There are various methods for studying the fluid flow of irrigants delivered via irrigation needles within the root canal. These include macroscopic methods such as visual observation and spectroscopic methods; the use of contrast media; microscopic methods such as real-time imaging of bioluminescent bacteria; and investigations of the ability of irrigants and devices to remove debris in simulated irregularities [16]. However, the above methods provide incomplete estimates of fluid flow dynamics [17]. This study employed CFD to investigate fluid flow in three distinct needle designs. CFD not only evaluates hydrodynamic performance but also predicts the effectiveness of irrigating needles [10, 16, 17].

As in other studies, in this study, the root canals were modelled as single or straight to avoid complications in the generation of mesh for CFD [9,10,11]. A standard length of 15 mm for the frustum of the cone was adopted for all the models to simplify the geometry [11]. The apical size was kept corresponding to a #30 file size, as studies have shown this to be the minimum diameter for efficient debridement and disinfection of the canal. This diameter allows the positioning of a 30 G needle nearer to the working length [14, 18]. Due to certain limitations of the model, the apical foramen was considered a rigid and impermeable barrier. Because of such assumptions, the possibility of irrigants being extruded from the apex is excluded [11].

For safe and effective instrumentation, continuous tapering of the root canal with a small diameter at the apex and nominal coronal flaring are required as the fracture resistance of the tooth increases [12, 13]. Newer NiTi files were introduced with a continuous taper (4%) for conservative preparation of the root canal and preservation of the pericervical dentin [15, 19]. In the literature, there are limited CFD studies performed on root canal models with 4% taper canal preparation [20]. Therefore, in this study, different root canal preparations, such as 4%, 6%, and 8% taper canal preparations, were compared.

The different needle designs used in the study were open-ended, single-side-vented, and double-side-vented. For standardization in this study, all three different needle designs used were 30G [10]. The dimensions of the first two needles were reproduced from the study by Boutsioukis et al. [21]. The last needle studied was the Maillefer Dentsply 30G plastic needle with 2 lateral openings [22]. This study is one of the first to investigate the flow dynamics of root canal irrigant using these newly introduced irrigating needles. In the present study, two irrigant flow rates, 0.083 ml/sec and 0.26 ml/sec, were compared; these are clinically realistic flow rates that help in evaluating the effect of an increase in velocity on different fluid dynamics parameters, such as apical pressure, shear wall stress and irrigant flow pattern [11, 23].

A larger volume of fluid penetrates the apical anatomy when an open-ended needle is used, which leads to an increase in kinetic energy and, in turn, an increase in apical pressure. This was in accordance with the findings of Shen et al. [10] and Boutsioukis and his colleagues [11, 24]. Therefore, the utilization of OEN increases the likelihood of irrigant extrusion, especially in high-flow-rate cases [11]. The less tapered canals had greater apical pressure, which may be related to the reduced room at the needle's tip for reverse flow in the direction of canal opening [20]. Additionally, the apical pressure was directly proportional to the depth of insertion of the needle. This finding is consistent with the findings of a previous study that showed greater apical pressure when the needle was positioned closer to the working length [24].

When the DSVN was 1 mm shy of the working length, even at a lower flow rate, the apical pressure was 1074.7 kPa, while the pressure on the SSVN was 111.7 kPa. The probable reason for this marked apical pressure could be the 4% taper of the needle, as opposed to the parallel nature of the other needles tested [13]. This taper provides less space for the backflow of the irrigants toward the orifice, leading to increased apical pressure. The results of this study for apical pressure in a 6% tapered root canal were similar to those of Shen et al., in which the pressure in the closed-ended needle was 2.5–3 times lower than that in the open-ended needle [10]. However, the apical pressure observed in this study surpassed that reported in prior research [10, 24]. This is probably because of the variations in the experimental settings and the CFD model. Overall, in all the situations studied here, the SSVN and DSVN were preferable to the OEN, except in minimally tapered preparations. Similar to the open-ended needle, in both side-vented needles tested, the apical pressure was greater when the flow rate increased, the canal taper decreased, and the needle penetrated deeper. Hence, caution is advised in these scenarios to reduce the risk of irrigant extrusion through the apical foramen [21].

In the present study, the shear stress generated by the OEN was the highest, followed by that induced by the SSVN and then that induced by the DSVN. In the SSVN and DSVN, the stress was mainly concentrated on the walls facing the needle outlet. These observations are similar to those of previous studies [9, 18]. With the increase in needle penetration depth and velocity in all three types of needles, there was an increase in shear stress as well as in the shear wall stress region, which was in accordance with the findings of previous studies [24]. Although the wall shear stress generated by the DSVN was drastically low, the area of stress concentration observed on the walls opposite to the openings of the double-side vented needle was greater. This could lead to a larger area of debris and biofilm detachment, thereby enhancing cleaning [21]. Rotation of these needles while irrigating could also prove beneficial for more effective canal debridement [25, 26].

The magnitude of the irrigant velocity within the root canal is a crucial parameter when assessing flow parameters. It allows the evaluation of the irrigant replacement and reverse flow of the irrigant coronally [2, 11]. Efficient canal cleaning relies on favourable irrigant flow to effectively carry debris from the canals out of the orifice [9]. The irrigant replacement and reverse flow were better with open-ended needles than with closed-ended needles, especially when the flow rate was higher and canals were less tapered. However, from a clinical perspective, the risk of irrigant replacement far outweighs that of irrigant replacement [24]. Hence, when employing an open-ended needle under high irrigant flow rates in minimally tapered canals, operators must exercise caution.

The results of this study also demonstrated that none of the needles tested with various parameters could effectively deliver the irrigant up to the apex. The OEN demonstrated greater irrigant penetration than did the side-vented needles. This result was supported by previous studies in which side-vented needles reported limited irrigant exchange [9, 11]. The single-sided vented needle showed better apical penetration of the irrigant than did the double-sided vented needle. For the DSVN, the irrigant barely crossed the tip of the needle. The inability of all the needles examined in this study to effectively replace irrigants even when the tip is 1 mm from the apical construction contradicts the findings of previous studies [2, 20]. The conflicting findings could be attributed to the varied experimental settings and variations in the numerical models used [2]. As mentioned above, all the irrigating needles demonstrated a stagnant dead water zone apical to the tip [27]. Therefore, it is imperative to agitate irrigants or even slowly move the needle up and down within canals to effectively clean the root canal to its terminus and to improve fluid replacement dynamics. [28, 29].

The closed-ended double-side-vented needle tested in this study differed from that used in previous studies [2, 9, 21] in that the openings in this needle are positioned back-to-back rather than at the top and bottom. In the latter double-vented needle, the flow between the two openings is not equally distributed. The fluid passes more through the vent closer to the tip of the needle [21]. However, in the back-to-back side vented needle, the flow from both vents is equally distributed. The overall size of the opening could also account for the lower wall shear stress and irrigant velocity. The advantage of the back-to-back double-side-vented needle tested in the current study is that it has a flexible body made of polypropylene that allows the needle to access the apical regions without resistance or damage to the dentinal walls even in curved canals [22, 30]. Therefore, further research is required with regard to the irrigant flow and efficiency in curved canals using these syringe needles.

The limitations of the study include the design of the root canal model, which was considered to be closed and straight, with impermeable and rigid walls. Surface irregularities were also not incorporated into the model design. The parameters of irrigation dynamics can easily be affected by such changes. Additionally, it is difficult to maintain constant inlet flow velocities in clinical scenarios. Therefore, the quantitative data obtained from these studies cannot be directly applied in clinical situations.