Dental implant surgery is a highly invasive. Development of a Drilling Simulator for Dental Implant Surgery. Use of Technology in Dental Education

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1 Use of Technology in Dental Education Development of a Drilling Simulator for Dental Implant Surgery Hideaki Kinoshita, DDS, PhD; Masahiro Nagahata; Naoki Takano, MEng, PhD; Shinji Takemoto, PhD; Satoru Matsunaga, DDS, PhD; Shinichi Abe, DDS, PhD; Masao Yoshinari, PhD; Eiji Kawada, DDS, PhD Abstract: The aim of this study was to develop and evaluate a dental implant surgery simulator that allows learners to experience the drilling forces necessary to perform an osteotomy in the posterior mandibular bone. The simulator contains a force-sensing device that receives input and counteracts this force, which is felt as resistance by the user. The device consists of an actuator, a load cell, and a control unit. A mandibular bone model was fabricated in which the predicted forces necessary to drill the cortical and trabecular bone were determined via micro CT image-based 3D finite element analysis. The simulator was evaluated by five dentists from the Department of Implantology at Tokyo Dental College. The ability of the evaluators to distinguish the drilling resistance through different regions of the mandibular bone was investigated. Of the five dentists, four sensed the change in resistance when the drill perforated the upper cortical bone. All five dentists were able to detect when the drill made contact with lingual cortical bone and when the lingual bone was perforated. This project successfully developed a dental implant surgery simulator that allows users to experience the forces necessary to drill through types of bone encountered during osteotomy. Furthermore, the researchers were able to build a device by which excessive drilling simulates a situation in which the lingual cortical bone is perforated a situation that could lead to negative repercussions in a clinical setting. The simulator was found to be useful to train users to recognize the differences in resistance when drilling through the mandibular bone. Dr. Kinoshita is Assistant Professor, Department of Dental Materials Science, Tokyo Dental College, Tokyo, Japan; Mr. Nagahata is a master course student, Graduate School of Science and Technology, Keio University, Kanagawa, Japan; Dr. Takano is Professor, Department of Mechanical Engineering, Keio University, Kanagawa, Japan; Dr. Takemoto is Assistant Professor, Department of Dental Materials Science, Tokyo Dental College, Tokyo, Japan; Dr. Matsunaga is Associate Professor, Department of Anatomy, Tokyo Dental College, Tokyo, Japan; Dr. Abe is Professor, Department of Anatomy, Tokyo Dental College, Tokyo, Japan; Dr. Yoshinari is Professor, Oral Health Science Center and Department of Dental Materials Science, Tokyo Dental College, Tokyo, Japan; and Dr. Kawada is Professor, Department of Dental Materials Science, Tokyo Dental College, Tokyo, Japan. Direct correspondence to Dr. Hideaki Kinoshita, Department of Dental Materials Science, Tokyo Dental College, Misaki-cho, Chiyoda-ku, Tokyo , Japan; kinoshitahideaki@tdc.ac.jp. Keywords: dental education, dental implants, implant dentistry, computer simulation, educational research, dental simulator Submitted for publication 10/21/14; accepted 6/13/15 Dental implant surgery is a highly invasive procedure involving mucosal incision and bone drilling, which requires proper training and sufficient knowledge. 1 Training needs are especially acute in the mandibular molar region because the sublingual and submental arteries pass through the lingual side of the mandibular bone, and accidental trauma of these arteries may occur if the lingual cortical bone is perforated. 2 Therefore, bone morphology must be evaluated before carrying out implant treatments. The indices of bony tissue that are commonly used in implant treatments are the classifications of Lekholm and Zarb 3 and Misch. 4 Nevertheless, these evaluations are controversial due to the fact that they are influenced by the subjectivity of the clinician. Several universities are currently working on various educational methods relating to predoctoral implant dentistry For example, detailed mandibular bone models are being used to teach implant placement surgery for students in practical training. 11,12 However, the models used for training are expensive and are not suitable for repeated practice. Most of these models are made from epoxy resin, which simulates only a single pattern of tactile sensation. 13 Therefore, live implant surgery is the only opportunity for the user to experience the variability in drilling forces necessary to drill different types of bone. A number of studies have researched the drilling force and resistance experienced during osteotomy. Among them, Friberg et al. reported a positive January 2016 Journal of Dental Education 83

2 correlation between radiographic bone density of pig ribs and drilling resistance. 14 Sugaya found that the force required and torque experienced when drilling mandibles of cadavers positively correlated with increasing bone mineral density. 15 However, drilling resistances felt by the clinician s fingers are subjective; thus, it is difficult to quantify these sensations to educate young inexperienced dentists on drilling resistance sensed during osteotomy. Recent advances in simulators have led to the development of a variety of devices that are currently being used in many academic dental institutions Haptic devices have been developed to provide tactile sensations of cutting through teeth or drilling bone. 20,21 However, simulators that use haptic devices do not use the same handpieces as those used in actual surgery, so they are unrealistic in terms of weight, sounds (e.g., motor noise), and physical sensations (e.g., vibrations). In addition, movements controlled by mechanical devices are different from actual human movements required during a surgical operation. The aim of this study was to develop and evaluate an implant surgery simulator that allows learners to experience the dynamic drilling forces necessary to perform an osteotomy in the posterior mandibular bone. In order to reproduce the intricate features of the cortical and trabecular bone, micro CT images of cadaver mandibles were analyzed, and threedimensional finite element methods were employed. Materials and Methods This study was approved by the research ethics committee of Tokyo Dental College, Tokyo, Japan (Permission number 00356). The effectiveness of a newly developed simulator was evaluated by five dentists from the Department of Implantology at Tokyo Dental College, each with fewer than five years of clinical experience. The subjects were given a detailed explanation in advance of the system and the mandibular bone model prior to performing the drilling simulation. The subjects were asked to report when they had perforated the upper cortical bone, when they had made contact with the lingual cortical bone, and when they had perforated the lingual cortical bone. They were also asked to give their overall impressions after experiencing the simulation. Development of a Force-Sensing Device Panel a of Figure 1 shows the system configuration of the implant surgery simulator that was developed. The force-sensing device (Figure 1, Figure 1. Overview of developed implant surgery simulator: system configuration (panel a), force-sensing device (panel b), decentering jig (panel c) 84 Journal of Dental Education Volume 80, Number 1

3 panel b) consists of a load cell, where input force is measured, and an actuator, which is the motor responsible for the counterforce. This device uses the same handpiece and motor that is used during implant surgery (INTRAsurg 300, KaVo Dental, Charlotte, NC, USA). The force applied in a downward direction by the user is measured by the load cell, which then connects to the actuator that controls the counterforce or resistance transmitted to the fingers. A program implemented in the control unit controls the movement of the actuator by comparing the applied force to a database of drilling resistance derived from cadavers. If the applied force exceeds the necessary drilling force, the actuator moves downward and, while moving, gives the user resistance according to its location in the mandibular bone. A decentering jig is used instead of an actual drill to imitate the vibration experienced during drilling (Figure 1, panel c). By using this simulator, learners can repeatedly practice osteotomy of the mandibular bone. The starting point is already prepared with a round bur. The simulator mainly focuses on the following step, which is implant bed formation where the bone is drilled into the trabecular bone from the cortical bone using a twist drill. Calculation of Drilling Resistance Drilling resistance reflects bone conditions and morphologies, especially in the trabecular bone region. The database created consists of sets of drilling depths and corresponding counterforce data. An edentulous cadaver mandible, stocked for training purposes by the Department of Anatomy at Tokyo Dental College, was scanned with a micro CT system (HMX-225 Actis4, Tesco Co., Tokyo, Japan) with a resolution of mm and 0.05 mm per slice. The two-dimensional slice data obtained from the micro CT system were used to measure distribution of bone volume fraction. Bone volume fraction measurement was carried out by using bone morphology measurement software (TRI/3D-BON, Ratoc System Engineering Inc., Tokyo, Japan). A three-dimensional finite element (FE) model of the mandibular molar region was generated automatically using cubic elements (voxel elements) on the basis of the slice data from the scan. The total number of elements was 2,257,574 when 0.1 mm sized voxel elements were used. The image-based FE analysis software (VOXELCON, Quint, Tokyo, Japan) was used in this study. The drilling process in the trabecular bone region was calculated by sequential linear static analyses giving the prescribed displacement of 0.01 mm to the tip of a twist drill model simplified by a cylinder. 22 The bottom part of the bone model was constrained. Our analysis assumed isotropic material because the orthotropic material axes have not been clarified in the entire mandibular region. 23 The Young s modulus of the bone was 15 GPa, and that of the drill was 200 GPa. The cylindrical drill model was surrounded by thin layered soft material with Young s modulus of 15kPa to simplify the model into a linear one. This simplification contributed to the reduction of computational time. The Poisson s ratio was set at 0.3 for all materials. Figure 2 shows the simulation model with the schematic drilling path (panel a) and the crosssectional drilling path (panel b). The sequential linear static analyses were carried out at intervals of 0.5 mm. For experimental purposes, in addition to drilling to the appropriate depth for implant placement, drilling was continued along the same path until lingual perforation at a depth of 7 mm. FE analyses were applied to the trabecular bone region, but the force in the cortical bone region was adjusted based on the experiences of an expert, a professor in the Department of Implantology, Tokyo Dental College. Results The force required to drill the mandibular trabecular bone region at constant speed was calculated from drilling resistance (Figure 3). The required drilling force was lower than 4 N until the depth of 4 mm. As the drill approached the lingual cortical bone, this value increased. Small fluctuations in forces were derived, which reflected the inconsistent morphology of trabecular microarchitecture. The force and depth curve was programmed so that the actuator did not allow the drill to move deeper if the applied force was less than the curve. If the applied force was greater than the curve, then the actuator allowed the drill to move faster in proportion to the difference between applied force and required force. The drilling speed ranged from 0.3 to 0.9 mm/s depending on the applied force and the bone condition. Figure 4 shows the distribution of bone volume fraction measurements. This graph demonstrates that between 0 mm and 6 mm, where no cortical bone exists, the average bone volume fraction fluctuated around 10%. Figure 5 shows the January 2016 Journal of Dental Education 85

4 Figure 2. Modeling of perforation of lingual side of cortical bone: schematic of drilling path (panel a) and cross-sectional display of drilling path (panel b) Note: In panel b, (1) shows the starting point to drill trabecular bone; (2) shows trabecular bone region; and (3) shows the perforated point. Figure 3. Drilling force under constant drilling speed in case of perforation of lingual side of cortical bone Note: Drilling in trabecular bone region was calculated by finite element method. 86 Journal of Dental Education Volume 80, Number 1

5 Figure 4. Distribution of bone volume fraction in trabecular bone region Note: The left panel shows the 3D microarchitecture in drilling path in Figure 3. Figure 5. Correlation between drilling force under constant speed and bone volume fraction in trabecular bone region January 2016 Journal of Dental Education 87

6 relationship between the bone volume fraction and the calculated drilling force under constant speed. In the evaluation described in region 2-3 (Figure 3), four out of five subjects were able to detect the sudden drop in resistance in the upper cortical bone region. All the subjects were able to sense the contact to the lingual side of the cortical bone and when the lingual cortical bone was perforated. Some comments provided by the subjects were as follows: I could notice when I was drilling through the upper and lingual cortical bone, it felt incredibly real ; The vibration felt through the bur was extremely realistic ; and I could sense the increase in drill resistance as the bur approached the lingual cortical bone. There were also negative comments such as I could not tell the difference between the cortical and trabecular bone and The sensation when passing through the upper cortical bone is more noticeable during actual surgery than this simulator. Discussion In recent years, the spread of CBCT-guided surgery has led to a decreasing incidence of adverse outcomes in implant surgery. 24 However, serious complications such as bone perforation, nerve damage, and hemorrhage can still arise during osteotomy in oral implant surgery if the surgeon lacks experience or technical skills. A significant advantage with simulators is that they allow students to familiarize themselves with difficult or risky procedures by repetitively practicing the procedure before actually performing it on a patient. The simulator described in this study was developed for the purpose of providing students with a device that teaches them the sensations involved during the osteotomy in the mandibular bone. In addition, this model allows students to experience the sensations transmitted to the fingers when the lingual cortical bone is accidentally perforated; in that case, they should seek immediate attention to prevent any lethal complications. In this study, drilling resistance was numerically analyzed with an FE model by reconstructing the internal trabecular structure of the mandibular bone. Stegaroiu et al. compared the influence of implant load on the mandibular bone through two FE models: one regarded the trabecular bone as a homogeneous material, and the other replicated the actual trabecular structure. 25 Those researchers observed that stress was dispersed across a greater range in the actual trabecular model than the homo- geneous structure. This finding implies that a precise model that takes into account the intricate structure of the trabecular bone is required when performing a biomechanical analysis. Biomechanical analysis of bone structure has become possible in recent years as a result of the development of micro CT systems and improved performance of analytical systems For example, Matsunaga et al. performed an analysis using an FE model that closely reproduced trabecular structure in three dimensions; they reported that the peri-implant trabecular structure was closely connected with its biomechanical role. 28 In the past, we have also used a 3D FE model that replicated trabecular structure in order to study the association between the periimplant bone structure and stress distribution around the mandibular canal. 29 In this study, by using an FE model that closely reproduced the trabecular bone structure, we were also able to calculate the curve of reaction force at each depth when drilling through the trabeculae. Sugaya et al. measured cutting force at an intraosseous puncture and investigated its relation to bone mineral content and reported a strong correlation between cutting force and bone mineral density. 15 Our results also demonstrated a strong correlation between bone volume fractions and required drilling force for most areas. However, from 2 mm to 3 mm of depth, the two lines were especially different, and the patterns did not coincide. From a mechanical viewpoint, this difference was most likely due to the network architecture that determines resistance in the area. Even if the bone volume fraction is high, resistance would be weak if the network architecture is unorganized. Our FE analysis model took this factor into consideration. Programming with FE analysis is a convenient method to store and increase the database. Many cases can be simulated using different locations and/ or angles from a single bone model. Our goal is to analyze more cadavers and upgrade the program so that parameters such as drilling paths and bone conditions can be changed according to the practitioner. More cases should also be analyzed and added to the database in the future if this device is to be used for dental implant education. In the process of drilling during osteotomy, there is usually a sudden change in resistance when the drill comes in contact with the cortical bone or when the drill completely penetrates the cortical bone. This simulator aims to familiarize students with the sensations that are encountered during osteotomy 88 Journal of Dental Education Volume 80, Number 1

7 to successfully prepare the mandible for implant placement or to recognize accidents that could lead to negative outcomes if not brought to attention immediately. In our study, almost all of the subjects (who had fewer than five years of experience in placing implants) were able to recognize the changes in resistance at the necessary locations; however, it was apparent from the comments they provided that further improvements need to be made. Nevertheless, the subjects reported that drilling sensations were very realistic. These results suggest that, for dental students or young dentists with little or no experience with actual surgery, this educational simulator can help them become familiar with the drilling process and the resistance felt during osteotomy. Limitations of this study include the fact that FE analysis was used to reproduce the trabecular bone and not the cortical bone and only one model has been replicated for this device. Another limitation is that the way to hold the drill was not specified, which could have affected the efficiency of drilling. Also, in a clinical setting it would be difficult to identify the border between the cortical and trabecular bone at the extraction site due to the uncertain degree of bone healing. Perhaps there needs to be a standard force-sensing range established in order to distinguish this boundary. Finally, since this study was conducted with a small number of faculty members at one academic dental institution, the results may not necessarily be applicable more broadly. Conclusion Within the limitations of this study, the following conclusions were drawn. A drilling simulator was developed that allowed users to experience drilling different types of bone encountered during osteotomy. The changes of drilling resistance when the drill made contact with lingual cortical bone and when it perforated this bone were recognized by all subjects participating in this study. 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