Mechanisms of Atomization from Rotary Dental Instruments and Its Mitigation

Abstract

Since the onset of coronavirus disease 2019, the potential risk of dental procedural generated spray emissions (including aerosols and splatters), for severe acute respiratory syndrome coronavirus 2 transmission, has challenged care providers and policy makers alike. New studies have described the production and dissemination of sprays during simulated dental procedures, but findings lack generalizability beyond their measurements setting. This study aims to describe the fundamental mechanisms associated with spray production from rotary dental instrumentation with particular focus on what are currently considered high-risk components-namely, the production of small droplets that may remain suspended in the room environment for extended periods and the dispersal of high-velocity droplets resulting in formites at distant surfaces. Procedural sprays were parametrically studied with variables including rotation speed, burr-to-tooth contact, and coolant premisting modified and visualized using high-speed imaging and broadband or monochromatic laser light-sheet illumination. Droplet velocities were estimated and probability density maps for all laser illuminated sprays generated. The impact of varying the coolant parameters on heating during instrumentation was considered. Complex structured sprays were produced by water-cooled rotary instruments, which, in the worst case of an air turbine, included droplet projection speeds in excess of 12 m/s and the formation of millions of small droplets that may remain suspended. Elimination of premisting (mixing of coolant water and air prior to burr contact) resulted in a significant reduction in small droplets, but radial atomization may still occur and is modified by burr-to-tooth contact. Spatial probability distribution mapping identified a threshold for rotation speeds for radial atomization between 80,000 and 100,000 rpm. In this operatory mode, cutting efficiency is reduced but sufficient coolant effectiveness appears to be maintained. Multiple mechanisms for atomization of fluids from rotatory instrumentation exist, but parameters can be controlled to modify key spray characteristics during the current crisis.

Keywords: SARS-CoV-2; aerosol; aerosol-generating procedure; dental drill; imaging; infection control.

Figure 1.

Air turbine visualizations using broadband and LED light sources. (A) Still frame from high-speed imaging of an unobstructed spray from an air turbine possessing a high-velocity spray, a turbulent shear layer at the periphery of the core, and recirculation regions close to the burr tip. (B) Same image false-colored with the central spray core in orange, a shear layer region located outside of the central core region in yellow, and recirculation regions in blue. A group of high-droplet velocity straight trajectories is shown by the red streaks toward the top of the image. (C) When the same instrument is placed inside a simulated oral cavity (palatal to the maxillary central incisors), a turbulent fine mist of a reduced but significant velocity is produced (principal direction indicated by the yellow arrow).

Figure 2.

Air turbine visualizations using laser sheet optics. (A) An instantaneous image of the spray formed by an air turbine unobstructed running in a steady state. (B) Probability distribution of the spray droplet concentration based on >2,000 images. Pixels that are red indicate a 100% chance of encountering a droplet at any point in time, and pixels that are black represent 0%. (C) Standard deviation, plotted on an equivalent scale.

Figure 3.

Quantification of spray generation using laser sheet optics. (A) Probability density function (PDF) and (B) associated SD maps of droplet concentration for a micromotor with a 5:1 speed-increasing handpiece run with no “chip air” rotating between 200,000 and 20,000 rpm. PDF maps are based on >2,000 images for each modality with the instrument running unobstructed in steady state. At 200,000 rpm, most of the droplet velocity is ~1.4 m/s, which is an order of magnitude less than an air turbine. PDFs of droplet concentration (C, top line) and SD (C, bottom line) of the concentration fluctuations for a micromotor run with no “chip air” rotating at decreasing speed with the burr tip in contact with wet enamel. Distributions are again based on >2,000 images for each modality. Spray distribution and trajectories are modified by tooth contact. There is evidence at higher speeds that atomization near the tooth surface occurs; however, with decreasing speed, the coolant largely streams over the tooth surface with a limited number of low-velocity droplets being deposited within the imaging field of view. Scale bars are equivalent to 50 mm in all images.

Figure 4.

Quantification of spray generation as a function of speed of burr revolution. (A) Proportion of image field where the probability density function of droplet concentration is greater than 1% plotted against micromotor speed, for the entire imaging field in (B) (black-colored bars) and the left side of the imaging field (gray-colored bars), which is ~90 mm from the burr tip. For comparison, equivalent values for an air turbine are plotted as diamonds at the right of the histogram. We observe that reduction of micromotor speed to 60,000 rpm results in, at a distance of >90 mm away from the burr, only 0.1% of the imaged pixels having >1% chance of encountering a droplet. This represents a ~280-fold reduction compared with an air turbine. (C–E) Representative sprays with an elimination of radial atomization below 100,00 rpm. (F, G) Histograms showing droplet particles sizes associated with non-premisted micromotor speed (rpm) compared with ambient baseline (BL) and air turbine (AT) with premisted coolant.

Figure 5.

Representative images showing spray formation when mechanisms identified to cause atomization are mitigated, achieved here by running an electric micromotor with a 5:1 speed-increasing handpiece with “chip air” blocked and revolutions restricted to 60,000 rpm. The burr was held in contact with the palatal surface of right maxillary central incisor (A), lingual surface of right mandibular central incisor (B), and the occlusal surface of the right mandibular first molar (C).