1, development history As early as 1916 Custer began the exploration of electrical measurement of root canal length, in 1942 Suzuki found that the resistance value between the periodontal membrane and oral mucosa is a constant value, in 1958 Sandy further research to confirm this constant value of 6.5KΩ, and first use the DC electrode to measure the root canal length, because the DC electrode is prone to polarization and inaccurate measurement results, so in 1969 Sandy switched to 150 Hz AC as the measurement current. In 1973, Inoue proposed the aural method, which showed the measurement result by the change of tone. Since the accuracy of electrical measurement of root canal length is easily affected by the electrolytes in the root canal, in 1979 Hasegawa used 400 Hz high frequency electricity as the measuring current and wrapped an insulating sleeve around the electrode, this method improved the accuracy of the measurement, but there are problems such as the insulating sleeve is easily broken and the detection is limited in fine root canals. 1983 In 1984, Yamaoka et al. proposed the frequency response difference method, which was reported to be able to accurately measure root canal length in root canals containing electrolytes, but not dry root canals. 1991, Kobayashi et al. reported the ratio method, which was claimed to be able to perform accurate measurements under different conditions in the root canal. Over the past decades, many significant advances have been made in theoretical and practical aspects of root canal length electrometry, from measuring absolute resistance values to measuring relative resistance values, from impedance-dependent to frequency-dependent, from the past electrometer showing the contact between the probing electrode and the periodontal ligament at the apical foramen to the new generation of electrometer showing the narrowest point of the canal diameter in the apical narrowing area, and the accuracy of measurement has been greatly improved. 2, principle, method and instrumentation Root canal length electrometry is a method to determine the location of the apical foramen or the length of the root canal of a living tooth by measuring the law of impedance change in the root canal with an electronic instrument. The electronic instrument used to measure the root canal length is called the electronic canal length measuring device (ECLMD) or the electronic apex locator (EAL). The various principles, methods and instruments used for electrical canal length measurement are described below. 2.1 Impedance method There are two views on the basic principle of impedance method, one is the theory of biological properties proposed by Suzuki and Sandy, etc. The theory is that the resistance value or impedance value (R) between periodontal membrane and oral mucosa is constant and not affected by the patient’s age, gender and tooth position, etc. Through actual measurement and statistics, R is 6.5 KW. Another point of view is the theory of electrical properties advocated by Lizi Huang, that is, the basic constancy of the resistance value between the apical foramen and the oral mucosa is not related to the biological properties of the periodontal membrane and the oral mucosa, but is a physical phenomenon, and the law of impedance change during the electrical measurement of the root canal length can be proved by physical experiments or expressed and calculated by electrical formulas; the size of R value is closely related to the area of the apical foramen, and its value is in the range of 4.5-7.0 KW. 4.5-7.0 KW range, while 5.6 KW is the optimal value[1] , a theory that has attracted the attention of many scholars. The impedance method uses a single current to detect the change of impedance in the root canal, and the commonly used power sources are DC, AC and HF. EALs designed according to this method include Root Canal Meter, Endodontic Meter, C.L. Meter, Pio , Roots, Rooty, Endometer, Dentometer, Foramatron, Apex Finder, Exact-a-pex, etc. Endocater is an EAL produced by Hygenic, which uses 400KHz high-frequency electricity as the measurement current, and wraps about 0.04mm thick insulating sleeve around the anode finder file, and also designs a handheld cathode. The audiometric method actually belongs to the impedance method, which uses a low-frequency audible signal to indicate the resistance base boundary between the oral mucosa and the periodontal membrane. The EALs designed according to this method are Sono-Explorer, Sono-Explorer Mark Ⅲ, Neosono D, Forameter and Neosono MC, etc. According to the basic principle of impedance method, China has developed KGC-I root canal length EAL, KDY oral multi-purpose EAL and Y dental sounding detector. 2.2 Voltage gradient method The voltage gradient method, also known as Ushiyama’s method, is based on the principle that when a constant current is passed through a root canal containing electrolytes, the current density is highest at the apical narrowing (small hole) and lowest at the apical hole (large hole). When the electrodes were located at the apical narrowing, the current density was highest and the measured voltage was highest. This method has not been applied due to its design defects. 2.3 Relative values of frequency response method This method is derived from the voltage gradient method, so it is also called the gradient impedance method, and its basic principle is: when measuring the resistance of a place in the root canal, if the frequency of the measured current is different, the measured resistance value will be different. The Endex and Apit EALs were designed based on this principle and use 1 KHz and 5 KHz currents to measure the difference in resistance between these two currents to detect the narrowest part of the root tip [2]. 2.4 The ratio method The basic principle of the ratio method is that by simultaneously measuring the resistance of two currents of different frequencies through the root canal, the ratio of the two resistances is calculated, and the magnitude of the ratio reflects the position of the electrode in the root canal, which is not affected by the electrolyte in the root canal-[3]. It determines the position of the file tip in the root canal by measuring the ratio of the resistance of two different frequencies of current (8 KHz and 0.4 KHz) through the root canal, the ratio is almost equal when the file tip is far from the root tip, and it decreases to 0.66 when the file tip is close to the apical narrowing [4, 5]. 3. Study of the accuracy of the root canal length electrographic method and the factors influencing it 3.1 Accuracy of the root canal length electrographic method The accuracy of the electrographic method is usually judged by the accuracy of the EAL in determining the position of the apical foramen in the range of ±0.5 mm. In the past, it was mostly evaluated by the radiographic method, but now it is based on the actual measurement of the extracted teeth. The accuracy of EAL varies between design types, with 55-75% accuracy for impedance type EAL [6]. 89.64% accuracy for Endex [7] and 82.97-96.2% accuracy for Root ZX [8, 9]. 3.2 Influence of root canal contents on the accuracy of electrometry The degree of dryness in the root canal, the nature of the rinsing solution and the pulp vitality status may all be factors affecting the accuracy of electrometry, and different types of instruments are affected to different degrees. The impedance EAL is susceptible to the influence of electrolytes in the root canal, and the canal should be as dry as possible during electrical measurements; otherwise, the measured length is easily shorter than the actual length. the Endocater is less affected by electrolytes in the root canal due to its special design [10]. the Endex and Apit are difficult to measure in dry root canals because the scale cannot be accurately calibrated in dry root canals, and Fouad [ 11] concluded that the accuracy of Endex measurements in root canals containing electrolytes is higher than that of impedance-based instruments.Shabahang[9] and Danlap[12] showed that the accuracy of Root ZX measurements is not affected by the root canal contents, but Meridith[13] suggested that the degree of root canal dryness may have an effect on the accuracy of Root ZX measurements. 3.3 Influence of apical foramen size, location and root canal morphology on the accuracy of the electrical measurement method Apical foramen size is one of the elements affecting the accuracy of the electrical measurement method, and the length of the electrical measurement was negatively correlated with the area of the apical foramen. The two factors of apical foramen size and the degree of dryness in the root canal are interrelated. In dry root canals, apical foramen size has no effect on the accuracy of each type of EAL measurement; while in root canals containing electrolytes, when the apical foramen diameter exceeds 0.3 mm or 0.4 mm, the accuracy of impedance type EAL measurement will be affected, and when the apical foramen diameter exceeds 0.62 mm, the accuracy of Endex measurement will also is affected, i.e., the electrical measurement length is shorter than the actual length. There are two types of apical foramen locations, namely apical apex type (56.53%) and collateral type (43.47%). When the apical foramen location was determined by Root ZX, the measurement error was smaller for the apical apex type than for the collateral type [13]. The incidence of apical foramina was 21.14% (93.48%), and the effect of the presence of apical foramina or lateral root canals on the accuracy of the electrometric method has not been reported. The degree of curvature of the root canal had no significant effect on the accuracy of electrometry. 3.4 Influence of operational factors on the accuracy of electrometry Poor contact between the probing file and the file holder, between the labial clamp (hook) and the oral mucosa, and other circuit connections can affect the accuracy of the measurement, making the probing file exceed the apical foramen. The cervical leakage of the measurement current due to metal restorations, cervical caries or conductive rinsing fluid can also make the results of electrometry inaccurate [14]. 4. Advantages and disadvantages The ideal root canal length measurement method requires: accurate measurement, easy operation, avoidance of radiation, doctor-patient comfort, and reasonable price. There is no method that meets all these conditions. The commonly used radiographic method is cumbersome, time consuming, radiographic contamination, and is limited in patients with severe vomiting reflexes or in pregnant women. Compared to radiographic methods, electrodiagnostic methods have the advantages of simplicity, speed, accuracy, and reduced X-ray exposure, but also have some disadvantages, such as the need for special instruments, the accuracy is affected by the electrolytes in the root canal, it is difficult to accurately measure teeth with large apical foramina, and it is contraindicated in patients with cardiac pacemakers. In addition to estimating the length of the tooth, the radiographic method also enables the clinician to observe the whole picture of the treated tooth, especially the anatomical form of the crown and root, the crown-root relationship, and the size, shape and position of the root. 5.Application prospects The root canal length electrometry can also be used for other purposes: (1) checking the lateral penetration of the root canal; (2) detecting the lateral penetration of the staple; (3) controlling the position of the file tip in the root canal in the root canal ultrasonic preparation system; (4) using in the root canal motorized preparation system to control the position of the file tip during the whole preparation process to prevent over-preparation; (5) monitoring the filling when the root canal is filled with a conductive dental adhesive tip. (5) monitoring of the filling when the root canal is filled with a conductive adhesive tip. As the performance of the EAL improves (e.g., accurate length determination and sensitive digital display), the EAL is shifting from single-use to multi-purpose, and the clinical application of root canal length electrometry will become increasingly widespread.