Keyong Yuan 1, Chenguang Niu 1, Qian Xie 2, Wenxin Jiang 1, Li Gao 1, Zhengwei Huang 1, Rui Ma 1

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1 Eur J Oral Sci 2016; 124: DOI: /eos Printed in Singapore. All rights reserved Ó 2016 Eur J Oral Sci European Journal of Oral Sciences Comparative evaluation of the impact of minimally invasive preparation vs. conventional straight-line preparation on tooth biomechanics: a finite element analysis Keyong Yuan 1, Chenguang Niu 1, Qian Xie 2, Wenxin Jiang 1, Li Gao 1, Zhengwei Huang 1, Rui Ma 1 1 Department of Endodontics, Ninth People s Hospital, School of Medicine, Shanghai Key Laboratory of Stomatology, Shanghai JiaoTong University, Shanghai, China; 2 Department of Endodontics, University of Illinois at Chicago, Chicago, IL, USA Yuan K, Niu C, Xie Q, Jiang W, Gao L, Huang Z, Ma R. Comparative evaluation of the impact of minimally invasive preparation vs. conventional straight-line preparation on tooth biomechanics: a finite element analysis. Eur J Oral Sci 2016; 124: Eur J Oral Sci Minimally invasive endodontics emphasizes preservation of a maximal amount of healthy tooth tissue. However, whether the tooth structure preserved by minimally invasive endodontics can maintain higher fracture resistance is unclear. This study aimed to compare the biomechanics on teeth after minimally invasive (MI) preparation and straight-line (SL) preparation using finite element analysis. Six finite element analysis models of a mandibular first molar were constructed and divided into two groups (MI and SL). Two loads of 250 N, one vertically stimulating the vertical masticatory force and the other given 45 to the longitudinal axis of the tooth, were applied. Stresses in the teeth were calculated and analyzed. Under both vertical and 45 loads, the greatest stresses were located at the margin of the cavities on the occlusal surfaces. The stress concentration areas of teeth with minimally invasive access cavities were smaller than those of teeth prepared with straight-line opening in coronal and cervical areas. The stress concentration points in the cervical areas increased with the increase of canal taper in the coronal third. Minimally invasive access preparation reduced the stress distribution in crown and cervical regions. A smaller taper cervical enlargement caused lower stress in the cervical region. Rui Ma, Department of Endodontics, Ninth People s Hospital, School of Medicine, Shanghai Key Laboratory of Stomatology, Shanghai JiaoTong University, 500 Quxi Road, Shanghai, China marui1723@sina.cn Key words: finite element analysis; stress; tooth preparation Accepted for publication August 2016 Root canal therapy is one of the most efficacious therapeutic options for pulpal and periapical diseases. The main objectives of root canal therapy are to prepare and clean all pulp spaces and to fill the spaces with inert materials (1). In the conventional root canal therapy paradigm, straight-line access from the pulp chamber to the root canal system was the prevailing approach. However, this usually results in excessive removal of tooth tissue as a side effect. The extensive loss of hard tissue could weaken the rigidity of a tooth (2) and decrease its fracture resistance (3). Minimally invasive endodontics, which has been used for several years (4), has challenged the conventional approach. Minimally invasive endodontics emphasizes preservation of a maximal amount of healthy tooth tissue and can be embodied in the diagnosis of a carious lesion, the decision-making process, the design of an endodontic access cavity, and the preparation of the root canals or the restorations (4 6). In the process of root canal therapy, access is one of the most important steps and a prerequisite for a good endodontic prognosis (1). With the new generation of nickel titanium instruments, microscopy, and cone-beam computed tomography, dentists can make the access cavity as minimally invasive as possible and preserve most of the treated tooth structure. Previous studies have demonstrated that cervical dentin is more important than occlusal tooth structure for tooth longevity and optimal function (7, 8). As the minimally invasive access design is mainly concerned with preserving a greater amount of occlusal tooth structure, the role of the minimally invasive access preparation for increasing the resistance of endodontically treated teeth to fracture remains controversial (3, 9, 10). The question of whether a tooth prepared with minimally invasive endodontics retains higher fracture resistance compared with a tooth prepared with conventional straight-line opening needs to be clarified. Therefore, the aim of the present study was to investigate, using finite element analysis, the biomechanical fracture resistance of teeth (in both access cavity and

2 592 Yuan et al. cervical portion) prepared using a minimally invasive technique. Material and methods In this study, a minimally invasive access cavity and a conventional straight-line opening superimposed on 0.04, 0.10, and 0.12 taper canal preparations in the coronal third of root canals were simulated and analyzed. The study was approved by the Ethics Committee of Shanghai Jiao Tong University. After obtaining consent from patients, the intact, caries-free, mandibular left first molar was extracted and scanned using a microcomputed tomography (micro-ct) scanner (lct80; Scanco Medical, Zurich, Switzerland), operating at 70 kvp, 114 la, with a slice thickness of 10 lm. Then, the micro-ct images were imported into Mimics software (Materialise, Leuven, Belgium). The initial threedimensional (3D) models of enamel, dentin, and pulp space were reconstructed with different CT gray values using this software. Fig. 2. The filled composite resin space. (A) Minimally invasive access cavity space. (B) Straight-line access opening space. (C) Subtraction of the volume of coronal preparations. Cavity access design The line from the coronal third of every canal was extended in the 3D models using Mimics software. These lines intersected the occlusal surface and generated four cross points. The geometric shape of the minimally invasive access cavity was then created by connecting these points. A minimally invasive access cavity allows access to and instrumentation of every canal, while preserving as much of the hard tooth structure as possible. A straightline access opening was designed so that the entire roof of the pulp chamber was removed and a straight-line path was created from the access opening to the coronal part of the canal (Fig. 1). Creation of finite element models The 3D models reconstructed using Mimics software were transferred into Geomagic Studio software (Raindrop Geomagic, Rock Hill, SC, USA). These models were converted into non-uniform rational basis spline surfaces and saved as initial graphics exchange specification (IGES) files. The 3D solid models of all parts were generated using Unigraphics NX (UG) software (Siemens PLM Software, Plano, TX, USA) from the IGES files. Subsequently, Boolean calculations carried out in UG software were used to create the two types of access cavities described above, and the cavities were filled with simulated resin composites Fig. 3. Three types of root canal preparations. (Fig. 2A,B). A size 35, 0.04 taper instrumentation space was created to simulate the clinical situation. Three types of root-canal preparations were simulated in UG software, namely the coronal third of the canals were simulated with 0.04 taper, 0.10 taper, and 0.12 taper, and the remaining part of the canals of all three preparations were simulated with 0.04 taper (Fig. 3). The prepared root-canal models were then simulated to be filled with gutta-percha from the apex up to 1 mm short of the canal orifices. Six treated models were assembled and divided into two groups: the minimally invasive group MI 0.04,MI 0.10, and MI 0.12 (i.e. minimally invasive access cavity with 0.04, 0.10, or 0.12 tapered canals in the coronal third); and the straightline group SL 0.04, SL 0.10, and SL 0.12 (i.e. straight-line access opening with 0.04, 0.10, or 0.12 tapered canals in the coronal third). The periodontal ligament was modeled as a 200-lm-thick layer surrounding the root, which finished 1.5 mm from the cemento enamel junction. A cube of alveolar bone, of 15 cm, was modeled around the periodontal ligament. Meshing models All models were meshed in C3D10, tetrahedral type, using Hypermesh software (Altair, Troy, MI, USA). In the alveolar bone model, the element size was 0.2 mm. To reduce the number of elements, the element size of the alveolar bone model was altered from 0.2 mm in inner surfaces to 2 mm in outer surfaces. All surface contacts were modeled as co-nodes, without relative movement. The number of nodes and elements varied between the different models (496, ,000 elements and 88,000 94,000 nodes). Fig. 1. The cavity designs. (A C) The minimally invasive access cavity design. (D) The straight-line access opening design. Boundary conditions and loads of models The mesial and distal surfaces of the alveolar bone segment were constrained in 6 degrees of freedom. In this study, two different tooth-loading patterns were simulated with a 250 N force (11), and were applied to an area of

3 Finite element analysis of tooth biomechanics mm 2 (12). One load was applied to the central fossa, vertically simulating the vertical masticatory force, while another load was applied to the lingual plane of the buccal cusp at 45 to the longitudinal axis of the tooth to simulate the lateral masticatory force. Static linear analysis was performed using Abaqus software (3DS, Waltham, MA, USA). The isotropic properties of all materials were defined. The properties of all simulated materials used in this study are listed in Table 1. Results The dimensions of the two types of access cavities were measured. The area of the minimally invasive access cavity was approximately 3.2 mm 2, while that of the straight-line access opening was approximately 13.7 mm 2. The volumes of the two access cavities were approximately 43.7 and mm 3, respectively (Fig. 2A,B). The volume difference between them was approximately 65.6 mm 3, as shown in Fig. 2C. The von Mises equivalent stress distributions of teeth under vertical and lateral loads are shown in Figs 4 and 5. On the occlusal surfaces of the models (Fig. 4), the highest stress concentration, approximately 60 MPa, was located at the margin of the cavities, and the areas with a stress concentration of over 10 MPa in the teeth with a minimally invasive access cavity (MI Material Table 1 Properties of the materials used in the study Young s modulus (MPa) Poisson s ratio Reference Enamel (13) Dentin (14) Resin composite (15) Gutta-percha (16) Pericementum (17) Cortical bone (18) group) were 38.5 mm 2, while these of teeth with a straight-line path access (SL group) were 35.5 mm 2 under vertical load; under lateral loads, the areas of stress concentration over 15 MPa of the MI group were 14.2 mm 2, while these of the SL group were 10.1 mm 2. In the same access cavity group, even with different taper enlargement in the cervical regions, the stress distributions on the occlusal surfaces were similar (Fig. 4). The cross-sections (L1 L3) in Fig. 5 were taken starting from the floor of the pulp chamber at intervals of 0.5 mm (Fig. 5A). In the cervical region (L1 L3), the stress areas were 4.5 and 7.5 MPa under vertical and lateral loads (Fig. 5B,C, Table 2), respectively, and these were smaller in the teeth with a minimally invasive access cavity (MI group) than those with s straight-line path access (SL group). The stress levels and the areas of stress distribution in L1 L3 were different depending on the enlargement of the canal orifice in the same access cavity group (Fig. 5B,C). The stress concentration areas exceeding 4.5 MPa under vertical load (Fig. 5B) and 7.5 MPa under lateral load (Fig. 5C) gradually expanded with the increasing taper preparations (Table 2). Discussion Tooth fractures are closely related to stress distribution (19). However, it is difficult to measure stresses in a tooth with a complex shape using a measuring device. In either in vitro or in vivo biomechanical studies, the wide variability of anatomy and configurations of extracted teeth within groups may make it difficult to show statistically significant impacts of therapeutic procedures. Finite element analysis, which can keep all irrelevant factors uniform during analysis (20), has recently become a widely applicable numeric method, with reliable results, for analyzing complex dental stress-related problems (21, 22). Consequently, the results of our study should be generally applicable in the clinical setting. Fig. 4. The occlusal stress distribution of models under different loads. (A) Vertical load. (B) Lateral load. MI, minimally invasive; SL, straight line.

4 594 Yuan et al. Fig. 5. Distribution of the von Mises stress of teeth in the cervical regions plane under vertical and lateral loads. (A) Positions of the three planes L1, L2, and L3. (B) Vertical load. (C) Lateral load. MI, minimally invasive; SL, straight line. Cross section Table 2 Areas of stress distributions in cervical regions (mm 2 ) Group Vertical load (>4.5 MPa) Lateral load (>7.5 MPa) 0.04* 0.10* 0.12* 0.04* 0.10* 0.12* L1 MI SL L2 MI SL L3 MI SL *Taper size. MI, minimally invasive; SL, straight line. This study sought to evaluate the biomechanical effects of minimally invasive preparation of teeth using finite element analysis. It may be helpful to show the clinical significance of minimally invasive endodontics. A 3D model can display the anatomic morphology of a tooth very clearly and accurately, especially that of the root canals. These advantages will make diagnoses based on 3D models more common in the future. In this study, a minimally invasive access cavity was designed by the 3D reconstruction of a tooth. The occlusal projections of the root canal orifices were precisely identified using a 3D model. The dimension of the minimally invasive access cavity, which was formed by linking the projections, was reduced by 76.6% compared with that of straight-line opening access (3.2 mm 2 vs mm 2 ). Approximately 60.0% additional coronal tissue was retained when using the minimally invasive access approach than when using the straight-line access approach, whilst all canal orifices were well exposed to facilitate preparation. In root canals instrumented with a 0.04 tapered file, less hard tissue was removed in the coronal third than in those prepared by 0.10 and 0.12 tapered files, as in conventional straight-line canal orifice preparations. Obviously, this demonstrates that the minimally invasive access cavity strategy preserves more tooth structure than conventional straight-line access opening. The results from all simulated cases revealed that the highest stress concentrations were at the margins of the access cavities on the occlusal surfaces (Fig. 4). The hard tissue or resin composite at these sites has a higher risk of fracture. A minimally invasive access cavity has a shorter margin than a conventional access opening, and this implies that the high-risk areas of a minimally invasive access cavity will be smaller than those of a straight-line opening cavity. The patterns of stress distribution suggest that these sites were those most likely to be fractured. The stress distribution findings of this study show that occlusal surface areas in which stresses are over 10 MPa with a minimally invasive access cavity (Fig. 4A,B) are smaller than those with a straight-line

5 Finite element analysis of tooth biomechanics 595 access opening under both vertical and lateral loads. This may indicate that a minimally invasive access cavity could reduce crown stress and further reduce the predisposition to crown fracture, in contrast to a straight-line access opening. This finding is consistent with the results of an in vitro study (9), in which teeth with standard mesial-occlusal-distal cavity preparation and superimposed endodontic access showed inferior fracture resistance when compared with teeth with access opening only. It is possible that the remaining crown tissue of a tooth with a minimally invasive access cavity decentralizes the force, and that the force could be spread out rather than concentrated in a limited area. It is interesting that minimally invasive access can reduce the stress distribution, not only on the occlusal surface but also on the cervical area of a tooth. The stress analysis of models showed that the high stress areas in the cervical region were smaller in teeth with minimally invasive access cavities than in teeth with straight-line access opening (Fig. 5B,C; Table 2). The possible reason for this might be that a larger part of the tooth was obturated with restorative material in the SL group compared with that of the MI group. When the tooth was subject to masticatory force, more force from the occlusal surface was transmitted directly to the cervical region and this induced greater stress concentrations. For the MI group, some forces were dispersed via the tooth structure preserved during treatment. As a result, the fracture susceptibility was lower in the cervical region of the tooth after minimally invasive access endodontic treatment. It is well known that the retention of cervical dentin is important for tooth longevity and optimal function (7, 8). Therefore, this finding will be of great significance in clinical practice. In the coronal third of the root canals, excessive canal enlargement with a 0.10 or 0.12 taper seemed to induce greater stress concentrations in the cervical tissue (Fig. 5B,C; Table 2). Previous studies have similarly found that the stress levels at the root increased with the preparation size or taper increase (3, 23 25). Increased size of canal preparations causes larger stress distributions that are expanded in the cervical parts of the tooth (Fig. 5, L1 L3), but has little influence on crown stress distributions (Fig. 4A,B). Moreover, larger taper instrumentation in the coronal thirds of canals led to a reduction of dentin thickness. In endodontically treated teeth, high stress was observed in the cervical region (26, 27). Removal of dentin was a significant factor in increasing fracture susceptibility (23). These results indicate that small taper preparation of the coronal third of root canals could reduce the fracture risk in the cervical part of the tooth. This study confirmed that preserving more tooth structure could reduce the stress concentration in a tooth, especially in the cervical regions, and might reduce fracture susceptibility. Although the minimally invasive access cavity approach preserved more dental hard tissue, it resulted in shrinkage of both the operating space and the visual range. It may be challenging to clean and shape the root canals with such access configuration. Our aim was to develop a basic understanding of the general effects of access cavity preparation. As stress distributions are influenced by geometric shape, mechanical properties, loading, and constraining conditions, the results may vary and further simulations or clinical trials are needed to confirm the significance of minimally invasive endodontics. In summary, a minimally invasive access cavity preserved a higher amount of coronal hard tissue of the treated teeth compared with the conventional method. The minimally invasive procedure not only reduced the stress of the occlusal area but also reduced the stress concentration of the cervical structure. Compared with a straight-line canal orifice opening, small taper canal preparations also preserved more cervical tooth tissue and preserved better fracture resistance of the cervical structure. Therefore, use of a minimally invasive access cavity reduced the stress distribution in crown and cervical regions, and a small taper cervical enlargement resulted in low stress in the cervical region, and little effect in the crown. Acknowledgements This work was supported by a grant from the National Natural Science Foundation of China, no / The authors are immensely grateful to Dr Zhang Kai and Dr Yang Biao, Department of Mechanics, Tongji University, Shanghai, for their invaluable help with the FEA of the tooth models. Conflicts of interest The authors deny any conflicts of interest related to this study. References 1. KENNETH MH, STEPHEN C. 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