An Investigation of the Accuracy and Reproducibility of 3D Printed Transparent Endodontic Blocks

Due to a broad spectrum of endodontic rotary instruments on the market and no standardised protocol for comparing their mechanical properties, it can be challenging for clinician to choose proper instruments. In vitro studies using resin blocks with artificial canals can offer many valuable information because of their uniformity compared to studies performed on extracted teeth. T o improve precision and reproducibility of artificial canals, 3D printing was used in this study to manufacture endodontic test block samples. 20 commercially available endodontic blocks Endo-Training-Bloc-J by Dentsply Sirona were tested. The mean values of the measured parameters were used for a 3D CAD model of their replicas. 20 copies of the endodontic training blocks were printed from acrylic resin (VeroClear-RGD810, Stratasys, Eden Prairie, USA) using the 3D printer Objet30 Pro (Stratasys, Eden Prairie, USA). The key dimensions of the commercial blocks and the 3D printed blocks were measured under and compared using t – test and Levene’s test for equality of variances. The profiles of the 3D printed artificial canals showed significantly lower dimensional variability when compared with the commercial blocks. 3D polyjet printing proved to be a precise and reproducible method for production of blocks for testing endodontic rotary instruments.

endodontic training block; 3D printing; additive manufacturing; artificial root canal; J shape resin block; PolyJet printing AUTHOR AFFILIATIONS 1 Department of Medical Biophysics, Faculty of Medicine in Hradec Králové, Charles University, Hradec Králové, Czech Republic * Corresponding author: Charles University, Faculty of Medicine in Hradec Králové, Department of Medical Biophysics, Hradec Králové, Czech Republic; e-mail: [email protected] Received: 1 July 2022 Accepted: 8 September 2022 Published online: 2 December 2022 Acta Medica (Hradec Králové) 2022; 65(2): 59–65 https:/ /doi.org/10.14712/18059694.2022.19 © 2022 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution License (http:/ /creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. AM_2_2022.indd 59AM_2_2022.indd 59 05.12.2022 9:2305.12.2022 9:23 60 Martin Smutný et al.  Acta Medica (Hradec Králové)

Maintaining the original anatomy of a root canal during preparation is one of the goals of the root canal treatment. It is challenging especially in curved canals (1). To achieve this goal, NiTi (nickel-titanium) systems using the super- elastic properties of the nickel-titanium alloy have been introduced. NiTi rotary instruments are more flexible, and preparation is less time consuming when compared with stainless steel instruments (2). Probably the biggest disad- vantage of this type of treatment is a risk of an abrupt sep- aration of a tip of the instrument. This can prevent prop- er irrigation of a system of root canals and subsequently jeopardize the success of root canal treatment (3). Many NiTi endodontic systems came to the market during last decades and choosing proper system is essential for every clinical situation. Changing parameters, such as different cross section, conicity , surface treatment, can improve the performance of NiTi instruments (4, 5). Some procedures as heat treatment of alloy are proprietary (6). There is no standardised protocol that compares prop- erties of NiTi endodontic instruments. Some studies used extracted teeth to better simulate clinical situation but range of variations in the anatomy of root canals of teeth makes the study results difficult to interpret and reproduce (7, 8). Therefore, some other studies used artificial canals made of clear cast resin for their uniformity (9, 10). Trans- parent simulated root canal models also allow for overlap- ping pre- and post- instrumentation images to evaluate the preparation (11). However, the artificial root canals lack an- atomical irregularities, three-dimensional curvatures, and other qualities of clinical situation. The market offers only few variations in the shape of a canal. Furthermore, the conventionally manufactured simulated root canal models have production-related deviations, so even these models are not sufficiently identical and lack standardization (12, 13). Nevertheless, the resin blocks are broadly used not only in theoretical studies but also for endodontic training, ed- ucational purposes, and laboratory assessment of qualities of endodontic instruments, where it is essential to have precise morphology of the artificial root canal. There are many classification systems describing root morphology. Cross-sectional shapes include round, oval, round oval, ribbon, irregular and C-shaped canals (14). Shape of root curvature can be classified as straight, J-shaped, entirely curved and S-shaped (15). For most purposes, the J-shaped artificial canals with round cross-section are the most rele- vant ones due to widely established test methodology (9, 11, 16). A commercially available endodontic block with J-shape round cross-section artificial canal with dimensions corre- sponding to the average physiological dimensions of the root canal is Endo-Training-Bloc-J by Dentsply Sirona. The advance in the additive manufacturing makes it possible to create transparent endodontic blocks with ar- tificial root canals of any conceivable shape with sufficient accuracy and reproducibility (17, 18). Current drop on drop 3D techniques allows for printing objects with resolu- tion of 16 μm per layer. However, to this date, there is no study that has properly analysed the J-shaped endodontic training block and 3D printed its replica. The aim of this study was to investigate the possibilities of 3D printing of endodontic blocks with artificial root canals using the 3D printer Object 30 by Stratasys Ltd. and to compare the accuracy and reproducibility of the 3D printed endodon- tic blocks (further referred to as the 3DP blocks) with the conventional commercially available endodontic blocks Endo-Training-Bloc-J by Dentsply Sirona (further referred to as the original resin blocks).

A total of 20 ready-made transparent epoxy resin blocks – Plastic Practice Blocks .02 taper – Oblong for 15 file (En - do-Training-Bloc-J Dentsply Sirona, Ballaigues, Switzer- land) were photographed under binocular microscope (DSZS 1112-300, Arsenal, Prague, Czech Republic). The key parameters of the artificial canals, i.e., the Canal profile, Canal thin, Canal thick, Cone length, Canal angle, Cone angle, and Pitch angle, shown in Figure 1, were measured with the help of NIS-Elements 3.20 (Nikon Instruments Inc., Melville NY , USA). The canal profile is defined by the perpendicular distances from the base line, measured at 100 to 1000 pixels positions with the help of the 100 × 100-pixel grid. The base line is the line tangent to the out- put cylinder surface, starting at point where the thin tip of the canal intersects this surface (Figure 1). The mean value of each parameter was used to design 3D drawings in CAD software (Cloud Powered 3D CAD/ CAM Software for Product Design | Fusion 360, 2018, Au- todesk, San Rafael, USA). Subsequently , 20 prototypes of endo blocks were printed from acrylic resin (Vero- Clear-RGD810, Stratasys, Eden Prairie, USA) using the 3D precision printer Objet30 Pro (Stratasys, Eden Prairie, USA). The Objet30 Pro uses the so called PolyJet or Drop on drop printing process where the object is built by selec- tive spraying drops of a photopolymer in ultra-thin layers. Each photopolymer layer is cured with UV light after it is jetted, producing fully cured models that can be handled and used immediately , without post-curing (19). High printing precision with the layer thickness of 16 μm was used. The printer used a soluble support material (SUP706B, Stratasys, Eden Prairie, USA), which should al- low for hands-free, water-jet removal without damaging the delicate structures. A combined glossy , for transparent outer walls, and matte, for precise inner structures, print- ing technique was used. This should allow to observe the printed inner artificial canal structure through the trans- parent wall. To enhance optical properties of the transpar- ent wall eliminating the undesirable light scattering on the still rough surface of the resin block, Evetric Bond (Ivoclar Vivadent AG, Schaan, Liechtenstein) with straight stain- less steel dental matrix were used to create smooth surface and improve the transparency . The printed prototypes were then instrumented us- ing ISO 10 stainless steel K-file (Micro-Mega, Besançon, France) and ethanol 96% as irrigation to remove the soluble support material (SUP706B, Stratasys, Eden Prairie, USA). The same procedure of measuring as for manufactured models was used for these prototypes. However, due to a small shift in the focal plane of the 3DP blocks relative to the original resin blocks, a difference in the micrograph AM_2_2022.indd 60AM_2_2022.indd 60 05.12.2022 9:2305.12.2022 9:23 Investigation of 3D Printed Transparent Endodontic Blocks 61 calibration occurred between the original and the 3DP block, i.e., 100 pixels in the micrographs of the original res- in blocks corresponds to 852.2 μm while 100 pixels in the micrographs of the 3DP blocks corresponds to 743.8 μm. This mismatch led to a different “true positions”, at which the canal profile was measured, and need for fitting the original profile values with a polynomial function to calcu- late the profile values as if measured at the true positions in the 3DP micrographs. For this purpose, the profile data of the original blocks were fitted with a fourth–degree poly- nomial using the MS Excel 2016 (Microsoft Corp., Redmond W A, USA) polynomial trend line function. Nevertheless, as all the micrographs were calibrated using the same calibra- tion slide, the measured distances were not affected. The results were compared, processed, and statistical- ly analysed using MS Excel 2016 (Microsoft Corp., Red- mond W A, USA) and NCSS 10 statistical software (2015, NCSS, LLC., Kaysville, Utah, USA, ncss.com/software /ncss). Since all the data showed to be from normally dis- tributed populations, we opted for using the mean and standard deviation of the sample (± SD) for the data de- scription. We tested the printing accuracy by comparing the printed blocks’ parameters with the values used in the 3D CAD design using 1-sample t -test. We compared the reproducibility of the commercially available endodontic blocks (Endo-Training-Bloc-J Dentsply Sirona, Ballaigues, Switzerland) with the printed blocks using Levene’s test for equality of variances. In all the tests, a value of p < 0.05 was considered as significant. RESUL TS Table 1 shows the measured key parameters of the original resin blocks and the corresponding rounded values used as an input for the CAD model of the 3D printed blocks (avail- able in the supplementary materials on the article web page). For the comparison purposes of the original resin blocks with the 3DP blocks, Table 1 also shows the correct- ed values of the original resin blocks obtained using the fitted fourth–degree polynomial function. The course of the function and its parameters are shown in Figure 2. Table 2 shows the measured key parameters of the 3D printed blocks. When compared with the original resin blocks, the artificial canal profiles of the 3D printed blocks have significantly lower dimensional variability as shown by the results of the Levene’s test (Table 3). The dimension- al variability of the 3D printed blocks represented by co- efficients of variation (Table 2) was 2 – 5 x lower than the variability of the original resin blocks (Table 3). All the measured values of the canal profile of the 3D printed blocks were significantly different compared to the original resin blocks as proved by the t  – test (one sample t – test, Table 3). The larger the measured profile value, the larger the observed difference. The maximum profile difference was observed at the “true” position of 7438 μm where the mean measured 3DP block profile val- ue was by 8% smaller (Ratio of Means 3DP / Original, Ta- ble 3) than the corresponding profile value of the original resin block. Apart from the Canal thin parameter, the other mon- itored parameters also differed significantly (one sample t – test, Table 3). Considering the data variability , the Ca - nal thin, Canal thick Canal angle, and Pitch angle of the 3DP block did not differ significantly from the original resin block. Only in the case of the cone angle parameter, the variability of the measured data of the 3DP block was significantly larger, more than four times (Coefficient of Variation ratio 3DP / Original, Table 3), than that of the original resin block. Fig. 1 3D printed block with highlighted measured parameters. 100 pixels correspond to a distance of 743.8 μm. AM_2_2022.indd 61AM_2_2022.indd 61 05.12.2022 9:2305.12.2022 9:23 62 Martin Smutný et al.  Acta Medica (Hradec Králové) Tab. 1 Key parameters of the original epoxy resin block – Plastic Practice Blocks .02 taper – Oblong for 15 file (Endo-Training-Bloc-J Dentsply Sirona, Ballaigues, Switzerland) – 1 ) measured values, 2 ) rounded mean values used as the input for the CAD model (available in the supple - mentary materials on the article web page), and 3 ) corrected values used for comparison purposes. dimension Original block 1 (μm) Coefficient of Variation 1 (%) Model input Canal profile (pixel) true position (μm) Original 2 (μm) Corrected 3 (μm) @ 100px @ 852.1 μm 1208.0 ± 85.3 7.06 1208.0 1090.4 @ 200px @ 1704.3 μm 1954.3 ± 120.3 6.15 1954.3 1791.1 @ 300px @ 2556.5 μm 2467.8 ± 130.4 5.28 2467.8 2296.4 @ 400px @ 3408.6 μm 2820.9 ± 136.6 4.84 2820.9 2658.1 @ 500px @ 4260.8 μm 3065.1 ± 130.6 4.26 3065.1 2918.5 @ 600px @ 5112.9 μm 3249.4 ± 121.3 3.73 3249.4 3111.2 @ 700px @ 5965.0 μm 3381.4 ± 110.2 3.26 3381.4 3260.6 @ 800px @ 6817.2 μm 3481.8 ± 99.8 2.87 3481.8 3382.0 @ 900px @ 7669.3 μm 3590.8 ± 220.3 6.14 3590.8 3481.9 @ 1000px @ 8521.5 μm 3592.6 ± 82.2 2.29 3592.6 3557.4 Canal thin 136.9 ± 11.3 8.24 136.9 – Canal thick 314.3 ± 15.2 4.84 314.3 – Cone length 6033.4 ± 196.7 3.26 6033.4 – (deg) (%) (deg) – Canal angle 125.5 ± 2.8 2.22 125.5 – Cone angle 29.4 ± 0.3 1.01 2 9 . 4 – Pitch angle 58.2 ± 2.2 3.74 58.2 – Fig. 2 Profile data of the artificial canals. The blue dots indicate the means of the measured profile data of the original blocks, the red crosses mark the corrected profile data of the original blocks as if measured at the same “true” positions as the 3DP blocks, the green dots indicate the means of the measured profile data of the 3DP blocks, and the blue dotted line depicts the fitted fourth–degree polynomial function. The formula represents the fourth–degree polynomial function fitted on the mean profile data of the original blocks with the resulting function parameters. AM_2_2022.indd 62AM_2_2022.indd 62 05.12.2022 9:2305.12.2022 9:23 Investigation of 3D Printed Transparent Endodontic Blocks 63 Tab. 2 Key parameters of the 3D printed endodontic blocks. dimension 3DP block (μm) Coefficient of Variation (%) Canal profile (pixel) true position (μm) @ 100px @ 743.8 μm 1121.0 ± 47.7 4.26 @ 200px @ 1487.6 μm 1759.3 ± 41.1 2.34 @ 300px @ 2231.4 μm 2225.8 ± 34.7 1.56 @ 400px @ 2975.2 μm 2535.9 ± 30.7 1.21 @ 500px @ 3719.0 μm 2756.9 ± 34.3 1.24 @ 600px @ 4462.8 μm 2921.0 ± 32.5 1.11 @ 700px @ 5206.6 μm 3053.7 ± 30.0 0.98 @ 800px @ 5950.4 μm 3138.0 ± 35.3 1.13 @ 900px @ 6694.2 μm 3211.0 ± 43.7 1.36 @ 1000px @ 7438.0 μm 3261.9 ± 39.6 1.21 Canal thin 130.9 ± 13.1 10.0 Canal thick 348.1 ± 17.2 4.95 Cone length 5459.2 ± 82.6 1.51 (deg) (%) Canal angle 127.5 ± 2.3 1.76 Cone angle 28.7 ± 1.2 4.21 Pitch angle 54.2 ± 2.3 4.23 Tab. 3 Comparison of the key parameters of original epoxy resin block – Plastic Practice Blocks .02 taper – Oblong for 15 file (Endo-Train - ing-Bloc-J Dentsply Sirona, Ballaigues, Switzerland) with the 3D printed endodontic blocks. The asterisk (*) indicates insignificant differences of the respective parameters of the 3DP blocks in comparison with the original resin blocks. dimension Ratio of Means 3DP / Original (%) one sample t – test Coefficient of Variation ratio 3DP / Original (%) Levene’s test p – value Canal profile true position (μm) p – value power @ 743.8 μm 103 = 0.010 0.777 56 = 0.018 @ 1487.6 μm 98 = 0.003 0.907 34 > 0.001 @ 2231.4 μm 97 > 0.001 1 27 > 0.001 @ 2975.2 μm 95 > 0.001 1 22 > 0.001 @ 3719.0 μm 94 > 0.001 1 26 > 0.001 @ 4462.8 μm 94 > 0.001 1 27 > 0.001 @ 5206.6 μm 94 > 0.001 1 27 > 0.001 @ 5950.4 μm 93 > 0.001 1 35 = 0.003 @ 6694.2 μm 92 > 0.001 1 20 = 0.037 @ 7438.0 μm 92 > 0.001 1 48 = 0.027 Canal thin 96 = 0.055* 0.493 116 = 0.488* Canal thick 111 > 0.001 1 113 = 0.272* Cone length 90 > 0.001 1 42 = 0.050 (%) (%) Canal angle 102 > 0.001 0.958 81 = 0.392* Cone angle 98 = 0.015 0.715 408 > 0.001 Pitch angle 93 > 0.001 1 105 = 0.876*

In our study , we fabricated 20 transparent endodontic blocks of the same design as the commercially available Plastic Practice Blocks .02 (Endo-Training-Bloc-J Dentsply Sirona, Ballaigues, Switzerland). In previous studies (20–22), the endodontic resin blocks with a  J-shaped artificial canal are described by the length, angle, and radius of curvature of the artificial canal. Nevertheless, especially in the case of determina- tion of the canal angle, there are many different methods (according to Weine, Schneide, Luiten, etc.) (23) providing AM_2_2022.indd 63AM_2_2022.indd 63 05.12.2022 9:2305.12.2022 9:23 64 Martin Smutný et al.  Acta Medica (Hradec Králové) significantly different results, even for the same tested res- in block. Thus, these methods are not sufficiently accurate and reproducible. Therefore, we used a larger number of clearly defined parameters (Figure 1) to precisely measure, copy , and 3D print the endodontic blocks. We measured 20 original resin blocks and used the mean values of the se- lected parameters to design the new 3DP blocks. Despite the printing resolution of 16 μm per layer, the resulting roughness of the block surface was relatively high, preventing direct observation of the artificial canal in the optical microscope. Polishing with different gums and pastes did not bring any improvement. The use of Ev- etric Bond (Ivoclar Vivadent AG, Schaan, Liechtenstein) with straight stainless steel dental matrix led to the for- mation of a smooth transparent layer that allowed obser- vation of the internal structures of the printed block with a minimum distortion. To observe the artificial canals, the newly printed blocks also required removement of the support mate- rial, which turned out to be quite difficult. We used high pressure water, which is the recommended method by the printer manufacturer. Nevertheless, this method did not remove the support material from the artificial canal. In many studies involving the investigation of the resin blocks, water (11, 24), isopropyl alcohol (25), or different types of alcohols were used as irrigation solutions. For us, 96% ethanol worked best as it helped to disrupt and re- move the partially alcohol soluble support material. We found that ethanol was even a better irrigant than NaOCl or EDTA (17). Furthermore, in order to fill canal with ir- rigant and ensure patency , the canals were instrumented with stainless-steel files. It was necessary to prebend the stainless-steel instruments to protect the canals from al- ternating the design during their instrumentation. We proved a high reproducibility of the 3D printing process of the endodontic training blocks printed from acrylic resin (VeroClear-RGD810, Stratasys, Eden Prairie, USA) using the 3D precision printer Objet30 Pro (Strata- sys, Eden Prairie, USA). The dimensional variability of the 3DP blocks was significantly lower (3–5 times) for most of the tested parameters. It is the most important study output because only the blocks with a low dimen- sional variability allow for a consistent and reproduc- ible results of the tests of endodontic instruments. The accuracy of the 3DP blocks was also sufficient. Although the measured dimensions of the 3DP blocks were statis- tically significantly different from the dimensions of the original resin blocks, the maximum difference was 11% and can be easily eliminated by modifying the CAD model (available in the supplementary materials on the article web page). A limitation of this study is the effect on the artificial canal profile by removing the support material. Even with the use of a fine and pre-bent ISO 10 stainless steel K-file, the canal was straightened, which is a typi- cal problem when using rotary files (26) and is shown by a decrease in the pitch angle and a relative increase in the values of the canal profile measured at distances of 100, 200, 300 and 400 px. The use of a finer tools – e.g. ISO 06 and 08 – could not remove the support material from the artificial canal. Also, the proximal and distal diameters of the artificial canal of the 3DP blocks were affected. Alter- native possibilities of the support material removal from the artificial canals should be further investigated. Despite certain difficulties associated with the elimina- tion of the support material, PolyJet printing is currently the only available 3DP method that can print such narrow (100 μm) curved cavities. For comparison, the SLA (ste- reolithography) method does not allow the printing of supports from different material that that of the object printed, the FFD (filament feeder) 3DP method does not allow such a fine resolution (usually not less than 50 μm per layer, e.g., TRILAB DeltiQ 2 by TRILAB Group s.r.o., Hradec Králové, Czech Republic) and is also limited by the minimum width of the printed line (usually not less than 250 μm, e.g., TRILAB DeltiQ 2 by TRILAB Group s.r.o., Hra- dec Králové, Czech Republic), the SLS (selective laser sin- tering) method is limited by the grain size of the sintered

(usually not less than 100 μm, e.g., Polyamid 12 PA 2200 by EOS GmbH, Krailling, Germany).

3D polyjet printing is a promising method of manufactur- ing endodontic test blocks. Its main advantages are high reproducibility of printing and the possibility of produc- ing artificial root canals of any desired shape. The profiles of the 3D printed artificial canals showed statistically significantly lower dimensional variability (2–5 times) when compared with the original resin blocks. The disadvantage of the printing method used is the difficult support material removal negatively affecting the profile of the printed artificial canal. Alternative methods to remove the support material should be further investi- gated.

1. Capar ID, Ertas H, Ok E, Arslan H, Ertas ET. Comparative study of different novel nickel-titanium rotary systems for root canal prepa- ration in severely curved root canals. J Endod 2014; 40(6): 852–6. 2. Crespo S, Cortes O, Garcia C, Perez L. Comparison between rotary and manual instrumentation in primary teeth. J Clin Pediatr Dent 2008; 32(4): 295–8. 3. Simon S, Machtou P , Tomson P , Adams N, Lumley P . Influence of frac- tured instruments on the success rate of endodontic treatment. Dent Update 2008; 35(3): 172–4, 176, 178–9. 4. Versluis A, Kim HC, Lee W , Kim BM, Lee CJ. Flexural stiffness and stresses in nickel-titanium rotary files for various pitch and cross-sectional geometries. J Endod 2012; 38(10): 1399–403. 5. Bui TB, Mitchell JC, Baumgartner JC. Effect of electropolishing Pro- File nickel-titanium rotary instruments on cyclic fatigue resistance, torsional resistance, and cutting efficiency . J Endod 2008; 34(2): 190–3. 6. Staffoli S, Grande NM, Plotino G, et al. Influence of environmental temperature, heat-treatment and design on the cyclic fatigue resist- ance of three generations of a single-file nickel-titanium rotary in- strument. Odontology 2019; 107(3): 301–7. 7. Vahid A, Roohi N, Zayeri F. A comparative study of four rotary NiTi instruments in preserving canal curvature, preparation time and change of working length. Australian Endodontic Journal 2009; 35(2): 93–7. 8. Elsherief SM, Zayet MK, Hamouda IM. Cone-beam computed tomog- raphy analysis of curved root canals after mechanical preparation with three nickel-titanium rotary instruments. J Biomed Res 2013; 27(4): 326–35. 9. Çelik G, Maden M, Savgat A, Orhan H. Shaping ability of the profile 25/0.06 and protaper F2 in rotary motion, and reciproc in simulated canals. PeerJ 2018;6:e6109. AM_2_2022.indd 64AM_2_2022.indd 64 05.12.2022 9:2305.12.2022 9:23 Investigation of 3D Printed Transparent Endodontic Blocks 65 10. Ceyhanli KT, Kamaci A, Taner M, Erdilek N, Celik D. Shaping ability of two M-wire and two traditional nickel-titanium instrumentation systems in S-shaped resin canals. Niger J Clin Pract 2015; 18(6): 713. 11. Ba-Hattab R, Pröhl AK, Lang H, Pahncke D. Comparison of the shap- ing ability of GT® Series X, Twisted Files and AlphaKite rotary nick- el-titanium systems in simulated canals. BMC Oral Health 2013; 13: 72. 12. Dummer PMH, Alodeh MHA, Al-Omari MAO. A method for the con- struction of simulated root canals in clear resin blocks. Int Endod J 1991; 24(2): 63–6. 13. Hartmann RC, Fensterseifer M, Peters OA, de Figueiredo J a. P , Gomes MS, Rossi-Fedele G. Methods for measurement of root canal curva- ture: a systematic and critical review. Int Endod J 2019; 52(2): 169–80. 14. Razumova S, Brago A, Howijieh A, Barakat H, Kozlova Y , Baykulova M. Evaluation of Cross-Sectional Root Canal Shape and Presentation of New Classification of Its Changes Using Cone-Beam Computed To- mography Scanning. Appl Sci 2020; 10(13): 4495. 15. Nagy CD, Szabó J, Szabó J. A mathematically based classification of root canal curvatures on natural human teeth. J Endod 1995; 21(11): 557–60. 16. Troiano G, Dioguardi M, Cocco A, Zhurakivska K, Ciavarella D, Muzio LL. Increase the glyde path diameter improves the centering ability of F6 Skytaper. Eur J Dent 2018; 12(1): 89–93. 17. Christofzik D, Bartols A, Faheem MK, Schroeter D, Groessner-Schrei- ber B, Doerfer CE. Shaping ability of four root canal instrumentation systems in simulated 3D-printed root canal models. PLOS ONE 2018; 13(8): e0201129. 18. Soo WKM, Thong YL. Construction Of Standardised Simulated Root Canals In Resin Blocks For Pre-clinical Teaching. Ann Dent UM 2002; 9(1): 7–10. 19. Coward C. 3D Printing. New York (US) Penguin Group; 2015: 74. 20. Keskin C, Demiral M, Sarıyılmaz E. Comparison of the shaping abili- ty of novel thermally treated reciprocating instruments. Restor Dent Endod 2018; 43(2): e15. 21. Van Pham K, Vo CQ. A new method for assessment of nickel-titanium endodontic instrument surface roughness using field emission scan- ning electronic microscope. BMC Oral Health 2020; 20(1): 240. 22. Uslu G, İnan U. Effect of glide path preparation with PathFile and Pro- Glider on the cyclic fatigue resistance of WaveOne nickel-titanium files. Restor Dent Endod 2019; 44(2): e22. 23. Nagmode PS, Chavan KM, Rathi RS, Tambe VH, Lokhande N, Kapse BS. Radiographic evaluation of root canal curvature in mesiobuccal canals of mandibular molars by different methods and its correlation with canal access angle in curved canals: An in vitro study . J Conserv Dent 2019; 22(5): 425–9. 24. Silva EJNL, Muniz BL, Pires F, et al. Comparison of canal transpor- tation in simulated curved canals prepared with ProTaper Universal and ProTaper Gold systems. Restor Dent Endod 2016; 41(1): 1–5. 25. Goldberg M, Dahan S, Machtou P . Centering Ability and Influence of Experience When Using WaveOne Single-File Technique in Simulat- ed Canals. Int J Dent 2012; 2012: e206321. 26. Loizides A, Eliopoulos D, Kontakiotis E. Root canal transportation with a Ni-Ti rotary file system and stainless steel hand files in simu- lated root canals. Quintessence Int 2006; 37(5): 369–74. AM_2_2022.indd 65AM_2_2022.indd 65 05.12.2022 9:2305.12.2022 9:23