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Shear Bond Strength of Three Resin Based Sealers to Dentin With and Without the Smear Layer
Basic Research—Technology
Shear Bond Strength of Three Resin Based Sealers to Dentin With and Without the Smear Layer Ayce Unverdi Eldeniz, DDS, PhD, Ali Erdemir, DDS, PhD, and Sema Belli, DDS, PhD Abstract Bond strength of root canal sealers to dentin is an important property for the integrity of the sealings of root-canals. The purpose of this study was to test shear bond strength of three endodontic sealers (Diaket, AH Plus and Endo-REZ). The coronal two thirds of ninety extracted human third molars were removed. The smear layer of the exposed dentin surfaces were removed using 17% EDTA followed by 5.25% NaOCl and the teeth were randomly divided into two groups (n ⫽ 45). Group 1 was kept as control and in group 2, uniform smear layer was created using waterproof polishing papers. Three-mm long sections of polyethylene tubing were filled with freshly mixed sealer and placed on the dentin surfaces for conducting a shear bond strength test. The data was calculated as MPa and was statistically analyzed using a two way ANOVA, KruskalWallis and Mann-Whitney U tests. A significant difference was found among the bond strength of the sealers, smear layer, and control groups. AH Plus sealer showed the highest bond strength in smear layer removed surfaces (p ⬍ 0.05). Pretreatment with EDTA/ NaOCl affected bond strength of AH Plus. AH Plus had the highest bond to dentin with or without smear layer.
From the Department of Endodontics, Faculty of Dentistry, University of Selcuk, Konya, Turkey. Address request for reprints to Ayce Unverdi Eldeniz, Selcuk University, Faculty of Dentistry, Department of Endodontics, Konya, Turkey; E-mail address: [email protected]. Copyright © 2005 by the American Association of Endodontists
T
he smear layer is a layer of debris on the root canal wall and has been shown to be packed into some of the dentinal tubules (1–3). This layer is a direct result of canal instrumentation and is not present in uninstrumented canals (1–2). The make-up of the smear layer is primarily inorganic particles of calcified tissue. It is also believed to contain some organic material, including necrotic, and viable pulp tissue, odontoblastic processes, bacteria, and blood cells (2). Much of the attention that the smear layer has received over the past decade has been in relation to methods of smear layer removal (2, 4, 5). Goldman et al. (5) reported that the most effective way to remove the smear layer is using NaOCl and EDTA. Smear layer covers prepared areas and prevents medicaments and filling material from penetrating the dentinal tubules or even contacting the canal wall. Some authors state that this layer should be removed (1, 2, 6). Others, Pashley et al. (7) state that the smear layer should be left intact, as it can actually form a protective barrier. The smear layer may be responsible for excluding bacteria from dentinal tubules and as well as restricting the surface area available for the diffusion of many molecules. Also, it has been shown in the literature that some of the sealers’ bond strength on dentin was better with the smear layer present (8). The adhesion, or in other words, the bond of endodontic sealers to dentin (9, 10) has been the subject of several studies in the last decade (11–13). The removal of the smear layer before filling the root canal system may enhance the ability of filling materials to enter dentinal tubules (1). This may increase the adhesive strength of sealers to dentin and may also improve the sealing ability of the filling. Smear layer removal is proved to be beneficial in leakage studies (14 –18). By studying the adhesive strength of root canal sealers, whether or not the smear layer is present, some additional insight may be gained on the need to remove this layer. The purpose of this investigation was to measure the relative adhesive strength of three resin based root canal sealers to dentin by shear test, with or without the presence of smear layer.
Materials and Methods Ninety human third molar teeth were scaled to remove all adhering soft tissue and debris, washed under running tap water, placed in distilled water, and refrigerated at 4°C. The coronal two-thirds of 90 extracted human third molars were removed with a low speed diamond saw (Isomet, Buehler Ltd., Lake Bluff, NY) and the exposed dentin surfaces were used. The teeth were then fixed with cold acrylic resin to a plastic cylindrical ring (2 cm in diameter and 2.5 cm deep). The rings were filled with autopolymerizing polymethyl methacrylate (PMMA- Meliodent, H.Kulzer, Berkshire) mixed in accordance with the manufacturer’s instructions to embed the tooth with its coronal surface exposed. After the PMMA had set, to get a flat, superficial layer of dentin, coronal surface of tooth was grounded on a water-irrigated grinding wheel (Buehler Ltd.) with the sequential use of #180 and #320 SiC paper.
Dentin Conditioning The smear layer of the exposed dentin surfaces was removed by rinsing for three minutes with 17% EDTA and followed by 5.25% NaOCl. The teeth were divided into two groups and smear layer was reproduced under water irrigation by #600 SiC paper in the first group. The surfaces were then rinsed with water for 60 s and dried. Each group was then divided into three subgroups (n ⫽ 15) according to the sealer that was used: Diaket, AH Plus, and Endo-REZ (Table 1).
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Shearing Bond Strength of Endo-REZ
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Basic Research—Technology TABLE 1. Root canal sealers tested Sealer
Manufacturer
Lot no.
Constituents
Diaket
ESPE, Seefeld, Germany
FW0067705
AH Plus
Dentsply, De Trey, Germany
0005001130
EndoREZ
Ultradent Corp., South Jordan, UT
53HO
Powder: ZnO 97%; bismuth phosphate 3% Liquid: Propionylacetophenone 76%, copolymers of vinyl acetate, vinyl chloride, and vinylisobutyl ether 23%, 2.2⬘-dihydroxy-5.5⬘-dichlorodiphenylmethane 0.5%, triethanolamine 0.2% Paste A(epoxy): diglycidil-bisphenol-A-ether, calcium tungsten, zirconium oxide, aerosol, iron, oxide. Paste B (amina): Amina 1-adamantane, N, Ndibenzyl-5-oxanonandiamine-1, 9, TCD-diamine, calcium tungsten, zirconium oxide, silicone oxide 30% urethane dimethacrylate
Application of Resin Sealers Polyethylene tubes were cut to form 3 mm high cylinders. These cylinders were used to apply the sealers on to the dentin with a constant surface area of 3.45 cm2. To accurately restrict the sealer to a particular area of dentin, adhesive Teflon tape with a hole with respect to the size of the cylinder’s contact area was affixed to the dentin. A window to the predetermined bond area was provided by this hole. The sealers were mixed according to the manufacturer’s instructions, at room temperature at 23°C and a humidity of 37%. To eliminate air bubbles, the polyethylene cylinders were filled from the bottom with a plastic instrument and then vibrated for 15 s to eliminate air bubbles. Preparation and placement of each material did not exceed 5 min. All sealer cylinders were allowed to bench set for 2 h to ensure that initial setting reaction had taken place. Specimens were then stored at 100% humidity at 37°C for a period of 1 wk. The specimens were removed from the incubator, air dried, and then were perpendicularly engaged at their bases with a custom probe (Ultradent Products Inc., South Jordan, UT) in a universal testing machine (Micro 500, Universal Testometric Co., Ltd., UK) at a crosshead speed of 0.5 mm/min. The probe was positioned so that the chisel would travel parallel to the dentinal surface and contact the sealer cylinder at its interface with this surface. The shear force required to separate the cylinder from the dentin was recorded in Newtons (N) for each specimen, then divided by the contact surface area to determine the shear bond strength in Mega Pascals (MPa). After testing, the fracture modes were examined in a dissecting microscope with 22⫻ magnification (SZ-TP Olympus, Japan). Statistical Analysis Two-factor ANOVA followed by Kruskal-Wallis and Mann-Whitney U tests were used to analyze the data. Significance was established at the 5% level.
Results The mean shear bond strengths for the various subgroups are illustrated in Table 2. Mean shear bond strengths ranged from 0.81 MPa (Endo-REZ) to 6.09 MPa (AH Plus). AH Plus had the highest bond strength values to dentin regardless of the effect of smear layer (p ⬍ 0.05). The differences in bond strength between Diaket and Endo-REZ were not statistically significant (p ⬎ 0.05). The removal of the smear layer significantly increased the adhesion of AH Plus. The inspection of fractured surfaces by stereomicroscope under 22⫻ magnification revealed that the bond failure to be mainly adhesive for Endo-REZ and Diaket. The failure appeared to be predominantly cohesive within the sealer for AH Plus, with some areas debonding in the sealer-dentin interface. 294
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TABLE 2. The effect of absence or presence of smear layer on the shear bond strength of three resin based sealers to dentin Groups Diaket AH Plus EndoREZ
Smear Layer
n
Mean ⴞ SD (MPa)*
Smear (⫺) Smear (⫹) Smear (⫺) Smear (⫹) Smear (⫺)
15 15 15 15 15
1.12 ⫾ 0.47c 0.91 ⫾ 0.33c 6.09 ⫾ 1.33a 3.29 ⫾ 1.14b 1.17 ⫾ 0.36c
Smear (⫹)
15
0.81 ⫾ 0.33c
* The same letters in the column are not statistically significant (p ⬎ 0.05).
Discussion Adhesion of the root-canal filling on the dentinal walls seems to be advantageous for two main reasons. In a static situation, it should eliminate any space that allows the percolation of fluids between the filling and the wall (13). In a dynamic situation, it is needed to resist dislodgement of the filling during subsequent manipulation (19). The adhesion depends on a multitude of interacting factors including the surface energy of the adherent (dentine or gutta-percha); the surface tension of the adhesive (sealer), its ability to wet the surfaces and the cleanliness of the adherent surface (20). The sealer must also have cohesive strength to hold the obturation together (20). The American Dental Association (21) established a series of regulations and tests for the study of the physical properties of root canal sealers. However, because of the lack of consensus among researchers, adhesion tests were not standardized. Adhesion tests measure either tensile bond strength, where the bond is broken by a force perpendicular to the interface between material and surface, or shear strength, where the force is parallel to the interface between the material and surface. None of the adhesion tests has been generally accepted, therefore, the shear bond test was used in this study. The shear test was developed for measuring the bond of endodontic sealers to dentin and guttapercha, and has been proven to be effective and reproducible (10, 22). The test model does not imitate clinical conditions. Attempts to closely duplicate these have resulted in complicated models that are difficult to reproduce, and sometimes even to interpret (23, 24). Root dentin is not uniform and the surface of the canal walls that has been prepared during the endodontic treatment may differ widely. This is true not only between specimens, but also between sites in the same root, according to the level or even the direction of the wall-proximal or faciolingual (24 –25). Therefore, coronal, rather than root dentin, was used for better reproducibility. The endodontic smear layer forms over the surface of dentinal walls when the root canals are instrumented (3), and has received considerable amount of attention over the past decade. Not only can the
JOE — Volume 31, Number 4, April 2005
Basic Research—Technology smear layer act as a reservoir or substrate for microorganisms (26), but it can also obstruct the extension of sealer tags into the dentinal tubules and thereby decreasing adhesion by micromechanical forces (27). Several in vitro studies have shown less leakage into filled root when the smear layer was removed (14 –16). So smear layer removal methods were taken much attention (2–3, 28). A variation of the method of Goldman et al. (5) for removing the smear layer, making it applicable to split teeth, rather than instrumented canals was used in this study, and was shown to effectively remove the smear layer (Fig. 1, A and B). It is believed that for good adhesion, the adherent surface should be clean and smooth to enable intimate contact between it and the adhesive. The present investigation does not support this idea but it is in agreement with Gettleman et al. (30) and Pecora et al. (31), as the highest adhesive strength resulted when the smear layer was removed. Scanning electron micrographs revealed that smear layer removal exposed the dentinal tubules creating a much more irregular surface, compared with those samples where the smear layer was left intact. All the sealers tested showed measurable adhesive properties. This result is in agreement with the data of Orstavik et al. (13), but differs from the results of McComb and Smith (12) who tested AH 26, Diaket and ProcoSol and did not find any adhesion to dentine in the case of Diaket. This discrepancy may be explained by the fact that McComb and Smith (12) stored the test specimens in distilled water, not in a humid atmosphere as in the present study. Saleh et al. (20) found lower bond strength values with AH Plus sealer (1.19 – 0.47 MPa) on dentin surfaces treated with distilled water and 0.55 MPa on dentin surfaces treated with only 17% EDTA. These findings are in disagreement with our study and this may be a result of not using 5.25% NaOCl irrigation after EDTA as a final rinse when removing smear layer. Gettleman et al. (30) investigated the effect of smear layer removal on adhesion and reported smaller mean values for AH 26 than our study. Lee et al. (32) also evaluated the adhesion of AH 26 to dentin and also reported lower bond strength values than this study. The different mean values reported in these two studies may be explained by the different method used to measure bond strength (tensile strength) (24), the different substrates (33), and higher adhesive properties of AH Plus when compared to AH 26. Endo-REZ showed the lowest bond strength to dentin. Two tentative assumptions may explain this result without negating our basic premise that a strong bond to dentin is an important property of a sealer. One explanation may be that the cohesive strength of the set sealer may be lower than its adhesive strength. The second explanation could be
Figure 1. (A and B) Photomicrographs of two sectioned teeth showing the ability to produce and remove the smear layer.
JOE — Volume 31, Number 4, April 2005
that some sealers behave differently, depending on the thickness of the layer that is being tested (34). The fact that the failure of the shear tested specimens was seen mainly through the sealer rather than at the dentin-sealer interface indicates that adhesion of the AH Plus sealer to the dentin is stronger than the sealer itself. Clinically, this suggests that there may be a reduced chance of disturbing the seal of the root canal when the material is manipulated after it has set, compared with sealers that have less adhesiveness to dentin. The presence of the hybrid layer suggests that there could be increased resistance to bacterial ingress, compared with instances where the hybrid layer is absent. However, both of these possibilities are speculative, and further investigation is required before definitive statements can be made. The determination of the adherence of an endodontic sealer on dentin represents a preliminary step in establishing the efficacy of filling materials and methods. Further studies, however, must establish if increased adherence equates with increased efficacy, particularly with improved resistance to bacterial ingress.
References 1. White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by plastic filling materials. J Endod 1975;10:558 – 62. 2. Mader CL, Baumgartner C, Peters DD. Scanning electron microscopic investigation of the smeared layer on root canal walls. J Endod 1984;10:477– 83. 3. Mc Comb D, Smith DC. A preliminary scanning electron microscopic study of root canals after endodontic procedures. J Endod 1974;1:238 – 42. 4. Goldman LB, Goldman M, Kronman JH, Lin PS. The efficacy of several irrigating solutions for endodontics: a scanning electron microscopic study. Oral Surg 1981; 52:197–204. 5. Goldman M, Goldman LB, Cavaleri R, Bogis J, Lin PS. The efficacy of several endodontic irrigating solutions: a scanning electron microscopic study. Part 2. J Endod 1982; 8:487–92. 6. White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by endodontic filling materials. Part 2. J Endod 1987;13:369 –74. 7. Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer removal. J Prosthet Dent 1981;46:531–7. 8. Lalh MS, Titley K, Torneck CD, Friedman S. The shear bond strength of glass ionomer cement sealers to bovine dentine conditioned with common endodontic irrigants. Int Endod J 1999;32:430 –5. 9. Anusavice KJ. Phillips’ science of dental materials, 10th ed. Philadelphia: W.B. Saunders Co., 1996:25–30. 10. Tagger M, Tagger E, Tjan AHL, Bakland LK. Measurement of adhesion of endodontic sealers to dentin. J Endod 2002;28:351– 4. 11. Grossman LI. Physical properties of root canal cements. J Endod 1976;2:66 –75. 12. Mc Comb D, Smith DC. Comparison of physical properties of polycarboxylate-based and conventional root canal sealers. J Endod 1976;2:28 –35. 13. Orstavik D, Eriksen HM, Beyer-Olsen EM. Adhesive properties and leakage of root canal sealers in vitro. Int Endod J 1983;16:59 – 63. 14. Abramovich A, Goldberg F. The relationship of the root canal sealer to the dentine wall. An in vitro study using scanning electron microscope. J Br Endod Soc 1976;9: 81– 6. 15. Tidmarsh BG. Acid-cleansed and resin-sealed root canals. J Endod 1978;4:117–21. 16. Clark-Holke D, Drake D, Walton R, Rivera E, Guthmiller JM. Bacterial penetration through canals of endodontically treated teeth in the presence or absence of the smear layer. J Dent 2003;31:275– 81. 17. von Fraunhofer JA, Fagundes DK, McDonald NJ, Dumsha TC. The effect of root canal preparation on microleakage within endodontically treated teeth: an in vitro study. Int Endod J 2000;33:355– 60. 18. Economides N, Liolios E, Kolokuris I, Beltes P. Long-term evaluation of the influence of smear layer removal on the sealing ability of different sealers. J Endod 1999;25: 123–5. 19. Stewart GG. A comparative study of three root canal sealing agents (Part 1). Oral Surg 1958;11:1029 – 41. 20. Saleh IM, Ruyter IE, Haapasalo M, Ørstavik D. The effects of dentine pretreatment on the adhesion of root canal sealers. Int Endod J 2002;35:859 – 66. 21. ANSI/ADA specification no. 57 for endodontic filling materials. J Am Dent Assoc 1984;108:88. 22. Tagger M, Tagger E, Tjan AHL, Bakland LK. Shearing bond strength of endodontic sealers to gutta-percha. J Endod 2003;29:191–3. 23. Wennberg A, Ørstavik D. Adhesion of root canal sealers to bovine dentine and guttapercha. Int Endod J 1990;23:13–9.
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Basic Research—Technology 24. Galun EA, Saleh N, Lewinstein I. Diametral tensile strength and bonding to dentin of type 1 glass ionomer cements. J Prosthet Dent 1994;72:424 – 8. 25. Weiger R, Heuchert T, Hahn R, Lost C. Adhesion of a glass ionomer cement to human radicular dentine. Endod Dent Traumatol 1995;11:214 –9. 26. Pashley DH. Smear layer: physiological considerations. Oper Dent Suppl 1984;3:13–29. 27. Kouvas V, Liolios E, Vassiliadis L, Parissis-Messimeris S, Boutsioukis A. Influence of smear layer on depth of penetration of three endodontic sealers: an SEM study. Endod Dent Traumatol 1998;14:191–5. 28. Moodnik RM, Dorn SO, Feldman FJ, Levey M, Borden BG. Efficacy of biomechanical instrumantation: a scanning electron microscopic study. J Endod 1976;2:261– 6. 29. McComb D, Smith DC. Comparison of physical properties of polycarboxylate-based and conventional root canal sealers. J Endod 1976;1:228 –35.
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30. Gettleman BH, Messer HH, ElDeeb ME. Adhesion of sealer cements to dentin with and without the smear layer. J Endod 1991;17:15–20. 31. Pecora JD, Cussioli AL, Guerisoli DMZ, Marchesan MA, SousaNeto MD, Brugnera JA. Evaluation of Er:YAG laser and EDTAC on dentin adhesion of six endodontic sealers. Braz Dent J 2001;12:27–30. 32. Lee KW, Williams MC, Camps JJ, Pashley DH. Adhesion of endodontic sealers to dentin and gutta-percha. J Endod 2002;28:684 – 88. 33. Schilke R, Bauss O, Lisson JA, Schuckar M, Geurtsen W. Bovine dentin as a substitute for human dentin in shear bond strength measurements. Am J Dent 1999;12: 92– 6. 34. Pommel L, About I, Pashley D, Camps J. Apical leakage of four endodontic sealers. J Endod 2003;29:208 –10.
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Shear Bond Strength of Three Resin Based Sealers to Dentin With and Without the Smear Layer Ayce Unverdi Eldeniz, DDS, PhD, Ali Erdemir, DDS, PhD, and Sema Belli, DDS, PhD Abstract Bond strength of root canal sealers to dentin is an important property for the integrity of the sealings of root-canals. The purpose of this study was to test shear bond strength of three endodontic sealers (Diaket, AH Plus and Endo-REZ). The coronal two thirds of ninety extracted human third molars were removed. The smear layer of the exposed dentin surfaces were removed using 17% EDTA followed by 5.25% NaOCl and the teeth were randomly divided into two groups (n ⫽ 45). Group 1 was kept as control and in group 2, uniform smear layer was created using waterproof polishing papers. Three-mm long sections of polyethylene tubing were filled with freshly mixed sealer and placed on the dentin surfaces for conducting a shear bond strength test. The data was calculated as MPa and was statistically analyzed using a two way ANOVA, KruskalWallis and Mann-Whitney U tests. A significant difference was found among the bond strength of the sealers, smear layer, and control groups. AH Plus sealer showed the highest bond strength in smear layer removed surfaces (p ⬍ 0.05). Pretreatment with EDTA/ NaOCl affected bond strength of AH Plus. AH Plus had the highest bond to dentin with or without smear layer.
From the Department of Endodontics, Faculty of Dentistry, University of Selcuk, Konya, Turkey. Address request for reprints to Ayce Unverdi Eldeniz, Selcuk University, Faculty of Dentistry, Department of Endodontics, Konya, Turkey; E-mail address: [email protected]. Copyright © 2005 by the American Association of Endodontists
T
he smear layer is a layer of debris on the root canal wall and has been shown to be packed into some of the dentinal tubules (1–3). This layer is a direct result of canal instrumentation and is not present in uninstrumented canals (1–2). The make-up of the smear layer is primarily inorganic particles of calcified tissue. It is also believed to contain some organic material, including necrotic, and viable pulp tissue, odontoblastic processes, bacteria, and blood cells (2). Much of the attention that the smear layer has received over the past decade has been in relation to methods of smear layer removal (2, 4, 5). Goldman et al. (5) reported that the most effective way to remove the smear layer is using NaOCl and EDTA. Smear layer covers prepared areas and prevents medicaments and filling material from penetrating the dentinal tubules or even contacting the canal wall. Some authors state that this layer should be removed (1, 2, 6). Others, Pashley et al. (7) state that the smear layer should be left intact, as it can actually form a protective barrier. The smear layer may be responsible for excluding bacteria from dentinal tubules and as well as restricting the surface area available for the diffusion of many molecules. Also, it has been shown in the literature that some of the sealers’ bond strength on dentin was better with the smear layer present (8). The adhesion, or in other words, the bond of endodontic sealers to dentin (9, 10) has been the subject of several studies in the last decade (11–13). The removal of the smear layer before filling the root canal system may enhance the ability of filling materials to enter dentinal tubules (1). This may increase the adhesive strength of sealers to dentin and may also improve the sealing ability of the filling. Smear layer removal is proved to be beneficial in leakage studies (14 –18). By studying the adhesive strength of root canal sealers, whether or not the smear layer is present, some additional insight may be gained on the need to remove this layer. The purpose of this investigation was to measure the relative adhesive strength of three resin based root canal sealers to dentin by shear test, with or without the presence of smear layer.
Materials and Methods Ninety human third molar teeth were scaled to remove all adhering soft tissue and debris, washed under running tap water, placed in distilled water, and refrigerated at 4°C. The coronal two-thirds of 90 extracted human third molars were removed with a low speed diamond saw (Isomet, Buehler Ltd., Lake Bluff, NY) and the exposed dentin surfaces were used. The teeth were then fixed with cold acrylic resin to a plastic cylindrical ring (2 cm in diameter and 2.5 cm deep). The rings were filled with autopolymerizing polymethyl methacrylate (PMMA- Meliodent, H.Kulzer, Berkshire) mixed in accordance with the manufacturer’s instructions to embed the tooth with its coronal surface exposed. After the PMMA had set, to get a flat, superficial layer of dentin, coronal surface of tooth was grounded on a water-irrigated grinding wheel (Buehler Ltd.) with the sequential use of #180 and #320 SiC paper.
Dentin Conditioning The smear layer of the exposed dentin surfaces was removed by rinsing for three minutes with 17% EDTA and followed by 5.25% NaOCl. The teeth were divided into two groups and smear layer was reproduced under water irrigation by #600 SiC paper in the first group. The surfaces were then rinsed with water for 60 s and dried. Each group was then divided into three subgroups (n ⫽ 15) according to the sealer that was used: Diaket, AH Plus, and Endo-REZ (Table 1).
JOE — Volume 31, Number 4, April 2005
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293
Basic Research—Technology TABLE 1. Root canal sealers tested Sealer
Manufacturer
Lot no.
Constituents
Diaket
ESPE, Seefeld, Germany
FW0067705
AH Plus
Dentsply, De Trey, Germany
0005001130
EndoREZ
Ultradent Corp., South Jordan, UT
53HO
Powder: ZnO 97%; bismuth phosphate 3% Liquid: Propionylacetophenone 76%, copolymers of vinyl acetate, vinyl chloride, and vinylisobutyl ether 23%, 2.2⬘-dihydroxy-5.5⬘-dichlorodiphenylmethane 0.5%, triethanolamine 0.2% Paste A(epoxy): diglycidil-bisphenol-A-ether, calcium tungsten, zirconium oxide, aerosol, iron, oxide. Paste B (amina): Amina 1-adamantane, N, Ndibenzyl-5-oxanonandiamine-1, 9, TCD-diamine, calcium tungsten, zirconium oxide, silicone oxide 30% urethane dimethacrylate
Application of Resin Sealers Polyethylene tubes were cut to form 3 mm high cylinders. These cylinders were used to apply the sealers on to the dentin with a constant surface area of 3.45 cm2. To accurately restrict the sealer to a particular area of dentin, adhesive Teflon tape with a hole with respect to the size of the cylinder’s contact area was affixed to the dentin. A window to the predetermined bond area was provided by this hole. The sealers were mixed according to the manufacturer’s instructions, at room temperature at 23°C and a humidity of 37%. To eliminate air bubbles, the polyethylene cylinders were filled from the bottom with a plastic instrument and then vibrated for 15 s to eliminate air bubbles. Preparation and placement of each material did not exceed 5 min. All sealer cylinders were allowed to bench set for 2 h to ensure that initial setting reaction had taken place. Specimens were then stored at 100% humidity at 37°C for a period of 1 wk. The specimens were removed from the incubator, air dried, and then were perpendicularly engaged at their bases with a custom probe (Ultradent Products Inc., South Jordan, UT) in a universal testing machine (Micro 500, Universal Testometric Co., Ltd., UK) at a crosshead speed of 0.5 mm/min. The probe was positioned so that the chisel would travel parallel to the dentinal surface and contact the sealer cylinder at its interface with this surface. The shear force required to separate the cylinder from the dentin was recorded in Newtons (N) for each specimen, then divided by the contact surface area to determine the shear bond strength in Mega Pascals (MPa). After testing, the fracture modes were examined in a dissecting microscope with 22⫻ magnification (SZ-TP Olympus, Japan). Statistical Analysis Two-factor ANOVA followed by Kruskal-Wallis and Mann-Whitney U tests were used to analyze the data. Significance was established at the 5% level.
Results The mean shear bond strengths for the various subgroups are illustrated in Table 2. Mean shear bond strengths ranged from 0.81 MPa (Endo-REZ) to 6.09 MPa (AH Plus). AH Plus had the highest bond strength values to dentin regardless of the effect of smear layer (p ⬍ 0.05). The differences in bond strength between Diaket and Endo-REZ were not statistically significant (p ⬎ 0.05). The removal of the smear layer significantly increased the adhesion of AH Plus. The inspection of fractured surfaces by stereomicroscope under 22⫻ magnification revealed that the bond failure to be mainly adhesive for Endo-REZ and Diaket. The failure appeared to be predominantly cohesive within the sealer for AH Plus, with some areas debonding in the sealer-dentin interface. 294
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TABLE 2. The effect of absence or presence of smear layer on the shear bond strength of three resin based sealers to dentin Groups Diaket AH Plus EndoREZ
Smear Layer
n
Mean ⴞ SD (MPa)*
Smear (⫺) Smear (⫹) Smear (⫺) Smear (⫹) Smear (⫺)
15 15 15 15 15
1.12 ⫾ 0.47c 0.91 ⫾ 0.33c 6.09 ⫾ 1.33a 3.29 ⫾ 1.14b 1.17 ⫾ 0.36c
Smear (⫹)
15
0.81 ⫾ 0.33c
* The same letters in the column are not statistically significant (p ⬎ 0.05).
Discussion Adhesion of the root-canal filling on the dentinal walls seems to be advantageous for two main reasons. In a static situation, it should eliminate any space that allows the percolation of fluids between the filling and the wall (13). In a dynamic situation, it is needed to resist dislodgement of the filling during subsequent manipulation (19). The adhesion depends on a multitude of interacting factors including the surface energy of the adherent (dentine or gutta-percha); the surface tension of the adhesive (sealer), its ability to wet the surfaces and the cleanliness of the adherent surface (20). The sealer must also have cohesive strength to hold the obturation together (20). The American Dental Association (21) established a series of regulations and tests for the study of the physical properties of root canal sealers. However, because of the lack of consensus among researchers, adhesion tests were not standardized. Adhesion tests measure either tensile bond strength, where the bond is broken by a force perpendicular to the interface between material and surface, or shear strength, where the force is parallel to the interface between the material and surface. None of the adhesion tests has been generally accepted, therefore, the shear bond test was used in this study. The shear test was developed for measuring the bond of endodontic sealers to dentin and guttapercha, and has been proven to be effective and reproducible (10, 22). The test model does not imitate clinical conditions. Attempts to closely duplicate these have resulted in complicated models that are difficult to reproduce, and sometimes even to interpret (23, 24). Root dentin is not uniform and the surface of the canal walls that has been prepared during the endodontic treatment may differ widely. This is true not only between specimens, but also between sites in the same root, according to the level or even the direction of the wall-proximal or faciolingual (24 –25). Therefore, coronal, rather than root dentin, was used for better reproducibility. The endodontic smear layer forms over the surface of dentinal walls when the root canals are instrumented (3), and has received considerable amount of attention over the past decade. Not only can the
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Basic Research—Technology smear layer act as a reservoir or substrate for microorganisms (26), but it can also obstruct the extension of sealer tags into the dentinal tubules and thereby decreasing adhesion by micromechanical forces (27). Several in vitro studies have shown less leakage into filled root when the smear layer was removed (14 –16). So smear layer removal methods were taken much attention (2–3, 28). A variation of the method of Goldman et al. (5) for removing the smear layer, making it applicable to split teeth, rather than instrumented canals was used in this study, and was shown to effectively remove the smear layer (Fig. 1, A and B). It is believed that for good adhesion, the adherent surface should be clean and smooth to enable intimate contact between it and the adhesive. The present investigation does not support this idea but it is in agreement with Gettleman et al. (30) and Pecora et al. (31), as the highest adhesive strength resulted when the smear layer was removed. Scanning electron micrographs revealed that smear layer removal exposed the dentinal tubules creating a much more irregular surface, compared with those samples where the smear layer was left intact. All the sealers tested showed measurable adhesive properties. This result is in agreement with the data of Orstavik et al. (13), but differs from the results of McComb and Smith (12) who tested AH 26, Diaket and ProcoSol and did not find any adhesion to dentine in the case of Diaket. This discrepancy may be explained by the fact that McComb and Smith (12) stored the test specimens in distilled water, not in a humid atmosphere as in the present study. Saleh et al. (20) found lower bond strength values with AH Plus sealer (1.19 – 0.47 MPa) on dentin surfaces treated with distilled water and 0.55 MPa on dentin surfaces treated with only 17% EDTA. These findings are in disagreement with our study and this may be a result of not using 5.25% NaOCl irrigation after EDTA as a final rinse when removing smear layer. Gettleman et al. (30) investigated the effect of smear layer removal on adhesion and reported smaller mean values for AH 26 than our study. Lee et al. (32) also evaluated the adhesion of AH 26 to dentin and also reported lower bond strength values than this study. The different mean values reported in these two studies may be explained by the different method used to measure bond strength (tensile strength) (24), the different substrates (33), and higher adhesive properties of AH Plus when compared to AH 26. Endo-REZ showed the lowest bond strength to dentin. Two tentative assumptions may explain this result without negating our basic premise that a strong bond to dentin is an important property of a sealer. One explanation may be that the cohesive strength of the set sealer may be lower than its adhesive strength. The second explanation could be
Figure 1. (A and B) Photomicrographs of two sectioned teeth showing the ability to produce and remove the smear layer.
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that some sealers behave differently, depending on the thickness of the layer that is being tested (34). The fact that the failure of the shear tested specimens was seen mainly through the sealer rather than at the dentin-sealer interface indicates that adhesion of the AH Plus sealer to the dentin is stronger than the sealer itself. Clinically, this suggests that there may be a reduced chance of disturbing the seal of the root canal when the material is manipulated after it has set, compared with sealers that have less adhesiveness to dentin. The presence of the hybrid layer suggests that there could be increased resistance to bacterial ingress, compared with instances where the hybrid layer is absent. However, both of these possibilities are speculative, and further investigation is required before definitive statements can be made. The determination of the adherence of an endodontic sealer on dentin represents a preliminary step in establishing the efficacy of filling materials and methods. Further studies, however, must establish if increased adherence equates with increased efficacy, particularly with improved resistance to bacterial ingress.
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30. Gettleman BH, Messer HH, ElDeeb ME. Adhesion of sealer cements to dentin with and without the smear layer. J Endod 1991;17:15–20. 31. Pecora JD, Cussioli AL, Guerisoli DMZ, Marchesan MA, SousaNeto MD, Brugnera JA. Evaluation of Er:YAG laser and EDTAC on dentin adhesion of six endodontic sealers. Braz Dent J 2001;12:27–30. 32. Lee KW, Williams MC, Camps JJ, Pashley DH. Adhesion of endodontic sealers to dentin and gutta-percha. J Endod 2002;28:684 – 88. 33. Schilke R, Bauss O, Lisson JA, Schuckar M, Geurtsen W. Bovine dentin as a substitute for human dentin in shear bond strength measurements. Am J Dent 1999;12: 92– 6. 34. Pommel L, About I, Pashley D, Camps J. Apical leakage of four endodontic sealers. J Endod 2003;29:208 –10.
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