# Working Length Transfer in the Endodontic Clinical Practice: A Comparative Study
## 4 Discussion  
Based on the results from the present study, the WL transfer that matches the instruments side by side from tip to shank in opposite directions may be more accurate than the use of a manual endodontic gauge during endodontic clinical practice.  
The correct WL should be preserved in order to avoid inner or outer apical transportation, particularly when shaping instruments of augmented taper are used [15; 16]. Over-instrumentation may cause an irregular foramen widening, a lack of sealing with a high rate of debris extrusion and post-operative discomfort [17; 18]. Alternatively, under-instrumentation can create ledges, blocks and other canal aberrations that may lead to inadequate shaping and filling, negatively affecting the disinfection and the long-term prognosis of the root canal treatment [1; 3; 15; 19]. Therefore, over- and under-instrumentation should be both equally reduced in order to achieve antimicrobial and sealing efficiency [10; 11; 12; 13].  
However, the canal anatomy and the coronal flaring may influence WL variations during shaping, especially in the mesial root of the mandibular molars and in the mesio-buccal root of the maxillary molars [20; 21]. In those roots, where the original anatomy is particularly complex, the preliminary flaring of the canal coronal portion is suggested in order to facilitate WL achievement [22; 23; 24]. Nevertheless, the electronic apex locator's accuracy is improved by canal flaring [25; 26; 27]. However, it can lead to a variation in the initial WL, which is independent from the root canal preparation technique [28; 29; 30; 31]. The present findings reported that severe canal curvatures (CC factor) may lead to larger negative DWL, confirming the influence of canal anatomy on WL transfers, as discussed in a  
Figure 3: Interval plot of WL difference values for the canal curvature (CC) factor.  
previous study [31]. Therefore, the WL measure transfer is constantly repeated during clinical practice and a possible error may dramatically influence the quality of the endodontic treatment [31].  
The precise transfer of a correct WL from the manual KF to the subsequent shaping instruments is mandatory for an adequate shaping and filling of the root canal system. However, it could be challenging even for an expert operator using magnification devices. No previous studies have reported the precision of different WL transfer clinical methods between endodontic instruments.  
Clinically, most operators use a manual endodontic gauge to report the WL measurement from the KF to the following shaping instruments, but this method can lead to a mean error of 0.20 mm, even with the use of a proper optical magnification. On the contrary, the matching of the instruments side by side to compare the WL (method II) may decrease this discrepancy to a negligible value of 0.09 mm, which is reasonably too low to influence the clinical outcomes. Instrument matching is intended as a visual appraisal of the position of the silicone stops for both instruments. Therefore, the two instruments are placed side by side in order to compare the position of the silicone stops until they are perfectly congruent and to prevent the shift in the silicone stop using the tips of the files. This way, it is possible to prevent any risk of puncture wounds, addressing safety concerns for the clinician. However, with respect to the maximum precision in WL maintenance, it is important to consider the necessity to maintain the silicone stop perpendicular to the instruments' axis and prevent touching of the coronal reference point during shaping.  
Reducing the errors in WL transfer during clinical practice may be significant for the improvement of the endodontic long-term prognosis [32; 33; 34]. Both over- and under-instrumentation are equally detrimental for treatment success and, therefore, they should be evenly prevented [16; 17].