{"paper_id":"9313e001-15dd-422c-9dcf-27bb76c5a929","body_text":"original article 59\nAn Investigation of the Accuracy  \nand Reproducibility of 3D Printed  \nTransparent Endodontic Blocks\nMartin Smutný 1 , Martin Kopeček 1 , Aleš Bezrouk 1, *\nABSTRACT\nDue to a broad spectrum of endodontic rotary instruments on the market and no standardised protocol for comparing their mechanical \nproperties, it can be challenging for clinician to choose proper instruments. In vitro studies using resin blocks with artificial canals can \noffer many valuable information because of their uniformity compared to studies performed on extracted teeth. T o improve precision \nand reproducibility of artificial canals, 3D printing was used in this study to manufacture endodontic test block samples. 20 commercially \navailable endodontic blocks Endo-Training-Bloc-J by Dentsply Sirona were tested. The mean values of the measured parameters were \nused for a 3D CAD model of their replicas. 20 copies of the endodontic training blocks were printed from acrylic resin (VeroClear-RGD810, \nStratasys, Eden Prairie, USA) using the 3D printer Objet30 Pro (Stratasys, Eden Prairie, USA). The key dimensions of the commercial blocks \nand the 3D printed blocks were measured under and compared using t – test and Levene’s test for equality of variances. The profiles of \nthe 3D printed artificial canals showed significantly lower dimensional variability when compared with the commercial blocks. 3D polyjet \nprinting proved to be a precise and reproducible method for production of blocks for testing endodontic rotary instruments.\nKEYWORDS\nendodontic training block; 3D printing; additive manufacturing; artificial root canal; J shape resin block; PolyJet printing\nAUTHOR AFFILIATIONS\n1  Department of Medical Biophysics, Faculty of Medicine in Hradec Králové, Charles University, Hradec Králové, Czech Republic\n* Corresponding author: Charles University, Faculty of Medicine in Hradec Králové, Department of Medical Biophysics, Hradec Králové, \nCzech Republic; e-mail: bezrouka@lfhk.cuni.cz\nReceived: 1 July 2022\nAccepted: 8 September 2022\nPublished online: 2 December 2022\nActa Medica (Hradec Králové) 2022; 65(2): 59–65\nhttps:/ /doi.org/10.14712/18059694.2022.19\n© 2022 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution License  \n(http:/ /creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium,  \nprovided the original author and source are credited.\nAM_2_2022.indd   59AM_2_2022.indd   59 05.12.2022   9:2305.12.2022   9:23\n\n\n60 Martin Smutný et al.  Acta Medica (Hradec Králové)\n \nINTRODUCTION\nMaintaining the original anatomy of a root canal during \npreparation is one of the goals of the root canal treatment. \nIt is challenging especially in curved canals (1). To achieve \nthis goal, NiTi (nickel-titanium) systems using the super-\nelastic properties of the nickel-titanium alloy have been \nintroduced. NiTi rotary instruments are more flexible, and \npreparation is less time consuming when compared with \nstainless steel instruments (2). Probably the biggest disad-\nvantage of this type of treatment is a risk of an abrupt sep-\naration of a tip of the instrument. This can prevent prop-\ner irrigation of a system of root canals and subsequently \njeopardize the success of root canal treatment (3). Many \nNiTi endodontic systems came to the market during last \ndecades and choosing proper system is essential for every \nclinical situation. Changing parameters, such as different \ncross section, conicity , surface treatment, can improve the \nperformance of NiTi instruments (4, 5). Some procedures \nas heat treatment of alloy are proprietary (6).\nThere is no standardised protocol that compares prop-\nerties of NiTi endodontic instruments. Some studies used \nextracted teeth to better simulate clinical situation but \nrange of variations in the anatomy of root canals of teeth \nmakes the study results difficult to interpret and reproduce \n(7, 8). Therefore, some other studies used artificial canals \nmade of clear cast resin for their uniformity (9, 10). Trans-\nparent simulated root canal models also allow for overlap-\nping pre- and post- instrumentation images to evaluate the \npreparation (11). However, the artificial root canals lack an-\natomical irregularities, three-dimensional curvatures, and \nother qualities of clinical situation. The market offers only \nfew variations in the shape of a canal. Furthermore, the \nconventionally manufactured simulated root canal models \nhave production-related deviations, so even these models \nare not sufficiently identical and lack standardization (12, \n13). Nevertheless, the resin blocks are broadly used not only \nin theoretical studies but also for endodontic training, ed-\nucational purposes, and laboratory assessment of qualities \nof endodontic instruments, where it is essential to have \nprecise morphology of the artificial root canal. There are \nmany classification systems describing root morphology. \nCross-sectional shapes include round, oval, round oval, \nribbon, irregular and C-shaped canals (14). Shape of root \ncurvature can be classified as straight, J-shaped, entirely \ncurved and S-shaped (15). For most purposes, the J-shaped \nartificial canals with round cross-section are the most rele-\nvant ones due to widely established test methodology (9, 11, \n16). A commercially available endodontic block with J-shape \nround cross-section artificial canal with dimensions corre-\nsponding to the average physiological dimensions of the \nroot canal is Endo-Training-Bloc-J by Dentsply Sirona.\nThe advance in the additive manufacturing makes it \npossible to create transparent endodontic blocks with ar-\ntificial root canals of any conceivable shape with sufficient \naccuracy and reproducibility (17, 18). Current drop on drop \n3D techniques allows for printing objects with resolu-\ntion of 16 μm per layer. However, to this date, there is no \nstudy that has properly analysed the J-shaped endodontic \ntraining block and 3D printed its replica. The aim of this \nstudy was to investigate the possibilities of 3D printing \nof endodontic blocks with artificial root canals using the \n3D printer Object 30 by Stratasys Ltd. and to compare the \naccuracy and reproducibility of the 3D printed endodon-\ntic blocks (further referred to as the 3DP blocks) with the \nconventional commercially available endodontic blocks \nEndo-Training-Bloc-J by Dentsply Sirona (further referred \nto as the original resin blocks).\nMATERIALS AND METHODS\nA total of 20 ready-made transparent epoxy resin blocks – \nPlastic Practice Blocks .02 taper – Oblong for 15 file (En -\ndo-Training-Bloc-J Dentsply Sirona, Ballaigues, Switzer-\nland) were photographed under binocular microscope \n(DSZS 1112-300, Arsenal, Prague, Czech Republic). The key \nparameters of the artificial canals, i.e., the Canal profile, \nCanal thin, Canal thick, Cone length, Canal angle, Cone \nangle, and Pitch angle, shown in Figure 1, were measured \nwith the help of NIS-Elements 3.20 (Nikon Instruments \nInc., Melville NY , USA). The canal profile is defined by the \nperpendicular distances from the base line, measured at \n100 to 1000 pixels positions with the help of the 100 × \n100-pixel grid. The base line is the line tangent to the out-\nput cylinder surface, starting at point where the thin tip of \nthe canal intersects this surface (Figure 1).\nThe mean value of each parameter was used to design \n3D drawings in CAD software (Cloud Powered 3D CAD/\nCAM Software for Product Design | Fusion 360, 2018, Au-\ntodesk, San Rafael, USA). Subsequently , 20 prototypes \nof endo blocks were printed from acrylic resin (Vero-\nClear-RGD810, Stratasys, Eden Prairie, USA) using the \n3D precision printer Objet30 Pro (Stratasys, Eden Prairie, \nUSA). The Objet30 Pro uses the so called PolyJet or Drop on \ndrop printing process where the object is built by selec-\ntive spraying drops of a photopolymer in ultra-thin layers. \nEach photopolymer layer is cured with UV light after it is \njetted, producing fully cured models that can be handled \nand used immediately , without post-curing (19).\nHigh printing precision with the layer thickness of 16 \nμm was used. The printer used a soluble support material \n(SUP706B, Stratasys, Eden Prairie, USA), which should al-\nlow for hands-free, water-jet removal without damaging \nthe delicate structures. A combined glossy , for transparent \nouter walls, and matte, for precise inner structures, print-\ning technique was used. This should allow to observe the \nprinted inner artificial canal structure through the trans-\nparent wall. To enhance optical properties of the transpar-\nent wall eliminating the undesirable light scattering on the \nstill rough surface of the resin block, Evetric Bond (Ivoclar \nVivadent AG, Schaan, Liechtenstein) with straight stain-\nless steel dental matrix were used to create smooth surface \nand improve the transparency .\nThe printed prototypes were then instrumented us-\ning ISO 10 stainless steel K-file (Micro-Mega, Besançon, \nFrance) and ethanol 96% as irrigation to remove the soluble \nsupport material (SUP706B, Stratasys, Eden Prairie, USA).\nThe same procedure of measuring as for manufactured \nmodels was used for these prototypes. However, due to \na small shift in the focal plane of the 3DP blocks relative \nto the original resin blocks, a difference in the micrograph \nAM_2_2022.indd   60AM_2_2022.indd   60 05.12.2022   9:2305.12.2022   9:23\n\nInvestigation of 3D Printed Transparent Endodontic Blocks 61\ncalibration occurred between the original and the 3DP \nblock, i.e., 100 pixels in the micrographs of the original res-\nin blocks corresponds to 852.2 μm while 100 pixels in the \nmicrographs of the 3DP blocks corresponds to 743.8 μm. \nThis mismatch led to a different “true positions”, at which \nthe canal profile was measured, and need for fitting the \noriginal profile values with a polynomial function to calcu-\nlate the profile values as if measured at the true positions in \nthe 3DP micrographs. For this purpose, the profile data of \nthe original blocks were fitted with a fourth–degree poly-\nnomial using the MS Excel 2016 (Microsoft Corp., Redmond \nW A, USA) polynomial trend line function. Nevertheless, as \nall the micrographs were calibrated using the same calibra-\ntion slide, the measured distances were not affected.\nThe results were compared, processed, and statistical-\nly analysed using MS Excel 2016 (Microsoft Corp., Red-\nmond W A, USA) and NCSS 10 statistical software (2015, \nNCSS, LLC., Kaysville, Utah, USA, ncss.com/software  \n/ncss). Since all the data showed to be from normally dis-\ntributed populations, we opted for using the mean and \nstandard deviation of the sample (± SD) for the data de-\nscription. We tested the printing accuracy by comparing \nthe printed blocks’ parameters with the values used in the \n3D CAD design using 1-sample t -test. We compared the \nreproducibility of the commercially available endodontic \nblocks (Endo-Training-Bloc-J Dentsply Sirona, Ballaigues, \nSwitzerland) with the printed blocks using Levene’s test \nfor equality of variances. In all the tests, a value of p < 0.05 \nwas considered as significant.\nRESUL TS\nTable 1 shows the measured key parameters of the original \nresin blocks and the corresponding rounded values used as \nan input for the CAD model of the 3D printed blocks (avail-\nable in the supplementary materials on the article web \npage). For the comparison purposes of the original resin \nblocks with the 3DP blocks, Table 1 also shows the correct-\ned values of the original resin blocks obtained using the \nfitted fourth–degree polynomial function. The course of \nthe function and its parameters are shown in Figure 2.\nTable 2 shows the measured key parameters of the 3D \nprinted blocks. When compared with the original resin \nblocks, the artificial canal profiles of the 3D printed blocks \nhave significantly lower dimensional variability as shown \nby the results of the Levene’s test (Table 3). The dimension-\nal variability of the 3D printed blocks represented by co-\nefficients of variation (Table 2) was 2 – 5 x lower than the \nvariability of the original resin blocks (Table 3).\nAll the measured values of the canal profile of the 3D \nprinted blocks were significantly different compared to \nthe original resin blocks as proved by the t  – test (one \nsample t – test, Table 3). The larger the measured profile \nvalue, the larger the observed difference. The maximum \nprofile difference was observed at the “true” position of \n7438 μm where the mean measured 3DP block profile val-\nue was by 8% smaller (Ratio of Means 3DP / Original, Ta-\nble 3) than the corresponding profile value of the original \nresin block.\nApart from the Canal thin parameter, the other mon-\nitored parameters also differed significantly (one sample \nt – test, Table 3). Considering the data variability , the Ca -\nnal thin, Canal thick Canal angle, and Pitch angle of the \n3DP block did not differ significantly from the original \nresin block. Only in the case of the cone angle parameter, \nthe variability of the measured data of the 3DP block was \nsignificantly larger, more than four times (Coefficient of \nVariation ratio 3DP / Original, Table 3), than that of the \noriginal resin block.\nFig. 1  3D printed block with highlighted measured parameters. 100 pixels correspond to a distance of 743.8 μm.\nAM_2_2022.indd   61AM_2_2022.indd   61 05.12.2022   9:2305.12.2022   9:23\n\n62 Martin Smutný et al.  Acta Medica (Hradec Králové)\n \nTab. 1 Key parameters of the original epoxy resin block – Plastic Practice Blocks .02 taper – Oblong for 15 file (Endo-Training-Bloc-J Dentsply \nSirona, Ballaigues, Switzerland) – 1 ) measured values, 2 ) rounded mean values used as the input for the CAD model (available in the supple -\nmentary materials on the article web page), and 3 ) corrected values used for comparison purposes.\ndimension\nOriginal block 1  (μm) Coefficient of \nVariation 1  (%)\nModel input\nCanal profile\n(pixel) true position (μm) Original 2  (μm) Corrected 3  (μm)\n@  100px @  852.1 μm 1208.0 ± 85.3 7.06 1208.0 1090.4\n@  200px @ 1704.3 μm 1954.3 ± 120.3 6.15 1954.3 1791.1\n@  300px @ 2556.5 μm 2467.8 ± 130.4 5.28 2467.8 2296.4\n@  400px @ 3408.6 μm 2820.9 ± 136.6 4.84 2820.9 2658.1\n@  500px @ 4260.8 μm 3065.1 ± 130.6 4.26 3065.1 2918.5\n@  600px @ 5112.9 μm 3249.4 ± 121.3 3.73 3249.4 3111.2\n@  700px @ 5965.0 μm 3381.4 ± 110.2 3.26 3381.4 3260.6\n@  800px @ 6817.2 μm 3481.8 ± 99.8 2.87 3481.8 3382.0\n@  900px @ 7669.3 μm 3590.8 ± 220.3 6.14 3590.8 3481.9\n@ 1000px @ 8521.5 μm 3592.6 ± 82.2 2.29 3592.6 3557.4\nCanal thin 136.9 ± 11.3 8.24  136.9 –\nCanal thick 314.3 ± 15.2 4.84  314.3 –\nCone length 6033.4 ± 196.7 3.26 6033.4 –\n(deg) (%) (deg) –\nCanal angle 125.5 ± 2.8 2.22  125.5 –\nCone angle 29.4 ± 0.3 1.01   2 9 . 4 –\nPitch angle 58.2 ± 2.2 3.74   58.2 –\nFig. 2  Profile data of the artificial canals. The blue dots indicate the means of the measured profile data of the original blocks, the red \ncrosses mark the corrected profile data of the original blocks as if measured at the same “true” positions as the 3DP blocks, the green dots \nindicate the means of the measured profile data of the 3DP blocks, and the blue dotted line depicts the fitted fourth–degree polynomial \nfunction. The formula represents the fourth–degree polynomial function fitted on the mean profile data of the original blocks with the \nresulting function parameters.\nAM_2_2022.indd   62AM_2_2022.indd   62 05.12.2022   9:2305.12.2022   9:23\n\nInvestigation of 3D Printed Transparent Endodontic Blocks 63\nTab. 2 Key parameters of the 3D printed endodontic blocks.\ndimension\n3DP block (μm) Coefficient  \nof Variation (%)\nCanal profile\n(pixel) true position (μm)\n@  100px @  743.8 μm 1121.0 ± 47.7  4.26\n@  200px @ 1487.6 μm 1759.3 ± 41.1  2.34\n@  300px @ 2231.4 μm 2225.8 ± 34.7  1.56\n@  400px @ 2975.2 μm 2535.9 ± 30.7  1.21\n@  500px @ 3719.0 μm 2756.9 ± 34.3  1.24\n@  600px @ 4462.8 μm 2921.0 ± 32.5  1.11\n@  700px @ 5206.6 μm 3053.7 ± 30.0  0.98\n@  800px @ 5950.4 μm 3138.0 ± 35.3  1.13\n@  900px @ 6694.2 μm 3211.0 ± 43.7  1.36\n@ 1000px @ 7438.0 μm 3261.9 ± 39.6  1.21\nCanal thin 130.9 ± 13.1 10.0\nCanal thick 348.1 ± 17.2  4.95\nCone length 5459.2 ± 82.6  1.51\n(deg) (%)\nCanal angle 127.5 ± 2.3  1.76\nCone angle 28.7 ± 1.2  4.21\nPitch angle 54.2 ± 2.3  4.23\nTab. 3 Comparison of the key parameters of original epoxy resin block – Plastic Practice Blocks .02 taper – Oblong for 15 file (Endo-Train -\ning-Bloc-J Dentsply Sirona, Ballaigues, Switzerland) with the 3D printed endodontic blocks. The asterisk (*) indicates insignificant \ndifferences of the respective parameters of the 3DP blocks in comparison with the original resin blocks.\ndimension Ratio of Means 3DP / \nOriginal (%)\none sample t – test Coefficient of Variation \nratio 3DP / Original (%)\nLevene’s test\np – value\nCanal profile\ntrue position (μm) p – value power\n@  743.8 μm 103 = 0.010 0.777  56 = 0.018\n@ 1487.6 μm  98 = 0.003 0.907  34 > 0.001\n@ 2231.4 μm  97 > 0.001 1  27 > 0.001\n@ 2975.2 μm  95 > 0.001 1  22 > 0.001\n@ 3719.0 μm  94 > 0.001 1  26 > 0.001\n@ 4462.8 μm  94 > 0.001 1  27 > 0.001\n@ 5206.6 μm  94 > 0.001 1  27 > 0.001\n@ 5950.4 μm  93 > 0.001 1  35 = 0.003\n@ 6694.2 μm  92 > 0.001 1  20 = 0.037\n@ 7438.0 μm  92 > 0.001 1  48 = 0.027\nCanal thin  96 = 0.055* 0.493 116 = 0.488*\nCanal thick 111 > 0.001 1 113 = 0.272*\nCone length  90 > 0.001 1  42 = 0.050\n(%) (%)\nCanal angle 102 > 0.001 0.958  81 = 0.392*\nCone angle  98 = 0.015 0.715 408 > 0.001\nPitch angle  93 > 0.001 1 105 = 0.876*\nDISCUSSION\nIn our study , we fabricated 20 transparent endodontic \nblocks of the same design as the commercially available \nPlastic Practice Blocks .02 (Endo-Training-Bloc-J Dentsply \nSirona, Ballaigues, Switzerland). \nIn previous studies (20–22), the endodontic resin \nblocks with a  J-shaped artificial canal are described by \nthe length, angle, and radius of curvature of the artificial \ncanal. Nevertheless, especially in the case of determina-\ntion of the canal angle, there are many different methods \n(according to Weine, Schneide, Luiten, etc.) (23) providing \nAM_2_2022.indd   63AM_2_2022.indd   63 05.12.2022   9:2305.12.2022   9:23\n\n64 Martin Smutný et al.  Acta Medica (Hradec Králové)\n \nsignificantly different results, even for the same tested res-\nin block. Thus, these methods are not sufficiently accurate \nand reproducible. Therefore, we used a larger number of \nclearly defined parameters (Figure 1) to precisely measure, \ncopy , and 3D print the endodontic blocks. We measured 20 \noriginal resin blocks and used the mean values of the se-\nlected parameters to design the new 3DP blocks.\nDespite the printing resolution of 16 μm per layer, the \nresulting roughness of the block surface was relatively \nhigh, preventing direct observation of the artificial canal \nin the optical microscope. Polishing with different gums \nand pastes did not bring any improvement. The use of Ev-\netric Bond (Ivoclar Vivadent AG, Schaan, Liechtenstein) \nwith straight stainless steel dental matrix led to the for-\nmation of a smooth transparent layer that allowed obser-\nvation of the internal structures of the printed block with \na minimum distortion.\nTo observe the artificial canals, the newly printed \nblocks also required removement of the support mate-\nrial, which turned out to be quite difficult. We used high \npressure water, which is the recommended method by \nthe printer manufacturer. Nevertheless, this method did \nnot remove the support material from the artificial canal. \nIn many studies involving the investigation of the resin \nblocks, water (11, 24), isopropyl alcohol (25), or different \ntypes of alcohols were used as irrigation solutions. For us, \n96% ethanol worked best as it helped to disrupt and re-\nmove the partially alcohol soluble support material. We \nfound that ethanol was even a better irrigant than NaOCl \nor EDTA (17). Furthermore, in order to fill canal with ir-\nrigant and ensure patency , the canals were instrumented \nwith stainless-steel files. It was necessary to prebend the \nstainless-steel instruments to protect the canals from al-\nternating the design during their instrumentation.\nWe proved a high reproducibility of the 3D printing \nprocess of the endodontic training blocks printed from \nacrylic resin (VeroClear-RGD810, Stratasys, Eden Prairie, \nUSA) using the 3D precision printer Objet30 Pro (Strata-\nsys, Eden Prairie, USA). The dimensional variability of \nthe 3DP blocks was significantly lower (3–5 times) for \nmost of the tested parameters. It is the most important \nstudy output because only the blocks with a low dimen-\nsional variability allow for a consistent and reproduc-\nible results of the tests of endodontic instruments. The \naccuracy of the 3DP blocks was also sufficient. Although \nthe measured dimensions of the 3DP blocks were statis-\ntically significantly different from the dimensions of the \noriginal resin blocks, the maximum difference was 11% \nand can be easily eliminated by modifying the CAD model \n(available in the supplementary materials on the article \nweb page). A limitation of this study is the effect on the \nartificial canal profile by removing the support material. \nEven with the use of a fine and pre-bent ISO 10 stainless \nsteel K-file, the canal was straightened, which is a typi-\ncal problem when using rotary files (26) and is shown by \na decrease in the pitch angle and a relative increase in the \nvalues of the canal profile measured at distances of 100, \n200, 300 and 400 px. The use of a finer tools – e.g. ISO 06 \nand 08 – could not remove the support material from the \nartificial canal. Also, the proximal and distal diameters of \nthe artificial canal of the 3DP blocks were affected. Alter-\nnative possibilities of the support material removal from \nthe artificial canals should be further investigated. \nDespite certain difficulties associated with the elimina-\ntion of the support material, PolyJet printing is currently \nthe only available 3DP method that can print such narrow \n(100 μm) curved cavities. For comparison, the SLA (ste-\nreolithography) method does not allow the printing of \nsupports from different material that that of the object \nprinted, the FFD (filament feeder) 3DP method does not \nallow such a fine resolution (usually not less than 50 μm \nper layer, e.g., TRILAB DeltiQ 2 by TRILAB Group s.r.o., \nHradec Králové, Czech Republic) and is also limited by the \nminimum width of the printed line (usually not less than \n250 μm, e.g., TRILAB DeltiQ 2 by TRILAB Group s.r.o., Hra-\ndec Králové, Czech Republic), the SLS (selective laser sin-\ntering) method is limited by the grain size of the sintered \nmaterial (usually not less than 100 μm, e.g., Polyamid 12 \nPA 2200 by EOS GmbH, Krailling, Germany).\nCONCLUSION\n3D polyjet printing is a promising method of manufactur-\ning endodontic test blocks. 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Root canal transportation \nwith a Ni-Ti rotary file system and stainless steel hand files in simu-\nlated root canals. Quintessence Int 2006; 37(5): 369–74.\nAM_2_2022.indd   65AM_2_2022.indd   65 05.12.2022   9:2305.12.2022   9:23","source_license":"CC-BY-4.0","license_restricted":false}