Effective chemo-mechanical preparation of the root canal space is of paramount importance for a successful root canal treatment . Nonetheless, shaping procedures can leave residual smear layer on dentinal walls , which may have a role in endodontic failure [3, 4]. Another crucial aspect to consider for the long-term success is dentin preservation . Therefore, a challenge for present endodontics is to provide enhanced cleanliness efficiency while saving as much restorable tooth as possible. This in vitro study aimed the evaluation of the effectiveness of sonic (EndoActivator), ultrasonic (EndoUltra) and two lasers irrigant activation techniques (PIPS and SWEEPS) on the smear layer removal from the apex to the coronal third in minimally instrumented canals. Smear layer is usually removed by irrigants able to dissolve both its organic and inorganic components . In the present study EDTA followed by NaOCl was used as it is currently considered the most effective irrigating clinical protocol [4,5,6]. Attempts were made to limit variables as much as possible. In fact, it was possible to standardize volume, rate, time of irrigation and irrigants activation. During irrigant activation, root canals were continuously replenished to maintain constant irrigants level . A continuous flow of irrigants provides the advantage of a constant replacement of irrigant fluid  and it is crucial for the success of LAI techniques . In contrast with previously published studies [20,21,22], in our investigation, canal shaping was kept conservative (apical size #25, taper 0.06), in order to save as much dentinal tissue as possible. Considering present conditions, PUI may result ineffective. In fact, it was suggested that small canal shapes may hinder its efficacy, as this technique relies on the ability of the activated instrument to oscillate freely, while when this does not happen, the acoustic streaming can lose intensity . Nonetheless, in the present investigation there were no significant differences between PUI and SWEEPS efficacy at 1, 3, 5 and 8 mm from the apex, even though SWEEPS obtained better scores. This may be explained by the small EndoUltra tip size (15/0.02) used. In fact, the thinner the vibrating instrument, the higher the frequency. This leads to a higher streaming speed and a more intense acoustic streaming . For standardization purpose, in the present study, a 15/0.02 tip was also selected for EA activation, as the 15/0.02 EndoUltra tip is the only one currently available on the market. Sonic activation operates at a lower frequency than ultrasonic activation. The positive relationship between frequency and streaming velocity should justify a better efficiency of PUI versus sonic activation . Conversely, in our study, the differences between PUI and EA scores were small, though favouring PUI activation. In fact, no statistically significant differences were noticed between EA and PUI at any distance, but both the techniques removed significantly more smear layer at 3, 5 and 8 mm from the apex compared to the control group. This was coherent with previous papers [6, 20]. Conversely, Uroz-Torres et al. reported EA to remove no significant more smear layer than conventional irrigation . These results might be attributed to the lower volume of irrigant and the shorter time of irrigation used compared to the present study . Schmidt et al. showed that PUI did not remove more smear layer than conventional syringe irrigation , and this may be also attributed to the lower NaOCl volume and concentration used. Rödig et al. showed that EA obtained even better cleanliness result than ultrasonic activation . This may be justified by the choice to use a K-file, instead of a blunt tip, for ultrasonic activation, which could have itself produced new undesirable smear layer. Similar results were already described, in which EA removed significantly greater smear layer than PUI at 3 mm from the apex [20, 22]. This could be explained by the choice to increase the apical shaping to a ProTaper F4 (apical size 0.40, taper 6% in the last 3 mm) in order to improve the volume exchange of irrigants. Conversely, PIPS and SWEEPS techniques seem to require no particular canal enlargement [13, 25]. Cavitation bubbles reach the bottom of the root canals even when the laser tip is maintained without walls contact in the access cavity, allowing canals to be shaped to considerably smaller sizes [11, 25]. Nonetheless, the question of ideal canal size and taper has yet to be addressed . Peeters et al. suggest that it is possible to minimize the invasiveness of the treatment when using LAI, as in their study promising results were obtained in the apical region with minimal canal enlargement . Laser activated irrigation using PIPS has been shown to be effective in significantly better cleaning of the root canal walls in comparison with conventional irrigation procedures [11, 21, 26, 27]. To the best of our knowledge, only a few studies compared PIPS with sonic or ultrasonic activation in terms of smear layer removal [21, 27]. Akyuz et al. showed how PIPS and PUI obtained similar cleanliness results . Similarly, Arslan et al. found no significant difference between PIPS and EA in terms of cleanliness in the middle third of the canal . These findings may be mainly explained by the larger canal shaping (ProTaper F4 as final instrument) used that could have facilitated EA and PUI action. In the present study PIPS was significantly more efficient than control group, EA and PUI at 1, 3, 5 and 8 mm from the apex. As mentioned before, PIPS and PUI efficacy is based on acoustic streaming and cavitation effects [9, 11]. These phenomena have an important role in the smear layer removal. In fact, PIPS has been found to be effective even when the activated solution was saline, which alone does not affect the smear layer . Accordingly, in the present study, the choice to leave irrigants for 30 s of resting time as part of the post-shaping activation protocol was justified by the intention to rely also on their chemical action. SWEEPS has been developed in order to improve PIPS efficacy even further, aiming at producing shock waves even in spatially confined reservoirs . Despite that, in the present study, there were no statistically significant differences between PIPS and SWEEPS at any distance from the apex. Nonetheless SWEEPS never removed significantly more smear layer than PUI, while it was always statistically more effective than control group and EA. The present study is the first one investigating SWEEPS in terms of smear layer removal. Therefore, it was not possible to discuss our findings with other reports, and we can only speculate that SWEEPS performance has been influenced by the present experimental conditions that, somehow, have affected its efficacy. Indeed, SWEEPS should be further tested in the future. This study has several limitations. Traditional SEM investigations have been reported to be not trustworthy. In fact, longitudinal observations, in which a given dentin area can be observed at different times, are to be considered as a more reliable study model . Despite that, SEM evaluations are still the most used in research [2, 6, 11, 14, 20,21,22, 30] and, an ideal experimental model to assess smear layer removal seems not to be available at the moment . An ideal experimental setting should also take into consideration the amount of sclerotic dentin . In this study samples were taken from young patients. This may limit the occurrence to mistake sclerotic dentin, which increases with aging, for smear-layer covered dentin. Another issue is the qualitative scoring method used, even though it is one of the most used in
literature [2, 11, 14, 20,21,22]. Computational systems, able to automatically extract quantitative data, are likely to minimize human bias . Nonetheless, in our investigation, the use of a direct qualitative scoring system, by multiple calibrated examiners with concordance between them (Kappa test), together with the large number of observations made, may considerably increase the reliability of the results [6, 8].