2.4. Methods

Twenty PTU F1 pins in each group were fixed on the silicone rubber template in accordance with numbered sequence, which ensured that the tips of PTU F1 pins were leveled with each other (Fig. ​(Fig.1).1). After the laser marking program was started, the fiber laser mode was selected. Six laser marking lines were set up at a 3-mm interval and a 0.1-mm line width, and marking parameters were set to speed: 200 mm/s, power: 30%, and frequency: 500 KHz. The laser indicator was opened and made to level with the tips of PTU F1 (Fig. ​(Fig.1A).1A). Before laser marking, the first and second laser lines (blue solid lines) were not selected in the program, but other laser markers (red dotted lines) were selected to work, which enabled accurate and uniform laser marking at 6 mm, 9 mm, 12 mm, and 15 mm (Fig. ​(Fig.11B).

The method of laser marking.

The PTU F1 was removed from the silicone rubber template according to the numbered sequence to carry out root canal preparation experiment at room temperature. The stop ring was placed at 18 mm, and the motor of X-Smart plus was set at 350 rpm/min with a torque of 3 Ncm. The rotation was clockwise and unidirectional. The vertical pressure of each preparation to the root canal side was kept at 10 N, so that the filing needle could stay for 2 seconds in the root canal each time. At the same time, the flusher was used, and the needle was continuously flushed outside the root canal at about 1 cm perpendicular to the needle. The liquid flow rate was set to 220 ml/min, so that the working length of the root canal can be prepared repeatedly to 18 mm. In this experiment, only the control group did not undergo extra-canal irrigation for root canal preparation. However, none of the five groups changed the existing clinical irrigation methods (five times after nickel-titanium instruments entered the root canal, and the root canal was dredged with No. 10 enlarged needle and irrigated with lateral opening needle). Each root canal was prepared by the above methods until the needle was broken and a new needle was replaced, and the above operations were repeated (Fig. ​(Fig.22).

Method of root canal preparation. Note: (1) refers to 0N when no force is applied, (2) refers to 10N when pressing down.