Primary-to-first-rebound bubble dynamics of plasma-mediated Nd:YAG and thermally driven Er:YAG configurations in 17% EDTA solution

Technical Summary: Primary-to-First-Rebound Bubble Dynamics in 17% EDTA

Problem Statement
Endodontic irrigation relies on chelating agents like 17% ethylenediaminetetraacetic acid (EDTA) to remove the inorganic smear layer. While laser-activated irrigation (LAI) utilizes expanding and collapsing bubbles to enhance fluid exchange in anatomical recesses, the physical response of the irrigant is contingent on both the activation source and the liquid composition. Previous studies have established that solute-dependent optical breakdown and bubble dynamics differ between distilled water and EDTA, yet specific comparisons between plasma-mediated Nd:YAG and thermally driven Er:YAG activation modes within 17% EDTA were lacking. Furthermore, the relationship between the primary bubble and its first rebound—specifically whether the rebound is a uniform geometric scaling of the primary event—had not been characterized for these configurations in this specific preparation.

Methodology
The study employed high-speed imaging (50,000 frames/s) to analyze 90 independent bubble events in an unconfined 17% EDTA solution within a quartz cuvette. The experimental design compared nine configurations across two initiation families:

1. Plasma-mediated Nd:YAG (LIPA): Q-switched 1064-nm laser with pulse energies of 100, 200, and 300 mJ (10 ns pulse duration).
2. Thermally driven Er:YAG: 2940-nm laser with two pulse modes:
   • Ultra-short-pulse (USP): 25 µs duration at 10, 15, and 20 mJ.
   • Super-short-pulse (SSP): 50 µs duration at 10, 15, and 20 mJ.

Note that LIPA was not energy-matched to the Er:YAG configurations.

Bubble dynamics were quantified using software-exported ellipse-equivalent projected area ("Size"), equivalent diameter, timing (time to maximum, lifetime), and beam-referenced axial and transverse lengths. Statistical analysis included Kruskal–Wallis tests, Mann–Whitney comparisons with Holm adjustment, and factorial analysis for the Er:YAG modes. Rebound geometry was analyzed via rebound-to-primary diameter ratios and paired log-aspect changes to assess shape shifts.

Key Contributions and Results
The study characterized distinct primary-to-first-rebound profiles across the tested configurations:

• USP vs. SSP (Shared Settings): At every shared energy setting (10, 15, and 20 mJ), the USP mode produced significantly larger primary and first-rebound projections compared to SSP. USP exhibited higher rebound/primary diameter ratios (sample Cliff's δ = 1.00; adjusted p ≤ 0.0057).
  • Primary Size: USP primary area medians increased from 4.310 mm² (10 mJ) to 6.513 mm² (20 mJ).
  • Rebound Size: USP rebound area medians increased from 2.520 mm² to 3.540 mm².
  • Timing: USP events demonstrated longer time-to-maximum and longer lifetimes for both primary and rebound phases compared to SSP.
• SSP Energy Response: While SSP primary-area medians increased by 41.9% from 10 to 20 mJ, the rebound-area medians remained relatively stable (1.150 mm² at 10 mJ vs. 1.139 mm² at 20 mJ). Consequently, the rebound/primary diameter ratio for SSP declined significantly from 0.629 to 0.535 (adjusted trend p = 0.000300).
• LIPA Characteristics: LIPA configurations occupied larger absolute size ranges than the Er:YAG modes but were not energy-matched, serving as a reference rather than a controlled mechanism test.
• Geometric Anisotropy: The study found that rebound geometry is not a uniform scaling of primary geometry.
  • LIPA: Rebounds were axially dominant (aspect ratio > 1).
  • USP: Rebounds were transversely dominant (aspect ratio < 1).
  • SSP: Exhibited a shift from transverse dominance at 10 mJ to axial dominance at 20 mJ.
  • Paired shape analysis confirmed significant transverse shifts in all USP groups and SSP10, and axial shifts in SSP20 and LIPA100/200.

Significance and Claims
The paper claims to provide a preparation-specific physical baseline for laser-generated bubbles in 17% EDTA, distinguishing between plasma-mediated and thermally driven initiation pathways. The authors emphasize that:

1. Initiation Mechanisms: While distinct profiles were observed across initiation families, the authors explicitly state that these labels do not establish persistent plasma, pure vapor in a rebound, or the absence of heating during LIPA. The observations do not isolate initiation mechanisms as the sole cause of the differences.
2. Non-Uniform Scaling: The first rebound is not a simple geometric scaling of the primary bubble; absolute rebound size, relative size, and projected anisotropy respond differently to changes in pulse energy and duration.
3. Limitations of Extrapolation: The authors explicitly state that these findings do not establish clinical efficacy, energy retention, pressure safety, or an EDTA-versus-water effect.
4. Contextual Role: The findings serve as a foundational dataset for future confined-canal studies but do not directly measure cleaning efficacy or dentine safety, as the experiment was conducted in an unconfined liquid without a mineral substrate.

The study concludes that configuration-specific profiles must be considered when evaluating laser-activated irrigation, and that the relationship between primary and rebound bubbles is complex and mode-dependent.