An in vivo experimental study was conducted on 120 canals from 98 maxillary premolar teeth scheduled for endodontic treatment from July 2013 to June 2014. The study was performed in the Department of Conservative Dentistry and Endodontics, Rural Dental College, Loni, Maharashtra, India, after obtaining an institutional ethical clearance (Reg No. RDC/RC/2013/45). Sample size was calculated using Raosoft online sample size calculator [8]. The power of the study was set at 80%.

Two variables such as the type of metal and the apical size of the instruments were evaluated. Teeth with open apices, internal or external root resorption, severe curvatures, calcified canals (confirmed by using preoperative Radiovisiograph) and patients with heart pacemakers or had a contributory medical history were excluded.

After detailed case history, patient was informed about the procedure and consent was taken. Teeth were isolated under rubber dam and standard access cavities were prepared. After checking the patency of the root canal with #10K file, the coronal portion of each canal was preflared using sequential Gates Glidden drills #4, #3, and #2 (Mani Inc. Japan) later the canal was irrigated with 10 ml 2.6% Sodium hypochlorite solution (Chen Chems, Chennai, India) and pulp extirpated with a barbed broach (Spirocolorinox, Dentsply). Working Length was estimated by using Root ZX apex locator as per manufacturer’s instructions with stainless steel and NiTi #8, #10 and #15 ISO K files.

Electronic measurements were made (in sequence) until there were stable reading for at least five seconds and the values were recorded. The actual working length was established by Direct Digital Radiographic Method (Radiovisiography or RVG) for each canal which is 0.5 mm short of the radiographic apex by using Ingle’s method and recorded separately [9].

The results were analysed using one-way ANOVA and Post-hoc Tukey HSD tests. Significance was set at p<0.05. The analysis was carried out using Statistical Package for the Social Sciences (SPSS 16.0, SPSS Inc., Chicago, IL, USA).

[Table/Fig-1] shows mean and standard deviation of the various groups and [Table/Fig-2] show a highly significant difference between Ingle’s method and apex locator measured with stainless steel and NiTi #8 files and significant difference between Ingle’s method and apex locator measured with NiTi#10 files (p <0.110).

There was no significant difference found between Ingle’s method and apex locator measured with stainless steel and NiTi #15 files and stainless steel #10 file (p <0.643). Stainless steel files showed more accurate results compared to their corresponding NiTi files. In intergroup comparison #15 files showed more accurate length and #8 files showed least accurate length in both stainless steel and NiTi file system.

The accuracy of the measurements performed with the use of electrical methods depends on several factors including the type and size of the root canal instrument used for taking the measurement, the shape of the root canal, the diameter of the apical foramen and the type of irrigation solution [10]. Many authors report that the width of the apical foramen and the type of endodontic instrument determine the obtained result. Ebrahim’s study showed that, the working length with the smaller size files determined shorter working length when root canal diameter was increased [11]. Herrera M et al., also did not find any significant influence of the width of the apical opening which was above 0.2mm on the accuracy of determination of the working length [12].

The size of the foramen, surface of contact of the active electrode with the walls and enlargement of the apical foramen diameter can contribute to the errors in determining working length [13].

Maxillary premolars were selected for this study as Root ZX located the minor apical diameter more correctly in anterior and premolar teeth than molar teeth [14].

The type of instrument and the material it is made of may also have an influence on the accuracy of the measurements. Various instruments were used in endodontic treatment which is made of stainless steel (73% iron, 9% nickel, and 18% chromium), carbon steel, chromium and nickel alloys and nickel titanium (54% nickel and 46% titanium) alloys. In the present study, stainless steel files showed more accurate result than NiTi files. Reason for this might be NiTi files effect voltage gradient of apex locator more compared to stainless steel files [15]. Nekoofar MH et al., evaluated the accuracy of Neosono Ultima EZ apex locator using nickel-titanium and stainless steel files. The accuracy of the nickel-titanium and stainless steel was 94% and 91%, respectively, and there was no statistically significant difference [16]. In this study, In comparison to the steel instruments, NiTi instruments showed more deviation in the working length determination this may be due to increased flexibility of NiTi instruments [4]. So, our null hypothesis was rejected.

The Root ZX and Foramatron D10 showed significantly better scores than the Apex NRG and Apit 7, and may be reliable to determine the working length of teeth with a wide apical foramen if a tight-fit file is used [17]. McDonald NJ recommended the use of files with sizes comparable with the root canal diameter, claiming that this would result in more accurate readings. The length of the enlarged canals was measured using small-sized files and large size files matching the canal diameter. They found that the Root ZX was accurate even when the file was much smaller than the diameter of the canal and the measured lengths obtained with small and large size files were comparable [18]. The results of the present study showed significant differences when smaller NiTi and Stainless steel files (ISO no. #8 NiTi and Stainless steel files, ISO no. #10 NiTi files) were used. The ISO no.15 NiTi and Stainless steel files were more reliable in estimating the working length.

Further, the canals were initially preflared to improve the accuracy of determining working length by increasing the Canal patency and decreasing the accumulation of dentin debris. These factors may disrupt the electrical resistance between the inside of the canal and the periodontal ligament leading to inconsistent EAL readings. Constant recapitulation was also performed as it ensures accurate electronic length readings during instrumentation [19].

The limitation of the study were the inabilty to include different groups of teeth and failing to evaluate the effect of the various test variables in different root canal enviroments. Further studies are required to evaluate the effect of instrument size and type on the accuracy of EAL on various groups of teeth and in different root canal environments.

Under the limitations of this invivo study, stainless steel and NiTi instruments with apical size similar to diameter of apical constriction yielded the best results. Stainless steel files yielded better results when compared to NiTi files. The knowledge of these factors may help in correct determination of the working length which facilitate proper root canal treatment; prevent complications, and help to obtain satisfactory results.