Evaluation of removal efficiency of capping materials used in pulp revascularization in vitro

Pulp necrosis in an immature tooth can not only have a low prognosis with traditional treatment techniques but also have risks for complications [1]. Banchs & Trope [2] have suggested a ‘biologically-based treatment approach, namely pulpal revitalization (PR), for the management of the non-vital tooth with open apices instead of traditional apexification or single visit apical plug treatments. In the Glossary of Endodontic Terms, PR defines biologically-based stepwise processes developed to physiologically substitute damaged tooth structures, including pulp-dentin complex and root structures [3].

In PR, an infected root canal with immature teeth is recommended to be copiously irrigated with the standard cleaning solution at a low concentration (e.g., 1.5–3% sodium hypochlorite, NaOCl). In this stage, the disinfection is continued with calcium hydroxide or a low concentration of triple antibiotic paste dressing, then temporarily sealed for 1–4 weeks. At the following visit, the antibiotic mixture used in paste form is removed, the canal surface is conditioned with chelating agents, and apical bleeding is induced into the canal. At the final stage, the blood clot (tissue scaffold) is capped with a hydraulic tricalcium silicate cement (hTCS) positioned at 3–4 mm thickness or at the cervical third of the root, and a coronal restoration is placed after the initial setting reaction of hTCS has been completed [4].

Mineral trioxide aggregate (ProRoot MTA, Dentsply Tulsa Dental Specialties, Johnson City, TN, USA) is the first endodontic bioactive repair cement [5‐7]. Today, the updated guidelines demonstrate the benefits of fast-setting hTCS counterparts as a capping material in PR [8]. However, the fast-setting counterparts of hTCSs can present differences in their adhesive properties to dentin substrate due to their varying ingredients [9‐13]. Over the past decade, an extensive amount of research has been conducted on the bond strength of hTCS to dentin, which has yielded sufficient data to suggest that there can be significant differences in adhesive properties between their counterparts [9‐13]. The effect of fast-setting hTCS counterparts is still unclear on PR procedures. A recent study mentioned that further studies on the manipulation effects of hTCS are necessary [4]. BioDentine (Septodont, Saint-Maur-des-Fosses, Cedex, France), known as a dentine substitute, is a fast-setting hTCS representative [14]. BioDentine contains Tricalcium silicate, dicalcium silicate, calcium carbonate, zirconium oxide, calcium oxide, and iron oxide [15]. RetroMTA (BioMTA, BioMTA, Yuseong-gu, Daejon, Korea) is a relatively newer hTCS that contains calcium carbonate, silicon oxide, aluminum oxide, hydraulic calcium zirconia complex, and water [16]. Due to their fast initial setting times and the fact that they do not induce discoloration of the teeth, it has been suggested to use them in vital pulp therapies and PR for both hTCSs [17].

The rationale of the endodontic retreatment is to completely remove old root canal filling material to avoid decreasing disinfectant cleaning efficiency. Accordingly, one of the main requirements of a root canal filling material is that it can be removed when necessary [18]. If there are signs or symptoms of persistent infection, additional antimicrobial visits in the scope of PR or traditional treatment alternatives must be considered in this event of failure [19, 20]. The clinical failure rate of PR was reported as 25% in immature incisors in a recent study [21]. In this context, it is still unclear whether completely removing the capping materials will be possible if any complications occur in PR. Furthermore, practitioners’ awareness and knowledge about removing solid (completed final setting reaction) hTCS from the cervical third of the canal are essential to managing the worst-case scenario in PR failures. Boutsioukis et al. [22] reported that some drawbacks might originate from removing hTCS from the root canal. These facts and justifications lead to base on the research question of this study. To the authors’ knowledge, no study has evaluated this research question in the literature. Based on the relevant question and the gap in the literature, the investigation of the removal efficiency of hCSCs is fundamental. Thus, the authors considered that this evaluation was necessary, particularly comparing capping materials with different properties.

This study aimed to evaluate the removal efficiency of different capping materials used in PR in a failure scenario. The null hypothesis of this study was there were no differences in the quantity of hTCS residues on the canal surface after the recommended removal techniques were applied.

The dataset is available in the citation [25]. The analyzed data for µCT volumes and durations are given in Table 2.

In the first examined µCT data, the mean (SD) capping material volumes of the PMTA, BD, and RMTA were 6.447 µm³ (1.086), 8.771 µm³ (0.491), and 8.114 µm³ (2.447), respectively.

In the last examined µCT data, the median (IQR) residual volumes of the PMTA, BD, and RMTA were 0.051 µm³ (0.1), 0.313 µm³ (0.5), and 0.124 µm³ (0.1), respectively Fig. 3. A significant difference was found between BD and PMTA in the residual volumes (p < 0.05).

This study evaluated the removal efficiency of different capping materials used in PR in a failure scenario. To achieve this aim, for the first time, research was conducted on a method of hTCS removal that was considerably less invasive. High-speed rotary devices could be used to remove hTCSs; however, because immature teeth have thin root dentin, this could pose a threat to the teeth’s ability to survive. The white-colored capping materials present low contrast to the dentine substrate. This may lead to difficulties in the optical discrimination of small residues by operators. The highest median value of residue material was seen in the BD, significantly different from PMTA. Therefore, the null hypothesis was rejected.

A single operator experienced in manipulating hTCS materials placed the capping material in the PD simulation. Although the root canal spaces of tooth samples were standardized, the volume of the capping materials was seen to vary in the first µCT examination. This could be originate from the differences in their handling characteristics of hTCSs [8].

The incisor teeth were preferred in the PR simulation due to being frequently affected by dental trauma in school-age children and therefore having a risk of losing vitality [26]. The authors noted that the 2-week-old capping material needs 20–25 min of chair time for the removal procedures in clinical practice. Considering school-age children, this duration could be described as long.

In PR guidelines, hTCS cement has been suggested as a capping material [1, 4]. To undertake biologically-based healing, the prominent rationale for the current suggestion is based on the good bioactivity, biocompatibility, and adhesion properties of hTCSs [5‐8, 27]. In our findings, the differences in the amount of residual capping material were attributed to the differences in adhesive properties between PMTA, BD, and RMTA. Thus, literature-based adhesive data for PMTA, BD, and RMTA, and their physical characteristics and ingredient details affecting adhesion were discussed in this part.

The precursor hTCS counterpart, PMTA, has a relatively slow initial setting time (74 min) and has been reported to cause tooth discoloration [28]. On the contrary, its successors, RMTA or BD, have a flash-initial setting time and do not cause discoloration [28]. More specifically, the initial setting time of RMTA and BD has been reported as 3 min [16] and 12 min [15], respectively. Hench reported that the flash initial setting reactions of the bioceramics could induce more mineral attachment on the surface of the dentin substrate [29]. Comparative in vitro studies reported that BD had a higher bond strength than PMTA to dentin [30‐33]. The shear bond strengths of 2-week-old BD and PMTA were reported as 9.34 ± 1.01 MPa and 4.96 ± 4.54 MPa in a previous study (p < 0.05) [30]. Likely, the push-out bond strength of 7-day-old PMTA was reported as 4.75 ± 1.71 MPa and 9.0 ± 0.9 MPa in previous studies [31, 32].

Furthermore, the previously reported relatively smaller granular size and uniform powder distribution of BD might contribute to the micromechanical anchoring to the dentine since the cement characteristics affect the marginal adaptation of hydrated cement materials [13, 34]. Moreover, different failure modes were reported between BD and PMTA in a previous study [30]. Accordingly, it has been reported that BD has a more cohesive type failure mode; whereas, PMTA has a more adhesive type failure mode [30].

It has been reported that the presence of residual irrigation solutions, antibiotics or calcium hydroxide medicaments, and the smear layer affect the bond strength of hTCS [11, 13]. The simulation of the capping material placement and storage did not fully mimic the clinical conditions due to the experiment’s absence of blood plasma or blood clot products. This could be considered a limitation of the study.

When PMTA was first introduced, it was not considered for retrieval after hTCS administration as it was a retrograde filling and repair material [35]. However, in the case of contemporary non-invasive treatment failures, the set hTCS material must be completely removed to allow conventional treatments to be performed. Old canal filling residues affect the subsequent sealing properties, especially the tubule penetration depth [36]. Following the conservative technique utilized, the evaluation suggests that hTCS removal is most likely to be complete with an invasive approach such as Gates Glidden rotary instruments. However, invasive methods would have risks of complication due to the weakness of root dentin in immature teeth. In addition, the authors suggested that removing hTCS after failed regenerative endodontic procedures could be a problem. Further studies are needed to better understand the effect of hTCS removal on the outcome of further treatment.

Within the limitations of the presented study, it was concluded that the capping materials were largely removed from the root canals using a non-invasive approach. Nevertheless, this duration of the retrieving could be described as long.