Introduction
Regenerative endodontic procedures (REPs) involve combining scaffolds, stem cells, and signalling factors and then implanting them into the pulp cavity of an affected tooth to replace damaged pulp tissue via tissue engineering [
1,
2]. This process promotes the regeneration of blood vessels and nerves in the root canal system and restores the original function of the pulp-dentin complex. Postoperative evaluation of the results of pulp regeneration can be divided into three goals [
3,
4]. The first goal is the absence of clinical symptoms and bone tissue healing; the second is an increase in root canal length and root canal wall thickness (desired but not necessary); and the third is a positive pulp vitality test result.
The three key elements of REPs are stem cells, scaffolds and signalling molecules. The ideal scaffold material should have the following characteristics: the ability to provide biological and mechanical support for stem cells, i.e., an environment conducive to cell adhesion, migration, proliferation and differentiation; the ability to facilitate the transportation of nutrients, oxygen and metabolites; and a degradation rate consistent with tissue regeneration. Furthermore, scaffolds are better able to evoke a small inflammatory reaction than other materials and are easy to prepare [
5‐
7]. Scaffolds in common clinical practice include blood clots (BCs), platelet-rich plasma (PRP), platelet-rich fibrin (PRF), and hyaluronic acid.
Different scaffold materials have their own advantages and disadvantages. The BC is the most traditional and most popular type of material used in REPs. It was reported to be related to a high success rate, simplicity, economy, and lack of allergic reactions [
8]. However, a BC might not induce true pulp-dentin complex regeneration. The mechanical structure of a BC is relatively fragile and may not be able to fill the root canal during treatment, eventually leading to coronal sealing collapse [
9]. PRP is a first-generation autologous platelet concentrate (APC) rich in growth factors obtained by centrifugation of autologous whole blood. Platelets in PRP release important growth factors, including vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), and epidermal growth factor (EGF) [
10]. Several research results suggest that PRP induces regeneration of periodontal tissue rather than dental pulp tissue [
10,
11].
Currently, there is still controversy regarding the postoperative evaluation of various scaffolds for pulp regeneration. For example, several scholars have observed good effects on tooth root growth and canal wall thickening after using BCs alone, or PRP or PRF alone [
12,
13]. The expected effect of tooth root growth and thickening accouts for the majority of cases [
14‐
16]; however, according to Bezgin et al. [
17], root growth and thickening are observed after using BCs but not after using PRP alone. Several studies have reported that a BC in combination with PRP is effective as a dental pulp regeneration scaffold [
18]. Therefore, this meta-analysis aimed to access whether other scaffolds, used alone or in combination with BC, are more effective than BC in regenerative endodontic procedures and to provide a reference for clinical scaffold selection.
Discussion
In our systematic review, the clinical and imaging results of the BC group and other exogenous scaffold groups were comparable, and the overall success rates were both greater than 90%, similar to the rates in previous clinical trials [
35,
36]. Notably, the success criteria for pulp regeneration may differ across clinical trials. According to the American Association of Endodontists (AAE) clinical guidelines, the success criteria include the disappearance of clinical symptoms and apical radiolucency, further root development, and a positive pulp vitality test [
4]. In the included studies, for example, Ulusoy et al. divided the judgement criteria into three levels: failure; only the absence of clinical symptoms along with radiographic evidence of osseous healing; radiographic root development and a positive pulp vitality test result [
27]. The latter two levels were both categorized as success, which was not consistent with the AAE criteria. In addition, some scholars believe that crown discolouration and root canal calcification are inevitable in REPs and should not be considered a failure [
13]. Others believe that preventing tooth discolouration, especially in the aesthetic area, should also be included in the success criteria [
35]. When using BCs, the unpredictability of blood clotting increases the likelihood of tooth discolouration [
37]. Unclear or varying success criteria for the included trials may bias the final analysis of the results.
This meta-analysis revealed that there was no statistically significant difference in the effects of BCs or other exogenous scaffolds on the further development of tooth roots. Some scholars believe that residual bacteria after REPs affect the development of dentine root canal wall thickness [
37,
38]. Conventional BC methods involve limited antibacterial media, but in theory, APCs contain a high concentration of growth factors, which promote stem cell migration, proliferation and differentiation, as well as strong and stable fibrous matrix and antibacterial properties. Similarly, in the included trials, Rizk et al. hypothesized that the root development of the PRF group would be better than that of the BC group [
29]. In addition to the above reasons, the author's explanation is that the thrombin contained in PRF can create equal-sided junctions in polymerized fibrin so that signalling molecules can discharge continuously and the fibrin network is mouldable, ultimately forming a proper microenvironment for cell migration. Notably, according to the clinical guidelines of the AAE, an increase in root canal wall thickness is usually observed 12-24 months after treatment [
4]. Nevertheless, we observed that the maximum follow-up period of some included studies was only 12 months, and some of them were analysed in forest plots regarding root development [
28,
32]. Therefore, the impact of differences in follow-up time on the final results was also not meausrable. In addition, we noticed that for the calculation of imaging data, many experiments did not consider the impact of changes in camera angles into consideration, and their methods of correction were also different, making it impossible to ensure the comparability of data between groups.
The AAE proposes that a positive pulp vitality test (cold test or electric vitality test) is the highest goal of REPs [
4]. However, the results of the pulp vitality test in the included trials varied greatly, and in some studies, neither the control group nor the experimental group exhibited a positive result. Therefore, we selected only studies with discrepant data for the two groups for meta-analysis. Electric vitality testing may result in false negatives for young permanent teeth with an open apical foramen, which means that the true result was based on the closure of the apical foramen. Moreover, when cold testing is used, as the pulp-capping material is often placed slightly below the plane of the cemento-enamel junction, vital pulp regeneration will not occur in the crown region. This means that when cold stimuli are applied to a crown, they cannot be transferred to the tissue under the capping material, which also leads to false negatives [
34].
The adverse outcome reports of REPs have not been widely been considered. At present, the most common complication is calcification in the root canal, for which the incidence rate is approximately 50% [
17,
32,
33]. In a previous study by Chen et al., the incidence rate was similar to that of 35% [
39]. Biological analysis of the causes of root canal calcification has shown that blood from the apical foramen may bring periodontal stem cells and alveolar bone-derived bone marrow stem cells, ultimately inducing the formation of bone or cementum structures in the root canal [
40‐
42]. In addition, some studies have shown that residual plaque biofilms and antigens are related to root canal calcification, and that in the presence of both, stem cells in the apical papilla stably express osteoblast-like markers [
43]. Currently, studies on the long-term prognosis of patients with root canal calcification after REPs are rare. Therefore, whether calcification must be avoided is yet unclear. We suggest taking calcification into consideration when choosing the scaffold, as root canal calcification, especially obliteration, is widely accepted to be detrimental when root canal treatment is needed.
Regarding the ROB assessment, double-blinding of patients and personnel may have been impossible because the APC group needed to undergo the treatment process of blood drawing treatment process, and the patients would have realized that they were assigned to the APC group [
27]. Rizk solved this problem through the design of "a split mouth" to ensure that each participant needed to draw blood, and the two affected teeth of a participant underwent different groups of operations [
28,
29]. Whether double-blinding affects the results of REPs cannot be determined, and we recommend that researchers conducting subsequent trials take note of this.
Among the current studies screened, there was only one network meta-analysis focused on the comparison of exogenous scaffolds and traditional BC methods in dental pulp regeneration, but it included many non-RCTs, which decreased the certainty of the evidence [
44]. In contrast, this review included only RCT studies, excluding the possibility that cases using exogenous scaffolds were due to poorer tooth conditions or unsuccessful bleeding, thus ensuring the validity of the results and conclusions.
This review also has certain limitations. Due to the different study designs, data standards, data integrations and outcome measurement methods of the included trials, comparison of teeth root development between BCs and other scaffolds have been limited. In addition, we simply categorized all the exogenous scaffolds into groups for comparison with BC, which was due to the limited number of studies available. This could have resulted in significant heterogeneity. At present, more clinical trials are still needed to verify the effect of exogenous scaffolds compared with traditional BC methods.
Conclusion
Based on the limited evidence of this review, we draw the following conclusions.
For clinical REPs, the most commonly used scaffolds include BC, PRP and PRF. There is high-level evidence that these scaffolds all have high clinical success rates, and the differences are not statistically significant. The methods used to measure the increase in root length and root canal wall thickness, the measurement methods are highly heterogeneous, and based on the currently limited data, there was no significant difference between the use of exogenous scaffolds and traditional BCs, regardless of whether the former was applied alone or in combination with BC, Pulp vitality testing is still not taken seriously by some researchers. Cold testing and electrical vitality testing are recommended methods, but attention should be given to avoid false negatives. For REPs of young permanent teeth with pulp necrosis, clinicians can choose reasonable scaffolds based on the equipment conditions and patient conditions.
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