|Year : 2012 | Volume
| Issue : 3 | Page : 115-130
The accuracy of Root ZX electronic apex locator
Osama S Alothmani
Division of Endodontics, Department of Conservative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia
|Date of Web Publication||30-May-2013|
Osama S Alothmani
Assistant Professor, P.O. Box 9216, Makkah
Source of Support: None, Conflict of Interest: None
The aim of this review was to evaluate studies assessing the accuracy of Root ZX when used for working length determination in permanent teeth and to identify factors affecting the device's precision. An electronic search for articles published in English language since 1994 was conducted on the Medline via Ovid interface. All issues of the International Endodontic Journal, Journal of Endodontics and Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontology since 1994 were manually searched. The reference lists of review articles were cross-referenced to identify any potential publications. A total of 76 publications fulfilled the inclusion criteria. The studies varied in their methodologies, and most of them did not adhere to the manufacturer's recommendations while operating the device. The Root ZX failed to consistently detect the apical constriction or the apical foramen. Nonetheless, it mostly allowed file tip placement in the area bounded by these two landmarks, especially when the 0.5 mark of its digital meter was adopted. Tooth-related factors potentially influencing the precision of Root ZX included pre-operative pulp status, tooth type, position of the apical foramen, canal obliteration, and the size of apical diameter. Operative factors including coronal pre-flaring, presence or absence of irrigants, file size, file alloy, and mode of file operation could also influence the performance of Root ZX. In conclusion, adopting the 0.5 mark of the digital meter of the Root ZX reduces chances of violating the apical foramen. Further, factors influencing the precision of Root ZX must be considered while operating it.
Keywords: Accuracy of Root ZX, ex vivo accuracy of Root ZX, factors influencing the accuracy of Root ZX, in vivo accuracy of Root ZX
|How to cite this article:|
Alothmani OS. The accuracy of Root ZX electronic apex locator. Saudi Endod J 2012;2:115-30
| Introduction|| |
Electronic apex locators (EAL) have gained huge popularity in recent years as they are commonly used for working length (WL) determination and several EALs, differing in their electrical basis of function, are available in the market. EAL devices do not detect apices, and they should be more precisely labeled as electronic root canal length measuring apparatuses. ,,,, These devices help in locating the root canal terminus utilizing certain electrical properties of the tooth and the surrounding periodontium. , EAL devices function by completing an electrical circuit through the body. One side of the circuit is connected to a root canal instrument, while the other end is connected to a lip clip. The circuit becomes partially completed as the file is introduced and advanced into the root canal. The changes in the electrical characteristics become less attenuated as the file contacts the periodontal tissue at the apical foramen (AF). 
The Root ZX EAL (J. Morita Co., Kyoto, Japan) has been extensively tested in vivo and ex vivo and has become the gold standard to which new devices are compared. , The circuit of Root ZX is mainly based on detecting the changes in electrical capacitance in the vicinity of the AF. It works by calculating the impedance ratio of two simultaneously produced frequencies (0.4 and 8 kHz). The ratio represents a definite value that reflects the file position inside the canal. This ratio decreases as the AF is approached, and it has a value of 0.72 at the apical constriction (AC), 0.5-mm short of AF.  It is noteworthy that the authors assumed that the AC was at 0.5 mm from the AF without confirming that histologically. Hence, it is more accurate to consider that the ratio of 0.72 is found at 0.5 mm short of the AF; the AC might or might not be located at this area.
Considerable decline in impedance ratio was observed as passing the AC and approaching the AF, indicating high sensitivity of the ratio method.  Impedance decreases as the file penetrates the canal with its value markedly dropping at the AF. This is attributed to the fact that the electrical current conveniently passes through shorter dentinal tubules within thinner dentinal mass as the file advances apically, leading to the foreseen fall in impedance.  Impedance is a product of resistance and capacitance, both being demonstrated in human teeth. , In dry canals, the resistance value decreases towards the apex, while the value of capacitance increases.  The presence of electrolytes inside the canal further reduces its resistance ,,, and increases its capacitance, , i.e., at the apical area, resistance is low while capacitance is high; the presence of fluids further accentuates this condition. These circumstances favor the circuit of the Root ZX because it relies on detecting the change in electrical capacitance (rather than resistance) near the AF regardless of the presence of electrolytes.  Nonetheless, 100% accuracy should not be expected.  The performance of the Root ZX might be influenced by different factors that are difficult to detect and interpret. 
The aim of this review was to evaluate studies assessing the performance of Root ZX in permanent teeth. Potential factors affecting its precision were also highlighted. Studies that evaluated the newer versions of Root ZX (Root ZX II, Tri Auto ZX, Root ZX mini, and Dentaport ZX; all produced by the same manufacturer and function on the same principle) were also considered. The Tri Auto ZX is a cordless handpiece that allow root canal preparation with rotary nickel titanium (NiTi) files with concomitant length control. It can also be used in a non-revolving mode with hand files. Root ZX II and Dentaport ZX come with two attachments: One for hand files and another one for the handpiece. Either option can be used.
An electronic search using Medline via Ovid was conducted to identify publications in peer-reviewed journals considering the accuracy of Root ZX. The following search terms were used: Electronic length determination, EALs, Root ZX, and permanent human teeth. The Medical Subject Heading (MeSH) option was used to include keywords related to search terms. The search was limited to English language and to publications from 1994 onwards. Furthermore, all issues of the International Endodontic Journal, Journal of Endodontics and Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontology since 1994 were manually searched for relevant articles. The reference lists of review articles were cross-referenced to identify any potential publications.
The search strategy gave 79 articles, of which 52 studies were conducted on extracted teeth. In another 22 studies, the electronic measurements were obtained in vivo. Two studies had both ex vivo and in vivo measurements. , Few studies evaluated the accuracy of Root ZX radiographically. ,,,, Precise canal length determination is only possible histologically. Radiographic determination of EAL accuracy is an inaccurate method due to the inability of radiographs to exactly demonstrate the location of the AC and AF .,,,, Thus, three studies that depended on the radiographic verification of Root ZX performance were excluded ,, and only the radiographic accuracy results of two studies were excluded. , Hence, a total of 76 studies were considered for this review.
| Methodologies Assessing the Accuracy of Root ZX|| |
Accuracy reflects the ability of the method to detect the target apical limit.  The accuracy of Root ZX has been extensively tested ex vivo and in vivo. The findings of these studies are difficult to compare as a result of variations in the methodologies and descriptive terms used.
Several in vivo studies followed a protocol that involved locking and fixing the file in place at the length given by the EAL then longitudinally shaving away apical cementum and dentine until the file tip could be visualized. This allowed the calculation of the distance between the file tip and different apical reference points including the AF and AC/minor diameter, ,,,,,,,, the cemento-dentinal junction (CDJ),  the most coronal border of the AF, ,, or 0.5-mm coronal to the AF. , Several ex vivo studies also followed the protocol of file fixation. ,,,,,,,,
Alternatively, the distance between the fixed file and the AF was calculated after extracting the teeth and rendering them transparent. , Instead of locking the file, another approach determined the electronic length in vivo and then measured the gold standard length after extraction utilizing the same coronal reference point used for electronic length determination. These measurements were done in relation to the AC,  AF,  or to a level coronal to the AF by 0.5 mm. 
Wrbas et al., secured the file at the electronically identified position within a light-cured composite pattern that allowed an accurate file repositioning. Hence, several EAL devices could be tested in the same tooth. Another unique approach assessed the relation between the file tip and the AC by taking an impression of the apical area only if the file protruded beyond the AF. Otherwise, root shaving was performed.  A distinctive scheme involved longitudinal shaving of the apical 4 mm of the roots after root canal filling and then relating the apical extent of the root filling to the position of the CDJ under the surgical microscope. 
Many ex vivo studies assessed the device accuracy by comparing the Root ZX measurement against the length required to place the file exactly at the AF, ,,,,,,,, 0.5-mm shorter than this length, ,,,, 1-mm shorter than this length, , or at the most coronal border of the AF. ,,,, In another study, the teeth were split after recording the electronic length. The distance from the coronal reference point to the AC was calculated and related to the electronic length. 
In vivo studies
The manufacturer recommends advancing the file to the APEX mark as indicated on the digital display and then withdrawing the file to the bar located between the APEX mark and (1) mark  . This bar is commonly known as the 0.5 mark.
[Table 1] lists in vivo studies reporting the accuracy of Root ZX in relation to the AC when the 0.5 mark was used. Only three studies adhered to the manufacturer's recommendation. ,, In the remaining studies, file advancement was stopped once the 0.5 mark was reached.
|Table 1: In vivo studies reporting the accuracy of Root ZX in relation to the apical constriction|
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Based on [Table 1], the mean distance between the file tip and the AC ranged from 1.35 (±0.95) mm coronal to the AC to 0.51 (±0.65) mm beyond it. The range of readings precisely locating the AC was 0-89.1%, while the range of readings violating the AC was 6.2-88.9%. A minimum of 35% of measurements were within the strict tolerance limit of ± 0.5 mm, whereas at least 88.9% of the recordings were within the lenient tolerance limit of ± 1 mm.
Other in vivo studies evaluated the accuracy of Root ZX in relation to the AF [Table 2]. Four of the listed studies used the device according to the manufacturer instructions. ,,, Based on [Table 2], adopting the APEX mark was associated with an increased likelihood of establishing the file tip beyond the AF.
|Table 2: In vivo studies reporting the accuracy of Root ZX in relation to the apical foramen|
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Grimberg et al., related the file tip to a position coronal to the AF by 0.5 mm and found that the performance of the two modes of the Tri Auto ZX was identical. Another study related the file tip position to the most coronal border of the AF in vital and necrotic teeth and found no significant difference in the performance of Root ZX II.  Pagavino et al., evaluated the Root ZX performance only in vital teeth and found that most of the readings were beyond the most coronal border of the AF. Only one in vivo study related the file tip position to the CDJ and reported that the mean distance of the file tip was 0.09 (±0.02) mm coronal to the CDJ in 10 mandibular premolars. 
Ex vivo studies
Embedding teeth in media possessing electrical properties similar to the periodontium can be useful for testing and comparing the performance of EAL devices.  Different embedding media have been used to reproduce the conductivity of periodontium. , Aurelio et al., suggested embedding extracted teeth in agar mixed with phosphate-buffered saline. For a cheaper model, replacement of agar with gelatin was proposed. 
Another proposed medium is alginate. ,, Alginate provided more firmness and better tooth manipulation than the gelatin model. However, this model required constant moistening and refrigeration when not in use. Furthermore, canal irrigation with NaOCl was prohibited because of potential deterioration of alginate. ,, Others found that NaOCl did not adversely affect this model. ,, However, teeth were dislodged out of the model when canal preparation was done with large files.  Thus, stabilizing the tooth with a layer of acrylic poured over the top of alginate has been advised.  The instability of alginate's electrical resistivity has also been criticized as the resistance increases considerably after 5 min from mixing.  Regardless of the popularity of this model, the duration for which the mixed alginate was used differed among studies. Many studies identified a 2-h timeline for using the freshly mixed alginate. ,,,, One study specified a shorter timeline of 30 min.  On the contrary, the same alginate mix was used for 45 days with the model being kept moist and r efrigerated when not in use.  In several studies, this time interval was not mentioned. ,,,,,,,,,,,, The extent to which this variability has affected the findings of these studies, if any, is unknown.
In other studies, saline has been used to complete the electrical circuit. ,,,,, With a saline model, there is a chance for a faulty short reading because of the possibility of saline leaking into the canal.  A digital mounting model for reliable length measurements in which teeth were situated in saline has been introduced.  This mounting model has been frequently adopted to eliminate potential errors arising from accidental rubber stopper movement and imprecise length measurements when hand rulers are used. ,,, Another model is designed to insert the tooth in a porous plastic block, usually used for flower arrangement, filled with Ringer's solution to complete the circuit.  A recently proposed model utilized an endodontic training kit filled with an electroconductive gel. , This simple and affordable model provides convenient handling of the tooth because the roots are secured by the training kit holder. Furthermore, the steadiness of the resistive properties of the electroconductive gel over a period time has been demonstrated. 
The mean length indicated by the Root ZX  and Dentaport ZX  varied when the teeth were placed in different embedding media. However, the differences were not statistically significant. The best length determination for anterior teeth was obtained when an alginate model was used. For premolars and molars, the saline and gelatin models were the optimum, respectively.  It must be kept in mind that ex vivo studies permit controlled testing conditions  and that the electrical circuit of EAL would be subjected to more variables in vivo.
Few ex vivo studies reported the accuracy of Root ZX in relation to the AC [Table 3]. Two studies adhered to the manufacturer's instructions. , Based on [Table 3], at least two-thirds of the readings were within ± 0.5 mm from the AC, while all of them were within ± 1 mm. Adhering to the manufacturer's instructions led to exact localization of the AC in 42% of the teeth.
|Table 3: Ex vivo studies reporting the accuracy of Root ZX in relation to the apical constriction|
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Few other studies selected a point coronal to the AF by 0.5 mm as the apical reference point to which the accuracy of Root ZX was compared [Table 4]. Three of the listed studies used the Root ZX according to the manufacturer instructions. ,, Based on [Table 4], the range of readings within the strict tolerance limit was 80-97.4%, while all the readings were within the lenient tolerance limit.
|Table 4: Ex vivo studies reporting the accuracy of Root ZX in relation to 0.5 mm coronal to the apical foramen|
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Many ex vivo studies focused on the accuracy of Root ZX in relation to the AF utilizing the 0.5 mark [Table 5]. Four of the listed studies adhered to the manufacturer instructions. ,,, Based on [Table 5], the average file tip position was always coronal to the AF except in one study. The AF was precisely located at a maximum of 30%. At least 50% of the recordings were within the strict tolerance limit, whereas a minimum of 90% of the measurements were ±1 mm from the AF.
|Table 5: Ex vivo studies reporting the accuracy of Root ZX in relation to the apical foramen when the 0.5 mark was used|
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Studies using the APEX mark to assess the precision of the Root ZX in relation to the AF are listed in [Table 6]. The mean length was beyond the AF in only three studies; two of them used the same model. The range of precise readings was 13.2-85%, while 7.7-46.55% of the readings were beyond the AF. At least 58.7% of the readings were within the strict tolerance limit. The range of readings within the lenient tolerance limit was 97.4-100%.
|Table 6: Ex vivo studies reporting the accuracy of Root ZX in relation to the apical foramen when the APEX mark was used|
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One ex vivo study related the file tip position to the CDJ and found that using the APEX mark of Root ZX allowed the placement of the file tip at an average distance of 0.45 (±0.33) mm coronal to the CDJ without any incidence of over-instrumentation beyond the AF. 
Which apical landmark does the Root ZX detect?
According to Kobayashi and Suda,  the inventors of the Root ZX, the device functions by calculating the impedance ratio of 0.72 detectable at 0.5-mm short of the AF. Several studies refuted this claim and recommended that the Root ZX should be used to detect the AF. ,,,
The user manual of the Root ZX states that the machine detects the AC.  The Root ZX was able to detect the narrowest part of the canal with high consistency even when the AC was eliminated, enlarged by instrumentation, or shifted to the AF. ,, These studies collectively showed that the Root ZX targets the narrowest part of the canal regardless of this being the AC or being created by root canal preparation.
Based on [Table 1], [Table 2], [Table 3], [Table 4], [Table 5] and [Table 6], it is clear that the Root ZX fails to consistently detect either the AC or AF. However, it appears that the majority of the measurements would lie between these two boundaries, i.e. the length recorded would be mostly apical to the AC and coronal to the AF, especially when the 0.5 mark is selected as the furthest apical limit.
More studies are needed in which the recordings of Root ZX are related to the topography of the AC and the morphology of the AF in order to further elucidate the exact landmark the Root ZX targets. It is possible that the use of hand (or rotary) files during electronic measurements might alter the apical morphology. Weiger et al., recommended using loosely fitted hand files throughout the procedure to avoid any alteration in the morphology of the apical area, while Hör et al., used Miller's needles to evade this possibility.
Does the digital marking of the Root ZX Reflect file position inside the canal?
The impedance ratio calculated by Root ZX indicates file position inside the canal.  Others concluded that the numbers on the digital screen of the Root ZX are arbitrary indication that the file is moving toward the AF. , This is in accordance with the manufacturer's statement. 
The indication on the Root ZX digital display screen when the file is placed at predetermined lengths short of the AF would clarify this issue. This approach was used utilizing the Dentaport ZX and it was found that the discrepancy between numeric display and actual file position inside the canal decreased as the file approached the AF. This correlation was accurate only within 0-1 mm short of AF.  In contrast, only 13.51% of the readings were exactly 1-mm short of the AF when the 1 mm mark of Dentaport ZX was used.  Using the 1-mm mark of the Root ZX resulted in about 40% of the measurements being outside the range of strict tolerance limit. However, the precision of the digital display improved when the APEX mark was used and only 19% of the measurements were out of the ± 0.5 mm range.  Stoll et al., reported that the Dentaport ZX demonstrated higher correlation between its digital display and the actual intra-canal file position as compared to the Root ZX mini. They recommended considering the digital meter display of the 0-3 mm zone of Dentaport ZX to be coincident with the intra-canal file position, with the correlation being the most accurate when the tip reaches the AF.
The consistency of the auto-reverse function of the Tri Auto ZX and Root ZX II has been investigated. This feature allows controlling the apical extent of canal instrumentation as the handpiece can be set to auto-reverse when the file tip approaches a certain apical point depending on the clinician's preference. The least consistent auto-reverse setting was the 2 mm option, and the use of the 1 mm setting has been advised.  This is in line with the findings of another ex vivo study that reported that setting the Tri Auto ZX to auto-reverse at 1-mm coronal to the AF resulted in a mean distance of 0.699 mm (rather than exactly 1 mm) between the file tip and the AF. The average tip distance between the file tip and the AF was 1.384 mm short of the AF when the Tri Auto ZX was set to auto-reverse at 2 mm from the AF.  Testing the Root ZX II in vivo showed that auto-reversing the device at 1-mm short of the AF established the WL at a mean distance of 0.46 ± 0.77 mm coronal to the AF. Meanwhile, the file tip stopped coronal to the AF by an average 2.09 ± 1.2 mm when the device auto-reversed at 2 mm from the AF.  The large standard deviation of the latter mean should not be overlooked as it indicates inconsistent performance. In fact, 6 recordings were coronal to the AF ranging between 2.05 mm and 4.14 mm. In another in vivo investigation, the frequency of measurements violating the AF when the Root ZX II was set to auto-reverse at 0.5, 1, and 1.5 marks was 70%, 40%, and 0%, respectively. All measurements obtained by the 1.5 mark were coronal to the AF in a range of 1.01-3.11 mm, which was considered unacceptable. 
In another in vivo evaluation, the majority of readings were coronal to the AF by more than 0.5 mm despite the Tri Auto ZX being set to auto-reverse at the 0.5 mark.  Even ex vivo testing of the Tri Auto ZX auto-reverse function at 0.5-mm short of the AF demonstrated its inconsistency.  The Root ZX II auto-reversed exactly at the corresponding point to its auto-reverse setting (2, 1, and 0.5 mm) in 4/65, 14/65, and 13/65 of the sample, in that order. 
It seems that there is weak correlation between the actual intra-canal position and the auto-reverse function of Tri Auto ZX and Root ZX II. This can be explained by the fact that the handpiece auto-reverses the file when the file tip passes (rather than when it reaches) the particular apical point set.  The performance of Tri Auto ZX might also be influenced by the application of pecking motion during instrumentation and by the screwing-in effect, resulting in longer file advancement than intended. ,
Impedance is inversely related to the frequency used,  indicating that the frequency might affect the accuracy of the EAL.  Furthermore, EAL devices differ in their calibration qualities that could potentially influence their performance.  Such differences might explain the findings of Wrbas et al., and Miletic et al., who reported that different devices verified different lengths for the same root canals. Hence, the results reported in this review should not be extrapolated to that for other EAL devices.
Possible factors influencing the accuracy of Root ZX
Capacitance is affected by the electrical properties of the materials involved in the test system and by the system geometry itself.  This implies that the accuracy of Root ZX might be affected by several factors that could be divided into tooth-related factors (pre-operative pulp status, tooth type, position of the AF, canal obliteration, and the size of apical diameter) and operative factors (coronal pre-flaring, presence or absence of irrigants, file size, file alloy, and mode of file operation, i.e. rotary or non-revolving).
Precision of the Root ZX in relation to pre-operative pulp status
Several in vivo studies assessed the accuracy of Root ZX only in vital teeth. Measurements falling within the strict tolerance limit of ± 0.5 mm from the AC ranged between 84-100%. ,, The range of readings within the strict tolerance limit from the AF was 82.75-96.2%. , All the recorded measurements were within the lenient tolerance limit of ± 1 mm with the mean electronic measurement being 0.395 (±0.215) mm beyond the AF. 
The performance of Root ZX in vital and necrotic teeth has been compared and the findings indicated that the device was not affected by the pre-operative status of pulp. ,,, Nine readings were exactly at the AC or coronal to it in vital teeth corresponding to four similar readings in necrotic teeth. Mean file tip position was beyond the AC in vital teeth by 0.21 (±0.19) mm and by 0.51 (±0.65) mm in necrotic cases. The difference was not significant.  Eleven readings were exactly at the AF or coronal to it in vital teeth corresponding to five similar readings in necrotic teeth. The mean file tip position was coronal to the AF by 0.11 (±0.218) mm in vital teeth, while it was beyond the AF by 0.12 (±0.373) mm in necrotic teeth. The difference was, again, not significant.  Another study concluded that the Root ZX was significantly more accurate in necrotic cases. 
External root resorption altered the morphology of AF, and it was more frequently associated with the roots of necrotic teeth. The altered AF morphology could complicate WL determination.  Two in vivo studies reported that the Root ZX was capable of maintaining the files tips within the roots limits despite the presence of external root resorption. , Conversely, the only long measurement recorded when the Tri Auto ZX was set to auto-reverse at the 0.5 mark was in a tooth associated with internal/external resorption.  In teeth with simulated apical root resorption, 94% of Root ZX measurements were within the lenient tolerance limit. Only 1.3% of the measurements were beyond the AF.  The occurrence of long readings in this study was low, although they used the sponge model associated with the highest frequency of long readings [Table 5].
Accuracy of the Root ZX in re-treatment cases
The residual root filling material occluding the dentinal tubules predisposes to a reduction in electrical conductivity and an increase in impedance, a factor potentially enhancing the electrical detection of the AF.  WL determined by the Root ZX before obturating the root canals and after removing the root fillings was not significantly different. The performance of the Root ZX was also not influenced by the type of the root filling material as the device performed similarly when the canals were previously filled with Resilon or gutta percha.  Root ZX measurements were within ± 0.5 mm from the AF in 95% of the re-treated teeth and always within ± 1 mm from the AF.  The Dentaport ZX was also highly accurate with 92% and 100% of the measurements being within ± 0.5 mm and ± 1 mm from AF, respectively. 
Alternatively, the Tri Auto ZX used with hand file size 15 gave readings beyond the AF more frequently after root filling removal. It was suggested that the Tri Auto ZX could detect the AF only when it outdoes the root filling material.  The frequency of long readings with Tri Auto ZX in re-treatment cases might be influenced by the selected mode; 60% of the measurements were beyond the AF with the rotary setting, whereas only 20% were outside the canals in the non-revolving mode. The interval between the detection of the AF by the device and stopping the handpiece might lead to such over-instrumentation.  The use of the auto-reverse function of Tri Auto ZX during re-treatment was not recommended because it frequently resulted in over-instrumentation, while the use of the non-revolving mode frequently kept the file tip within the canal confines. 
Performance of Root ZX across different teeth
It has been recently suggested that the complexity of apical canal anatomy (namely the number of canal termini) could manipulate the impedance features of root canals leading to variations in electronic length determination.  Mancini et al., found the Root ZX to be significantly more accurate in premolars than in anteriors and molars. Conversely, the performance of the Root ZX  and Dentaport ZX  was similar across different tooth types.
In a clinical study of 482 canals of 160 vital maxillary and mandibular teeth, the Root ZX exactly located the minor diameter in 74%, 53%, and 58% of anteriors, premolars, and molars, respectively. All measurements were within ± 0.5 mm of the minor diameter regardless of tooth type.  Another clinical study evaluated the performance of Root ZX in 693 canals of 245 vital maxillary and mandibular teeth. The device exactly located the AC in 89.1%, 75%, and 69% of the anteriors, premolars, and molars, correspondingly. All measurements were within ± 0.5 mm of the AC regardless of tooth type.  In both of these in vivo studies, the Root ZX was not used according to the manufacturer instructions.
Effect of the position of AF on Root ZX measurements
For teeth with AFs centered over the root apex, the mean length given by the Root ZX was 0.268 (±0.135) mm longer than the most coronal border of the AF. For teeth with deviated AF, the mean length was 0.531 (±0.205) mm beyond the most coronal border of the AF. The difference was statistically significant.  In contrast, another study reported that the Root ZX was significantly more accurate when the AF was deviated. The median length measurement of Root ZX was coronal to the most coronal border of the AF by 0.359 mm when the AF was centered over the root tip, while the same value in teeth with deviated AF was 0.209 mm. The difference was statistically significant.  The former study was conducted in vivo where they utilized the APEX mark. File fixation and root shaving protocol was followed and a mean measurement was reported. The latter study was an ex vivo investigation utilizing the digital mounting model. The Root ZX was used according to the manufacturer instructions and a median measurement was reported. A third study concluded that the precision of the Root ZX was unaffected by the deviation of the AF. 
Effect of canal obliteration on Root ZX function
The accuracy of Root ZX in canals with apical obliteration was compromised.  Unstable Root ZX measurements were related to canal obliteration in vivo. The inability of the digital meter display of the device to reach the "zero-reading" despite the apical advancement of the file suggests that the canal is electrically patent but mechanically blocked. However, the absence of any meter movement reflects the lack of electrical current and the absence of canal patency. 
Effect of apical diameter on the Root ZX accuracy
The total area of the minor diameter affected the performance of Root ZX with the device being significantly more accurate in teeth with smaller minor diameters.  The enlargement of the AC to 0.32 and to 0.62 mm caused differences in Root ZX measurements, which could not be measured. Only when the size of the AC was 1.02 mm did discrepancies appear depending on the size of the file used. 
The Root ZX recorded shorter lengths as the size of the AF increased.  Root ZX precision was better in canals enlarged to sizes 50-60 than in canals instrumented to sizes 70-90 with a frequency of short readings of 2% and 18%, respectively. The device never gave long readings.  A recent study found that the Root ZX was not affected by enlarging the size of the AF to 0.6 mm. The device was considered inaccurate in teeth with apical foramina larger than 0.9 mm. 
It might be crucial to adequately dry the canal before using the Root ZX in immature teeth.  Ebrahim et al., related the accuracy of Root ZX in enlarged root canals to the type of fluid present (NaOCl or blood) and to the size of file used. All teeth were enlarged to size 40 hand files and divided into two groups: One group was irrigated with NaOCl, while the other was irrigated with blood. Teeth were sequentially enlarged to apical sizes 60 and 80. As the size of canals irrigated with blood increased, the Root ZX gave shorter readings when used with smaller files. However, when the canals were irrigated with NaOCl, the Root ZX was very accurate regardless of canal diameter or file size.
Influence of coronal pre-flaring on Root ZX precision
Two ex vivo studies compared the performance of Root ZX in coronally pre-flared canals and when no pre-flaring was done. They reported that coronal pre-flaring significantly improved the accuracy of the device. However, the differences between the two groups were small and probably clinically irrelevant. ,
Performance of the Root ZX in dry and irrigated canals
Most of the clinical studies testing the accuracy of Root ZX and its variants used NaOCl. ,,,,,,,,,,,, In one study, 3% hydrogen peroxide (H 2 O 2 ) was employed followed by properly drying the canals with paper points prior to using the Root ZX. Higher accuracy was reported when the canals were dried rather than when residual humidity was present.  Another study used saline. 
If the canal is too dry, the meter reading might not move until the file is in close proximity to the apex.  Unstable Root ZX measurements have been linked to canal dryness. ,, Nonetheless, when operated in dry canals, the Root ZX gave only six measurements beyond the AF by 0.5 mm, while the remaining 24 were exactly at the AF. 
Measurement precision of the Root ZX was not affected when the canals were dry or irrigated with different NaOCl concentrations. , Compared to dry canals, the Root ZX frequently gave shorter readings in canals irrigated with 5.25% NaOCl, 0.1% chlorhexidine (CHX), 15% EDTA, or saline. The least variability was with CHX.  The accuracy of Dentaport ZX in the presence of 3% NaOCl or 0.9% saline was not significantly different.  Although the measurements were not significantly different, Root ZX measurements were closer to the AF when EDTA or saline were used and they were further away when the canals were dry or irrigated with xylol. 
Irrigation with 0.9% saline significantly reduced the accuracy of the Tri Auto ZX set to auto-reverse at the 0.5 mark. The mean file tip position was coronal to the AC by 1.35 (±0.95) mm and only 35% of the measurements were within ± 0.5 mm from the AC. When the canals were left dry, irrigated with 2.5% NaOCl, 3% H 2 O 2 , 0.2% CHX, 17% EDTA, or Ultracaine, the mean length indicated by the Tri Auto ZX was coronal to the AC in the range of 0.26 (±0.3) mm to 0.38 (±0.29) mm and 85% of the measurements were within ± 0.5 mm from the AC. 
The precision of the APEX mark and the 0.5 mark of the Root ZX was unaltered in the presence of 3% H 2 O 2 or 1% NaOCl.  Comparable and reliable performances of the Root ZX were found when the canals were flooded with saline, 3% H 2 O 2 , 5% NaOCl, 0.12% CHX, xylocaine, or RC-Prep. Measurements in the presence of H 2 O 2 were the closest to the AF with an average 0.117 (±0.11) mm beyond the AF. Measurements in the presence of NaOCl were the furthest away with an average 0.433 (±0.11) mm beyond the AF.  A potential factor influencing these results might be that the researches enlarged the canals to size 20 after measuring the actual length of the canal. Instrumentation shortens WL. ,, Furthermore, canal instrumentation leads to reduction in impedance values as a result of reduction in the dentinal thickness and length of dentinal tubules.  However, the influence of this factor would have affected all the irrigants groups. Flooding the canals with the irrigant might also have affected their findings.
Effect of file size on the performance of Root ZX
Usually small-size files are used for electronic WL determination. This might potentially expose more of the metallic surface of the file to the surrounding electrolytes. However, the use of tightly fitted files might help in providing more contact between the file and the canal wall. These variations could affect the electrical circuit of the device and hence its accuracy. 
The use of a snugly fitted file is not necessary for optimum performance of the Root ZX. ,, The Root ZX was used with a size 15 hand file to measure the length of canals with apical diameters ranging between 0.5-0.9 mm. About 90% of the measurements were 0-1 mm short of the actual length, while the rest of the recorded lengths were 1-2 mm short of it. 
The Dentaport ZX recorded significantly higher precision when used with hand file size 15 rather than size 10, while the precision of the Root ZX mini was not influenced by the hand file size. The frequency of unstable measurements for both devices was higher when they were used with size 10, especially when the file tip was coronal to the AF by more than 2 mm.  Others recommended using snugly fitted files to measure the length of canals in the presence of blood. In the meantime, the influence of the file size on Root ZX accuracy vanished when canals were irrigated with NaOCl, although the use of snugly fitted files provided slighter accuracy. 
Two studies investigated the interaction of the file size and canal size. When the diameter of the AC was enlarged to 0.32 and 0.62 mm, an insignificant difference between initial length measured with size 10 and final WL measured with file sizes up to 60 was found. The effect of file size appeared only when the AC was enlarged to size 1.02 mm, with larger files giving more accurate recordings.  The Root ZX was accurate when it was used in teeth with AF enlarged to 0.6 mm regardless of the file size. Enlarging the AF to 0.7 mm required the use of files size 45 or larger to achieve acceptable accuracy. Further increase in the AF size to 0.8 mm necessitated the use of at least file size 70 to maintain device accuracy. Root ZX precision fluctuated when the AF size was increased to 0.9 mm or more. In general, smaller files established shorter lengths. Nevertheless, the Root ZX never gave lengths beyond the AF regardless of the file or foramen size. 
Accuracy of the Tri Auto ZX set to auto-reverse at the 0.5 mark when used with F3 ProTaper, 30/0.05 Mtwo, or 30/0.02 FlexMaster rotary files was compared and found to be not affected by the type of the file when the file tip position was related to the AF. However, when the file tip position was related to the AC, the FlexMaster was significantly less accurate than the others with its mean length measurement being most coronal to the AC.  The FlexMaster files were the smallest among the tested files.
Effect of file alloy (stainless steel or NiTi), file type (hand or rotary), and mode of operation (rotary or non-revolving) on Root ZX precision
An ex vivo comparison of Root ZX length measurements when used with stainless steel hand files, NiTi hand files, Lightspeed, and 0.04 ProFile rotary NiTi files showed that the differences were clinically insignificant. 
The Tri Auto ZX used in vivo with a ProFile size 20/0.04 always auto-reversed at the same point the device determined when used with hand file size 15 indicating consistent device performance regardless of the file type.  On the other hand, when the Tri Auto ZX auto-reversed at the 1 mm mark, one-half of the readings was equal to the lengths measured with the non-revolving mode. The other half was shorter by 0.2 mm. The range of readings with the 1.5 mm auto-reverse settings was 0.2-1.2 mm shorter than the non-revolving mode length. Too broad range was obtained with the 2 mm auto-reverse function (0-4.2 mm short of the non-revolving length). The auto-reverse length was shorter than the non-revolving length by a mean distance of 0.1 (±0.1) mm, 0.3 (±0.2) mm, and 1.3 (±1.04) mm, correspondingly. The auto-reverse function was tested using 0.04 ProFile series 29 files, while the non-revolving lengths were measured using number 1 ProFile series 29 hand files.  The findings of this study might have been influenced by their decision to coronally flare the canals after obtaining the length with the non-revolving mode and before testing the auto-reverse function.
The in vivo performance of 0.04 Profile rotary files was poorer than that of the conventional stainless steel hand files when Root ZX II was used. The rotary files violated the minor diameter in 79% of the teeth and passed the AF in 28.6% of the sample.  For Dentaport ZX, the mean length for the rotary mode acquired using the S1 ProTaper rotary files was 0.22 mm shorter than the length acquired for the non-revolving option using size 15 hand file. The potential differences in the electrical conductivity between these two files might have modified the length determination of the two modes.  In fact, there was no significant difference in the performance of Tri Auto ZX coupled with Hero642 rotary file size 20/0.02 when used in the rotary or non-revolving modes. 
| Conclusions|| |
Heterogeneity of the studies evaluating the performance of Root ZX complicated the comparison of their results. Nevertheless, the apical advancement of the file until the digital meter of Root ZX indicates that reaching the 0.5 mark would reduce the chances of violating the AF. Several operative and tooth-related factors potentially influencing the precision of Root ZX must be considered when the device is used.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
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