Following the randomization procedure, patients will be divided into 2 groups (12 immature molars in each group): Group I will be treated with MTA as apical plug, whereas group II will be treated with Biodentine as apical plug. After working length determination, instrumentation and sterilizing root canals, apical plug will be done as following:

Biodentine Group: Biodentine will be placed into the apical portion of canals. After 12 min the coronal and middle third of the root canal will be filled with gutta-percha and AH Plus. The coronal restoration will be completed with GIC, composite, and stainless-steel crown.

MTA Group: MTA will be placed into the apical portion of canals, then a moist cotton pellet will be placed and the access cavity will be restored with temporary filling. Next day, temporary filling and the cotton pellet will be removed, then the coronal restoration will be performed as Biodentine group.

Patients of both groups will be recalled for clinical and radiographical follow-up within 12 months after treatment.

Following the randomization procedure, patients will be assigned sequential numbers in the order of enrolment. Group I will include 12 immature molars treated with MTA (White ProRoot MTA, Dentsply, Tulsa, OK, USA) as apical plug, whereas group II will include 12 immature molars treated with Biodentine (Septodont, Saint-Maur-des-Fosses, France) as apical plug. All the apexification treatments will be performed by the researcher. Under local anesthesia and rubber dam isolation, all superficial caries will be removed with carbide fissure bur mounted on a low-speed hand-piece. Access cavity will be prepared using an endo-z bur mounted on high-speed hand-piece, and the cavity will be rinsed with 2.5% NaOCl. Working length will be calculated radiographically with K-files (Mani, INK, Japan) and recorded as reference. root canals will be instrumented gently with Fanta AF3 (Fanta Dental Materials Co., LTD, Shanghai, China) and copious irrigation with 1.3% sodium hypochlorite (NaOCI) (Merck, Darmstadt, Germany) by a 30- gauge endodontic irrigating needle (Sybron Endo, Crop, Orange, CA, USA). As final irrigation all canals will be filled up with NaOCl 1.3% and activated with #40 U-file ultrasonic tip (Zipperer Co., Munchen, Germany) for 30 seconds for each canal, then all canals will be irrigated with normal saline, then all canals will be filled up with Q-mix (Dentsply Tulsa Dental, Tulsa, OK, USA) and activated with #40 U-file ultrasonic tip (Zipperer Co., Munchen, Germany) for 10 seconds. After drying with large sterile paper points apical plug will be done according to randomization MTA with 2 visits or Biodentine with one visit.

Biodentine Group (performed in a single clinic visit of approximately 45 minutes): Biodentine will be placed with MAP-One System (Produits Dentaires, sa vevey, switzerland) into the apical portion of canals with about 4 mm thickness and adapted to the canal walls with an endodontic hand plugger (Dentsply, Tulsa, OK, USA). Correct placement of the Biodentine plugs will be verified with a radiograph. After 12 min the setting of Biodentine will be detected gently with #40 k-file (Mani, INK, Japan). The coronal and middle third of the root canal will be filled with gutta-percha in conjunction with AH Plus sealer (Dentsply Sirona Endodontics) using lateral condensation technique. The coronal restoration will be completed with GIC Fuji IX (GC Corporation, Tokyo, Jappan), then bonded resin composite (3M ESPE, Dental Products, St. Paul, MN, USA), and stainless-steel crown (3M ESPE, Dental Products, St. Paul, MN, USA).

MTA Group (performed in 2 clinic visits, each visit will be approximately 30 minutes): MTA will be placed with MAP-One System (Produits Dentaires, sa vevey, switzerland) into the apical portion of canals with about 4 mm thickness and adapted to the canal walls with an endodontic hand plugger (Dentsply, Tulsa, OK, USA). Correct placement of the MTA plugs will be verified with a radiograph. After inserting MTA, a moist cotton pellet will be placed in the canal, and the access cavity will be restored with temporary filling (TG, Germany). Next day, under local anesthesia and rubber dam isolation, the temporary filling and the cotton pellet will be removed and the setting of MTA will be detected gently with #40 k-file (Mani, INK, Japan). The coronal and middle third of the root canal will be filled with gutta-percha in conjunction with AH Plus sealer (Dentsply Sirona Endodontics) using lateral condensation technique. The coronal restoration will be completed with GIC Fuji IX (GC Corporation, Tokyo, Jappan), then bonded resin composite (3M ESPE, Dental Products, St. Paul, MN, USA), and stainless steel crown (3M ESPE, Dental Products, St. Paul, MN, USA).

Patients of both groups will be recalled for radiographical follow-up after the end of treatment at 1, 3, 6 and 12 months. Periapical radiographs will be obtained under standard exposure conditions (60 kVp, 7 mA and 0.32 s) using a dental X-ray machine (Gendex GX, Lake Zurich, IL, USA) and intraoral sensor (VATECH. Gyeonggi-do, Korea) . The radiographic assessment will be done with two pre-trained independent investigators. Each investigator will determine the size of apical lesion using "Image J" program and scored in both groups blindly, independently and will repeat the radiographic scoring after 1 month to assess the intraobserver reliability. Any disagreement on apical lesion size for a particular root resulted in joint evaluation until agreement will be reached.

Patients of both groups will be recalled for clinical follow-up after the end of treatment at 1 day, 3 days, 1 week, 2 weeks, 1 month, 3 months, 6 months, 12 months to detect pain, swelling, tenderness to percussion, abscess, fistula, and abnormal tooth mobility Finally, the numerical data of the apical lesions sizes and ranked data of clinical variables will be statistically analyzed using SPSS, and any statistically significant values will be investigated.

Inclusion Criteria:

• Cooperative children.
• Healthy children without any systemic disease or compromised immune status.
• Children with first immature mandibular molar with at least one canal sized #60 or more with an apical lesion.

Exclusion Criteria:

• Patients with advanced periodontitis (more than 5 mm periodontal attachment and bone loss).
• Molars that cannot be restored (root fracture, unrestorable tooth, massive internal or external).
• Molars that had previous endodontic treatment.

Triple Masking: Patients, Investigators and Statistician