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Int. J. Oral Maxillofac. Surg. 2014; xxx: xxx–xxx http://dx.doi.org/10.1016/j.ijom.2014.09.018, available online at http://www.sciencedirect.com
Research Paper Trauma
Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?
S. Antic, I. Saveljic, D. Nikolic, G. Jovicic, N. Filipovic, Z. Rakocevic, M. Djuric Laboratory for Anthropology, Institute of Anatomy, University of Belgrade – School of Medicine, Belgrade, Serbia
S. Antic, I. Saveljic, D. Nikolic, G. Jovicic, N. Filipovic, Z. Rakocevic, M. Djuric: Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?. Int. J. Oral Maxillofac. Surg. 2014; xxx: xxx–xxx. # 2014 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Abstract. It has been suggested that unerupted lower third molars (M3) increase the fragility of the mandibular angle and simultaneously decrease the risk of condylar fracture. However, it is unknown whether this applies regardless of the direction and point of impact of the traumatic force. The aim of this study was to investigate the impact of an unerupted M3 on the fragility of the angle and condyle in terms of a force acting from different directions and affecting different regions of the mandible. Computed tomography scans of a human mandible and finite element methodology were used to obtain two three-dimensional models: a model with, and the other without an unerupted M3. A force of 2000 N was applied to three different regions of the models: the symphysis, ipsilateral body, and contralateral body, respectively. When the force was applied to the mandibular body, the results revealed increased angle fragility in cases with unerupted M3. When the force was applied to the symphysis, the condyle region showed higher fragility, irrespective of the presence of an unerupted M3. In summary, fragility of the angle and condyle regions depends on the presence of an unerupted M3 and on the direction and point of impact of the force.
The association of unerupted lower third molars (M3) and angle fractures of the lower jaw has been the subject of many recent epidemiological studies. It is suggested that the presence of the M3, especially in cases where it is impacted or partially impacted and angulated, increases the risk of fracture in the region of the mandibular angle two- to four-fold.1–5 0901-5027/000001+05
Nevertheless, previous studies have shown that when the M3 is not present, the fracture is more likely to occur in the region of the mandibular condyle.6,7 This suggests that the presence of the M3 might prevent condylar fracture. Takada et al.8 used a finite element analysis (FEA) method to analyse the distribution of stress during the simulation
Keywords: third molar; mandible; fracture; finite element. Accepted for publication 25 September 2014
of traumatic force in the region of the mandibular angle, and compared two cases: with half-impacted M3 and without M3. They found differences in the distribution of stress, and also in average and maximum values of stress. However, that study showed the stress distribution only in the case where force acted in the region of the ipsilateral mandibular body. The
# 2014 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
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Antic et al.
Fig. 1. Process for 3D model generation using CT images.
FEA study of Bezerra et al.9 showed that erupted M3 weaken the angle region after a blow simulated to the chin region. The aim of this study was to evaluate the influence of the unerupted M3 on the distribution of stress and fragility in the regions of the mandibular angle and condyle, in three standard trauma situations: a simulated blow to the region of the symphysis, the ipsilateral body, and the
contralateral body. Further, we sought to determine the pattern of biomechanical behaviour in each case. Materials and methods
The mandible of a middle-aged male with partially impacted M3 was imaged with diagnostic computed tomography (CT) (SOMATOM Sensation 16; Siemens) in
Table 1. Mechanical properties of the materials used. Material Cortical bone Trabecular bone Teeth
Elastic modulus (GPa) 13.70 1.37 18.60
Poisson ratio 0.30 0.30 0.31
transversal planes, with slices of 0.75 mm in thickness. The slices obtained were used for the creation of the following computer models of the mandible: (1) A model of the human mandible without M3. In this model, M3 was removed by computer manipulations; the pixels of the M3 were converted from initial tooth structure mask to cortical and medullar bone masks on each CT slice, with respect to the anatomic structure. (2) A model of the human mandible with partially impacted M3 in the mesioangular position. Symmetrical models based on the left side were created by mirror-imaging in order to avoid the possible influence of differences in the sides on the distribution of stress.
Fig. 2. Three-dimensional model of the mandible with boundary conditions and loading. (a) Simulated blow to the region of the symphysis; (b) simulated blow to the region of the ipsilateral body; (c) simulated blow to the region of the contralateral body.
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
Unerupted M3 and risk of mandibular fracture These models were studied using FEA methodology, a numerical method that is widely used in biomedical sciences and engineering as a tool for providing an accurate prediction of the mechanical response of complex structures when submitted to loading. This method is based on subdivision of the complex geometry into smaller elements of finite dimensions, which, when combined, form the mesh model of the analysed structure.10 As a first step, we used Mimics visualisation software (version 10.1; Materialise, Leuven, Belgium) for three-dimensional (3D) reconstruction of the CT images (Fig. 1). Based on image density thresholding, three different masks were obtained: a cortical mask, a trabecular mask, and a mask for all teeth. Using the Mimics STL+ module, cortical bone, trabecular bone, and teeth were converted into stereolithography files. To reduce triangles and fix the quality of the triangles that were not appropriate for the FEA, we used the REMESH module attached to Mimics. TetGen was then used to create a 3D mesh. Four-node tetrahedral elements were used as the final element, where each node had three degrees of freedom: translation in the nodal x, y, and z directions. The average size of elements from each group was used, as follows: 0.5 mm for cortical bone, 1.0 mm for trabecular bone, and 2.0 mm for teeth. Non-linear FEA was performed using PAKC (BioIRCBioengineerng Research and Development Center, Kragujevac, Serbia) software. Linear and homogeneous material behaviours were assumed for mandibular bone and teeth. The Young modulus and Poisson ratio values, based on Lotti et al.,11 are given in Table 1. Figure 2 shows the 3D mandible model with boundary conditions and loadings, where actions of traumatic forces in three different directions and points of action were simulated using the FEA method. 2000 N was applied perpendicularly to the frontal plane (in the region of symphysis) and laterally (in the region of the mandibular body) on a circular area of 1 cm in diameter. The results were evaluated qualitatively using a colour scale by assessing the distribution of von Mises equivalent stress12 along the mandible as a response to the loading condition, and quantitatively by calculating the average and the maximum stress in the regions of interest. The von Mises stress scores were calculated automatically by the software. Results
The results are based on the analysis of the distribution of von Mises equivalent
stress on a colour scale and by measurement of the average and maximum stress (Figs. 3 and 4; Table 2). The greatest amount of stress in the angle region was observed when the force was applied to the region of the ipsilateral body, both in cases with and without M3. The stress was greater than when the force was applied to the contralateral body and symphysis, respectively. In the region of
3
the mandibular condyle, the highest stresses were recorded when the force was applied to the region of the symphysis, and then the ipsilateral and contralateral body, respectively. When the third molar was present, average and maximum values of the concentrated von Mises equivalent stress increased slightly in the angle region, and simultaneously decreased in the condyle
Fig. 3. Model of the human mandible without M3. Colour scale analysis of the von Mises stress distribution in the regions of the mandibular angle and condyle shown to the right. (a) Simulated blow to the region of the symphysis; (b) simulated blow to the region of the ipsilateral body; (c) simulated blow to the region of the contralateral body. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
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Antic et al. slightly more concentrated in the condyle region. When the blow was directed to the symphysis (Fig. 3a and Fig. 4a), conceivably more of the stress concentrated in the region of the condyle, irrespective of the presence of the M3. Discussion
Fig. 4. Model of the human mandible with partially impacted M3 in the mesioangular position. Colour scale analysis of the von Mises stress distribution in the regions of the mandibular angle and condyle shown to the right. (a) Simulated blow to the region of the symphysis; (b) simulated blow to the region of the ipsilateral body; (c) simulated blow to the region of the contralateral body. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
region, regardless of the direction of the blow. In contrast, the model without M3 demonstrated a slight increase in von Mises equivalent stress in the condyle region and decrease in stress in the angle region. Subsequently, after simulated blows from different directions, we compared concentrated Von Mises stresses in the angle and condyle region in order to predict
the risk of fracture in these different regions. After applying force to the region of the ipsilateral and contralateral body (Fig. 3b and c, Fig. 4b and c), a slightly higher stress concentration was detected in the angle region when M3 was partially impacted, compared to the model without M3. In the latter case, the stress was
Previous studies have pointed to the association of impacted, or partially impacted, M3 and the weakness of the angle region of the mandible.1–7 Some authors have even suggested removing the unerupted M3 in order to prevent fractures of the mandibular angle, especially in persons engaged in contact sports.1,2 However, an evaluation of the reported clinical data revealed a higher risk of condylar fracture in patients without M3,6,7,13,14 directly contradicting the practice of extraction as a preventive measure. In addition, fractures of the condylar region are more difficult to treat because of problems of accessibility and the higher risk of complications. Furthermore, several studies have shown that surgical extraction of the impacted M3 itself can occasionally lead to fracturing of the mandibular angle and carries some degree of risk.15–18 Several studies3–7 on the influence of the position and state of eruption of M3 on mandibular angle and condylar fractures, have reported a statistically significant association between unerupted M3 (often class B in mesioangular position) and angle fractures,6,13 and between a missing M3 and fractures of the condyle. These data were taken into account when selecting the mandible with partially impacted M3 for creating the models presented in this study. In the present study, the FEA method was used to analyse the influences of the M3 and the direction of force on the fragility of the angle and condyle regions of the mandible. The increase in von Mises stress in the angle region when M3 was present, and conversely in the condyle region when it was missing, indicates that the unerupted M3 contributes to the greater fragility of the mandibular angle. Simultaneously, the presence of an M3 is expected to decrease the risk of a condylar fracture. These results are in agreement with clinical data1–7,13,14,19 and with an experimental study on monkey mandibles.20 The observed impact of the M3 on angle fragility was found to be of varying significance for different directions of the acting force. After applying force to the region of the ipsilateral and contralateral mandibular body, the influence of M3 was
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
Unerupted M3 and risk of mandibular fracture
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Table 2. Quantitative analysis of the distribution of von Mises equivalent stress in the regions of the mandibular angle and condyle.
Model
Region of the acting force
von Mises stress in the region of the angle (MPa)
von Mises stress in the region of the condyle (MPa)
Average
Maximum
Average
Maximum
Model 1 Mandible without M3
Symphysis Ipsilateral body Contralateral body
41.63 128.54 70.98
53.09 149.19 85.93
129.07 149.21 80.08
142.00 165.00 99.87
Model 2 Mandible with partially impacted M3
Symphysis Ipsilateral body Contralateral body
49.06 145.86 101.08
61.74 165.00 122.54
121.17 123.99 58.63
142.00 146.36 92.44
more pronounced, with a higher risk of angle fracture when M3 was present. After simulating a blow to the symphysis, conceivably higher values of concentrated stress were recorded in the condyle region, despite the influence of the M3. Unreported in previous studies, a new finding emerged that the influence of M3 is almost negligible when the force is directed towards the symphysis. It is expected that other factors, such as surface area at the point of impact, action of the masticatory muscles, teeth occlusion, and condylar position during the injury, might influence the fracture pattern of the mandibular angle and the condyle. In the present study, those factors, as well as force intensity, were kept constant in order to analyse the impact of the direction of the force and the presence of M3 under constant conditions. These factors should be the focus of future investigations. In summary, when exposed to a traumatic force, the fragility of the angle and the condyle regions of the mandible depends on the presence and position of the M3, but also on the direction and action point of the impact force. Given our findings, the extraction of an unerupted lower third molar in order to prevent angle fractures is not recommended. Funding
The authors acknowledge the support of the Ministry of Science of the Republic of Serbia (45005). Competing interests
We have nothing to declare. Ethical approval
This study was submitted to and approved by the local committee on human research of the School of Dentistry, University of Belgrade and registered under protocol 36/ 2 on 15.01.2013.
References 1. Hanson BP, Cummings P, Rivara FP, John MT. The association of third molars with mandibular angle fractures: a meta-analysis. J Can Dent Assoc 2004;70:39–43. 2. Schwimmer A, Stern R, Kritchman D. Impacted third molars: a contributing factor in mandibular fractures in contact sports. Am J Sports Med 1983;11:262–6. 3. Safdar N, Meechan JG. Relationship between fractures of the mandibular angle and the presence and state of eruption of the lower third molar. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1995;79:680–4. 4. Tevepaugh DB, Dodson TB. Are mandibular third molars a risk factor for angle fractures? A retrospective cohort study. J Oral Maxillofac Surg 1995;53:646–9. 5. Lee JT, Dodson TB. The effect of mandibular third molar presence and position on the risk of an angle fracture. J Oral Maxillofac Surg 2000;58:394–8. 6. Thangavelu A, Yoganandha R, Vaidhyanathan A. Impact of impacted mandibular third molars in mandibular angle and condylar fractures. Int J Oral Maxillofac Surg 2010;39:136–9. 7. Duan DH, Zhang Y. Does the presence of mandibular third molars increase the risk of angle fracture and simultaneously decrease the risk of condylar fracture. Int J Oral Maxillofac Surg 2008;37:25–8. 8. Takada H, Abe S, Tamatsu Y, Mitarashi S, Saka H, Ide Y. Three-dimensional bone microstructures of the mandibular angle using micro-CT and finite element analysis: relationship between partially impacted mandibular third molars and angle fractures. Dent Traumatol 2006;22:18–24. 9. Bezerra TP, Silva Junior FI, Scarparo HC, Costa FW, Studart-Soares EC. Do erupted third molars weaken the mandibular angle after trauma to the chin region? A 3D finite element study. Int J Oral Maxillofac Surg 2013;42:474–80. 10. Korioth TW, Versluis A. Modeling the mechanical behavior of the jaws and their related structures by finite element (FE) analysis. Crit Rev Oral Biol Med 1997;8:90–104. 11. Lotti RS, Machado AW, Mazzieiro ET, Landre Jr J. Aplicabilidade cientı´fica do me´todo dos
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elementos finitos. Rev Dent Press Ortodon Ortop Facial 2006;11:35–43. Ulrich D, van Rietbergen B, Laib A, Ru¨egsegger P. Load transfer analysis of the distal radius from in-vivo high-resolution CT-imaging. J Biomech 1999;32:821–8. Fuselier JC, Ellis 3rd EE, Dodson TB. Do mandibular third molars alter the risk of angle fracture. J Oral Maxillofac Surg 2002;60: 514–8. Zhu SJ, Choi BH, Kim HJ, Park WS, Huh JY, Jung JH, et al. Relationship between the presence of unerupted mandibular third molars and fractures of the mandibular condyle. Int J Oral Maxillofac Surg 2005;34: 382–5. ¨ zc¸akir-Tomruk C, Arslan A. Mandibular O angle fractures during third molar removal: a report of two cases. Aust Dent J 2012;57: 231–5. Ethunandan M, Shanahan D, Patel M. Iatrogenic mandibular fractures following removal of impacted third molars: an analysis of 130 cases. Br Dent J 2012;212:179–84. Cankaya AB, Erdem MA, Cakarer S, Cifter M, Oral CK. Iatrogenic mandibular fracture associated with third molar removal. Int J Med Sci 2011;8:547–53. Grau-Manclu´s V, Gargallo-Albiol J, Almendros-Marque´s N, Gay-Escoda C. Mandibular fractures related to the surgical extraction of impacted lower third molars: a report of 11 cases. J Oral Maxillofac Surg 2011;69: 1286–90. Patil PM. Unerupted lower third molars and their influence on fractures of the mandibular angle and condyle. Br J Oral Maxillofac Surg 2012;50:443–6. Reitzik M, Lownie JF, Cleaton-Jones P, Austin J. Experimental fractures of monkey mandibles. Int J Oral Surg 1978;7:100–3.
Address: Marija Djuric Laboratory for Anthropology Institute of Anatomy University of Belgrade – School of Medicine Dr Subotica 4/2 11000 Belgrade Serbia. Tel.: +381 112686172 E-mail: [email protected]
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
Int. J. Oral Maxillofac. Surg. 2014; xxx: xxx–xxx http://dx.doi.org/10.1016/j.ijom.2014.09.018, available online at http://www.sciencedirect.com
Research Paper Trauma
Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?
S. Antic, I. Saveljic, D. Nikolic, G. Jovicic, N. Filipovic, Z. Rakocevic, M. Djuric Laboratory for Anthropology, Institute of Anatomy, University of Belgrade – School of Medicine, Belgrade, Serbia
S. Antic, I. Saveljic, D. Nikolic, G. Jovicic, N. Filipovic, Z. Rakocevic, M. Djuric: Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?. Int. J. Oral Maxillofac. Surg. 2014; xxx: xxx–xxx. # 2014 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Abstract. It has been suggested that unerupted lower third molars (M3) increase the fragility of the mandibular angle and simultaneously decrease the risk of condylar fracture. However, it is unknown whether this applies regardless of the direction and point of impact of the traumatic force. The aim of this study was to investigate the impact of an unerupted M3 on the fragility of the angle and condyle in terms of a force acting from different directions and affecting different regions of the mandible. Computed tomography scans of a human mandible and finite element methodology were used to obtain two three-dimensional models: a model with, and the other without an unerupted M3. A force of 2000 N was applied to three different regions of the models: the symphysis, ipsilateral body, and contralateral body, respectively. When the force was applied to the mandibular body, the results revealed increased angle fragility in cases with unerupted M3. When the force was applied to the symphysis, the condyle region showed higher fragility, irrespective of the presence of an unerupted M3. In summary, fragility of the angle and condyle regions depends on the presence of an unerupted M3 and on the direction and point of impact of the force.
The association of unerupted lower third molars (M3) and angle fractures of the lower jaw has been the subject of many recent epidemiological studies. It is suggested that the presence of the M3, especially in cases where it is impacted or partially impacted and angulated, increases the risk of fracture in the region of the mandibular angle two- to four-fold.1–5 0901-5027/000001+05
Nevertheless, previous studies have shown that when the M3 is not present, the fracture is more likely to occur in the region of the mandibular condyle.6,7 This suggests that the presence of the M3 might prevent condylar fracture. Takada et al.8 used a finite element analysis (FEA) method to analyse the distribution of stress during the simulation
Keywords: third molar; mandible; fracture; finite element. Accepted for publication 25 September 2014
of traumatic force in the region of the mandibular angle, and compared two cases: with half-impacted M3 and without M3. They found differences in the distribution of stress, and also in average and maximum values of stress. However, that study showed the stress distribution only in the case where force acted in the region of the ipsilateral mandibular body. The
# 2014 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
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Antic et al.
Fig. 1. Process for 3D model generation using CT images.
FEA study of Bezerra et al.9 showed that erupted M3 weaken the angle region after a blow simulated to the chin region. The aim of this study was to evaluate the influence of the unerupted M3 on the distribution of stress and fragility in the regions of the mandibular angle and condyle, in three standard trauma situations: a simulated blow to the region of the symphysis, the ipsilateral body, and the
contralateral body. Further, we sought to determine the pattern of biomechanical behaviour in each case. Materials and methods
The mandible of a middle-aged male with partially impacted M3 was imaged with diagnostic computed tomography (CT) (SOMATOM Sensation 16; Siemens) in
Table 1. Mechanical properties of the materials used. Material Cortical bone Trabecular bone Teeth
Elastic modulus (GPa) 13.70 1.37 18.60
Poisson ratio 0.30 0.30 0.31
transversal planes, with slices of 0.75 mm in thickness. The slices obtained were used for the creation of the following computer models of the mandible: (1) A model of the human mandible without M3. In this model, M3 was removed by computer manipulations; the pixels of the M3 were converted from initial tooth structure mask to cortical and medullar bone masks on each CT slice, with respect to the anatomic structure. (2) A model of the human mandible with partially impacted M3 in the mesioangular position. Symmetrical models based on the left side were created by mirror-imaging in order to avoid the possible influence of differences in the sides on the distribution of stress.
Fig. 2. Three-dimensional model of the mandible with boundary conditions and loading. (a) Simulated blow to the region of the symphysis; (b) simulated blow to the region of the ipsilateral body; (c) simulated blow to the region of the contralateral body.
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
Unerupted M3 and risk of mandibular fracture These models were studied using FEA methodology, a numerical method that is widely used in biomedical sciences and engineering as a tool for providing an accurate prediction of the mechanical response of complex structures when submitted to loading. This method is based on subdivision of the complex geometry into smaller elements of finite dimensions, which, when combined, form the mesh model of the analysed structure.10 As a first step, we used Mimics visualisation software (version 10.1; Materialise, Leuven, Belgium) for three-dimensional (3D) reconstruction of the CT images (Fig. 1). Based on image density thresholding, three different masks were obtained: a cortical mask, a trabecular mask, and a mask for all teeth. Using the Mimics STL+ module, cortical bone, trabecular bone, and teeth were converted into stereolithography files. To reduce triangles and fix the quality of the triangles that were not appropriate for the FEA, we used the REMESH module attached to Mimics. TetGen was then used to create a 3D mesh. Four-node tetrahedral elements were used as the final element, where each node had three degrees of freedom: translation in the nodal x, y, and z directions. The average size of elements from each group was used, as follows: 0.5 mm for cortical bone, 1.0 mm for trabecular bone, and 2.0 mm for teeth. Non-linear FEA was performed using PAKC (BioIRCBioengineerng Research and Development Center, Kragujevac, Serbia) software. Linear and homogeneous material behaviours were assumed for mandibular bone and teeth. The Young modulus and Poisson ratio values, based on Lotti et al.,11 are given in Table 1. Figure 2 shows the 3D mandible model with boundary conditions and loadings, where actions of traumatic forces in three different directions and points of action were simulated using the FEA method. 2000 N was applied perpendicularly to the frontal plane (in the region of symphysis) and laterally (in the region of the mandibular body) on a circular area of 1 cm in diameter. The results were evaluated qualitatively using a colour scale by assessing the distribution of von Mises equivalent stress12 along the mandible as a response to the loading condition, and quantitatively by calculating the average and the maximum stress in the regions of interest. The von Mises stress scores were calculated automatically by the software. Results
The results are based on the analysis of the distribution of von Mises equivalent
stress on a colour scale and by measurement of the average and maximum stress (Figs. 3 and 4; Table 2). The greatest amount of stress in the angle region was observed when the force was applied to the region of the ipsilateral body, both in cases with and without M3. The stress was greater than when the force was applied to the contralateral body and symphysis, respectively. In the region of
3
the mandibular condyle, the highest stresses were recorded when the force was applied to the region of the symphysis, and then the ipsilateral and contralateral body, respectively. When the third molar was present, average and maximum values of the concentrated von Mises equivalent stress increased slightly in the angle region, and simultaneously decreased in the condyle
Fig. 3. Model of the human mandible without M3. Colour scale analysis of the von Mises stress distribution in the regions of the mandibular angle and condyle shown to the right. (a) Simulated blow to the region of the symphysis; (b) simulated blow to the region of the ipsilateral body; (c) simulated blow to the region of the contralateral body. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
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Antic et al. slightly more concentrated in the condyle region. When the blow was directed to the symphysis (Fig. 3a and Fig. 4a), conceivably more of the stress concentrated in the region of the condyle, irrespective of the presence of the M3. Discussion
Fig. 4. Model of the human mandible with partially impacted M3 in the mesioangular position. Colour scale analysis of the von Mises stress distribution in the regions of the mandibular angle and condyle shown to the right. (a) Simulated blow to the region of the symphysis; (b) simulated blow to the region of the ipsilateral body; (c) simulated blow to the region of the contralateral body. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
region, regardless of the direction of the blow. In contrast, the model without M3 demonstrated a slight increase in von Mises equivalent stress in the condyle region and decrease in stress in the angle region. Subsequently, after simulated blows from different directions, we compared concentrated Von Mises stresses in the angle and condyle region in order to predict
the risk of fracture in these different regions. After applying force to the region of the ipsilateral and contralateral body (Fig. 3b and c, Fig. 4b and c), a slightly higher stress concentration was detected in the angle region when M3 was partially impacted, compared to the model without M3. In the latter case, the stress was
Previous studies have pointed to the association of impacted, or partially impacted, M3 and the weakness of the angle region of the mandible.1–7 Some authors have even suggested removing the unerupted M3 in order to prevent fractures of the mandibular angle, especially in persons engaged in contact sports.1,2 However, an evaluation of the reported clinical data revealed a higher risk of condylar fracture in patients without M3,6,7,13,14 directly contradicting the practice of extraction as a preventive measure. In addition, fractures of the condylar region are more difficult to treat because of problems of accessibility and the higher risk of complications. Furthermore, several studies have shown that surgical extraction of the impacted M3 itself can occasionally lead to fracturing of the mandibular angle and carries some degree of risk.15–18 Several studies3–7 on the influence of the position and state of eruption of M3 on mandibular angle and condylar fractures, have reported a statistically significant association between unerupted M3 (often class B in mesioangular position) and angle fractures,6,13 and between a missing M3 and fractures of the condyle. These data were taken into account when selecting the mandible with partially impacted M3 for creating the models presented in this study. In the present study, the FEA method was used to analyse the influences of the M3 and the direction of force on the fragility of the angle and condyle regions of the mandible. The increase in von Mises stress in the angle region when M3 was present, and conversely in the condyle region when it was missing, indicates that the unerupted M3 contributes to the greater fragility of the mandibular angle. Simultaneously, the presence of an M3 is expected to decrease the risk of a condylar fracture. These results are in agreement with clinical data1–7,13,14,19 and with an experimental study on monkey mandibles.20 The observed impact of the M3 on angle fragility was found to be of varying significance for different directions of the acting force. After applying force to the region of the ipsilateral and contralateral mandibular body, the influence of M3 was
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
YIJOM-3001; No of Pages 5
Unerupted M3 and risk of mandibular fracture
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Table 2. Quantitative analysis of the distribution of von Mises equivalent stress in the regions of the mandibular angle and condyle.
Model
Region of the acting force
von Mises stress in the region of the angle (MPa)
von Mises stress in the region of the condyle (MPa)
Average
Maximum
Average
Maximum
Model 1 Mandible without M3
Symphysis Ipsilateral body Contralateral body
41.63 128.54 70.98
53.09 149.19 85.93
129.07 149.21 80.08
142.00 165.00 99.87
Model 2 Mandible with partially impacted M3
Symphysis Ipsilateral body Contralateral body
49.06 145.86 101.08
61.74 165.00 122.54
121.17 123.99 58.63
142.00 146.36 92.44
more pronounced, with a higher risk of angle fracture when M3 was present. After simulating a blow to the symphysis, conceivably higher values of concentrated stress were recorded in the condyle region, despite the influence of the M3. Unreported in previous studies, a new finding emerged that the influence of M3 is almost negligible when the force is directed towards the symphysis. It is expected that other factors, such as surface area at the point of impact, action of the masticatory muscles, teeth occlusion, and condylar position during the injury, might influence the fracture pattern of the mandibular angle and the condyle. In the present study, those factors, as well as force intensity, were kept constant in order to analyse the impact of the direction of the force and the presence of M3 under constant conditions. These factors should be the focus of future investigations. In summary, when exposed to a traumatic force, the fragility of the angle and the condyle regions of the mandible depends on the presence and position of the M3, but also on the direction and action point of the impact force. Given our findings, the extraction of an unerupted lower third molar in order to prevent angle fractures is not recommended. Funding
The authors acknowledge the support of the Ministry of Science of the Republic of Serbia (45005). Competing interests
We have nothing to declare. Ethical approval
This study was submitted to and approved by the local committee on human research of the School of Dentistry, University of Belgrade and registered under protocol 36/ 2 on 15.01.2013.
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Address: Marija Djuric Laboratory for Anthropology Institute of Anatomy University of Belgrade – School of Medicine Dr Subotica 4/2 11000 Belgrade Serbia. Tel.: +381 112686172 E-mail: [email protected]
Please cite this article in press as: Antic S, et al. Does the presence of an unerupted lower third molar influence the risk of mandibular angle and condylar fractures?, Int J Oral Maxillofac Surg (2016), http://dx.doi.org/10.1016/j.ijom.2014.09.018
