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microleakage in restorative dentistry/rotary endodontic courses by indian dental academy
 

microleakage in restorative dentistry/rotary endodontic courses by indian dental academy

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    microleakage in restorative dentistry/rotary endodontic courses by indian dental academy microleakage in restorative dentistry/rotary endodontic courses by indian dental academy Document Transcript

    • Smear Layer & Microleakage 113 Microleakage in Restorative Dentistry: Microleakage is a major cause of restorative failure. There are atleast two or three routes by which substances can leak into the pulp. It can occur because there are microscopic gaps at the interface of the filling material and the tooth. Even if there were no gap between dentin and a restorative material, bacterial products could theoretically diffuse around the material via small channels and interstices within the smear layer. Pashley et al (1989) observed an extensive reticular network of microchannels around restorations that had been placed in cavities with smear layer. The thickness of these channels was 1-10 m. Smear layer may thus present a passage for substances to leak around or through its particles at the interface between the filling material and the tooth structure. Pashley & Depew (1986) found that microleakage decreased after the removal of the smear layer, but dentin permeability increased. Fig 40 Schematic representation of the interface of dentin and restorative material in a typical cavity. The granular constituents of the smear layer have been exaggerated out of their normal proportion for emphasis. Three theoretical routes for microleakage are indicated by arrows.
    • Smear Layer & Microleakage 114 Unfortunately, one cannot perfectly adapt amalgam or any other restorative material to the walls of a prepared cavity. Thus, there are voids and spaces between amalgam and dentin that allow considerable microleakage (Going, 1972). Most clinicians use a cavity varnish or liner to “seal” dentin. These organic films are placed on moist dentin, which microscopically, has pools of liquid on it, which produce an uneven layer of film of variable thickness and permeability. One wonders how well these films adapt to dentin and how well the restorative material adapts to them. Each layer provides potential route for microleakage. Jodaikin & Austin (1981) tested whether the smear layer restricts the adaptation of freshly condensed dental amalgams to unvarnished tooth cavity surfaces and whether the smear layer plays an essential role in the sealing mechanism which develops around aged dental amalgam restorations. It was thought that the smear layer may play a chemical role in the sealing mechanism by providing a substrate (which interacts with the amalgam substrates or other substances which migrate into the microcrevices at the amalgam-tooth interface) or by providing an environment which is conducive to the initiation and progression of the sealing mechanism. The smear layer may also play a physical role by restricting the dentinal fluid from flushing the molecules which effect the seal from the amalgam-tooth interface. It was found that all the short-term restorations leaked and the long-term restorations demonstrated some sealing. Thus the smear layer does not appear to restrict the adaptation of freshly condensed amalgams to tooth cavity surfaces. The improved marginal seal, which was obtained around the aged restoration margins of the etched cavities, suggests that the smear layer may in fact, hinder the initial sealing process. The reason for this maybe that the smear layer is unstable and leaches out and this leaching process may tend to widen the amalgam-tooth microcrevice and interfere with the sealing mechanism. Srisawasdi et al (1988) evaluated the effect of removal of smear layer on microleakage of Class V restorations using 3 restorative techniques: a composite resin with its dentin bonding agent, a composite based with glass-ionomer lining
    • Smear Layer & Microleakage 115 cement, and a glass-ionomer restorative cement. 10% polyacrylic acid was used for the removal of the smear layer. The results of the study favour the removal of the smear layer only when a glass-ionomer liner is used under the composite restoration. The microleakage of the glass-ionomer restorations was greater than either the composite or composite based with glass-ionomer liner. In a study done by Yu et al (1992) on a composite restorative material that utilizes a smear layer-mediated dentinal bonding agent, it was found that microleakage occurred at the smear-layer dentin interface and progressed into both the smear layer and dentinal tubules, suggesting that the smear layer acts as a pathway for microleakage. Viewed in this theoretical, perspective, if one could produce a truly adhesive filling material that had no shrinkage upon polymerization and a coefficient of thermal expansion close to that of tooth structure, then one would want to remove the smear layer and omit the use of any cavity liner or varnish that did not react chemically with both the dentin and the resin. Microleakage in Endodontics: Another important consideration in endodontics is the ultimate seal of root canals in order to prevent possible microleakage which may be the cause of the future failure of the root fillings. Microleakage is defined as the passage of bacteria, fluids and chemical substances between the root structure and fillings of any type. It occurs because there are microscopic gaps at the interface of the filling material and the tooth. Microleakage in root canals is a more complicated subject as many variables may contribute such as anatomy and instrument size of the root canal, irrigating solutions, root-filling techniques, physical and chemical properties of the sealers, and the infectious state of the canal. Allen (1964), Ingle (1976) and Strindberg (1956) have shown inadequate obturation of the root canal system to be one of the major causes of endodontic failure. Ingle (1976) determined that 63% of failures resulted form inadequate obturation. Cohen &
    • Smear Layer & Microleakage 116 Burns (1987) said that an inadequate seal at the apex accounts for 60% of failures of root canal therapy. However, coronal leakage is now considered to be a more important reason for failure (Madison et al, 1987; Madison & Wilcox, 1988; Saunders & Saunders, 1990). It has been shown that the quality of the permanent restoration plays a significant role in the success of endodontically treated teeth, probably more so than the quality of root canal filling. When the coronal portion of the root canal system is exposed to oral flora, it may allow ingress of bacteria to the periapical tissues. Coronal leakage provides a constant source of microorganisms and nutrients that initiate and maintain periradicular inflammation and may very well be the largest cause of failure of non-surgical endodontic therapy (Saunders & Saunders, 1994). Since the path of leakage may be affected by the presence of smear layer, it is seen that the smear layer could influence coronal leakage. Prepared dentin surfaces should be very clean to increase sealing efficiency of obturation (McComb & Smith, 1975; Combe, 1986). Smear layer on root canal walls acts as an intermediate physical barrier and may interfere with adhesion and penetration of sealers into dentinal tubules. Its absence makes the dentin more conducive to a better and closer adaptation of the gutta percha to the canal wall. Lester & Boyde (1977) found that ZOE-based root canal sealer failed to enter into dentinal tubules in the presence of smear layer. In 2 consecutive studies, White et al (1984, 1987) observed that plastic filling materials and sealers penetrated into the dentinal tubules after removal of the smear layer. Oksan et al (1993) also found that smear layer obstructed the penetration of filling materials, and that penetration in smear-free groups ranged from 40-60 m. It may be concluded that such tubular penetration may increase the interface between the filling and the dentinal structures, and this process may improve the ability of a filling material to prevent leakage (White et al, 1984). This mechanical lock between the gutta percha and the canal wall, coupled with the increased surface area at the interface between filling and canal wall, should create an impermeable seal. Dye tests, however failed to substantiate this thesis
    • Smear Layer & Microleakage 117 when tested after the removal of the smear layer. The dye penetrated accessory canals and spread laterally along the filling canal interface. Thus the injection of thermoplasticized gutta percha should be accompanied by the use of a sealer regardless of whether or not the smear layer has been removed. Follow-up dye tests, with the smear layer intact and the use of sealer and lateral condensation, showed no dye penetration. If the aim is maximum penetration into the dentinal tubules to avoid microleakage, root canal filling materials should be applied at a surface free of smear layer and they should have a low surface activity (Aktener et al, 1989). However, there has been no direct correlation between microleakage and penetration of filling materials into dentinal tubules. The presence or absence of a smear layer may play an important role in the adhesiveness of some sealers to the root canal walls. Studies have shown a significant increase in adhesive strength and resistance to microleakage of AH26 sealer when the smear layer was removed (Gettleman et al, 1991; Economides et al, 1999). Gettleman et al (1991) did not find any change in adhesive strengths when Sultan And Sealapex sealers were evaluated with or without the smear layer intact. Several investigators have shown less dye leakage after removal of the smear layer with various obturation techniques and root canal sealers. It was found that 80% of obturated teeth will leak after 96 hours regardless of the presence or removal of the smear layer (Goldman et al, 1986). With the smear layer intact, apical leakage will be significantly increased. Without the smear layer, leakage will still occur but at a decreased rate (Kennedy et al, 1986). Kennedy et al (1986) also stated that the use of a chelating agent on the smear layer would increase apical microleakage. Furthermore, he stated that 7-day duration between instrumentation and obturation allows for an increased amount of apical leakage. He concluded that removal of the smear layer would improve gutta percha seals if the master cones were softened with chloroform and used with a sealer and lateral condensation. Cergneux et al (1987) demonstrated that when the smear layer was not eliminated there was a tendency for greater
    • Smear Layer & Microleakage 118 infiltration of dye. The complete elimination of the smear layer improved the seal of the root canal obturation. Karagoz-Kucukay (1994) also showed by means of an electrochemical technique, that the incidence of apical leakage reduced significantly in the absence of the smear layer. Goya et al (2000) evaluated the removal of smear layer at the apical stop by pulsed Nd:YAG laser irradiation with or without black ink, and the degree of apical leakage after obturation in vitro. Irradiation vaporized the debris and tissue remnant from root canal surface and smear layer was evaporated, melted, fused and recrystallized. Root canal walls were left clean. The results of this study suggest that pulsed Nd:YAG laser irradiation with black ink increased the removal of the smear layer compared with that without black ink, and reduces apical leakage after obturation significantly. Several investigations done regarding coronal leakage also showed that smear layer removal is beneficial and that it resulted in less leakage than those in which smear layer was left intact (Saunders & Saunders, 1992 & 1994; Vassiliadis et al, 1996; Taylor et al, 1997- as demonstrated by dye leakage models). Leonard, Gutmann &Guo (1996) found that there was a significantly better seal in both the apical and coronal directions when using a dentin bonding agent and resin obturation material (C & B Metabond) following smear layer removal and dentin conditioning with 10:3 citric acid-ferric chloride solution, as compared to obturation using a glass-ionomer sealer. The C & B Metabond interface revealed the presence of the characteristic hybrid layer along with microtags of resin penetrating deep into the dentinal tubules. Behrend et al (1996) and Clark-Holke et al (2003) showed that removal of smear layer reduced the leakage of bacteria through the root canal system. De Souza et al (2005) found that the use of procedures to remove the smear layer (17% EDTA or Er:YAG laser) led to less microleakage because this permits greater contact of the sealer with the dentine wall. They also suggested that use of liquid adhesives reduced coronal microleakage significantly.
    • Smear Layer & Microleakage 119 Fig 41 (Leonard et al, 1996) Cross-sectional SEM view of the dentine adhesive interface hybrid layer (H), resin filling material (R), and demineralized dentin are visible with resin tags extending deep into the dentinal tubules. x 940. Other investigators have reported that the removal of the smear layer did not have any significant effect on the microleakage of the root canals when various sealers and obturation techniques were used. Evans & Simons (1986) evaluated the apical seal produced by injected thermoplasticized gutta percha in the absence of smear layer, and found that smear layer had no significant effect on apical seal whereas the sealer was necessary to prevent apical leakage. Madison & Krell (1984) also found no difference in apical leakage after use of chelating agent irrigation. Timpawat et al (1998) also found that there was no difference in the apical leakage in canals obturated with Thermafil with different sealers, with or without the smear layer. Froes et al (2000) found that found that there was no significant difference in the degree of apical leakage with and without the smear layer when 4 different obturation techniques were compared. Cook et al (1976) and Biesterfield & Taintor (1980) examined apical leakage of canals after potentially affecting the smear layer with a chelating agent. Cooke et al (1976) found that apical leakage increased with chelating agent use. Biesterfield & Taintor (1980) found that apical leakage increased in specimens obturated 1 week after instrumentation. Neither study documents smear removal, apical patency prior to obturation, or effective leakage evaluation techniques. Chailertvanitkul et al (1996) found no significant difference in microbial coronal leakage of obturated root canals when the smear layer was removed or intact.
    • Smear Layer & Microleakage 120 In contrast to these findings, Timpawat et al (2001) have reported that removal of the smear layer has adverse effects on the apical microleakage of filled root canals and in fact caused more microleakage than when the smear layer was left intact. These conflicting results might be attributable to differences in the types of sealer and obturation techniques, the means of producing a smear layer, different forms of chelating agents to remove the smear layer, and the diversity of bacteria used under various laboratory conditions. When the smear layer is not removed, the durability of the apical seal should be evaluated over a long period. Since this layer is a nonhomogenous and weakly adherent structure (Mader et al 1984), it may slowly disintegrate, dissolving around a leaking filling material, thus creating a void between the root canal wall and the sealer. If the endodontist does not wish to worry about these possible disadvantages, the smear layer can be removed in ways that will be discussed later. However, it should also be borne in mind that there is a risk of reinfection of dentinal tubules by microleakage if the seal should fail after removal of the smear layer (Brannstrom, 1984). Another important factor is that, the greater the degree of canal preparation, the smaller the amount of apical leakage (Yee et al, 1984). It is still inconclusive whether or not the presence of dentinal filings will enhance the seal of a root canal filling. When it was noted that stoppage of leakage occurred, it was related to smaller file sizes. With situations in which apical leakage existed in the presence of dentin plugs, it must be concluded that the plugs were permeable Their porosity allowed them to fall short of the goal of creating a hermetic apical seal (Jacobson et al, 1985). In addition to being porous, dentin plugs allowing microleakage exhibited large amounts of shrinkage. Scanning electron microscopic examination of unsatisfactory apical plugs always showed marginal and structural defects (Yee et al, 1984). Further considerations for advocating smear layer removal in Endodontics are the importance of creating a good apical plug and the effects the two main types of sealers have on the canal walls.