# Working Length Transfer in the Endodontic Clinical Practice: A Comparative Study
## 2 Materials and Methods  
Thirty root canals of extracted permanent teeth with fully formed apex (first and second upper molar buccal canals, and first and second lower molar mesial canals) that had not undergone prior endodontic treatment were selected. The patients signed an informed consent for the treatment and for the use of the extracted tooth for research purposes and the Local Scientific Committee authorized the in vitro study. A sample size of 15 per group was calculated with G*Power 3.1.4 (Kiel University, Kiel, Germany) with alpha error = 0.05 and \(\beta\) = 0.95. After debriding the root surface, specimens were immersed in a 0.01% NaOCl solution for 1 h and then stored in saline solution until preparation. Pre-operative X-rays were taken (Planmeca Intra--Helsinki, Finland) using photostimulable phosphor imaging plates and were processed and archived by a dedicated scanner and software interface (OpTime Soredex, Helsinki, Finland). Root canals were divided according to canal curvature factor (CC) into moderate (10\({}^{\circ}\)-25\({}^{\circ}\)) and severe (>25\({}^{\circ}\)) groups.  
After access cavity preparation, the appropriate reference cusp and the root terminus were flattened using 3.5\(\times\) Galilean loupes with a grinding/polishing wheel to provide reproducible and accurate measurements. A flat glass surface was placed in contact with the flattened root end and the WL was established by an expert endodontist under microscopic vision (OPMI Pro Ergo, Carl Zeiss, Oberkochen, Germany) at 10\(\times\) magnification when the tip of a #10 k-file was visible at the root tip that was in contact with the glass surface. WL was obtained with K-files (KF) and the silicone stop position was fixed with cyanoacrylate cement. Root canals were divided according to canal length factor (CL) into short (15.56-17.88 mm), medium (17.89-19.29 mm) and long (19.48-20.45 mm) groups. Afterwards, WL was transferred to the NiTi reciprocating files (WaveOne Primary, W1) using both clinical methods.  
In method I, WL was checked with a traditional endodontic millimetric gauge with a bar for each millimeter; the reference K-file was measured using a graduated ruler ([PERSON], [PERSON], York, PA, USA) and then a W1 instrument was set to the same length with the same gauge.  
In method II, corresponding files were matched by placing them side by side from tip to shank in opposite directions in order to compare the length of the two instruments. The silicone stop position was adjusted until the right measure was reached (Figure 1).  
The digital images of the KF and the corresponding two W1 files, one for each method, were acquired by a digital reflex camera (Canon EOS 350D (8 Mpx resolution, ISO 100, f/18, 1/60 s)). For each instrument and method, two measures were taken by two examiners using Rhino (ver. 4.0, McNeel, Seattle, WA, USA) software for a total of 360 measures. The agreement between operators was calculated by applying the Cohen weighted kappa coefficient (SPSS v.24.0; IBM Corp, Armonk, NY,USA). The kappa results were interpreted as follows: values \(\leq\)0 indicated no agreement, 0.01-0.20 none to slight agreement, 0.21-0.40 fair agreement, 0.41-0.60 moderate agreement, 0.61-0.80 substantial agreement and 0.81-0.99 almost perfect agreement. Analysis of variance was performed by taking the difference in length (Delta WL, DWL) between files of the same canal. For each canal, seven DWL were independently performed for each method, which resulted in a total of 420 DWL measures. A balanced hierarchical ANOVA was used to evaluate the influence of the three main factors (method, CC, and CL). Other root canal properties included in CC and CL were considered by taking into account a further random factor canal (C). The significance level was set to 5% (\(p<0.05\)). All statistical analyses were performed using the Minitab 15 software package (Minitab Inc., State College, PA, USA).