Antimicrobial Efficacy of Indocyanine Green-Mediated Photodynamic Therapy Against Infected Root Canals with Diverse Cross-Sectional Morphologies

Objective This study aims to assess the efficacy of indocyanine green-mediated antimicrobial photodynamic therapy (ICG-aPDT) in enhancing disinfection following chemomechanical preparation in root canals infected with Enterococcus faecalis, characterized by varying cross-sectional morphologies. Methods A total of sixty extracted single-rooted teeth were categorized into groups based on canal morphology—round, oval, long-oval, or flattened—determined by the ratio of the maximum to minimum canal diameter measured 3 mm from the apical foramen (n = 15 per group). Following the establishment of a 70-day Enterococcus faecalis biofilm model, microbial samples were collected at three stages: prior to treatment, post-chemomechanical preparation using ProTaper Gold instruments, 3% sodium hypochlorite, and 17% EDTA, and subsequent to ICG-aPDT. During ICG-aPDT, indocyanine green was applied for 1 minute and irradiated with an 808 nm diode laser in pulsed-wave mode at 2.5 W for two 30-second cycles. Colony-forming units were quantified, antibacterial efficacy was calculated, and representative specimens were analyzed using scanning electron microscopy. The irradiation protocol was determined through external root-surface thermography and DPBF-based reactive oxygen species detection. Results Under the specified irradiation conditions, the peak external root-surface temperature reached 44.7°C, and a time-dependent decline in DPBF absorbance was observed, indicating the generation of reactive oxygen species. Baseline bacterial counts were statistically similar across the four groups (p = 0.987). Following chemomechanical preparation, the antibacterial rates were 84.4 ± 2.1% for round canals, 78.4 ± 3.4% for oval canals, 66.8 ± 3.0% for long-oval canals, and 53.8 ± 5.4% for flattened canals, with statistically significant differences among the groups (p < 0.05). Post-ICG-aPDT treatment, the antibacterial rates increased to 97.0 ± 0.7%, 94.4 ± 1.0%, 94.2 ± 0.9%, and 93.8 ± 1.1%, respectively. The final antibacterial rate remained highest in round canals, while no significant differences were observed among the three non-round canal groups (p = 0.268). Conclusion The cross-sectional morphology of root canals significantly affected the antibacterial efficacy of chemomechanical preparation. ICG-aPDT further reduced viable bacterial counts across all canal groups and minimized the differences in final antibacterial efficacy among oval, long-oval, and flattened canals. These findings warrant further exploration of ICG-aPDT as an adjunctive disinfection strategy.