The aim of this study was to compare the length between the mesio-buccal and mesio-lingual canal of the mandibular molars before and after early coronal flaring at the different measuring time using several electronic apex locators. Fifty mandibular molars with complete apical formation and patent foramens were selected. After establishing the initial working length of the buccal and lingual canal of the mesial root using a surgical microscope (Carl Zeiss Co. Germany) at 25X with #15 K-file tip just visible at the foramen, radiographs were taken for the working length. After measuring the length of mesio-buccal and mesio-lingual canal (control group), the electronic lengths were measured at different times using several electronic apex locators (experimental groups; I-Root ZX, II-Bingo, III-Propex, IV-Diagnostic). After early coronal flaring using the K³ file, the additional electronic lengths were measured using the same manner.

The results were as follows: One canal has a correct working length for the mesial root of the mandibular molar, it can be used effectively for measuring the electronic working length of another canal when the files are superimposed or encountered at the apex. In addition, the accuracy of the electronic apex locators was increased as the measurement was accomplished after the early coronal flaring of the root canal and the measuring time was repeated.

The objective of this in vitro study was to evaluate the efficacy of a Ca(OH)₂ removal before and after early coronal flaring using different types of instruments. 100 plastic blocks with 30° artificial curved canals were used in this study and randomly divided into a control group and 4 experimental groups(GG, OS, GT, PT Group) 20 teeth each. The canals were instrumented, and Ca(OH)₂ was temporary filled into the each canal. Irrigation was performed with Max-i-Probe 25-, 30-gauge probes before and after recapitulation.

The results of this study were as follows:

1. There were no significant difference among the groups in size of irrigating needle(p<0.05).

2. There was a significant difference between before and after recapitulation regardless size of irrigating needle(p<0.05).

3. Before recapitulation, there was a significant difference between 25- and 30-gauge needle in all groups(p<0.05).

4. After recapitulation, there was a significant difference between 25- and 30-gauge needle in the control group(p<0.05). But there were no significant difference among the experimental groups.

It is concluded that the effectiveness of canal irrigation was decided to the depth of irrigating needle into the canal. The effect of canal irrigation tend to facilitate by the early coronal flaring. The recapitulation was the most effective during canal irrigation regardless the size of irrigating needle. Therefore, the recapitulation is a mandatory way to facilitate the effectiveness of canal irrigation during canal enlargement.