(a) Micro-computed tomography axial section of a tooth with the root canals filled. The material appears in black color and shows a good filling of the root canals. (b) Micro-computed tomography axial section of the same tooth showing a lack of flow of the filling material in the apical isthmus between the root canals and the presence of some voids (in white). (c) Microcomputed tomography cross section of the same tooth within the two root canals in the apical third, showing a good filling of the prepared root canals. (d) Micro-computed tomography cross section of the same tooth within the two root canals and the isthmus in the coronal third showing a good filling of the endodontic space
A second left lower molar has been treated showing a root canal filling flush to the apex and obtaining a good seal without any filling material extrusion
Unfortunately, root canal filling does not invariably provide a complete seal of root canals [12], although it may entomb many microorganisms remained within them. Nevertheless, the combined effects of both host immune responses and operating procedures allow to obtain high percentages of success of endodontic treatment [13–15]. However, in relation to the endodontic treatment, complications may occur, some of which may also be caused by the filling techniques and the materials used.
Some complications of endodontic treatment depend on the difficulties with tools and materials that dental practitioners have to use within complex spaces [16–19]. A correct diagnosis, a treatment plan, and the knowledge of anatomical variations of the root canal system are necessary to obtain a favorable outcome of the endodontic treatment [20, 21].
Many studies have demonstrated that mechanical instrumentation and intra-canal irrigation during root canal therapy result in a reduction of bacterial counts [22–24]. Indeed, most of the studies fail to adequately report clinically important and potentially patient-relevant outcomes [25], and there is currently insufficient reliable evidence showing the superiority of any one individual irrigant [25]. Moreover, there are no studies showing that the use of irrigants currently available, as well of any instrumentation technique, allows for a root canal space free of bacteria [25–28]. During mechanical instrumentation, an amorphous layer of tissue remnants is deposited on the surface of the root canal. This “smear layer,” consisting of organic and inorganic materials as well of bacteria and their by-products, may prevent adhesion of sealers to the canal wall and seems to serve as a substrate for bacterial growth. The current opinion is that removing the smear layer is useful and that the removal should be carried out before root canal filling [25–27, 29–32]. White et al. [33] observed that plastic filling materials and sealers penetrated dentinal tubules after removal of smear layer. Okşan et al. [34] also found that smear layer prevented the penetration of sealers into dentinal tubules. Tubular penetration increases the interface between the filling and the dentinal structures, which may improve the ability of a filling material to prevent leakage [35].
6.2 Root Canal Filling Materials
6.2.1 Characteristics of Ideal Root Canal Filling Materials
The quality guidelines for endodontic treatment of the European Society of Endodontology state that materials used to fill the root canal system should be biocompatible, dimensionally stable, able to seal, unaffected by tissue fluids, insoluble, nonsupportive of bacterial growth, radiopaque, and easily removed from the canal if retreatment is needed [2, 36].
- 1.
It should be easily introduced into the root canal.
- 2.
It should seal the canal laterally as well as apically.
- 3.
It should not shrink after being inserted.
- 4.
It should be impervious to moisture.
- 5.
It should be bacteriostatic or at least not encourage bacterial growth.
- 6.
It should be radiopaque.
- 7.
It should not stain tooth structure.
- 8.
It should not irritate periradicular tissues.
- 9.
It should be sterile, or easily and quickly sterilized, immediately before insertion.
- 10.
It should be removed easily from the root canal, if necessary.
Historical Background
The Beginning of the Use of the Gutta-Percha
Information about the use of the gutta-percha as root canal filling material before the beginning of the twentieth century are scarce and vague.
In the 1800s and before, materials such as medicated cotton pellets, tin foil, oxychloride of zinc, lead foil, plaster of Paris, gold foil, wood, spunk, oxyphosphate of zinc, zinc oxide, paraffin, copper points, and various others were used to fill root canals.
In 1847, Hill developed the first root canal filling material containing gutta-percha, called “Hill’s stopping”. The compound, which mainly contained bleached gutta-percha, calcium carbonate, and quartz, was patented in 1848 and then made available for dental use [38, 39].
In 1867, Bowman was credited with using gutta-percha points to fill root canals by the St. Louis Dental Society [38, 39].
In 1883, Perry declared that he had used a point of gold wire coated with softened gutta-percha, a sort of anticipation of modern techniques for the use of the gutta-percha delivered by a carrier. He also softened strips of gutta-percha with a lamp and rolled them, thus preparing gutta-percha points. Then the points were heated and compacted into the canals in which it introduced alcohol, realizing something similar to a chemical softening technique [40].
In 1887, the S.S. White Company began to produce gutta-percha points [41]. In 1893, Rollins introduced gutta-percha to which was added vermilion, which contains mercury. However, the use of this material was not considered acceptable [42].
Callahan in 1914 introduced the use of resin to soften and dissolve gutta-percha so that it could be used as a cementing agent [43].
The Silver Points
In 1933, Jasper introduced the silver points [44] that were widely used to fill the root canals in the 1930s to the 1960s, particularly in smaller canals. They had the same size as instruments used in the preparation of the canal. Silver points could be inserted easily and allow for easy working length control. The main disadvantage of silver points is that they do not seal laterally or apically due to their lack of plasticity. They left too much space to be filled by sealer, thus leading to leakage, that caused corrosion of the silver points and formation of cytotoxic silver salts [45–47].
Silver points used in smaller canals could be successful, but their use in larger canals was inappropriate and gave rise to failures. The silver point use declined because of their inherent disadvantages, and currently there are no indications for their use.
The Medicated Materials
The use of X-rays progressively is made possible to better evaluate the root canal fillings, and it was evident that other materials suitable to fill the observed gaps should be used. At first zinc oxide- and eugenol-based cements, which were able to harden in the root canals, were used, but the results obtained with these materials were unsatisfactory. So it was thought appropriate to propose the use of fillings with antiseptics and medicated cements, and pastes were proposed, containing phenol, formaldehyde, antibiotics, and endomethasone.
In 1965, Sargenti introduced a paste originally marketed as N-2, containing 6.5% paraformaldehyde, lead, and mercury [48–50]. Lead has subsequently been reported in distant organ systems when N-2 is placed within the radicular space [51]. In another study, the investigators reported the same results regarding systemic distribution of the paraformaldehyde component of N-2 [51, 52]. Removal of the heavy metals from N-2 resulted in a new formulation: RC2B. Other paraformaldehyde sealers include endomethasone, SPAD, and Reibler’s paste.
In general, the toxic and adverse in vivo effects of these materials on the pulp and periapical tissues have been demonstrated over time [53, 54]. In addition to the toxic nature of these materials, clinicians placed them with a lentulo spiral. Overextension often resulted in osteomyelitis and paresthesia. One clinician reported irreversible neurotoxicity, manifested as dysesthesia, in cases where paraformaldehyde pastes were forced through the apical foramen into the periapical tissues [55].
6.2.2 Characteristics of an Ideal Root Canal Filling
An ideal root canal filling three-dimensionally fills the entire root canal system as close to the cemento-dentinal junction as possible. Indeed, on the basis of microscopic analysis and clinical tests, it has been reported that optimum filling is achieved when canals are instrumented and filled 0 to 2.0 mm short of the root apex [56, 57]. Root canal sealers, most of which have been shown to be biocompatible or tolerated by the tissues in their set state, are used in conjunction with a core filling material to establish an adequate seal. Radiographically, the root canal filling should have the appearance of a dense, three-dimensional filling that extends up to 0–2 mm from the radiographic root apex [2] and should be extended to the terminus of the root canal system without extruding materials into the surrounding tissues [14]. In fact, complete periradicular healing after endodontic treatment may be influenced by not only the effectiveness of the microbial control procedures but also the apical extent of the root filling materials, the composition, the biocompatibility, and the performance of these materials [58, 59].
6.2.3 Contemporary Filling Materials
At the present time, gutta-percha is the most popular core material used for obturation. Gutta-percha is utilized in combination with a root canal sealer that fills the minor irregularities [60] and acts as a lute between gutta-percha and the canal wall [61].
6.2.3.1 Endodontic Sealers
Sealers are used between dentin surfaces and core materials to fill spaces that are created due to the physical inability of the core materials to fill all areas of the canal. Traditionally, desirable characteristics were to adhere to dentin and the core material as well as to have adequate cohesive strength. Various types of sealers have been proposed, including zinc oxide-eugenol, as well as polymer resins, silicon-based materials, calcium hydroxide, glass ionomer, bioglass, and calcium silicate [36]. Newer generation of sealers is being engineered to improve the ability to penetrate into dentinal tubules and bond to, instead of just adhering to, both the dentin and core material surfaces. Various types of delivery systems such as auto-mix syringes have improved not only the efficiency of mixing but also the quality of the mix and ultimately the properties of the set material.
Zinc Oxide-Eugenol Sealers
Zinc oxide-eugenol sealers have a history of successful use in root canal obturation for over 100 years. They are widely used root canal sealers because of their plasticity, slow setting time in the absence of moisture, and small volumetric change on setting. They get resorbed if extruded into the periapical tissue. The zinc oxide-eugenol sealers have antimicrobial activity and popularity among clinicians, especially when used with thermoplasticized obturation technique [62]. However, eugenol is found to leak and is known to induce toxic effect and decrease the transmission in nerve cells. The effect is persistent even after setting. Localized inflammation with zinc oxide-eugenol sealers has been seen, both in soft tissue and in the bone [63].
Polymer Resin Sealers
Epoxy resin sealer exhibits reduced solubility [64] and disintegration [65] and microretention to root dentine with higher bond strength than other root canal sealers [66, 67] as well as adequate dimensional stability [68]. However, these sealers have shown no bioactive properties [69] or osteogenic potential [70]. Some studies reported that epoxy resin-based sealers showed toxicity and mutagenicity before setting and decreasing with time [71], but other authors [72–74] found no cytotoxic effect and moreover aged specimens appeared to induce cellular proliferation. New resin sealers have been designed to improve the adhesion of the sealer to dentine, both in combination with a dentine primer and without it [72–74]. For these new resin sealers, various studies reported toxic effects that did not decrease with time [65, 73, 75, 76].
Silicon-Based Sealers
These materials have been developed as root canal sealers, and laboratory and clinical data are promising [65, 73, 75–78]. Some of them contain gutta-percha powder. In different studies, silicone-based sealers, both fresh and aged, demonstrated slight cytotoxic effects [75, 79, 80].
Calcium Hydroxide-Based Sealers
They promote hard tissue formation but tend to dissolve over time and may thus compromise the endodontic seal [75, 79–82].
Glass-Ionomer Sealers
They might exhibit long-term adhesion to dentin, which would be an obvious advantage over zinc oxide-eugenol-type or epoxy resin-type sealer cements [83, 84]. However, it has been reported that pretreatment with phosphoric acid or citric acid should be used in association with glass-ionomer root canal sealers to achieve the most effective removal of the smear layer and to provide better adhesion [83–85].
Bioglass-Based Sealers
In contrast to calcium hydroxide, bioactive glasses not only have antibacterial but also bioactive/remineralizing effects [86]. One most promising field for the application of composite dental materials containing alkaline bioactive glass powders is endodontology. Adding particles of bioactive glasses to endodontic sealers could be a valuable add-on because of Ca/P deposition, bioactivity, and pH increase induction, mainly responsible for the antimicrobial effect of bioactive glasses [87].
Calcium Silicate-Based Sealers
In addition to antibacterial activity [87–89], they show cytocompatibility [90], good sealing ability [91], and good bonding to root canal dentin even under various conditions of dentin moisture [92, 93]. Bioceramic sealers have been recently developed for orthograde root canal obturation as a consequence of the success of some basic calcium silicate materials such as mineral trioxide aggregate (MTA), being today the material of choice for perforation repair, root-end fillings, pulp caps, pulpotomies, and obturation of immature teeth with open apices [94, 95].
These materials have been specifically designed as a nontoxic calcium silicate cement that is easy to use as an endodontic sealer while simultaneously taking advantage of its bioactive characteristics. In addition to its excellent physical and mechanical properties, some of the advantages are the following: they increase the pH value (up to 12.8) during the initial 24 h of the setting process (which is strongly antibacterial); they are hydrophilic, not hydrophobic; they have enhanced biocompatibility; they do not shrink or resorb (which is critical for a sealer-based technique); they are radiopaque; they have excellent sealing ability; they set quickly (3–4 h); they have good handling properties and are easy to use (particle size is so small; it can be used in a syringe to improve the convenience and delivery method).
Additionally, and this is very important in endodontics, bioceramics will not result in a significant inflammatory response if an overfill occurs during the obturation process or in a root repair. A further advantage of the material itself is its ability (during the setting process) to form hydroxyapatite and ultimately create a bond between dentin and the filling material. A significant component of improving this adaptation to the canal wall is the hydrophilic nature of the material [96–98].
Bioceramics are biocompatible, nontoxic, non-shrinking, and usually chemically stable within the biological environment [99]. A further advantage of these materials is their ability to form hydroxyapatite and ultimately create a bond between dentin and the material [100]. The majority of papers show favorable properties for bioceramic materials including biocompatibility, bioactivity, and antimicrobial properties, and they have sealing properties similar to MTA. While in vitro studies are promising, it is not clear if any of these results influence clinical success. Only well-designed, prospective outcome studies can answer this question [96, 98].
6.2.3.2 Core Materials
Gutta-Percha
Gutta-percha is a hydrocarbon polymer, i.e., a trans-1,4-polyisoprene, and is an isomer of natural rubber [101]. It is obtained from the coagulation of latex produced by trees of the Sapotaceae family and mainly derived from Palaquium gutta bail [102]. An important characteristic of gutta-percha and of clinical importance is the fact that when it is exposed to air and light over time, it becomes more brittle [103, 104]. Storage of gutta-percha in a refrigerator extends the shelf life of the material.
Brittleness, stiffness, tensile strength, and radiopacity have been shown to depend primarily on the proportions of organic (gutta-percha polymer and wax/resins) and inorganic (zinc oxide and metal sulfates) components [102]. Zinc oxide is also responsible for the antibacterial activity of gutta-percha points [105]. The particular percentages of components vary according to the manufacturer. It is evident that since the cones differ in their composition, they may differ in their physical properties and even in their biological effect [106].
Gutta-percha is a thermoplastic polymer material within which segments of the polymer molecules may be sufficiently aligned and associated to form crystalline segments randomly dispersed among the rest of the disordered, amorphous volume [107, 108]. The crystalline phase appears in two forms: (1) the alpha phase and (2) the beta phase. The forms differ only in the molecular repeat distance and single carbon-bond configuration [8, 109]. Employing X-ray methods, the degree of crystallinity of gutta-percha was reported to be 55–60% [110]. The crystal structure of pure gutta-percha has been reported in detail [107, 108], and it is known that the application of heat or mechanical energy will increase the mobility of the long molecules, perhaps increasing the size of some of the ordered, crystalline segments but, in general, increasing the proportion of the disordered, amorphous volume [111].
Thus, gutta-percha is rigid at room temperature, becomes pliable at 25–30 °C, softens at 60 °C, and melts at 100 °C with partial decomposition [112]. Gutta-percha undergoes phase transitions when heated from beta to alpha phase at around 46 °C. At a range between 54° and 64 °C, the softening point of gutta-percha, an amorphous phase is reached (Goodman 1981). When cooled at an extremely slow rate, the material will recrystallize to the alpha phase. However, this is difficult to achieve, and under normal conditions, the material returns to the beta phase. The phase transformation is important in thermoplastic obturation techniques. Gutta-percha is soluble in chloroform, eucalyptol, halothane, carbon disulfide, benzene, and xylem and less soluble in turpentine. This property of gutta-percha allows it to be removed for post preparation and in the retreatment of non-healing cases.
Any method manipulating gutta-percha using heat or solvent will result in some shrinkage (1–2%) of the material. Shrinkage of the core material is not desirable when attempting to seal a canal. Dental gutta-percha is not in its pure form or even mostly gutta-percha. Its major component is zinc oxide (50–79%), heavy metal salts (1–17%), wax or resin (1–4%), and only 19–22% actual gutta-percha [113]. The variations in content are because of different manufacturers and distributors desiring different handling properties. Some formulations are softer than others. Some clinicians choose the brand of gutta-percha depending on the technique being used. Compaction with spreaders, condensers, or carriers is usually the means used to attempt to compensate for this shrinkage of the core material [114]. In any case, some means of compensation for this shrinkage must be incorporated into the technique being used. Schilder et al. [113] postulated that vertical pressure must be applied in all warm gutta-percha techniques to compensate the volume changes. However, it has to be underlined that this assumption has never been demonstrated. Meyer et al. [115] suggested that methods for the compensation of shrinkage in root canal obturation should be evaluated.
Resilon
Resilon, a synthetic methacrylate-based resin polycaprolactone polymer, has been developed as a gutta-percha substitute to be used with Epiphany (Pentron Clinical Technologies, Wallingford, CT, USA) [116], a new methacrylate-based sealer, in an attempt to form an adhesive bond between the sealer, the core material, and the canal dentin walls. Advocates of this technique propose that the bond to the canal wall and to the core material creates a “monoblock” [117]. It is capable of being supplied in standardized ISO sizes and shapes, conforms to the configuration of the various nickel-titanium rotary instruments, and is available in pellet form for injection devices. The manufacturer states that its handling properties are similar to those of gutta-percha, and therefore it can be used with any obturation technique. Resilon contains polymers of polyester, bioactive glass, and radiopaque fillers (bismuth oxychloride and barium sulfate) with a filler content of approximately 65% [116]. It can be softened with heat or dissolved with solvents like chloroform. This characteristic allows the use of various current treatment techniques. Being a resin-based system makes it compatible with current restorative techniques in which cores and posts are being placed with resin-bonding agents [7, 118]. Some studies reported that Resilon may be degraded by pathogenic bacteria [119–121]. These findings suggest that the seal and integrity of root canal fillings obturated with Resilon may be impaired by a microbial insult.
6.2.3.3 Coated Gutta-Percha Points
This process has been developed in an attempt to achieve similar results as those claimed by Resilon, a bond between the canal wall, the core, and the sealer. Two versions of coating gutta-percha are available. In the first one, the surface of gutta-percha cones is coated with a resin (Ultradent, South Jordan, Utah, USA) [119, 121–124]. A bond is formed when the resin sealer contacts the resin-coated gutta-percha cone, and the manufacturer claims that this will inhibit leakage between the solid core and sealer; with this new coated solid core material, the technique calls for the use of EndoRez sealer (Ultradent, South Jordan, Utah), a methacrylate-based material in two components, dual-cure, whose hydrophilic characteristics allow the penetration into the dentin tubules [125]. Another manufacturer has coated gutta-percha cones with glass ionomer (Brasseler USA, Savannah, GA). This system is called Active GP Plus, and cones are designed for use with their glass-ionomer sealer [126].
To date, manufacturers of bioceramic sealers used for orthograde root canal filling are also producing their proprietary coated gutta-percha cones to bond with the correspondent bioceramic sealer in an attempt to achieve a bond between the core and the sealer [96]. Bioceramic sealer when combined with coated cones offers a new obturation technique (Synchronized Hydraulic Condensation) [97, 98]. Some experimental gutta-percha containing bioactive phosphate glasses have been recently developed with significant improvements, particularly in self-adhesiveness to root dentine and the release of alkaline species in an aqueous environment [127–130].
Injectable Materials
The GuttaFlow (Colténe/Whaledent, Altstätten, Switzerland) is an injectable silicone cement. It consists of a matrix of polydimethylsiloxane highly filled with fine powder of gutta-percha (size less than 30 μ). The GuttaFlow and GuttaFlow2, respectively, incorporate nano- and microparticles of Ag (silver) with antibacterial properties. It is applied in the root canal before inserting the cone of gutta-percha, but can be also injected without any core material. The manufacturer emphasizes the insolubility, the biocompatibility, the slight expansion consequent to the setting, the great fluidity, and the ability to be disposed in thin layer [131, 132]. The last version of this material, the GuttaFlow bioseal, is claimed to be a bioactive material that may provide natural repair constituents such as calcium and silicates.
Mineral Trioxide Aggregate (MTA)
MTA is composed of tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and bismuth oxide [133]. MTA has a proven track record in clinical and laboratory research. Its biocompatibility [73, 134–137] and bioactive properties are recognized [94]. In addition, MTA is a relatively nontoxic material with a high pH, is insoluble in tissue fluids [94, 138], and is capable of depositing a hydroxyapatite-like layer upon exposure to physiologic tissue fluids [139, 140]. Continuous leaching of calcium, phosphate, and hydroxyl ions not only allows MTA to participate in the process of regeneration and remineralization of hard tissues but may also enhance the sealability of MTA apical plugs by deposition of hydroxyapatite crystals into voids and potential spaces between the dentin and root filling material [141].
Many studies have reported successful long-term clinical outcomes associated with MTA apexification procedures [136, 142, 143]. Even if MTA has been already proposed as the material of choice for orthograde root canal filling owing to its advantageous properties [144], disadvantages of traditional MTA for this application include its long setting time [145–147], difficulty in handling due to its sandy consistency [148], potential to discolor teeth and soft tissues, and the presence of potentially toxic elements in its compositions [94].
6.3 Filling the Root Canal System in Three Dimensions
Successful filling of root canals requires the use of materials and techniques capable of densely filling the entire root canal system and providing a fluid tight seal from the apical segment of the canal to the cavosurface margin in order to prevent reinfection. This also implies that an adequate coronal filling or restoration be placed to prevent oral bacterial microleakage. When preparing the root canals to allow for a three-dimensional obturation and an effective sealing of the root canal space to its proper apical extent, the clinician’s inability to maintain the original anatomy in terms of working length, shape, and position of the apical foramen and to create an optimal canal geometry with a continuous, progressive, and uniform conical shape with a circular base within the canal can result in procedural errors. These may include a lack of compaction and adaptation of the filling materials and excessive apical extrusion of these materials into the periapical tissue [94, 149, 150].
6.3.1 Techniques for Filling the Root Canal System
6.3.1.1 Single Cone
The single-cone technique consists of matching a master point to the prepared root canal. For this technique, a type of canal preparation is advocated so that the size of the cone and the shape of the preparation are closely matched. The cone is cemented in place with a root canal sealer, which is advocated to fill the spaces. This technique is simple, but does not fill completely root canals that are seldom round throughout their length [36].
6.3.1.2 Chemoplasticized Gutta-Percha
Especially in the past, solvents such as chloroform, eucalyptol, and xylene have been used to chemically soften gutta-percha. Solvents were used to soften the outer surface of the custom cone as if making an impression of the apical portion of the canal. Indeed, chemical solvent use implies many problems. The techniques that make use of solvents have been criticized because they cause the contraction of the material after evaporation of the solvent itself [151]. The techniques using solvents are also inadvisable for the poor stock control and the related risk of extrusion [151–154]. Chloroform was found carcinogenic [152, 154, 155], and although its use has never been surely correlated with the onset of cancer, in the USA it has been deleted from the list of solvents used in dentistry in 1979 [153]. Chloroform has been widely used in the past for the removal of the gutta-percha from the root canals. For this use, in its replacement, alternative solvents are currently preferred [156, 157], i.e., halothane, which compared to chloroform is more volatile and showed equally effective solvent properties [157, 158]. The eucalyptol shows slower action but provides less damage to the tissues [159, 160].
6.3.1.3 Cold Lateral Compaction
Lateral compaction provides for the cold compaction of gutta-percha: a master cone corresponding to the final instrumentation size and length of the canal is coated with sealer, inserted into the canal, and is laterally compacted with a spreader. Additional accessory cones are placed against it and are laterally compacted as well by means of a spreader. This technique provides a good control of the working length. On the other side, the cold compacted gutta-percha shows poor plastic deformability, a modest degree of elastic deformation, which acts negatively, and no ability to flow. The filling obtained is a mass of leaning cones, with a large amount of interposed spaces which are filled by root canal cement. The poor cold formability would require the compression of the material very close to the apical limit, according to Allison et al. [161]. The lateral compaction requires that relatively high forces are applied by the spreader with some risks of root fracture [162].
6.3.1.4 Warm Lateral Compaction
A master cone corresponding to the final instrument size of the canal is coated with sealer, inserted into the canal, heated with a warm spreader, laterally compacted with spreaders, and filled with additional accessory cones. Some devices use vibration in addition to the warm spreader [163].
6.3.1.5 Warm Vertical Compaction
A master cone corresponding to the final instrument size and length of the canal is fitted, coated with sealer, heated, and compacted vertically with pluggers until the apical 4–5 mm segment of the canal is filled. Then the remaining root canal is backfilled using warm pieces of core material or by injection systems.
The warm vertical compaction technique as described by Schilder [8] provides for the introduction in the root canal of a gutta-percha cone coated by a thin layer of endodontic sealer. The gutta-percha is warmed by means of a heat spreader and compacted vertically by pluggers with multiple waves of condensations from the crown to the apex. This technique has the aim to provide a three-dimensional filling of the root canal system with all its complexities. The compaction of the material is important not only to allow its adaptation to the endodontic space, but also, it is claimed by Schilder et al. [113], to compensate for changes in volume of the gutta-percha that always corresponds to a certain degree of contraction.
6.3.1.6 Continuous Wave of Condensation
Continuous wave [164] is essentially a vertical compaction (down-packing) of core material and sealer in the apical portion of the root canal using commercially available heating devices that use pluggers as heat carrier and then backfill the remaining portion of the root canal with thermoplasticized core material using injection devices. The main difference with the classic Schilder technique is that this technique performs the compaction in only one continuous wave of compaction from crown to the apex. A good apical stop is necessary for both these techniques to prevent apical extrusion of the filling.
6.3.1.7 Injection Technique
It provides that preheated, thermoplasticized, injectable core material is injected directly into the root canal, using specific injection devices [165–167]. Sealer is placed in the canal before injection. As a master cone is not used, it is difficult to control the apical extent of the filling with high risk of under- or overfilled root canal obturations. A cold, flowable matrix that is triturated, GuttaFlow® (Coltene Whaledent, Cuyahoga Falls, OH), consists of gutta-percha added to a resin sealer, RoekoSeal. The material is provided in capsules for trituration. The technique involves injection of the material into the canal and placing a single master cone [168].
6.3.1.8 Thermomechanical Compaction
A cone coated with sealer is placed in the root canal, engaged with a rotary instrument mounted on contra-angle handpiece that frictionally warms, plasticizes, and compacts it into the root canal [169]. The compactors are similar to the Hedström files but have spirals oriented in the opposite direction, and the gutta-percha is heated and simultaneously pushed both apically and laterally. This technique can easily cause extrusion. On the other hand, it can be a good backfilling technique [170].
An alternative technique (MicroSeal) uses thermoplasticized softened core material delivered directly by the mechanically activated condenser into the space created on the side of a master point laterally condensed with a mechanical spreader. It will unify the advantages of the apical control of filling material related to the use of a master point and of the effective filling of the lateral space with a thermoplasticized core material forced into the canal [170–172].
6.3.1.9 Carrier-Based
Carrier-based thermoplasticized warm gutta-percha on a plastic or gutta-percha carrier, heated in an oven, is delivered directly into the canal as a root canal filling [173, 174]. Even if root canal walls are only painted with sealer before the insertion of the obturator, the apical control of the filling material is difficult and extrusions common. Furthermore, plastic carriers are not so easy to be removed in the case of retreatment or post-space preparation.
In the carrier-based sectional technique, a sized and fitted section of gutta-percha with sealer is inserted into the apical 4 mm of the root canal. The remaining portion of the root canal is filled with injectable, thermoplasticized gutta-percha using an injection gun. An example is SimpliFill (Discus Dental, Culver City, CA) [116, 175].
6.3.1.10 Apical Barrier
Apical barriers are important for the obturation of canals with immature roots with open apices, whose endodontic management is challenging for the clinician because of the lack of resistance and retention form associated with a blunderbuss apex. A blunderbuss configuration of the apex makes delivery of root filling materials difficult, potentially leading to overextension and/or overfilling of the root canal. Historically, nonsurgical treatment of immature teeth was achieved by calcium hydroxide apexification to induce physiologic formation of a hard tissue barrier before obturation. However, significant improvements to endodontic biomaterials, placed using proper carriers, have allowed for more convenient and efficacious treatment of these teeth in a single-visit procedure [142, 143, 176]. At this time mineral trioxide aggregate (MTA) is generally considered the material of choice for the obturation of canals with immature roots with open apices because of its ability to act as an osteoconductive apical barrier [138, 140].
6.4 Complications Due to Obturation Procedures
Obturation errors often are a result of inadequate cleaning and shaping or preparation errors. Ledges, blockages, perforations, separated instruments, debris present apically, apical or canal transportation, inaccurate working lengths, and underprepared or overprepared canals are frequent errors encountered during root canal instrumentation that create an unpredictable shape of the root canals and that consequently may negatively influence the quality of root canal obturation. These have all been already discussed in detail in the previous chapter.
Complications during or after the endodontic treatment might be prevented by careful preoperative examination, good-quality radiographs, good instrumentation, irrigation, and obturation techniques. In fact, obturation-related complications may be mainly due to the chemical or physical negative consequence of a gross extrusion of filling materials beyond the apex into the periapical tissues.
6.4.1 Prevention of Extrusion of Filling Materials
Prior to the obturation phase, the clinician must establish the proper shape and size of the root canal. Proper canal shaping should create a continuous tapered shape from the apex to the coronal opening to obtain an apical resistance or retention form. The consequences of compression of gutta-percha within a root canal will differ with the physical composition of the gutta-percha, the temperature, the taper of the canal, and the point of application of the compacting force.
The available commercial [103, 177–179] gutta-percha products vary greatly in their composition [177, 178], although not all have been chemically or physically analyzed [103, 178, 179]. These properties affect the adaptation of the gutta-percha to the endodontic space. Schilder [8] declared that a tapered root canal allows the compaction of the gutta-percha cone and its close fitting to the canal walls, minimizing the risk of producing extrusion. With lateral condensation, cold gutta-percha is used. If gutta-percha is compacted at body temperature, only very small elastic and plastic deformation may occur, and the probability that it would flow into irregular spaces is unrealistic. Alternative techniques described above use warm gutta-percha in different ways, applied [8] or frictional heat plasticizes gutta-percha [169], allowing for better adaptation to canal walls and a higher degree of homogeneity [109, 179, 180]. The compacting of heated amorphous gutta-percha is difficult to control, especially close to the apex [181], both because of the frequent lack of apical constriction [182] and the need to enlarge the apical portion of the canal to obtain mechanical debridement [183].
The main difficulty for the practitioner is to adjust the compaction procedure to the softening of the gutta-percha. A plastic mass of gutta-percha forced to flow will deform on contact with the internal surface of the root canal. Thus, when compacting heated amorphous gutta-percha, the major force is directed toward the apex, and the flowing gutta-percha may extrude, if apical patency has been maintained during instrumentation. Extrusion has been reported to be a complication of thermo-softened techniques [181, 184].
Warm vertical compaction [8] and the “continuous wave technique” [164] offer techniques that use the apical control of the internal placement of a cold point while providing the homogeneous, three-dimensional filling advantages of the thermo-softened techniques. Marlin and Schilder stated [151] that heating and compaction may be performed to a distance of 5–7 mm from the endpoint of the gutta-percha. When using the warm vertical condensation technique, a temperature increase to 40–42 °C usually occurs in the apical gutta-percha, and it should not exceed 45 °C to avoid the volume variations dependent on phase changes [185]. In this condition, from a rheological viewpoint, despite the apical patency being maintained, the compacted gutta-percha has no or reduced risk to extrude. In fact, at the interface between gutta-percha and canal wall, reaction forces can be distinguished: normal reaction forces act perpendicular, and shear reaction forces act parallel (friction) to surfaces in contact. When the applied force exceeds the maximum static friction force, surfaces move relative to each other and dynamic friction force occurs [186]. Dynamic friction depends on force squeezing objects together, and on the nature of materials in contact, and has a direction opposite to the motion or the impending motion. However, these frictional phenomena can develop only if the gutta-percha is not too deformable, i.e., in crystalline state [186]. Only if the cone is in crystalline state, the vertical compaction can force it to fit more apically. Thus, a reversible elastic strain may occur, able to increase the force normal to the canal wall, to compress the gutta-percha against the canal walls and squeeze the endodontic cement, and to fill the lateral canals [186].
An increase of friction also occurs that counteracts the cone motion and thus its extrusion [186]. The apical gutta-percha, at the temperatures recorded and advocated by Marlin and Schilder [151], maintains its more crystalline state and shows low plasticity. If gutta-percha is compacted at those temperatures, only very small elastic and plastic deformation may occur, and the probability that it would flow and extrude is less. Obviously, if plasticized gutta-percha is compacted close to the apex, the probability that it would flow and extrude is very high. In contrast, when using carrier-based obturators, thermomechanical compaction [169], and the injection techniques close to the patent apical foramen, gutta-percha is heated to higher temperatures, increasing the proportion of the amorphous phase as happens with all semicrystalline thermopolymers [111] and thus increasing the risk of extrusion of the softened gutta-percha. Furthermore, attention must be paid when placing the sealer in the canal. Placing the sealer with a lentulo spiral or a syringe or activating it with sonic or ultrasonic instruments may promote extrusion of the material because this technique lacks apical control. Placing the sealer with a file and spinning it counterclockwise do not seem to provide an ideal distribution of the sealer on the root canal walls. Placing the sealer with a master cone and pumping it up and down in the canal seem to be the most predictable technique for the application of sealers in root canals, if a filling technique using a master point will be performed. When placing sealer in general, the clinicians should use particular care if the canal presents an open apex, to avoid extrusion of material.
6.4.2 Complications Due to Extrusion of Filling Materials
Clinical image (a) and radiograph (b) of a gross overfilling appearing under the subcutaneous tissues, probably due to the lack of the buccal cortical bone plate. Despite the big amount of filling material extruded, the patient did not complain any pain or discomfort
A first left mandibular molar has been endodontically treated due to chronic apical periodontitis (a). Despite the overfilling in the mesial root (b), complete healing has been obtained at the 2-year control (c)
On left, CBCT examination of an upper lateral incisor endodontically treated. It is possible to appreciate the filling material extruded in the periapical bone. At the baseline the patient had no symptoms related with this tooth. On the right, after 20 months the patient presented an acute apical abscess symptom of pain and swelling. A possible association between extrusion of infected material and development of pathology can be hypothesized
6.4.2.1 Anatomical Areas Involved
Maxillary Sinus Damage
CBCT evaluation of an upper second right molar with extrusion of filling material from the apex of the mesiobuccal root into the maxillary sinus that caused a fibrotic reaction that encapsulated the material
A cleared root of an extracted upper canine showing a possible clinical complication with carrier-based filling techniques: the extrusion of the plastic carrier in the periapical tissues
A case report of extreme overextension of gutta-percha within the maxillary sinus after endodontic retreatment on tooth #14 was provided by Brooks and Kleinman [205]; the mesiobuccal canal had only been filled with thermoplasticized injectable warm gutta-percha, the origin of the extrusion; imaging with three-dimensional cone beam computed tomography was performed for localization of the gutta-percha, the patient underwent a Caldwell-Luc approach for removal of the extruded material, the gutta-percha was successfully removed intact, and the patient had an unremarkable postoperative course; however, the patient continues to have mild tenderness in the sinus region. Several root canal sealers containing zinc have also been implicated with the development of an aspergillosis infection and fungus ball formation within the maxillary sinus [205–208].
Neurological Damage
(a) A panoramic radiograph, following endodontic treatment of 3.4 and 3.7. (b) CBCT exam (coronal slices/cuts) of the 37 revealing filling material extruded in the mandibular canal. (c) CBCT sagittal slice/cut through the filling material present into the mandibular canal. (Reprinted with permission by: G. Gambarini, G. Plotino, N. M. Grande, L. Testarelli, M. Prencipe, D. Messineo, L. Fratini, F. D’Ambrosio. Differential diagnosis of endodontic-related inferior alveolar nerve paraesthesia with cone beam computed tomography: a case report. Int Endod J 2011;44:176–181)
In mandibular teeth posterior to the mental foramen, extrusion of filling material can be responsible for damage to the inferior alveolar nerve, and labiomandibular paresthesia is the most frequent complication that can occur [187, 188]. Most cases have been reported in connection with mandibular second molars, but cases related to first molars and premolars have also been described [213]. In fact, because of the proximity of the mental foramen, the mental nerve is usually affected by endodontic-related complications in mandibular premolars [214]. Endodontic materials can spread to the periradicular tissues theoretically in four different ways. These include migration toward the mandibular nerve bundle, drainage through lymphatic vessels, systemic diffusion within a periapical vein, and progression between the bone and mucosal membrane toward soft tissues [215]. Anatomy of the mandible favors the introduction of endodontic materials into the mandibular canal area and thus the paresthesia to occur. Especially in the posterior area of mandible, the trabecular, vacuole-rich cancellous bone facilitates the diffusion of different materials into the surrounding tissues [188]. This alone is sufficient to cause paresthesia in case of overfill. Periapical infection weakens further the loose trabecular pattern of the bone, which raises the probability of diffusion of endodontic materials and results in paresthesia together with widened apical foramen [216].
Attention should also be paid to the distance between the apices and the mandibular canal. According to one study, this distance varies between 1 and 4 mm in the case of the first mandibular molar; it is less than 1 mm with the second and third mandibular molars [188]. With mandibular premolars, the proximity of mental foramen should always be taken into consideration [214]. Toxic root canal filling materials include sealers and paraformaldehyde-containing pastes [217], but almost all of the endodontic materials are neurotoxic at some level [218]. Neurotoxic materials are able to initiate a host-dependent inflammatory process, which causes damage to cells, ulceration, and hemolysis when in contact with vital tissue [216]. This process might culminate in necrosis of the tissue [217]. Free eugenol, a dissolution product of zinc eugenol, hydrolyzes cell membrane and inhibits cellular respiration [215, 217, 219]. Because of the potential for chemical degeneration of the nerve axons, the use of eugenol-containing pastes is not recommended for endodontic obturation at all [215, 217, 219]. This applies especially when forces are directly connected to the inferior alveolar nerve bundle [220]. In cases of paraformaldehyde paste overfill and introduction to the periradicular tissues, the risk of permanent tissue damage is high. These cases are widely reported in literature. In addition, endodontic-related paresthesia can result from mechanical pressure and ischemia or bacterial toxins involved in periapical pathosis of a nonvital tooth [220–222].
The recovery potential of the nerve depends on the extent of the damage (both mechanical and chemical) and rapidity of cause removal [164, 218, 223]. In some cases a clear overfill with sealer within the mandibular canal can be observed on a radiograph, but the patient is nevertheless free of symptoms [218]. Gutta-percha is traditionally considered as an inert root filling material, and the paresthesia cases involving gutta-percha usually result from overfill of thermoplastic gutta-percha [215]. In these circumstances the resulting paresthesia is of a mechanical or thermal nature. Clinically, the response is often seen as sudden onset of excruciating pain, swelling, and numbness of the affected region. The teeth might be tender to percussion or on palpation, and the opening of the mouth might be limited [216, 217]. It is difficult to ascertain the etiology for paresthesia after treatment with overfill into neurovascular tissue. The clinician must also consider the neurotoxicity of the materials, the possibility of direct mechanical damage due to instrumentation [216, 217, 224, 225], the compression of core materials such as gutta-percha [225, 226], and the possibility of epineural fibrosis resulting in neuroma [227].
In the case of paresthesia or anesthesia in which apical extrusion of endodontic materials is observed, however, because the paresthesia can be of both mechanical and chemical nature, removal of the excess material might not be sufficient if the nerve fibers have already undergone degeneration in a chemical manner. Paresthesia caused by a brief irritation of the nerve, for instance, on overinstrumentation, usually subsides within days. If no signs of healing are seen within 6 months, the chances of healing are considered much lower, although normal sensory function might still return after this [188, 212, 227]. The majority of tissue damage cases are treated by nonsurgical methods such as analgesics, cold packs, corticosteroids, and antibiotics to inhibit secondary infections [228, 229]. Dexamethasone is a corticosteroid that has been widely used in dentistry, and it seems to decrease periapical inflammation caused by a foreign body [220]. Even though care should be taken with the use of corticosteroids, with the correct indications and adequate dosage, the adverse effects seem to be rare [230]. Paresthesia can sometimes result from postoperative infection, in which case antibiotics are the treatment of choice. If the facial nerve has been damaged, physiotherapy might provide additional help [217].
Paresthesia resulting from local infection usually subsides through elimination of infection by root canal treatment, extraction, antibiotics, and/or periapical surgery [231]. The patient must be informed of the nature and possible duration of paresthesia as well as the importance of regular control visits [232]. In some cases surgical exploration is required to remove the foreign material from the periapical area as soon as possible, preferably within 48 h [218]. Surgery is indicated when neurotoxic material migrates along the mandibular nerve bundle, and the nerve must be exposed for debridement [218, 232, 233].
Other possible invasive methods of treatment are extraction of the tooth or incision and drainage [228]. The clinical examination that results in a diagnosis of anesthesia or increasing painful dysesthesia unresponsive to nonsurgical therapy should help guide this decision [234, 235]. It is suggested that the decision to intervene surgically should include the high suspicion of injury resulting in the loss of conduction within the nerve because of suspected chemical toxicity and mechanical compression. The favorable results for long-term spontaneous recovery require thoughtful considerations for taking a “wait-and-observe” approach. When a peripheral nerve is injured, a nonsurgical management that supports spontaneous neurosensory recovery and promotes patient tolerance of the sensory loss is a viable option [234–236]. The most compelling reason to wait is that a majority of injuries are known to recover spontaneously to some degree. Higher levels of recovery can also be expected when the patient is young and healthy. Ørstavik et al. [237] reported that out of 24 patients with paresthesia affecting the mandibular nerve, 14 showed no healing from 3 months to 18 years after the injury; all of the reported cases were lower molars or second premolars. Radiographs are often useful in monitoring the area of tissue damage in relation to hard tissues (e.g., the size and location of lesion or the location of the extruded material in relation to mandibular canal) [188, 189, 212]. It is not always possible to make a precise diagnosis of extrusion into the nerve by showing the contact of the filling material with the alveolar nerves using traditional endodontic radiographs.
One of the major problems is that intraoral radiographs only reveal limited information. The amount of information gained from analogue and digital periapical radiographs is incomplete because the three-dimensional anatomy of the area being radiographed is compressed into a two-dimensional image or shadowgraph [238]. Patel et al. [239] demonstrated CBCT’s superior diagnostic accuracy compared with intraoral radiographs [238]. Cone beam computed tomography can be considered an effective radiographic diagnostic device when endodontic-related inferior alveolar nerve or mental foramen paresthesia is suspected, and it is especially useful in the planning of surgical procedures [240–243].
6.4.3 The Management of the Extrusion of Filling Materials and Its Complications
In cases of overextension with the lateral compaction technique or of a thermoplastic core carrier, the filling material can often be teased back through the foramen, provided the sealer has not hardened. If the sealer has hardened, it may still be possible to retrieve the gutta-percha. In cases of overextension, when retraction of the filling material through the apical foramen is difficult, the routine and immediate use of surgical intervention is neither indicated nor justified. In most cases, the periradicular tissues will heal, and the patient will be symptom-free. If, however, the patient exhibits signs or symptoms of periradicular inflammation, surgery may be indicated. Treatment of endodontically related paresthesia remains controversial, varying from a wait-and-see approach [237] to early [244], if not immediate [245], surgical debridement of the inferior alveolar nerve via a number of possible approaches. These include extraction of the tooth and approaching the nerve through the socket [244], decortication of the mandible achieved laterally [245] from an intraoral [246] and extraoral [227] approach, and sagittal splitting of the mandible to expose the nerve within the split [233]. The use of biocompatible materials did not suggest an immediate surgical approach, but rather a wait-and-see approach, even when the maxillary sinus may be involved. It is well known that toxicity tends to reduce and the extruded filling material undergoes resorption over time. In any case, the dangers of any extruded materials during root canal treatment should be highlighted, even if only a limited overfilling occurs, especially in proximity to important anatomical areas.
In conclusion, although proper techniques have been followed, occasionally gutta-percha, resin-bonded filling materials, or root canal sealer may be unintentionally pushed beyond the confines of the root canal system. However, the periradicular tissues generally tolerate these materials. Although sealers may provoke an initial inflammatory response to a greater or lesser degree over a short period, the macrophage scavenger system eliminates the excessive material from the periradicular tissues. In any case, the mere placement of filling material outside the canal system is not a major cause for alarm if the canal space is three-dimensionally obturated. If excessive amounts of materials are extruded, the patient should be informed, and periodic reexaminations are indicated.
6.4.4 Complications Due to Chemical Effects
6.4.4.1 Toxic Effects of Root Canal Filling Materials
Since they are classified as medical devices, root canal sealers and core materials need to meet biocompatibility requirements, which include the evaluation of cytotoxicity potential, in addition to exhibiting proper chemical, physical, and mechanical properties.
During the last three decades, biological properties, or biocompatibility, of a dental material have become increasingly important. Currently, there are mandatory regulatory requirements as well as voluntary standards at national and international levels [247–249]. The European Society of Endodontology guidelines for endodontic therapy state that “the objective of any (endodontic) technique used should be to apply a biocompatible hermetically sealing canal filling that obturates the prepared canal space from pulp chamber just to its apical termination” [250]. An ideal root canal filling material, in addition to have suitable chemical and physical properties, should be biologically compatible and well tolerated by the periapical tissues, avoiding any possible modification and delay of the healing process [251].
The biocompatibility of root canal filling materials has been of concern to dentistry for many decades now because they can come into contact with the connective periapical tissue [252], and the components released from these materials could produce irritation or even degeneration of the surrounding tissues [253]. The periapical tissues can react to the presence of a sealer and a GP point in several ways. It can cause an inflammatory reaction, it can be regarded as a foreign body and encapsulated, and it can be present without causing inflammatory reactions and is not encapsulated, and the sealer can be resorbed over time, with or without an inflammatory reaction.
It has been reported that the extrusion of root filling substances can cause a foreign body reaction leading to the development of periapical lesions that may be refractory to endodontic therapy [229, 254]. Large quantities of excess filling materials in the periapical tissues caused necrosis of bone followed by bone resorption and absorption of the filling materials, despite gutta-percha having a low degree of toxicity when compared with other components used in endodontic obturation [229]. In fact, gutta-percha is the most common component used in root canal filling materials because it is well tolerated from host tissues [255], but other compounds such as zinc oxide-eugenol are capable of inducing cytotoxic effects [178, 256]. On the other side, sealers and components of sealers are recognized by the scientific literature as toxic or highly irritating, especially in their freshly mixed states, and produces an initial acute inflammatory reaction in the connective tissues [178, 257].
However, after setting or curing, some sealers become relatively inert, and it has to be underlined that pastes and sealers will be absorbed more rapidly than solid core materials [258]. If over a relatively short period of time (up to 30 days) a mild inflammation is present and it has diminished over time, a material with otherwise favorable properties can be considered acceptable [259].
Elution of components has been recognized [260], and the inflammatory process as a result of this is the body’s response to irritation. Fibrous encapsulation is the body’s response to isolate an otherwise biocompatible material. Furthermore, a material, usually small-size particles, can be present in periapical tissues, cause no inflammation, and be present without encapsulation.
Calcium hydroxide sealers. Some studies in rats [261] and dogs reported neurotoxic effects of hydroxide, with partial recovery after various periods of time [262, 263]. A study on human fibroblasts showed early severity cytotoxic effects of calcium hydroxide in the first 48 h with significant reduction in toxicity between the third and fifth day [264].
Paraformaldehyde pastes. Paraformaldehyde paste materials cause mummification and fixation of pulp tissue [265]. In 1959, Sargenti and Richter introduced N2 and subsequently other paraformaldehyde paste formulations, because of their consistent antimicrobial activity when used as root canal filling materials [266]. Thus N2, RC2B, endomethosone, and other paraformaldehyde paste formulations were recommended as the sole filling material. Unfortunately, absorbability and toxicity are serious considerations with paraformaldehyde pastes. A large number of studies reported paresthesia and other complications of the inferior alveolar nerve due to the neurotoxicity of these paraformaldehyde compounds [55, 224, 237, 246, 266–270].
Others have demonstrated the systemic distribution of paraformaldehyde and disintegration products in periapical and periodontal tissues, as well in the blood, regional lymph nodes, kidney, and liver [271]. Moreover several clinical reports of extreme complications have been published [55, 227, 237, 272]. At the present time, the use of N2 or similar type pastes is contraindicated, because of the higher risks associated with these paraformaldehyde-containing endodontic materials.
Polymer and resin sealers. The most commonly known sealers within this category are AH26 and AH26 Plus (Caulk/Dentsply, Milford, DE, USA). The sealer has been reported to be very toxic upon initial mixing [229, 273], due mainly to the formation of a very small amount of formaldehyde as a result of the chemical setting process [229, 273]. However toxicity resolves rapidly within 24 h during the setting process [229]. Diaket (ESPE, Seefeld, Germany) is a polyketone compound adhesive sealer. It has been demonstrated that it is relatively toxic during setting and these effects are persistent [273]. Resorcinol-formalin resin is a paste filling material that is commonly used in Russia, China, and India for the treatment of pulpitis. Although there are many variations of the resin pastes that are used, the main ingredients are resorcinol and formaldehyde [274]. When set, this material creates an almost impenetrable barrier, with loss of the retreatment option [275]. If the paste material is extruded beyond the apical foramen, severe toxic effects may arise. In the event of an overfill into the sinus or neurovascular bundle, irreversible damage may also occur [274].
6.4.4.2 Discoloration
Almost all materials used in modern endodontics may stain teeth. However, for a wide range of materials currently available on the market, there is only scarce or no evidence available on their staining ability.
Endodontic sealers. Parsons et al. [276] assessed coronal discolorations produced by four different sealers (Sealapex, Roth’s 801, AH26, and Kerr Pulp Canal Sealer) and reported that all of the experimental teeth revealed coronal discoloration; this effect was attributed to the silver ions that were part of the composition of both materials. A follow-up study published by the same group assessed the penetration depth of the same four sealers into the dentin [277]. All four sealers showed only minimal sealer penetration and no evidence of discoloration of the exposed dentinal surfaces. The findings were assumed to be a result of the smear layer not having been removed, which may prevent the materials from diffusing into the dentin. However, the 2-year set sealers in the endodontic cavity showed marked levels of discoloration compared to fresh mixes.
A recent study assessed the degree of staining in tooth crowns caused by commonly used endodontic sealers via a computer analysis method [278]. Discoloration induced by the root canal sealers AH26, Endofill, Tubuli Seal, zinc oxide-eugenol (ZnOE), Apatite Root Canal Sealer III, gutta-percha, and Cavizol (a filling material containing ZnOE) was assessed on extracted human premolars. After 3, 6, and 9 months, the order of severity of tooth discoloration (from the highest to lowest values) was as follows: amalgam = Endofill > ZOE > Tubli Seal > AH26 > gutta-percha > Apatite Root Sealer III > Cavizol > distilled water. For all groups, the discoloration was most evident in the cervical third of the crown and on the cervical root surface. Elkhazin investigated the discoloration effects of AH Plus, EndoRez, Sealapex, and Kerr Pulp Canal. The teeth were root-filled with gutta-percha with one of the four materials. After 6 and 8 weeks, all four sealers showed significant coronal discoloration, which increased with time [279].
Portland cement-based materials. Mineral trioxide aggregate (MTA) is based on portland cement and was introduced to the field of endodontics in 1993. Owing to its high level of biocompatibility and its good sealing properties, it is regarded as the material of choice in cases of vital pulp therapy (pulp capping, partial pulpotomy) or to seal pathways of communication between the root canal system and the external root surface (perforation, apexification, or retrograde filling) [95, 280]. One of the main drawbacks of MTA is its discoloration potential [94]. The gray-colored formula, which was first introduced to the market, led to visible color changes on the outer surface. When it was used as a pulpotomy agent in primary molars, discoloration occurred in 60% of all cases [281].
To reduce the discoloration potential, the chemical composition of MTA was changed and an improved formulation was later introduced as white MTA. The most significant difference between the two types of MTA is the lack of iron ions in white MTA [282, 283]. However, it has been reported that white MTA may cause discoloration as well [282, 283]. Some authors state that the discoloration induced by MTA may be attributed to bismuth oxide, which is added to improve the radiopacity in both gray and white formulations [282, 283]. Mineral trioxide aggregate (MTA) has been successfully used in perforations of the root canal. Bortoluzzi et al. [284] reported a clinical case of a root perforation sealed with gray MTA that resulted in discoloration of the marginal gingiva. Treatment consisted of replacing gray MTA with white MTA with the aid of a dental operating microscope, producing satisfactory esthetic results.
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