| Temporomandibular joint(TMJ)is burden-bearing joint,undertaking the force from the contraction of the masticatory muscles during occlusion.It's closely related to the function of occlusion..The study of the biomechanical characteristics of temporomandibular joint is of great significance to the study of clinical diagnosis and treatment of oral diseases.Three-dimensional model of right mandibular first molar and that of bilateral joint and mandible were established based on CT image data of an adult male volunteers in this work.Different designed occlusal loads were simulated using three-dimensional finite element technology.The stress value and stress concentration area of tooth,periodontal and bilateral joint were collected and analyzed,to evaluate the effects of different loading sites,areas and directions on the stress of tooth,periodontal and bilateral joints.The experiment was consist of three parts:The first part--The establishment of bilateral temporomandibular joint and mandible modelBased on an adult male volunteer's maxillofacial CT data,a model withreconstruction of enamel,dentin,pulp,periodontal membrane,mandible,condylar cartilage,articular disc and the articular surface of temporal bone(part structure)had been established.Then the 3D models were processed by reverse engineering software NX Imageware13.2 and 3D CAD software Solidworks2015 to get assembly model.Finally,the assembly model was imported into the three-dimensional finite element software ANSYS Workbench16.0.The 3D finite element model was then established.The second part--The bilateral condylar stress under specific occlusal loadsTo observe bilateral condyle stress with the bilateral TMJ and mandible model built in the first experiment.5 teeth(41,43,44,46,48)were chosen to perform vertical and inclined(4 directions)loads.The results showed that the stress concentration area of the anterior slope of condyle was greater than that of the posterior slope,and the stress concentration was rarely showed up at the inner and external pole of the condyle top area.The distribution of the maximum stress on the left and right sides of the condylar cartilage and the maximum stress value were influenced by different loading positions.The stress of condylar cartilage was more sensitive to buccal-lingual loading change in the posterior teeth region,but was more sensitive to mesial-distal loading change in the anterior teeth region.The third part--The tooth and periodontal stress distribution of the right mandibular first molar under simulated occlusal loadsTo study tooth and periodontal stress of the right mandibular first molar using the model built in first experiment.17 sites were chosen to perform different loads.Concentrated and dispersed modes,vertical and inclined(8 directions)directions were designed to simulated occlusal conditions.The following content were observed and analyzed:1.Stress values and stress concentration area in vertical loads at one site of two types of modes;2.Stress values and stress concentration area in concentrated loads in vertical and 8inclined directions;3.Stress values and stress concentration area in concentrated loads at different sitesin vertical direction;4.Stress values and stress concentration area in concentrated loads at different sites in 8 inclined directions;5.The difference of stress values and stress concentration area between inner part and outside surface of tooth and periodontal membrane;6.Stress values and stress concentration area in concentrated loads at different combinations of chosen sites.The results were as follow:1.Results of single site loads:(1)Loading area-size: The maximal tooth stress value was influenced greatly by the loading area-size(concentrated mode or dispersed mode),the bigger the lower,but the maximal periodontal stress value and the distribution of tooth and periodontal stress concentration were not.(2)Directions: When loading on the same site in different directions,the sizes of both dental and periodontal stress concentration areas changed with the loading directions.In both dental and periodontal regions,the closer the loading was directed to the center of the tooth occlusal surface,the smaller the stress concentration area.(3)The location of loading site: Compared all single site loads,when loading on distal buccal cusp tip,the value of the maximal periodontal stress was minimal;when loading on distal lingual cusp tip,the value of the maximal periodontal stress was maximal;when loading on the bottom of central fossa,the area of tooth and periodontal stress concentration were the smallest.(4)Internal and external stress: In general,the tooth stress had a lower magnitude and smaller area in the inner side than outer side.The stress concentration area in dentin was smaller than that in enamel,and the stress values was lower in dentin than that of in enamel under the same loading condition.While the periodontal stress had a larger area in the inner side but without obvious difference in terms of stress magnitude.(5)The location of the maximum periodontal stress: For vertical loading,the maximum periodontal stress was found at the mesial cervix of periodontal membranewhen the loading site was located in the mesial third of the occlusal surface;otherwise,it occurred at the apex of the distal root of periodontal membrane.2.Results of multi-sites loads:(1)Resultant Force: A parameter named the Resultant Force was used to describe the multi-sites loading.It was based on the principle that the Resultant Force on the occlusal surface would get a similar moment as loading on the combined sites it simulated.The location of the Resultant Force was related to the results of that multi-sites load.(2)The location of the Resultant Force: When the Resultant Point was near to the distal buccal cusp tip,the value of the maximal periodontal stress was minimal;when the Resultant Point was near to the bottom of central fossa,the area of tooth and periodontal stress concentration were the smallest.(3)Internal and external stress: Tooth stress concentration could be found around all the loading sites.It had a lower magnitude and smaller area in the inner side than outer side.The stress concentration area in dentin was smaller than that in enamel,and the stress values was lower in dentin than that of in enamel under the same loading condition.While the periodontal stress had a larger area in the inner side but without obvious difference in terms of stress magnitude.(4)The location of the maximum periodontal stress: The maximum periodontal stress was found at the mesial cervix of periodontal membrane when the Resultant Point was located in the mesial third of the occlusal surface;otherwise,it occurred at the apex of the distal root of periodontal membrane.Therefore,the following conclusions were obtained from this work:1.The occlusal contact and the masticatory force direction had obvious influences on the stress of the tooth,periodontal membrane,condylar cartilage and mandibular condyle.2.The stress concentration in the outside surface of tooth occurred mostly at the loading sites and the root branch region.With the depth increased,the dental stress concentration area and magnitude decreased.Tooth stress rarely reached the pulp cavity.The stress concentration area of the outside surface of periodontal membrane mostly occurred at the root branch region,while the stress concentration of the inside surfacealways occurred at the apical regions of two roots.The area of the outside surface was smaller than that of the inside surface.No significant difference of stress value was noticed between the outside and inside surface of the periodontal membrane.The stress concentration area of the anterior slope of condyle was greater than that of the posterior slope,and the stress concentration was rarely showed up at the inner and external pole of the condyle top area.3.The stress of tooth was sensitive to the loading site,the area-size of loading region and loading direction,the stress of periodontal membrane was sensitive to the loading site and direction.The stress of condylar cartilage was more sensitive to buccal-lingual loading change in the posterior teeth region,but was more sensitive to mesial-distal loading change in the anterior teeth region.The stress of mandibular condyle was sensitive to change of loading tooth or loading direction. |
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