5.1 Apical Plug

Endodontic treatment of teeth with immature root apices and necrotic pulp has always been challenging [1]. The risk of not sealing the root canal correctly, together with the high risk of inadequate root canal instrumentation and failure to achieve an adequate apical stop [2], has led scientific research to identify techniques and materials that can increase the success rate without sacrificing the balance between time and success, typical characteristics of endodontic practice.

Apexification, a traditional procedure for immature, nonvital teeth, is a method for inducing a calcified barrier in a root with an open apex or for further apical development of teeth with incomplete roots and necrotic pulp [3]. Apical plug, also known as one‐step apexification, is an appropriate technique for immature teeth with pulp necrosis to create an apical barrier and achieve adequate stopping. Together with revitalization, it represents a therapeutic approach for eliminating signs and symptoms of infection and preserving affected teeth in the long term [4]. For many years, calcium hydroxide (CH) has been proposed as a successful material for the formation of a calcified barrier [5]. Although this material is effective and provides predictable results owing to its ability to form a calcified tissue at the root end, it has several disadvantages, such as the need for multiple sessions requiring high patient compliance, the high susceptibility of the tooth to fracture given the thin and fragile roots often characteristic of teeth with incomplete roots [6], the vulnerability of the provisional coronal restoration to re‐infection [7], the long‐term duration of CH use, which has been shown to weaken the tooth and increase the likelihood of dental fractures [8–12], and the long time required to form the calcified apical barrier (approximately six months) [13], as well as the nature of the barrier which is calcified but porous and sometimes contains a small amount of soft tissue.

Therefore, this technique has been replaced by a more powerful approach based on hydraulic calcium silicate cement, which reduces the treatment time to one or two visits, the risk of poor patient compliance, and root fractures. Mineral trioxide aggregate (MTA), used initially for root‐end fillings and mainly composed of tricalcium silicate, tricalcium aluminate, tricalcium oxide, and silica, is a powder of fine hydrophilic particles which form a colloidal gel that solidifies into a rigid structure [6]. MTA is considered an optimal allay for the application of the apical plug technique. It offers many advantages, such as the ability to cure even in the presence of blood, a high pH (10.2–12.5 three hours after setting) [6], and can be used as an apical plug in immature, nonvital teeth because of its ability to form a cementitious tissue in the periradicular area of the tooth [6]. Overall, using MTA as an apical plug simplifies the complex procedure and promotes hard tissue, resulting in a biological seal of the cementum over the material [14].

In contrast to CH, MTA showed less inflammation and more hard tissue formation [15], whereas MTA is more effective when combined with an intracanal combination of CH for one week [16]. In addition, MTA has a good sealing ability, can be packed with a carrier, injected in batches into the apical third, and condensed vertically with a hand plugger to form apical plugs at the end of immature roots with and without periapical lesions [17]. Clinical indications for performing an apical plug include: (1) apex greater than 50–60 ISO in diameter, (2) immature apex, (3) irregular apical morphology, (4) canal morphology with parallel walls, (5) internal resorptions, (6) and orthograde root canal filling in case of subsequent apical surgery.

5.2 MTA Apical Plug Outcome

Since the introduction of MTA as an apical plug and an alternative to CH dressing, several studies have reported higher success and survival rates of the MTA apical plug compared with the conventional apexification technique using CH. In outcome assessment, success rates of CH ranged from 79 to 96% [10, 18], while higher healing rates (81–100%) were reported for MTA [19–21]. In direct comparisons between the two techniques, MTA resulted in less persistent disease [19]. Studies evaluating MTA success rates after a period of CH intracanal medication (1–6 weeks) showed a success rate of more than 90% in the median period (follow‐up between 12 and 35 months) [22]. Sarris et al. reported a clinical success rate of 94.1% and a radiographic success rate of 76.5% at a mean follow‐up of 12.5 ± 2.9 months in 17 nonvital permanent incisors treated with CH for at least one week followed by 3–4 mm MTA [23]. According to a systematic review and meta‐analysis comparing the efficacy of CH and MTA for apexification of immature permanent teeth, which included only four studies (RCTs and observational studies), there was no difference in clinical and radiographic success rates or in the ratio of apical barrier formation between CH and MTA (clinical success rate: pooled odds ratio [OR] = 3.03, 95% CI: 0.42–21.72, p = 0.271; radiographic success rate: pooled OR = 4.30, 95% CI: 0.45–41.36, p = 0.206; apical barrier formation rate: pooled OR = 1.71, 95% CI: 0.59–4.96, p = 0.322). The actual difference between the two approaches is due to the time required for apical barrier formation, which is shorter for MTA than for CH (pooled difference in means = −3.58, 95% CI: from −4.91 to −2.25, p < 0.001) [24]. In a previous study, Chala et al. demonstrated that although similar results can be achieved with both techniques, the success rate of the MTA apical plug at final follow‐up is 100%, while the success rate of apexification with CH reaches 92% [25]. Considering these two works, the choice of apexification techniques seems to depend only on the time needed to achieve clinical outcome and patient compliance. In the study by Pradhan et al., the total treatment time for MTA apical plug was 0.75 ± 0.5 months, whereas the time required to achieve a similar result with CH can be more than seven months (7 ± 2.5 months) [26]. Similar results were obtained in the study by Damle et al., which showed that the mean time for the apical barrier in the MTA group was 4.5 ± 1.6 months and for completion of the lamina dura was 4.1 ± 1.5 months, significantly shorter than in the CH group, which required 7.9 ± 2.5 and 6.4 ± 2.5 months, respectively (p < 0.01) [27]. In the retrospective study by Witherspoon et al., which analyzed the healing rate in a subgroup of 119 immature teeth treated with MTA apical plug in one (60.3% of the sample) or two appointments (39.7%), the percentage of healing was higher in the one‐appointment group (96.5%) than in the two‐appointment group (89%) [15]. In a recent systematic review comparing the success and survival rates of MTA apical plug and regenerative endodontic treatment (RET), the authors reported a pooled survival rate of 97.1% (95% CI: 93.7–100) and pooled success rates of 94.6% (95% CI, 90.2–99.1) for MTA apical plug and 97.8% (95% CI: 94.8–100) and 91.3% (95% CI, 84.5–98.2) for RET [28].

5.3 Other Apical Plug Materials

Although MTA is considered the gold standard for apical plug technique, it has several shortcomings such as long setting time (from 2 hours and 45 minutes to 4 hours), high cost, difficult handling, and the risk of post‐treatment dyschromia, which is particularly detrimental in anterior teeth of young patients and because of the bismuth oxide added to MTA to improve radiopacity [29, 30]. For these reasons, and given the need for single‐session treatment in patients requiring pharmacologic behavior management techniques such as sedation or anesthesia [31], bioceramics have been proposed as an alternative to MTA.

These materials have the same chemical and physical properties as MTA in terms of biocompatibility and for inducing the formation of a mineralized barrier. However, they offer additional advantages in terms of integration with the dentin component and replacement of bismuth oxide with zirconium oxide to improve the radiopacity of the material and ensure the chromatic stability of the elements for a better aesthetic outcome [32–34]. The main feature of these materials is bioactivity [35], the ability of the material to release calcium ions, electroconductivity, and the formation of a mineralized biological barrier between the cement and the dentin wall with deposition of HA [35–37]. Biodentine was introduced in 2010 as a calcium silicate‐based bioactive cement with dentin‐like mechanical properties and was formulated using MTA‐based cement technology. It exhibits good sealing and mechanical properties with setting time of approximately 12 minutes. It can be used for a single visit apexification procedure, does not cause discoloration [38, 39], and has less radiopacity compared to MTA [40]. In a study conducted on 80 maxillary anterior teeth evaluating the apical microleakage of Biodentine compared to MTA in orthograde apical plugs and the effect of Biodentine thickness on sealability, Biodentine showed similar results to MTA in terms of microleakage and sealability without considering the thickness of the apical plug [41]. In a recent study, Abbas et al. showed that a 4‐mm‐thick Biodentine apical plug had the lowest bacterial leakage, followed by 2‐mm MTA and 4‐mm MTA, whereas a 2‐mmthick Biodentine apical plug had the highest bacterial leakage [42]. However, the current literature is inconclusive regarding the efficacy of Biodentine when used as an apical plug. In a study, investigating the sealing ability of Biodentine with and without phosphate‐buffered saline, Cechella et al. reported a lower sealing ability of Biodentine compared to MTA [43].

Positive results on the ability of Biodentine come from two recent randomized clinical trials. In the first study, Yadav et al. demonstrated apexification in 100% of teeth treated using Biodentine or MTA on a sample of 60 patients (age: 6–15 years). In the most recent randomized clinical trial [44], Tolibah et al. evaluated the radiographic and clinical outcomes of Biodentine apical plugs versus MTA in 30 immature roots of 24 permanent lower first molars with apical lesions. They showed no statistically significant differences in the periapical index scale between the two groups at 6‐ and 12‐months after treatment, as well as the presence of an apical calcified barrier at 12 months in the Biodentine group [17]. In recent years, a calcium‐enriched mixed cement (CEM cement), a hydrophilic tooth‐colored cement that releases CH during and after setting, has been proposed as an apical plug that has similar sealing ability to MTA but offers some advantages over MTA because of its ability to set in an aqueous environment with a short setting time [45].

5.4 Apical Plug Technique

The apical plug technique aims to seal large and irregular apexes permanently. Achieving this goal is particularly complex given the anatomical variability of immature teeth, making it difficult to determine the working length [46, 47]. The MTA apical plug procedure is performed in two sessions.

5.4.2 In the Second Session

After completion of irrigation, a 4‐mm MTA apical plug is placed 1 mm from the radiographic apex using a carrier or plugger [49].

However, some clinicians prefer to place the MTA apical plug in a single session to take advantage of the production of CH during the mixing phase of the material itself and to reduce the risk of difficulty in removing CH during the second session, a difficulty that can lead to a defect in the seal if the material is resorbed over time.

MTA has a “wet sand” consistency and is difficult to handle when placed in the root canal. To overcome this disadvantage, some instruments have been proposed, of which the Micro Apical Placement System (MAP system) (Produits Dentaires SA, Switzerland) is the most used (Figure 5.1). The system consists of a tip with a plugger and a bayonet connection into which a plastic piston is inserted for the placement of the material, and a screw attachment with accessories that can be adapted to the clinical situation (see Online Video). The tips, often made of NiTi and available in three outer diameters of 0.90, 1.10, and 1.30 mm, can be bent into the desired shape and return to their original shape when heated.

After preparation, the MTA is loaded into the tip and extruded in the shape of a 4/5‐mm cylinder. In 2013, Giovarruscio et al. proposed a new technique for filling root canals with an apical diameter greater than 0.4 mm. In this method, three Thermafil carriers (Maillefer, Switzerland) of increasing size are used as flexible pluggers starting 1, 2, and 3 mm short of the apex. This technique allows to check the consistency of the MTA during insertion to avoid a hard, shaped material that can lead to a short apical plug [50] and can be inserted in curved canals [51]. In addition, other accessories such as cones and microbrushes with a cylindrical shape are available to handle MTA directly in the canal.

After the MTA is placed, a cotton pellet, paper tip, or moistened brush head is placed over the MTA layer to supply the moisture needed to activate the material and thus for its hardening. After a few days, the MTA is checked with an endodontic instrument, and if it is not correctly hardening, the procedure is repeated; otherwise, the conventional obturation procedure can be continued. Table 5.1 showcases few of the procedural challenges faced with their predictable solutions. Solutions adopted in case of procedural challenges are shown in Figure 5.2.

Problem	Solution
Short apical plug or with gap and non‐hardened material	Irrigate the canal with sterile water and make a hole through the material using a 20 k‐File. Hydrate the material, which will be enough to recompact it to the correct length.
Short apical plug with hardened material	Remove by ultrasound
Abundant extrusion of MTA beyond the apex	Irrigate over the apex with saline Repeat the apical plug procedure
Material not hardened after 72 h	Remove the MTA Check there is no spontaneous drainage from the canal Repeat the apical plug procedure using the same or a different material