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. Discussion.
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).
Aurelio JA, Nahmias Y, Gerstein H (1983) A model for demonstrating an electronic canal-length measuring device. Journal of Endodontics 9, 568-9.
Czerw RJ, Fulkerson MS, Donnelly JC (1994) An in vitro test of a simplified model to demonstrate the operation of electronic root-canal measuring devices. Journal of Endodontics 20, 605-6.
Donnelly JC (1993) A simplified model to demonstrate the operation of electronic root-canal measuring devices. Journal of Endodontics 19, 579-80.
Fouad AF, Krell KV, McKendry DJ, Koorbusch GF, Olson RA (1990) A clinical evaluation of five electronic root-canal length measuring instruments. Journal of Endodontics 16, 446-9.
Katz A, Kaufman AY, Szajkis S (1992) An in vitro model for testing the accuracy of apex locators. Revue Francaise D'endodontie 11, 67(abstract).
Kobayashi C, Suda H (1994) New electronic canal measuring device based on the ratio method. Journal of Endodontics 20, 111-4.
Pagavino G, Pace R, Baccetti T (1998) A SEM study of in vivo accuracy of the Root ZX electronic apex locator. Journal of Endodontics 24, 438-41.
Shabahang S, Goon WWY, Gluskin AH (1996) An in vivo evaluation of Root ZX electronic apex locator. Journal of Endodontics 22, 616-8.