Using a new endodontic tooth model as an alternative in clinical education course during 2 the Covid-19 pandemic

Background The COVID-19 pandemic massively impacts endodontic teaching, and a 35 dramatically reduced number of patients is registered in clinical courses. This could be 36 countered with suitable training aids. Based on treatment errors made by students in the last 37 five years of endodontic courses at RWTH Aachen University (Germany), a new artificial root 38 canal treatment model (DRSK RCT) was developed. The model was aimed to be radiopaque 39 and to simulate the tactile feel during instrumentation in a realistic manner. Unlike already 40 existing 3D-printed tooth replicas, the RCT has anatomical root canals with a narrow lumen 41 with its width matching an ISO size 6 endodontic file. 42 Methodology 35 fourth-year students and seven dental demonstrators performed endodontic 43 treatments on both the DRSK RCT and extracted teeth. Students and demonstrators answered 44 a questionnaire on a scale ranging from 1 to 7 (poor to high) for different items (part 1). After 45 the first study, changes in the materials and root canal anatomy were applied to the model. Then, 46 the whole study was repeated and evaluated (part 2). Finally, it was evaluated whether the 47 models could replace patient treatment during the Covid-19 pandemic. Results Ratings by students and dental instructors (5.1 ± 0.4 and 5.3 ± 1.5 [mean ± SD], 49 respectively) in the first study increased after modifications of the DRSK RCT (5.5 ± 0.5 and 50 6.2 ± 0.8, respectively). Radiographs of the models were excellently assessable. The properties of the DRSK RCT were found to be realistic, thus allowing students to perform a satisfactory 52 simulation of root canal treatment and being rated sufficient in substituting patient treatment during COVID-19 pandemic. Conclusion The analysis suggests that the DRSK RCT has the ability to improve endodontic technique and education. Visible root canals enable students to observe the treatment process. All steps of a regular root canal treatment can be simulated. Further studies are needed investigate the outcome of the first patient after the DRSK RCT.


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Background 59 Similar to any other field of knowledge, dentistry constantly embraces novel and innovative 60 modes of training that could potentially facilitate and improve the learning curve for new 61 students. For endodontic education, attempts were made to establish guidelines for the 62 undergraduate training in dental schools [1], which can ensure the achievement of a certain 63 level of competence. To further improve the endodontic education, new forms of teaching 64 methods are required. A study from the United Kingdom showed that teaching methods 65 developed during the last decade led to a wider variation of methods [2]. Despite this, several 66 studies point out that the quality of root canal treatments performed by undergraduate students 67 is often not satisfactory [3,4,5,6,7,8]. Thereby, the way of teaching endodontics has a major 68 influence on the quality of root canal treatments performed by students [9]. 69 With regards to endodontics, the traditional preclinical training involved practicing the 70 procedure on extracted natural teeth. Nevertheless, this practice was fraught with concerns over 71 infection control and required disinfection of the teeth [10]. Some materials traditionally used 72 for the purpose of efficient disinfection, such as formalin, proved to have hazardous effects of 73 their own [11]. Furthermore, the supply of natural teeth is not infinite, and combined with the 74 dwindling number of extracted intact teeth -probably as a result of improvement in health 75 standards -it presents a problem for instructors and students in preclinical endodontics [12]. 76 The COVID-19 pandemic emphasizes the importance of alternative teaching methods in dental 77 education and both, students and instructors prefer these methods over having a non-semester 78 [13]. Current protective measures have a massive impact on university teaching [14,15]. 79 Especially the field of endodontics is affected, since teaching in clinical courses includes patient 80 treatments that cannot be simply substituted by e-learning programs. Although safety concepts 81 may not completely prohibit patient treatments, a very reduced number of patients is evident 82 due to the pandemic. Effective training models that enable the implementation of a realistic root 83 canal treatment, could adequately meet these challenges. 84 The simplest of the alternative endodontic training models come in the shape of endodontic 85 blocks with a built-in conduit that approximates in its shape and diameter the root canals of 86 natural teeth [16]. Because they do not represent the external anatomy of the crown and root, 87 these endodontic blocks are of limited educational value. These models do not allow the 88 students to learn how to avoid the procedural problems pertaining to the distance between the 89 canal and external surface of the tooth and lateral or apical perforations of the roots. 90 Further advances in 3D printing technology promised more sophisticated models simulating a 91 complete tooth including a hollow space representing the root canal system. Apart from their 92 application in pre-clinical courses, these models can be also used in the context of researchers 93 requiring a simulation of the internal anatomy of the teeth [17]. In recent years, different brands 94 of such models have been brought to the market, in turn prompting researchers and academics 95 to perform studies and investigate their properties and suitability as training tools, thus replacing 96 the extracted teeth. Another study found artificial tooth models suitable for endodontic training 97 [18]. Nevertheless, the results of these studies suggest that complete replacement of natural 98 teeth with artificial teeth for endodontic training should be regarded with caution. The physical 99 properties of the models have been of special concern to preclinical endodontics instructors. To 100 be suitable for the desired learning experience, these models are expected to feature physical 101 properties as similar as possible to those of a natural tooth. However, studies showed that, 102 despite advantages of artificial teeth, their physical characteristics are not yet completely 103 satisfactory [19,20]. In case of one particular model made from a hydroxyapatite-based matrix, 104 a study showed that it is similar to a natural tooth in regard to many physical properties [21]. 105 This study focused exclusively on the physical properties of the model. It did not provide any 106 information on the actual experience of practicing root canal treatment on synthetic teeth made 107 from this material. 108 Finally, a recent study introduced the concept of 3D-printed replicas of extracted teeth for 109 endodontic training [22] and recommended it as a practice that dental schools have to embrace. 110 In this study, the researchers produced exact copies of natural teeth and measured their 111 properties and accuracy. However, there remain limitations associated with this concept, and 112 therefore there is a continued demand for commercially produced models. Any modification of 113 the model design, including shape, curvature, length, and width of the canals, falls beyond the 114 scope of the proposed simple reproduction of a micro-computed tomography (micro-CT) file. 115 It can only be achieved by employing computer-aided design software applications, which -116 owing to their sophistication -normally necessitates enlisting the help of expert personnel with 117 computer design skills. 118 The present study aimed to evaluate the artificial root canal treatment model (DRSK RCT), 119 which is based on errors made by students in the last 5 years. In contrast to already existing 3D-120 printed tooth replicas, the DRSK RCT aims to have root canals with a narrow lumen with its 121 width matching an ISO size 6 endodontic file so as to enable students to negotiate the root canal 122 system with endodontic scouting files. We evaluated whether the DRSK RCT is radiographable 123 and also to what extent it permits practicing various shaping and cleaning techniques. In 124 addition to measuring the physical properties of this model, the study aimed to assess the 125 subjective experience of the users (including both students and instructors). By questioning 126 participants about every stage of the root canal treatment performed on this model, the present 127 study evaluated the educational value of DRSK RCT from a practical standpoint. The 128 participating students and dental instructors assessed the DRSK RCT model with regard to its 129 physical properties and its suitability to realistically imitate an endodontic treatment, to 130 optimize the tooth model according to the results of the evaluation for endodontology courses. all existing root canals of a tooth, especially when the number or configuration of root canals 172 deviates from the norm. As an example of a premolar, a mandibular first premolar with a single 173 root canal was chosen. It has been observed that students tend to straighten root canals during 174 mechanical preparation by not bending the files used. The chosen anatomy of the premolar in 175 the DRSK RCT model makes it inevitable to carefully scout the root canal with a bent file to 176 preserve its anatomy and not be stuck in the apical third of the model root canal without entering 177 the apical constriction. 178 A maxillary first molar with three roots and four root canals was selected to demonstrate molars. 179 The molar model featured a second mesiobuccal root canal (MB2) corresponding to the 180 established predominance of this root canal configuration for maxillary first molars [23]. While 181 trying to locate the MB2, students -owing to their lack of experience -may cause perforations 182 in the pulp chamber floor. Unsatisfactory anatomical skills lead to drilling in wrong places to 183 locate the MB2. This tooth model will provide knowledge of the location of MB2 and assist in 184 developing the skills to locate it. The study consisted of two parts, as described below. 185 186 Part 1 (students) 187 Before using DRSK RCT, all students practiced on extracted teeth. Then, the participants were 188 asked to perform a routine root canal procedure on the DRSK RCT models, starting by drilling 189 an access cavity with Endo-Z burs (Dentsply Sirona). A secondary access cavity was prepared 190 using Mueller drills (Komet dental) and Gates-Glidden drills (Komet dental). 191 After taking the radiograph (Figure 3), hand files (K-Files, SybronEndo) were used for 192 mechanical root canal preparation. 193 RCT. For the second part, the same questionnaire was used. In this manner, the differences in 227 ratings could be analyzed. 228 229

Evaluation of students' performances 230
Finally, the seven dental instructors evaluated these simulated treatments. The success of the 231 students' performance was determined by the correct length and shape of the root canal fillings 232 as well as their homogeneity. The correct length was determined by the obturation ending 0.5 -233 1 mm before the radiographic apex. All seven dental instructors blindly evaluated all treated 234 DRSK RCT. Only tooth models with correct length, shape, and homogeneity of the root canal 235 filling were rated as "positive" (Table 1)

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All radiographs of the DRSK RCT were clearly assessable; there was a sufficient contrast 247 between the model and gutta-percha. Moreover, the outlines of roots were completely visible. 248 For the first part of the study, student's ratings of the DRSK RCT ranged from 4.4 to 5.9 on a scale of 1 to 7, with the mean of their ratings being 5.1 ± 0.4 (mean ± SD). For the second part 250 of the study performed after improving the RCT, students' ratings ranged from 4.6 to 6.1. The 251 mean of their ratings increased to 5.5 ± 0.5 (mean ± SD). 252 Students' ratings of the pulp anatomy, the ability of the model to simulate a natural tooth, ability 253 to flare canals, and preference for using the DRSK RCT in preclinical evaluations were 254 significantly higher (p < .05) for the improved DRSK RCT than for the model used before 255 modifications. The average rating of the pulp anatomy increased from 5.4 ± 1.1 for the first 256 DRSK RCT to 5.9 ± 0.9 for the improved model. Likewise, the rating of the model's ability to 257 simulate natural teeth increased from 4.6 ± 1.3 for the first DRSK RCT to 5.3 ± 1.2 for the 258 modified DRSK RCT. Furthermore, students found that flaring of root canals could be 259 performed in a significantly better manner on the modified DRSK RCT; the ratings increased 260 from 4.8 ± 1.6 to 5.6 ± 1.0 after the material was modified. In the first part, the mean rating of 261 the suitability of DRSK RCT as an evaluation tool in preclinical course was 5.0 ± 1.7, whereas 262 after modifications, it increased to 5.9 ± 1.1 (Figure 4). The mean ratings by dental instructors increased from 5.3 ± 1.5 to 6.2 ± 0.8 following model 275 improvement ( Figure 5). The simulated hardness felt during instrumentation, in particular, was 276 rated significantly higher (p < .05) after DRSK RCT modifications (from 5.0 ± 0.7 to 6.3 ± 0.4). 277 When asked about the student's readiness to work on a real patient after practicing on RCT, the 278 mean rating was 4.8 ± 2.3, whereas this rating increased to 6.6 ± 0.7 for the enhanced model 279

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In this study, we used an artificial root canal treatment model (DRSK RCT) with an 303 anatomically accurate design. According to its manufacturer, this model can facilitate practicing 304 various shaping and cleaning techniques. Instead of merely measuring the physical properties 305 of this model, the study evaluated the subjective experience of the users (including both students 306 and instructors). The DRSK RCT was designed virtually based on errors made in the last five 307 years by students of endodontic courses. In addition, the anatomy of extracted teeth was used 308 as an overall reference. 309 When drilling an access cavity, the material used for the tooth model must provide sufficient 310 resistance such that its difference from the material filling the pulp chamber can be perceived 311 clearly. Some authors already mentioned that resin used for endodontic tooth models does not 312 match the hardness of dentine. Therefore, drilling an access cavity becomes more complicated 313 because the perceived difference in resistance is insufficient [20,24,25]. The hardness of 314 dentine-imitating resin and the accuracy of anatomical reproduction affect the degree of 315 difficulty of working with endodontic models in general [26]. Soft resin complicates its 316 distinction from the soft material imitating the pulp, whereas excessively hard resin leads to 317 canal blockage [20]. 318 The DRSK RCT model allows a close simulation of performing endodontic treatment on a 319 natural tooth. This is evidenced by the good ratings achieved in our study. One major alteration 320 to the model involved changing the material used in the manufacturing of DRSK RCT. 321 Consequently, access drilling, root canal preparation, and obturation were performed more 322 easily because the use of harder material offers more resistance. When entering the pulp 323 chamber, the tactile feel is crucial for not harming the pulp floor by accident. This is important 324 for beginners, as they are not used to paying attention to the difference between dentine and the 325 hollow space of the pulp chamber. Sticky debris produced while shaping the root canals blocks 326 the canals and cannot be flushed out easily; however, the stickiness of the debris is reduced 327 when a harder material is used. A material with hardness similar to that of dentine and cement 328 of natural teeth accurately represents the tactile feel while performing root canal treatment on 329 patients, which is the actual reason for using endodontic training aids. 330 Students highly rated DRSK RCT for potential use in preclinical training, which may be 331 because each model has the exact same anatomy and therefore creates equal and fair conditions 332 for every student. However, there are no two natural teeth with the exact same anatomy. Owing 333 to the large variation observed in the anatomy of natural teeth, a further major alteration made 334 to the studied model included modifying the shapes of the root canals to create a more complex 335 and realistic root canal system. Considering that each tooth requires a unique mode of 336 endodontic intervention, any evaluation procedure based on the use of natural teeth would 337 become complicated, and questions may be raised over its fairness. These difficulties emphasize 338 the need to seek alternative training models such as the DRSK RCT for practicing root canal 339 treatment during preclinical courses. 340 The ambition to have models with translucent roots has been on record since as early as 1975 341 [27]. To that end, different methods have been devised to increase the transparency of the model 342 by applying chemical agents. However, this is often accompanied by undesired effects such as 343 altered physical properties. In the present study, the transparent roots of the model may also 344 have influenced the students' opinion about the DRSK RCT as a training aid in preclinical 345 courses. Because endodontic treatments are performed inside root canals and obscured from 346 view, students often feel insecure when they are unable to see what they are doing. Making the 347 treatment procedure visible allows inexperienced students to gain a deeper understanding of the 348 process. If any error occurs, the cause can thus be quickly identified [21], which is essential for 349 the students' learning process. Such models may also benefit research and testing of certain 350 devices and equipment when it is required to have a direct view of the canals and observe how 351 the equipment functions inside. 352 In root canal treatments on previous training models, endodontic files of ISO size 15 were used 353 for scouting. Owing to technical considerations, printing tooth models for endodontic training 354 with optimal root canal diameter was challenging [22]. In a study that compared several 355 endodontic training aids, a model named TrueTooth by DELabs, built on the basis of micro-CT 356 scans of natural teeth, was favored owing to its anatomy and material properties being closer to 357 reality [26]. For instance, diameters of the MB2 of TrueTooth #19 (02) are indicated to be 0.12 358 mm (apical part) and 0.28 mm (coronal part) on its manufacturer's website. Because the initially 359 used file for root canal treatment in real patient is often of a significantly lower ISO size (6,8,360 or 10), it is part of our department's protocol of root canal treatment to use c-pilot files of ISO 361 size 6, 8, or 10 for scouting the root canal system. This needs to be practiced in preclinical 362 training as well and thus requires models with very narrow root canals. 363 After modification, the DRSK RCT received higher ratings for most points from both cohorts. 364 Owing to the changes in the material, various aspect of simulated root canal treatment such as 365 the stickiness of debris, the feel of drilling the access cavity, or the perceived hardness during 366 filing were improved. Furthermore, radiographs of the tooth model were easier to read as the 367 material became more radiopaque. 368 The 3D print technology used for producing the DRSK RCT made it possible to change the 369 shapes of the root canals and modify them as desired without incurring substantial costs. The 370 similarity of the DRSK RCT to a natural tooth makes it an ideal training tool in endodontics. 371 The ratings by students and instructors alike point to the suitability of the studied model to be 372 used in endodontic courses. During the COVID-19 pandemic it could be advantageous to 373 incorporate this training aid into the clinical curriculum as supplying students with suitable 374 patients to perform endodontic treatments becomes challenging. Many patients currently avoid 375 the student course out of fear of infection and postpone treatments. In addition, as the number 376 of cases increases, it is becoming more common for patients to be quarantined, making it 377 impossible to carry out treatment properly. As many universities exclusively train students on 378 phantom heads during the current pandemic the tooth model would perfectly fit into their 379 curriculum. Both, students and tutors rated the model as suitable for training and evaluation in 380 preclinical courses and students also felt well prepared for their first root canal treatment on 381 patients. The realistic anatomy and properties of the model allow students to learn the 382 difficulties of endodontic treatments and enable them to practice any clinical step of the 383 procedure. Ratings by the dental instructors indicate its potential use in clinical courses. 384 Reduced patient numbers or concerns about the safety of patients, students and staff in the face 385 of high infection rates might force universities to completely substitute patient treatments with 386 the work on phantom heads or models [28]. Even though training on models can never replace 387 training on patients, it is probably the best alternative given the current circumstances. 388 In the last part of the study, seven dental instructors evaluated the results of endodontic 389 treatments performed on the DRSK RCT. The length of the gutta percha filling was taken as 390 the main measure of a successful treatment. The success rate ranged between 82.86% to 391 88.57%, indicating that the students were able to perform proper endodontic treatments on the 392 developed tooth model. As radiopacity is the key to determine the correct working length, it is 393 crucial that the apical third is clearly visible on radiographs, and thus the correct length of the 394 gutta percha filling can be perceived [26]. The DRSK RCT can be used as an alternative to the old-school extracted teeth or as an 405 additional tool for improving dental education. Specifically, the good contrast on radiographs 406 and the realistic and freely modifiable anatomy of root canals makes the DRSK RCT a desirable 407 teaching aid. Furthermore, during COVID-19 pandemic these tooth models could help to ensure 408 teaching endodontics when patient treatment is not possible. 409

Authors' contributions 437
All authors contributed to the study conception and design. Material preparation, data collection 438 and analysis were performed by SYM and CF. The first draft of the manuscript was written by 439 SYM. All authors (SYM, CF, AH) edited and commented on previous versions of the 440 manuscript. All authors (SYM, CF, AH) read and approved the final manuscript. 441 Line: standard deviation/ asterisk: p < 0.05 550