Influence of temporal delay in SWEEPS dual-pulse Er: YAG laser-activated irrigation on fluid dynamics in confined...

The shock wave-enhanced emission photoacoustic streaming (SWEEPS) technique delivers synchronized dual-pulse Er: YAG laser energy to enhance fluid dynamics during laser-activated irrigation. The temporal delay (Tp) between consecutive pulses is a key parameter, but its influence on fluid dynamics within a confined root canal environment remains unclear. This in vitro study investigated the effects of different Tp settings on bubble morphology and transient apical peak pressure (TAPP) in a simulated root canal model. A transparent single-rooted resin anterior tooth model (apical diameter: 0.30 mm, taper: 6%, root canal length: 12 mm) was fabricated. The root canal was filled with deionized water. SWEEPS dual-pulse Er: YAG laser irrigation (20 mJ/pulse, 20 Hz) was performed at five Tp settings: 120, 210, 300, 400, and 500 µs. Bubble dynamics within the root canal were recorded using a high-speed camera system (200,000 fps). The grayscale integral of bubble images was calculated using MATLAB as a surrogate for relative bubble volume. Simultaneously, TAPP at the root apex was measured using a high-frequency pressure sensor (50 kHz). Four repeated measurements were performed per Tp group. Data were analyzed using one-way ANOVA followed by Tukey's HSD post-hoc test (α = 0.05). Both bubble morphology and TAPP varied significantly with Tp. At Tp = 120 µs, the bubbles generated by the first and second pulses coalesced into a single larger bubble, whereas at Tp = 300-500 µs, two independent bubbles formed sequentially. The grayscale integral (secondary cavitation volume surrogate) was significantly higher at Tp = 120 µs ((0.074 ± 0.010) ×10⁶ pixel) than at 210 µs ((0.052 ± 0.012) ×10⁶ pixel), P < 0.05), and significantly higher at 500 µs ((2.767 ± 0.195) ×10⁶ pixel) than at 300 µs ((1.926 ± 0.06) ×10⁶ pixel) and 400 µs ((1.986 ± 0.135) ×10⁶ pixel, both P < 0.05). TAPP followed a similar pattern: 120 µs produced significantly higher apical pressure than 210 µs (3.520 ± 0.291 vs. 2.835 ± 0.047 kPa, P < 0.05), and 500 µs produced significantly higher pressure than 300 and 400 µs (3.398 ± 0.068 vs. 2.807 ± 0.099 and 3.128 ± 0.182 kPa, both P < 0.05). No significant differences in TAPP were observed between 120 and 500 µs (P > 0.05). In this in vitro resin root canal model with deionized water, SWEEPS dual-pulse Er: YAG laser-activated irrigation with Tp = 120 or 500 µs produced greater secondary cavitation volume and higher apical pressure than intermediate Tp values (210-400 µs), indicating that Tp significantly influences fluid dynamics under the tested experimental conditions.