The root canals were gently instrumented with K-files and copious irrigation was done with 2.5% sodium hypochlorite(NaOCI) by means of a 30 gauge endodontic irrigating needle . After drying with large sterile paper points, calcium hydroxide(CH) paste was mixed with saline and applied to the root canal with a lentulo spiral filler at low speed. A cotton pellet was used to gently compress CH into the root canal and its placement was examined radiographically before placing ZOE as temporary restoration into the access cavity. After one week, CH was removed from the canal by using both the files and the irrigation with 2.5% NaOCI and 17% ethylenediaminetetraacetic acid (EDTA). A final irrigation was made with 2% chlorhexidine (CHX) before obturation. Following drying the root canal with sterile paper points, MTA was placed with a MTA Endo Gun into the apical portion of canals with a minimum 4-mm thickness and adapted to the canal walls with an endodontic hand plugger.

After using the same biomechanical root canal preparation protocol, the root canal was filled to working length with CH paste. Both clinical and radiographical examinations were performed to evaluate the barrier formation and periapical healing. When a continuous hard tissue barrier was observed apically on radiographs that was verified by clinical probing and complete or significant periapical healing was noticed, the root canal was obturated and coronary restorations were completed as done in MTA group

Apexification treatment was done with mineral trioxide aggregate

Follow-up visits were performed 12 to 48 months. Changes in apical bone density was evaluated with the modified PAI radiographic healing criteria. The clinical and radiographic healing classification was as follows:

Healed - the tooth was asymptomatic and the radiograph showed PAI 1 or 2 Healing - the tooth was asymptomatic and the radiograph showed PAI 3 or 4, with score improved at follow up from immediate post-treatment radiograph Not healed - the tooth was either symptomatic or the tooth was asymptomatic but the radiograph presented no decrease or an increase in the size of the pre-existing radiolucency at follow up from immediate post-treatment radiograph (PAI 3-5).

The treatment outcome was assessed according to the last follow-up data. And all analyses were performed according to the last follow-up data.