Accuracy of Root ZX in teeth with simulated root perforation in the presence of gel or liquid type endodontic irrigant

Root perforations that result in a communication of the root space with the periodontal tissues occasionally occur during endodontic procedures. They may be induced iatrogenically, by resorptive process, or by caries.1 Identification of root perforations is possible by direct observation of bleeding, indirect bleeding assessment using a paper point, radiography, and an electronic apex locator (EAL).2 The EAL's principle was initially introduced to clinical practice by Sunada.3 The EALs are considered as accurate tools for determining canal working lengths, and are valuable aids in clinical endodontics.4 EALs may detect root fracture that reaches the pulpal chamber and should detect the fracture as an 'apex' from the beginning of the periodontal communication at the fracture site.5 The accuracies of EALs in fractured or resorpted root cases have been evaluated in a few studies, but confusing results have been reported.6-8 In case of perforation, some authors recommended the use of EALs, but there were some difficulties in measuring the exact working length under different irrigation solutions in perforated roots.2, 9, 10

Effects of various irrigants, such as saline, hydrogen peroxide, sodium hypochlorite (NaOCl) solution, RC-Prep (Premier Dental Products, Plymouth Meeting, PA, USA), and ethylenediaminetetraacetic acid (EDTA) solution, on EALs' performance have been investigated.11-13 Chlorhexidine is a potent antimicrobial agent that has been used in endodontic treatment as both a root canal irrigant and an intracanal medicament.14-16 Because of the good biocompatibility of chlorhexidine, it has been recommended as a useful alternative to NaOCl, especially in patients allergic to NaOCl, or in patients with open apices.14, 17 Recently, chlorhexidine gel has been introduced as an endodontic lubricant.18 Chlorhexidine gel has several advantages: it is biocompatible, water-soluble, and viscous, thereby facilitating instrumentation. The material acts like a lubricant, allowing increased mechanical removal of organic tissues and decreased smear layer formation as other irrigants.19 The antimicrobial ability of 2% gel chlorhexidine gluconate was either no different from, or better than, 5.25% NaOCl in limiting Enterococcus faecalis counts 7-day after biomechanical instrumentation.20

RC-Prep, which is composed of glycol, urea peroxide and EDTA in a special water soluble base helps to remove calcifications and lubricates the canal to permit more efficient instrumentation. RC-Prep is also excellent for use with apex locators as it permits consistently reliable readings.12

Electrical conductivity is the ability of different types of matter to conduct an electric current. The electrical conductivity of a material is defined as the ratio of the current per unit cross-sectional area to the electric field producing the current.21 It is an intrinsic property of a substance, which is dependent not on the amount or shape, but on the temperature and chemical composition.

To date, a few studies have examined the accuracy of EAL in perforated roots in the presence of gel or liquid type endodontic irrigants. In addition, electrical conductivities of each irrigant are not yet fully determined. The purpose of this study was to evaluate the accuracy of the Root ZX (J Morita Corp, Tokyo, Japan) in the presence of various gel or liquid type endodontic irrigants in perforated teeth and also to determine the electrical conductivities of these endodontic irrigants.

Post-treatment endodontic disease is often a result of root perforation.23 In perforated cases, endodontists may face difficulties in measuring of working length with EALs. The prognosis of perforated teeth depends on the location, size, duration of perforation and feasibility of sealing the perforation, and an accurate detection of the location of root perforation is a key factor for successful treatment.2, 10, 24

In this study, the perforation situation was different from the previous study of Fuss et al.10 The perforation was made by transportation in their study. However, in this study perforations were made by drilling from the external surface, like an external resorption or a big lateral canal, while original canal of apical third was remained. The shapes and positions of our simulated root perforations, which were created iatrogenically by round bur with an 1 mm diameter at 4 mm above apical foramen, may be unrealistic. In addition, dentin thickness from inner root canal to outer root surface may vary with each tooth. Although we made perforation on mesial or distal root surfaces instead of buccal or lingual root surfaces to minimize differences of dentin thickness between each tooth, varying dentin thickness could have influenced the detection of root perforation location.

The lubricants like RC-Prep or chlorhexidine gels have been used and also they may act as electric conductors in the root canal. Dual-frequency EAL, Root ZX, simultaneously uses two wave forms, a high (8 kHz) and a low (400 Hz) frequency wave forms.25 Even though Root ZX was accurate in the presence of electrolytes, electrical conductivity of irrigant affected the determination of working length.12 Electrical conductivity in the liquid state is generally due to the presence of ions. The conductivity of a one molar electrolyte is of the order of 0.01 siemens per meter, far less than that of a metal, but still very much larger than that of typical insulators. As might be expected, the conductivity of a dissolved electrolyte depends on its concentration.21 It is well known that electric conductivity of tap water is 100 to 1,000 µS/cm and 5% sodium chloride solution is 70,000 µS/cm. Similarly, in this study, electrical conductivity of physiologic saline and 1% NaOCl solution were 44,940 µS/cm and 172,420 µS/cm, respectively. Therefore different concentration of endodontic preparations may present dissimilarities to another. Gel types were different from solution types. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state.26 By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional cross-linked network within the liquid. Therefore, electrical conductivities in gel have complexities. They may be influenced by three-dimensional structure of gel, concentration of ions and water content.

In this study, RC-Prep had the lowest conductivity. It is composed of glycol, urea peroxide and EDTA in a special water soluble base. It might be different from hydrogel like chlorhexidine gel. Chlorhexidine gel is used as an endodontic lubricant, antiseptic agent and irrigation medicament of canal preparation.18 Clinically, this agent is very useful for canal enlargement. However, further long-term studies are needed to determine its effectiveness as a canal irrigant or preparatory material.

Working length measurements tended to be slightly shorter in solutions of higher electrical conductivity, such as NaOCl solutions.12 In our study, the electrical conductivity of the 1% NaOCl solution was four-fold higher than that of physiologic saline. In the previous study, the influence of chlorhexidine liquid on EALs' accuracies was tested, which was found to be similar to that of NaOCl.27 However, in this study, resulting data with chlorhexidine liquid were very different from other liquid type irrigants such as saline and NaOCl. Electrical conductivity of chlorhexidine liquid was much lower than that of other liquid type irrigants and similar to that of gel type irrigants. It can be presumed to be main cause of our results.

Another study showed that the Root ZX reliably measured the canal lengths to within 0.31 mm under RC-Prep.12 However, in the present study, we found that there were statistically significant differences between liquid type irrigants of saline and NaOCl and gel type irrigants of chlorhexidine gel and RC-Prep. Gel type irrigants were very stable in determining real working length in perforated teeth. Similarly, in the other study, the largest deviation from the actual canal length was reported with NaOCl.28 Obviously, both type of irrigants can be extruded over the apical foramen excessively. However, the possibility and amount of excessive extrusion is greater in liquid types than in gel types. This excessive extrusion may explain why gel types showed higher accuracy than liquid types in case of measuring AWL. On the contrary, liquid types showed higher accuracy than gel types in case of locating root perforation. Because simulated perforation was made to proximal root plane, good flowing liquid types were advantageous to reach the outer proximal root surface. This could make liquid types more accurate than gel types when we located root perforation.

Shabahang et al. suggested that a 1.0 mm tolerance can be considered clinically acceptable.28 This tolerance level would be considered acceptable, especially when the determination of the apical limit becomes more difficult because of the existence of perforation.7 Most of the results obtained were between perforation length (AP) and real working length (AWL), and there was no unacceptable data more than 1 mm shorter from perforation.

Further evaluation of the accuracy of the Root ZX in perforated tooth in clinical conditions is indicated and should be needed in future in vivo studies.

Different canal irrigants with different electrical conductivities may affect the accuracy of the Root ZX in perforated teeth. In AWL measurement, gel type irrigants showed higher accuracy than liquid type irrigants such as saline and NaOCl. On the contrary, liquid type irrigants such as saline and NaOCl showed higher accuracy than gel type irrigants when locating root perforation. Chlorhexidine gel and RC-Prep gel had the lowest electrical conductivities.