0099-2399/91/1701-0015/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1990 by The American Association of Endodontists
Printed in U.S.A.
VOL. 17, NO. 1, JANUARY1991
Adhesion of Sealer Cements to Dentin with and without the Smear Layer Bradley H. Gettleman, DDS, MS, Harold H. Messer, MDSc, PhD, and Mahmoud E. EIDeeb, BDS, MS
be amorphous with an irregular and granular surface (2). Although some believe that smear layer to be only loosely adherent to the canal wall (2-4), it can also be argued that because it is difficult to remove using physical methods it adheres well to dentin. Much of the attention that the smear layer has received over the past decade has been in relation to methods of smear layer removal (2-6). Goldman et al. (6), using NaOC1 and EDTA, developed what has become recognized as the most effective way to remove the smear layer. Some consider that it is desirable to remove this layer, as it covers prepared areas and prevents medicaments and filling material from penetrating the dentinal tubules or even contacting the canal wall (1, 2, 7). As of today, no material has been shown to enter dentinal tubules with the smear layer present. Removal of the smear layer is also considered to be desirable by others because, in addition to inorganic particles, it may contain some organic material, as well as viable bacteria (2). In contrast, other investigators (8) consider that the smear layer should be left intact, as it may actually form a protective barrier. This layer has been shown to increase microleakage in class I amalgam restorations (9). The smear layer may be responsible for excluding bacteria from dentinal tubules as well as restricting the surface area available for the diffusion of many molecules. One study (10) has shown that the smear layer will prevent bacterial penetration (Streptococcus mutans), but will permit fluid filtration. Another study (11) has revealed that the smear layer simply delays the penetration of some bacteria (Proteus vulgaris) rather than preventing it. Many articles have been written on the physical properties of root canal sealers, including their adhesive strength to dentin (12, 13) and to gutta-percha (12). Adhesive strength measurements may be important to clinical usage, because higher adhesive strengths may reduce leakage in clinical situations. Removal of the smear layer prior to filling the root canal system may enhance the ability of filling materials to enter dentinal tubules (1). This may actually increase the adhesive strength of sealers to dentin and improve the sealing ability of the filling. Few studies (14-16) have been published relating apical seal to the presence or absence of the smear layer where consistent removal of the smear layer was verified by a scanning electron microscope. Two studies (14, 15) revealed that apical leakage was significantly reduced in those teeth obturated with gutta-percha when the smear layer was removed. However, Hydron-obturated canals showed no difference in leakage, whether the smear layer was present or absent.
The influence of a smear layer on the adhesion of sealer cements to dentin was assessed in recently extracted human anterior teeth. A total of 120 samples was tested, 40 per sealer; 20 each with and without the smear layer. The teeth were split longitudinally, and the internal surfaces were ground flat. One-half of each tooth was left with the smear layer intact, while the other half had the smear removed by washing for 3 min with 17% EDTA followed by 5.25% NaOCI. Evidence of the ability to remove the smear layer was verified by scanning electron microscopy. Using a specially designed jig, the sealer was placed into a 4-mm wide x 4-mm deep well which was then set onto the tooth at a g0~ angle and allowed to set for 7 days in 100% humidity at 37~ This set-up was then placed into a mounting jig which was designed for the Instron Universal Testing Machine so that only a tensile load was applied without shearing or applying preloading forces. The set-up was subjected to a tensile load at a crosshead speed of 1 mm per min. The results show significant differences (p < 0.001) among AH26, Sultan, and Sealapex, with AH26 being the strongest and Sealapex being the weakest. The only significant difference with regard to the presence or absence of the smear layer was found with AH26, which had a stronger bond when the smear layer was removed.
The smear layer, as it relates to the root canal system, 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 thickness of this layer varies; however, it will generally be about 1 to 2 um (2). The depth of tubular packing also varies and has been shown to be as much as 40 ~m (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/or viable pulp tissue, odontoblastic processes, bacteria, and blood cells (2). This layer cannot be seen with the naked eye, but under a scanning electron microscope it appears to 15
16
Gettlemen et al.
Journal of Endodontics
Another study (16) reported no significant difference in apical leakage in teeth obturated with laterally condensed guttapercha or by injected thermoplasticized gutta-percha, whether or not the smear layer was removed. In each of the aforementioned studies, only one root canal sealer was used. Both studies used a zinc oxide- and eugenol-based sealer. Therefore, how each of the different basic types of root canal sealers will influence apical leakage when the smear layer is removed has yet to be investigated. Leakage studies where the smear layer has been removed have yet to prove that removal is consistently beneficial. By studying the adhesive strength of root canal sealers, with and without the presence of the smear layer, some additional insight may be gained on the need to remove this layer. Some sealers may respond differently than others once the smear layer is removed. If there is an increase with only one type of sealer and no difference with another, it may be desirable to remove the smear layer only while using that specific sealer. Increasing the adhesive strength of a sealer may reduce the leakage in root canals obturated using that particular sealer. The purpose of this investigation was to measure the relative adhesive strength of root canal sealers to dentin with, as well as without, the presence of the smear layer.
MATERIALS AND M E T H O D S
Selection and Preparation of Teeth Teeth used in this investigation were noncarious, singlerooted, h u m a n anterior teeth which were stored for various periods of time in physiological saline after extraction. These teeth were extracted for various reasons, and none had received endodontic therapy prior to extraction. The teeth were split longitudinally in a buccolingual direction, and the portions of the root surface where the canal had been located were ground until they were smooth and flat using #27 Vulcarbo 7/8-inch separating diamond discs (S. S. White, Philadelphia, PA) at 200 to 300 rpm. This was done in order to create a smear layer. The external surfaces had three 1-mm grooves prepared into them using a high-speed handpiece and a #35 inverted cone bur. These grooves were made on the mesial and distal aspects of the tooth in order to aid in specimen stabilization during testing. One-half of each tooth was left with the smeared layer intact, while the other half was washed with 17% EDTA buffered to a pH of 7.7 for 3 min and then washed with 5.25% NaOCI for 3 min in order to remove the smeared layer. Washing was done by placing the teeth in a sealed flask containing the appropriate solution and shaking by hand. The samples were then rinsed with water and air dried. Evidence of the presence or absence of the smear layer was verified with a scanning electron microscope (Figs. 1 and 2).
FIG 1. Photomicrograph of a sectioned tooth showing the ability to produce a smear layer.
Preparation for Testing The teeth were placed ground surface up onto a metal washer with an 11-mm internal diameter opening and a 26m m external diameter. The teeth were shortened incisally and apically if they were longer than 23 mm. The teeth were secured to the washer with the use of sticky wax. Next, a 10cm long a l u m i n u m cylinder with an external diameter of 26
FIG 2. Photomicrograph of a sectioned tooth showing the ability to remove the smear layer.
Effect of Smear Layer on Sealer Adhesion
Vol. 17, No. 1, January 1991
m m and an internal diameter of 23 m m was placed onto the washer and secured with cyanoacrylate. The void between the internal diameter of the washer and the tooth was sealed with sticky wax in order to prevent the extrusion of dental stone. The entire unit was then inverted, and the well created by placing the aluminum cylinder onto the washer was filled with dental stone which filled the grooves prepared into the external surfaces of the tooth and aided in stabilization. This was allowed to set at 37~ in 100% humidity for 24 h. Three root canal sealer cements were tested for their adhesive strengths. One was a zinc oxide-eugenol based (Sultan), one was an epoxy-resin based (AH26), and one was a calcium hydroxide based (Sealapex). Each of the sealer cements was mixed according to the manufacturer's instructions. The sealers were then poured into a 4-ram deep a l u m i n u m well with a 4-mm internal diameter which had grooves prepared into its internal surface in order to aid in sealer retention. These wells also had bilateral vents which were 1 to 2 m m in diameter to allow for sealer cement expansion and to further aid in forming a mechanical bond between the sealer and the aluminum well. Prior to filling these wells, the rim of each well was coated with a very thin layer of Vaseline. This was done in order to prevent the flow of sealer in this area, as this could have resulted in bonding between the well rim and the tooth, which would have led to inconsistent and inaccurate results. Therefore, the sealer was only able to contact the tooth in a 4-ram diameter circle, or what was 12.5/100 cm 2. Once the sealers were poured into these wells, they were pressed onto the prepared surfaces of the teeth with finger pressure (Fig. 3). The wells which were to receive Sealapex were lined with a paste mixture of dentinal shavings and saline. This was done because a pilot study revealed that even at 37~ in 100% humidity for 1 wk Sealapex would not set. However, when the wells were lined by this mixture, Sealapex would come to
q
I)
a final set. All samples were then allowed to set for 1 wk at 37~ in 100% humidity.
Adhesive Strength Testing Each sample was allowed to bench cool for 1 h prior to testing, in order to reach room temperature. The bond between the sealer and the tooth was then tested for adhesive strength with an Instron (Universal Testing Machine). A mounting device was made for the Instron, which prevented subjecting the specimen to any shearing forces. The jig consisted of a lower aluminum hinge unit, which did not permit any preloading forces, and an upper steel support unit, which permitted an equal distribution of the tensile load as well as easy insertion and reproducibility from one sample to the next (Fig. 4). The specimen was subjected to a tensile load, increasing at a constant crosshead speed o f a 1 m m per rain. The force (in kilograms) at which the bond between the tooth surface and the sealer cement ruptured was recorded and used to calculate the adhesive strength in kilograms per square centimeter.
Sample Groups A total of six groups with 20 samples per group was tested. The groups were: A, AH26 sealer-cement with the smeared layer intact; B, AH26 sealer-cement without the smeared layer; C, Sultan with the smeared layer intact; D, Sultan without the smeared layer; E, Sealapex with the smeared layer intact; and F, Sealapex without the smeared layer, A total of 30 samples, 5 from each group, was tested on each of the 4 days in which data were collected.
\.\ \\
\ ',\\
(i!!!.,:,.,:,s
C
,>>>>)) ,>)>>>> ,))>>>>
q
i
,i
p>))>>) ,~))))) ~>))>))
.))))))
,>>>~,))
,>)>))) ,))))))
,). ,)>
~>) ,)) ,>)
,>>>>>Y q
,>>>>>>'
~PPP,
)>>>>>:
,>)>>)~ ,>>>>>>
,}}>>>F r A
23m~, Z6cnm
F~G 3. Diagram of tooth and sealer-containing wells.
17
L
v
Ib Fue 4. Diagram of apparatus to measure adhesive strength.
18
Journal of Endodontics
GetUemen et al.
Location of Bond Rupture After all samples were tested for their adhesive strengths, a random sampling of the tested teeth were prepared for, and evaluated by, the use of a scanning electron microscope. This was done in order to evaluate where the bond failed when the smear layer was left intact, as well as any sealer penetration of the dentinal tubules by sealer when the smear layer was removed.
Statistical Analysis The overall effect of smear layer removal on adhesive strength was evaluated using two-way analysis of variance. The inter- and intragroup comparisons were evaluated using tests of simple main effects. RESULTS A total of 120 samples were tested, 40 per sealer; 20 with the smear layer and 20 without the smear layer. The force, in kilograms, at which the bond between the tooth structure and sealer ruptured was recorded and used to calculate the tensile adhesive strength in kilograms per square centimeter (kg/ cm2). The results are presented in Table 1. The data demonstrate the relative adhesive strengths of the three sealers, as well as how each sealer compares with itself when adhered to dentin with the smear layer and to dentin without the smear layer. AH26 had the greatest adhesive strength, resulting in 20.38 kg/cm 2 without the smear layer and 12.42 kg/cm 2 with the smear layer. Tests of simple main effects showed this difference to be highly significant (p < 0.001). Sultan proved to have the second greatest overall adhesive strength, exhibiting 6.22 kg/cm 2 with the smear layer and 7.27 kg/cm 2 without the smear layer; there was no significant difference with this sealer whether or not the smear layer was present. Overall, Sealapex showed the lowest adhesive strength, having 0.93 kg/cm 2 with the smear layer and 1.24 kg/cm 2 without the smear layer. The difference here also proved to be statistically insignificant. The difference between any two of the three sealers was highly significant (p < 0.001, tests of simple main effects). Although the overall effect of smear layer removal appeared to be highly significant, resulting in an increase in adhesive strength upon smear layer removal (p < 0.001, by ANOVA), this difference was solely related to AH26. Tests of simple main effects showed that AH26 was the only sealer with a
significant difference in adhesive strength when the smear layer was intact, compared with when the smear layer was removed.
Bond Failure Location In all samples with the smear layer present, the result was an adhesive failure, rather than a failure at the smear layerdentinal tubule interface (Fig. 5). This was consistent for all three sealers tested. In those samples with the smear layer removed, the bond failed at the sealer-dentinal tubule interface for each of the Sultan and Sealapex samples (Fig. 6). However, when AH26 was used, 5 of 20 samples actually had a cohesive failure. DISCUSSION The smear layer, which is a direct result of endodontic instrumentation, has received a considerable amount of attention over the past decade. Much of this attention has been in relation to methods of smear layer removal (2-6). A variation of the method of G o l d m a n et al. (6) for removing the smear layer, making it applicable to longitudinal split teeth, rather than instrumented canals, was used in this study, and was shown to effectively remove the smear layer. The method of testing the relative adhesive strength of sealers to dentin used in the present investigation proved very successful. Extreme care was used when handling the speci-
TABLE 1. Sealer adhesive strengths in relation to the presence or absence of the smear layer (n = 20)
Sealer
Smear Layer (Present/Absent)
Mean Adhesive Strength (_+SEM) in kg/cm2
AH26 AH26 Sultan Sultan Sealapex Sealapex
Present Absent Present Absent Present Absent
12.42 (0.75) 20.38 (1.5) 6.22 (0.37) 7.27 (0.47) 0.93 (0.16) 1.24 (0.20)
FiG 5. Photomicrograph showing the smear layer remaining on the tooth after testing.
Vol. 17, No. 1, January 1991
FiG 6. Photomicrograph showing clean dentinal tubules on a tooth, with the smear layer removed, after testing.
mens to be tested. Orstavik et al. (12) described the problems in testing the adhesive strength of root canal sealers. The authors stated that tensile bonds of low magnitude, as those of the root canal sealers which they tested, are highly susceptible to transverse forces. Thus, the specimens must be handled extremely carefully during mounting in the testing machine, to avoid influences from transverse forces. This was taken into careful consideration during this investigation. The jig used for this study was designed to reduce the influences of transverse or shearing forces during both mounting and testing. However, regardless of the jig design, as well as the care taken during any handling of the samples, it is virtually impossible to totally eliminate these shearing forces. Some investigators (13) believe 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, 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. This irregular surface produced greater adhesion, especially when AH26 was used. As shown in Table 1, smear layer removal did allow greater adhesive strengths for all of the sealers tested; however, as stated previously, there was no significant difference with Sultan and Sealapex whether the smear layer was removed or left intact. AH26 having the greatest adhesive strength of the root canal sealers is consistent with other adhesion studies ( 12, 13,
Effect of Smear Layer on Sealer Adhesion
19
17). This is the only sealer of the three evaluated in this investigation for which previously reported adhesive strengths are available. We found the adhesive strength of AH26 to be 20.38 kg/cm 2 to dentin without the smear layer and 12.42 kg/cm 2 to dentin with the smear layer. McComb and Smith (13), in 1976, reported similar adhesive strength to AH26 to dentin (16.5 kg/cm2). No comment was made as to whether or not this dentin had a smear layer, but the authors made no mention of removing this layer, so most likely it was present. Orstavik et al. (12), making no mention as to the presence or absence of the smear layer, found the adhesive strength of AH26 to be 25.5 kg/cm 2. They made no mention of heating AH26, which could have contributed to this difference since heating will increase the flow rate of AH26. In addition, they used a specific powder to liquid ratio (1.75 g: lg), while we mixed to a consistency that would separate when lifted 1.5 to 2.5 cm from the glass slab when carried with a spatula prior to heating, as stated in the manufacturer's instructions. Grossman (17), in 1976, also evaluated the adhesive strength of AH26, although he did not use the Instron or dentin. Rather, he used a pulley which had 100 g added to it every 10 to 15 s until the sealer was dislodged from a glass microscope slide. The adhesive strength averaged 3.57 kg/ cm 2, which is much lower than the results of all investigations using dentin. The only information from Grossman's study which is presently relative is the relative adhesive strengths of the sealers, and AH26 did show the greatest adhesive strength. No previously published study has reported the adhesive strength of Sultan, the zinc oxide and eugenol sealer we examined. Nevertheless, other studies (12, 13, 17) have reported the adhesive strengths of various other zinc oxide and eugenol sealers. McComb and Smith (13) assessed the adhesive strengths of two zinc oxide and eugenol sealers: Kerr Antiseptic Pulp Canal Sealer and ProcoSol nonstaining cement. They did not report specific values for these sealers; they simply classified their adhesive strengths to be "nil.'" This may be attributed to their using a constant crosshead speed of 10 m m per min rather than 1 m m per minute, at which we found the adhesive strength of Sultan to be 6.22 kg/cm 2 with the smear layer present and 7.23 kg/cm 2 without the smear layer. The zinc oxide and eugenol sealers may not be able to withstand any tensile load which is initiated at this high rate. In addition, the slight variation in constituents, or percentage of specific constituents, between zinc oxide and eugenol sealers may also be attributed to this difference, as only the exact sealers can accurately be compared between studies. Orstavik et al. (12), also studied various other zinc oxide and eugenol sealers including Endomethasone, ProcoSol, and N2 Normal. They found that Endomethasone and N2 Normal had virtually no adhesive strength while ProcoSol had an adhesive strength of almost 15 kg/cm 2. Again, it is very difficult to compare absolute values for zinc oxide and eugenol sealers from one study to another. However, it is clear that Sultan had an adhesive strength above the average of many similar sealers, although it may not be the most adhesive of the zinc oxide and eugenol sealers. Grossman (17) also examined the adhesive strengths of various zinc oxide and eugenol sealers: Kerr, Mynol, N2, N2 No-lead, ProcoSol, Roth 801, Roth 811, RC2B, Tubli-Seal, and zinc oxide and eugenol cement. His results varied from 0 to 2.15 kg/cm 2, all
20
Gettlemen et al.
of which are lower than we found for Sultan. However, as mentioned earlier, due to the different methods and adherent surfaces used, results cannot be easily compared between the two investigations. As far as calcium hydroxide-based sealers are concerned, no published studies have looked at their adhesive strengths. First, there are very few available, the most common being CRCS and Sealapex. In addition, these are relatively new on the market. The adhesive strength of Sealapex was 0.94 kg/ cm 2 with the smear layer present and 1.24 kg/cm 2 without the smear layer. It is obvious that this low adhesive strength is not a very advantageous aspect of this sealer. Adhesive strength is only one aspect of root canal sealers. Further investigation of various aspects of root canal sealers is necessary. Which sealers seal better in the presence as well as the absence of the smear layer is one specific area which needs further evaluation. In addition, studies are needed regarding which sealer works best in specific situations such as open apices, apical deltas, ledged canals, and with specific obturation technique. Also, evaluation on which sealers work best utilizing specific obturating techniques should be completed. The present evaluation examined only one aspect of the question of which sealer is best in terms of adhesion to dentin with and without the smear layer present.
This study was partially funded by a grant from the American Association of Endodontists, Endowment and Memorial Foundation. Drs. Gettleman, Messer, and EIDeeb are affiliated with the School of Dentistry, Division of Endodontics, University of Minnesota, Minneapolis, MN. Address requests for reprints to Dr. Mahmoud E. EIDeeb, Division of Endodontics, School of Dentistry, University of Minnesota, 515 Delaware Street, SE, Minneapolis, MN 55455.
Journal of Endodontics References 1. White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by plastic filling materials. J Endodon 1975;10:55862. 2. Mader CL, Baumgartner C, Peters DD. Scanning electron microscopic investigation of the smeared layer on root canal walls. J Endodon 1984;10:47783. 3. McComb D, Smith DC. A preliminary scanning electron microscopic study of root canals after endodontic procedures. J Endodon 1974;1:238-42. 4. Moodnik RM, Dorn SO, Feldman MJ, Levey M, Borden BG. Efficacy of biomechanical instrumentation: a scanning electron microscopic study. J Endodon 1976;2:261-6. 5. 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. 6. 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 Endodon 1982;8:487-92. 7. White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by endodontic filling materials. Part 2. J Endodon 1987;13:369-74. 8. Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer removal. J Prosthet Dent 1981 ;46:531-7. 9. Pashley DH, Depew D. Effects of smear layer, copalite, and oxalate on microleakage. Oper Dent 1986;11:95-102. 10. Michelich V, Schuster GS, Pashley DH. Bacterial penetration of human dentin. J Dent Res 1980;59:1398. 11. Williams S, Goldman M. Permeability of the smeared layer by a strain of Proteus vulgaris. J Endodon 1985;11:385-8. 12. Orstavik D, Ericksen NM, Beyer-Olsen EM. Adhesive properties and leakage of root canal sealers in vitro. Int Endod J 1983;16:99-107. 13. McComb D, Smith DC. Comparison of physical properties of polycarboxylate-based and conventional root canal sealers. J Endodon 1976;1:22835. 14. Cergneux M, Ciocchi B, Dietschi JM, Holz J. The influence of the smear layer on the sealing ability of canal obturation. Int Endod J 1987;20:228-32. 15. Kennedy WA, Walker WA, Gough RW. Smear layer removal effects on apical leakage. J Endodon 1986;12:21-7. 16. Evans JT, Simon JHS. Evaluation of the apical seal produced by injected thermoplasticized gutta-percha in the absence of smear layer and root canal sealer. J Endodon 1986;12:101-7. 17. Grossman LI. Physical properties of root canal cements. J Endodon 1976;2:166-75.
Printed in U.S.A.
VOL. 17, NO. 1, JANUARY1991
Adhesion of Sealer Cements to Dentin with and without the Smear Layer Bradley H. Gettleman, DDS, MS, Harold H. Messer, MDSc, PhD, and Mahmoud E. EIDeeb, BDS, MS
be amorphous with an irregular and granular surface (2). Although some believe that smear layer to be only loosely adherent to the canal wall (2-4), it can also be argued that because it is difficult to remove using physical methods it adheres well to dentin. Much of the attention that the smear layer has received over the past decade has been in relation to methods of smear layer removal (2-6). Goldman et al. (6), using NaOC1 and EDTA, developed what has become recognized as the most effective way to remove the smear layer. Some consider that it is desirable to remove this layer, as it covers prepared areas and prevents medicaments and filling material from penetrating the dentinal tubules or even contacting the canal wall (1, 2, 7). As of today, no material has been shown to enter dentinal tubules with the smear layer present. Removal of the smear layer is also considered to be desirable by others because, in addition to inorganic particles, it may contain some organic material, as well as viable bacteria (2). In contrast, other investigators (8) consider that the smear layer should be left intact, as it may actually form a protective barrier. This layer has been shown to increase microleakage in class I amalgam restorations (9). The smear layer may be responsible for excluding bacteria from dentinal tubules as well as restricting the surface area available for the diffusion of many molecules. One study (10) has shown that the smear layer will prevent bacterial penetration (Streptococcus mutans), but will permit fluid filtration. Another study (11) has revealed that the smear layer simply delays the penetration of some bacteria (Proteus vulgaris) rather than preventing it. Many articles have been written on the physical properties of root canal sealers, including their adhesive strength to dentin (12, 13) and to gutta-percha (12). Adhesive strength measurements may be important to clinical usage, because higher adhesive strengths may reduce leakage in clinical situations. Removal of the smear layer prior to filling the root canal system may enhance the ability of filling materials to enter dentinal tubules (1). This may actually increase the adhesive strength of sealers to dentin and improve the sealing ability of the filling. Few studies (14-16) have been published relating apical seal to the presence or absence of the smear layer where consistent removal of the smear layer was verified by a scanning electron microscope. Two studies (14, 15) revealed that apical leakage was significantly reduced in those teeth obturated with gutta-percha when the smear layer was removed. However, Hydron-obturated canals showed no difference in leakage, whether the smear layer was present or absent.
The influence of a smear layer on the adhesion of sealer cements to dentin was assessed in recently extracted human anterior teeth. A total of 120 samples was tested, 40 per sealer; 20 each with and without the smear layer. The teeth were split longitudinally, and the internal surfaces were ground flat. One-half of each tooth was left with the smear layer intact, while the other half had the smear removed by washing for 3 min with 17% EDTA followed by 5.25% NaOCI. Evidence of the ability to remove the smear layer was verified by scanning electron microscopy. Using a specially designed jig, the sealer was placed into a 4-mm wide x 4-mm deep well which was then set onto the tooth at a g0~ angle and allowed to set for 7 days in 100% humidity at 37~ This set-up was then placed into a mounting jig which was designed for the Instron Universal Testing Machine so that only a tensile load was applied without shearing or applying preloading forces. The set-up was subjected to a tensile load at a crosshead speed of 1 mm per min. The results show significant differences (p < 0.001) among AH26, Sultan, and Sealapex, with AH26 being the strongest and Sealapex being the weakest. The only significant difference with regard to the presence or absence of the smear layer was found with AH26, which had a stronger bond when the smear layer was removed.
The smear layer, as it relates to the root canal system, 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 thickness of this layer varies; however, it will generally be about 1 to 2 um (2). The depth of tubular packing also varies and has been shown to be as much as 40 ~m (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/or viable pulp tissue, odontoblastic processes, bacteria, and blood cells (2). This layer cannot be seen with the naked eye, but under a scanning electron microscope it appears to 15
16
Gettlemen et al.
Journal of Endodontics
Another study (16) reported no significant difference in apical leakage in teeth obturated with laterally condensed guttapercha or by injected thermoplasticized gutta-percha, whether or not the smear layer was removed. In each of the aforementioned studies, only one root canal sealer was used. Both studies used a zinc oxide- and eugenol-based sealer. Therefore, how each of the different basic types of root canal sealers will influence apical leakage when the smear layer is removed has yet to be investigated. Leakage studies where the smear layer has been removed have yet to prove that removal is consistently beneficial. By studying the adhesive strength of root canal sealers, with and without the presence of the smear layer, some additional insight may be gained on the need to remove this layer. Some sealers may respond differently than others once the smear layer is removed. If there is an increase with only one type of sealer and no difference with another, it may be desirable to remove the smear layer only while using that specific sealer. Increasing the adhesive strength of a sealer may reduce the leakage in root canals obturated using that particular sealer. The purpose of this investigation was to measure the relative adhesive strength of root canal sealers to dentin with, as well as without, the presence of the smear layer.
MATERIALS AND M E T H O D S
Selection and Preparation of Teeth Teeth used in this investigation were noncarious, singlerooted, h u m a n anterior teeth which were stored for various periods of time in physiological saline after extraction. These teeth were extracted for various reasons, and none had received endodontic therapy prior to extraction. The teeth were split longitudinally in a buccolingual direction, and the portions of the root surface where the canal had been located were ground until they were smooth and flat using #27 Vulcarbo 7/8-inch separating diamond discs (S. S. White, Philadelphia, PA) at 200 to 300 rpm. This was done in order to create a smear layer. The external surfaces had three 1-mm grooves prepared into them using a high-speed handpiece and a #35 inverted cone bur. These grooves were made on the mesial and distal aspects of the tooth in order to aid in specimen stabilization during testing. One-half of each tooth was left with the smeared layer intact, while the other half was washed with 17% EDTA buffered to a pH of 7.7 for 3 min and then washed with 5.25% NaOCI for 3 min in order to remove the smeared layer. Washing was done by placing the teeth in a sealed flask containing the appropriate solution and shaking by hand. The samples were then rinsed with water and air dried. Evidence of the presence or absence of the smear layer was verified with a scanning electron microscope (Figs. 1 and 2).
FIG 1. Photomicrograph of a sectioned tooth showing the ability to produce a smear layer.
Preparation for Testing The teeth were placed ground surface up onto a metal washer with an 11-mm internal diameter opening and a 26m m external diameter. The teeth were shortened incisally and apically if they were longer than 23 mm. The teeth were secured to the washer with the use of sticky wax. Next, a 10cm long a l u m i n u m cylinder with an external diameter of 26
FIG 2. Photomicrograph of a sectioned tooth showing the ability to remove the smear layer.
Effect of Smear Layer on Sealer Adhesion
Vol. 17, No. 1, January 1991
m m and an internal diameter of 23 m m was placed onto the washer and secured with cyanoacrylate. The void between the internal diameter of the washer and the tooth was sealed with sticky wax in order to prevent the extrusion of dental stone. The entire unit was then inverted, and the well created by placing the aluminum cylinder onto the washer was filled with dental stone which filled the grooves prepared into the external surfaces of the tooth and aided in stabilization. This was allowed to set at 37~ in 100% humidity for 24 h. Three root canal sealer cements were tested for their adhesive strengths. One was a zinc oxide-eugenol based (Sultan), one was an epoxy-resin based (AH26), and one was a calcium hydroxide based (Sealapex). Each of the sealer cements was mixed according to the manufacturer's instructions. The sealers were then poured into a 4-ram deep a l u m i n u m well with a 4-mm internal diameter which had grooves prepared into its internal surface in order to aid in sealer retention. These wells also had bilateral vents which were 1 to 2 m m in diameter to allow for sealer cement expansion and to further aid in forming a mechanical bond between the sealer and the aluminum well. Prior to filling these wells, the rim of each well was coated with a very thin layer of Vaseline. This was done in order to prevent the flow of sealer in this area, as this could have resulted in bonding between the well rim and the tooth, which would have led to inconsistent and inaccurate results. Therefore, the sealer was only able to contact the tooth in a 4-ram diameter circle, or what was 12.5/100 cm 2. Once the sealers were poured into these wells, they were pressed onto the prepared surfaces of the teeth with finger pressure (Fig. 3). The wells which were to receive Sealapex were lined with a paste mixture of dentinal shavings and saline. This was done because a pilot study revealed that even at 37~ in 100% humidity for 1 wk Sealapex would not set. However, when the wells were lined by this mixture, Sealapex would come to
q
I)
a final set. All samples were then allowed to set for 1 wk at 37~ in 100% humidity.
Adhesive Strength Testing Each sample was allowed to bench cool for 1 h prior to testing, in order to reach room temperature. The bond between the sealer and the tooth was then tested for adhesive strength with an Instron (Universal Testing Machine). A mounting device was made for the Instron, which prevented subjecting the specimen to any shearing forces. The jig consisted of a lower aluminum hinge unit, which did not permit any preloading forces, and an upper steel support unit, which permitted an equal distribution of the tensile load as well as easy insertion and reproducibility from one sample to the next (Fig. 4). The specimen was subjected to a tensile load, increasing at a constant crosshead speed o f a 1 m m per rain. The force (in kilograms) at which the bond between the tooth surface and the sealer cement ruptured was recorded and used to calculate the adhesive strength in kilograms per square centimeter.
Sample Groups A total of six groups with 20 samples per group was tested. The groups were: A, AH26 sealer-cement with the smeared layer intact; B, AH26 sealer-cement without the smeared layer; C, Sultan with the smeared layer intact; D, Sultan without the smeared layer; E, Sealapex with the smeared layer intact; and F, Sealapex without the smeared layer, A total of 30 samples, 5 from each group, was tested on each of the 4 days in which data were collected.
\.\ \\
\ ',\\
(i!!!.,:,.,:,s
C
,>>>>)) ,>)>>>> ,))>>>>
q
i
,i
p>))>>) ,~))))) ~>))>))
.))))))
,>>>~,))
,>)>))) ,))))))
,). ,)>
~>) ,)) ,>)
,>>>>>Y q
,>>>>>>'
~PPP,
)>>>>>:
,>)>>)~ ,>>>>>>
,}}>>>F r A
23m~, Z6cnm
F~G 3. Diagram of tooth and sealer-containing wells.
17
L
v
Ib Fue 4. Diagram of apparatus to measure adhesive strength.
18
Journal of Endodontics
GetUemen et al.
Location of Bond Rupture After all samples were tested for their adhesive strengths, a random sampling of the tested teeth were prepared for, and evaluated by, the use of a scanning electron microscope. This was done in order to evaluate where the bond failed when the smear layer was left intact, as well as any sealer penetration of the dentinal tubules by sealer when the smear layer was removed.
Statistical Analysis The overall effect of smear layer removal on adhesive strength was evaluated using two-way analysis of variance. The inter- and intragroup comparisons were evaluated using tests of simple main effects. RESULTS A total of 120 samples were tested, 40 per sealer; 20 with the smear layer and 20 without the smear layer. The force, in kilograms, at which the bond between the tooth structure and sealer ruptured was recorded and used to calculate the tensile adhesive strength in kilograms per square centimeter (kg/ cm2). The results are presented in Table 1. The data demonstrate the relative adhesive strengths of the three sealers, as well as how each sealer compares with itself when adhered to dentin with the smear layer and to dentin without the smear layer. AH26 had the greatest adhesive strength, resulting in 20.38 kg/cm 2 without the smear layer and 12.42 kg/cm 2 with the smear layer. Tests of simple main effects showed this difference to be highly significant (p < 0.001). Sultan proved to have the second greatest overall adhesive strength, exhibiting 6.22 kg/cm 2 with the smear layer and 7.27 kg/cm 2 without the smear layer; there was no significant difference with this sealer whether or not the smear layer was present. Overall, Sealapex showed the lowest adhesive strength, having 0.93 kg/cm 2 with the smear layer and 1.24 kg/cm 2 without the smear layer. The difference here also proved to be statistically insignificant. The difference between any two of the three sealers was highly significant (p < 0.001, tests of simple main effects). Although the overall effect of smear layer removal appeared to be highly significant, resulting in an increase in adhesive strength upon smear layer removal (p < 0.001, by ANOVA), this difference was solely related to AH26. Tests of simple main effects showed that AH26 was the only sealer with a
significant difference in adhesive strength when the smear layer was intact, compared with when the smear layer was removed.
Bond Failure Location In all samples with the smear layer present, the result was an adhesive failure, rather than a failure at the smear layerdentinal tubule interface (Fig. 5). This was consistent for all three sealers tested. In those samples with the smear layer removed, the bond failed at the sealer-dentinal tubule interface for each of the Sultan and Sealapex samples (Fig. 6). However, when AH26 was used, 5 of 20 samples actually had a cohesive failure. DISCUSSION The smear layer, which is a direct result of endodontic instrumentation, has received a considerable amount of attention over the past decade. Much of this attention has been in relation to methods of smear layer removal (2-6). A variation of the method of G o l d m a n et al. (6) for removing the smear layer, making it applicable to longitudinal split teeth, rather than instrumented canals, was used in this study, and was shown to effectively remove the smear layer. The method of testing the relative adhesive strength of sealers to dentin used in the present investigation proved very successful. Extreme care was used when handling the speci-
TABLE 1. Sealer adhesive strengths in relation to the presence or absence of the smear layer (n = 20)
Sealer
Smear Layer (Present/Absent)
Mean Adhesive Strength (_+SEM) in kg/cm2
AH26 AH26 Sultan Sultan Sealapex Sealapex
Present Absent Present Absent Present Absent
12.42 (0.75) 20.38 (1.5) 6.22 (0.37) 7.27 (0.47) 0.93 (0.16) 1.24 (0.20)
FiG 5. Photomicrograph showing the smear layer remaining on the tooth after testing.
Vol. 17, No. 1, January 1991
FiG 6. Photomicrograph showing clean dentinal tubules on a tooth, with the smear layer removed, after testing.
mens to be tested. Orstavik et al. (12) described the problems in testing the adhesive strength of root canal sealers. The authors stated that tensile bonds of low magnitude, as those of the root canal sealers which they tested, are highly susceptible to transverse forces. Thus, the specimens must be handled extremely carefully during mounting in the testing machine, to avoid influences from transverse forces. This was taken into careful consideration during this investigation. The jig used for this study was designed to reduce the influences of transverse or shearing forces during both mounting and testing. However, regardless of the jig design, as well as the care taken during any handling of the samples, it is virtually impossible to totally eliminate these shearing forces. Some investigators (13) believe 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, 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. This irregular surface produced greater adhesion, especially when AH26 was used. As shown in Table 1, smear layer removal did allow greater adhesive strengths for all of the sealers tested; however, as stated previously, there was no significant difference with Sultan and Sealapex whether the smear layer was removed or left intact. AH26 having the greatest adhesive strength of the root canal sealers is consistent with other adhesion studies ( 12, 13,
Effect of Smear Layer on Sealer Adhesion
19
17). This is the only sealer of the three evaluated in this investigation for which previously reported adhesive strengths are available. We found the adhesive strength of AH26 to be 20.38 kg/cm 2 to dentin without the smear layer and 12.42 kg/cm 2 to dentin with the smear layer. McComb and Smith (13), in 1976, reported similar adhesive strength to AH26 to dentin (16.5 kg/cm2). No comment was made as to whether or not this dentin had a smear layer, but the authors made no mention of removing this layer, so most likely it was present. Orstavik et al. (12), making no mention as to the presence or absence of the smear layer, found the adhesive strength of AH26 to be 25.5 kg/cm 2. They made no mention of heating AH26, which could have contributed to this difference since heating will increase the flow rate of AH26. In addition, they used a specific powder to liquid ratio (1.75 g: lg), while we mixed to a consistency that would separate when lifted 1.5 to 2.5 cm from the glass slab when carried with a spatula prior to heating, as stated in the manufacturer's instructions. Grossman (17), in 1976, also evaluated the adhesive strength of AH26, although he did not use the Instron or dentin. Rather, he used a pulley which had 100 g added to it every 10 to 15 s until the sealer was dislodged from a glass microscope slide. The adhesive strength averaged 3.57 kg/ cm 2, which is much lower than the results of all investigations using dentin. The only information from Grossman's study which is presently relative is the relative adhesive strengths of the sealers, and AH26 did show the greatest adhesive strength. No previously published study has reported the adhesive strength of Sultan, the zinc oxide and eugenol sealer we examined. Nevertheless, other studies (12, 13, 17) have reported the adhesive strengths of various other zinc oxide and eugenol sealers. McComb and Smith (13) assessed the adhesive strengths of two zinc oxide and eugenol sealers: Kerr Antiseptic Pulp Canal Sealer and ProcoSol nonstaining cement. They did not report specific values for these sealers; they simply classified their adhesive strengths to be "nil.'" This may be attributed to their using a constant crosshead speed of 10 m m per min rather than 1 m m per minute, at which we found the adhesive strength of Sultan to be 6.22 kg/cm 2 with the smear layer present and 7.23 kg/cm 2 without the smear layer. The zinc oxide and eugenol sealers may not be able to withstand any tensile load which is initiated at this high rate. In addition, the slight variation in constituents, or percentage of specific constituents, between zinc oxide and eugenol sealers may also be attributed to this difference, as only the exact sealers can accurately be compared between studies. Orstavik et al. (12), also studied various other zinc oxide and eugenol sealers including Endomethasone, ProcoSol, and N2 Normal. They found that Endomethasone and N2 Normal had virtually no adhesive strength while ProcoSol had an adhesive strength of almost 15 kg/cm 2. Again, it is very difficult to compare absolute values for zinc oxide and eugenol sealers from one study to another. However, it is clear that Sultan had an adhesive strength above the average of many similar sealers, although it may not be the most adhesive of the zinc oxide and eugenol sealers. Grossman (17) also examined the adhesive strengths of various zinc oxide and eugenol sealers: Kerr, Mynol, N2, N2 No-lead, ProcoSol, Roth 801, Roth 811, RC2B, Tubli-Seal, and zinc oxide and eugenol cement. His results varied from 0 to 2.15 kg/cm 2, all
20
Gettlemen et al.
of which are lower than we found for Sultan. However, as mentioned earlier, due to the different methods and adherent surfaces used, results cannot be easily compared between the two investigations. As far as calcium hydroxide-based sealers are concerned, no published studies have looked at their adhesive strengths. First, there are very few available, the most common being CRCS and Sealapex. In addition, these are relatively new on the market. The adhesive strength of Sealapex was 0.94 kg/ cm 2 with the smear layer present and 1.24 kg/cm 2 without the smear layer. It is obvious that this low adhesive strength is not a very advantageous aspect of this sealer. Adhesive strength is only one aspect of root canal sealers. Further investigation of various aspects of root canal sealers is necessary. Which sealers seal better in the presence as well as the absence of the smear layer is one specific area which needs further evaluation. In addition, studies are needed regarding which sealer works best in specific situations such as open apices, apical deltas, ledged canals, and with specific obturation technique. Also, evaluation on which sealers work best utilizing specific obturating techniques should be completed. The present evaluation examined only one aspect of the question of which sealer is best in terms of adhesion to dentin with and without the smear layer present.
This study was partially funded by a grant from the American Association of Endodontists, Endowment and Memorial Foundation. Drs. Gettleman, Messer, and EIDeeb are affiliated with the School of Dentistry, Division of Endodontics, University of Minnesota, Minneapolis, MN. Address requests for reprints to Dr. Mahmoud E. EIDeeb, Division of Endodontics, School of Dentistry, University of Minnesota, 515 Delaware Street, SE, Minneapolis, MN 55455.
Journal of Endodontics References 1. White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by plastic filling materials. J Endodon 1975;10:55862. 2. Mader CL, Baumgartner C, Peters DD. Scanning electron microscopic investigation of the smeared layer on root canal walls. J Endodon 1984;10:47783. 3. McComb D, Smith DC. A preliminary scanning electron microscopic study of root canals after endodontic procedures. J Endodon 1974;1:238-42. 4. Moodnik RM, Dorn SO, Feldman MJ, Levey M, Borden BG. Efficacy of biomechanical instrumentation: a scanning electron microscopic study. J Endodon 1976;2:261-6. 5. 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. 6. 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 Endodon 1982;8:487-92. 7. White RR, Goldman M, Lin PS. The influence of the smeared layer upon dentinal tubule penetration by endodontic filling materials. Part 2. J Endodon 1987;13:369-74. 8. Pashley DH, Michelich V, Kehl T. Dentin permeability: effects of smear layer removal. J Prosthet Dent 1981 ;46:531-7. 9. Pashley DH, Depew D. Effects of smear layer, copalite, and oxalate on microleakage. Oper Dent 1986;11:95-102. 10. Michelich V, Schuster GS, Pashley DH. Bacterial penetration of human dentin. J Dent Res 1980;59:1398. 11. Williams S, Goldman M. Permeability of the smeared layer by a strain of Proteus vulgaris. J Endodon 1985;11:385-8. 12. Orstavik D, Ericksen NM, Beyer-Olsen EM. Adhesive properties and leakage of root canal sealers in vitro. Int Endod J 1983;16:99-107. 13. McComb D, Smith DC. Comparison of physical properties of polycarboxylate-based and conventional root canal sealers. J Endodon 1976;1:22835. 14. Cergneux M, Ciocchi B, Dietschi JM, Holz J. The influence of the smear layer on the sealing ability of canal obturation. Int Endod J 1987;20:228-32. 15. Kennedy WA, Walker WA, Gough RW. Smear layer removal effects on apical leakage. J Endodon 1986;12:21-7. 16. Evans JT, Simon JHS. Evaluation of the apical seal produced by injected thermoplasticized gutta-percha in the absence of smear layer and root canal sealer. J Endodon 1986;12:101-7. 17. Grossman LI. Physical properties of root canal cements. J Endodon 1976;2:166-75.
