Revascularization of immature permanent teeth has become an important part of endodontic treatment modalities. Despite its high success rate clinically and radiographically, the concerns about post-revascularization tooth discoloration were increasing . This outcome is unfavorable and significantly impacts the quality of life in children negatively, therefore materials with the lowest possible staining potential should be considered in the esthetic zone.
The cause of post-revascularization discoloration is still debatable, but most fingers point to the presence of bismuth oxide in the conventional WMTA and its reaction with dental tissues, blood products, and irrigants used in revascularization procedures [16,17]. Although WMTA was developed to overcome tooth discoloration encountered with the use of the Grey mineral trioxide aggregate (GMTA), several in vitro studies had reported evidence of tooth discoloration [14, 18].
Accordingly, NeoMTA was suggested as coronal plug material in the intervention group, since it is a pure WMTA that is marketed by having a new non-staining formulation, due to replacing the radiopacifier (Bi2O3) with (Ta2O5), moreover, it has been reported that it has better handling characteristics and forms putty consistency upon mixing . Therefore, it was recommended to investigate the material performance in various clinical applications.
The inclusion and exclusion criteria of the present study have been determined according to the AAE  clinical considerations for regenerative procedures. Teeth showing severe coronal fracture were excluded from the study because the pulp space won’t be available for post retention and the VITA Easyshade probe’s diameter is 5 mm so, at least 5 mm of intact cervical enamel was needed to evaluate color changes . Teeth with luxation injuries were excluded since the stem cells from the apical papilla (SCAPs) and Hertwig epithelial root sheath (HERS) might be damaged .
The exclusion of cases with unacceptable discoloration compared with the contralateral tooth was done in order not to affect the evaluation of discoloration that might occur. The unacceptable discoloration limit for exclusion of cases (ΔE more than 5) was determined by following the intraoral perceptibility and acceptability tolerances for color mismatch reported in the literature, a difference of ΔE starting from 5.5 to 6.8 was required for dentists to find the color mismatch sufficiently unacceptable to suggest further treatment to improve it [21, 22].
The radiographic standardization was done by the construction of a radiographic stent for bite registration to facilitate consistent positioning of the film. Moreover, (XCP) alignment system was used for a paralleling technique to allow comparisons of the digital radiographs [2, 23]. A radio-opaque marker of known dimension was embedded in the acrylic resin stent to help in the determination and compensation of the magnification factor of the radiographic images thus minimizing errors in the measurement of root length .
Treatment procedures of the selected teeth were performed according to AAE  clinical considerations for a regenerative procedure. Lower concentrations of NaOCl (1.5 % concentration) were advised and recommended by AAE  to keep the balance between sufficient disinfection and preservation of SCAPs. Since it has been proven that using higher concentrations of NaOCl has a profound negative effect on the survival and differentiation of SCAPs or expression of proteins of the dentin matrix that are involved in promoting tissue regeneration [24,25,26].
It was recommended by the AAE  to set the second appointment within 1–4 weeks from the first appointment according to the assessment of the response to initial treatment, Since it was a must to standardize the time of the second appointment for all patients to limit any variables the maximum time interval given by the AAE was set for all patients to be able to take a decision either to proceed for a second visit or consider additional treatment time with antimicrobial.
The baseline color assessment was done at the beginning of the second appointment before the teeth are exposed to dehydration as recommended by Burki et al.  since most dental procedures lead to teeth dehydration which alters tooth shade and shows perceivable color change causing errors in color assessment.
Clinical follow-up protocol was planned to be after 1 week, 1 month, 3 months, 6 months, and 12 months in accordance with previous literature assessing and evaluating discoloration quantitatively [10, 28, 29].
The covid-19 pandemic lockdown caused unique challenges for the clinical trial community around the world . The effect of lockdown was noticed in the present study in the 6 months’ records as all patients were not able to attend the clinic. Also, it led to the drop out of 5 children having 8 treated teeth as their parents were unwilling to come for completion of the follow-up period even after the end of lockdown although they didn’t have their final composite build-up. Since the dropout was within the number calculated in the sample size, there was no need to compensate any of the dropout patients.
In the present study, boys to girls distribution were 18 boys (60%) and 12 girls (40%) respectively. This finding could be related to the results observed by El-Kenany et al.  that indicated a gender difference in injury rates among 8–12 years old school children in Egypt. This might be related to boys’ tendency of choosing more energetic, active, and vigorous outdoor games.
Regarding the clinical outcomes, both groups were (100%) clinically successful as there were a complete absence of pain on biting, pain on percussion, swelling, sinus/fistula, and mobility in both groups during all follow-ups. This could be explained by the standardized disinfection protocol and the good coronal seal used in both groups. The clinical outcomes were consistence with the findings of Aly et al.  and Rizk . On the contrary, a study done by Linsuwanont et al.  reported a lower clinical success rate as (76%) of cases were successful, the author explained that the root canal disinfection protocol was not effective.
Evaluation of discoloration was done in the present study by 3 methods. Concerning the discoloration as reported by the parents, only one parent (9.1%) in Group (N) and surprisingly only three parents (27.3%) in Group (W) reported discoloration at the end of 12 months. This result was not in line with Nazzal et al.  who reported that 7 parents out of 12 observed discoloration although double antibiotic paste (DAP) and non-bismuth-containing Portland cement were used.
Visual assessment of discoloration by the two assessors at 12 months was similar to the results reported by parents. However, assessors reported discoloration at an earlier time this was due to the fact reported by Vichi et al.  that skilled operators perceive color difference at earlier levels than untrained observers.
Although NeoMTA showed less discoloration potential compared to WMTA angelus, there was no statistically significant difference between both groups. Little information is currently available on NeoMTA, the results of the NeoMTA group were in line with the conclusion of the invitro study done by Camilleri,  that NeoMTA has better color stability than conventional WMTA because of its new formulation that replaced the bismuth oxide with tantalum oxide. The result of discoloration outcome of the WMTA in the present study was in agreement with Nagata et al.  that reported only 3 teeth out of 11 (27.3%) showed discoloration in revascularization protocol using non-staining intracanal medication and WMTA (Angelus) that was placed above collagen matrix. On the contrary, other clinical studies reported a higher percentage of discoloration in revascularization procedures using non-staining intracanal medication and WMTA as Kahler et al.  reported discoloration in 10 cases out of 16 and Aly et al.  reported discoloration in (53.84%) of cases.
These contradictory results might be explained by slightly different clinical procedures. First, it was found that in the present study WMTA was not in direct contact with the blood clot, it was placed on collagen matrix, while in the aforementioned clinical studies, WMTA was in direct contact with blood without collagen matrix. The use of synthetic resorbable matrix over the blood clot, separating it from the barrier materials was suggested by Žižka et al.  and Wei X et al.  as a way for minimizing discoloration in REP, but no available clinical trials confirmed this postulation.
The later postulation was supported by Lenherr et al.  and Žižka et al.  that WMTA and all calcium silicate-based cements containing bismuth oxide or other radiopacifiers showed exacerbated discoloration after contact with blood due to absorption of blood disintegration products into the porosities of freshly unset materials, Thus, concluded that allowing the coronal plug materials to set away from blood is beneficial in decreasing discoloration.
The second explanation for the conflicting results of WMTA might be related to the position of WMTA in relation to CEJ. In the present study, WMTA was placed accurately just below CEJ with the aid of collagen matrix, While Kahler et al.  and Aly et al.  reported that controlling the position of WMTA above the blood clot was technically difficult and the authors justified that one of the possible causes of discoloration was the presence of WMTA above the CEJ. This was in line with Žižka et al.  recommendation that placing the coronal barrier just below CEJ makes the discoloration of tooth structure be covered by bone or gingiva.
Concerning the quantitative assessment of color change. The smaller the ΔE value, the lower the color difference between the initial and final color of the tooth over time. The mean ΔE value showed no statistically significant difference between both materials during the 12 months, this might be explained by that ΔE quantifies the color change but not the direction. It is only when the different components of color, L* a * b*, are analyzed individually that the nature of the color change can be identified.
The direction of color change in the WMTA group towards decreased luminosity, and reduction of redness and yellowness thus increasing the tendency towards green and blue direction agreed with Ioannidis et al.  and Esmaeili et al. , however, the present study showed less values of change. This difference in values could be explained by different study settings, as those studies were performed in laboratory conditions, which do not reflect the actual clinical conditions. As in the present study WMTA was applied carefully in areas of aesthetic concern just below CEJ to reduce the risk of material-induced tooth discoloration and the discoloration of tooth structure might be covered by bone or gingiva.
Esmaeili et al.  clarified that variable amounts of color change were reported with the same formulation of WMTA would be as a result of different thicknesses of the remaining tooth structure, colorimetric method of measurement, and material application methods. Accordingly, direct comparisons and interpretation of the available data concerning ΔE and different components of color, L* a * b* values were difficult to be performed due to the different experimental methodologies.
The proposed limit for color difference adopted in this study was set at 3.7 ΔE units (perceptibility threshold), as there was a lack of consensus in the literature about the perceptibility threshold set value ranging between 2.6 and 3.7, it was set at the highest value in the present study since it was suggested that clinicians are more tolerant of color difference in a clinical scenario than under controlled in vitro conditions . Therefore, although the mean ΔE value was more in the WMTA group than the NeoMTA group both materials were below the perceptibility threshold.
In the present study, the ΔE value of WMTA was in agreement with the invitro study conducted by Rouhani et al.  and Beatty & Svec,  that reported the ΔE value of WMTA was 3.54, and 2.6 respectively. While in terms of visual perception, they were not consistence together since the perceptibility threshold value was different.
On the contrary Ioannidis et al.  reported a higher ΔE value for WMTA than the present study this might be explained by Žižka et al.  that if recommendations are meticulously followed the level of discoloration of teeth can be minimized below the human eye threshold.
On the other hand, a clinical study done by Nazzal et al.  reported that post-revascularization discoloration occurred with ΔE value = 7.9, although the intracanal medication was minocycline free and the coronal plug material was non-bismuth containing Portland cement that was placed directly above blood clot. This supports the postulation that the cause of discoloration is multifactorial and doesn’t depend solely on the radio pacifier of coronal plug material or the intra-canal medication.
Regarding the radiographic outcomes, the mean initial pre-operative root length of teeth showed no statistically significant difference between both groups, which indicates that the 2 groups were comparable and there was no variability between the selected teeth in both groups.
The change in root length results were generated in millimeter units and as percentage change from preoperative values as well, this was to provide a more conservative analysis as each case is normalized to its pre-operative measurement. In addition, the units of percentage change provide a clinically meaningful outcome when considering the impact of REPs .
Continued root lengthening observed in this study was in consistence with the results of Nagy et al. 2014  reported the mean increase in root length was 0.8 ± (0.5) mm, 5% and also was comparable to Aly et al.  results as the mean increase in root length was 0.7 (± 0.23) mm, 5.02% in immature teeth treated with the same protocol as the present study using (DAP) and WMTA.
On the contrary Mittmann et al.  reported that the percentage of increase in root length was 0.96% during 22 months follow-up. This contradictory data can be explained by different case selections as the aforementioned study done by Mittmann et al.  included luxation injuries and replanted avulsed teeth that have a higher risk of (HERS) or apical papilla injury, which are described as the most important elements controlling continuing of root development.
There was no statistically significant difference in the increase in root length in mm and percentage between the two groups. This might be explained by the postulation of Kharchi et al.  that regardless of the REP details, the single most important variable for further root development in the regeneration procedure is the disinfected root canal, whenever a sufficiently disinfected environment was created this allows a biological response of appropriate cells to support the continued root development.
All teeth in both groups were free of internal and external root resorption. However, Mittmann et al.  reported (56.3%) of teeth developed root resorption, but this could be explained by different case selections as resorption is more likely to be due to great damage to periodontal ligament due to luxation injuries rather than by the revascularization treatment.
All teeth in both groups showed a complete absence of any radiographic signs of infection, accordingly, all the teeth in both groups were having 100% radiographic success and 100% overall success rate, this was in agreement with Kahler et al.  and Rizk,  reported 100% overall clinical and radiographic success rate, and was comparable to systematic review and meta-analysis done by Koç & Del Fabbro,  reported that overall success rate of revascularization procedures in teeth with traumatic injuries was 94.8%. This shows that the primary outcome of revascularization (elimination of clinical symptoms and resolution of apical periodontitis) can be easily achieved as long as the infection is controlled .
There was no statistically significant difference in the overall success rate between the two groups but this could be explained by the fact reported by Khan et al.  that NeoMTA is a pure MTA with a new formulation having alternative radiopacifiers, so it is exhibiting physical and chemical properties compared with conventional WMTA.
COVID-19 pandemic lockdown affected the attendance of patients to follow-up.
Comparing and interpreting the results of the present study with other clinical studies was difficult because clinical protocols among the previous studies were widely varied.
The data from in-vitro or in-vivo studies on NeoMTA was scarce.
What this paper adds
Both NeoMTA and conventional WMTA were successful coronal plug materials in the revascularization of non-vital immature permanent teeth achieving a high level of clinical and radiographic success.
NeoMTA is a promising coronal plug material that can be used for revascularization procedures in the esthetic zone as it showed less discoloration potential compared with conventional WMTA, however, there was no statistically significant difference between both groups.
Coronal discoloration in the regenerative endodontic procedure is a multifactorial process not only related to the composition of materials used but other factors might minimize or exacerbate the discoloration potential.
If clinical guidelines and recommendations were scrupulously followed, the degree of teeth discoloration might be reduced below the human eye perceptibility threshold.