Effectiveness of Direct Pulp Capping Bioactive Materials in Dentin Regeneration: A Systematic Review

Background: Regenerative endodontics aims to restore normal pulp function in necrotic and infected teeth, restoring protective functions, such as innate pulp immunity, pulp repair through mineralization, and pulp sensibility. The aim of this systematic review was to assess the dentin regeneration efficacy of direct pulp capping (DPC) biomaterials. Methods: The literature published between 2005 and 2021 was searched by using PubMed, Web of Science, Science Direct, Google Scholar, and Scopus databases. Clinical controlled trials, randomized controlled trials, and animal studies investigating DPC outcomes or comparing different capping materials after pulp exposure were included in this systematic review. Three independent authors performed the searches, and information was extracted by using a structured data format. Results: A total of forty studies (21 from humans and 19 from animals) were included in this systemic review. Histological examinations showed complete/partial/incomplete dentin bridge/reparative dentin formation during the pulp healing process at different follow-up periods, using different capping materials. Conclusions: Mineral trioxide aggregate (MTA) and Biodentine can induce dentin regeneration when applied over exposed pulp. This systematic review can conclude that MTA and its variants have better efficacy in the DPC procedure for dentin regeneration.


Introduction
Vital dental pulp exposure may be caused by caries removal (caries exposure), cavity preparation where pinpoint exposure to the dental pulp (mechanical exposure), and accidental coronal pulp injury (traumatic exposure). The preservation of pulp vitality is important in all of these situations for tooth viability, nutrition, innervation, and immune defense. Dentin acts as a protective barrier that protects the tooth pulp from direct contact with potentially tissue-damaging external stimuli [1]. The formation of tertiary dentin in response to various noxious stimuli can increase the thickness of the dentin barrier [2]. The odontoblasts, which are the cells responsible for dentinogenesis, are found on the periphery of the dental pulp. These odontoblast cells could be destroyed due to severe external stimuli, such as deep dental caries. Consequently, the recruitment of progenitors

Search Strategy
This systematic review was performed in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) guidelines [16]. All the authors reached a consensus on the search strategy. Then three independent authors pre-selected the articles as per titles and abstracts and submitted them for the other authors' approval. After completing this extraction, four independent and experienced authors critically checked, extracted, and confirmed the data. Articles from 2005 to 2021 were reviewed, and the literature published until 2021 was systematically searched. The search encompassed articles (full text) that have been published in peer-reviewed journals and written in English related to DPC material used for dentin regeneration.

Study Selection
Here, the primary concern was finding out the type of DPC material, formation/regeneration of dentin, quality of dentin formation, and outcomes. The case reports and the letters to the editors were also excluded from this review. The titles and abstracts of identified studies were independently evaluated to ensure if the studies met the inclusion criteria.

Inclusion and Exclusion Criteria
Inclusion criteria included the following:
Studies on permanent teeth in clinical conditions; 3.
Direct pulp capping.
Studies with insufficient information; 5.
Non-English publications.

Data Extraction and Organization
Data extraction was performed on the studies that met the inclusion criteria. The following data were collected: the first author's name; the year of publication; the age range; sample size; where this research was carried out; type of teeth; intervention/control material used for DPC; and follow-up, finding, and outcomes. The data were extracted and double-checked by the four independent authors, using a standard format. Disagreements during data extraction were resolved by means of discussion and consensus by a fifth author (MKA).

Quality Assessment
The Cochrane collaboration's tool [17] for human studies and SYRCLE's risk of bias tool [18] for animal studies were used to assess the methodological quality. For human studies, the following 6 domains were assessed: sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, and selective reporting. For animal studies, the following 5 studies were assessed: sequence generation, baseline characteristics, blinding of outcome assessment, incomplete outcome data, and selective reporting. Using the Revman software, version 5.3, each domain was evaluated for a low, unclear, and high risk of bias.

Selection of Studies
A total of 4583 papers from databases, including PubMed, Web of Science, Science Direct, Google Scholar, and Scopus, were initially identified by using this research search strategy. After removing the 2017 articles from consideration (duplicate studies, review, case repots, editorial letters, and comments), a second round of screening was conducted on the 2566 papers that remained. A total of 127 studies were considered worthy, and 87 studies were excluded because of an unacceptable data format. Thus, 40 studies (21 humans and 19 animals) were included in this study (Figure 1), with the complete text of all of the included studies being obtained based on the research goal and inclusion and exclusion criteria.  Table 1 summarizes the key characteristics of the human studies that were included in this systematic review. All of the included studies were journal articles and were conducted on adults. Among these 21 studies, five are from India, four from Brazil, three from Poland, and two from Egypt. Iran, Japan, China, Korea, UK, USA, and Turkey each had one study. Table 2 summarizes the key characteristics of the animal studies that were included in this systematic review. All of the included studies were journal articles and were conducted on animals. Among these 19 studies, five are from Japan, four from China, two Thailand, and two from Greece. Egypt, Korea, Portugal, Belgium, Tehran, and Germany had one each.  Table 1 summarizes the key characteristics of the human studies that were included in this systematic review. All of the included studies were journal articles and were conducted on adults. Among these 21 studies, five are from India, four from Brazil, three from Poland, and two from Egypt. Iran, Japan, China, Korea, UK, USA, and Turkey each had one study. Table 2 summarizes the key characteristics of the animal studies that were included in this systematic review. All of the included studies were journal articles and were conducted on animals. Among these 19 studies, five are from Japan, four from China, two Thailand, and two from Greece. Egypt, Korea, Portugal, Belgium, Tehran, and Germany had one each.   MTA group showed complete dentin-bridge formation, whereas all-in-one adhesives group showed incomplete or partial dentin-bridge formation.  Figure 2 summarizes the assessment of the risk of bias in the included human studies. All assessed studies exhibited low attrition and reporting bias, whereas selection bias (random sequence and allocation concealment) and performance bias had a high and unclear risk of bias. Figure 3 summarizes the assessment of the risk of bias in the included animal studies. Studies exhibited low selection (baseline characteristics), attrition, and reporting bias, whereas selection bias (random sequence) and detection bias had a high risk of bias.  Figure 2 summarizes the assessment of the risk of bias in the included human studies. All assessed studies exhibited low attrition and reporting bias, whereas selection bias (random sequence and allocation concealment) and performance bias had a high and unclear risk of bias. Figure 3 summarizes the assessment of the risk of bias in the included animal studies. Studies exhibited low selection (baseline characteristics), attrition, and reporting bias, whereas selection bias (random sequence) and detection bias had a high risk of bias.

Discussion
The aim of this current systematic review was to assess the efficacy of various DPC materials that are used in dentin regeneration. This systematic review employed a risk of bias assessment of the included studies, which revealed that some of them had poor methodological quality. These studies investigated different age groups, gender, and tooth type at the population level. The quality of the included studies ranged from low to moderate, and many of them were associated with a high risk of bias. The primary goal of DPC is to maintain the pulpal tissue's full integrity under various pathological conditions of exposure [57]. An ideal DPC material should not cause pulpal inflammation, which can lead to necrosis, and should regenerate good quality dentin at the exposure area [4]. It has been demonstrated that the use of calcium silicate-based materials as DPC agents can effectively treat dental pulp. CH has long been regarded as the gold standard of DPC material because of its biocompatibility, high pH, antibacterial effect, and ability to form a  Figure 2 summarizes the assessment of the risk of bias in the included human studies. All assessed studies exhibited low attrition and reporting bias, whereas selection bias (random sequence and allocation concealment) and performance bias had a high and unclear risk of bias. Figure 3 summarizes the assessment of the risk of bias in the included animal studies. Studies exhibited low selection (baseline characteristics), attrition, and reporting bias, whereas selection bias (random sequence) and detection bias had a high risk of bias.

Discussion
The aim of this current systematic review was to assess the efficacy of various DPC materials that are used in dentin regeneration. This systematic review employed a risk of bias assessment of the included studies, which revealed that some of them had poor methodological quality. These studies investigated different age groups, gender, and tooth type at the population level. The quality of the included studies ranged from low to moderate, and many of them were associated with a high risk of bias. The primary goal of DPC is to maintain the pulpal tissue's full integrity under various pathological conditions of exposure [57]. An ideal DPC material should not cause pulpal inflammation, which can lead to necrosis, and should regenerate good quality dentin at the exposure area [4]. It has been demonstrated that the use of calcium silicate-based materials as DPC agents can effectively treat dental pulp. CH has long been regarded as the gold standard of DPC material because of its biocompatibility, high pH, antibacterial effect, and ability to form a

Discussion
The aim of this current systematic review was to assess the efficacy of various DPC materials that are used in dentin regeneration. This systematic review employed a risk of bias assessment of the included studies, which revealed that some of them had poor methodological quality. These studies investigated different age groups, gender, and tooth type at the population level. The quality of the included studies ranged from low to moderate, and many of them were associated with a high risk of bias. The primary goal of DPC is to maintain the pulpal tissue's full integrity under various pathological conditions of exposure [57]. An ideal DPC material should not cause pulpal inflammation, which can lead to necrosis, and should regenerate good quality dentin at the exposure area [4]. It has been demonstrated that the use of calcium silicate-based materials as DPC agents can effectively treat dental pulp. CH has long been regarded as the gold standard of DPC material because of its biocompatibility, high pH, antibacterial effect, and ability to form a new dentin bridge at the exposure site [57]. The use of CH as a DPC material was proven to have a higher clinical success rate according to studies that followed patients for more than 10 years [58]. CH has high alkalinity, which leads to necrosis and inflammation to the pulp [9]. Besides its high solubility and lack of adhesion with hard tissues, it does not provide an optimal seal, even though the dentin bridge appears to be fully formed by the time of its complete dissolution [59,60]. CH presents tunnel defects in the dentin bridge, but there is evidence to suggest that the appearance of these defects improves with increased dentin-bridge thickness [57]. In comparison to CH, MTA has a higher rate of clinical success and can result in dentin-bridge formation that is much thicker [61,62]. Based on calcium oxide, CH and MTA both react to carbon dioxide in tissues, which is a similar mechanism of action. At the exposure site, calcite granulations are formed, and fibronectin accumulates, promoting cellular migration, proliferation, adhesion, and differentiation, resulting in the formation of hard tissue [57,63]. Bioactive molecules are released during this process that facilitate regeneration of dental pulp and are integrated into the dentin matrix during the process of dentinogenesis [64][65][66].
Nowadays, MTA has proven to be a suitable choice for DPC material because of its good sealing ability and biocompatibility [4]. Studies demonstrated that dentinal-bridge formation by MTA was higher quality, less porous, thicker, and caused less pulpal inflammation than CH [3]. Furthermore, MTA has been demonstrated to induce adhesion, migration, and attachment of undifferentiated cells in order to form a dentinal bridge while having minimal inflammatory effect on the pulp [5,67]. However, the MTA has some disadvantages, including high cost, difficulty in handling, and long setting time [4]. In a practice-based research network, confirmatory evidence for MTA's superior performance as a DPC agent emerged when it was compared to CH in a randomized clinical trial [68]. The dental pulp capped with MTA had a 92.5 to 97.96 percent success rate in clinical trials, according to a review of the few clinical observations [69,70]. According to a histological study, MTA application directly affects the dental pulp's regeneration potential and is associated with an increase in TGF-1 secretion by the pulp cells [71]. These cells migrate to the material-pulp interface, where they are stimulated to differentiate into odontoblastic cells, which secrete reparative dentin, affecting the quality of the dentin-bridge formation [72]. Further histological examination revealed that the hard-tissue barrier formation after DPC with MTA is not the result of the differentiation of true odontoblast and does not have the properties of regular dentin [73]. These findings recommend that the calcified tissue formation should be considered as a reparative process rather than a real regeneration process. As a result, regular dentin could not be regenerated, and a fast-setting pulp-capping material could not be used in regenerative dentistry, due to its inadequate bioactive potential [72]. Furthermore, MTA and Biodentine, in contrast to calcium hydroxide, have favorable metabolic activity and stimulate almost similar desired cellular response, resulting in a higher rate of clinical success [74]. When comparing the MTA and Biodentine groups in terms of the formation of dentin bridge, micro-CT imaging demonstrated that the MTA group had a more regular pattern of reparative dentin layer which is homogenous and uniform thickness. These findings revealed that both MTA and Biodentine have the ability to induce the dentin-bridge formation, with MTA being the most effective at improving the quality of dentin [75]. Therefore, MTA is the preferred material for DPC [72].
Biodentine is a newer calcium silicate-based DPC material having properties similar to CH and MTA, as well as favorable effects on the dental-pulp cells that promote the formation of tertiary reparative dentin [62]. By releasing TGF-β1 and stimulating odontoblasts, Biodentine promotes pulpal healing and mineralization [3,76]. Biodentine also releases silicon ions that play a significant role during the process of mineralizing the dentinal bridge [3]. It has been demonstrated that the formation of the dentin bridge by Biodentine is similar to that of MTA with no pulpal inflammatory response [77]. This is due to the anti-inflammatory effect, which inhibits the secretion of pro-inflammatory substances and reduces the recruitment of inflammatory cells [77]. According to Nowicka et al.'s findings, Biodentine and MTA induced homogeneous reparative dentin formation, whereas CH induced a more porous formation, implying that calcium silicates induce higher tissue-repair efficacy as compared to CH [27]. Jalan et al. found similar superior outcomes for dental pulp capped with Biodentine when compared to Dycal [26]. Therefore, Biodentine material has great potential as a pulp-capping agent because of its proper setting time and restorative properties. However, studies suggested that long-term clinical research is still required to check the efficacy of Biodentine [3].
Adhesive systems have been investigated as suitable DPC materials because of their ability to adhere to dentin to protect the pulp from bacterial contamination [1]. On the other hand, bonding agents have been shown to have direct cytotoxic effects on dentalpulp cells [1]. These materials did not show favorable responses when compared to MTA in terms of pulpal inflammation and hard-tissue formation [57]. A new dentin-bridge formation was observed in all MTA specimens, whereas no hard-tissue deposition was observed even if the pulp tissue showed no symptoms of inflammation in the polymericbased materials group, or the adhesive materials group only induced a few hard-tissue depositions with pulpal necrosis and inflammation [78,79].

Conclusions
The findings of this systematic review, based on the available information, conclude that MTA and its variants have a higher success rate in dentin regeneration. MTA and its variants are more likely to form a homogenous dentinal bridge than CH and other DPC materials.