Journal of Endodontics Research -
A simple model to demonstrate the electronic apex locator
By JofER editor
Published on 11/14/2008
A.C. Tinaz, T. Alacam & O. Topuz
Department of Operative Dentistry and Endodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey.

To describe and evaluate a newly developed model for demonstrating and teaching the use of electronic apex locators.

Key learning points.
  • A simple, inexpensive model can be manufactured from plastic dental jaws, natural teeth and alginate impression material to demonstrate electronic working length measurement.
  • The model is stable for many hours and provides consistent results with different concentrations of NaOCl in the canal and various apical diameters.
  • The model is a useful teaching aid but needs further evaluation and refinement before use in research applications.

Introduction - Materials and methods.
A.C. Tinaz, T. Alacam & O. Topuz
Department of Operative Dentistry and Endodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey.

Electronic root canal length measuring devices should be validated for accuracy and reliability in vitro before clinical use. Reliable in vitro models are also needed for teaching. Such a model was described by Aurelio et al. (1983) who tested three electronic devices.  This model employed agar (Bacto-Agar; Difco Laboratory, Detroit, MI, USA; 2 g in 100 mL of phosphate buffered saline containing NaCl, 9 g; NaHPO4_H2O, 1.43 g; KH2PO4, 0.18 g; and H2O, 1 L) poured molten into a polystyrene culture tube unit, the test tooth being secured with a snap cap assembly. After setting the agar in a refrigerator, a 0.5-in. nail was placed through the base of the tube for attachment of the lip clip, whilst the other electrode was attached to an endodontic instrument in the usual way.
Donnelly (1993) suggested a simplified and less expensive model using sugar-free Jell-O (Kraft General Foods Inc., White Plains, NY, USA) mixed with 0.9% sodium chloride USP (Travenol Laboratory Inc., Deerfield, IL, USA) in 100-mL plastic specimen cups. After refrigeration to jell the liquid, a metal lip clip was placed into the gel and secured with wax. The test tooth was held in place by the dentist.
Katz et al. (1992) described a simpler model where teeth were embedded in alginate in a small plastic box. The teeth were embedded to CEJ level and the lip clip inserted before setting. This study was designed to construct and validate a further simple model for demonstrating, teaching and possibly testing electronic apex locators.

Materials and methods.

Three single-rooted human premolar and canine teeth with single root canals were chosen for study. Before the tests, teeth were stored in formalin solution (10%) and placed into 5.25% sodium hypochlorite solution for 2 h to remove the periodontal ligament. After rinsing in tap water they were transferred again to formalin solution. Standard access cavities were established and actual lengths determined with a size 10 K- file (Zipperer UDM, West Palm Beach, FL, USA) fitted with a rubber stop and inserted until the tip was just visible at the apical foramen. The distance from file tip to the base of the rubber stop was measured with a caliper to the nearest 0.02 mm. The model was tested with these three teeth in six different ways: Groups 1 and 4 comprised of a second premolar prepared with a size 15 reamer until the tip was visible 1 mm through the apical foramen to obtain an apical opening of 0.20 mm diameter. Groups 2 and 5 comprised of a first premolar prepared similarly with a size 25 reamer to produce a 0.30-mm opening. Groups 3 and 6 comprised of a canine prepared with a size 40 reamer to obtain an apical diameter of 0.45 mm. Groups 1–3 were irrigated with 2.65% NaOCl; groups 4–6 were irrigated with 5.25% NaOCl whilst making electronic measurements.

The mandibular premolars and canine were removed from their sockets in a plastic master model (Frasaco, Tettnang, Germany). The sockets were enlarged with a bur until the human teeth could be adapted and fitted easily (Fig. 1). Alginate (Blue-print, De Trey, Surrey, UK) was prepared according to the manufacturer’s instructions and packed into the sockets for immediate tooth insertion. More material was then placed under the master model and the lip clip electrode inserted (Figs 2 and 3). Excess alginate around the teeth and master model was removed with a knife before mounting the model in a phantom head. The apex locator (Root ZX, Morita, Tokyo, Japan) was then connected to the lip electrode. All root canals were irrigated with 2.65% NaOCl and canal lengths determined electronically with size 20 K-files (group 1), size 30 K-files (group 2), and size 45 K-files (group 3). Files were inserted slowly until the signal on the LCD screen display’s bar reached the ‘0.5’ mark, indicating the apical foramen (Kobayashi & Suda 1994). Measurements were repeated thrice and the whole procedure repeated every 3 h up to 45 h. After each series of measurements, canals were dried with paper points sizes 25–40 (Roeko, Langenau, Germany). After testing with 2.65% NaOCl, the same canals were irrigated with 5.25% NaOCl and lengths established in a similar manner. Records were made as previously for groups 4–6 for teeth with foramen diameters of 0.20, 0.30 and 0.45 mm. Measurements within 1–13 h were designated Period A. Period B included measurements during 16–28 h and Period C measurements during 31–45 h (Table 1). All electronic measurements were made at 19–248C and the model kept in 100% humidity throughout the investigation. Statistical evaluation was completed using the Analysis of Variance and Turkey multiple range test (P = 0.05).

Figure 1. Human teeth fitted to their sockets on the master model.

Human teeth fitted to their sockets on the master model

Figure 2. Lip clip inserted into unset alginate.

Lip clip inserted into unset alginate

Figure 3. The model ready for use.

The model ready for use

Table 1. Results of the electronic measurements in different concentrations of NaOCl and different time periods.

Results of the electronic measurements in different concentrations of NaOCl and different time periods

Results - Discussion - References.
The model was simple to use. Table 1 shows that most measurements were within 1 mm of actual length (range: _2.2 to 0.21 mm). Electronic lengths tended to increase with time. However, this observed increase was statistically significant only in period C for groups 3 (P = 0.006) and 6 (P = 0.006), where the apical diameter was 0.45 mm. Increases for groups 1, 2, 4 and 5 (teeth with apical diameters of 0.3 mm or less) were not significant when periods A–C measurements were compared (P > 0.05). NaOCl concentration had no significant influence on length measurements.

This report describes the manufacture and evaluation of a simple new in vitro model for teaching electronic working length determination. This model could provide service for demonstration for periods of up to 45 h provided it was kept moist.
This study did not seek to investigate the accuracy of the Root ZX whose performance and reliability has been reported previously (Shabahang et al. 1996, Pagavino et al. 1998). Rather, this reliable apex locator was employed to validate the model as a teaching tool.
Our model required no special materials and had advantages over the models of Aurelio et al. (1983) and Donnelly (1993). This latter model had the added disadvantage that it was cumbersome and required close attention because the investigator had to hold the experimental tooth. Bias was also possible because the tooth apex and emergent file were visible to the user. Root apices in our model could not be seen. Additionally, in the previous models, problems were encountered in teeth with open apices. It is possible that molten gelatine or phosphate buffered saline solution moved into the canal on insertion of the tooth causing premature electronic readings (Fouad et al. 1990, Czerw et al. 1994). A similar scenario did not arise in our model, probably owing to the relative stiffness of the alginate mould. However, when the diameter of the foramen was 0.45 mm and measurements were made after 28 h, the electronic measurements were beyond the apex. This could have resulted from shrinkage or deterioration of the alginate mould.
At the other extreme, readings in the tooth with a narrow foramen (groups 1 and 4) were consistently more than 1 mm short during the first 19 h of testing. The explanation for this is unclear, but its occurrence in both the 2.65 and 5.25% NaOCl groups suggests some form of equilibration taking place between the alginate and canal contents. No attempt was made to investigate if differing the file size for testing would have influenced results. After 19 h, readings fell within 1 mm of root end. There may, therefore, be merit in preparing the demonstration model 24 h before use if teeth with normal root-end architecture are to be employed. If used with immature teeth, the model may be used immediately after construction.
The stability of teeth in our model, and the potential to mount it in the phantom head brings a measure of realism for use in preclinical courses and even with cordless endodontic handpieces with a built-in apex locator such as the Tri Auto ZX (Morita, Tokyo, Japan).


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