Scanning electron microscopic evaluation of two compaction techniques using a composite resin as a root canal filling material

0099-2399/95/2112-0594503.00/0 Printed in U.S.A. Vot. 21, No. 12, DECEMBER1995

JOORNALOF ENOOOONTtCS

Copyright © 1995 by The American Association of Endodontists

Scanning Electron Microscopic Evaluation of Two Compaction Techniques Using a Composite Resin as a Root Canal Filling Material Ivica Ani6, DDS, PhD, Tetsuya Shirasuka, DDS, PhD, and Koukichi Matsumoto, DDS, PhD

The canals of 20 human dental roots were instrumented using a step-back technique. The smear layer was removed, and the canals were obturated with composite resin. Two different techniques of compaction were used: either vertical or lateral motions were used to condense composite resin inside the root canal that was then photopolymerized layer by layer using the argon laser (488 rim). The beam was delivered into the root canal by means of an optical fiber 320/~m in diameter. Longitudinal and cross-sections of the samples and resin replicas of the root canals were examined using a light and scanning electron microscope. Scanning electron microscopic examination revealed that laterally compacted resin fillings showed fewer voids than those obtained by vertical compaction. In both experimental groups, adhesion of the resin to the dentin walls, pulled-out resin tags, microfailure, and resin fracture, leaving a layer of resin associated with the wall surface were observed.

ported by Kelsey et al. (5). Another advantage of the argon laser is that the beam can be delivered through a small optic fiber, thus facilitating delivery of the laser light energy into the root canal (6). It has been shown that argon laser polymerization used in conjunction with dentin adhesive systems seems to improve adhesion (7). In addition, the argon laser has the Food and Drug Administration marketing clearance for oral soft tissue procedures (8). Soon after the ability of the argon laser to cure composite resin was demonstrated, Potts and Petrou (9) reported successful polymerization of the composite resin inside the root canal. When the artificial material has been introduced into the canal, the major concern is the intimate adaptation of the material to the dentin walls. During instrumentation, a smear layer is created on the surface that may interfere with the adaptation of obturation materials to the canal walls. Oksan et al. (10) demonstrated that smear layer obstructed the penetration of the tubules by sealer. White et al. (11) demonstrated penetration of filling materials (pHEMA, silicone, and laterally condensed gutta-percha) with sealer into the tubule when the smear layer was removed before obturation. Consequently, removal of the smear layer might aid in intimate adaptation of filling materials to the walls and even penetration of the plastic materials into the dentinal tubules (12, 13). The objective of the present study was to compare, using light and scanning electron microscopy, the effectiveness of the vertical and lateral techniques for introduction and compaction of the composite resin inside the instrumented root canal.

In endodontics, the most commonly used core filling material is gutta-percha. Recently, it has been demonstrated that light-cured composite resins can be used as filling materials. Experimental root canal obturation using plastic resin-based materials has been demonstrated by Saunders et al. (1). Hasegawa et al. (2) reported that bonding agents might improve the sealing ability of the dentinal apical plug. Currently used composite resins employ a diketone initiator (camphoroquinone) and reducing agent (tertiary amine) to initiate polymerization (3). If this photoinitiator system is exposed to light in the blue region of the visible spectrum, free radicals are formed, thus inducing the polymerization. The peak area of photoinitiating activity centers in the 470 nm range. The wavelength of the argon laser is in the blue spectrum (488 and 514.5 nm), and it would appear to be in the ideal range for initiation of composite polymerization (4). The enhancement of the physical properties of composite materials photopolymerized with argon laser was re-

M A T E R I A L S AND M E T H O D S Twenty human single-rooted teeth were collected and stored in a 10% formalin solution. To standardize the conditions, the crowns of the teeth were removed at the cementoenamel junction, or lower toward the root apex, depending on the length of the root. Thus, every root sample was 20 mm in length. The root canals were enlarged using a step-back technique to a master K-file #50. Gates Glidden drills #2 through #5 were used to establish straight-line access. During instrumentation, the canal was irrigated with copious amounts of 5% NaOC1 and 3% H202. Following instrumentation the roots were divided into the two experimental groups (groups A and B) of 10 samples each. 594

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Laser Devices Argon laser HGM (Medical Laser Systems, Salt Lake City, UT) was used. The 488 nm wavelength laser beam was delivered through a flexible quartz optical fiber 300/xm in diameter. Single exposures were performed at a laser power of 1 W in continuous mode (CW).

Obturation Techniques GROUP A The smear layer was removed with 15% EDTA and 5% NaOC1. Each canal was irrigated with EDTA for 4 min, followed by irrigation with NaOCI for 4 rain using a syringe and plastic needle with a blunt tip. After that, a #60 paper point was immersed into the 70% ethyl alcohol and left in the canal for 15 s. The canal was dried using a corresponding paper point. The bonding agent was introduced into the canal using a small square sponge from the Clearfil complete box. The sponge was positioned at the canal orifice and plugged into the canal by hand plugger presized to within 2 mm of the apical stop. After sponge withdrawal using a rat-tail file, a #50 paper point was inserted into the canal, and the excess bonding agent was removed. The argon fiber was positioned at the canal orifice, and the laser was started before the fiber was inserted into the canal. Immediately after the laser action was started, the fiber was introduced into the canal and slowly pushed toward the apical stop. The bonding agent was cured a total of two times by 1 W/2 s/CW. After that, a small amount of the composite was positioned at the tip of the master file and introduced at the apical stop. The master file was spun counterclockwise and removed, leaving the resin at the apical portion. The composite was plugged using a hand plugger equal to the master file, presized to 1 mm short of the apical stop. The fiber was inserted - 1 mm from the composite, and the resin was cured using two pulses of 1 W/2 s/CW each. After withdrawing the fiber, an additional amount of the composite resin was positioned at the canal orifice. The resin was plugged into the canal and then a plugger, one size smaller than the master file, was inserted through the composite inside the canal to the apical cured composite plug. At that position, the plugger was rotated and slightly pressed laterally against the walls (360 degrees). During withdrawal, the instrument was rotated. This technique left the composite resin condensed against the wall and, in the center of the composite mass, a hole of a diameter larger than the plugger used. The fiber was positioned at the orifice, and the laser was again started immediately before the fiber was inserted into the canal. During 2 s of the lasing, the fiber reached its desired position (2 mm from the apical stop). The second lasing was performed during withdrawal of the fiber. A small amount of additional composite resin was positioned at the orifice, and the compaction and lasing was performed again in the same manner. This procedure was repeated until the whole canal was obturated.

GROUP B The removal of the smear layer, introduction of the bonding, and apical sealing with the composite were performed in the same manner described in group A. Following the curing of the apical composite plug, a small amount of additional composite resin was plugged into the canal using a hand plugger equal to the master

FIG 1. Apical bonding plug. Margin between bonding plug and composite filling can be seen clearly (arrow). point. The curing by argon laser (2 × 1 W/2 s/CW) proceeded in the same manner as in group A. In contrast to group A, in group B each additional composite layer was condensed using only vertical motions.

Preparation of Samples for Examination Following obturation, all samples were immediately stored in physiological solution for 48 h at 37°C. Five specimens from each group were sectioned transversally using a low-speed saw Isomet (Buehler, Lake Bluff, IL) under water cooling conditions. The speed control was set at position 5, and the root samples were cut every 1 mm starting from the apex. The sections were polished manually under water using polishing stones and then submerged into the demineralization solution [100 ml content: 7 g A1 (H20)6; 5 ml H202; 8.5 ml HC1; distilled water] for 1 h. After that, the samples were washed with distilled water and dried. Three samples from each group were embedded in epoxy resin to allowed manipulation of the samples during cutting with a low-speed saw. The samples were sectioned longitudinally and polished using the same procedure described herein. Finally, to dissolve the dentin and cementum, the last two samples from each group were submerged in the demineralization solution (previously mentioned) for 48 h at 60°C following 24 h in 5% NaOC1 at the same temperature. The surfaces of resultant resin replicas of the root canals were examined. After preparation, all samples were dehydrated in ascending grades of alcohol (70% toward absolute). After drying, specimens were photographed at X 4 magnification under the light microscope Nikon HFX-IIA (Hitachi, Tokyo, Japan), and then prepared for examination by electron microscopy by mounting on stubs and coated with 20/xm of platinum layer. Scanning electron microscopy was performed on these specimens using the JEOL JSM T220A (Jeol Technic Co., Ltd., Tokyo, Japan) at 20 kV.

RESULTS The scanning electron micrographs revealed that removal of the smear layer allowed the bonding agents and composite resin to enter dentin tubules. An apical bonding plug, photopolymerized before the composite filling material was introduced into the canal, partially prevented overextrusion of the composite if lateral corn-

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FJG2. Lateral canal filled with the composite resin compacted using lateral motions (black arrow). Note the space appearing between first and second composite layers (small arrows). Shape corresponds to the hand plugger one size smaller than the master file. White arrow indicates shrinking of the bonding plug.

FLG 4. Resin replica of the dentin canal. Note overextrusion of the resin beyond the apical stop into the noninstrumented canal space.

FIG 3. Resin tags remaining after dissolution of the dentin. Gap occurred due to the contraction forces inside the dentin tubules. Only a few tags bridge the gap. C, composite filling.

paction was performed (group A). The margin between the bonding plug and the composite is clearly seen (Fig. 1). Composite resin plugged with a plug one size smaller than master file, obturated the main as well as the lateral canals if present (Fig. 2). The bonding plug was observed to shrink somewhat in this specimen. Furthermore, contraction of the composite mass and fracture of the resin tags were observed. The fracture occurred - 2 0 txm inside the dentinal tubules (Fig. 3). Longitudinal and cross-sections, as well as resin replicas of the samples in group B, showed large amounts of space inside as well as at the periphery of the filling. Fewer voids and irregularities in the obturation were found in the samples from group A than in group B. The typical composite impression of the apical portion after dissolving the dentin is shown at Figs. 4 and 5. At the replicas' surface, areas with bundle of tags (Fig. 6), as well as areas without tags, were found. Some space was observed between the bonding agent and dentin surface and between the composite and the bonding agent (Fig. 7).

FIG 5. Scanning electron microscopic view of the apical part of the resin replica. Note that no tags can be seen.

DISCUSSION In this study, root canals were obturated by composite resin cured layer by layer inside the canal by means of an argon laser. The two different techniques were applied and compared. According to the scanning electron microscopic findings, the technique used in group A achieves better results than the technique performed in group B. The composite adaptation was better, and the

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FIG 6. Bundles of the resin tags remaining at the composite replica after dentin was dissolved.

FrG 7. Space between bonding agent and composite mass and between bonding and dentin. D, dentin; A, bonding agent; C, composite resin.

voids, if any, are positioned in the central part of the filling. In contrast, in group B, empty spaces were randomly present inside the filling mass; however, they were usually located between the dentin wall and the composite filling. Such findings can be explained by the techniques used. In group A, the composite was condensed using lateral motions, leaving a thinner layer of the composite on the dentin walls. Use of a plugger smaller than master file and circular motion during compaction, enable better penetration of the materials into the tubules and, at the same time, reduce the possibility o f air entrapment inside the filling mass. Vertical condensation of the composite with a plugger equal to the master point, prevents escape of trapped air and thus, the empty spaces are found inside the filling mass. A pilot study showed that laser action should be started in front of the canal orifice before insertion of the fiber into the canal. If this is not done, the fiber can become trapped inside the hardening resin during polymerization. When this occurred, additional force was necessary to withdraw the fiber from the canal that led to damage of the fiber tip. We also observed, inside the root canal, the intensity of the laser light decreased very quickly. This can be explained by melting of the fiber tip and/or by coating of the optical fiber tip with vaporizing steam. The local rise in temperature is a consequence of absorption of laser energy and exothermic reaction of photoactivated composites. Care should be taken to

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recognize that a hard polymerized top composite surface does not necessarily indicate that the deeper areas are also adequately polymerized. Therefore, frequent cutting of the fiber tip aids in achieving the best result. Longer exposure to the laser should be avoided, and lasing can be repeated after the temperature decreased. Potts and Petrou (9) reported that no charring, vaporization of the surface, or overheating occurred during argon laser polymerization of composite resin inside the root canal. However, in that study, the fiber was not introduced to the apical portion, and the composite was not polymerized layer by layer. It was evident that demineralization increased the diameter of dentin tubules, and the massive penetration of the resin inside the tubules occurred. The resin penetrated >250 ~m into the tubules. In the same section, however, a space was often found between composite and the dentin wall. This finding is in agreement with those reported by Youngson and Grey (14). The defect was visible as break off the surface composite layer with composite tags remaining in situ (15), or less frequently, as loss of adhesion to the tubule's wall, leading to incomplete "pull out" of the tags (14). These findings imply that, during laser polymerization of the composite inside the canal, contractile forces occur. Furthermore, finding of a crumpled image of the bonding agent at the apical stop could be explained by the fact that 8 to 10 s of exposure time was necessary for bonding hardening. (This observation was confirmed by additional experiments performed after the results of the main study were evaluated.) In the main study, the bonding layer received only 4 s (2 × 2 s); the first composite layer subsequently received the same amount of lasing. Thus, insufficiently hardened bonding agent was compressed during compaction motions, causing the crumpling of agent. Complete hardening of the bonding agents was achieved after polymerization of the first resin layer. Following dissolution of the root, areas with no tags could be seen at the surface of the composite replicas. This could be because fine composite tags were destroyed by quick dissolution of the dentin. At the other areas, bundles of resin tags were noticed. The lateral compaction of the composite inside the root canal results in fewer voids and more compact filling mass compared with vertical compaction. Argon laser can be easily delivered into the root canal using an optical fiber. The wavelength and the intensity of the radiation used enable efficient photopolymerization of the composite. The resin penetrates into the tubules; however, contraction occurring during polymerization may affect adhesion to the dentin. Dr. Ani~ is assistant professor, Department of Dental Pathology, School of Dentistry, University of Zagreb, Zagreb, Croatia. Dr. Shirasuka is assistant professor and Dr. Matsumoto is professor and chairman, Department of Endodontics, School of Dentistry, Showa University, Tokyo, Japan. Address requests for reprints to Dr. Ivica Ani~, Department of Dental Pathology, School of Dentistry, University of Zagreb, Gundufi6eva 5, 41000 Zagreb, Croatia.

References 1. Saunders WP, Saunders EM, Herd D, Stephens E. The use of glass ionomer as a root canal sealer--a pilot study. Int Endod J 1992;25:238-44. 2. Hasegawa M, Tanaka S, Satake S, Shimizu A, Yoshioka W. An experimental study of the sealing ability of a dentinal apical plug treated with bonding agent. J Endodon 1993;19:570-2. 3. Council of Dental Materials, ~nstruments and Equipment. Visible light cured composites and activating units. J Am Dent Assoc 1985;110:100-3. 4. Cook WD. Spectral distribution of dental photopolymerization sources. J Dent Res 1982;61:1436-8. 5. Kelsey WP, Blankenau RJ, Powell GL, Barkmeier WW, Cavel WT, Whisenant BK. Enhancement of physical properties of resin restorative materials by laser polymerization. Lasers Surg Med 1989;9:623-7.

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6. Kelsey WP, Blankenau RJ, Powell GL. Application of the argon laser to dentistry. Lasers Surg Med 1991 ;11:495-8. 7. Blankenau RJ, PoweU GL, Barkmeier WW, Kelsey WP. Argon laser effects on dentin adhesive systems [Abstract 1117]. J Dent Res 1989;68:321. 8. Kim Kutsch V. Laser in dentistry: comparing wavelengths. J Am Dent Assoc 1993;124:49-54. 9. Potts TV, Petrou A. Laser photopolymerization of dental materials with potential endodontics applications. J Endodon 1990;16:265-8. 10. Oksan T, Aktener BO, Sen BH, Tezel H. The penetration of root canal sealers into dentinal tubules. A scanning electron microscopic study. Int Endod J 1993;26:301-5. 11. White RR, Goldman M, Sun Lin P. The influence of the smeared layer

Journal of Endodontics upon dentinal tubule penetration by endodontic filling materials. Part I1. J Endodon 1987;13:369-7. 12. Gutmann JL. Adaptation of injected thermoplasticized gutta-percha in the absence of dentinal smear layer. Int Endod J 1993;26:87-92. 13. White RR, Goldman M, Sun Lin P. The influence of the smeared layer upon dentinal tubule penetration by plastic filling materials. J Endodon 1984; 10:558-62. 14. Youngson CC, Grey NJA. An in vitro comparative analysis: scanning electron microscopy of dentin/restorative interfaces. Dent Mater 1992;8:252-8. 15. Wang T, Nakabayashi N. Effect of 2-(methacryloxy)ethyl phenyl hydrogen phosphate on adhesion to dentin. J Dent Res 1991;70:59-66.

You Might Be Interested The cover feature of the January 26, 1995 issue of the prestigious journal, Nature, features a picture of "a carnivorous sponge" and the accompanying article describes the "discovery" of the first such organism in a marine cave in the Mediterranean. Nonsense! With the same caption they could have run a picture of my brother-in-law eating a hamburger.

William Cornelius