Explaining the working mechanism of laser-activated irrigation and its action on microbial biofilms: A high-speed imaging study.

Aim: Laser-activated irrigation (LAI) using pulsed erbium lasers has been studied with regard to canal cleaning, but its working mechanism remains poorly understood. This study sought to unravel the method of action of LAI and to assess its effect on bacterial biofilms in a root canal model, by means of high-speed imaging.

Methodology: A root canal model consisting of dentine and glass walls was used. Visualization of the canal space during activation was achieved with a high-speed camera, capturing 20-s activation series at 50 000 frames per second. Recordings were made of canal models filled with water, models filled with water containing glass microspheres, and models with a biofilm (an undefined biofilm originating from oral samples, a 1-week-old Enterococcus faecalis biofilm or a 11-day-old multispecies biofilm) grown on the dentine walls. LAI parameters were 2940 nm, 15 Hz, 50 μs, 20 mJ and 400 μm conical tip held at orifice level. Quantitative (measurement of size, life time and timing of cavitation bubbles; velocity and amplitude of root canal content movement) and qualitative (descriptive) analysis of the intracanal events was performed using imaging software.

Results: During the implosion of the primary bubble, smaller cavitation bubbles emerged throughout the entire canal. This process began in the coronal canal part and continued in the apical direction. Expansion of these bubbles was followed by an implosion, and this volumetric change over a time span of a few 100 μs resulted in a very rapid vertical movement of the canal content with a mean amplitude of 900 μm. The succession of these movements with every pulse, resulted in biofilm detachment from the root canal walls and the gradual displacement of fragments coronally, until their complete removal. The pattern of the biofilm removal was the same for all groups. LAI was able to remove biofilm from the root canal models.

Conclusions: The hydrodynamic effect of LAI is based on the generation of small cavitation bubbles throughout the entire canal, far from the primary bubble. Their volumetric oscillation results in a small yet very fast vertical movement of the root canal content and local liquid streaming on each pulse, resulting in biofilm detachment and coronal displacement.