## Materials and Methods
### Root Canal Preparation and Introduction of Artificial Debris  
Following glidepath preparation with K-files to size 20.02 (VDW), both mesial root canals were instrumented with rotary files to size 30.07 (Protater Next X1 to X3, Dentsply Sirona, York, PA, USA). After each use of files, the root canals were rinsed with NaOCl (5%, 1 mL, preheated to 60 \({}^{\circ}\)C; SPEIKO, Munster, Germany). The irrigating solution was introduced by a 30-gauge double-side-vented needle (Endo irrigation needle, Transcodent, Kiel, Germany) that was inserted 1 mm short of working length, and a syringe pump created a flow rate of 1 mL/min (Orchestra Module DPS, Fresenius Vial Infusion Technology, Brezins, France). All endodontic procedures were carried out using a surgical microscope (PROergo, Carl Zeiss Surgical, Oberkochen, Germany). Subsequently, excess liquid was removed with paper points, and a second \(\mu\)CT-scan was carried out as described above. During pilot experiments, it was observed that already copious irrigation without activation led to nearly complete removal of debris. In order to enable a standardized test system for the different activation techniques, an artificial hard tissue debris model was developed. For this purpose, dentine powder from human molars was fabricated as described previously [39]. Therefore, enamel and root cementum were removed by a diamond bur, and the pulp tissue was separated from the teeth after cracking. Subsequently, dentine was ground using a mixer mill (Mixer Mill MM 200, RETSCH, Haan, Germany). The resulting powder was cleared of particles larger than 50 \(\mu\)m by sieving (Stainless-Steel Test Sieve 50 \(\mu\)m, Thermo Fisher Scientific, Waltham, MA, USA) and mixed with NaOCl as well as 15 wt% BaSO4 (Sigma-Aldrich, Steinheim, Germany) to a homogenous, paste-like consistency. The artificial debris was applied to the root canals with a lentulo (Dentsply Sirona) and compacted with two simultaneously inserted guttapercha points (Protaper Next X3, Dentsply Sirona) until full working length was reached. This procedure ensured  
Figure 1: **(a)** CAD model of the holding and rinsing device, consisting of an upper part (mint) and a lower part (ocher), which were connected via a plug connection. **(b)**\(\mu\)CT-scan of the mesial root of a mandibular molar with isthmus area after root canal preparation. This data was used to create the individual opening in the respective holding device. **(c)** Tight fit of the root sample in the upper part, enabling good sealing of the reservoir and stable fixation of the sample during manipulation. **(d)** Experimental setup with the syringe pump for irrigant supplies, as well as the saliva ejector for irrigant aspiration, connected to the rinsing device via infusion tubes. that isthmuses were blocked with debris, but the canal lumina were kept open, which allowed for regular action of irrigants and activation systems (Figure 2). Excess liquid was removed with paper points (Protaper Next X3, Dentsply Sirona), and the canal orifice was sealed with PTFE tape (Scotch 4702, 3M, St. Paul, MI, USA). The teeth were stored moist in Bijou-Tubes (Sterilin 7 mL Bijou tube, Thermo Fisher Scientific). A third \(\mu\)CT-scan was performed to quantify the amount of artificial debris and verify blockage of the isthmus and lateral canals. Due to the distinct difference in radiographical density between the artificial debris containing BaSO4 and normal dentine, a clear and simple discrimination was possible during image processing. Therefore, a histogram with the x-axis showing grey values and the y-axis showing the number of voxels was extracted from the \(\mu\)CT-scans. Four clearly distinguishable grey scale peaks were obtained for background (air and PTFE), dentin and radiopaque debris. Based on these peaks, distinct grey scale thresholds were defined for all samples for the background (0-0.07), dentin (0.07-0.15) and for radiopaque artificial debris (0.15-0.40).