Treatment outcome of regenerative endodontic procedures in mature permanent teeth compared to nonsurgical endodontic treatment: A systematic review and meta-analysis.

Background: Regenerative endodontic procedures (REP) have the advantage of restoring root canal's native defense ability by re-establishing vital pulp-like tissue. This review aims to determine the overall clinical and/or radiographic success rate (O) of REP (I) in mature permanent teeth (P) and to compare it (C) with nonsurgical endodontic treatment (NSET). Materials and
Methods: Sources: PubMed, Web of Science, Embase, EBSCO, Cochrane Central Register of Controlled trials, ClinicalTrials.gov, Clinical Trials Registry-India and OpenGrey. Inclusion: Randomized clinical trials and single-arm prospective studies evaluating the treatment outcomes of REP in mature permanent teeth. Exclusion: Incomplete trials/studies, in vitro studies, animal studies, case reports/series, conference proceedings. Cochrane ROB2.0 and ROBINS-I tools were used to assess the risk of bias. Risk difference (R.D.) between NSET and REP was determined by meta-analysis of the randomized clinical trials. The overall success rate of REP was calculated using data from both randomized clinical trials and single-arm prospective studies. Sensitivity analysis and subgroup analysis were performed.
Results: Ten studies (n = 552) were included. R.D between REP and NSET was 0.032 (95% C.I: 0.023-0.087; P = 0.258). Overall success rate of REP was 96.0% (95% confidence interval: 94%-98%). No significant difference was found in sensitivity analysis (P = 0.551), or any of the subgroup analysis (P > 0.05). Discussion: A limited number of randomized clinical trials were available, and only two of them had a low risk of bias. Consistent results were obtained in both types of included studies.
Conclusion: Based on a limited number of comparative studies, REP has a similar success rate to NSET in mature permanent teeth. Other: Funding: Nil. Registration: PROSPERO (CRD42020204882). Copyright:

INTRODUCTION

Regenerative endodontics is defined as biologically based procedures designed to physiologically replace damaged tooth structure, including dentin and root structures, as well as cells of the pulp–dentin complex.[1] This procedure has been extensively applied for the management of nonvital immature tooth since it offers the benefit of root lengthening, thickening, and subsequent apical closure which is not achievable with conventional nonsurgical endodontic treatment (NSET). The pooled survival and success rate of the regenerative endodontic procedure (REP) in immature teeth has been reported to be 97.8% and 91.3%, respectively.[2]

NSET is the preferred treatment modality for irreversible pulpitis or pulp necrosis in mature permanent teeth. According to strict criteria (complete healing), its mean success rate varies between 72% and 83% depending on the pulpal and periapical diagnosis.[3] However, despite significant advancements in technology, devices, and materials, no significant improvement in treatment outcome has been observed.[4] In addition, loss of proprioception and destruction of tooth structure occurs as a consequence to NSET in mature permanent teeth.[56]

This has urged the researchers to divert their focus to REP in mature permanent teeth. The earliest contribution in this direction was made in 2012.[7] Since then, several researchers have documented the successful application of REP in mature permanent teeth.[8910111213] The procedure includes cell homing (active recruitment of endogenous stem/progenitor cells into an anatomic compartment), and cell based (transplantation of ex vivo cultivated stem/progenitor cells) approaches.[14] The current protocol for apical revascularization via blood clot (BC) induction is based on the concept of cell homing. The success of REP is determined by the achievement of its primary objective, namely the abolition of symptoms and evidence of bony healing, which serve as functional indicators of tooth retention in long term.[15] Overall, REPs restore homeostasis and natural defense with NK cells, lymphocytes, and antibodies, which may promote tooth survival. Given the advantages of REPs over the conventional NSET, the rationale is to assess the clinical and radiographic outcomes of REPs for the treatment of mature permanent teeth. The objective of the current systematic review is to determine the overall clinical and/or radiographic success rate of REPs in mature permanent teeth and to compare it to the success rate of the conventional NSET.

MATERIALS AND METHODS

The current review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines. The review protocol was registered in the PROSPERO database (CRD42020204882).[16]

Research question

What is the overall clinical and/or radiographic success rate (O) of REP (I) in mature permanent teeth (P), and how does it compare (C) to the success rate of NSET?

Search strategy and inclusion criteria

A literature search was performed in four databases: PubMed, Embase, EBSCO, and Web of Science until March 2021 and updated in June 2021, using the search terms [Supplementary Table 1]. Unpublished/gray literature was electronically searched from Cochrane Central Register of Controlled Trials, ClinicalTrials.gov, Clinical Trials Registry-India, and OpenGrey. The search was extended to reference lists of screened studies and published reviews. The duplicate articles were removed using EndNote X9 tool (EndNote, Thomson Reuters, Philadelphia PA, USA). This was followed by screening the title and abstract for suitable studies and full-text reading by two independent reviewers (DN, AS). In case of any disagreement, a third reviewer (SS) was consulted. The articles were selected based on the following inclusion criteria: Randomized clinical trials (for comparison) and single-arm prospective studies (for overall success rate) that evaluated the treatment outcomes of REP to manage mature permanent teeth were included. Studies published in the English language only were selected. Incomplete trials/studies, in vitro studies, animal studies, case reports, case series, comments, conference proceedings, and those concerned with evaluating parameters other than outcomes of REP in mature permanent teeth, for example, inflammatory markers, were excluded.

Supplementary Table 1

Search strategy

Database	Keyword
PUBMED	(regenerative endodontic OR regenerative endodontics OR regenerative endodontics/methods OR regenerative endodontics’ AND (mature AND (“tooth” OR “tooth” OR “teeth”)) Revascularization AND (mature AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) revitalization AND (mature AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) ((“dental pulp”[MeSH Terms] OR (“dental” AND “pulp” OR “dental pulp” OR “pulp” AND (“regeneration”[MeSH Terms] OR “regeneration”)) AND (mature AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) (regenerative endodontic OR regenerative endodontics OR regenerative endodontics/methods OR regenerative endodontics’) AND (permanent AND (“adult”[MeSH Terms] OR “adult”) AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) Revascularization AND (permanent AND (“adult”[MeSH Terms] OR “adult”) AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) Revitalization AND (permanent AND (“adult”[MeSH Terms] OR “adult”) AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) ((“dental pulp”[MeSH Terms] OR (“dental” AND “pulp”) OR “dental pulp” OR “pulp”) AND (“regeneration”[MeSH Terms] OR “regeneration”)) AND (permanent AND (“adult”[MeSH Terms] OR “adult”) AND (“tooth”[MeSH Terms] OR “tooth” OR “teeth”)) (regenerative endodontic OR regenerative endodontics OR regenerative endodontics/methods OR regenerative endodontics’) AND (closed AND “apex”) Revascularization AND (closed AND “apex”) Revitalization AND (closed AND “apex”) ((“dental pulp”[MeSH Terms] OR (“dental” AND “pulp” OR “dental pulp” OR “pulp”) AND (“regeneration”[MeSH Terms] OR “regeneration”)) AND (closed AND “apex”)
EMBASE	regenerative AND endodontic* AND (‘mature teeth’ OR (mature AND (‘teeth’/exp OR teeth))) (‘revascularization’/exp OR revascularization) AND mature AND (‘teeth’/exp OR teeth) revitalization AND mature AND teeth; pulp AND regeneration AND mature AND teeth regenerative AND endodontic* AND permanent AND adult AND teeth; revascularization AND permanent AND adult AND teeth; revitalization AND permanent AND adult AND teeth pulp AND regeneration AND permanent AND adult AND teeth regenerative AND endodontic* AND closed AND apex; revascularization AND closed AND apex; revitalization AND closed AND apex; pulp AND regeneration AND closed AND apex
EBSCO	Regenerative Endodontic* AND mature teeth; Revascularization AND mature teeth; Revitalization AND mature teeth; Pulp Regeneration AND mature teeth Regenerative Endodontic* AND permanent adult teeth; Revascularization AND permanent adult teeth; Revitalization AND permanent adult teeth; Pulp Regeneration AND permanent adult teeth Regenerative Endodontic* AND closed apex; Revascularization AND closed apex; Revitalization AND closed apex; Pulp Regeneration AND closed apex
Web of Science	(Regenerative Endodontic* AND mature teeth); (Revascularization AND mature teeth); (Revitalization AND mature teeth); (Pulp Regeneration AND mature teeth) (Regenerative Endodontic* AND permanent adult teeth); (Revascularization AND permanent adult teeth); (Revitalization AND permanent adult teeth); (Pulp Regeneration AND permanent adult teeth) (Regenerative Endodontic* AND closed apex); (Revascularization AND closed apex); (Revitalization AND closed apex); (Pulp Regeneration AND closed apex)

Outcome measure

The outcome of REP was assessed in mature permanent teeth and compared with that of conventional NSET. Success was defined clinically as the absence of any signs and symptoms, and radiographically as the absence or reduction of periapical radiolucency at 12 months or the maximum follow-up period possible if <12 months. The pulp sensibility tests were not considered for outcome evaluation since their use is questionable in the presence of restorative material in the pulp chamber. Furthermore, since pulp vitality tests were only performed in a few studies, they were not included as an outcome measure in this systematic review.

Data extraction

The following data were recorded in an Excel spreadsheet (Microsoft, Redmond, WA, USA): demographic characteristics, study design, sample size, follow-up time, protocol followed (type of stem cell approach, type of scaffold, biomaterial, intracanal medicament, final irrigant used), and study findings, such as success rate, vitality response, and any other investigations like cone-beam computed tomography (CBCT) or magnetic resonance imaging (MRI) if present.

Quality assessment

The quality of selected randomized clinical trials and single-arm prospective studies was assessed by applying the Cochrane risk of bias (RoB 2.0)[17] and ROBINS-I tool[18], respectively. This was done by the same two independent reviewers (DN, AS). A third reviewer (SS) was consulted in case of any disagreement.

Data synthesis

The meta-analysis was performed using STATA I/C Version 16.0 (StataCorp, College Station, Texas, 77845, USA). The primary outcome was success rate, and the differences in success rate between the two arms (REP and NSET) were compared in terms of risk difference (R. D.), that is one among various effect sizes for binary data in the meta-analysis. Using the fixed effect model with the Mantel–Haenszel method, individual study-specific and overall R.D. estimates with its 95% confidence interval (CI) were calculated as a weighted (inverse of the variance of the treatment effect) average of the individual summary statistic shown in the forest plot. The presence of true heterogeneity was checked using the Q test, which follows a Chi-square distribution with k–1 degrees of freedom (k = the number of studies). The I2 index was also used in percentage values for checking heterogeneity: A random effect model was fitted when the I2 index was more than 25%. Publication bias was assessed between the studies using funnel plot (using common effect model with the inverse-variance method) and Egger regression test (using a fixed-effect model with the inverse-variance method).

To assess the overall success rate for REP, a single proportion meta-analysis was performed using “metaprop” module of STATA by taking the studies of both randomized clinical trials and single-arm prospective studies. The analysis was carried out using standard generic inverse variance methods for the combination of single proportions based on transformed proportions using Freeman–Tukey double arcsine transformations. To confirm the consistency of the result, a sensitivity analysis was carried out by study types (randomized clinical trials vs. single-arm prospective studies). A subgroup analysis was carried out based on stem cell strategy (cell based vs. cell homing), biomaterial (MTA vs. Biodentine vs. calcium sulfate), intracanal medicament (ICM) (antibiotic paste vs. calcium hydroxide), and final irrigant (EDTA vs. no EDTA). P < 0.05 was considered for statistical significance.

RESULTS

Search results

Figure 1 depicts the entire search process. One thousand and seventy-seven studies were identified during the initial search, followed by the exclusion of 1049 studies after the title and abstract screening. Finally, 28 studies were selected for full-text assessment. Subsequently, 18 studies were excluded as they included case reports, case series, and a study evaluating mesenchymal stem cell (MSC) markers.[19] A total of ten studies (five randomized clinical trials and five single-arm prospective studies) were included.[7121320212223242526]

Figure 1

Preferred Reporting Items for Systematic Reviews and Meta-Analyses flowchart summarizing the systematic review process in the identification of the included studies

Study characteristics

Table 1 presents the detailed characteristics of the included studies. A total of 362 (184 for REP, 178 for NSET) and 190 patients were followed up in the included randomized clinical trials and single-arm prospective studies, respectively. The mean age ranged between 9 and 46 years and included both genders. Six studies included both anterior and posterior teeth.[71321242526] Rest of the studies included just the anterior teeth.[12202223] Table 2 depicts the quality assessment (ROB 2.0 and ROBINS-I) of the included studies.

Table 1

Study characteristics and outcome measures evaluated

Author, year		Study type	Age (mean in years)			Gender		Tooth types	Diagnosis	Follow-up (months)
						Male	Female
Shah and Logani, 2012		Single-arm prospective study	44.7			11	7	Both anterior and posterior teeth	Pulp necrosis	6-29
Shah, 2016		Single-arm prospective study	46.84			76	40	Both anterior and posterior teeth	Any (pulp necrosis/irreversible pulpitis)	Till 72
Nageh et al. 2017		RCT	18-40					Anterior teeth	Pulp necrosis	12
Nakashima et al. 2017		Single-arm prospective study	28.6±10.0			3	2	Incisors - 2 Premolars - 3	Irreversible pulpitis	7-8
Nageh et al. 2018		Single-arm prospective study	25					Anterior teeth	Pulp necrosis	12
El-Kateb et al. 2018		Single-arm prospective study	25.5			7	11	Anterior teeth	Pulp necrosis	12
Arslan et al. 2019		RCT	RCT - 20.66±1.27 REP - 20.58±2.53			RCT - 13 REP - 22	RCT - 7 REP - 4	Anterior teeth	Pulp necrosis	12
Jha et al. 2019		RCT	9 - 15					Both anterior and posterior teeth	Pulp necrosis	18
Brizuela et al. 2020		RCT	28 - RCT 27 - REP			RCT - 5 REP - 6	RCT - 13 REP - 12	Incisors - 14 (RCT); 16 (REP) Canine - 2 (RCT) 1 (REP) Mandibular premolar - 2 (RCT) 1 (REP)	Pulp necrosis	12
Shah 2012		RCT	28			75	87	Incisors - 4 (RCT); 4 (REP) Premolars - 8 (RCT); 8 (REP) Molars - 92 (RCT); 92 (REP)	Any (pulp necrosis/irreversible pulpitis)	12
Author, year	Protocol followed					Sample size	Primary outcome: Success (healed/total)		Additional Outcomes
	Intracanal medicament	Final irrigant	Scaffold	Stem cell strategy	Biomaterial		Clinical	Radiographic	Pulp sensibility	Others
Shah and Logani, 2012	TAP	AMS	Blood clot	Cell homing	Calcium sulfate	18	18/18 (100%)			CBCT - lesion size (CBCT-PAI) and bone and cementum density (in HU)
Shah, 2016	CaOH	AMS	Blood clot	Cell homing	Calcium sulfate	134	125/134 (93.28%)
Nageh et al. 2017	DAP	EDTA	PRF	Cell homing	MTA	30	NSET - 15/15 (100%) REP - 15/15 (100%) 4/5 (80%)	--		Clinical and radiographic assessment - Cervical root fracture
Nakashima et al. 2017	Minocycline or 0.5% levofloxacin	EDTA	MDPSCs with G-CSF in atelocollagen	Cell based	Gelatin sponge	5			EPT - 4/5 (80%)	MRI - Signal intensity measurement CBCT - Lateral dentin formation, and volume of root canal
Nageh et al. 2018	DAP	EDTA	PRF	Cell homing	MTA	15	15/15 (100%)		Cold - 9/15 (60%) EPT - 9/15 (60%)
El-Kateb et al. 2018	CaOH	EDTA	Blood clot	Cell homing	Biodentine	18	18/18 (100%)		Cold - 15/18 (83.33%) EPT - 14/18 (77.78%)	MRI - Signal intensity measurement
Arslan et al. 2019	RCT - CaOH REP - TAP	EDTA	Blood clot	Cell homing	MTA	56 (46 patients were followed up)	NSET - 16/20 (80%) REP - 24/26 (92.31%)		EPT - 13/26 (50%)
Jha et al. 2019	TAP	EDTA	Blood clot	Cell homing	Calcium sulfate	30	NSET - 12/15 (80%) REP - 13/15 (86.67%)
Brizuela et al. 2020	CaOH	EDTA	UC-MSCs + PPP	Cell based	Biodentine	36	NSET - 18/18 (100%) REP - 18/18 (100%)		Cold - 10/18 (55.56%) EPT - 9/18 (50%)	CBCT - Dimensions (height, anteroposterior, mesiodistal) of the apical lesions LDF - Perfusion unit percentage
Shah 2012	CaOH	AMS (Betadine)	Blood clot	Cell homing	Calcium sulfate	220	NSET - 100/110 (90.91%) REP - 102/110 (92.72%)			CBCT - lesion size (CBCT-PAI) and bone and cementum density (in HU)

TAP: Triple antibiotic paste, CaOH: Calcium hydroxide, DAP: Double antibiotic paste, AMS: Antimicrobial solution, EDTA: Ethylenediaminetetraacetic acid, HU: Hounsfield unit, CBCT: Cone beam computed tomography, MRI: Magnetic resonance imaging, PRF: Platelet-rich fibrin, NSET: Nonsurgical endodontic treatment, RCT: Randomized clinical trial, REP: Regenerative endodontic procedure, EPT: Electric pulp test, G-CSF: Granulocyte colony-stimulating factor, MDPSCs: Mobilized dental pulp stem cell, PAI: Periapical index, LDF: Laser doppler flowmetry

Table 2

(a) Quality assessment of randomized clinical trials using ROB 2.0 tool (Summary of risk of bias) (b) ROBINS-I tool for quality assessment of single-arm prospective studies

(a) ROB 2.0 tool
Author, year	Experimental	Comparator	Randomization process	Deviation from intended interventions	Missing Outcome data	Measurement of the outcome	Selection of the reported result	Overall
Arslan, 2019	REP	NSET	Low risk	Some concerns	Low risk	Low risk	Some concerns	Some concerns
Jha, 2019	REP	NSET	Some concerns	Low risk	Low risk	Low risk	Some concerns	Some concerns
Brizuela, 2020	REP	NSET	Low risk	Low risk	Low risk	Low risk	Low risk	Low risk
Nageh, 2017	REP	NSET	Low risk	Low risk	Low risk	High risk	Some concerns	High risk
Shah, 2012	REP	NSET	Some concerns	Low risk	Low risk	Low risk	Low risk	Some concerns
El-Kateb, 2020	REP 1	REP 2	Low risk	Low risk	Low risk	Low risk	Low risk	Low risk
(b) ROBINS - I tool
Author, year	Bias due to confounding	Bias in selection of participants into the study	Bias in classification of interventions	Bias due to deviations from intended interventions	Bias due to missing data	Bias in measurement of outcomes	Bias in selection of the reported result	Overall Bias
M.Nakashima 2017	Serious	Low	Low	Low	Low	Low	Low	Serious
M. Nageh 2018	Moderate	Low	Low	Low	Low	Moderate	Low	Moderate
N. Shah 2012	Serious	Low	Low	Low	Moderate	Moderate	Low	Serious
N. Shah 2016	Serious	Low	Low	Low	Moderate	Moderate	Low	Serious

NSET: Nonsurgical endodontic treatment, REP: Regenerative endodontic procedure

Risk difference meta-analysis

Among the five randomized clinical trials, the individual study size varied between 15[23] and 110.[25] Of the 184 patients treated with REP, the success rate for individual studies ranged between 86.7% and 100% accounting for an overall success rate of 93.5% (95% CI: 88.9%–96.6%). Of 178 patients treated with NSET, the overall success rate was 90.4% (95% CI: 85.1%–94.3%). The success rate among the individual studies for NSET was between 80%[12] and 100%.[1323]

Meta-analysis results showed that the R.D. between REP and NSET studies was 0.032 (95% CI: −0.023–0.087), indicating no significant difference (P = 0.258) between the success rates of two procedures. Individual studies also showed similar results, as evidenced by P > 0.05. The I2 statistics was shown to be 0.00, indicating that there was no heterogeneity between the studies. The forest plot showing the R.D. of individual and overall studies is given in Figure 2a.

Figure 2

Forest plot depicting (a) Risk difference between success rates of REP and NSET (b) Overall Success rate of REP based on randomized clinical trials and single-arm prospective studies (c) Sensitivity analysis performed between the two study types

Publication bias

The funnel plot showed no publication bias, as evidenced by the symmetric distribution of effect sizes close to zero value. The Egger regression analysis also showed that the intercept was not statistically significant (P = 0.830), confirming the absence of publication bias.

Single proportion meta-analysis

A total of ten studies (five randomized clinical trials and five single-arm prospective studies) were eligible for assessing the impact of REP on success rate. The sample size (number of patients) in studies varied between 5 and 134. Further, two studies were cell based,[1324] and the remaining eight were cell-homing-based REP.[712202122232526] The observed success rate varied between 80%[24] and 100%.[713202223] Meta-analysis results showed an overall success rate of 96.0% (95% CI: 94%–98%). The heterogeneity between the studies was not established and both fixed and random effect models resulted in the same effect size, i.e., 96.0%. Forest plot of individual study effect sizes is depicted in Figure 2b.

Sensitivity analysis

A sensitivity analysis was performed by study types (randomized clinical trials vs. single-arm prospective studies), and the results are depicted as a forest plot in [Figure 2c]. Success rate of REP in randomized clinical trials (95%; CI: 91%–98%) and single-arm prospective studies (97%; CI: 94%–100%) did not differ significantly and were consistent.

Subgroup analysis

Stem cell strategy

The success rate for cell-based studies (95.65%; CI: 88%–100%) and cell-homing studies (96%; CI: 93%–98%) did not differ significantly [Supplementary Figure 1a] (I2 = 0.18; P = 0.670).

Biomaterial used

Of the ten studies, three studies used MTA,[122223] four studies used calcium sulfate[7212526] and two studies used Biodentine.[1320] The respective success rates were 98% (CI: 91%–100%), 94% (CI: 91%–97%) and 100% (CI: 95%–100%), respectively [Supplementary Figure 1b]. In one study,[24] no biomaterial was used (gelatin sponge was used) (I2 = 5.04; P = 0.165).

Intracanal medicament paste

Six studies used antibiotic paste[71221222324] and four studies used calcium hydroxide[13202526] and their corresponding success rates were found to be 97% (CI: 92%–100%) and 95% (CI: 92%–98%) which did not differ significantly [Supplementary Figure 1c] (I2 = 0.04; P = 0.832).

Final irrigant

In seven studies, EDTA was used as the final irrigant,[12132021222324] and the remaining three studies did not use EDTA as a final irrigant.[72526] The forest plot [Supplementary Figure 1d] generated for these subgroups meta-analysis revealed that the success rates for the studies which used EDTA as a final irrigant was 98% (CI: 93%–100%) and for the studies which did not use EDTA as a final irrigant was 94% (CI: 91%–97%) (I2 = 0.57; P = 0.450).

No evidence of heterogeneity was established in any subgroup analysis.

DISCUSSION

The present meta-analysis critically appraises the emerging evidence on the outcome of REP in mature permanent teeth. The results of this analysis may provide a useful foundation for the development of sound regenerative endodontic strategies as an alternative treatment option to the conventional NSET for the management of mature permanent teeth.

The success rate of NSET reported in the included randomized clinical trials in this analysis ranged between 80% and 100%, which is in accordance with the published literature by Friedman.[27] However, this conventional approach renders the tooth nonvital, thereby discontinuing the tooth's innate immunity and leading to increased chances of reinfections. Furthermore, there is an increased destruction of tooth structure and loss of proprioception which might result in tooth fracture. These drawbacks may be overcome by employing dentin–pulp regeneration through REP.[14] In the present analysis, the success rate of REP was found to be comparable to that of the conventional NSET. However, the included studies have variable follow-up periods. The average follow-up period considered in this analysis was 12 months. The follow-up period is a crucial factor in prospective studies. The success rate of an intervention may vary over time, and the short-term outcome might differ from the long-term outcome.[28]

The clinical and radiographic measures have been considered for evaluating the primary treatment outcome of REP in mature permanent teeth which is similar to the criteria followed for conventional NSET. Additional measures like pulp sensibility or vitality testing, increased dentin width, and histologic evaluation of the regenerated tissues have also been evaluated by a few studies since REP involves dentin–pulp regeneration in the empty root canal space. This review found that positive pulp sensibility results were not consistent across the studies (60%–83% for cold test; 50%–80% for electric pulp test.[1213202224] This could be attributed to the presence of layered coronal restoration. Brizuela et al.[13] assessed pulpal regeneration by laser doppler flowmetry, which exhibited increased perfusion unit percentage from 60.6% to 78.1% between baseline and 12 months. Another additional test conducted for assessment of pulp regeneration was signal intensity (S.I.) measurement by MRI,[20] which revealed that the S.I. of the regenerated tissues in the treated teeth approached to a value similar to that of the normal contralateral teeth after a 12-month follow-up. Many REP studies have considered 3D imaging to assess periapical healing and root development in immature teeth.[293031] In mature teeth, four of the included studies have utilized CBCT scans for the assessment of periapical healing[71325] and dentin deposition.[24] Increased width of lateral dentinal walls was documented at 28 weeks by Nakashima et al.[24] In a case report, Meza et al.[32] observed dentin bridge formation in the middle third and calcification of the canal in the apical third.

The primary goal of REP is to form new tissue with a similar structure and function as dentin and pulp. Therefore, only a histology-based assessment can be considered as an accurate predictor of pulp regeneration. A case report recently presented histologic evaluation after a REP in human mature permanent central incisor.[33] The fibrous connective tissue that contains bone-like tissue, vascular structures, and inflammation was found in the same report. An animal study[34] investigated regenerated dental pulp in mature canine teeth after combining an induced BC, platelet-rich plasma (PRP), and bone marrow aspirate. The new vital tissue was characterized as connective, cementum-like, or bone-like tissue but not as pulp-like tissue.

Different stem cell strategies are being employed in REP to achieve predictability in the type of regenerated tissue formed in the empty root canal space.[35] Broadly, there are two such strategies. The first is cell homing, in which the patient's resident cells migrate toward the site of tissue injury. It includes the induction of BC by placing a sterile ISO # K/#H file beyond the apex. Studies in mature teeth have used ISO #10K-#40K file sizes, and #15H-#35H file, depending on the tooth type. However, Nosrat et al.[36] reported that the stem cells in the BC can originate from different sources other than the apical papilla and might not differentiate into odontoblasts. Thus, alternate scaffolds were suggested, such as using scaffolds impregnated with growth factors,[37] PRP,[3839] and platelet-rich fibrin (PRF), to regenerate pulp tissue. In the present review, while most studies had used BC as the scaffold, PRF was utilized (adjunct to BC) in two studies.[2223] The second strategy is a cell based one, in which stem cells are transplanted from ex vivo cultivated stem/progenitor cells. In preclinical animal models, fractions of DPSCs mobilized by granulocyte-stimulating growth factor have shown to produce dental pulp and dentin.[40] In the present analysis, a study conducted by Brizuela et al.[13] employed the use of umbilical cord-derived MSCs encapsulated in a plasma-derived biomaterial (platelet poor plasma). Mobilized DPSCs with G-CSF in atelocollagen were used by Nakashima et al.[24] for pulp regeneration. The clinical efficacy and safety of these cell-based strategies has been well investigated in the published literature.[1324324142] In the present meta-analysis, no significant difference was found between the two strategies (cell homing vs. cell based). However, further research is required as the number of studies with a high level of evidence is inadequate.

Thorough disinfection of the root canal system is pertinent for achieving a successful outcome in REP. REP does not involve obturation of the root canal space, and the regeneration of new tissue in the root canal system takes time during which the residual bacteria might proliferate and initiate disease.[43] Hence, REP requires an enhanced level of bacterial disinfection in comparison to conventional NSET. The protocol recommended by AAE for REP in immature roots includes disinfection of root canal system by irrigants and intracanal medicament, with minimal mechanical instrumentation. However, no such defined guidelines are available for REP in mature roots. Mechanical instrumentation, similar to conventional NSET, has been performed in all the included studies. Some of the included studies advocated the use of modified instrumentation techniques like the apical clearing technique and apical foramen widening.[7212526] The concentration of sodium hypochlorite used in the reported studies was 1%,[12] 1.5%,[22] 2.5%,[7132021232526] and 6%.[24] EDTA was used as the final irrigant in seven studies[12132021222324] in a concentration of 3%,[24] 5%,[12] and 17%.[1320212223] EDTA is shown to liberate transforming growth factor-beta (TGF-ß) from root dentin[44] which controls cell migration, proliferation, and differentiation of pluripotent cells into odontoblast. Recently, Ivica et al.[45] have shown the possibility of TGF-ß release from mature roots. Following instrumentation and irrigation, the use of ICM has been widely advocated to eradicate the remaining bacteria within the root canal system.[46] The medicaments used in the included studies were calcium hydroxide,[13202526] triple antibiotic paste (TAP),[71221] double antibiotic paste (DAP),[2223] and minocycline/0.5% levofloxacin powder.[24] Fahmy et al.[43] assessed the revascularization in necrotic mature dog teeth following different protocols. They concluded that disinfection with ciprofloxacin or DAP resulted in significantly better corono-apical tissue ingrowth and significantly lower inflammation than modified TAP. In the present analysis, similar success rates were observed in all the included studies irrespective of the final irrigant's choice, or the inter-appointment ICM used. A limited number of studies and smaller sample size might be responsible for this finding.

Overall, the results of the present meta-analysis further strengthen the scientific evidence generated by another systematic review, which also showed that there is no difference in relative risk for successful/unsuccessful treatment outcome between REP and NSET, based on just four randomized clinical trials.[47]

Limitations

Due to a lack of sufficient randomized clinical trials, the current review included both randomized clinical trials and single-arm prospective studies. Only randomized trials were used to compare the two methods (REP and NSET). We included single-arm studies to determine the overall success rate of REP. A sizable proportion of the cases were derived from an unpublished study[25] that raised “some concerns” in the RoB 2.0 tool. The bias was addressed, however, by conducting sensitivity analyses for each study type. Two of the randomized controlled trials employed a cell-based strategy, which is a different approach. As a result, a subgroup analysis was conducted, and no differences in the two regenerative strategies were discovered. Despite the fact that age has been identified as a significant predictor of success in REP, no statistical comparisons were made because the majority of studies included patients in a similar age range. In addition, only English-language articles were considered. As a result, the meta-analysis findings are based on limited evidence.

CONCLUSION

Based on the limited comparable studies, REP has a similar outcome to NSET in the management of mature permanent teeth. However, there is diversity in the treatment protocols as well as variation in the outcome assessment. Hence, it would be prudent to conduct well-designed randomized clinical trials with larger sample size and longer follow up period. Multicentric trials comparing cell-based versus cell-homing approaches would help in establishing the superior protocol. Advanced imaging techniques (CBCT, MRI) should be utilized for evaluating the outcome. Efforts should be directed for developing uniform guidelines and techniques. This review provides an insight into the presently available evidence and provides a foundation for further research to address the knowledge gap.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Forest plots comparing success rates for (a) Stem cell approach (cell based vs. cell homing) (b) Biomaterial (MTA vs. Biodentine vs. Calcium Sulfate), (c) Intracanal medicament (Antibiotic paste vs. Calcium Hydroxide), and (d) Final irrigant