Post-treatment apical periodontitis is commonly associated with persistent intracanal infection following root canal treatment. Effective disinfection of the complex root canal system during nonsurgical retreatment remains challenging due to anatomical complexities that may harbor microbial biofilms. Conventional retreatment protocols typically involve nickel-titanium rotary instrumentation combined with irrigation using sodium hypochlorite and activation techniques such as Passive Ultrasonic Irrigation (PUI).

The GentleWave System (GWS) is a minimally invasive irrigation technology designed to enhance tissue dissolution and microbial removal through multisonic energy, advanced fluid dynamics, and negative pressure. Although preliminary studies suggest promising results in root canal disinfection and patient outcomes, high-level clinical evidence comparing GWS with conventional retreatment techniques remains limited, particularly in molar teeth with post-treatment apical periodontitis.

This randomized clinical study aims to compare the disinfection efficacy of GWS with a conventional nickel-titanium rotary instrumentation protocol combined with PUI in nonsurgical retreatment of molars with post-treatment apical periodontitis. Intracanal bacterial load will be quantified using quantitative polymerase chain reaction (qPCR), and microbial composition will be analyzed using next-generation sequencing (NGS). Residual organic debris will be evaluated using the chairside diagnostic device Endocator and facultative bacterial culturing. Postoperative pain levels will also be recorded to assess short-term clinical outcomes.

The results of this study will provide clinical evidence regarding the comparative effectiveness, safety, and potential advantages of advanced irrigation technology for root canal disinfection in complex retreatment cases.

Post-treatment apical periodontitis in root canal-treated teeth is caused by persistent, secondary, or recurrent intra-canal infection. The complex anatomy of the root canal system-including isthmuses, fins, and lateral canals-can limit the effectiveness of conventional instrumentation and irrigation, allowing microbial biofilms to persist. As a result, nonsurgical endodontic retreatment often presents a lower success rate compared with primary root canal treatment.

Conventional retreatment protocols typically involve mechanical debridement using nickel-titanium rotary instrumentation combined with irrigation using sodium hypochlorite. Irrigation activation techniques such as passive ultrasonic irrigation (PUI) are commonly used to improve irrigant penetration and enhance removal of debris and bacteria from complex canal anatomy. However, these techniques may still be limited in their ability to completely eliminate microbial biofilms and may carry a risk of irrigant extrusion.

The GentleWave System (GWS) is an advanced irrigation technology that uses multisonic energy, negative pressure, and optimized fluid dynamics to enhance tissue dissolution and microbial removal throughout the root canal system. This system allows effective disinfection while minimizing the need for extensive mechanical instrumentation, potentially supporting minimally invasive endodontic approaches. Although studies have demonstrated promising results, well-designed randomized clinical trials evaluating its effectiveness in retreatment cases remain limited.

This randomized clinical trial will compare the disinfection efficacy of GWS with a conventional retreatment protocol consisting of nickel-titanium rotary instrumentation combined with PUI in molars diagnosed with post-treatment apical periodontitis. Microbiological samples will be collected before treatment and prior to obturation. Intracanal bacterial load will be quantified using quantitative real-time polymerase chain reaction (qPCR), and microbial community composition will be analyzed using next-generation sequencing (NGS). In addition, residual organic debris will be evaluated using the chairside device Endocator, which provides a rapid quantitative Endoscore to determine the degree of disinfection of each group.

The primary outcomes of the study are reduction in intracanal bacterial load and changes in microbial composition following disinfection. Secondary outcomes include the efficacy of the chairside device Endocator in disinfection of the root canal, compared with PCR-based methods as well as facultative bacterial culturing, and short-term postoperative pain assessment. These findings from this study aim to provide high-quality clinical evidence regarding the comparative effectiveness of advanced irrigation technologies versus conventional disinfection protocols in endodontic retreatment.

Inclusion Criteria:

• Molars showing radiographic evidence of post-treatment apical periodontitis: a lesion that exceeds 3 mm in diameter on one or more roots.
• Evidence from periapical and CBCT examination suggests that retreatment is a viable option, i.e., there are low risks of vertical root fracture, no major root canal obstruction, evidence of coronal leakage, and potential improvement on the previous treatment.
• The initial root canal therapy was performed at least 2 years previously, or more than 1 year if the patient has signs and symptoms or symptomatic disease (pain, swelling, or sinus tracts).
• Patients show no chronic systemic disease that may influence the outcome of care, such as uncontrolled diabetes, autoimmune disease, malignancy treated with chemotherapy or patients who are on immunosuppressive medications or require antibiotic prophylaxis before dental treatment.
• Patients who are able to read and consent in English.

Exclusion Criteria:

• Patients with a tooth requiring RCT retreatment that also presents with periodontal pocket(s) deeper than 5mm
• Patients with a tooth requiring RCT retreatment that also presents with severe crown destruction that would prevent proper rubber dam isolation or an adequate restoration
• Patients with a tooth requiring RCT retreatment that also presents with evidence of vertical root fracture, or cracks with probing defects > 5 mm
• Patients with a tooth requiring RCT retreatment that is deemed unrestorable
• Patients who received antibiotic therapy within the previous 3 months.
• Patients under the age of 16 or those incapable of providing informed consent
• Patients with a tooth requiring RCT retreatment that also presents with immature or open apices
• Patients with a tooth requiring RCT retreatment that also presents with apices in the maxillary sinus or those teeth where the apical lesion has eroded the bone of the maxillary sinus floor
• Patients with a tooth requiring RCT retreatment that also presents with apices that are less than 1 mm from the mandibular canal
• Patients with a tooth requiring RCT retreatment that also presents with internal or external root resorption

Due to the nature of the interventions, the treating clinician cannot be blinded to the treatment allocation. However, laboratory personnel performing microbiological analyses, including quantitative polymerase chain reaction (qPCR) and next-generation sequencing (NGS), will be blinded to group assignment. In addition, investigators assessing Endocator results and analyzing study outcomes will be blinded to the intervention group to minimize potential bias.