Push-out of root canal filling using the material testing stage (MTS) inside a Micro-CT: preliminary observations

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1 Push-out of root canal filling using the material testing stage (MTS) inside a Micro-CT: preliminary observations G.B. Leoni, 1 M.Brito-Júnior, 1 E.A. Gomes, 2 J.Chaves, 1 Y.T.C. Silva-Sousa, 2 M.D.Sousa-Neto 1 1 Department of Restorative Dentistry, Dental School of Ribeirão Preto, University of São Paulo, Brazil 2 Faculty of Dentistry,University of Ribeirão Preto UNAERP, Ribeirão Preto - São Paulo, Brazil Aims Mechanical testing has been performed to evaluate the bond strength of root canal fillings to dentin surface of root canal. In this scenario, the push-out testing is widely used, although the results from several studies are sometimes inconsistent (1-4). The variability of specimen s geometry and thickness; and diameters of tip applying the load are some factors that can to alter the stress distribution and consequently the results (4,5). In addition, the fixation and parallelism of specimens during push-out testing is difficult to obtain (6). These drawbacks claim for a more rigorous protocol to perform the bond-strength testing (5). Micro-computed tomography (μct) is a non-destructive tool producing three-dimensional images used for different purposes in dental researches (7,8). A recently developed technology is able to provide specimen s images during the mechanical tests. For this purpose, a material testing stage (MTS) is fitted inside a μct scanner (9). Since MTS records load-displacement data during the test, a digital push-out micromodel can be created by using μct permitting to evaluate with more accuracy the bond-strength values. In addition, μct datasets once converted into numerical models facilitates the finite element analysis (FEA) of the material and its geometrical structure (9). Thus, this study presents some preliminary observations on push-out bond strength of root canal filing using MTS technology associated to a μct. FEA was used to provide additional information on this micromechanical experimental model. Method Sample selection and preparation Ten straight single-rooted human maxillary canine teeth with complete root formation were used in this study. The inclusion criteria were: straight roots with similar lengths and without pronounced flattening, with a single root canal verified radiographically. The teeth were examined under light at 10x magnification, and those with cracks or fractures were discarded. Afterwards, each tooth was horizontally sectioned at the cementoenamel junction with a diamond disc keeping a root length of 15 mm. The root canals were instrumented using a R50 Reciproc file attached to a torque-limited electric motor. Preparation was performed under irrigation with 1% NaOCl followed by smear layer removal with a 17% EDTA solution. The canals were irrigated again with 1% NaOCl and dried with paper points. Root canal fillings were performed with a R50 single-cone gutta-percha slightly coated with one of following sealers: AHPlus (n=5) or Epiphany SE (n=5). The sealers were used according to the manufacturers instructions and inserted into the canals using a K-file size 40 in a counterclockwise rotation. All canal preparations and root filling procedures were completed by the same operator. The teeth were stored at 37 C and 100% humidity for 1 week. After this period, the specimens were fixed on acrylic plates with wax and then sectioned in a precision cutting machine obtaining four 2-mm-thick slices (cervical and medium level) from each root sample. The first slice of middle root third was selected for the push-out test.

2 Micromechanical push-out model The push-out test was carried out using the MTS fitted in a μct scanner (SkyScan 1174v2; Bruker-microCT). As the MTS was not designed to push-out test, some adjustments were necessary. Two polyurethane devices were fabricated to facilitate the mechanical testing. The former, equipped with a 0.45 mm-diameter stainless steel cylindrical plunger, was precisely fitted on the disc inside the plexi tube of the MTS (Figure 1A). Another device, with a 2mm central hole, was fitted on the lower MTS disc. This enabled the positioning of the specimen apicalcoronally on central hole (Figure 1B). An acrylic tube with a plunger was fabricated to provide the correct specimens parallelism (Figure 1 C). Afterwards, the plexi tube of the MTS was attached on its lower disc. An initial image of the apparatus was acquired to check the alignment of the specimen relative to plunger fixed to the upper base (Figure 1C). Before push-out testing, the apparatus was scanned at an isotropic resolution of 13.0 µm. The X-ray tube was operated at 50 kv and 80 ma (0.5 mm Al filter), and the scanning was performed by 360º rotation around the vertical axis. Sequentially, the parameter for object continuous compression was chosen in the MTS software. The test started with the lower disc moving upward to the plunger, which penetrated on the filling material leading to its displacement. At this moment the stop button was activated. The results were showed on screen in a graph, as well as displayed in quantitative data on load and displacement. The final load (N) was recorded. Push-out bond strength mean values in each group were used for statistical analysis (t-test, P<0.05). The specimen was submitted to a final µct scan using the initial parameter settings. Figure 1: (A and B) Polyurethane devices attached in the plexi tube and in the lower disc of the MTS; (C) The apparatus fitted in the μct for scanning under push-out test.

3 3D finite element analysis The image of a specimen obtained from the initial scan was reconstructed with dedicated software (NRecon v.1.6.3; Bruker-microCT). Based on this image, a model was established for 3D finite element analysis simulating a push-out test. The μct data were imported by the ScanCAD software for performing the segmentation and fabrication of masks in Stl format. Also in Simpleware software using ScanFE, the mechanical properties of the material, namely elastic modulus [E] and Poisson's ratio [v], were defined from data described in the literature. A finite element mesh was genereted using linear tetrahedral elements of C3D4 type, with the final model had elements and 9277 nodes (Figure 2). The finite element mesh was imported into the finite element software (baqus EF1 A). A 1.9 N pressure load was applied on the top surface simulating the steel rod. This load represented the bond strength mean value verified experimentally for the AHPlus group. The model was supported by the dentin in the three axes (x = y = z = 0). Equivalent von Mises stress (σvm) was obtained, which demonstrates energy transmission in a structure, that is, where there is a higher concentration of energy. Figure 2: Mesh generation and load application with the simulated cylindrical plunger. Results Bond strength mean values and standard deviations (in N) for filling material displacement from root canals are given in Figure 3. There were no statistically significant (P = 0.137) differences f o r A HP l u s a n d Epiphan y S E r o o t c a n a l s e a l e r s. Figure 3: Bond strength mean values verified for AHPlus and Epiphany SE groups.

4 The displacement after push-out test can also observed by 3D models (Figure4). Figure 4: Examples of reconstructed 3D images showing the displacement of filling materials after push-out test. The results of finite element analysis using the von Mises are shown in Figure 4. High concentration of tensile stress was verified in the filling material/dentin interface. This indicates the possible sites of failure initiation during load application. Figure 5. (A and B) The von Mises stress distribution between filling material and dentin interface in the presence of the plunger; (C and D) Interfacial stress distribution without the plunger. Highest and lowest stress values are indicated by red and blue colors, respectively.

5 Conclusion Based on these preliminary observations, the micromechanical model using the MTS technology seems a suitable method to test bond strength of root filling materials to root dentin. References: 1. De-Deus G, Di Giorgi K, Fidel S, Fidel RA, Paciornik S. Push-out bond strength of Resilon/Epiphany and Resilon/Epiphany self-etch to root dentin. J Endod, 35, , Neelakantan P, Varughese AA, Sharma S, Subbarao CV, Zehnder M, De- Deus G. Continuous chelation irrigation improves the adhesion of epoxy resinbased root canal sealer to root dentine. Int Endod J, 45, , Carneiro SM, Sousa-Neto MD, Rached FA Jr, Miranda CE, Silva SR, Silva- Sousa YT. Push-out strength of root fillings with or without thermomechanical compaction. Int Endod J, 45, 821-8, Nagas E, Uyanik O, Durmaz V, Cehreli ZC. Effect of plunger diameter on the push-out bond values of different root filling materials. Int Endod J, 44, 950-5, Chen WP, Chen YY, Huang SH, Lin CP. Limitations of push-out test in bond strength measurement. J Endod, 39, 283-7, Van Meerbeek B, Peumans M, Poitevin A, Mine A, Van Ende A, Neves A, De Munck J. Relationship between bond-strength tests and clinical outcomes. Dent Mater, 26, , Swain MV, Xue J. State of the art of Micro-CT applications in dental research. Int J Oral Sci, 1, , Versiani MA, Pécora JD, Sousa-Neto MD. Microcomputed tomography analysis of the root canal morphology of single-rooted mandibular canines.int Endod J, 46, 800-7, Tsafnat N, Wroe S. An experimentally validated micromechanical model of a rat vertebra under compressive loading. J Anat, 218, 40-6, 2011.