Accuracy of a new apex locator in ex-vivo teeth using scanning electron microscopy

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Drs. Maria Bonilla, Taner Cem Sayin, Brenda Schobert, and Patrick Hardigan compare the accuracy of root canal working lengths in 200 ex-vivo teeth determined using a fourth-generation electronic apex locator and a new fifth-generation electronic apex locator

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A key factor affecting the success of endodontic treatment is the establishment of an accurate root canal working length. The ideal cleaning, shaping, and disinfection of the root canal system depends on the accurate determination of the root canal anatomy from canal orifice to the canal-dentinal-cement (CDC) junction.


The apical anatomy of root canals has been investigated in several research studies and review articles (Kuttler, 1955; Ricucci, 1998; Green, 1956; Pineda, Kuttler, 1972). The apical CDC junction, also defined as the minor diameter, is the anatomical landmark that segregates the pulp tissue from periodontal tissues. Dummer, et al., described the morphological variations of apical CDC junctions in 1984. Many of these variations cannot be determined radiographically. The distance between the major diameter and the minor diameter of the apex can vary, but usually it is between 0.5 mm to 1 mm (Ricucci, 1998; Green, 1956; Pineda, Kuttler 1972).

To preserve the vitality of the periapical tissues, the ideal cleaning, shaping, and root canal filling materials have to be limited to the apical CDC junction. Therefore, it has become the preferred landmark for the apical endpoint for root canal therapy (Nekoofar, et al., 2002). 

Procedural errors — such as over-instrumentation or under-instrumentation — can occur because of inaccurate estimates of root canal length. Over-instrumentation can damage the anatomy of the root end and also injure the periodontal tissues. On the other hand, under-instrumentation may create a suitable environment for bacteria that might cause a less favorable outcome of the endodontic treatment. Therefore, the accurate determination of the working length is an important goal for the success of the root canal treatment. Several methods can be used to measure the root canal working length.

Radiographs can visualize the root canal but are limited to two dimensions and are technique-sensitive to operator inputs (Cox, et al., 1991). A study by Brunton, et al., (2002) showed that electronic apex locators (EALs) could be used to reduce the radiation exposure time to the patients by requiring less radiographs. Some studies found that there were no significant differences between the accuracy of EALs and radiographs (Hoer, Attin, 2004; Vieyra, Acosta, 2011). A study by Real, et al., (2011) found that EALs were significantly more accurate than digital sensors.

The use of EALs for determining the root canal working length has become an indispensable part of endodontic treatment. More accurate EALs have evolved in recent years by improving the basic principles upon which the measurements are performed. In 1918, Custer proposed the development of electronic devices to determine the working length. In 1942, Suzuki presented the first generation of EAL to use the electrical resistance properties of the root canal to determine its working length. Sunada (1962) determined the electrical resistance value constantly at 6.5 ohms. This theory considered the electrical resistance between the oral tissues and the periodontal ligament to remain constant.

The second generation of EAL had the peculiarity of working with impedance principles. An example of the third-generation EAL is the Root ZX® (J. Morita) which worked with a constant frequency principle. A fourth-generation EAL was created by Gordon and Chandler (2004), which worked with multiple frequencies. 

The first version of Root ZX EAL used the average measurements of two frequencies of 0.4kHz and 8kHz. Kobayashi and Suda (1994) described this method as the EAL frequency ratio. The most recent version of Root ZX uses multiple frequencies and can be classified as a fourth-generation EAL (Kobayashi, Suda, 1994). 

The fifth generation of EAL also uses multiple frequencies, in addition to calculating the root mean square (RMS) values of the electric signals. The RMS represents the energy of the electric signals, and therefore, it is claimed to be less affected by electrical noises affecting other physical parameters such as amplitude or phase of electrical signal that are used by other EALs. An example of a fifth-generation EAL is the Propex Pixi, which is a newer version of recently designed EAL Propex (Dentsply Maillefer, Switzerland).

Aims and objectives

The aim of this study was to compare the accuracy of root canal working lengths in 200 ex-vivo teeth determined using a fourth-generation EAL (the Root ZX II) with a fifth-generation EAL (the Propex Pixi). The Propex Pixi and Root ZX II use signals at two different frequencies to calculate the file tip position relatively to root apex. Furthermore, the technology utilized in Propex Pixi differs from the technology used in Root ZX II: Propex Pixi by measuring the RMS of the electric signal, which is further used for calculations. Because of these technology differences, there is a need to compare the accuracy of the Propex Pixi with the Root ZX II to determine root canal working lengths. 

Materials and methods

After IRB approval was obtained, an archive of 200 sound human permanent teeth with completely formed apices was used in this study. The teeth were disinfected by submerging them in a 6% sodium hypochlorite (NaOCl) solution for 15 minutes. They were then rinsed for 10 minutes with distilled water. This disinfection cycle was repeated 3 times for each tooth. The teeth were stored in 20-ml sterile scintillation vials filled with distilled water in a refrigerator at 5ºC until use. 

Prior to inclusion in this study, the root surfaces and apices of each tooth were examined under x16 magnification using a surgical microscope (Global Surgical Corp.) for a possible fracture or resorptive areas. If any defects were observed in a tooth, it was discarded from this study. The outer surfaces of the teeth were cleaned by removing tissues with a 15c scalpel (Aspen Surgical). Photographs were taken of each tooth in a buccolingual as well as a mesiodistal view (Figure 1). Digital radiographs (Schick Technologies) for each tooth in a buccolingual and a mesiodistal direction were also taken as pre-operatory procedure (Figure 2).

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Access cavities were prepared with a high-speed handpiece and a fissure bur (Maillefer, Switzerland) with water coolant, under the surgical operating microscope. Pre-flaring of the root canals was not performed. The root canals were irrigated with 6% NaOCl before the introduction of any file. Patency was established by introducing a No. 6 or No. 8 hand file (Maillefer, Switzerland) until it emerged in the apical foramen, and this was corroborated by visualization using the surgical microscope. Each of the teeth was embedded in a dental device for training purposes with alginate. The 200 teeth were randomly assigned to the Propex Pixi (n = 100) group or the Root ZX II (J. Morita) (n = 100) group. 

The root canal working length measurements were carried out according to the manufacturers’ instructions. The lip clip electrode was attached to the device, and the other electrode was attached to a file that fit snugly in the apical portion of the root canal. Digital radiographs for each tooth in a buccolingual and a mesiodistal direction were taken to corroborate radiographically that the working length had been established. The files were then withdrawn from the canals to measure them with an endodontic ruler (Maillefer, Switzerland). The reference points were marked with silicone stoppers. All the working lengths were measured using the same endodontic ruler. The working lengths were recorded on a spreadsheet.

The files were reinserted into the root canal and cemented with a flowable composite resin to avoid any movements from within the root canal. The apical 4-mm portion of the root canals was carefully shaved in a longitudinal direction using a fine diamond bur (Maillefer, Switzerland) and a scalpel under a Olympus SZX7® stereomicroscope at x8 magnification to prevent touching the files with the diamond bur. 

The apical portion of the teeth and files were observed in micrographs at x40 magnification using an FEI Quanta 200 FEG Environmental Scanning Electron Microscope in the low-vacuum mode, and the distance from the file tip to the CDC junction was measured with Scandium image software (FEI Company) (Figure 3). A Welch’s t-test test was used to compare the accuracy of the working lengths determined by the two EALs at a significance level of P<.05.


The mean distance from the final working length to the file tip was 0.21 ± 0.25 mm for the Propex Pixi EAL while it was 0.08 ± 0.22 mm for the Root ZX II EAL (Table 1, Figure 2). A difference of 0.13 mm (95%: 0.23 to 0.47) was found between the Propex Pixi and Root ZX II EALs. The Propex Pixi was accurate 88% of the time to ± 0.5 mm and 98% accurate within ± 1.00 mm (Table 2). The Root ZX II was accurate 97% of the time to ±0.50 mm and 99% accurate within ±1.00 mm (Table 2). There was no significant difference in the accuracy of the working lengths determined by the two EALs (P > 0.05).

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This study is the first to investigate the accuracy of the root canal working length measurements of a new fifth-generation EAL called the Propex Pixi. Given the importance of accurate root canal working length measurements to the outcome of endodontic treatment, it is essential that all new EALs be evaluated for their accuracy. 

The multiple frequency processing technology, and use of RMS incorporated into the Propex Pixi may have theoretical advantages for increasing the accuracy of the working length measurements, by reducing the electrical noises affecting other physical parameters like amplitude or phase of electrical signal that are used by other EALs. But the technology improvements were not enough to make the Propex Pixi significantly more accurate than the Root ZX II (P > 0.05), which appears to be an extremely accurate fourth-generation EAL. 

The Propex Pixi and Root ZX II gave root canal working lengths of 0.21 and 0.08 mm, which were accurate 88% and 97% of the time within 0.5 mm of the actual root canal length. These high levels of accuracy appear to be beneficial to the practice of endodontics, and since both EALs had similar levels of accuracy, both the Propex Pixi and Root ZX II EALs can be recommended for use in endodontics.

Traditionally, a radiographic evaluation has been the primary technique to determine the vertical limit of instrumentation, irrigation, and obturation in endodontic therapy (Fouad, Rivera, Krell, 1993). However, El Ayouti, et al., (2005) concluded that radiographic evaluation was not accurate enough and causes over-instrumentation, especially in 56% of premolars. Williams, et al., (2006) concluded that the files that seem to be beyond the apex were longer by an average of 1.2 mm. In contrast, files that seemed to be short of the apex on the radiographs were 0.47 mm closer to the apical foramen. 

The new technologies in EALs appear to make them more accurate; they are more accurate than radiographs, which are only useful to corroborate the EAL readings. Radiographs are useful for visualizing the existence of pathology, the amount of root to treat, and the direction of curvatures in the root canal system (Ricucci, 1998; Dummer, McGinn, Rees, 1984; Gordon, Chandler, 2004). The use of EAL reference points has been controversial. The major diameter reference point has been claimed as the more reliable and accurate reference point than minor diameter because the minor reference point is more difficult to locate (Martinez-Lozano, et al., 2001; Lee, et al., 2002). Lee, et al., (2002) recommended using the major foramen as reference point to determine the accuracy of EALs. The anecdotal evidence suggests it is extremely important to follow manufacturers’ EAL instructions without any deviation and to always have a high battery charge. Some previous studies discovered that EALs can only detect the major foramen (Mayeda, et al., 1993; Ounsi, Naaman, 1999). Therefore, the present study used the CDC junction as the measuring point for both EALs.

The Propex Pixi is a new EAL, and no literature is available to compare its working length accuracy with the present study. The results of the present study did demonstrate that it has a similar accuracy to the Root ZX II. The accuracy of the Root ZX II has been successful to determine the root canal working length within 1 mm in 96.5% of the cases observed by Shabahang, et al., (1996). The accuracy of the Root ZX II was confirmed in a study by Pagavino (1998), which had an 82.75% success in locating the root canal working length with a 0.5 mm tolerance. In a study by El Ayouti (2005), the Root ZX II also showed 90% accuracy within a 1 mm range when compared to Raypex® (VDW) (74%) and Apex Pointer (Micro-Mega) (71%).

Welt, et al., (2003) also found that Root ZX II was 90.7% accurate within 0.5 mm at the apical constriction. An in-vivo study by Silveira, et al., (2011) found that the Root ZX II was 91.7% accurate in locating the apical constriction. On the other hand, percentages for accuracy in Tselnik’s 2005 study were around 75% for Root ZX. The first generation of Propex also showed similar accuracy for determining the apical constriction with Root ZX II in Plotino’s (2006) study. The accuracy of the Root ZX II measurement to within 0.50 mm of the root canal working length 97% of time, and within 1 mm 99% of the time in the present study, appears consistent with previous research. Some of the accuracy variations may be due to differences in operator technique sensitivity, handling of the EALs, placement of files, and radiographic angulation visualization of the file inside the root canal. 

Stainless-steel hand files were used in the present study. The file sizes were different in each root canal because of differences in root canal sizes. According to Herrera (2007), the Root ZX II EAL is more accurate if the diameter size of the file is less than a No. 60 (0.6 mm). The largest apical file diameter used in the present study was 0.30 mm. Shaving the apical portion of the canal also gave a clear visibility of the CDC junction, and it seemed to allow more accurate measurements from radiographs using the SEM. 

The results of the present study suggest that our improvements to the methodology for measuring working length accuracy can help improve the reliability of EALs — in the case of the Root ZX II, up to 99% of the time within 1 mm. Root canal irrigation with 6% NaOCl was used in the present study to dissolve the necrotic pulp around the orifice and the coronal portion of the canals before determining the working lengths. The antimicrobial activity and the removal of the organic remnants by irrigants are very important for the success of endodontic treatment. Previous studies showed that some EALs had inaccurate measurements when used with other irrigation solutions (Kaufman, Keila, Yoshpe, 2002; Haffner, et al., 2005). The present study confirmed that both EALs can provide accurate measurements in the presence of 6% NaOCl. We recommend further modification of EALs to select the type and dilution of irrigation solutions to avoid this problem, and to help improve the accuracy of EALs under all types of operating conditions. While using the Propex Pixi, we did appreciate its smaller size compared to traditional sized EALs; this gave a little more space in the clinical setup. 


The multiple frequency processing technology and the use of RMS incorporated into the Propex Pixi may have theoretical advantages for increasing the accuracy of the working length measurements. But the technology improvements were not enough to make the Propex Pixi significantly more accurate than the Root ZX II (P > 0.05), which appears to be an extremely accurate fourth-generation EAL. These high levels of accuracy appear to be beneficial to the practice of endodontics, and since both EALs had similar levels of accuracy, both the Propex Pixi and Root ZX II EALs can be recommend for use in endodontics.


The authors thank Dr. Armando Lara from University of Tlaxcala, Mexico. 




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