What is the best long-term treatment modality for immature permanent teeth with pulp necrosis and apical periodontitis?

Different techniques have been adopted to prevent overfilling and to stimulate the closure of the apical area—s.c. apexification techniques. A widely accepted endodontic approach in the treatment of immature teeth with pulp necrosis and apical periodontitis is the apexification technique. This technique consists of multiple and long-term applications of calcium hydroxide (CaOH₂) (Petti et al. 2018). Many studies show continued apical formation by hard tissue deposition where the teeth are asymptomatic with radiographical signs of closure of the apical foramen (Hecova et al. 2010; Tsilingaridis et al. 2012). The main advantage of this technique is the excellent disinfecting effect of the root canal due to the high pH of CaOH₂ (Kakoli et al. 2009; Orstavik and Haapasalo 1990). On the other hand, this technique often requires 9–24 months of treatment (Sheehy and Roberts 1997). In addition, CaOH₂ may weaken the tooth structure, which often results in cervical root fractures (Andreasen et al. 2002). Understandingly, many studies discuss the consequences of premature tooth loss and highlight the urgent need for alternative treatment strategies for this patient group (Orstavik and Haapasalo 1990; Trope 2010; Sheehy and Roberts 1997; Andreasen et al. 2002; Dawood et al. 2017).

As an alternative to apexification with CaOH₂, another technique has been introduced that stimulates apical hard tissue formation to aid obturation of teeth with open apices—the s.c. MTA-apical plug technique (mineral trioxide aggregate, MTA). Many studies using this technique report good results in periapical healing and signs of closure of the apical foramen (Mozynska et al. 2017; Chueh and Huang 2006). The MTA technique has the advantage of shorter treatment time as well as being biocompatible, which improves the interaction with the periapical tissue such as induction of cell proliferation and differentiation (Murray et al. 2007; Hargreaves et al. 2008). However, MTA does have several drawbacks. The MTA’s interaction with collagen causes tooth discoloration (Dawood et al. 2017), an undesirable consequence that may be the result of bismuth oxide and iron contamination of the blood (Mozynska et al. 2017). In addition, little is known about how the material influences the fracture resistance in immature traumatised teeth (Hecova et al. 2010; Tsilingaridis et al. 2012).

Neither calcium hydroxide nor MTA-apical plug apexification improves root formation and thickening of the dentin walls and therefore does not lead to the strengthening of the immature tooth and better long-term survival (Trope 2010; Sheehy and Roberts 1997). Hence, it is of high priority to replace conventional endodontic procedures with a new biological approach that will stimulate regeneration of the periapical tissue and continued root formation (Chueh and Huang 2006; Murray et al. 2007; Hargreaves et al. 2008; Lovelace et al. 2011). The ultimate goal of the treatment of traumatised immature teeth is to prolong the life span of the traumatised tooth and to postpone or prevent the need for more extensive prosthetic alternatives (Op Heij et al. 2003).

Recently, regenerative endodontic treatments (RET) have gained much attention as biologically based treatment alternatives to the techniques described above. These regenerative endodontics, which are gradually becoming part of the endodontic treatment spectrum, replace necrotic and damaged tissues with a healthy functioning pulp–dentin complex (Garcia-Godoy and Murray 2012). As regenerative endodontic treatment is a relatively new treatment modality, little is known about the processes about the type of intra-canal tissues formed during revascularisation and about the factors involved in healing and regeneration (Žižka and Šedý 2016).

Although the interest in regenerative endodontic treatment is increasing significantly, it is currently not the first choice for the management of immature permanent teeth with pulp necrosis as scientific evidence has not established the efficacy of the method (Rafter 2005; Cvek 1992; Trope 2010) and little is known about the long-term outcome. Therefore, it would be beneficial to evaluate and assess the current knowledge about this technique as a meaningful treatment option. Many studies have investigated the field of endodontic regeneration, but unfortunately most of these are based on small study sizes (n = 1–5) (Asgary et al. 2016; Zhujiang and Kim 2016; Farhad et al. 2016; Nagaveni et al. 2016; Nagaveni et al. 2015; Wang et al. 2015) and/or short follow-up (6–12 months) (Hargreaves et al. 2013; Gelman and Park 2012; Kim et al. 2012; Jadhav et al. 2012). Short follow-ups are a limitation because severe complications, such as ankylosis, can occur as a consequence of traumatic injuries to immature permanent teeth the first 2–3 years after an injury (Lauridsen et al. 2019). Therefore, the dental trauma itself influences the long-term prognosis rather than the type of treatment performed. Consequently, an internationally accepted approach is to postpone the completion of the endodontic treatment until the growth of the jaws is complete. This fact emphasises the need for intervention studies with sufficient study sizes and follow-up periods in order to find evidence for best practices.

To date, several review articles have been written on the subject of regenerative endodontic treatment (Antunes et al. 2016; Nicoloso et al. 2019; Nazzal and Duggal 2017; He et al. 2017). These reviews investigate and propose different important outcomes, such as quantitative measurements of tooth-root development, resolution of apical periodontitis, and surrogate outcome measurements of survival or success. To our knowledge, no systematic reviews have focused exclusively on long-term studies of RET (≥ 24 months) with larger study sizes (≥ 20 patients). For the reasons outlined above, this review critically evaluates whether regenerative endodontics can routinely be recommended in the treatment of traumatised immature teeth with pulp necrosis and infection assuming that the treatment modality will benefit the long-term survival of traumatised teeth and eventually the well-being of the patient.

Prior to conducting the work with the current review, a scoping search was performed on the International prospective register of systematic reviews, PROSPERO in order to avoid duplication (registration number CRD42020204801). No matches were found on protocols related to the topic and the study was subsequently registered before completion (Stewart et al. 2012). The search process was carried out in two steps according to the guidelines for systematic reviews.

First, we mapped the subject (Grant and Booth 2009) as mapping provides a systematic description of the search, screening, and classification process, rendering a representative sample of published literature (Gough et al. 2012). The overall goal was to map, categorise, and synthesise the findings of the specific topic area.

Second, we appraised and synthesised the research evidence for endodontic treatment of traumatised immature teeth in order to insure that the literature study findings are an accurate representation of the studies it contains (Thomas 2005).

The eligible studies were categorised into two subgroups: studies of immature permanent teeth with pulp necrosis and open apex treated with regenerative endodontic treatment (RET studies) and studies of immature permanent teeth with pulp necrosis and open apex treated with CaOH₂ or MTA apexification techniques (CaOH₂ or MTA studies) (SBU 2017).

The results showed that the included studies in both groups were heterogeneous regarding the outcome measurements. Only three studies reported Cvek’s stage of root development at baseline—one study in the RET group (Chan et al. 2017) and two studies in the MTA group (Kandemir Demirci et al. 2019; Bucher et al. 2016). Based on the comparison of the mean age values between the two study groups, we assumed that RET studies included teeth with a lower stage of root development than the MTA studies (mean age RET 8.33 years; mean age MTA 18.39 years) (Table 7). As the average study size of the two included RET studies was small (28 in one and 88 in one divided into four treatment groups including 22 cases each) (Table 3), the differences in the intervention effects might have been overlooked or misinterpreted (Gluud 2006).

The outcome measures between the MTA and RET studies were not identical as different treatment modalities were the subject of evaluations. To address the aim of this study (What measures were used to assess the treatment outcomes of regenerative endodontic procedures and apexification techniques with calcium hydroxide or MTA/gutta percha), the following measurements were made in both study groups: healing of apical periodontitis, survival, success, failure, functional retention, adverse effects, and apical closure (apexification).

In the RET study group, the deviant outcome measures were root lengthening and root thickening, which basically refer to continued root development. In the MTA group, the deviant outcome measure was the quality of root filling. This finding makes it difficult to perform a complete statistical comparison of the outcomes between the two groups. If the studies were more homogenous, the results could have been combined in a meta-analysis with a goal of increasing statistical significance.

Although the studies included in this review revealed different disinfecting routines, the treatment protocols were essentially homogenous: disinfection, intra-canal antimicrobial dressing, and blood clot as a matrix for regeneration. Several treatment protocols were used and are addressed in detail in the discussion (Alobaid et al. 2014; Chen et al. 2012). Varying concentrations of irrigation with NaOCl were used in the majority of the studies, but no significant difference in the overall outcomes was reported. The same observation concerns the root canal dressings. The disinfecting agent triple antibiotic paste (TAP) was used in both of the RET studies. The Hoshino`s classic mixture of triple antibiotic paste (TAP) (Hoshino et al. 1996) was modified and applied (mTAP) (Thibodeau and Trope 2007). Calcium hydroxide was used as a disinfecting agent in the MTA study group. Even in this group, no significant difference in overall outcomes was observed between the studies. The methods for inducing an apical barrier were compared, apexification was compared to apical plug techniques, and both techniques were compared to no apexification treatment (conventional root canal obturation with gutta percha).

Only two of the included studies were randomised clinical trials—one of the RET studies (Ulusoy et al. 2019) and one of the MTA study group (Kandemir Demirci et al. 2019)—and the majority of the studies lacked control groups. Therefore, we could not use statistical meta-analysis models to estimate the effect size (Dixon-Woods et al. 2006).

The qualitative analyses of the included cases showed that the majority of the treated teeth were diagnosed with pulp necrosis. In the RET study group, the most common basis of diagnosing pulp necrosis was preoperative negative response to cold testing and electric pulp tests. Both of the RET studies were designed to record pulp sensitivity testing both preoperatively and postoperatively. The results were inconclusive as the first study showed that 86% of the regenerated cases responded positively to sensitivity tests at follow-up (Ulusoy et al. 2019). The second study showed that 100% of the regenerated cases did not respond positively to sensitivity tests at follow-up (Chan et al. 2017). In the MTA study group, the majority of the included studies did not state accurate information about pulp sensitivity before treatment. That is, four of the five included MTA studies were retrospective so the data for sensitivity testing before treatment might have been unavailable. Standardised examination methods, including sensitivity testing at baseline, were used in a MTA study with a RCT design (Kandemir Demirci et al. 2019). In fact, the retreatment cases and cases with irreversible pulpitis were also included in the MTA treatment group (Ree and Schwartz 2017).

Anterior teeth were the most common subject for treatment in both groups (see Tables 7, 8).

Traumatic dental injuries leading to pulp necrosis and apical periodontitis were the most common reason for treatment in both RET and MTA study groups. Exhaustive data about the specific type of trauma injury were reported in five of the included studies (Bucher et al. 2016; Ree and Schwartz 2017; Kandemir Demirci et al. 2019; Chan et al. 2017; Ulusoy et al. 2019), and two studies did not state the cause for the treatment (Demiriz and Hazar 2017; Mente et al. 2013). In both study groups, the overall trauma frequency varied between 46 and 100% (Table 9).

An important finding concerning both groups was the reported high rates of success and survival treatment (90–96%). Surprisingly, the presence of trauma did not seem to negatively influence the outcome of the treatment.

The overall drop-out rates were low (between 7 and 19%). The most common stated reason for the drop-out was missing follow-up appointments (patients refused to participate or patients could not be contacted). Several other less common reasons were also reported: choosing extraction instead of further treatment because of financial reasons, patients not returning for the final treatment, and the lack of retrievable radiographs in the studies with retrospective design.

Before this study started, a search in Cochrane Library and PROSPERO was conducted to map all published and in progress reviews. The whole search process was transparently described and defined with the help of a librarian from Karolinska Institute.

As the search question and search strategy defined, we identified the best matching terms, synonyms, controlled vocabularies (MeSH terms), Boolean operators, and truncations according to the ‘golden-standard’ article for mapping and extraction of appropriate terms (Nicoloso et al. 2019). After a pilot search was conducted to explore the sensitivity and specificity of the search, the field codes were defined more precisely. Advanced search functions were used in appropriate blocks corresponding to the aims of the review. As dentistry is an integrated part of the medical topic, we searched PubMed before adjusting search strategies for additional databases. These databases were scoped for suitability. Web of Science, Medline (Ovid), Cochrane Library, and Embase provided an exhaustive coverage of the literature in the medical field (Schlosser et al. 2007). All databases were practical and offered numerous hits. For example, Medline (Ovid) provided a more expanded spectrum of journals than PubMed, (E-pub Ahead of Print and In-Process). However, the searches in Web of Science, which is a multidisciplinary database, resulted in fewer hits (n = 49). This result could be explained by the way the citation analysis was designed: it was intended to satisfy the needs of users of citation analysis, a field discussed and debated by scientists for decades.

As five electronic databases were searched, the risk for database biases was minimised according to AMSTAR (Perry et al. 2017; Shea et al. 2017). Several authors and the Cochrane Collaboration state that multiple appropriate databases should be chosen so that the yield of relevant studies is maximised and the risk for database bias is minimised (Moher et al. 2009; Schlosser et al. 2007). In this review, however, we found that many of the retrieved studies were duplicates (approximately 40%), evidence that the broad search strategy had high specificity.

When it comes to the specific terms, it is worth clarifying that the terms ‘traumatised’ and ‘immature permanent teeth’ were chosen for the pilot search, a strategy that produced few results. Therefore, these terms were not added in the final search as the final search was prioritised to include ‘all immature permanent teeth’. Because the search was conducted recently and the author of this report receives weekly updates with RSS and alerts, it was decided that no ‘new’ search was needed. No grey literature search was conducted with regard to the inclusion criterium only published peer-reviewed articles (the background, objective, aim, eligibility criteria, method, and outcomes). To summarise, the search strategy was well targeted and successful, resulting in 21 matching hits.

Three authors conducted the screenings independently and blinded. The authors screened the articles by title and abstract and read the full text of the potentially eligible articles. A majority rule was adopted in the decision-making with respect to the eligibility of the studies. A fourth participant, the principal investigator with experience in systematic reviews, was consulted for a final decision about eligibility, discrepancies, and classification with regard to the inclusion and exclusion criteria applied in the process. Consequently, the fact that four persons were involved assures good structure and minimises the risk for source selection bias (Schlosser et al. 2007). The excluded articles were classified to assure transparency in the selection process (Table 5).

As Schlosser et al. state, ‘a systematic review can only be as sound as the included studies’; therefore, we find it very important to provide quality assessment as studies might have a different quality level (Schlosser et al. 2007). Several quality assessments suitable for the included studies were considered: the STROBE checklist consisting of 22 questions (STROBE Statement-checklist of items that should be included in reports of observational studies) and the CONSORT statement (SBU 2017). As the included studies had different study designs, we adopted the modified quality assessment developed by Lewin et al. (2015) (GRADE-CERQual) (Lewin et al. 2015). An appraisal of the research question was made to assess the correspondence with the results. Moreover, evaluation of interventions, possible limitations, adequacy of data, statistical methods, and conclusions were made to strengthen the scientific evidence of the findings.

Two different tools for assessing risk of bias were used: ROBINS-I tool (for assessing risk of bias in non-randomised studies of interventions) (Sterne et al. 2016) and RoB 2.0 tool (The Cochrane Collaboration’s tool for assessing risk of bias in RCTs) (Higgins et al. 2011). In accordance with the tools, the possible risk for bias in the included studies was organised and evaluated in different domains (i.e. random sequence generation and allocation concealment, blinding of participants and personnel regarding the treatment choice or the medicament choice, blinding of outcome data interpretation, and selective outcome data reporting). In particular, the lack of an adequate allocation sequence generation method is possible source of selection bias; that is, there is an uneven distribution between the different treatment groups that could result in overestimation of the treatment effect. Furthermore, the rating of several studies was challenging because blinding was not always feasible to personnel and participants and only one study reported predetermined power calculation of the sample size (Ulusoy et al. 2019). Online Appendix 2 specifies the domain questions considered during bias assessment with the ROBINS-I tool (see Tables 11, 12).

Subject characteristics such as age, sex, aetiology, and tooth type were presented thoroughly in all studies in both groups. The majority of the studies had a complete and convincing reporting of the outcome data. Efforts were made to apply descriptive statistical methods. The studies in the MTA group had a low drop-out rate, included a large amount of teeth, and had a follow-up period longer than the RET group.

As mentioned above, both of the included studies in the RET group used modified triple antibiotic paste (TAP) as an inter-appointment disinfecting agent. This method, developed by Hoshino et al. (1996), combines Ciprofloxacin, Metronidazole, and Minocycline (Hoshino et al. 1996). Although the previously mentioned paste has the ability to combat bacteria specific to endodontic infections, it can cause severe tooth discolorations. Therefore, Minocycline has been replaced with Cefaclor and was launched as a modified triple antibiotic paste (mTAP) (Thibodeau and Trope 2007).

TAP and mTAP have several drawbacks, including the lack of standardised mixing protocols. Different amounts of the drugs are ground into powders and mixed with sterile water or macrogol ex tempore and applied in the root canal. This application causes uneven distribution and excessive concentration problems, resulting in high cytotoxicity (Ruparel et al. 2012). Recently, several authorities discouraged the use of one of the components in mTAP, Ciprofloxacin, due to its severe side effects (The Swedish Medical Products Agency, The European Medical Agency, The Food and Drug Administration Agency).

Therefore, the current recommendations include calcium hydroxide as a disinfecting agent in regenerative endodontic treatment (Endodontology 2006). Calcium hydroxide has good antibacterial properties and has been shown not to be cytotoxic to periapical tissues and SCAP: s (Sjogren et al. 1991). In accordance with existing literature, the disinfection protocols used in the both study groups effectively eliminated bacterial infections as healing of apical periodontitis was achieved (Sjogren et al. 1991, 1997; Sundqvist et al. 1998). Nevertheless, little is known about the post-treatment microbiologic status of the treated teeth, because none of the included studies reported bacterial samplings as a standardised control method of disinfection.

All the included studies in the RET and MTA treatment groups were performed at specialist clinics either by residents supervised by experienced endodontists and paediatric dentistry specialists or by the specialists themselves. Moreover, several studies reported that endodontic and paediatric specialists collaborated to provide non-compliant patients nitrous oxide/oxygen analgesia. All treatment procedures were performed with the aid of a surgical operating microscope. In this context, the treatment providers might have influenced the results and the external validity of the studies as operator experience is a key factor when it comes to outcome in endodontics (Alley et al. 2004). Root canal treatments consist of a series of independent steps, including isolation, access preparation, mechanical debridement, chemical irrigation, root canal dressing, and obturation. These factors have the ability to influence the treatment outcome (Ng et al. 2008). When the technical standard of root fillings is adequate, the healing rates are high (Sjogren et al. 1990; Bierenkrant et al. 2008).

It is important to analyse outcome measures as these are basis for evaluation of the treatment outcome of immature permanent teeth with pulp necrosis and apical periodontitis. Traditionally, endodontic outcome studies use strict criteria for success. They define successful treatment as absence of signs and symptoms of apical pathology, where the contours, width, and structure of the periodontal ligament around the treated tooth are radiographically normal (Sjogren et al. 1990, 1997; Ng et al. 2007). Factors associated with better tooth survival are the quality of the root filling, the length of root filling in relation to apex, optimal coronal restoration, and healthy preoperative apical status (Ng et al. 2010). It is generally accepted that optimal root filling lacks voids and ends within 0–2 mm from the radiographical apex (Frisk et al. 2008). In a meta-analysis, Ng et al. (2008) identified additional key factors that significantly improve the outcome of primary root canal treatment. These are preoperative absence of periapical radiolucency and satisfactory coronal restoration (Ng et al. 2008). However, we did not find a correlation between the preoperative presence of periapical radiolucency and the rate of success.

Often, radiographic assessment of treatment results is the only objective method used to evaluate endodontic treatment. Assessment is usually performed with intraoral radiographs using a variety of criteria to determine health of periapical. These strict radiographic criteria are established as a golden standard in the field of endodontic outcome measures (Sjogren et al. 1990, 1997). The most often used structured criterion for measurement of periapical status is the PAI index, which consists of five scores with cut-off points between PAI 2 and PAI 3 score for periapical health and disease (Orstavik et al. 1986). The analyses of the included studies showed that none of the RET studies reported PAI index at baseline or at follow-up while three of the MTA studies reported the use of PAI index both at baseline and at follow-up (Bucher et al. 2016; Mente et al. 2013; Kandemir Demirci et al. 2019). Therefore, in the vast majority of the studies, absence or presence of apical periodontitis at baseline and follow-up was identified as a radiographic outcome measure.

A more forgiving approach for outcome measure has been proposed by Friedman and Mor (Friedman, Mor 2004). Healing of periapical disease was compared to functionality or merely retention when endodontic treatment was weighed against tooth extraction and replacement with implant to provide relevant comparisons between the two treatment outcomes. This definition proposed the terms ‘functional retention’ and ‘survival’ as a specific goals of the individual patient and it includes teeth that may be healing or show signs of apical pathology. The third term, ‘success’, was adopted for teeth that have healthy apical status and are free of symptoms. Furthermore, the above-mentioned terms were applied for long-term evaluations and are less appropriate for short-term evaluations (< 10 years) (Dammaschke et al. 2003). The follow-up for the included RET studies varied between 24 and 49 months; however, large preoperative apical lesions could need a longer period to heal than small lesions (Sjogren et al. 1990). Therefore, in this study, the radiographic interpretation in terms of ‘success’ and resolution of periapical radiolucency should be interpreted with caution.

Therefore, we can say that the adopted terms of ‘functional retention’ and ‘survival’ may be seen as more loose criteria in the definition of endodontic outcomes. In other words, when we compare the outcomes of RET and MTA groups, we should bear in mind that the used outcome measures focus on different issues. In the RET group, the key outcome measure is the continuation of root development (root wall lengthening and root wall thickening); in the MTA group, the key outcome measure is the quality of the root filling. It could be justifiable for the included RET studies to use surrogate measures of treatment outcomes. Such measures are healing signs of local infection (sinus tract and abscess), healing of apical pathology, increase of root wall thickness, increase of root length, closure of apical, and absence of subjective symptoms. The study analyses showed that the majority of the cases were treated successfully and were retained throughout the follow-up. Furthermore, there were no statistical differences between the study groups.

Little is known about the impact of extruded MTA on the healing capacity of traumatised teeth with apical pathosis. Unfortunately, this topic has resulted in few studies with high scientific value; the results from several case reports and case series on extrusion of MTA have been inconclusive and dubious outcomes have been reported (Tezel et al. 2010; Tahan et al. 2010; Nosrat et al. 2012; Chang et al. 2013).

We found that three of the studies from the MTA group reported the level of root filling in relation to the root apex (Bucher et al. 2016; Demiriz and Hazar 2017; Mente et al. 2013). For example, one of the studies found that 30% of the root-filled teeth were underfilled or overfilled, but this did not affect the healing rate as 90% showed clinical and radiographical success (Bucher et al. 2016). Additionally, one study found no significant difference in periapical healing between overfilled and optimally filled teeth (Demiriz, Hazar Bodrumlu 2017). Unlike other studies, the results of this review showed that the level of root filling in relation to the root apex in the MTA study group did not affect the success rate considerably. This finding could be partially explained by the fact the MTA study group had good biocompatibility (Solanki et al. 2018) and partially by the described ability of MTA to stimulate deposition of mineralised tissue at the interface of the placement (Dreger et al. 2012).

In this review, the most consistent radiographic findings for both study groups were apical closure (apexification) and healing of periapical radiolucency (81–100%). The clinically meaningful findings include teeth being free from signs of local infection (sinus tract and abscess) and being free of symptoms. According to the adopted outcome measures, both RET and MTA groups showed high survival rates in the short term, ≤ 24 months (95%). A critical appraisal of these outcome measures would mean that the more forgiving measures were applicable for the RET study group (Friedman and Mor 2004).

Cvek (1992) described the problem of root fractures after prolonged intra-canal medication with calcium hydroxide in immature teeth. Root fracture has been recognised as one of the major complications in management of young necrotic immature teeth. The earlier the root development stage, the greater the risk of cervical root fractures (ranging from 77% in teeth with the lowest root development stage to 27% in the teeth with the highest root development stage). Cvek observed that the incidence of root fracture was associated with the root wall thickness rather than with prolonged intra-canal medication with calcium hydroxide. These findings were supported by several other studies (Kahler et al. 2018; Al-Jundi 2004). Further search in the literature did not identify studies reporting increased frequency of root fractures in immature teeth with open apices medicated with calcium hydroxide over varying periods (Morfis and Siskos 1991; Thater and Marechaux 1988).

It has been known that the application of MTA in the root canal facilitates an apical seal but does not promote continued root development. Similarly, none of the included MTA studies reported continued root formation and maturation. On the contrary, both the included RET studies reported accurate information on root wall widening and lengthening, significantly increasing root dimensions. The findings from the included studies showed that the reported overall incidence of root fractures was low (Table 10). Another finding is that an evaluation of the root maturation post-treatment was not possible to perform because not all of the included studies provided this information. In conclusion, based on the included information, it was difficult to establish a correlation between total medication time, continued root development, and the risk for root fractures.

Previous studies report that RET treatment of non-traumatic dental injuries, such as caries or developmental anomalies, results in a better prognosis than trauma cases (Lin et al. 2017; Cehreli et al. 2013). Severe trauma to the periodontal ligament induces inflammatory root resorption and may cause damage to the Hertwig epithelial root sheath as well as the apical papilla, compromising the outcome of regenerative treatment. Obviously, it is not possible to draw any conclusions on the effect of trauma on the outcome because the RET study group consisted of only two studies. However, both of these studies included both severe and non-severe trauma. No differences in overall success and survival rates were observed between severe and non-severe traumatic aetiology. In addition, no differences in the rate of root wall lengthening and thickening could be correlated to the nature of the trauma.

The included studies indicated low frequency of occurrence of adverse effects. The most frequently reported drawback in both groups was tooth discoloration caused by MTA. Discoloration is an undesirable consequence of endodontic treatment and efforts should always be made to substitute agents that cause discolouring. Six out of seven of the included studies in both RET and MTA groups substituted grey MTA with white MTA. However, white MTA without bismuth oxide also causes discoloration in the presence of blood due to porosity of the cement or the iron absorption from haemoglobin (Torabinejad et al. 2018). Perhaps, MTA treatments are technique-sensitive procedures depending both on how experienced the treatment provider is and on the patient’s cooperation. Several difficulties—e.g. a child’s behaviour, proper visualisation, and the ideal placement of the material—are linked directly to the rate of discoloration.

A strength of this study is that it investigates a large number of immature permanent teeth with pulp necrosis and open apices (694 teeth) using both clinical and radiographic outcomes with a minimum follow-up of 24 months. In total, 116 teeth were treated in the included RET studies and 578 teeth were treated in the included MTA studies. In addition, the studies included in the MTA group also had longer follow-up and larger study sizes, resulting in better statistical significance.

The present review has, however, some limitations. The search was not expanded within unpublished literature (registries or theses). It was discussed that the risk for possible flaws in the design, conduct, analysis, and reporting of the results of unpublished studies would not have been detected unless submission to review process. Consequently, the authors agreed to include only published peer-reviewed studies.

Another limitation of this review is the fact that only seven studies matched the inclusion criteria of intervention studies on immature necrotic teeth with follow-up of at least 20 teeth for 24 months.

The number of included cases was unevenly distributed: compared to the RET group, the MTA group had approximately five times the total number of cases treated. In addition, the material that this review is based on consists of studies with different designs. Some studies had a retrospective design, which might limit the information if the treated teeth presented any signs or symptoms of pre-, intra-, or postoperative adverse effects. Another limitation is the fact that the periapical PAI index was not stated at baseline in all of the studies. The PAI index is considered to be accurate, reproducible, and suitable for observational studies (Orstavik et al. 1986). The index allows calibration between the observers and comparisons of the inter-observer validity (Huumonen 2002). In addition, the RET study group showed lack of standardised follow-up protocols despite the earlier suggested protocols (Jeeruphan et al. 2012; Jung et al. 2008; Wigler et al. 2013). These protocols have been adopted by the European Society of Endodontists. Feasible follow-up times are 3, 6, 12, 18, and 24 months and annually for five years (Galler et al. 2016). Unfortunately, few RET studies report follow-up periods longer than 24 months. To summarise, the heterogeneity of the study design of the included studies did not provide a definite answer for the outcomes of endodontic regenerative and apexification techniques.

In this review, clinical and radiographical outcomes of endodontic treatment in immature necrotic permanent teeth were compared with respect to regenerative and apexification techniques (calcium hydroxide and MTA apical plug). The knowledge gaps uncovered are based on the general absence of consensus within the research field. All the included studies claimed to have found one or more approaches for combating bacterial infection. Knowledge gaps were identified for regenerative endodontic treatment, apexification with MTA plug, and follow-up protocols. Further research is needed that examines treatment of traumatised immature permanent teeth with high-quality study design, larger study sizes, and longer follow-up periods. Future studies should predefine the root development stage and periapical index status (PAI index) at baseline and after treatment.