## 2 Materials and Methods  
The samples used include 50 S-shaped endo training blocks (Dentsply Maillefer, Ballaigues, Switzerland) made of resin, with a 0.15-0.30 mm \(\upomega\) and a 2% taper (Maillefer-Dentsply, Ballaigues, Switzerland) [25]. Each sample was shaped by a dentist, with at least three years of experience in the field of endodontics, using the following sequence of instruments to shape the root-canal system: a No. 10 K-file to establish the 16-mm working length (Maillefer-Dentsply, Ballaigues, Switzerland), pathfiles no. 1, 2, and 3 (Maillefer-Dentsply, Ballaigues, Switzerland) [26] to produce the glide path [26], and ProTaper Universal instruments S1-S2-F1-F2-F3 (Maillefer-Dentsply, Ballaigues, Switzerland) for the final phase of shaping [3]. The final shaped endo training blocks were divided into two groups, composed of 25 samples each, assigned to either MDA [27] or PUI [28].  
In the evaluation of MDA efficacy [29] performed on the endo training block similar to the close-ended systems whose terminal portions were closed using orthodontic wax, a watery solution was delivered inside the endo training blocks using a disposable syringe, with a tip-opened 25 gauge diameter needle located 5 mm beyond the entrance of the resin block. This technique is commonly known as positive pressure irrigation (PPI) (Figure 1) [30]. In the cases where vapor lock presence was demonstrated, the MDA [20] method was performed with a dedicated F3 gutta-percha cone, using back and forth movements inside the canal for 10 s. In the evaluation of PUI efficacy performed on the endo training block similar to the close-ended systems whose terminal portion was closed using orthodontic wax, a watery solution was delivered inside the endo training blocks using a disposable syringe, with a tip-opened 25 G diameter needle, located 5 mm beyond the entrance of the resin block; this technique is commonly known as PPI [31]. In the cases where vapor lock presence was demonstrated, the PUI method was performed using a Woodpecker ultrasonic tip (28/32 kHz) (Guilin Woodpecker Medical Instrument, Guilin, Guangxi, China) [32], with a 0.8 mm \(\upomega\) mounted on a Satelec handle(Satelec, Bordeaux, France), acting inside the block for 10 s.  
Figure 1: Sequence introduction of irrigants using positive pressure irrigation (PPI) with vapor lock formation in an endo training block with the apex closed by orthodontic wax: (**A**) liquid introduction, (**B**) initial formation of the vapor lock in the middle third, (**C**) vapor lock stabilization in the middle third, and (**D**) failed attempt to remove the vapor lock by re-treating the liquid using PPI.  
The photos were recorded with a digital camera (Canon 1100D, Tokyo, Japan) at a fixed position and magnification using stable supports for the digital camera and specimens. Two photos were captured for each block both before and after the use of the irrigation activation method (Figure 2). Each photo was analyzed on Image J software [33] to calculate the vapor lock area projection. Data expressed as a percentage of vapor lock reduction are reported in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test** & **Manual Dynamic Agitation (MDA)** & **Test** & **Passive Ultrasonic Agitation (PUI)** \\
**Number** & **Vapor Lock** & **Number** & **Vapor Lock** \\ \hline
**Un** & **Reduction** & **Removal** & **Un** & **Reduction** & **Removal** \\
**Removed** & **(\%)** & **Removal** & **Removed** & **(\%)** & **Removal** \\ \hline
**1** & & ++ \({}^{1}\) & 26 & \(-\)\({}^{2}\) & \\ \hline
**2** & & ++ & 27 & & 41.253 \\ \hline  
(-\) & & 48 & & ++ \\ \hline
**24** & & 92.073 & & 49 & 70.92 \\ \hline
**25** & & ++ & 50 & & 93.92 \\ \hline
**Total** & 1 & 9 & 15 & 7 & 17 & 1 \\ \hline \hline \end{tabular} \({}^{1}\) ++ = removal (100%); \({}^{2}\) = not removed (0%);  
\end{table}
Table 1: Results. Legend: ++ = removal (100%); \(-\) = not removed (0%); reduction expressed as a percentage.