Study setting
The study setup within the University of Glasgow Dental Clinical Research Facility consisted of a dental mannikin head attached to a dental unit, complete with standard built-in high and low-volume suction, coolant and air-turbine functions (Additional file 1), with a separate portable ultrasonic scaling unit. As part of the hospital ventilation assessment the room was assessed as zero air changes per hour and during procedures the windows were kept closed with no mechanical ventilation.
Particulate sensors positioning and data acquisition
Two Plantower PMS5003 particulate matter sensors (Plantower, China) [20] were used for the duration of the study. These sensors utilise laser scattering to determine particle sizes and detect particle sizes within the aerosol and droplet ranges, from 0.3 to 10 µm in diameter [21], with readings provided for every 0.1 L of air intake. The sensors were used in two different arrangements. Pilot experiments identified the optimum position for sensor placement, to both maximise detection of particles escaping from the oral cavity but also minimise restriction of access for both operator and assistant. The first of these arrangements, termed the ‘single sensor’ setup, involved a single sensor placed 10 cm vertically above the upper and lower central incisors with a funnel attachment to help maximise particle capture. The second arrangement, termed the ‘dual sensor’ setup, involved two sensors being placed in the nose and chin region, 5 cm from the upper and lower incisal edges, with readings taken simultaneously (Additional file 1). The sensors were connected to single board computers (Raspberry Pi, UK). Using a bespoke in-house Bash code (Additional file 2) the particle readings were assimilated into fixed width text files that were then analysed with both Microsoft Excel and GraphPad Prism.
Aerosol-generating devices
Ultrasonic scaling was carried out using a Cavitron Touch® Scaling System (Dentsply Sirona, US) connected to a Cavitron® DualSelect™ Dispensing System (Dentsply Sirona, US), with a Cavitron® Powerline® 1000 30 K Ultrasonic Insert (Dentsply Sirona US) (Additional file 3A). 0.89% saline was used as a coolant for all ultrasonic procedures, measured at a flowrate of 18.0 ml/min. The ultrasonic power was set to 100% for all procedures. High-speed turbine/handpiece treatment was carried out using a Midwest Stylus® Plus Handpiece (Dentsply Sirona, US) (Additional file 3B) with a flat end cylindrical diamond bur (ISO: 111-012 M (837)). 0.89% saline was used as a coolant for all high-speed handpiece procedures, measured at a flowrate of 68.0 ml/min.
Suction devices
A total of six dental unit suction devices were used for the ultrasonic scaling arm of the study. These were divided into ‘dynamic’ suction devices which closely follow the aerosol generating instrument throughout the procedure: Standard high-volume suction (referred to as ‘standard HVE’) (Additional file 3C), Purevac® HVE System [22] which included the lightweight hose and adapter (referred to as ‘Purevac (+H)’; Dentsply Sirona, US) and Purevac® HVE Mirror Tip connected directly to the suction port (referred to as ‘Purevac (−H)’; Dentsply Sirona, US) (Additional file 3D); and ‘static’ suction devices which are hands-free and maintained in the same position throughout the procedure: DryShield® Isolation System [23] (referred to as ‘DryShield’; DryShield, US) (Additional file 3E), Ivory® ReLeaf™ hands-free suction device [24] (referred to as ‘ReLeaf’; Kulzer, US) (Additional file 3F) and standard low-volume suction placed lingual to the lower central incisors (referred to as ‘standard LVE’) (Additional file 3G). For the high-speed handpiece arm of the study, only the standard HVE, Purevac (±H) and DryShield devices were used. In both arms of the study the efficacy of the devices was also compared against a negative control—the absence of suction (no suction) (Additional file 4).
Ultrasonic scaling protocol
As is common practise in dental hygienist treatment, both the ultrasonic scaling and the use of intra-oral suction were operated ‘solo’ by a single operator (PW). For the single sensor setup, the sensor was started and four minutes of background particle levels were recorded to give an initial baseline reading. The sensor was then restarted, and a half-mouth ultrasonic scale of the right-hand side was undertaken for four minutes (Fig. 1A)—this consisted of a predetermined consistent pattern of supragingival scaling, starting in the upper right quadrant at the upper right third molar and then moving in a mesial direction to finish at the upper right central incisor over a one minute period; whilst ensuring the active tip of the ultrasonic scaler was always kept parallel to the long axis of the tooth at the level of the gingival margin, with an equal amount of time spent scaling each tooth to ensure consistency. This was then repeated in the upper right quadrant palatally from the upper central incisor moving distally to the upper third molar. Finally, the same procedure was continued in the lower right quadrant lingually from the lower right third molar to the lower right central incisor, then buccally from the lower right central incisor to the lower right third molar. The sensor was then stopped and the reading saved. Following the half-mouth scale there was a four-minute period of fallow-time. This procedure was completed in the presence of each suction device, used as per the manufacturers’ instructions for use manual [22,23,24], followed by no suction, for a total of five replicates per device. For the single sensor setup only the right-hand side of the mouth was instrumented as it was not possible to effectively access the left-hand side of the mouth with the ultrasonic scaler, suction device and the funnel in-situ.
In the dual sensor setup, four minutes of background particle levels were recorded immediately prior to an eight-minute full-mouth scale (Fig. 1A)—this consisted of supragingival scaling as per the single sensor setup, with the identical procedure also carried out on the left-hand side of the mouth. A one-minute pause between the right and left sides was included to allow transfer of equipment. At the conclusion of the full-mouth scale, the sensor was stopped and the reading saved, followed by a four-minute period of fallow time. This procedure was completed in the presence of each suction device, used as per the manufacturers’ instructions for use manual, [22,23,24] followed by no suction, for a total of five replicates per suction device.
High-speed handpiece protocol
As is common practise in restorative dentistry, the high-speed handpiece procedures were carried out by both an operator (KP) and an assistant (RD). For both the single sensor and the dual sensor setup, the sensor was started and ninety seconds of background particle levels were recorded to given an initial baseline reading. Immediately following this was the use of the high-speed handpiece on the upper right central incisor (Fig. 1B)—this consisted of three depth cuts being made on the labial surface in a mesial to distal direction across a thirty-second period, ten seconds per depth cut, followed by the same procedure on both the incisal and palatal surfaces. This was to mimic the types of cuts used as part of an anterior crown preparation, requiring use of the handpiece in three distinct orientations. At the end of the 90 s the sensor was stopped and the reading saved. After each procedure there was a four-minute period of fallow-time. This procedure was completed in the presence of each suction device, used as per the manufacturers’ instructions for use manual [22, 23], followed by no suction, for a total of five replicates per suction device.
Data analysis and statistics
For each particle sensor, an output reading of all particles detected between 0.3 and 10 µm was provided for every 0.1 L of air intake. Once each saved file had been processed (Additional file 2), the area under the curve (AUC) for the duration of the AGP was calculated which allowed an evaluation of the variation in particle count as a function of time. Background particle levels showed variations that appeared independent of both temperature and humidity readings. Therefore, to accommodate changeable atmospheric particle levels between experiments, the AUC for the initial background reading was deducted from the AUC for the AGP reading to give a normalised AUC value for analysis, hereby referred to as ‘normalised particle count’.
Statistical analysis was carried out from the mean normalised particle counts ± standard deviation for each device (n = 5) using one-way analysis of variance (ANOVA) with Tukey’s post-hoc test. All data analysis and statistical tests were conducted using MS Excel and GraphPad Prism with a significance level set at 0.05.