## 1 Introduction  
Caries, dental trauma and tooth malformations frequently cause an infection of the dental pulp, which results, if untreated, in pulpitis, pulp necrosis and, finally, apical periodontitis. In this case, root canal treatment (RCT) is indicated, as the removal of necrotic pulp tissue and bacteria from the entire canal system enables healing of periapical pathosis [1, 2, 3]. During RCT, the endodontic canal system has to be explored, and the canals are enlarged and debrided by endodontic files. However, mere mechanical instrumentation results in insufficient cleanliness, as up to one-third of the canal surface remains untouched [4, 5, 6]. Furthermore, accumulated hard tissue debris (AHTD) during treatment locks isthmuses and lateral canals and, thus, impedes the transport of irrigation into these areas [1]. Therefore, there is a demand for copious irrigation to support mechanical instrumentation and dissolve debris. Sodium hypochlorite (NaOCI) has proven to be very effective, as it has both antibacterial and tissue-dissolving features [7]. If applied by needle irrigation only, thorough cleaning and disinfection of isthmuses, lateral canals and the apical segment of root canals fails [8; 9; 10; 11]. In addition, the endodontium can be considered a closed-end-system; thus, air is trapped in the apical region during irrigation, which prevents the penetration of disinfect even further [12; 13]. For this reason, various activation methods have been developed to optimize the penetration of disinfecting agents into the root canal system and enhance the removal of AHTD [14]. Several different activation methods are available to the endodontist. Among them, the sonic-based activation system EDDY (VDW, Munich, Germany) uses a polyamide tip, which oscillates at a frequency of 6 kHz. These oscillations create turbulences in the surrounding disinfecting solution, supposedly resulting in removal of AHTD and disruption of intracranial biofilm [15]. Various studies have reported sonic activation to be more effective than needle irrigation in terms of AHTD elimination [16; 17; 18; 19; 20; 21]. In recent years, newly developed laser systems have enabled the activation of disinfectants by photon-induced photoacoustic streaming (PIPS, FOTONA, Ljubljana, Slovenia). This method seems to be advantageous when cleaning confined spaces [22], where (ultra)sonic-based systems are considered less effective due to limited space for oscillation [23]. In addition, curvatures hinder the distribution of (ultra)sonic energy in the root canal [24], and rigid tips can damage dentine walls and thus alter the shape of the canal [25; 26]. Previous studies have shown that PIPS offers advantages over conventional needle irrigation and passive ultrasonic activation regarding removal of debris and the smear layer, as well as antibacterial effects [27; 28; 29]. Recently, a PIPS derivative, termed shock-wave enhanced emission photoacoustic streaming (SWEEPS), has been introduced to the market. An Er:YAG laser emits pulsed light at a wavelength of 2940 nm through a fiberglass tip, which transfers the light into the surrounding disinfection solution. Due to the high absorption of clear liquids at this wavelength, heat is generated locally and causes spontaneous evaporation at the tip. The resulting gas bubbles collapse and trigger shock waves that generate smaller secondary bubbles. In contrast to PIPS, SWEEPS emits two subsequent laser pulses. The timing is adjusted such that the second pulse is emitted at the time the gas bubble from the first pulse collapses. As a result, the first bubble is pushed deeper into the root canal, and secondary bubbles collapse due to the additional pressure [30]. This chain reaction may continue along the entire length of the root canal to cause microstreaming and, thus, increase the penetration depth of the disinfectant [30]. [PERSON] et al.