## 4 Discussion  
Under certain circumstances, revascularization may still occur in the space of the pulp cavities [13]. Revascularization of immature teeth with apical periodontitis depends mainly on: (a) an effective coronal seal; (b) a matrix for tissue ingrowth; and (c) thorough disinfection of the root canal [14]. Current revascularization commonly uses provoked apical bleeding into the pulp space as a possible source of stem cells and also for creating a blood clot [15]. It provides an autologous scaffold that contains the growth factors and has the advantages of low cost, clinical simplicity, and a short setting time [16]. However, invoking adequate apical bleeding may not always be possible, which may be related to unfavourable outcomes [17]. An alternative to creating a blood clot is the use of platelet-rich plasma, platelet-rich fibrin,  
Figure 5: Long term follow-ups. (**a**) 3-month follow-up; (**b**) 6-month follow-up; (**c**) 11-month follow-up; (**d**) 22-month follow-up; (**e**) 39-month follow-up; (**f**) and 53-month follow-up. or other autologous platelet concentrates [18]. CGF, the latest generation of autologous platelet concentrates, was first developed by Sacco in 2006. It has been widely used in the oral field as a barrier membrane, reservoir, or bio-scaffold [19]. Compared with platelet-rich plasma and platelet-rich fibrin, CGF is prepared by a simplified and optimized variable speed centrifugation procedure, which increases the probability of releasing various cytokines after rupture. CGF is also superior in degradation performance, strong clinical operability, and promotion of tissue healing [20]. NaOCI possesses favourable antimicrobial and cytotoxic effects. Lower concentrations of NaOCl are commonly used because of the fear of inadvertent extrusion into the periapical tissues [21]. [PERSON] et al. demonstrated that only 34% of the published pulp revascularization cases used full strength NaOCl (5-6%), which may be a reflection of operators' anxiety when treating immature necrotic teeth [22]. The selection of NaOCl concentration reflects the need for a balance between adequate disinfection and tissue preservation [23]. Therefore, the steps of revascularization procedures will be a compromise that allows for sufficient disinfection with minimum structural damage to cells. Lower concentrations of NaOCl (up to 3%) are advised by AAE. In this case, we further reduced the concentration of NaOCl and selected the concentration of 0.5% NaOCl because low concentrations (0.5 to 1%) of NaOCl can reduce its irritation, corrosiveness, and cytotoxicity to tissues [24]. NaOCl irrigation alone is not enough to reliably create the necessary conditions for revascularization of infectious and necrotic teeth [25]. Furthermore, there are still some limitations to syringe irrigation in addition to using a high volume of irrigate (approximately 20 mL) for more than 20 min, including the existence of the stagnation plane, difficulty in irrigating infiltration, and the disadvantages of laminar flow being able to impose shear stress on the canal walls and biofilm [26]. [PERSON] et al. reported that ultrasound-assisted irrigation was used to perform revascularization in a single visit [27]. In this case, we used an Er: YAG laser with a wavelength of 2940 nm to assist with the 0.5% NaOCl irrigation. Er: YAG laser is commonly used in root canal therapy to achieve better clinical effects [9]. The high absorption of the Er: YAG (2940 nm) by water, coordinated with the high peak power generated by a short pulse (50 u\(\upmu\)s), leads to the rapid formation, expansion, and subsequent collapse of a bubble [28]. The collapsing bubble then initiates the growth of secondary small cavitation bubbles, inducing turbulent fluid movement throughout the canal, which generates strong shear stresses against the root canal walls and improves the efficacy of debridement [9].