J Huazhong Univ Sci Technol [Med Sci] 34(1):108-113,2014 DOI 10.1007/s11596-014-1240-1 J Huazhong Univ Sci Technol [Med Sci] 34(1):2014

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Push-out Bond Strength of Self-adhesive Methacrylate Resin-based Sealers to Root Dentin Yan SUN (孙 燕), Yu-hong LI (李宇红), Ming-wen FAN (樊明文)# Key Laboratory for Oral Biomedical Engineering of Ministry of Education, School and Hospital of Stomatology, Wuhan University, Wuhan 430079, China © Huazhong University of Science and Technology and Springer-Verlag Berlin Heidelberg 2014

Summary: This study examined the adhesive strength of two self-adhesive methacrylate resin-based sealers (MetaSEAL and RealSeal SE) to root dentin and compared them with RealSeal and AH Plus in properties. A total of 48 extracted human single-rooted teeth were used to prepare the 0.9-mm thick longitudinal tooth slice (each per tooth). Standardized simulated canal spaces of uniform dimensions were prepared in the middle of radicular dentin. After treated with 5.25% sodium hypochlorite (NaOCl) and 17% EDTA, tooth slices were allocated randomly to four groups (n=12) in terms of different sealers used: MetaSEAL, RealSeal SE, RealSeal, and AH plus groups. The simulated canal spaces were obturated with different sealers in each group. There were 10 slabs with 20 simulated canal spaces (n=20) used in each group for push-out testing. The failure modes and the ultrastructures of fractured sealer-dentin interfaces were examined. The remaining 2 slabs in each group underwent partial demineralization for observation of the ultrastructure of resin tags. The results showed that the push-out bond strength was 12.01±4.66 MPa in MetaSEAL group, significantly higher than that in the other three groups (P0.05). Mixed failures were predominant in the fractured sealer-dentin interfaces in MetaSEAL and AH Plus groups, while adhesive failures were frequently seen in RealSeal SE and RealSeal groups. In conclusion, after complete removal of the smear layer, MetaSEAL showed superior bond ability to root dentin. The RealSeal SE is applicable in clinical practice, with its adhesive strength similar to that of AH Plus. The self-adhesive methacrylate resin-based sealer holds promise for use in endodontic treatment. Key words: endodontic sealer; push-out bond strength; adhesive interface; raidicular dentin

The objective of endodontic obturation is to achieve three-dimension filling in root canal system, so as to hermetically seal the root canal from the periapical tissues to prevent reinfection and promote the healing of periapical diseases[1]. Gutta-percha is commonly used with various endodontic sealers to obturate root canal system in clinical practice. Recently, Shipper et al[2] proposed that a solid monoblock should be created between the obtruation material and root canal wall to achieve and maintain a hermetical seal. However, conventional endodontic sealers bond to neither gutta-percha cone nor root dentin. Moreover, some sealers are not dimensionally stable after setting or they will dissolve partially over time, and therefore gaps develop among Gutta-percha, sealer and dentin walls, resulting in the microleakage and the failure of endodontic treatment[3]. In recent years, with the technological innovations in restorative dentistry, attemps are made to apply the resin-based obturating materials with bondabiltiy to endodontic treatment. Resilon, a thermoplastic polycaprolactone polymer-based solid root filling material with bonding capaYan SUN, E-mail: [email protected] # Corresponding author, E-mail: [email protected]

bility, has been used in clinical setting and is being considered to replace gutta-percha. It does not bond to traditional endodontic sealers, but its methacryloyl groups can combine to methacrylate resin-based sealers[4]. RealSeal is one of the earliest developed methacrylate-based sealers coupled with Resilon, but Cecchin et al[5] found that its adhesive property was not superior to some traditional sealers. RealSeal SE and MetaSEAL, the latest methacrylate resin-based sealers with self-adhesive properties, have recently been introduced to the dental market. They can creat a bond to both dentin and Resilon so that a solid “monoblock” will be produced within the root canal, where core material, endodontic sealer, and root canal dentin form a single cohesive unit to reduce microleakage[6] and provide greater fracture resistance to an endodontically treated tooth[7, 8]. The self-adhesive resin-based root canal sealers, analogous to self-adhesive resin luting cements, can eliminate the use of a separate self-etching primer. The compound of acid resin monomers decalcifies dentin substrates, and meanwhile flowable resin sealer infiltrates into dentine matrix and polymerizes to form a hybrid layer, which offers micromechanical retention. These materials fulfil acid etching and bonding in one step, making the application of these materials easier and more convenient. RealSeal SE is an

J Huazhong Univ Sci Technol [Med Sci] 34(1):2014

evolution of RealSeal (a self-etching methacrylate resin-based sealer). The new compound, 2-hydroxyethyl methacrylate (HEMA), which is highly hydrophilic, is a substitution for urethane dimethacrylate monomer (UDMA). New ingredient, acidic methacrylate resins, with self-etch property renders RealSeal SE etch and adhere to dentin substrates in one step. MetaSEAL is another commercialized sealer. 4-methacryloyloxyethyl trimellitate anhydride (4-META) is the key factor of MetaSEAL for self-adhension[9]. The acidic monomer 4-META consists of hydrophobic and hydrophilic groups, which can enhance the infiltration of monomers into demineralized surface and dentinal collgen fiber mesh to promote the formation of the hybrid layer[10, 11]. Patil[12], De-Deus[13], Costa[14] and Carneiro[15] evaluated the adhesion ability of RealSeal SE and MetaSEAL to radicular dentin via thin-slice push-out test. In their researches, the specimen was prepared by slicing root canals filled with sealers and core materials. By reviewing the results, we found that the adhesion failure mainly occured in the interface of sealer-core materials due to the weak interface bond between the core material and the sealer[16], which can not reflect the real bonding behavior between the sealer and root dentin. In this study, we prepared artificial standardized root canals in radicular dentine and filled the canals spaces with sealers solo in order to evaluate the push-out bond strength to radicular dentine produced by MetaSEAL and RealSeal SE versus that of RealSeal and AH Plus (a epoxy resin-based sealer). Furthermore, the morphology of root canal sealer-dentin interfaces and resin tags was also examined by scanning electon microscopy (SEM). 1 MATERIAL AND METHODS 1.1 Sample Collection The human single root teeth exacted for orthodontic reason were collected after the informed consents of patients were obtained. All samples were examined with naked eyes and by stereomicroscopy (Stemi 2000-C, Carl Zeiss Jena GmbH, Germany) at 12× magnification and X-ray scanning. The exclusion criteria for the teeth were as follows: (1) cracks on dentine; (2) multiple root canals; (3) previous endodontic treatment; (4) root surface caries or restorations. A total of 48 teeth were harvested based on the critreia. After removal of soft tissue and dental calculus, the teeth were stored in an aqueous solution of 1% chloramine-T at 4°C to inhibit bacterial growth. The period of preservation of the samples was within 3 months. 1.2 Root Canal Preparation A longitudinal tooth slice 0.90±0.05 mm in thickness was prepared from each tooth with an Isomet saw (Buehler Ltd., Evanston, IL, USA) under water cooling. The slices were polished with 1000-grit SiC carbide papers under running water. Two small holes perpendicular to the slice were carefully prepared in the middle of radicular dentine by a bur 0.7 mm in diameter. The distance from the hole to the cementum and to the canal wall was almost equal. Subsequently, the hole was enlarged using a size 40 0.04 taper ProFile nickel titanium rotary instrument (Dentsply Tulsa Dental Special-

109 ties, USA) under water cooling. Finally, the dimensionally identical, vertically oriented truncated artificial cavities were obtained. Its top was 1.04 mm in diameter and the bottom was 0.94 mm[17]. 1.3 Sealer Application Forty-eight teeth slices (1 mm in thickness) were randomly assigned to four groups in terms of different endodontic sealers: MetaSEAL (Parkell, Inc, USA), RealSeal SE (Sybron Dental Specialties, Inc, USA), RealSeal (Sybron Dental Specialties, Inc, USA), and AH Plus (DENTSPLY Maillefer, USA). For each group, 20 sealer-filled canal spaces and 10 slices were prepared for a thin-slice push-out test and the remaining two slices for the SEM evaluation of resin tags. Tooth slices with artificial canals were ultrasonicated in 5.25% sodium hypochlorite solution, 17% EDTA solution and distilled water sequentially for 2 min each to remove smear layers and organic debris. The canal spaces were dried with the tip of the paper. The dried slices were placed on the glass slides for sealer filling. The sealers were mixed according to the manufacturer’s instructions: MetaSEAL was mixed in the ratio of 3 drops of liquid to one level scoop of powder; AH Plus was mixed with equal volume of epoxide paste and amine paste; RealSeal and RealSeal SE were mixed separately using the auto-mix syringe tip. In RealSeal group, primer was applied on the simulated canal walls previously to the sealer filling. The mixed sealers were dispensed into dimensionally identical, simulated canal spaces until each hole was filled with excess sealer. The surface of the tooth slice was then covered with another glass slide. Because the remains of oxygen could inhibit polymerization of free radical and affect the setting of the sealer, the assembly was secured with binder clips so that excess sealer and air bubbles were excluded. All samples were stored in light-protected aerobic incubator for 2 h until the sealers had initially set, and then transferred into humidors with 100% humidity at 37°C for 1 week to allow the sealers to set completely. 1.4 Push-out Bond Strength Test The bond strength of the sealers to radicular dentin was evaluated with a modified thin-slice push-out test. After removal of the glass slides, the slices were polished with 1000-grit SiC papers under running water to remove the excess materials. Digitized photographs were taken from the coronal and apical aspects of the tested cavities under a stereomicroscope. The coronal (Cc) and apical (Ca) circumferences of each cavity were measured and calculated using Image J image analysis software. The thickness of each slice was measured by using the digital caliper. The slice was apical-coronally fixed to a metallic platform with a hole by using sticky wax. A cylindrical plunger 0.7 mm in diameter was positioned on the center of the apical side of the sealer-filled cavity. Each cavity was subjected to vertical compressive loading using a universal testing machine (Instron, USA) at a cross-head speed of 0.6 mm/s until dislodgement occurred (fig. 1). The load/displacement curve showed a sudden reduction in load when the filling sealer was extruded from the artifical canal. The peak force on the curve recorded in newtons (N) was the maximum load. The push-out strength of each specimen was calculated with the fol-

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J Huazhong Univ Sci Technol [Med Sci] 34(1):2014

lowing formula: Push-out strength (MPa)=N/[0.5(Cc +Ca)h], where h is the thickness of the slice in millimeters, Cc and Ca is the coronal and apical circumferences of each cavity respectively in millimeters. Each cavity was examined using stereomicroscopy at 40× magnification to determinate the failure modes after push-out testing: adhesive failure at the sealer-dentin interface, cohesive failure within the sealer, and mixed failure in both sealer and dentin (fig. 2).

axis of the simulated root canal using an Isomet saw under water cooling. Then, they were mounted on stubs, gold-sputter-coated and observed under a SEM (Quanta200, FEI, Netherlands) to examine the ultrastructures of fractured sealer-dentin interfaces. For each group, the remaining two slices were also sectioned under running water likewise. The dentine surfaces were polished with 1200-grit Sic paper to expose the sealer-dentin interfaces, which was followed by demineralization in 15% EDTA for 10 min and deproteinization in 5% NaOCl for 10 min. The morphology of resin tags and the penetration of sealer into dentinal tubules were also observed under the SEM. 1.6 Statistical Analysis The value of push-out bond strength in each simulated root canal was expressed as a statistical unit. Since the pooled data were not normally distributed, nonparametric statistical analysis (Kruskal-Wallis analysis of variance) with the SPSS 19.0 statistical package (SPSS, Chicago, IL, USA) was adopted, followed by a Mann-Whitney test. The statistical significance was set at P less than 0.05.

Fig. 1 Experimental device for the push-out test A tooth slice was fixed on a metallic platform with a central hole. A cylindrical plunger (diameter: 0.7 mm) was positioned on the center of the apical side of the filled cavity (diameter: 1.04 mm) and then vertical compressive load was applied until debonding occurred.

2 RESULTS 2.1 Push-out Strength The push-out strength of MetaSEAL was 12.01±4.66 MPa, significantly higher than that of AH Plus (7.34±2.83 MPa), RealSeal SE (5.43±3.68 MPa) and RealSeal (2.93±1.76 MPa). There was no significant difference in the push-out strength between the RealSeal SE and RealSeal groups or between the RealSeal SE and AH Plus groups (P>0.05 for both). RealSeal had significantly lower bond strength than AH Plus (P