prepared, and the posts were luted following the exact protocol which was previously described in detail (5). Bond strength testing

After embedding the specimens parallel to the long axis of the post (5), the pull-out tests were performed using a universal testing machine (Z2.5/TN1S, Zwick, Ulm, Germany, cross-head speed 5 mm min 1) and a custom-made jig (University mechanics shop, Erlangen, Germany). The post excess was inserted into a drill chuck while the embedded part was placed under a clamp (Fig. 2). The tensile force required for post dislodgement was recorded in Newtons (N). The bond strengths in MPa were calculated using the following equation: bond strength (MPa) = tensile force (N)/ bonding surface (mm2). Subsequently, failure mode analysis was performed for all groups, as described previously (5). Five failure types were defined: within the dentin, between the dentin and the luting system, within the luting system, between the luting system and the post, and within the post. Statistical analysis

All data were analyzed using the Statistical Package for Social Sciences program 19.0 (IBM Corp., Armonk, New York, NY, USA).

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Table 1. Characteristics of luting systems used in the study Luting system type

Luting system Company TM

Composition

LOT (Expiration date)

Chemical

Glass powder, pigments, polyethylene polycarbonic acid, tartaric acid, water, conservation acid (a) Citric acid (10%), distilled water (87%), iron(III) chloride (3%) (b) Aluminosilicate glass, polyacrylic acid, hydroxyethyl-methacrylate, urethanedimethacrylate, water Methacrylated phosphoric acid esters, triethyleneglycoldimehtacrylate, substituted dimethacrylate (a) Water, phosporic acid acrylate, HEMA, polyacrylic acid-modified methacrylate resin (b) Dimethacrylate, HEMA, barium glass, ytterbium triflouride, spheroid mixed oxide (a) Phosphoric acid (32%) (b) Acetone, biphenyldimethacrylate (c) Bis-GMA, silica, barium glass fillers, stabilizers, initiators (a) Phosphoric acid (37%) (b) PreBond: ethanol arylsulfinate solution; Primer A: bis-GMA, catalyst; Primer B: bis-GMA, benzoyl peroxide (c) Barium glass, pyrogenic silicic acid, nano fillers, zirconium oxide, bis-GMA

376840 (2012-09)

Conventional glassionomer cement (CGIC)

KC

Resin-reinforced glassionomer cement

Fuji Plus (FP)

Self-adhesive resin cement

RelyX Unicem

RelyXTM Unicem Aplicap 3M ESPE

Dual

Self-conditioning adhesive + resin cement (SCA+RC)

MLP_ML

(a) Multilinkâ Primer A&B (b) Multilinkâ Automix Ivoclar Vivadent, Schaan, Liechtenstein

Dual

Etch and rinse adhesive + resin cement (ERA+RC)

SBU_CC

(a) SealBond II Etching (b) SealBond UltimaTM (c) CoreCemTM RTD, St. Egr e ve,France (a) Etching Gelâ (b) LuxaBondâ (c) LuxaCoreâ Z Dual DMG, Hamburg, Germany

Light Dual

LB_LCZ

Ketac Cem Aplicap 3M ESPE, Seefeld, Germany (a) Fuji PlusTM Conditioner (b) Fuji PlusTM Capsule GC Europe, Leuven, Belgium

Curing mode

Chemical

Dual

0906241 (2011-06) 0907221 (2011-07)

375832 (2011-04)

M36892; M42208 (2011-12) M41970 (2012-01)

115910908 (2001-05) 0900005586 (2011-04) 7906605 (2011-07) 626113 (2010-11) 624741 (2010-11) 624696 (2011-06)

Table 2. Characteristics of the posts used in the study Post type

Post Company

Composition

Surface roughness

Diameter

LOT

Titanium post

TiP

Custom-made titanium post size 3 NTI, Kahla Germany

Titanium

Rz 5.38 Ra 0.79

Tip 1.1 mm end 2.2 mm

U10.001

Fiber-reinforced composite post

Plane surface XRO

Custom-made quartz-fiberreinforced composite post plane surface size 6 RTD, St. Egreve, France

Quartz Stretched fibers Epoxy resin

Rz 5.48 Ra 0.82

Tip 1.3 mm end 2.2 mm

119390910

Fiber-reinforced composite post

Macro-Lock Post Illusion X-RO

Macro-LockTM Post IllusionTM size 6 RTD, St. Egreve, France

Quartz Stretched fibers Epoxy resin

Rz 11.58 Ra 2.06

Tip 1.3 mm end 2.2 mm

116900909

Descriptive analysis was performed. The bond strengths in MPa were graphically displayed in boxplots. The results of the failure analysis were processed and shown in stacked bar charts. The general level of significance was set at a = 0.05. Mean values of bond strength were tested for normal distribution using the Kolmogorov–Smirnov test (KST). Normally, distributed data were evaluated with parametric tests. Equality of variances was tested using Levene’s test. To test the influence of the post design and the luting system in general, ANOVA/Welch was performed, followed by the Dunnet-T3 post hoc test (DT3). When comparing the posts within one luting system, t-test/Welch (TT) was applied. For multiple testing, the

Picture

Bonferroni–Holm procedure (BHP; a′ = a/number of tests) was used to offset the a-error accumulation. Results

Because the bond strength values of each group were normally distributed (KST, P > 0.05), parametric tests were used for statistical analysis. General influence of the post surface design

The post design significantly influenced the bond strength (ANOVA/Welch, P < 0.001; Fig. 3a). Pairwise comparison (post hoc test) of the bond strength © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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Fig. 1. Group classification with respect to luting system types, luting systems, and post types; KC_co, control (Ketac Cem with titanium post); KC, Ketac Cem; FP, Fuji Plus; RXU, RelyX Unicem; MLP_ML, Multilink Primer and Multilink; SBU_CC, SealBond Ultima and CoreCem; LB_LCZ, LuxaBond and LuxaCore Z; TiP, Titan Post; PSXRO, Plane surface XRO; MLXRO, Macro-Lock XRO.

(approximately 80% of all failures; Fig. 3b). In comparison, for the MLXRO samples, approximately 50% of failures were observed at the interface between the dentin and the luting system, and 40% of failures occurred at the interface between the post and the luting system (Fig. 3b). General influence of the luting system

Fig. 2. Schematic illustration of the pull-out test design.

between the three posts revealed statistically significant differences for all comparisons (DT3, P < 0.001: MLXRO: 12.8 (5.3) MPa > PSXRO: 10.7 (4.4) MPa > TiP 5.4 (1.9) MPa). The predominant failure for PSXRO and TiP occurred between the luting system and the post © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Overall, the luting system significantly influenced the bond strength (ANOVA/Welch; P < 0.001; Fig. 4a). Pairwise comparison using the Dunnett-T3 post hoc test revealed statistically significant differences when comparing the control group with all test groups (Fig. 4a; DT3, P ≤ 0.001). The pairwise comparisons of all other luting system groups revealed statistically significant differences for almost all groups (Fig. 4a; DT3, P < 0.05). No differences were observed when comparing RelyX Unicem (RXU) vs FP (Fig. 4a; DT3, P = 0.124) and SBU_CC vs MLP_ML (Fig. 4a; DT3; P = 0.999). Data regarding the failure modes per luting system are summarized in Fig. 4b. For the KC_co, KC, FP and LB_LCZ groups, the main failure mode was detected between the luting system and the post; the LB_LCZ group also exhibited failure within the post (approximately 10%). RXU failed predominantly at the interface between the luting system and the dentin (approximately 80%). MLP_ML and SBU_CC exhibited similar failure modes, with an equal distribution of failure at the dentin–luting system (40–50%) and the luting system–post interface (50–60%). Influence of post surface design within a single luting system

The bond strengths for the two post designs (PSXRO and MLXRO), generated within a single luting system group, were compared using TT in combination with the BHP (a’ = 0.05/6 = 0.008). No statistically

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(a)

(b)

Fig. 3. (a) Pull-out bond strength (MPa) of the different post types, independent of the luting systems. **Statistically significant difference (DT3, P < 0.001). (b) Failure mode analysis (%) of conventionally and adhesively luted posts to bovine root canal dentin, depending on the post type. TiP, Titan post; PSXRO, Plane surface XRO; MLXRO, Macro-Lock XRO.

(a)

(b)

Fig. 4. (a) Pull-out bond strength (MPa) of conventionally and adhesively luted posts, independent of the different post types. Φ no statistically significant difference (DT3, P > 0.05). (b) Failure mode analysis (%) of conventionally and adhesively luted posts to bovine root canal dentin, depending on the luting system type. KC_co, control (Ketac Cem with titanium post); KC, Ketac Cem; FP, Fuji Plus; RXU, RelyX Unicem; MLP_ML, Multilink Primer and Multilink; SBU_CC, SealBond Ultima and CoreCem; LB_LCZ, LuxaBond and LuxaCore Z.

significant differences between PSXRO and MLXRO were observed for RXU (TT/BHP, P = 0.051), MLP_ML (TT/BHP, P = 0.813), or SBU_CC (TT/ BHP, P = 0.190; Fig. 5a). For all other luting systems (KC, FP, and LB_LCZ), statistically significant differences were detected when comparing PSXRO and MLXRO (TT/BHP, P < 0.008; Fig. 5a). Data regarding the failure mode analysis of conventionally and adhesively luted posts to bovine root canal dentin are summarized in Fig. 5b. When comparing the failure patterns of the two FRCP surface designs per luting system, we observed that for all luting system groups, the failure rate at the interface between the luting system and the post was greater in combination with PSXRO than in combination with MLXRO. An exception was observed for LB_LCZ, in which the

main failure was detected between the luting system and the post for both PSXRO and MLXRO. In addition, for MLXRO, internal fractures within the post could be seen in up to 18% of failures. Influence of the luting system within a single post surface design

The bond strengths of different luting systems were analyzed within a single post design. The luting system selection significantly influenced the bond strength within PSXRO (ANOVA/Welch, P < 0.001) and MLXRO (ANOVA/Welch, P < 0.001; Fig. 5a). The pairwise comparison of all luting systems, used to insert PSXRO, with the control group, revealed statistically significant differences (Fig. 5a; DT3, © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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(a)

(b)

Fig. 5. (a) Pull-out bond strength (MPa) of conventionally and adhesively luted posts splits into two parts. The upper half shows the statistically significant differences between the post types with regard to the luting system. **Statistically significant difference (TT and Bonferroni–Holm procedure, P < 0.008). The lower half highlights the luting systems with no significant difference (Φ; DT3, P > 0.05) with regard to a single post type. (b) Failure mode analysis (%) of conventionally and adhesively luted posts to bovine root canal dentin, depending on the different post types and luting systems., KC_co, control (Ketac Cem with titanium post); KC, Ketac Cem; FP, Fuji Plus; RXU, RelyX Unicem; MLP_ML, Multilink Primer and Multilink; SBU_CC, SealBond Ultima and CoreCem; PSXRO, Plane surface XRO; MLXRO, Macro-Lock XRO.

P < 0.001), except when comparing the control group KC_co/Tip with KC/PSXRO (Fig. 5a; DT3, P = 1.000). The comparison of all other groups revealed statistically significant differences (Fig. 5a; DT3, P < 0.001), except for SBU_CC/PSXRO compared with MLP_ML/PSXRO (Fig. 5a; DT3, P = 1.000). When comparing the control group with all other systems used for luting MLXRO, significant differences were detected (Fig. 5a; DT3, P < 0.001). The pairwise comparison of all other groups revealed significant differences (Fig. 5a; DT3, P < 0.05), except when comparing MLP_ML/MLXRO with FP/MLXRO, MLP_ML/ MLXRO with SBU_CC/MLXRO, and SBU_CC/ © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

MLXRO with FP/MLXRO (Fig. 5a; DT3, P> 0.05 for all three comparisons). Failure mode analysis data are summarized in Fig. 5b. Generally, the main failure, when using PSXRO, occurred between the luting system and the post. The lone exception was RXU, which failed predominantly between the dentin and the luting system. In contrast, for MLXRO in combination with RXU, MLP_ML, and SBU_CC, the main failure occurred at the dentin–luting system interface. The failure mode of KC, FP, and LB_LCZ in combination with MLXRO was primarily between the luting system and the post.

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Discussion Methodological factors

The use of bovine teeth as a substrate as well as the methodological factors of the endodontic procedure was discussed in-depth in a previous paper from our group (5). In vivo studies describe post debonding as the main failure mode of postendodontic indirect restorations (24, 27). Regarding the bonding, the interface between the post and the luting system appears to be the ‘weakest link’ of the restoration (27). The resin matrix of prefabricated FRCPs is usually cured, and the post surface is machine-processed. Therefore, carbon double bonds that can be broken by free radicals, generated during polymerization of the adhesive luting systems, are scarcely available, and the compound between the post and the luting system is mainly based on mechanical retention (28). There are different approaches for improving the bonding properties to the post surface: silicatization (29) and/or silanization (30) can be performed to establish chemical bonding, and etching (26) and sandblasting (26) can be performed to improve mechanical bonding by increasing the surface roughness. In this study, we tested another approach to improve the mechanical bonding by introducing a macroretentive post design. The FRCPs used in this investigation provide superior mechanical properties (31). The individually manufactured PSXRO (plain surface design; RTD) and the commercially available MLXRO (macroretentive surface design; MACRO-LOCKTM POST ILLUSIONTM X-ROâ, RTD) feature similar post shape, material composition, mechanical properties, and surface roughness (Ra 0.8). Therefore, these parameters can be excluded as influencing factors on the bond strength results. The macroretentions of the MLXRO are spiral-shaped (Table 2), which results in superior mechanical stability and a higher cohesion of the quartz fibers within the post, in contrast to other posts in which the macroretentions are aligned perpendicular to the post axis and the fiber orientation. A titanium post with similar surface roughness, as the FRCPs (Ra 0.8), was selected as a control because postendodontic restoration with conventionally cemented metal posts was the gold standard for decades (10, 32). Chemical- or dual-curing luting systems can generally be recommended for post insertion (33, 34). In this study, the influence of the post surface design should be tested for different luting system types. Therefore, several luting systems with varying bonding mechanisms were selected with respect to their material categorization and increasing complexity: conventional glass-ionomer cement (CGIC), resin-reinforced glassionomer cement (RRGIC), self-adhesive resin cement (SARC), self-conditioning adhesive with resin cement (SCA + RC), and etch and rinse adhesive with resin cement (ERA+RC). Based on the results of a previous study (5), one luting system with superior bonding properties was selected from each luting system category. For decades, CGIC or zinc phosphate cement was successfully used in the clinic to insert metal posts (24, 32, 35). Therefore, a CGIC (Ketac Cem) was

selected for inserting the titanium post of the control group as well as for inserting the two FRCPs with the different surface design. Push-out, pull-out, and micro-tensile tests are standard methods with which to evaluate the bond strength of endodontic posts. The push-out test favors stress uniformity and allows the determination of regional differences in the root canal dentin. However, vibrations during the sectioning procedure can alter the compound and negatively influence the bond strength, especially when testing luting systems with low bond strength. The pull-out test is a simple and quick alternative for testing higher specimen amounts. In addition, the influence of post length and surface design can be evaluated. In this study, the pull-out method was performed to evaluate the influence of the macroretentive surface design of the MLXRO. A detailed failure mode analysis of the bonding interface (post/luting system and dentin/luting system) was possible by sectioning and separating the roots longitudinally, followed by a microscopic evaluation of the root halves and the post (5). Study outcome

Within the limits of this study, the tested hypotheses must be rejected. Both the post-surface design and the luting system influenced the bond strength. With the exception of KC/PSXRO, the bond strengths of the FRCPs in combination with all tested luting systems were significantly higher than that of the conventionally cemented titanium post (control). Post surface design

In general, the macroretentive MLXRO reached statistically significant higher bond strength compared with the plain surface post PSXRO (Fig. 3a). The enlarged post surface and mechanical retention of the MLXRO were beneficial regarding the bonding between the post surface and the luting system, as indicated by the reduced failure rate at the post/luting system interface when comparing MLXRO and PSXRO (Fig. 3b). These findings could be explained based on the production process of the FRCPs. The resin matrix of the FRCP is polymerized during manufacturing, the raw material is laced into the desired post shape, and no or only few free reactive double bonds are available on the post surface for chemical binding (5). Based on the findings of our study, the introduction of a macroretentive surface design seems to be a possible approach to increase the overall bonding performance. When comparing the two different post designs, per luting system, it was found that the bond strength of MLXRO, inserted with cement-based luting systems (KC and FP) or an etch and rinse adhesive in combination with a resin-based luting system (LB_LCZ), was significantly higher than for PSXRO (Fig. 5a). Therefore, for these luting systems, the macroretention improved the overall bonding properties. For all other luting systems (RXU, MLP_ML, and SBU_CC), no statistically significant differences in bond strength were observed. However, failure at the post/luting system © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

Postendodontic restoration with FRC-posts interface was considerably reduced when the MLXRO was used (Fig. 4b). Luting system

The selection of the luting system significantly influenced the bond strength (Fig. 4a) and the failure mode (Fig. 4b). In general, the adhesive resin-based luting systems (MLP-ML, SBU_CC, and LB_LCZ) generated greater bond strength than the cement-based materials (KC and FP). This finding is in accordance with the literature (33). The FRCPs luted with CGIC (KC) achieved significantly greater bond strength than the titanium post luted with Ketac cem (KC_co), influenced by the improved interface between the post and the luting system when using MLXRO (Fig. 5a). The RRGIC (FP) achieved significantly greater bond strength than the CGIC (KC). The reason for this may be the better mechanical properties of RRGIC (36) and the conditioning of the dentin with a mild acid (iron (III) chloride) to remove the smear layer, which allows direct chemical bonding of the functional groups of the glass-ionomer cement to the calcium ions of the dentin (37, 38). Especially in combination with MLXRO, FP generated greater bond strength than the resin-based SARC (RXU). For SARC, no additional adhesive system is used to condition the dentin, and the smear layer remains at the post cavity surface (39). The high failure rate at the dentin/luting system interface indicates reduced bonding ability of the luting system to the dentin (Figs 4b and 5b). Significant differences were detected between the SARC (RXU) and the SCA+RC (MLP_ML), which may be explained by the additional dentin-conditioning step when using SCA + RC. RXU achieved lesser bond strength than the multistep adhesive systems and greater bond strength than CGIC (KC). Ebert et al. (40) observed similar results for CGIC compared with RXU. In contrast to our results when comparing multistep adhesives with RXU, Ebert et al. (40) observed no significant differences. These discrepancies may be explained by the use of different dentinal substrates (bovine vs human; 41, 42). The SCA + RC (MLP_ML) and ERA + RS (SBU_CC and LB_LCZ) achieved significantly greater results than the cement-based luting systems [CGIC (KC) and RRGIC (FP)] and the SARC (RXU). The adhesive systems, used as coupling agents between the resin cement and the dentinal structures, enhance bonding at the dentin/luting system interface (28). When comparing luting systems that include a separate adhesive system (MLP_ML, SBU_CC, and LB_LCZ), no statistically significant differences were observed between the SCA + RC (MLP_ML) and the ERA + RC (SBU_CC), although statistically significant differences were detected between the two ERA + RC (SBU_CC and LB_LCZ). In contrast to all other luting systems used in this study, the SBU is a light-cured adhesive system. After the application of SBU to the post cavity dentin, the material was light-cured before post insertion. However, we suspect that the slightly lower bond strength of SBU_CC compared with LB_LCZ may be partly explained by incomplete polymerization of the adhesive SBU in the apical post © 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd

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cavity segment (43). For MLP_ML and SBU_CC, the failure mode differed distinctly when comparing PSXRO (main failure between the post and the luting system) and MLXRO (main failure between the dentin and the luting system) because of the enhanced macromechanical retention of the luting materials to MLXRO. In this study, exceptionally high bond strengths and low standard deviations were observed when LB_LCZ was used in combination with MLXRO. In this group, the relatively high rate of fractures within the post (~18%) indicates that the bonding at the interfaces is in some cases even higher than the compound of the fibers and the matrix within the post. However, the results for the SCA + RC (MLP_ML) and the ERA+RC (SBU_CC and LB_LCZ), evaluated on bovine dentin, should be cautiously interpreted and cannot indiscriminately be transferred to the clinical situation with human dentin (41). Studies on human dentin have found no statistically significant differences between RXU and LB_LCZ. Clinically, it is desirable to use simplified luting systems because they reduce time and material effort and may incur fewer application errors. However, when core buildup is required, RXU is not indicated according to the manufacturer’s instructions, and the use of materials such as SBU_CC or LB_LCZ can be recommended to create a monobloc within the post and the core buildup (8). The comparison of all luting systems to the control revealed statistically significant differences. The conventionally luted titanium post achieved the lowest bond strength compared with all other groups, except for PSXRO luted with KC. In the clinic, conventionally luted metal posts have been successfully used to restore endodontically treated teeth with extensive coronal hard tissue loss (7, 44). Therefore, regarding the bonding, it can be hypothesized that FRCPs luted with RRGIC or resin-based luting systems should reduce restoration failure by post debonding. Long-term in vivo studies must be conducted to prove this hypothesis under clinical circumstances. Conclusions

Within the limits of this in vitro study on bovine teeth, it can be concluded that the post surface design significantly influences the bond strength of conventionally and adhesively luted FRCPs. The macroretentive surface design of the MLXRO increases the bond strength significantly when using KC, FP, or LB_LCZ. The selection of the luting system also influences the bond strength. Etch and rinse adhesive systems in combination with resin cements generated the highest bond strength values. The conventionally and adhesively luted FRCPs achieved greater bond strength values compared with the conventionally luted titanium post of the control group. Acknowledgements

The invaluable help of Hans Loew of the dental workshop, University Erlangen, during the jig and mold

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manufacturing procedure is highly acknowledged. We want to thank Sergej Potapov of the FAU Institute for Medical Informatics, Biometry and Epidemiology, Erlangen, Germany for the valuable statistical advice. We highly appreciate the material support of 3M ESPE, DMG, GC Europe, Ivoclar Vivadent, NTI, RTD, and VOCO for this study.

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