Upon examination of studies focusing on apical extrusion, it becomes evident that there is considerable variation in methodology and study design [23]. A significant number of studies have employed extracted teeth as experimental specimens in the analysis of apical extrusion, as they more accurately reflect clinical circumstances [24,25,26]. Nevertheless, it is challenging to achieve reliable dimensional and morphological standardization when utilizing natural teeth. In the majority of studies [27, 28], the crowns of the teeth were decoronated at the cementoenamel junction to achieve specific root lengths for standardization purposes. However, it has been reported that the crown acts as a reservoir for root canal irrigants, particularly when employing activation techniques [27]. The presence of crown may enhance fluid dynamics during activation and the possibility of advanced delivery of irrigants to the apical third [28], which could result in a change in the amount of apical extrusion [29]. In conclusion, one potential disadvantage of studying irrigation methods is the difficulty in selecting natural teeth with robust crowns, standardized root canals, and strict measures in the apical region [23].

The utilization of 3D-printed tooth models, which were employed in numerous research studies [30, 31], presents a notable advantage in that it allows for the standardization of dimensions of root canals, thus facilitating the comparison of the different irrigation techniques in a more reproducible manner than is possible with the extracted teeth [30]. Furthermore, the capacity to produce teeth with open apex in varying sizes and shapes may represent a notable advantage, particularly in the studies on regenerative endodontics.

The utilization of 3D-printed teeth, comprising a robust crown structure that serves as reservoirs for irrigation solutions, has yielded more realistic results in extrusion studies than natural teeth that have undergone decoronation [23]. One potential limitation of studies utilizing acrylic models is the possibility that the heat generated by rotary instruments may soften the resin material, thereby compromising the reliability of the experimental system [32, 33]. Nevertheless, during the design phase, the teeth were designed with an open apex and shaped to the required width, and then produced in accordance with these parameters. This eliminated the necessity for instrumentation during the experimental process, thus overcoming the aforementioned limitation.

Advancements in irrigation activation technologies may lead to a decreasing trend in the required apical preparation width for root canal therapy. This belief may be true for mature teeth but is unacceptable for immature teeth with a large apical diameter. Hence, the risk of apical extrusion during irrigation becomes more pronounced in immature teeth, posing a challenge for clinicians. Most studies on extrusion have been conducted on permanent teeth with mature apices [34,35,36,37,38,39,40], with limited experiments performed on immature or open apex teeth. Currently, limited studies have examined the ER:YAG laser methods on apical extrusion in teeth with immature apex [41, 42]. However, this is the first study to demonstrate the effect of using novel SWEEPS technology on apical extrusion of NaOCI in immature teeth.

Several studies have failed to include the potential impact of periapical tissues when assessing the extent of extrusion into air-filled vials [40, 43, 44]. This technique leads to an unrealistic scenario where there is no resistance from periapical tissue. Prior research has recognized this problem as a factor that leads to an exaggerated estimation of irrigant extrusion [23]. The present study used an agar gel model with a concentration of 1.5%, which has a density of 1045 kg/m3. This density is within the range of human periapical tissues, which typically vary between 1000 and 1100 kg/m3. Additionally, using a certified contrast solution helps replicate clinical conditions [23].

The amount of apical debris extruded increased as the level of irrigation technique approached the working length [37]. According to our findings, although there was a tendency to extrude more irrigant as the closer to working length; for XP-F and CSI groups, there was not a significant difference among the methods when performing 1 mm and 2 mm short of the WL, thus rejecting the first null hypothesis. The current study's findings are consistent with a study [45], which showed that irrigant extrusion increases as the apical diameter increases, regardless of needle insertion depth.

A novel introduced SWEEPS is an Er: YAG laser model that has been specifically designed to enhance the cleaning and disinfection efficacy of the photon-induced-photoacoustic-streaming (PIPS) technique. By providing two subsequent laser pulses to the irrigation solution within a certain period of time, the bubbles generated by the laser collapse more quickly, allowing the photoacoustic shock wave to pass through narrower root canals [46]. Shock waves pass through the irrigation solution and engage with the surrounding tissue and root canal wall, penetrating deep into the accessory canals to remove debris and microorganisms [47]. The increased pressure within the root canal can facilitate the flow of irrigants toward the apical area, potentially leading to the extrusion of irrigants [34, 48]. The study's results clearly demonstrate that SWEEPS causes a significantly higher degree of apical extrusion when compared to CSI. This phenomenon can be confidently attributed to the shock waves generated by the SWEEPS method.

It is impossible to compare specifically this result as there is no other study in the literature evaluating the effect of SWEEPS on apical extrusion in immature teeth. In a study evaluated the effect of SWEEPS on apical extrusion in teeth with closed apices, it was found that syringe irrigation caused more apical extrusion compared to the SWEEPS method [35]. Variations in apical diameters could be the reason for the discrepancy between their and our results. One of the limited studies examining the effectiveness of the SWEEPS method in clinical use is related to postoperative pain [49]. Evidence of a direct relationship between postoperative pain and apical extrusion has been demonstrated in the previous studies [49,50,51]. It was reported that PIPS and SWEEPS approaches were more successful than EDDY and PUI in avoiding and decreasing postoperative pain [49]. However, when it comes to apical extrusion, our findings yielded opposite results. While the main cause of postoperative pain is apical extrusion of both debris and irrigation solutions, we focused solely on NaOCI extrusion after completion of irrigation procedures.

Concerns regarding the irrigation activation methods in immature teeth are still a reality in endodontic practice. Considering that preserving the viability and proliferation potential of stem cells should be a primary concern of RET. Although the SWEEPS may appear to be a more advantageous irrigation method for RET due to its requirement for a smaller access cavity and avoidance of contact with the canal walls, it is noteworthy that this method resulted in more apical extrusion compared to EDDY at a working length distance of 2 mm, and CSI at both working lengths, thus rejecting the second null hypothesis.

The current study was conducted under laboratory conditions, which may not reflect real-life situations where factors such as gravity and pressure from surrounding tissues could influence the results. This may be considered as a limitation of the current study. The model demonstrated a matching of the canal geometry following root canal preparation at a specified size, which is suitable for irrigation and activation. However, the root canal system did not fully correspond to the actual anatomy of a natural tooth, which has apical delta, lateral canals, anastomoses, and other characteristics. This may be considered a second limitation of the present study.

It is important to note that each approach and methodology presented and reviewed in the current research has its own limitations. Consequently, there is currently no ideal methodology that fully meets the parameters of an acceptable apical extrusion study [23]. However, the novel approach of utilizing 3D-printed models that simulate the morphology of natural teeth incorporates considerations such as the standardization of apical dimensions and shape [23]. This allows for the attainment of more reproducible results when compared to studies utilizing extracted teeth [23]. On the other hand, it appears that the introduction of new and efficient irrigation activation techniques is about to happen soon. To thoroughly investigate the efficacy of novel irrigation activation techniques on apical extrusion, further research is imperative. Future studies should encompass a multitude of variables and strive for enhanced standardization, utilizing 3D-printed teeth that closely resemble natural teeth including features, such as the isthmus, lateral canal, apical delta, curved canal, or multiple canals.