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Root canal obturation: hermetic or biological seal

Educational aims and objectives

The aim of this article is to review the classical obturation techniques and evaluate whether a paradigm shift is necessary for the clinical use of the hydraulic tricalcium silicate-based sealers.

Expected outcomes

Endodontic Practice US subscribers can answer the CE questions to earn 2 hours of CE from reading this article. Correctly answering the questions will demonstrate the reader can:

  • Realize that whichever technique and material selected to obturate a root canal, the objectives are always achieving a seal that is impervious to microbial recolonization.
  • Realize that with conservative materials and techniques, a hermetic seal is achievable by compaction of gutta percha, and inherent sealer properties lead to chemical bonding to the dentinal wall.
  • Discuss classical obturation techniques and evaluate whether a paradigm shift is necessary for the clinical use of the hydraulic tricalcium silicate-based sealers.
  • Identify the three primary functions of a root filling.

Dr. Josette Camilleri evaluates what type of seal is possible using classic obturation materials and techniques

Abstract

Root canal obturation is necessary when the pulp tissue is removed from the root canal system leaving a dead space that can be recolonized by microorganisms. After pulp removal, the root canal is cleaned, shaped, and irrigated after which it is obturated. For successful root canal obturation, the materials need to have specific properties, and the clinical procedures undertaken are complementary to the materials used. Root canal obturation has been undertaken with a combination of a solid cone/sealer technique. Gutta percha has been the most frequently used material in conjunction with various sealer types with different chemical compositions. The gutta-percha sealer combination can be compacted laterally and left unmodified or compacted vertically and heated. The appropriate irrigation protocol results in reduction of bacterial load and removal of smear layer. The ensuing obturation materials can thus bind to the root canal wall by sealer interlock in the dentinal tubules leading to a hermetic seal.

The hydraulic dental sealer cements have two basic properties, which are mainly their hydraulic nature; thus, their properties improve in the presence of moisture and the formation of calcium hydroxide as a byproduct of hydration, which makes the materials inherently antimicrobial. Furthermore, the sellers bind chemically to dentin. This leads to the query whether a paradigm shift is necessary for the use of these sealer cements and the whether the current clinical protocol needs to be reviewed to complement these materials.

Introduction

Pulp vitality is lost due to dental caries, trauma, tooth wear, and iatrogenic damage, which is extensive and thus involving the dental pulp. Dental materials in close proximity to the pulp can also lead to pulp damage. Occasionally, the dental pulp will have to be removed electively when the root canal space is needed to retain a dental restoration.

Whatever the cause, the pulp chamber and the root canal space need to be filled to prevent reinfection. The root canal space is cleaned mechanically and also by the use of chemical agents to eliminate microorganisms and also to remove the smear layer. The root canal is then obturated using a combination of solid cones and sealers. The aim of root canal obturation is to provide a hermetic seal and thus prevent reinfection of the root canal space, which will lead to treatment failure. The tricalcium silicate-based sealer cements were introduced due to their hydraulic nature. There is no specific protocol for their use, and currently, they are being used as any other sealer in conjunction with gutta percha. The aim of this article is to review the classical obturation techniques and evaluate whether a paradigm shift is necessary for the clinical use of the hydraulic tricalcium silicate-based sealers.

Classical obturation techniques

Root canal treatment methodologies are very old and have changed very little over the years. The obturation techniques mostly involved a solid cone and sealer combination. Initially, a single cone was used together with root canal sealer; then the techniques evolved to lateral condensation and warm vertical compaction to enhance the three-dimensional quality of the root canal filling (Schilder, 1967). The core acts as a piston on the flowable sealer, causing it to spread, fill voids, and to wet and attach to the instrumented dentin wall. It is the sealer that comes into contact with the dentin and periodontal tissues. It is thus important that the sealer possesses the ideal material properties as outlined by Grossman (Grossman, 1978).

The three primary functions of a root filling are the sealing against ingrowth of bacteria from the oral cavity, entombment of remaining microorganisms, and complete obturation at a microscopic level to prevent stagnant fluid from accumulating and serving as nutrients for bacteria from any source (Sundqvist and Figdor, 1998). To achieve a good obturation, the root canal needs to be chemo-mechanically cleaned. This is performed by a combination of mechanical root canal cleaning and shaping techniques and various irrigation protocols. The irrigation serves to eliminate the microorganisms and also remove the smear layer, thus leaving patent dentinal tubules. The canal is left clean and dry ready for obturation.

The choice of materials lies in the choice of the solid cone and the sealer type. It gives an indication of the type of obturation technique that can be employed. There are different types of solid cones that can be used. These include silver cones, gutta percha, gutta-percha-coated plastic/metal carriers, and resin cones. The silver cones were popular as they fitted the canal based on the master apical file size used in the canal in a standardized preparation (Kojima, et al., 1974). They can be used as a whole point filling the entire root canal or as sectioned points obturating the apical part of the canal (Eguren, 1966). The technique fell in disuse due to the corrosion of the silver points and questionable seal the technique provided (Gutmann, 1979).

The first gutta percha available for clinical use was manufactured by SS White in 1887. The dental gutta percha is mainly composed of zinc oxide, which accounts for its inherent antimicrobial properties. The gutta percha can be used unmodified or modified by heat (Markin and Schiller, 1973; Schilder, et al., 1974) or organic solvents (Magalhães, et al., 2007). Gutta percha can also be used to coat carriers for Thermafil® obturation technique (Lares and elDeeb 1990). This gutta percha is chemically modified and is found in the alpha-phase rather than in the standard beta-phase, which is found in all the gutta percha for dental use (Maniglia-Ferreira, et al., 2013). Alternatively, a resin core can be used as is available in the Resilon™ system (Shipper, et al., 2004). The choice of sealer depends on the type of core material in use. The silver cones and all types of gutta percha use various sealers with a range of compositions. The Resilon system comes complete with its own sealer and primer system.

The obturation technique varies on the type of core material chosen. The silver points and the gutta-percha-coated carriers in the ThermaFill system are used in a single cone technique. The gutta-percha can be used unmodified in the lateral condensed gutta percha obturation technique. The technique was first published by Bramante in 1972. This technique depends on the sealer’s ability of holding the individual cones together for its success. The technique is popular as it is easy and does not need any specific equipment. Over the years, the lateral condensation obturation technique became considered as the gold standard. The techniques using modified gutta percha are also popular. The solvent techniques result in shrinkage of the obturation in the long term due to the evaporation of the solvent. The application of heat also results in shrinkage as the gutta percha changes phase, but this can be counteracted by application of pressure. The gutta percha can be warmed outside the canal in the warm thermoplasticized injection molding techniques (Yee, et al., 1977), and the carrier based systems like ThermaFill (Lares and elDeeb, 1990; Chohayeb, 1992). Alternatively, intracanal warming using the warm vertical compaction technique can be undertaken (Wong, et al., 1981; Grossman, 1987). Warm vertical compaction of the master cone in the down-packing stage while using thermoplasticized injection molding technique for the back-packing stage would give the best outcome as it avoids gutta percha extrusion apically since the temperature of the master cone is quite stable in the apical third (Yared, et al., 1992). The types of techniques and new paradigms for filling the root canal are discussed by Ingle in 1995 (Ingle, 1995).

The heat profiles of gutta-percha are well researched (Marlin and Schilder, 1973; Schilder, et al., 1974). The heat carriers currently available on the market are set to deliver heat to 200ºC (Silver, et al., 1999) regardless the phase transformation of gutta percha occurring at 65ºC. The heat generated on the external surface of the root was within acceptable limits, thus caused no damage to the periodontal ligament and no bone necrosis (Lee, et al., 1998; Floren, et al., 1999). The dissipation of heat was dependent on the eternal media; thus, data procured in air like in in vitro studies may not be clinically relevant (Viapiana, et al., 2014). The temperature on the heat carrier was lower than that set on the machine dial (Venturi, et al., 2002, Viapiana, et al., 2014, 2015). Maximum temperatures recorded were 100ºC, and the temperature varied depending on the carrier size (Viapiana, et al., 2014). The temperatures generated did not affect the chemistry and properties of the gutta percha (Roberts, et al., 2017). However, root canal sealers were negatively affected by the rise in temperature generated during warm vertical compaction with AH Plus® (Dentsply), an epoxy resin-based sealer showing deterioration in both physical and chemical properties (Viapiana, et al., 2014, 2015, Camilleri 2015). Salicylate resin-based sealers (Camilleri, 2015) and zinc oxide eugenol-based sealers (Viapiana, et al., 2014) were more state to heat application and exhibited no changes in properties.

The synthetic resin core used with a resin-based sealer of the Resilon/Epiphany system promised to create a mono-block obturation (Raina, et al., 2007). The Resilon/ Epiphany system was not very successful as the synthetic resin was easily degraded by bacteria and their enzymes (Tay, et al., 2005, Hiraishi, et al., 2007). Thus, gutta percha was shown to be the best core material to date.

Root canal obturation with hydraulic sealers

A number of hydraulic sealer cements based on tricalcium and dicalcium silicate are available clinically (Table 1). These sealers are primarily composed of tricalcium and dicalcium silicate thus produce calcium hydroxide once in contact with water. The chemistry and presentation of these sealers varies considerably. The Portland cement-based sealers contain traces of heavy elements and an aluminum-based phase, and these features have been shown to be of concern as aluminum has been shown to accumulate in plasma, liver, and brain of test animals (Demirkaya, et al., 2015, 2016). The acid extractable levels of arsenic and chromium are high (Monteiro Bramante, et al., 2008, Schembri, et al., 2010, Matsunaga, et al., 2010; Chang, et al., 2011), and although there is no standard level of chromium for dental cements, the arsenic levels were higher than that set by ISO 6876 (2012) for sealer cements. The leached trace elements were low (Duarte, et al., 2005, Camilleri, et al., 2012), but no standard levels are set in international standards. Due to these concerns, the BioRoot™ RCS, iRoot SP and TotalFill®/EndoSequence® BC materials use pure tricalcium silicate. Interestingly, Endoseal MTA is composed of dicalcium silicate. This is slower to react than tricalcium silicate, but a deca-calcium aluminate is added to enhance the reactivity. Thus, the problem of aluminum incorporation is also present with Endoseal MTA.

All sealers contain a radiopacifier to be able to comply with ISO 6876 (2012). Most of the hydraulic sealers are bismuth oxide free unlike the original MTA formulation as bismuth oxide was shown to lead to material and tooth discoloration when in contact with sodium hypochlorite solution (Camilleri, 2014; Marciano, et al., 2015). The MTA Fillapex® excludes the bismuth oxide in the new generation and replaces it with calcium tungstate. CPM sealer and Endoseal MTA both contain the bismuth oxide added to another radiopacifier. All sealers also contain additives. These are present to enhance the material properties. The Endoseal MTA and TotalFill/EndoSequence and iRoot® SP are biphasic, thus contain another cementitious phase. The deca-calcium aluminate in the Endoseal MTA allegedly accelerates the hydration since the dicalcium silicate which is the main phase is a slow reaction. The calcium phosphate in TotalFill/EndoSequence and iRoot SP changes the material hydration with a reduction in pH and calcium ion release in the leachate. No crystalline calcium hydroxide was formed. A reduction in cell growth and proliferation was observed (Schembri-Wismayer and Camilleri, 2017).Other additives include fillers like silicon oxide and pozzolanic ash. These are added to enhance the long-term material physical properties since the silicon oxide races with the calcium hydroxide formed during hydration, and it is converted to calcium silicate hydrate. The depletion of calcium hydroxide may result in a deterioration of antimicrobial properties. The calcium chloride and water-soluble polymer present in the BioRoot RCS control the setting time and material flow.

As shown in Table 1, the sealers also use different vehicles and also vary in their presentation and delivery method. The CPM sealer and BioRoot RCS use a simple water/powder formulation; thus, the sealers are water based. MTA Fillapex uses a salicylate resin vehicle similar to that used in the calcium hydroxide based conventional sealers. In fact, the calcium ion release of MTA Fillapex is much lower than that of the other water-based sealers (Xuereb, et al., 2015). The iRoot SP, EndoSequence BC/TotalFill, and Endoseal MTA sealers are premixed. These sealers need moisture present in the root canal to set. A recent study where a low-pressure fluid column filled with simulated body fluid was applied to a root stump showed complete setting of EndoSequence BC sealer (Xuereb, et al., 2015). Thus, the back pressure of the tissue fluids in the root canal is enough to allow the setting of the premixed hydraulic sealers.

The obturation protocol for conventional root canal obturation includes irrigation with sodium hypochlorite to eliminate the microorganisms, followed by irrigation with a calcium chelator to remove the smear layer; thus, the seared can penetrate the dental tubules and enhance the bonding by producing resin tags. The irrigation with sodium hypochlorite is contraindicated in bismuth oxide-containing sealers due to the risk of sealer and tooth discoloration (Camilleri 2014, Marciano, et al.. 2015). Calcium chelators like ethylene diamine tetracetic aside (EDTA) effect the chemistry of these materials which are calcium containing. The EDTA reduces the interaction of calcium ions with dentin and the deposit of beta calcium phosphate in both BioRoot and EndoSequence BC sealers. The calcium ion depletion was more evident in BioRoot RCS (Harik, et al., 2016). So, the choice of irrigation protocol is important when using hydraulic tricalcium silicate-based sealers. The use of phosphate-buffered saline has been suggested as a final irrigant prior to root canal obturation. The push out bond strength of the obturation material is increased as the biomineralizing ability of the sealer is enhanced (Reyes Carmona, et al., 2010a, b). The use of phosphate-buffered saline final wash reduces the antimicrobial activity of the sealers. Even BioRoot which registers the highest pH compared to EndoSequence and double the calcium ion release (Xuereb, et al., 2015) still lost its antimicrobial activity when phosphate buffered saline was used as a final irrigant (Arias Moliz and Camilleri, 2016).

The hydraulic sealers can be used with either gutta-percha solid cones or with the bioceramic coated cones. These cones are only available from Brasseler USA® (Savannah,Georgia) and FKG (La ChauxdeFonds, Switzerland). The bioceramic coating of gutta percha is meant to enhance the bond strength of the sealer to the cone. There is still no definite data whether this is true. Hygroscopic points (CPoints) have also been suggested for use with biocermaic sealers. The pressure derived from hygroscopic expansion of CPoint or warm vertical condensation did not enhance penetration depths of the calcium silicate-based sealer. Sealer penetration into the dentinal tubules occurred independent of the obturation technique (Jeong, et al., 2017).

The single cone obturation technique has been suggested for use with hydraulic tricalcium silicate-based sealers. A comparison of single cone obturation with warm vertical compaction showed that the percentage volume of voids was similar in the two groups and was influenced by the obturation technique only in the cervical third (Iglecias, et al., 2017). A higher percentage of voids was shown in the cervical third when BioRoot was used in conjunction with gutta percha compared to AH Plus sealer (Viapiana, et al., 2016). Both techniques produced similar tubule penetration at both the 1 mm and the 5 mm level using tricalcium silicate-based sealers (McMichael, et al., 2016). Conversely, significantly less porosity was observed in root canals filled with the single-cone technique with porosity near the crown of the tooth reduced sixfold, whereas in the mid-root region porosity was reduced to less than 10% of values found in the lateral compaction filled teeth (Moinzadeh, et al., 2015). Single cone obturation resulted in better bond strength than warm vertical compaction with EndoSequence BC giving better results than an MTA based sealer (De Long, et al., 2015). Excessive heat in warm vertical compaction should be avoided as it tends to evaporate the water in the water-based sealers such as BioRoot RCS (Camilleri, 2015) and thus lead to changes in the physical properties, which may be detrimental to long-term success of the obturation. MTA Fillapex was shown to be very stable and resisted degradation when heated during the warm vertical compaction procedure (Viapiana, et al., 2014; Camilleri, et al., 2015).

The interaction of the tricalcium silicate-based sealers with the root canal wall is postulated to be a chemical bond. The sealers bond to dentin by a process known as alkaline etching, and a mineral infiltration zone develops at the interface of the dentin in contact with the material (Atmeh, et al., 2012). The presence of mineral infiltration zone and sealer tags was shown by confocal microscopy using fluorescent dyes to tag the sealer (Atmeh, et al., 2012; Viapiana, et al., 2016). The alkaline etching is caused by the sealer alkalinity. The development of the mineral infiltration zone has been discredited by other authors using micro-Raman and electron probe micro-analyses (Li, et al., 2016). The use of tricalcium silicate-based materials has been shown to cause softening of collagen in dentin (Leiendecker, et al., 2012) and a deterioration in flexural strength of the tooth (Sawyer, et al., 2012).

Conclusions

Whichever technique and material selected to obturate a root canal the objectives are always achieving a seal which is impervious to microbial recolonization. While conservative materials and techniques achieved a hermetic seal by compaction of gutta percha and sealer tags inside the dentinal tubules, the hydraulic cements based on tricalcium silicates aim at antimicrobial activity which is an inherent sealer property and chemical bonding to the dentinal wall. Therefore, the seal can be considered to be more biological. These materials have specific properties, and a proper clinical protocol is necessary to use the sealers with optimized properties.

Author Info

Professor Josette Camilleri, BCHD, MPHIL, PHD, FICD, FIMMM, FADM, is a dental surgeon specializing in dental and construction cements and is currently associate professor at the Faculty of Dental Surgery, University of Malta. She completed her doctoral degree, supervised by the late professor Tom Pitt Ford, at Guy’s Hospital, King’s College London. Dr. Camilleri has published over 90 papers in peer-reviewed international journals, and her work is cited over 3,800 times.

Declaration: The author declares no conflict of interest.

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