The accuracy of the auto-stop function of different endodontic devices in detecting the apical constriction

Loss of working length (WL) during preparation of root canals can lead to instrumentation beyond the predefined WL [1‐3]. This effect is mainly attributed to straightening of the root canal during instrumentation. Some studies showed, that this kind of over instrumentation as well as under instrumentation can negatively affect the outcome of the endodontic treatment [4‐6], while it remains unclear what exactly is the most favourable extent of the apical limit of root canal preparation [7, 8]. Moreover, all endodontic instruments produce apically extruded debris [9‐11], even when the preparation stays within the confines of the root canal. Therefore, preparations ending in the periapical tissue will lead to a greater amount of extruded debris that could trigger a neurogenic inflammatory response resulting from an irritation of the periodontal ligament [12] with following postoperative symptomatic apical periodontitis. For this reason, it seems favourable to control the determined WL during root canal preparation to avoid preparations ending in the periapical tissue.

The most commonly practiced electronic determination of endodontic WL is done manually. Usually a small sized stainless steel hand instrument, e.g. an ISO 10 or 15 K-file, is connected to an electronic apex locator (EAL) and introduced into the root canal. When the WL is determined, this length is transferred to the subsequent instruments, e.g. rotary nickel-titanium (NiTi) instruments. Normally WL must be repeatedly controlled manually from time to time to prevent instrumentation beyond the WL [7]. Endodontic motors with integrated apex locators offer a solution to the described problem. These motors continuously measure the WL during canal preparation and contain an auto stop function when WL is reached.

It has been demonstrated in many studies that WL determination with apex locators is generally reliable [13‐16]. Some older endodontic devices with continuous WL control have been evaluated earlier [17‐20]. The TriAuto ZX (J Morita, Tokyo, Japan) endodontic hand piece as well as the Root ZX II (J Morita, Tokyo, Japan) showed acceptable results in determining the WL during rotary root canal preparation [17, 18]. Recently, a study evaluated the accuracy of the newer VDW.Gold RECIPROC motor (VDW, Munich, Germany) WL determination during reciprocating motion and found no differences to other apex locators [21]. The EndoPilot (Schlumbohm, Brokstedt, Germany) is another new generation endodontic motor with continuous WL measuring function. The implemented apex locator uses a new detection method based on an impulse measuring system that should allow a greater accuracy to detect the apical constriction (AC), especially in the presence of sodium hypochlorite (NaOCl) in the root canal [22].

The aim of this study was to compare the accuracy to detect the AC during rotary root canal preparation with the VDW.Gold RECIPROC motor, the EndoPilot and the manual detection of the AC with Raypex 6 (VDW, Munich, Germany) and the following transfer of the measured length to rotary preparation. Our null hypothesis was: The detection of the AC with the three methods shows no differences.

In the RMM group 28 teeth, in the RCM group 29 and in the ECM group 30 teeth were included for statistical analysis. Three teeth were excluded, because the apical foramen was too large and did not allow a reproducible measurement.

The mean distances D_AC were −13.18 μm (SD 88.46 μm) for RMM, −22.70 μm (SD 91.57 μm) for RCM and 18.74 μm (SD 88.11 μm) for ECM. The differences were not statistically significant (P = 0.181) (Table 1). Negative values indicate a position of the root canal instrument beyond (apical to) the AC and positive values indicate a position above (coronal to) the AC. There were no statistically significant differences in the frequencies of preparation beyond WL (Chi² = 4.753, P = 0.096).

The manual technique with RMM as well as the auto-stop methods of RCM and ECM with rotary instruments detected the AC very precisely. There were no statistical differences between the different experimental groups in the mean aberrations from the AC. Therefore our null hypothesis was not rejected.

A lot of studies evaluate the electronically determined WL as the aberration within 0.5 mm to a predefined target area [24‐27], as this is a unit that can be measured clinically. In this study, the mean values of aberration from the AC were within the range of about ±10 to 20 μm and even the maximum aberrations in all test groups were well within the range of ±0.5 mm. One review reported mean aberrations of 0.25 mm (SD 0.17 mm) to 1.36 mm (SD 0.41 mm) from the instrument tip to the AC for manual measurements [8]. But the reviewed measurement devices were of older generations. One study compared the accuracy of the integrated apex locator of the RCM in reciprocating mode with two other manual apex locators [21]. There were no statistical differences between different measurement techniques, which is in concordance with our results. However, the target area was not the AC but a root canal with a standardized length. So far there is only one study of our research group that assessed the EAL function of the EndoPilot device [23]. Mean distances to the AC were not different for manual Raypex and EndoPilot measurements as well as for EndoPilot in rotary mode. But EndoPilot in rotary mode made significantly more preparations beyond WL than the other methods. In this study, ECM showed less tendency towards such over preparation than RCM and RMM, but the overall frequencies of preparation beyond WL were not statistically different.

Earlier generations of endodontic handpieces or motors with integrated EALs and auto-stop function have already shown acceptable results in different studies [17, 18, 25, 28]. Two studies found that the Tri Auto ZX endodontic handpiece (Morita, Kyoto, Japan) when used in the auto reverse mode with certain settings, lead to enlargement of the AC in a high frequency [17, 18]. We also detected a tendency to preparation beyond WL in continuous measuring mode with RCM (59%) and less pronounced with ECM (40%). Moreover, with ECM the mean D_AC had a positive value, indicating a position of the instrument tip coronal to the AC. Perhaps this can be explained by the higher frequency of measurements (that is about 50 times higher) compared to the other devices, so that the auto-stop function is being triggered faster. However, all preparations beyond WL were within a limit of 250 μm apical of the AC.

Interestingly, also the transfer of the manually determined root canal length with RMM to the rotary file and the following rotary preparation without auto-stop function did not lead to a higher inaccuracy in detecting the AC than the preparation techniques with auto-stop function. As the transfer of the measurements was done under simulated clinical conditions with only a precision of 0.25 mm of the endodontic ruler, a higher rate of inaccurate preparations was expected. To the authors’ best knowledge, there are no studies so far that investigated the transfer error from manual WL determination to rotary instrumentation.

A weakness of our study may be the in vitro study design. The clinical situation can alter the function of apex locators because of patient related factors like tooth morphology, metallic restorations, bleeding, suppuration of the root canal or even the irrigant used [28]. But models for evaluation of electronic apex locators are described widely in literature [14, 18, 29] and simulated the clinical situation sufficiently [30]. Moreover, the use of different types of roots may raise some concerns if the experimental conditions for the different test groups were homogenous enough. But it should be considered that under clinical conditions there is no possibility to assess the exact type of the measured AC. Therefore, the operator has to rely on the measurement of the apex locator regardless of the type of root. In this experimental model such different situations were simulated. The homogeneity of the groups was secured by block randomization of the different root types. Additionally, we observed a mean precision in our measurements being more accurate than the precision of previously published studies [8].

The strength of this study is the precision of our specially developed measurement method. It allows the detection of the tip of the measurement instrument in relation to the AC in terms of μm instead of mm. The method has also a high reproducibility [23] because of the automatically computed measurements and is therefore not prone to subjectivity, but does not necessarily reflect the imprecision of the clinical situation.

The manual detection of the AC with RMM as well as the detection of the AC during continuous measurement of WL in rotary mode with RCM and ECM resulted in a high degree of accuracy with only small aberrations from the targeted area. Therefore, the auto-stop function of the tested devices is a valuable addition to the endodontic armamentarium and enhances the safety of rotary root canal preparation.