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Until technology can regenerate infected pulpal tissues, they must be replaced with restorative material through endodontic therapy. The goal of endodontic therapy is the long-term functional retention of teeth with a history of pulpal or periapical pathology. Treatment includes total debridement of the pulpal tissues and shaping of the root canals to develop a fluid-tight seal at the apical foramen. One of the most important steps after canal preparation and disinfection in endodontic therapy is obturation of the root canal system.1 The entire root canal system must be 3-dimensionally filled as close to the cementodentinal junction as possible. To ensure an appropriate and acceptable seal, root canal sealers are used in conjunction with the core filling material.1 This article will expand on different materials used in root canal obturation systems that have been part of the history of endodontics.
History of Root Canal Obturation Systems
The history of endodontics can be traced to the 2nd or 3rd century BC. A skull found in the desert in Israel had a tooth with a bronze wire in one of the root canals.2 In 1,200 to 1,300 BC, ancient Egyptian papyrus suggested the "worm theory" as the source of dental pain, in which worms burrowed into the tooth and caused disease, resulting in pain. This theory of the origin of dental pain remained until Pierre Fauchard (1678-1761), considered the founder of modern dentistry, debunked the worm theory in his 1728 treatise "Le Chirurgien Dentiste," or "The Dental Surgeon."3
Over the following century, numerous contributions were made in the field of endodontics, including pulp capping and the use of different chemical agents, such as phenol, arsenic trioxide, and formalin. Other contributions were infection control, which occurred in culturing and obturation of the root canal system.4 Even though endodontic therapy was gaining in popularity, clinicians rendering endodontic treatment had no way of knowing what was taking place inside the root canal system. That changed in 1895 with the invention of x-rays by Röntgen. Commercial dental x-ray units were made available in 1919.5
In the past, various materials have been used to fill root canals. These materials included gold foil, amalgam, asbestos, iron, lead, bamboo, cement, copper, oxychloride of zinc, paraffin pastes, plaster of paris, resin, rubber, silver points, and even tin foil. None were ever considered to be ideal in providing the goal of complete obturation of the root canal system.1
Some of the earliest attempts to provide a hermetic seal of the root canal system came in the 19th century, when Koecker used a red-hot wire to cauterize the pulp and fill the canal with gold. Edward Hudson, in the early 1800s, used gold foil for root canal obturation. Although gutta-percha was discovered by John Tradescant in 1656, the introduction of gutta-percha into the field of endodontics did not occur until 1843 by Dr. William Montgomerie.6 Shortly thereafter, Hill developed the first root filling material in 1847, known as "Hill's stopping."7 The filling material consisted of bleached gutta-percha and carbonate of lime and was patented in 1848.
In 1867, Bowman demonstrated the use of gutta-percha as a filling material in an extracted molar.8 Perry, in 1883, used a pointed gold wire wrapped with gutta-percha.9 A new version of gutta-percha was introduced in 1893 by Rollins, who added vermilion, which was a red pigment made from mercury sulfide (cinnabar).10 In 1898, Gysi's Triopaste, a formaldehyde paste, was introduced by Gysi.
The softening and dissolution of gutta-percha using rosins was introduced by Callahan in 1914.11In 1930 Elmer Jasper introduced silver points as an obturation material.1 It was not until 1977 that Yee et al presented a thermo-plasticized injectable gutta-percha, which then was modified by W.B. Johnson, where he carried the thermo-plasticized gutta-percha on an endodontic file. This concept was further developed with heat softening and compacting gutta-percha by McSpadden in 1979. Then in 1984, Michanowicz introduced a low temperature (70° C) injectable gutta-percha.1
Root Canal Filling Material
The primary objective is to achieve a complete obturation of the root canal system that forms a fluid-tight seal at the apical, lateral, and coronal sections.12 Failures can be caused by incomplete mechanical debridement, persistence of bacteria left in the canals, incomplete obturation, over- and under-extension of the root canal filling, and coronal leakage.13
After shaping and cleansing the root canal system, the canals should be filled completely to prevent ingress of nutrients or oral microorganisms. Sealer cement and a central core material combined provide the basic elements to create an apical seal. The central core material acts like a piston that pushes the flowable sealer ahead to fill voids and attach to the dentinal wall of the canal. In most cases, the sealer is in contact with the dentin and not the gutta-percha.14 Therefore, the biocompatibility and sealing ability of the root canal sealer are important.
Essentials of Root Canal Obturation
According to the American Association of Endodontists (AAE), there are a number of acceptable materials and techniques for obturation of the root canal system. These include using a sealer alone (cement/paste/resin), using a sealer with a single cone of a stiff or flexible core material, using a sealer coating combined with cold compaction of the core material or with warm compaction, or using a sealer coating with a carrier-based core material.
Research has shown that preparation and disinfection of the root canal are the most important factors in the successful treatment of endodontic pathosis.15 Although the sealing of the root canal system is an essential step in the development of an apical seal, any single sealing technique cannot claim to be superior in the healing success.16,17
The primary functions of the root canal filling material are sealing against the ingrowth of bacteria, the entombment of any remaining microorganisms, and the complete obturation of the root canal, preventing the accumulation of stagnant fluids that serve as nutrients for bacteria from any source.18,19
Obturation Materials
Materials used for root canal obturation can be divided into 3 categories: plastics (gutta-percha and Resilon™), solids or metal cores (silver points, gutta-percha-coated cones, gold, stainless steel, and titanium), and cements (mineral trioxide aggregate [MTA] and calcium phosphate).
Today, the most common root canal obturation material is gutta-percha. The name gutta-percha is derived from two words in the Malay language: getah, meaning gum, and pertja, the name of a tree. Gutta-percha is extracted from trees of the genus Palaquium in the family Sapotaceae, which naturally inhabit Southeast Asia.7 The substance, derived from the sap of these trees, is a biologically inert, resilient, and electrically nonconductive thermoplastic latex. The material has minimal toxicity and minimal tissue irritability, and it is the least allergenic when contained in the root canal system.20 It is a polymer of isoprene, which forms a rubber-like elastomer. It has an approximately 60% crystalline form, which may occur in alpha or beta phase, depending on the temperature. At room temperature, gutta-percha is considered in beta phase. In this phase, gutta-percha is solid, is compactible, and can be elongated. Most commercial gutta-percha exists in the beta phase. If beta-phase gutta-percha is heated between 107° and 120° F, it will change to the alpha phase (how it exists in the tree naturally).21 In this phase, gutta-percha is sticky, runny, and noncompactible, and it cannot be elongated. As it cools, it returns to the beta phase, but with greater shrinkage than the degree of expansion seen during heating.1
When a solid material is heated, there is usually a linear increase in volume, resulting from the thermal excitation of individual atoms and molecules. After cooling, the dimensional change is reversible, so the volume of most substances is the same at any given temperature. However, this volume constancy does not apply to materials with crystalline phase changes. Therefore, the volume changes occurring from thermal manipulation must be considered and countered by a condensation procedure.1,21
Gutta-percha is available in different forms and sizes and with different coatings. Sizes and shapes are similar to International Organization for Standardization (ISO) standards, which have a 2% taper from sizes 15 to 140, although tapers of 4%, 6%, 8%, and 10% are available. There are also variable tapers used in conjunction with specific file systems.
Gutta-percha is used as a coating on either a metallic or plastic carrier that delivers the material into the canal. The gutta-percha-coated carriers are heated in an oven designed by the manufacturer to provide a constant heat source. Other versions of gutta-percha can be found in a powder form that can be incorporated into a resin-based sealer as well as pellets or bars that are heated and injected into the canal. Gutta-percha can also be medicated with calcium hydroxide, iodoform, or chlorhexidine.1
Although gutta-percha has a proven track record and is considered the "gold standard" for obturation, it does not bond to the dentinal canal wall and shrinks after cooling.22,23 In 2004, Resilon was introduced as an alternative to gutta-percha. It was described as a thermoplastic synthetic polymer that contained methacrylate resin, bioactive glass, barium sulphate, and bismuth oxychloride. It had an accompanying dual-cure resin-based root canal sealer.24 Resilon contained fillers of calcium hydroxide, bismuth oxychloride, barium glass, and silica. The total filler content was 70%.25 When compared with previous resin filling materials, Resilon was considered a viable option. In cases of retreatment, Resilon could be softened and dissolved with solvents.26In 2014, Resilon was removed from the market because the follow-up studies contradicted the in-vitro studies.27
Root Canal Sealers
The function of the sealer is to fill the spaces or voids left between the root canal obturation core (ie, gutta-percha cone) and the dentinal wall. Gutta-percha alone cannot seal the canal space because it has no adherence to dentin.28 Irregularities in the fill and voids between the canal walls, accessory canals, and minor foramina should be filled by the sealer.29 Chemically, root canal sealers should contain antimicrobial agents that are effective immediately after placement.30 Other necessary characteristics of sealers is that they be radiopaque, are biocompatible with periapical tissue, do not shrink on setting, and are insoluble in host tissue fluids but soluble in a solvent that allows for removal if needed.31 ISO 6876 and ADA specification No. 57 require less than 3% solubility, no more than 3% weight loss in distilled water.32
Classification of Root Canal Sealers
Commonly used sealers can be classified by their composition.29,30,33 Variations of sealer classifications can be found in peer-reviewed literature.
Group 1: Zinc oxide eugenol (ZOE)-based sealers. These sealers are commonly used and have a successful track record.33 There are two forms of these sealers: a mix of a powder and liquid or a two-paste preparation. ZOE sealers are antimicrobial and work well with heat carriers. They shrink slightly when set34and are marginally soluble.35 If extruded into the periapical tissues, ZOE will resorb.36
Group 2:Salicylate-based, or calcium hydroxide-based sealers. These sealers are usually referred to by their marketed therapeutic additives instead of by their composition.32 They have antibacterial properties that are dependent on dissolution of the material, calcium hydroxide, which then slightly reduces their efficacy.37 The solvation of calcium hydroxide is necessary to achieve the desired therapeutic effects.38 Calcium hydroxide sealers have been shown to have less antibacterial activity than ZOE, but they are also less toxic.39
Group 3: Epoxy resin-based sealers. These sealers have strong sealing ability; they adhere to dentin. They are antimicrobial and work well with resin-coated gutta-percha.33 AH Plus™(Dentsply Sirona) is considered the industry standard by which competitive manufacturing will test against. Unlike its predecessor, it does not release formaldehyde.40
Group 4: Silicone-based sealers. In 1972, Davis et al used an injectable polyvinyl silicone impression material in prepared root canals to study the internal anatomy of the canal. They found many anatomical variations, including lateral canals, webbing between canals, fins, and instrument markings. They also reported that the prepared canal was very dissimilar in shape, especially in the apical third, to the instrument used to prepare the canal.41Similar in composition to the polyvinyl silicone impression material, these sealers also set by additional reaction between vinyl groups, forming a polymer.42 These sealers have been shown to have clinical benefits in homogeneity and adaptation to the dentinal walls.43 Ørstavik et al44 reported that, due to the viscosity and elastic modulus, these silicone-based sealers absorb stress generated by mastication during root flexure. Savariz et al45 found that when compared with AH Plus, there was a significant improvement of the apical seal. However, Elias et al46 found similar sealing ability between both sealers. In a newer version, GuttaFlow® Bioseal (Coltene) has added bioactive glass calcium silicate. According to Gandolfi et al,47 this particular sealer can provide an alternative strategy for moist or bleeding apices with bone defects.
Group 5: Methacrylate resin-based sealers. The first methacrylate resin sealer was introduced in the mid-1970s. It was composed of 2-hydroxyethyl methacrylate polymer gel that was injected into the canal without the need for a core (gutta-percha). Due to handling problems and periapical tissue irritation, it was discontinued in the 1980s.48 The aim to achieve a bond between the dentin and sealer led to a second generation, which was a dual-cure sealer that did not require a dentin adhesive.49
Group 6 of root canal sealers combines all sealers that have bioactive capability. These types of sealers are often referred to as bioceramic sealers and have been classified as separate entities by other authors.32,50,51 For this article, this group of sealers includes calcium silicate phosphate-based and calcium phosphate-based sealers and MTA-based or bioceramic sealers.
Root canal sealers that are biocompatible, possess antibacterial properties, and produce hydroxyapatite on their surfaces in the presence of phosphate-buffered saline are considered to have bioactivity and fall into the category of bioceramics.52,53 The physical characteristics of these materials include nanocrystals, with diameters of 1 to 3 nm, that prevent bacterial adhesion. At times, fluoride ions are elements of apatite crystals and have antibacterial properties.54 GuttaFlow Bioseal was mentioned in Group 4 (silicone-based sealers); however, because of the addition of a bioactive component, calcium silicate, it can also be classified as a bioceramic.
Calcium silicate phosphate-based bioceramic sealers have strong sealing ability and little reaction to periradicular tissue in the case of extrusion past the apex.33 The materials are usually applied in a prefilled syringe and have similar viscosity to conventional sealers. Zhang et al55found these bioceramic sealers to have the potential to increase root strength after obturation due to their high alkalinity, biocompatibility, bioactivity, dimensional stability, and sealing ability.
The first bioceramic material successfully used in endodontics was MTA cement, which was introduced by Torabinejad in 1993. Except for the addition of bismuth oxide and lower levels of calcium aluminate and calcium sulfate, MTA is very similar in composition to Portland cement. Originally, Portland cement was invented and patented by Joseph Aspdin of Leeds, England. He produced the cement by heating powdered limestone mixed with clay in a furnace and then grinding the mixture into a powder. When mixed and set, it resembled a type of stone that was quarried on the Isle of Portland; hence, it was called Portland cement.56 MTA meets the criteria of bioactivity by possessing osteoconductivity, osseoinductivity, and biocompatibility. Initially, MTA was introduced as an apical filling material. Since then it has been used for pulp capping, pulpotomy, apexification, root perforation repair, and root canal filling material.57
Biocompatibility
Of all the properties of root canal sealers, biocompatibility is one of the most important.58When a material in contact with human tissue does not trigger an adverse reaction, it is said to be biocompatible.59 Fonseca et al50 conducted a systematic review of peer-reviewed literature on the biocompatibility of commercial root canal sealers. From a review of 1,249 studies, 73 in vitro and 21 in vivo studies were included in the analysis. In general, studies found that root canal sealers produce mild to severe toxic effects, and several factors may influence the sealers' biocompatibility. These factors include the material setting condition and time, material concentration, and type of exposure. The investigators concluded from the available evidence that all root canal sealers exhibit variable toxic potential at the cellular and tissue level. However, bioactive sealers had a lower potential in vitro.
Although root canal sealers tend to exhibit a certain degree of toxicity, especially at initial placement, the degree of toxicity decreases with the setting of the sealer. The cytotoxicity may arise from the unconverted monomers present in a freshly mixed sealer. Therefore, extrusion of the sealer into the periradicular tissue should be avoided.58
Komabayashi et al32 compared the biocompatibility and cytotoxicity of various root canal sealers. For example, they reported that ZOE sealers acted as an irritant and cytotoxic agent and that non-eugenol sealers exhibited a lesser inflammatory effect.60 Silicone-based sealers, when compared with epoxy resin sealers, demonstrated a lower cytotoxicity during the first 11 days after mixing.61 The investigators reported that methacrylate polymer demonstrated cytotoxicity only during the initial stages of polymerization, and even though it is considered to be the least toxic monomer used in dentistry, the methyl methacrylate monomer is still considered cytotoxic.62 Further research with specifically designed studies will broaden understanding of biocompatibility of root canal sealers.
As previously stated, the thorough cleaning and shaping of the root canal is vital to periapical healing. Sometimes overlooked, the restorative phase of an endodontically treated tooth is also critical to the success of treatment. Recontamination of the root canal system can arise from either apical or coronal leakage. There are conditions the clinician must be aware of that can promote coronal leakage. According to the AAE, clinicians can prevent coronal leakage in endodontically treated teeth through pre-endodontic tooth preparation, thoroughness of the root canal obturation technique, temporary seal of the root canal system during and after treatment, the choice and integrity of the final tooth restoration, the timeliness of the restoration, establishment of atraumatic occlusion, and long-term follow-up to ensure the integrity of the treatment.63 The endodontic success rate improves when the quality of the root canal obturation is accompanied by a quality restoration.64
Conclusion
Obturation of a root canal consists of three steps: shaping, cleansing, and sealing to create a fluid-tight seal. Although it has been said that shaping and cleansing are the most important steps in providing successful endodontic therapy, optimal success is based on the creation of an apical seal at the cementodentinal junction. If the first two steps are done correctly but there is no permanent apical seal in the final step of the process, the outcome may be negatively affected. However, that is only half of what is required for success. The coronal seal must not be overlooked after completion of root canal therapy. A permanent restoration should be placed as soon as possible to achieve long-term success no matter what sealer is used in the process.
About the Author
Gregg Helvey, DDS, MAGD, CDT
Adjunct Associate Professor
Virginia Commonwealth University School of Dentistry
Richmond, Virginia
Private Practice
Middleburg, Virginia
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