The accuracy of apical locator and the factors influencing it have been the subject of discussion. The accuracy of the root canal length electrometry is determined by two factors: the size of the apical hole diameter and the dryness inside the root canal; the two factors are interrelated; if the apical hole is small (<0.4 mm), the accuracy is not affected even if there is conductive fluid inside the root canal, and if the root canal is very dry, the accuracy is not affected even if the apical hole is large. Root ZX apical locator is designed according to the principle of ratio method. Since the ratio of resistance of two different frequencies of current through the root canal is measured simultaneously, which compensates for the decrease of resistance and increase of capacitance caused by strong electrolytes in the root canal, the measurement results of Root ZX should not be affected by the electrolytes in the root canal. It was found that the resistance ratio was affected by the degree of dryness in the root canal when currents of different frequencies were passed through the root canal, suggesting that the accuracy of Root ZX may be related to the presence or absence of electrolytes in the root canal or the dry and wet conditions. The aim of this experiment was to analyze the effects of root canal contents and apical narrowing size on the accuracy of Root ZX apical locator in measuring root canal length. 1. Materials and methods: 1.1. Establishment of in vitro study model: choose - finished transparent plastic box with the size of about 3×4×7 (cm3), and make - a plastic holder for fixing the tooth inside the box; mix 4 g of sugar-free agar (produced by Sigma, USA) and 2 mg of muscimol with 100 ml of 0.9% NaCl was mixed and heated, the hot solution was poured into the plastic box, the lid was closed, and finally the box was placed in the refrigerator for at least 2 hours to allow the liquid to solidify. 1.2, Experimental procedure: 40 single root canal isolated teeth with completed apical foramen development were selected, and the actual length of the teeth was measured with a No. 15 K file. Then, the teeth were fixed on a saline agar model and the root canal lengths were measured by Root ZX apical locator (made by Morita, Japan) under the conditions of root canal filling with 2.5% NaOCl, 0.9% NaCl, 17% EDTA, 3% H2O2 and distilled water and root canal drying, respectively. In addition, 23 teeth with varying sizes of apical reduction were selected, requiring complete root development, no defects or fractures, and a root canal curvature of 20° or less. The apical narrowing size was measured with Lightspeed instruments (made in the USA with an accuracy of 0.005 mm), and the root canal length was measured with Root ZX apical locator under dry and wet conditions in each tooth, respectively. The effect of different contents in the root canal on Root ZX was calculated by the column frequency table method and a two-factor one-way ANOVA was performed with SAS software. The relationship between the distance of the probe file tip from the apical foramen and the size of apical narrowing was analyzed by linear correlation, and the regression equation was written. 2. Results 2.1. Accuracy of root canal length measured by Root ZX The actual length of the root canal was subtracted from the electrical length measured by Root ZX, and the difference was the distance of the file tip from the apical foramen (IF). A negative value was obtained when the file tip was beyond the apical foramen, and a positive value was obtained when the file tip was inside the root canal. The root canal lengths were measured by Root ZX apical locator under the conditions of 2.5% NaOCl, 0.9% NaCl, 17% EDTA, 3% H2O2 and distilled water filling and root canal drying, respectively. 2.3, Relationship between IF values and the size of apical narrowing under the condition of root canal containing saline The measured IF values were positive in all cases, i.e., the electrically measured length was less than the actual length of the root canal. 2.4. The relationship between the size of apical narrowing and IF values under dry root canal conditions There was also a positive correlation between IF values and the diameter of apical narrowing under dry root canal conditions. 3, Discussion 3.1, About the in vitro test model of apical locator In 1983, Aurelio [4] et al. first used a phosphate buffer agar model as an in vitro test model of apical locator and found that this model could simulate in vivo conditions, but because of the cumbersome preparation of model materials and high cost, various simple models were mostly used instead in the study. Commonly used simple models are alginate model, sponge (with saline or 1% sodium hypochlorite) model, saline model, and agar gel (with saline or 1% sodium hypochlorite) model. In this experiment, the physiological saline agar gel model was used, which is simple to make and convenient to take. 3.2. Accuracy of Root ZX in measuring root canal length In the experiments on isolated teeth, there are three main criteria for judging the accuracy of the electric measuring instrument: ①, a range of ±0.5 mm from the apical foramen, ②, a range of ±0.5 mm from the apical narrowing, and ③, a range of 0~1 mm from the apical foramen. For most teeth, typical apical narrowing does not exist, and apical narrowing is difficult to observe, so the apical foramen becomes the only reliable marker for measuring root canal length, and the location of apical narrowing is often estimated through the apical foramen. Since root canal preparation and filling must be controlled within the apical narrowing, otherwise it is detrimental to the healing of the periapical tissue, exceeding the apical foramen is an unacceptable criterion, and Vajrahaya [8] considered 0-1 mm from the apical foramen as a more appropriate criterion. In the present experiment, the accuracy of Root ZX was 97.5% with 0~1mm from the apical foramen as the standard. If ±0.5 mm from the apical foramen is used as the standard, the accuracy is 70%, which is lower than the results of Pagavino [9] and Shabahang [10] et al. (82.75% and 96.2%, respectively), which may be related to the different reading values of Root ZX selected, which in this experiment was chosen as " 0.5" instead of "Apex". When the reading of Root ZX was "0.5", the experimental results showed that the average distance of the file tip from the apical foramen was 0.39 mm, and according to the product description of the instrument, the file tip should be 0.2~0.3 mm beyond the apical narrowing at this time, so it can be deduced that the average distance of the apical narrowing from the apical foramen was 0.59~0.69 mm. This value coincides with the results of the dental anatomical study by Kuttler [11] (0.5-0.7 mm), and it can be assumed that the Root ZX apical locator uses apical narrowing as the detection mark, which is in accordance with the design principle of Root ZX. Although some teeth do not have apical narrowing, it is safe and reliable to use the electrical measurement length of Root ZX minus 0.5mm as the working length of the tooth in clinical practice. 3.3. Influence of root canal contents on the accuracy of Root ZX In the experiment, 2.5% NaCl, 3% H2O2, 0.9% NaCl and 17% EDTA were the rinsing agents commonly used in root canal treatment with different conductive strengths. Distilled water and dry root canals were used as control groups, and each rinse solution was repeatedly rinsed with distilled water after testing to eliminate the influence of residual electrolytes, and the measurement of each group The order of the measurements was randomized to minimize errors. The experimental results showed that the nature of the rinsing agent in the root canal and the dry and wet condition of the root canal did not affect the Root ZX measurements. It was confirmed after experimental studies with isolated teeth that high ionic concentration in the root canal improves the accuracy of measurement when the apical hole diameter is 0.25 mm. In the experiments, it was found that Root ZX was not sensitive to the "0.5" position when the root canal was excessively dry in individual teeth. Therefore, when using Root ZX in clinical practice, it is not necessary to dry the root canal to avoid wasting operation time. Of course, when using the electric measuring instrument, pay attention to the moisture isolation of the cervical part of the tooth to prevent the leakage of electricity from the cervical part of the tooth to affect the measurement results. 3.4. Influence of the size of apical narrowing on the accuracy of Root ZX In the experiments, the size of apical narrowing was measured using Lightapeed instruments, which are made of nickel-titanium alloy with good flexibility and smooth, blunt tips without cutting action; the edges are short and about 0.274-1.75 mm long; the size of the offsets according to the diameter of the edges is 20#-100#. The precision of the instruments is high (0.005mm), because the root canal tends to be garden-shaped at the apical 2mm, so it is feasible to use Lightspeed instruments to measure the diameter of apical narrowing. Experiments showed that the size of apical narrowing was positively correlated with the IF value, i.e., the greater the apical narrowing, the greater the distance of the probe file from the apical foramen, or the decrease of the electrometric length with the increase of apical narrowing. Younong Wu et al [15] investigated the relationship between apical foramen size and electrical probe length in a simulated root canal using a single-frequency impedance-based electrometer and showed that the electrical probe length was negatively correlated with apical foramen size. Since the apical foramen on the simulated root canal is actually the narrowest diameter of the root canal, it can be assumed that the apical narrowing size is negatively correlated with the electrometric length. The distinction between apical foramen and apical narrowing is clinically important here because under normal physiological conditions, apical foramen size increases with age, but apical narrowing decreases with age, so it can be deduced from the results of this study that the length of the electrical measurements should increase with age, and the size of apical narrowing is a determining factor in the accuracy of Root ZX measurements. 4. Conclusion The accuracy of root canal length measurement by Root ZX is not affected by the root canal contents; the size of apical narrowing is the determining factor for the accuracy of Root ZX, and the electrical measurement length is negatively correlated with the size of apical narrowing.