Continuous absolute-time PIV of 1940-nm thulium fiber laser irrigation: recurrent bidirectional motion and diameter–position associations

Abstract Purpose To characterize recurrent root-canal motion across repeated thulium fiber laser (TFL) pulses and test associations of fiber diameter and orifice-relative tip position with sustained axial velocity activity. Methods A transparent chamber–canal model evaluated four fiber diameters (150, 200, 275, and 365 µm) at 1 mm above, at, or 1 mm below the canal orifice. Six independent 0.5-s acquisitions were analyzed per condition (72 total). Each acquisition contained 40 pulses (30 mJ, 80 Hz) and 5,000 frames at 10,000 frames/s without phase locking. The primary endpoint was noise-adjusted velocity–time magnitude over the 1.0–9.5-mm canal depth. A robust Gamma-log model tested diameter, position, and their interaction. Results All 72 acquisitions contained both apical and coronal 1-ms states above their run-specific signed-noise deadbands. The diameter–position interaction was significant (Wald χ²(6) = 403.821, p < 0.001). The highest activity occurred with the 365-µm fiber 1 mm below the orifice (26.224 mm; 95% CI 23.977–28.682). Position associations were supported for 200 µm (Holm-adjusted p = 0.009), 275 µm, and 365 µm (both p < 0.001), but not 150 µm (p = 0.490). The overall time-block test had p = 0.1434, and no late-versus-early contrast survived Holm adjustment. Conclusion Continuous absolute-time PIV documented acquisition-scale recurrence of both directions at the study-specific deadband and a diameter–position association. These hydrodynamic findings do not establish exchange, cleaning, pressure, safety, or clinical superiority.