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Influence of resin-based adhesive root canal fillings on the resistance to fracture of endodontically treated roots: an in vitro preliminary study
Vol. 103 No. 2 February 2007
ENDODONTOLOGY
Editor: Larz S. W. Spångberg
Influence of resin-based adhesive root canal fillings on the resistance to fracture of endodontically treated roots: an in vitro preliminary study Edgar Schäfer, Prof.DMD,a Tannaz Zandbiglari, DMD,b and Jens Schäfer,c Münster, Germany UNIVERSITY OF MÜNSTER
Objective. The aim was to investigate the root reinforcing capability of the resin-based RealSeal. Study design. In two groups (n⫽36) canals were instrumented with nickel-titanium rotary GTfiles or with hand K-files. Twelve teeth from each group were obturated with lateral compaction using either gutta-percha and AHPlus or RealSeal. The canals of twelve teeth of both groups were instrumented but not filled. Group 3 (n⫽12) acted as uninstrumented controls. The force required to fracture the roots was measured. ANOVA and Scheffé test were used for statistical analysis. Results. The intact roots were significantly stronger than both groups with instrumented and unobturated roots (P⬍.05). Between the roots of both groups obturated with RealSeal and the intact roots there were no significant differences (P⬎.05). The roots obturated with RealSeal were significantly stronger than those obturated with guttapercha and AHPlus (P⬍.05). Conclusions. An obturation with RealSeal significantly increases the fracture resistance of instrumented roots. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;103:274-9)
It is generally accepted that several endodontic treatment procedures, such as access preparation, instrumentation, and irrigation of the root canal with sodium hypochlorite (NaOCl),1 lead to a reduction of the fracture resistance of a tooth.2 Therefore, many attempts have been made in the past to reinforce an endodontically treated root.3-6 Although the use of gutta-percha with an insoluble root canal sealer can be seen as the gold standard of root canal fillings, the ability of these materials to reinforce an endodontically treated root is discussed with some controversy because in some studies different root canal filling materials were able to significantly strengthen the roots,3,6,7 whereas in other investigations these materials did not increase the fraca
Department of Operative Dentistry, University of Münster. Private Practice, Ibbenbüren, Germany. c Department of Prosthodontics, University of Münster. Received for publication May 16, 2006; returned for revision Jun 14, 2006; accepted for publication Jun 22, 2006. 1079-2104/$ - see front matter © 2007 Mosby, Inc. All rights reserved. doi:10.1016/j.tripleo.2006.06.054 b
274
ture resistance of root-filled teeth.4,5,8 In general it can be stated that the ability of conventional filling materials to reinforce the endodontically treated root is more then questionable owing to their inability to achieve an impervious seal along the dentinal walls of the root canal.9,10 Recently a new obturation material, resilon (representative brand names are Epiphany [Pentron Clinical Technologies, Wallington, CT], Next [Heraeus-Kulzer, Hanau, Germany], and RealSeal [SybronEndo, Orange, CA]) has been introduced to replace gutta-percha and conventional sealers.11 This system comprised resilon, which is a thermoplastic synthetic core material that contains bioactive glass, bismuth oxychloride, and barium sulfate as radiopaque fillers (content of fillers approximately 65 wt%). It is claimed that the handling properties of this resilon core material are highly comparable to that of gutta-percha.11 This system also comprised a dual-curing resin-based sealer. The matrix consists of BisGMA, ethoxylated BisGMA, urethane dimethacrylate, and hydrophilic difunctional methacrylates with a filler content of approximately 70 wt% (mainly calcium hydroxide, barium sulfate, barium
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glass, bismuth oxychloride, and silica).11 Furthermore, this system uses a priming agent which contains a sulfonic acid–terminated functional monomer, hydroxyethylmethacrylate, water, and a polymerization initiator.11 According to the manufacturer this system can be used with any of the most popular gutta-percha root canal obturation techniques such as lateral and vertical compaction, continuous wave, and thermoplasticized gutta-percha. According to recent reports, this adhesive root canal filling material penetrates into the dentinal tubules of the canal wall dentin and simultaneously develops a tight adhesion between the resilon cone and the sealer.9-11 Owing to these properties, this material forms what is called a “monoblock.”10-12 Because of this monoblock between the intraradicular dentin and the root canal filling material, the resilon-filled roots are supposed to be more resistant to both bacterial leakage9 and root fracture2 compared with similar roots that were filled with conventional filling materials. The purpose of this in vitro study was to determine whether the adhesive resin-based sealer RealSeal has the ability to reinforce endodontically treated roots. MATERIALS AND METHODS Instrumentation and obturation Eighty-four extracted mandibular single canal canine teeth were selected for this study. After the extraction, the teeth were stored in phosphate-buffered saline (PBS, pH 7.2) containing 0.1% sodium azide to inhibit bacterial growth for a maximum of 5 days. Coronal access was achieved using diamond burs, and the canals were controlled for apical patency with a size 10 root canal instrument. Only teeth with intact root apices, and whose root canal width near the apex was approximately compatible with size 15 were included. This was checked with silver points sizes 15 and 20 (VDW, Munich, Germany). The root length was between 15 and 18 mm, and the buccolingual diameter was between 5 and 7 mm. Soft tissue and calculus were removed from these teeth mechanically. All teeth were examined with a microscope (Leitz, Wetzlar, Germany) of 20⫻ magnification to rule out teeth with a preexisting root fracture. All unacceptable teeth were discarded. The crowns of all acceptable teeth were sectioned with a diamond disk at the cementoenamel junction under sufficient water cooling, and the cut surface was ground flat using carborundum abrasive paper. The roots were then immersed in a 2.5% NaOCl solution for 8 h to remove any remaining pulp tissue or periodontal ligament and stored in 100% humidity (immersed in physiologic saline) until treatment. In all groups, the roots were balanced with respect to the root length and the buccolingual diameter. The
Schäfer et al. 275
homogeneity of the experimental groups was examined with respect to the defined constraints using analysis of variance (ANOVA) with post hoc Scheffé test. The roots were assigned to the following groups: Group 1. Thirty-six canals were instrumented by rotary nickel-titanium System GT Rotary Files (Dentsply Maillefer, Ballaigues, Switzerland) to a size 40 at the apex with the following sequence using the crown-down technique: 12/35, 10/40, 8/40, 6/40, and 4/40. Group 2. Thirty-six canals were instrumented with stainless steel K-files (VDW) to a size 40 at the apex using a balanced force technique. No coronal flaring of the canals was performed. Group 3. In 12 canals, no instrumentation or obturation was performed (control group). All root canals were enlarged by only 1 operator to minimize operator variation. Otherwise, depending on the particular operator, roots may have been variably stressed with different applied lateral forces. The working length for all groups was obtained by measuring the length of the initial instrument (size 10 K-file) at the apical foramen minus 1 mm. After each instrument, the root canal was flushed with 5 mL 2.5% NaOCl solution and at the end of instrumentation with 5 mL NaCl using a plastic syringe with a gauge 30 closed-end needle (Hawe Max-I-probe; Hawe-Neos, Bioggio, Switzerland). The needle was inserted as deep as possible into the root canal without binding. Finally, all canals were irrigated with 10 mL 17% EDTA solution which was allowed to remain in the canals for 3 min to remove the smear layer.13 Thereafter, the canals were dried with paper points. The rotary nickel-titanium GT instruments were set into rotation with a 4:1 reduction handpiece (WD-66 EM; W & H, Buermoos, Austria) powered by a torquelimited electric motor (Endo IT motor; VDW). For each file the individual torque limit and rotational speed programmed in the file library of the Endo IT motor were used. The rotary nickel-titanium files were used in a crown-down manner according to the manufacturer’s instructions using a gentle in-and-out motion. Instruments were withdrawn when resistance was felt and changed for the next instrument. Before the obturation of the roots, the first 2 groups were subdivided into 3 subgroups of 12 roots each. This followed a pretest sample size calculation. It has been reported that the average maximum bite force in habitual occlusion ranged between 320 N14 and about 400 N.15 In the present study it was estimated that a fracture resistance ranging about 25% below the maximum physiologic bite force can be assessed as a clinically
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significant value (“effect size”). The type I error was set at .05 and the type II error at .10, and a required minimum sample size of 11 was calculated. In subgroup A an epoxy resin– based sealer, AH Plus (Dentsply Maillefer, Konstanz, Germany), was used. The teeth were obturated with lateral compaction by using gutta-percha and AH Plus sealer. A size 40 taper .02 gutta-percha master cone was fit to the working length with good tug-back, dipped in the sealer, and placed in the root canal. The diameter of the master cone was checked with a special gutta gauge (Dentsply Maillefer, Ballaigues, Switzerland). Then a size 30 nickel-titanium spreader (VDW) and fine-fine accessory gutta-percha cones (VDW) dipped in sealer were used for the lateral compaction until the accessory cone could not be introduced more than 2 mm into the canal. Excess gutta-percha was removed 1 mm below the cementoenamel junction and vertically condensed with a hot No. 11 plugger (Dentsply Maillefer, Ballaigues, Switzerland). In subgroup B the dual-curing resin-based RealSeal root canal sealant was used. The teeth were obturated with lateral compaction. Therefore, a polymer-based RealSeal point taper .02 size 40 was fit to the working length with good tug-back. The diameter of the master cone was checked with a special gutta gauge (Dentsply Maillefer). RealSeal primer was then introduced into the canal using special applicator brushes. Thirty seconds later, dry paper points were used to wick out the excess primer from the canal. The RealSeal master point was generously coated with RealSeal sealer and seated into the canal. Lateral compaction was accomplished using a size 30 nickel-titanium finger spreader and fine-fine RealSeal accessory cones, which were coated slightly with sealer before insertion into the canal until the accessory cone could not be introduced more than 2 mm into the canal. Excess RealSeal was removed 1 mm below the cementoenamel junction and vertically condensed with a hot No. 11 plugger (Dentsply Maillefer). The samples in subgroup C were left unfilled after instrumentation. In all of these 3 groups, the canal opening was sealed with Cavit (3M Espe, Seefeld, Germany). After the obturation, each sample was examined with a microscope at 20⫻ magnification to ensure that there were no cracks or craze lines in the roots. Moreover, in groups 1 and 2 postoperative radiographs were taken of all obturated roots from both proximal and clinical views to ensure that all root canals were properly obturated without voids. Canals that had not been adequately filled (2 roots) or specimens with cracks (1 root) were dismissed and replaced by a new sample. All
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roots were stored in 100% humidity for 2 weeks. All sealers were totally set. Strength testing Thereafter, the roots were prepared coronally by reducing the coronal lingual wall to 2 mm below the buccal wall with a fissure bur. The samples were mounted in copper rings (25 mm high and 10 mm in diameter) filled with acrylic resin (Palavit G; Heraeus Kulzer, Hanau, Germany). The apical root ends were embedded in the acrylic resin blocks, exposing 8 mm of the coronal buccal end of each root. The long axis of the root was vertically aligned using a protractor. The acrylic resin was allowed to polymerize for 1 h. The acrylic resin blocks with the prepared roots were stored in 100% humidity until they were ready for strength testing. The copper rings containing the roots were placed on a specially designed steel pad at an angle of 15° to the long axis of the roots. This steel pad was placed into a Universal Testing Machine (Lloyd Instruments, Hampshire, UK). A steel tip, 1 mm thick and 10 mm wide, was attached to the load cell and lowered to contact the root at the junction of the buccal wall and the root canal, as described in a previous study.8 The testing machine applied a slowly increasing force at a rate of 1.0 mm per minute until the fracture occurred. The force required to fracture the root was recorded in Newtons. The results were subjected to statistical analysis using 1-way ANOVA and post hoc Scheffé test to determine the differences between the groups. The level of significance was set at P ⬍ .05. RESULTS The statistical analysis of the root length (P ⫽ .841) and the buccolingual diameter (P ⫽ .978) of the roots revealed no significant differences between the groups. All roots fractured at the buccal aspect in a mesialdistal direction. With regard to the force required to fracture (Table I), the intact roots (group 3, control) were significantly stronger than all groups with instrumented and unfilled canals and all groups with canals obturated with gutta-percha (P ⬍ .05). The force required to fracture the roots filled with RealSeal (groups 1B and 2B) was significantly higher (P ⬍ .05) than that required to fracture the roots of all other groups except the intact roots (group 3). Comparing the forces required to fracture the roots filled with RealSeal and the intact roots, no significant differences were obtained (P ⬎ .05). Roots enlarged with GT files were weaker than those instrumented with hand instruments, although this difference was not statistically significant (P ⬎ .05).
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Table I. Means and standard deviations of forces required to fracture the roots of the different groups. The results are given in Newtons. Group 1 ⫽ GT files; group 2 ⫽ hand K-files; group 3 ⫽ intact teeth (controls); A ⫽ canals filled with gutta-percha and AH Plus; B ⫽ canals filled with RealSeal; C ⫽ canals instrumented but unfilled. Group
Mean
SD
Significance*
1A 1B 1C 2A 2B 2C 3
287.81 379.89 262.62 317.31 396.16 321.15 401.50
50.96 76.08 47.67 73.56 58.79 36.43 76.91
a b a a b a b
*Means with the same superscript letters were not statistically different at P ⫽ .05 (ANOVA and post hoc Scheffé test).
DISCUSSION In the past 20 years, several attempts have been made to improve the apical and coronal seal achieved with gutta-percha and conventional root canal sealers by using dentin-bonding agents.16-22 The major problem was the very short working time with these adhesives.22 Recently, a new material has been introduced to replace gutta-percha and conventional sealers for root canal obturation.11 According to the results of this in vitro study (Table I), the enlarged but unfilled roots (groups 1C and 2C) were found to be significantly weaker than the intact control roots (group 3). Around 20% of the root fracture resistance was lost after instrumentation with hand instruments and 35% after preparation with GT files. Thus, the force required to fracture the roots enlarged with the GT files (group 1C) was lower than that required to fracture the roots instrumented with hand instruments (group 2C), although this difference was not statistically significant (P ⬎ .05). This agrees only in part with a previous study which found the difference to be statistically significant.8 In addition, these results clearly emphasize that the instrumentation of root canals alone significantly weakens the roots and at the same time corroborates the results of previous studies.3,6,7 According to the present results, root canal obturation with RealSeal resulted in a significant increase in the resistance to fracture compared with the instrumented but unfilled roots and compared with roots obturated with gutta-percha and AH Plus (P ⬍ .05; Table I). These results are in good agreement with a previous study.2 In that study the fracture resistance of roots filled with gutta-percha and AH 26 was compared with resilon-filled roots using both lateral and vertical
compaction. The groups with the resin-based filling displayed significantly higher fracture loads than the gutta-percha groups independently of the filling technique used.2 In the present study, obturation of the canals with AH Plus did not significantly strengthen the roots compared with the instrumented but not obturated roots (P⬎ .05). These results corroborate those of several studies using gutta-percha and an epoxy resin– based sealer.2,6,8 Contradictory results were reported in another study which found that the use of the epoxy resin– based sealer AH 26 resulted in a significant strengthening of the instrumented roots.7 In conducting comparative in vitro studies such as the present one, it is important to obtain comparable and well standardized experimental groups. Therefore, each group comprised roots with similar dimensions to eliminate the dimension variation factor. According to the statistical analysis of the dimensions of the roots, there were no statistically significant differences between the lengths or the cross-sectional diameters in any of the groups. Thus, the experimental groups were well balanced concerning the defined constraints. In addition, the diameter of the root canal near the apex prior to instrumentation was approximately compatible with size 15 in all teeth. In the present study, a total of 84 teeth were allocated to different experimental groups. It was not known whether all stored teeth had comparable dentin in terms of strength and hardness. Moreover, the crowns of all teeth were removed before strength testing. This created a situation that is certainly not clinically relevant in most cases and might have additionally weakened the teeth.23 Thus, it has to be kept in mind that the reported force applied to the point of fracture are not absolute but only relative between the different groups, and thus they can not be transfered directly to the true clinical situation. In this study all canals were irrigated with a final rinse of EDTA after root canal preparation, as required by the manufacturer, although a recent investigation has pointed out that a final rinse with EDTA caused a collapse of the dentin matrix structure.24 Nevertheless, if an adhesive root canal filling material might have the ability to reinforce an endodontically treated root, it must bond to dentin. This adhesive joint presupposes that the material is able to infiltrate dentin. Therefore, the smear layer was removed with EDTA. This protocol of irrigation was employed also for the roots filled with gutta-percha to obtain a standardized experimental setup. Contrarily, it could be argued that EDTA may have some weakening effect on the dentin. Nevertheless, besides the reasons mentioned, EDTA was used as the final rinse to reduce the oxidizing effect of NaOCl
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on sealer polymerization.25,26 Owing to this precaution and the findings of a recent study,27 the effect of NaOCl on the polymerization of the RealSeal sealer should be negligible. The force was applied in this study at a 15° angle to the buccal wall, and thus the application of the loading force until the fracture occurred was similar to the technique used in previous studies,3,7 although in other investigations the force was directed at an angle of 0°, primarily resulting in a splitting stress applied above the access opening.4,5,23,28 Nevertheless, the first design was chosen consciously because under clinical conditions teeth are stressed not only vertically down the long axis of the root, but occlusal load also will be directed more likely at a certain angle.29 Although the present results concerning the ability of the adhesive root canal filling material RealSeal to reinforce an endodontically treated root, which are in accordance with already published results,2 are very promising, some care should be taken in the transfer of these findings to the long-term clinical situation, especially because recently the results of some studies pointed out that resilon seems to be biodegradable under the attack of hydrolytic ester bond– cleaving enzymes which may exist as a component of salivary enzymes or as extracellular enzymes from endodontically relevant pathogens such as Pseudomonas aeruginosa, Enterococcus faecalis, and several Actinomyces strains.30 Moreover, there is some evidence that resilon is also susceptible to alkaline hydrolysis.31 In addition, in other studies the formation of monoblocks is considered because a low shear bond strength of resilon to the methacrylate-based sealer was observed.10,32,33 It was assumed that the chemical coupling of the resin-based sealer to resilon is weak,32 which may be due to the fact that the amount or method of dimethacrylate incorporated in resilon may not be optimized for predictable chemical coupling.32,33 Thus, it should be kept in mind that the use of these resin-based materials has not had the same extensive evaluation that gutta-percha and conventional sealers have had.34 Clinical long-term studies are necessary to collect evidence-based data to support the confident use of these materials. Under the conditions of this in vitro study it can be concluded that the adhesive root-filling material RealSeal has the potential to strengthen endodontically treated roots to a level that is similar to that of intact teeth.30 REFERENCES 1. Sim TPC, Knowles JC, Ng YL, Shelton J, Gulabivala K. Effect of sodium hypochlorite on mechanical properties of dentine and tooth surface strain. Int Endod J 2001;34:120-32. 2. Teixeira FB, Teixeira ECN, Thompson JY, Trope M. Fracture
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resistance of roots endodonticially treated with a new resin filling material. J Am Dent Assoc 2004;135:646-52. Trope M, Ray HL. Resistance to fracture of endodontically treated roots. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1992;73:99-102. Apicella MJ, Loushine RJ, West LA, Runyan DA. A comparison of root fracture resistance using 2 root canal sealers. Int Endod J 1999;32:376-80. Johnson ME, Stewart GP, Nielsen CJ, Hatton JF. Evaluation of root reinforcement of endodontically treated teeth. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;90:360-4. Lertchirakarn V, Timyam A, Messer HH. Effects of root canal sealers on vertical root fracture resistance of endodontically treated teeth. J Endod 2002;28:217-9. ¨ ngör M, Belli S. The effect of two different root Cobankara FK, U canal sealers and smear layer on resistance to root fracture. J Endod 2002;28:606-9. Zandbiglari T, Davids H, Schäfer E. Influence of instrument taper on the resistance to fracture of endodontically treated teeth. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;101:126-31. Shipper G, Ørstavik D, Teixeira FB, Trope M. An evaluation of microbial leakage in roots filled with a thermoplastic synthetic polymer-based root canal filling material (resilon). J Endod 2004;30:342-7. Gesi A, Raffaelli O, Goracci C, Pashley DH, Tay FR, Ferrari M. Interfacial strength of resilon and gutta-percha to intraradicular dentin. J Endod 2005;31:809-13. Barnett F, Trope M. Resilon: A novel material to replace gutta percha. Contemp Endod 2004;1:16-9. Shipper G, Teixeira FB, Arnold RR, Trope M. Periapical inflammation after coronal microbial inoculation of dog roots filled with gutta-percha or resilon. J Endod 2005;31:91-6. Kokkas AB, Boutsiouskis CA, Vassiliadis LP, Stravrianos CK. The influence of the smear layer on dentinal tuble penetration depth by three different root canal sealers: an in vitro study. J Endod 2004;30:100-2. Laurell L. Occlusal forces and chewing ability in dentitions with cross-arch bridges. Swed Dent J Suppl 1985;26:160. Hagberg C, Agerberg G, Hagberg M. Regression analysis of electromyographic activity of masticatory muscles versus bite force. Scand J Dent Res 1985;93:396-402. Assouline LS, Fuss Z, Mazor Y, Weiss EI. Bacterial penetration and proliferation in root canal dentinal tubules after applying dentin adhesives in vitro. J Endod 2001;27:398-400. Mannocci F, Ferrari M. Apical seal of roots obturated with laterally condensed gutta-percha, epoxy resin cement, and dentin bonding agent. J Endod 1998;24:41-4. Hammond RMS, Meyers IA. A laboratory investigation of a composite resin/dentine bonding agent mixture used as a root canal sealer. Aust Dent J 1992;37:178-84. Leonard JE, Gutmann JL, Guo IY. Apical and coronal seal of roots obturated with a dentine bonding agent and resin. Int Endod J 1996;29:76-83. Torabinejad M, Kahn H, Bankes D. Isopropyl cyanoacrylate as a root canal sealer. J Endod 1984;10:304-7. Zidan O, ElDeeb M. The use of dentinal bonding agent as a root canal sealer. J Endod 1985;11:176-8. Ahlberg KMF, Tay WM. A methacrylate-based cement used as a root canal sealer. Int Endod J 1998;31:15-21. Wu MK, van der Sluis LWM, Wesselink PR. Comparison of mandibular premolars and canines with respect to their resistance to vertical root fracture. J Dent 2004;32:265-8. Garcia-Godoy F, Loushine RJ, Itthagarun A, Weller RN, Murray PE, Feilzer AJ, et al. Application of biologically-oriented dentin
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bonding principles to the use of endodontic irrigants. Am J Dent 2005;18:281-90. Morris MD, Lee KW, Agee KA, Bouillaguet S, Pashley DH. Effects of sodium hypochlorite and RC-prep on bond strengths of resin cement to endodontic surfaces. J Endod 2001;27:753-7. Ari H, Yasar S. Effects of NaOCl on bond strengths of resin cements to root dentin. J Endod 2003;29:248-51. Varela SG, Rabade LB, Lombardero PR, Sixto JM, Bahillo JD, Park SA. In vitro study of endodontic post cementation protocols that use resin cements. J Prosthet Dent 2003;89:146-53. Sedgley CM, Messer HH. Are endodontically treated teeth more brittle? J Endod 1992;18:332-5. Iwasaki LR, Thornton BR, McCall WS, Nickel JC. Individual variations in numerically modeled human muscle and temporomandibular joint forces during static biting. J Orofac Pain 2004;18:235-45. Tay FR, Pashley DH, Yiu CKY, Yau JYY, Yiu-fai M, Loushine RJ, et al. Susceptibility of a polycaprolactone-based root canal filling material to degradation. II. Gravimetric evaluation of enzymatic hydrolysis. J Endod 2005;31:737-41.
Schäfer et al. 279 31. Tay FR, Pashley DH, Williams MC, Raina R, Loushine RJ, Weller RN, et al. Susceptibility of a polycaprolactone-based root canal filling material to degradation. I. Alkaline hydrolysis. J Endod 2005;31:593-8. 32. Hiraishi N, Papcchini F, Loushine RJ, Weller RN, Ferrari M, Pashley DH, et al. Shear bond strength of resilon to a methacrylate-based root canal sealer. Int Endod J 2005;38:753-63. 33. Tay FR, Hiraishi N, Pashley DH, Loushine RJ, Weller RN, Gillespie WT, et al. Bondability of resilon to a methacrylatebased root canal sealer. J Endod 2006;32:133-7. 34. Gutmann JL. Biologic perspectives to support clinical choices in root canal treatement. Aust Endod J 2005;31:9-13.
Reprint requests: Prof. Dr. Edgar Schäfer Department of Operative Dentistry Waldeyerstr. 30 D-48149 Münster, Germany [email protected]
ENDODONTOLOGY
Editor: Larz S. W. Spångberg
Influence of resin-based adhesive root canal fillings on the resistance to fracture of endodontically treated roots: an in vitro preliminary study Edgar Schäfer, Prof.DMD,a Tannaz Zandbiglari, DMD,b and Jens Schäfer,c Münster, Germany UNIVERSITY OF MÜNSTER
Objective. The aim was to investigate the root reinforcing capability of the resin-based RealSeal. Study design. In two groups (n⫽36) canals were instrumented with nickel-titanium rotary GTfiles or with hand K-files. Twelve teeth from each group were obturated with lateral compaction using either gutta-percha and AHPlus or RealSeal. The canals of twelve teeth of both groups were instrumented but not filled. Group 3 (n⫽12) acted as uninstrumented controls. The force required to fracture the roots was measured. ANOVA and Scheffé test were used for statistical analysis. Results. The intact roots were significantly stronger than both groups with instrumented and unobturated roots (P⬍.05). Between the roots of both groups obturated with RealSeal and the intact roots there were no significant differences (P⬎.05). The roots obturated with RealSeal were significantly stronger than those obturated with guttapercha and AHPlus (P⬍.05). Conclusions. An obturation with RealSeal significantly increases the fracture resistance of instrumented roots. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;103:274-9)
It is generally accepted that several endodontic treatment procedures, such as access preparation, instrumentation, and irrigation of the root canal with sodium hypochlorite (NaOCl),1 lead to a reduction of the fracture resistance of a tooth.2 Therefore, many attempts have been made in the past to reinforce an endodontically treated root.3-6 Although the use of gutta-percha with an insoluble root canal sealer can be seen as the gold standard of root canal fillings, the ability of these materials to reinforce an endodontically treated root is discussed with some controversy because in some studies different root canal filling materials were able to significantly strengthen the roots,3,6,7 whereas in other investigations these materials did not increase the fraca
Department of Operative Dentistry, University of Münster. Private Practice, Ibbenbüren, Germany. c Department of Prosthodontics, University of Münster. Received for publication May 16, 2006; returned for revision Jun 14, 2006; accepted for publication Jun 22, 2006. 1079-2104/$ - see front matter © 2007 Mosby, Inc. All rights reserved. doi:10.1016/j.tripleo.2006.06.054 b
274
ture resistance of root-filled teeth.4,5,8 In general it can be stated that the ability of conventional filling materials to reinforce the endodontically treated root is more then questionable owing to their inability to achieve an impervious seal along the dentinal walls of the root canal.9,10 Recently a new obturation material, resilon (representative brand names are Epiphany [Pentron Clinical Technologies, Wallington, CT], Next [Heraeus-Kulzer, Hanau, Germany], and RealSeal [SybronEndo, Orange, CA]) has been introduced to replace gutta-percha and conventional sealers.11 This system comprised resilon, which is a thermoplastic synthetic core material that contains bioactive glass, bismuth oxychloride, and barium sulfate as radiopaque fillers (content of fillers approximately 65 wt%). It is claimed that the handling properties of this resilon core material are highly comparable to that of gutta-percha.11 This system also comprised a dual-curing resin-based sealer. The matrix consists of BisGMA, ethoxylated BisGMA, urethane dimethacrylate, and hydrophilic difunctional methacrylates with a filler content of approximately 70 wt% (mainly calcium hydroxide, barium sulfate, barium
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glass, bismuth oxychloride, and silica).11 Furthermore, this system uses a priming agent which contains a sulfonic acid–terminated functional monomer, hydroxyethylmethacrylate, water, and a polymerization initiator.11 According to the manufacturer this system can be used with any of the most popular gutta-percha root canal obturation techniques such as lateral and vertical compaction, continuous wave, and thermoplasticized gutta-percha. According to recent reports, this adhesive root canal filling material penetrates into the dentinal tubules of the canal wall dentin and simultaneously develops a tight adhesion between the resilon cone and the sealer.9-11 Owing to these properties, this material forms what is called a “monoblock.”10-12 Because of this monoblock between the intraradicular dentin and the root canal filling material, the resilon-filled roots are supposed to be more resistant to both bacterial leakage9 and root fracture2 compared with similar roots that were filled with conventional filling materials. The purpose of this in vitro study was to determine whether the adhesive resin-based sealer RealSeal has the ability to reinforce endodontically treated roots. MATERIALS AND METHODS Instrumentation and obturation Eighty-four extracted mandibular single canal canine teeth were selected for this study. After the extraction, the teeth were stored in phosphate-buffered saline (PBS, pH 7.2) containing 0.1% sodium azide to inhibit bacterial growth for a maximum of 5 days. Coronal access was achieved using diamond burs, and the canals were controlled for apical patency with a size 10 root canal instrument. Only teeth with intact root apices, and whose root canal width near the apex was approximately compatible with size 15 were included. This was checked with silver points sizes 15 and 20 (VDW, Munich, Germany). The root length was between 15 and 18 mm, and the buccolingual diameter was between 5 and 7 mm. Soft tissue and calculus were removed from these teeth mechanically. All teeth were examined with a microscope (Leitz, Wetzlar, Germany) of 20⫻ magnification to rule out teeth with a preexisting root fracture. All unacceptable teeth were discarded. The crowns of all acceptable teeth were sectioned with a diamond disk at the cementoenamel junction under sufficient water cooling, and the cut surface was ground flat using carborundum abrasive paper. The roots were then immersed in a 2.5% NaOCl solution for 8 h to remove any remaining pulp tissue or periodontal ligament and stored in 100% humidity (immersed in physiologic saline) until treatment. In all groups, the roots were balanced with respect to the root length and the buccolingual diameter. The
Schäfer et al. 275
homogeneity of the experimental groups was examined with respect to the defined constraints using analysis of variance (ANOVA) with post hoc Scheffé test. The roots were assigned to the following groups: Group 1. Thirty-six canals were instrumented by rotary nickel-titanium System GT Rotary Files (Dentsply Maillefer, Ballaigues, Switzerland) to a size 40 at the apex with the following sequence using the crown-down technique: 12/35, 10/40, 8/40, 6/40, and 4/40. Group 2. Thirty-six canals were instrumented with stainless steel K-files (VDW) to a size 40 at the apex using a balanced force technique. No coronal flaring of the canals was performed. Group 3. In 12 canals, no instrumentation or obturation was performed (control group). All root canals were enlarged by only 1 operator to minimize operator variation. Otherwise, depending on the particular operator, roots may have been variably stressed with different applied lateral forces. The working length for all groups was obtained by measuring the length of the initial instrument (size 10 K-file) at the apical foramen minus 1 mm. After each instrument, the root canal was flushed with 5 mL 2.5% NaOCl solution and at the end of instrumentation with 5 mL NaCl using a plastic syringe with a gauge 30 closed-end needle (Hawe Max-I-probe; Hawe-Neos, Bioggio, Switzerland). The needle was inserted as deep as possible into the root canal without binding. Finally, all canals were irrigated with 10 mL 17% EDTA solution which was allowed to remain in the canals for 3 min to remove the smear layer.13 Thereafter, the canals were dried with paper points. The rotary nickel-titanium GT instruments were set into rotation with a 4:1 reduction handpiece (WD-66 EM; W & H, Buermoos, Austria) powered by a torquelimited electric motor (Endo IT motor; VDW). For each file the individual torque limit and rotational speed programmed in the file library of the Endo IT motor were used. The rotary nickel-titanium files were used in a crown-down manner according to the manufacturer’s instructions using a gentle in-and-out motion. Instruments were withdrawn when resistance was felt and changed for the next instrument. Before the obturation of the roots, the first 2 groups were subdivided into 3 subgroups of 12 roots each. This followed a pretest sample size calculation. It has been reported that the average maximum bite force in habitual occlusion ranged between 320 N14 and about 400 N.15 In the present study it was estimated that a fracture resistance ranging about 25% below the maximum physiologic bite force can be assessed as a clinically
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significant value (“effect size”). The type I error was set at .05 and the type II error at .10, and a required minimum sample size of 11 was calculated. In subgroup A an epoxy resin– based sealer, AH Plus (Dentsply Maillefer, Konstanz, Germany), was used. The teeth were obturated with lateral compaction by using gutta-percha and AH Plus sealer. A size 40 taper .02 gutta-percha master cone was fit to the working length with good tug-back, dipped in the sealer, and placed in the root canal. The diameter of the master cone was checked with a special gutta gauge (Dentsply Maillefer, Ballaigues, Switzerland). Then a size 30 nickel-titanium spreader (VDW) and fine-fine accessory gutta-percha cones (VDW) dipped in sealer were used for the lateral compaction until the accessory cone could not be introduced more than 2 mm into the canal. Excess gutta-percha was removed 1 mm below the cementoenamel junction and vertically condensed with a hot No. 11 plugger (Dentsply Maillefer, Ballaigues, Switzerland). In subgroup B the dual-curing resin-based RealSeal root canal sealant was used. The teeth were obturated with lateral compaction. Therefore, a polymer-based RealSeal point taper .02 size 40 was fit to the working length with good tug-back. The diameter of the master cone was checked with a special gutta gauge (Dentsply Maillefer). RealSeal primer was then introduced into the canal using special applicator brushes. Thirty seconds later, dry paper points were used to wick out the excess primer from the canal. The RealSeal master point was generously coated with RealSeal sealer and seated into the canal. Lateral compaction was accomplished using a size 30 nickel-titanium finger spreader and fine-fine RealSeal accessory cones, which were coated slightly with sealer before insertion into the canal until the accessory cone could not be introduced more than 2 mm into the canal. Excess RealSeal was removed 1 mm below the cementoenamel junction and vertically condensed with a hot No. 11 plugger (Dentsply Maillefer). The samples in subgroup C were left unfilled after instrumentation. In all of these 3 groups, the canal opening was sealed with Cavit (3M Espe, Seefeld, Germany). After the obturation, each sample was examined with a microscope at 20⫻ magnification to ensure that there were no cracks or craze lines in the roots. Moreover, in groups 1 and 2 postoperative radiographs were taken of all obturated roots from both proximal and clinical views to ensure that all root canals were properly obturated without voids. Canals that had not been adequately filled (2 roots) or specimens with cracks (1 root) were dismissed and replaced by a new sample. All
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roots were stored in 100% humidity for 2 weeks. All sealers were totally set. Strength testing Thereafter, the roots were prepared coronally by reducing the coronal lingual wall to 2 mm below the buccal wall with a fissure bur. The samples were mounted in copper rings (25 mm high and 10 mm in diameter) filled with acrylic resin (Palavit G; Heraeus Kulzer, Hanau, Germany). The apical root ends were embedded in the acrylic resin blocks, exposing 8 mm of the coronal buccal end of each root. The long axis of the root was vertically aligned using a protractor. The acrylic resin was allowed to polymerize for 1 h. The acrylic resin blocks with the prepared roots were stored in 100% humidity until they were ready for strength testing. The copper rings containing the roots were placed on a specially designed steel pad at an angle of 15° to the long axis of the roots. This steel pad was placed into a Universal Testing Machine (Lloyd Instruments, Hampshire, UK). A steel tip, 1 mm thick and 10 mm wide, was attached to the load cell and lowered to contact the root at the junction of the buccal wall and the root canal, as described in a previous study.8 The testing machine applied a slowly increasing force at a rate of 1.0 mm per minute until the fracture occurred. The force required to fracture the root was recorded in Newtons. The results were subjected to statistical analysis using 1-way ANOVA and post hoc Scheffé test to determine the differences between the groups. The level of significance was set at P ⬍ .05. RESULTS The statistical analysis of the root length (P ⫽ .841) and the buccolingual diameter (P ⫽ .978) of the roots revealed no significant differences between the groups. All roots fractured at the buccal aspect in a mesialdistal direction. With regard to the force required to fracture (Table I), the intact roots (group 3, control) were significantly stronger than all groups with instrumented and unfilled canals and all groups with canals obturated with gutta-percha (P ⬍ .05). The force required to fracture the roots filled with RealSeal (groups 1B and 2B) was significantly higher (P ⬍ .05) than that required to fracture the roots of all other groups except the intact roots (group 3). Comparing the forces required to fracture the roots filled with RealSeal and the intact roots, no significant differences were obtained (P ⬎ .05). Roots enlarged with GT files were weaker than those instrumented with hand instruments, although this difference was not statistically significant (P ⬎ .05).
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Table I. Means and standard deviations of forces required to fracture the roots of the different groups. The results are given in Newtons. Group 1 ⫽ GT files; group 2 ⫽ hand K-files; group 3 ⫽ intact teeth (controls); A ⫽ canals filled with gutta-percha and AH Plus; B ⫽ canals filled with RealSeal; C ⫽ canals instrumented but unfilled. Group
Mean
SD
Significance*
1A 1B 1C 2A 2B 2C 3
287.81 379.89 262.62 317.31 396.16 321.15 401.50
50.96 76.08 47.67 73.56 58.79 36.43 76.91
a b a a b a b
*Means with the same superscript letters were not statistically different at P ⫽ .05 (ANOVA and post hoc Scheffé test).
DISCUSSION In the past 20 years, several attempts have been made to improve the apical and coronal seal achieved with gutta-percha and conventional root canal sealers by using dentin-bonding agents.16-22 The major problem was the very short working time with these adhesives.22 Recently, a new material has been introduced to replace gutta-percha and conventional sealers for root canal obturation.11 According to the results of this in vitro study (Table I), the enlarged but unfilled roots (groups 1C and 2C) were found to be significantly weaker than the intact control roots (group 3). Around 20% of the root fracture resistance was lost after instrumentation with hand instruments and 35% after preparation with GT files. Thus, the force required to fracture the roots enlarged with the GT files (group 1C) was lower than that required to fracture the roots instrumented with hand instruments (group 2C), although this difference was not statistically significant (P ⬎ .05). This agrees only in part with a previous study which found the difference to be statistically significant.8 In addition, these results clearly emphasize that the instrumentation of root canals alone significantly weakens the roots and at the same time corroborates the results of previous studies.3,6,7 According to the present results, root canal obturation with RealSeal resulted in a significant increase in the resistance to fracture compared with the instrumented but unfilled roots and compared with roots obturated with gutta-percha and AH Plus (P ⬍ .05; Table I). These results are in good agreement with a previous study.2 In that study the fracture resistance of roots filled with gutta-percha and AH 26 was compared with resilon-filled roots using both lateral and vertical
compaction. The groups with the resin-based filling displayed significantly higher fracture loads than the gutta-percha groups independently of the filling technique used.2 In the present study, obturation of the canals with AH Plus did not significantly strengthen the roots compared with the instrumented but not obturated roots (P⬎ .05). These results corroborate those of several studies using gutta-percha and an epoxy resin– based sealer.2,6,8 Contradictory results were reported in another study which found that the use of the epoxy resin– based sealer AH 26 resulted in a significant strengthening of the instrumented roots.7 In conducting comparative in vitro studies such as the present one, it is important to obtain comparable and well standardized experimental groups. Therefore, each group comprised roots with similar dimensions to eliminate the dimension variation factor. According to the statistical analysis of the dimensions of the roots, there were no statistically significant differences between the lengths or the cross-sectional diameters in any of the groups. Thus, the experimental groups were well balanced concerning the defined constraints. In addition, the diameter of the root canal near the apex prior to instrumentation was approximately compatible with size 15 in all teeth. In the present study, a total of 84 teeth were allocated to different experimental groups. It was not known whether all stored teeth had comparable dentin in terms of strength and hardness. Moreover, the crowns of all teeth were removed before strength testing. This created a situation that is certainly not clinically relevant in most cases and might have additionally weakened the teeth.23 Thus, it has to be kept in mind that the reported force applied to the point of fracture are not absolute but only relative between the different groups, and thus they can not be transfered directly to the true clinical situation. In this study all canals were irrigated with a final rinse of EDTA after root canal preparation, as required by the manufacturer, although a recent investigation has pointed out that a final rinse with EDTA caused a collapse of the dentin matrix structure.24 Nevertheless, if an adhesive root canal filling material might have the ability to reinforce an endodontically treated root, it must bond to dentin. This adhesive joint presupposes that the material is able to infiltrate dentin. Therefore, the smear layer was removed with EDTA. This protocol of irrigation was employed also for the roots filled with gutta-percha to obtain a standardized experimental setup. Contrarily, it could be argued that EDTA may have some weakening effect on the dentin. Nevertheless, besides the reasons mentioned, EDTA was used as the final rinse to reduce the oxidizing effect of NaOCl
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on sealer polymerization.25,26 Owing to this precaution and the findings of a recent study,27 the effect of NaOCl on the polymerization of the RealSeal sealer should be negligible. The force was applied in this study at a 15° angle to the buccal wall, and thus the application of the loading force until the fracture occurred was similar to the technique used in previous studies,3,7 although in other investigations the force was directed at an angle of 0°, primarily resulting in a splitting stress applied above the access opening.4,5,23,28 Nevertheless, the first design was chosen consciously because under clinical conditions teeth are stressed not only vertically down the long axis of the root, but occlusal load also will be directed more likely at a certain angle.29 Although the present results concerning the ability of the adhesive root canal filling material RealSeal to reinforce an endodontically treated root, which are in accordance with already published results,2 are very promising, some care should be taken in the transfer of these findings to the long-term clinical situation, especially because recently the results of some studies pointed out that resilon seems to be biodegradable under the attack of hydrolytic ester bond– cleaving enzymes which may exist as a component of salivary enzymes or as extracellular enzymes from endodontically relevant pathogens such as Pseudomonas aeruginosa, Enterococcus faecalis, and several Actinomyces strains.30 Moreover, there is some evidence that resilon is also susceptible to alkaline hydrolysis.31 In addition, in other studies the formation of monoblocks is considered because a low shear bond strength of resilon to the methacrylate-based sealer was observed.10,32,33 It was assumed that the chemical coupling of the resin-based sealer to resilon is weak,32 which may be due to the fact that the amount or method of dimethacrylate incorporated in resilon may not be optimized for predictable chemical coupling.32,33 Thus, it should be kept in mind that the use of these resin-based materials has not had the same extensive evaluation that gutta-percha and conventional sealers have had.34 Clinical long-term studies are necessary to collect evidence-based data to support the confident use of these materials. Under the conditions of this in vitro study it can be concluded that the adhesive root-filling material RealSeal has the potential to strengthen endodontically treated roots to a level that is similar to that of intact teeth.30 REFERENCES 1. Sim TPC, Knowles JC, Ng YL, Shelton J, Gulabivala K. Effect of sodium hypochlorite on mechanical properties of dentine and tooth surface strain. Int Endod J 2001;34:120-32. 2. Teixeira FB, Teixeira ECN, Thompson JY, Trope M. Fracture
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bonding principles to the use of endodontic irrigants. Am J Dent 2005;18:281-90. Morris MD, Lee KW, Agee KA, Bouillaguet S, Pashley DH. Effects of sodium hypochlorite and RC-prep on bond strengths of resin cement to endodontic surfaces. J Endod 2001;27:753-7. Ari H, Yasar S. Effects of NaOCl on bond strengths of resin cements to root dentin. J Endod 2003;29:248-51. Varela SG, Rabade LB, Lombardero PR, Sixto JM, Bahillo JD, Park SA. In vitro study of endodontic post cementation protocols that use resin cements. J Prosthet Dent 2003;89:146-53. Sedgley CM, Messer HH. Are endodontically treated teeth more brittle? J Endod 1992;18:332-5. Iwasaki LR, Thornton BR, McCall WS, Nickel JC. Individual variations in numerically modeled human muscle and temporomandibular joint forces during static biting. J Orofac Pain 2004;18:235-45. Tay FR, Pashley DH, Yiu CKY, Yau JYY, Yiu-fai M, Loushine RJ, et al. Susceptibility of a polycaprolactone-based root canal filling material to degradation. II. Gravimetric evaluation of enzymatic hydrolysis. J Endod 2005;31:737-41.
Schäfer et al. 279 31. Tay FR, Pashley DH, Williams MC, Raina R, Loushine RJ, Weller RN, et al. Susceptibility of a polycaprolactone-based root canal filling material to degradation. I. Alkaline hydrolysis. J Endod 2005;31:593-8. 32. Hiraishi N, Papcchini F, Loushine RJ, Weller RN, Ferrari M, Pashley DH, et al. Shear bond strength of resilon to a methacrylate-based root canal sealer. Int Endod J 2005;38:753-63. 33. Tay FR, Hiraishi N, Pashley DH, Loushine RJ, Weller RN, Gillespie WT, et al. Bondability of resilon to a methacrylatebased root canal sealer. J Endod 2006;32:133-7. 34. Gutmann JL. Biologic perspectives to support clinical choices in root canal treatement. Aust Endod J 2005;31:9-13.
Reprint requests: Prof. Dr. Edgar Schäfer Department of Operative Dentistry Waldeyerstr. 30 D-48149 Münster, Germany [email protected]