# Critical hydrodynamic force levels for efficient removal of oral biofilms in simulated interdental spaces

## TL;DR

This study finds that at least 20 Pa of hydrodynamic force is needed to start removing oral biofilms, but antiseptics are also needed for effective removal.

## Contribution

The study identifies critical hydrodynamic force thresholds for biofilm removal and highlights the need for antiseptics.

## Key findings

- Untreated biofilms had 7.7E7 CFU/harvest, significantly higher than treated groups.
- CFU reductions of two orders of magnitude occurred at hydrodynamic forces above 45 Pa.
- Hydrodynamic forces alone are insufficient; antiseptics are necessary for effective biofilm disruption.

## Abstract

Sonic toothbrushes generate hydrodynamic shear forces for oral biofilm removal on tooth surfaces, but the effective thresholds for biofilm removal remain unexplored. This in vitro study aimed to investigate various threshold values for hydrodynamic biofilm removal in vitro.

A specialized test bench was designed with a known water flow field within a gap, ensuring that hydrodynamic shear forces on the wall were solely dependent on the volume flow, which was quantifiable using an integrated flow meter and proven by a computational fluid dynamics simulation. A young 20 h supragingival six-species biofilm was developed on hydroxyapatite disks (∅ 5 mm) and applied into the test bench, subjecting them to ascending force levels ranging from 0 to 135 Pa. The remaining biofilms were quantified using colony forming units (CFU) and subjected to statistical analysis through one-way ANOVA.

Volume flow measures < 0.1 l/s: Error 1% of reading were established with the test bench. Untreated biofilms (0 Pa, no hydrodynamic shear forces) reached 7.7E7 CFU/harvest and differed significantly from all treated biofilm groups. CFU reductions of up to 2.3E6 were detected using 20 Pa, and reductions of two orders of magnitude were reached above wall shear forces of 45 Pa (6.9E5).

Critical hydrodynamic force levels of at least 20 Pa appear to be necessary to have a discernible impact on initial biofilm removal.

Pure hydrodynamic forces alone are insufficient for adequate biofilm removal. The addition of antiseptics is essential to penetrate and disrupt hydrodynamically loosened biofilm structures effectively.

The online version contains supplementary material available at 10.1007/s00784-024-05739-7.