Surgical procedure
CAVITY
All surgeries were performed under local anesthesia (2% lidocaine hydrochloride with 1:100,000 epinephrine) by a single operator using modern microsurgical techniques under a dental microscope (OPMI pico; Carl Zeiss, Gottingen, Germany). After flap elevation, an osteotomy was performed using rotary instruments to expose the root apex. The inflamed tissue was debrided using hand instruments. The apical 3 mm of the root end was resected using a high-speed bur and a 3-mm root-end cavity was prepared using an ultrasonic tip (JETip; B&L Biotech, Ansan, Korea). The retrograde filling was performed using a calcium silicate-based cement (Endocem; Maruchi, Wonju, Korea). In the experimental groups, the apical bone defects were covered with the respective materials in the form of membranes (PRF or CGF). The same surgical procedure was performed in the control group, except that no additional membrane was used. Finally, the flap was replaced and sutured using 6 − 0 nylon (Fig. 
Surgical procedure 