The main cause of endodontic disorders is microbial infection. From the infected root canal, microorganisms can penetrate into the deeper layers of root dentine and propagate a periapical tissue through the apical foramen and lateral canals. Eliminating microorgnims is the primary objective of endodontics in order to create an environment as free of bacteria as possible. This will favor the environment for healing without the need of periapical surgery in some resistant cases.

Disinfection in root canal retreatment (RCR) is much more difficult because persistent microorganisms have settled in the root canal. The resistant periapical lesions result from resistant bacteria to antimicrobial agents. They can survive for years around the filled root canals. Sodium hypochlorite (NaOCl) is the most widely used irrigant in root canal therapy due to its strong antibacterial effect and its ability to dissolve organic substances. Effective canal cleaning is difficult to achieve without the use of NaOCl at a sufficiently high concentration. However, NaOCl has several drawbacks, including its cytotoxicity which can lead to tissue damage and patient symptoms. Additionally, its strong oxidizing nature negatively affects the mechanical properties of dentin such as microhardness and elastic modulus. NaOCl should be used with caution in endodontic procedures to prevent hypochlorite accidents.

Calcium hydroxide (Ca (OH)2) is the most widely utilized intracanal medication. It has the potential to dissolve tissue, acts as a physical barrier and generates hydroxyl ions, creating an extremely alkaline environment. It has been shown to be quite effective in the treatment of teeth with persistent periapical lesions. To provide optimal endodontic treatment, the root canal system should be thoroughly cleaned of soft-tissue debris, smear layer, and bacteria. However, it is impossible to completely disinfect and clean debris that build up. That is why, adjunctive aids, such as the use of passive ultrasonic and sonic activation of the irrigant and lasers with varying wave lengths, have been introduced during conventional endodontic therapy in cleaning maneuvers.

[ Laser therapy for root canal disinfection has gained popularity since laser-assisted root canal treatment aims to remove the smear layer, penetrate deep into dentin and eliminate bacteria up to 1000 μm . Laser types have been shown to reduce bacterial load when used as an adjunct to conventional root canal treatment such as Nd:YAG , Er:YAG and diode laser . Diode laser exhibits a powerful antibacterial effect by altering the bacterial cell wall and damaging the cell membrane. Its photothermal action targets accessible bacteria, while its photodisruptive properties affect those in less accessible areas. This mechanism may not cause immediate bacterial death but instead induces sublethal damage that inhibits bacterial growth. By compromising the integrity of the cell wall and causing the accumulation of denatured proteins, bacterial growth is halted, eventually leading to cell lysis. These effects are achieved with minimal does of heat . Researches have demonstrated differing degrees of effectiveness in achieving root canal disinfection with the use of diode lasers.

Diode laser is highly well-suited for root canal therapy due to its infrared wavelength and the use of a thin, flexible optic fiber tip (200 µm), which enables deep penetration into the root canal system. This design allows for efficient delivery and distribution of laser energy, improving disinfection and reducing microbial load. Furthermore, the compact size and affordability of diode lasers make them practical for use in general dental clinics and increasing their adoption in routine endodontic procedures. A study demonstrated that 980nm diode laser achieved bactericidal effect ranging from 77 to 97% in root canals infected with E. faecalis using energy outputs of 1.7, 2.3 and 2.8 W. Antimicrobial effect was related to the amount of energy and dentin thickness. Furthermore, several studies have confirmed that diode laser can serve as an adjunct to conventional root canal therapy in cases with necrotic pulp and periapical radiolucency because of the bactericidal effect, the ability to penetrate through the dentinal thickness, and the ability to vaporize soft tissues .

Radiographic examination provides essential information on post-treatment periapical healing, where the absence of periapical radiolucency remains a primary marker of successful therapy. Intraoral digital periapical radiograph remains the most widely used imaging modality in endodontics due to their accessibility and reliability. These radiographs provide valuable insights into dentoalveolar structures, allowing clinicians to assess root morphology, canal anatomy, and quality of the treatment. However, the primary limitation lies in its two-dime](streamdown:incomplete-link)

Cone beam computed tomography (CBCT) has become a cornerstone in endodontic imaging, providing high-resolution, three-dimensional visualization of complex anatomical structures. CBCT enhances diagnostic precision and treatment planning beyond what is achievable with two-dimensional radiograph. It has been shown to be more accurate than digital periapical radiographs in the long-term assessment of root canal treat¬ment success. Moreover, CBCT demonstrates high accuracy in determining the volume of periapical pathosis making it a valuable tool in periapical healing evaluation. Therefore, this study aims to evaluate the effect of 810 nm diode laser on periapical healing after root canal retreatment in failed endodontic cases with chronic periapical lesion using CBCT. The null hypothesis is that there is no difference in periapical healing between the tested groups following root canal retreatment.

Inclusion Criteria:

• Patients have single rooted teeth with root canal form type I.
• Have previous endodontic therapy with failure.
• Have periapical radiolucency (PAI score of 3 or 4).
• Patients who provide written informed consent.

Exclusion Criteria:

• Patients who had received antibiotic therapy within the past month.
• Pregnancy and lactation.
• Systemic disease.
• Physical or mental disability.
• Non restorable teeth.
• Any signs of resorption, immature roots, fracture, perforation.
• Bone metabolism disease and/or patients using drugs that affect bone metabolism (such as steroids and bisphosphonates)