Influence Of The Mandibular Third Molar On Mandibular Angle And Condyle Fracture-A Retrospective Study And Finite Element Analysis

Objective:The purpose of this study was to analyze the relationship between the mandibular third molar(M3)and angle or condyle fractures in a group of patients treated for mandibular fractures;and to explore the effect of erupted types of M3 and angle or condyle fractures.Methods:(1)A retrospective study was conducted in patients treated for mandibular fractures in the Department of Oral and Maxillofacial Surgery,Affiliated Stomatology Hospital of Kunming Medical University,from June 2016 to February 2018.The study consisted of 157 patients with mandibular angle and condyle fractures who met the inclusion criteria.The study variables were presence or absence of third molars,if it was present,their position and angulation,were classified according to imaging findings.The outcome variables were angle and condyle fractures.Hospital records,computed tomography and Cone Beam Computed Tomography(CBCT)were used to determine and classify these variables.The data was analyzed using a statistical software(SPSS 22.0).(2)A three-dimensional(3D)mandibular model was obtained through finite element analysis(FEA)using CBCT with the geometry and mechanical properties to reproduce a normal mandibular structure.From computed tomographic scans of a human mandible with partially erupted M3,whose angulation was vertical,three additional virtual models were generated:a mandibular model with mesioangular impacted M3,a model with horizontal impacted M3 and a model without M3.An external force of 2000N was applied to the symphysis region and the right mental foramen area of the models respectively.The results were based on the chromatic analysis of the distributed von Mises and principal stresses using the software Abaqus 2017.Results:(1)Results showed that the risk of mandibular angle fracture was significantly lower in patients lacking M3(P<0.05),whereas the risk of condylar fracture was significantly higher in patients lacking M3(P<0.05).A relation between the position of M3 to angle or condylar fractures could not be found(P>0.05).(2)When the force was applied to the symphysis of the models,approximated von Mises equivalent stress of the mandibular angle region was 124.3MPa in the mandible with M3 and 45.6MPa in the mandible without M3.In the condylar region it was 144.8 MPa when the third molar was present and 136.2MPa when it was absent,which confirmed the results of case analysis.When the force acted on the right foramen area,approximated von Mises equivalent stress of the mandibular angle region was 279.9MPa in the mandible with M3 and 102.6MPa in the mandible without M3.In the condylar region it was 173.2 MPa when the third molar was present and 165.7MPa when it was absent.The results indicated that whenever the third molar was present,there was a greater concentration of tensions around the cervical part of condyle.Conclusion:1.It could be concluded that the presence of M3 increased the risk of angle fracture and simultaneously decreased the risk of condyle fracture.2.No relationship appeared to exist between M3 eruption position and mandibular angle and condyle fracture.