## 4 Discussion
Furthermore, previous \(\upmu\)CT-studies investigated how irrigation and activation does remove calcium hydroxide from the root canal [46, 47]. Radiopaque additives, like iodoforum or BaSO4, contained in these preparations were clearly differentiable from dentine in \(\upmu\)CT-images and are thus easy to quantify. On the other hand, calcium hydroxide has different qualities compared to dentine debris and may therefore not be a suitable surrogate in the context of root canal cleaning and disinfection. By combining human dentine powder with both sodium hypochlorite and BaSO4, we were able to unite the advantages of the previous approaches, resulting in debris formation that was technically quantifiable by \(\upmu\)CT.  
Clinically, the formation of AHTD during root canal preparation depends on various parameters, such as canal configuration and diameter, the constitution of isthmuses or the endodontic files and their mode of action [41, 42]. In contrast, artificial debris introduced into previously enlarged root canals offers great advantages in terms of standardization. A compaction of artificial debris with guttapercha facilitated homogeneous and consistent filling of isthmuses and voids right up to the apex. In particular the apical third of the canal constitutes a critical region that must be reached by disinfection and activation. In analogy to the results of this study, previous investigators reported AHTD volumes as low as 0.81 mm\({}^{3}\) after root canal preparation [48, 49]. However, the artificial radiopaque debris filled all voids and isthmuses with a free canal lumen, resulting in a median volume of 4.4 mm\({}^{3}\), which presented a greater but equitable challenge for final irrigation. From a methodical point of view, BaSO4, as a radiopaque additive to artificial debris, simplified optical segmentation in \(\upmu\)CT-scans at a detection limit of 0.01 mm\({}^{3}\) through a clear differentiation from root dentine. Another benefit of this method is the possibility to quantify debris automatically via thresholding. In contrast, previous studies mainly used co-registering and subtractive algorithms to determine the amount of debris, which entails systematic errors and inaccuracies due to multiple offsetting of volumes [1, 22, 44]. In another investigation, acrylic teeth were used for better differentiation; however, this does not correspond to a natural tooth due to differing surface qualities and microanatomy [50]. In terms of endodontic irrigation, CNI is widely accepted among clinicians, as there is no need for special equipment, and the procedure is easy to integrate in any workflow. Previous research has shown that an up and down motion of the needle in the canal during irrigation supports cleaning the dentine surface from collagen and allows for a better exchange of disinfectant towards the apex [51, 52]. However, it is also well accepted that CNI has shortcomings in regards to cleaning compared to activated irrigation in terms of AHTD removal [53, 54, 55] and comes with an increased risk of apical extrusion of the irrigant [56]. Instead, SAI causes a thorough mechanical agitation of the disinfectant in the root canal by \"three-dimensional tip movement\" [54]. Even though no cavitation effects are associated with sonic activation, both the tip oscillation and the microstreaming of the irrigant can loosen accumulated debris and pave the way for the irrigation to penetrate isthmunes and lateral canals [57]. The flexible and soft polyamide tip can move freely even in constrained and curved canals, and it enables sufficient disinfection without unwanted dentine preparation or alteration of the canal anatomy [17]. Although the efficiency can be controlled by the power settings of the sonic handpiece connected [58], caution must be exercised also with SAI because of the risk of instrument fracture [59] or apical irrigant extrusion causing postoperative pain [60; 61; 62].