| Objective: The skeletal mandibular retrusion mainly appears class?,skeletal malocclusion and has some influence on oral and maxillofacial functions a-nd appearance of the patients.To get the best individual occlusal states and coord-inated outlines of soft maxillofacial tissues,adult patients are always treated with the combined therapies of orthodontics and orthoganthic surgeries.Extensive re-searches have indicated that there was a close relationship between orthoganthic surgery and temporomandibular joint(TMJ),the influence on TMJ made by orthoganthic surgery is an issue continually concerned by some academic circles,however,they haven't formed a unified conclusion till now.The finite element method(FEM)has many advantages which the direct experiment analyses doesn't have,it has been used in the studies of dental biomechanics largely.In the ex-periment,the finite element(FE)models of TMJs,mandibles and masticatory muscle systems of the skeletal mandibular retrusion before and after orthognathic surgeries will be established by three-dimensional modeling softwares and the finite element analysis software,long-term effects on TMJ biomechanics caused by orthoganthic surgery will be basically made clear byFEM.Methods:One adult male patient was selected who diagnosed definitely with mere skeletal mandibular retrusion when the dental decompensation complete,the DICOM format data of plain CT scan and three-dimensional reconstruction of the maxillofacial region was obtained after the head was scanned by the 16 la-yer X ray computerized tomography device.Mimics,Geomagic Studio and Uni-graphics NX were synthetically applicated,all other images were wiped besides the images of mandible and the teeth on it,bilateral articular fossae and temporal bones,the model was sagittal split in the middle,one side(the left side)of the mo-del was selected for the research,periodontal membranes with 0.2 mm in thicken-ss were established on the clinical teeth roots on the left mandible,cartilages of 0.3mm on the condylar process and 0.5mm on the articular fossa in thickness,the disc was generated according to the gap between the condyle and the articular fossa,the condyle was divided into 4 regions-the front bevel,the transverse ridge,the back bevel and the neck of the condyle,the articular fossa was divided into 5 regions-the anterior,the posterior,the central,the medial and the lateral regions,adherent regions of the masseter,the temporalis,the musculi pterygoideus internus and the musculi pterygoideus were noted on the model and the directions of the masticatory muscles were confirmed,this model was recorded as thepreoperative model.In the softwares mentioned above,the states of the complete healing of the discontinuous regions after the plates for fixations and screws were removed which caused by sagittal split ramus and advancements were simulated on the preoperative models,the mandible fronted distances are successively4,8,10 mm,the primary directions of the masticatory muscles were become to the ult-imate along with the changing of masticatory muscle attachment regions,the new discs were generated according to the gaps between the condyles and the articular fossae,the corresponding models were respectively recorded as the postoperative model 1,2,and 3.In Workbench,contact types,interface types,friction coefficients of all components in the preoperative model and the postoperative models1,2 and 3 were set and the the tetrahedral meshing was complete,then the masticatory muscle strengths and the regions of the models where the strengths acted on were set according to the maximum masticatory stengths of human masseter,tempora-lis,musculi pterygoideus internus and tmusculi pterygoideus after the fixed constraints and the material properties were set.Results:1.The FE models of TMJs,mandibles and masticatory muscle systems of the skeletal mandibular retrusion were established before and after orthognathic sur-geries when the discontinuous regions healed absolutely after the plates for fixations and screws were removed which caused by sagittal split ramus and advancements,the mandible fronted distances were successively 4,8,10 mm,the FE mod-els had preferable geometrical similarities with the real TMJs and mandibles,the meshing was very detailed and could reflect the true states to a great extent when stressed.2.The Von Mises stress nephograms of TMJs and the maximum Von Mises stresses of the components of the TMJs of skeletal mandibular retrusion in the states of the maximal masticatory muscle strengths were obtained before and aft-er orthognathic surgeries when the the discontinuous regions healed absolutely after the plates for fixations and screws were removed which caused by sagittal split ramus and advancements,the mandible fronted distances were successively 4,8,10 mm mentioned above.The maximum stress areas were located on the posteromedial regions of the condylar neck before and after orthognnathic surgeries all the way,the maximum Von Mises stress was the highest-106.26 MPa when the mandible fronted distance was 4mm.The maximum Von Mises stress areas of the condylar processes were located on the lateral regions of the edges of the front bevels and the necks before and after orthognathic sugeries,the maximum VonMises stress was the highest-21.898 MPa when the mandible fronted distance was 8mm.The maximum Von Mises stress area of the articular fossa was located on the anterior region,the maximum Von Mises stress was the highest-15.729 MPa when the mandible fronted distance was 10 mm.The Von Mises stresses of the posterior and lateral regions of the articular fossae were increased along with the increasing of the mandible fronted distance.The ranges of the maximum Von Mi-ses stresses of the transverse ridge,the posterior bevel and the central region of the articular fossa were narrower than other regions.The range of the increased maximum Von Mises stresses of the medial region of the articular fossa was wid-er than other fronted distances when the mandible fronted distance was from 4mm to 8mm.The range of the increased maximum Von Mises stresses of the m-edial region of the articular fossa was wider than other fronted distances when t-he mandible fronted distance was from 4mm to 8mm.The range of the decreased maximum Von Mises stresses of the condylar neck was wider than other fronted distances when the mandible fronted distance was from 8mm to10 mm.The Von Mises stresses of the condyles were significantly greater than those of the articul-ar fossae in total.Conclusions:The FE models with preferable geometrical and mechanical si-milities of TMJs,mandibles and masticatory muscle systems of the skeletal ma-ndibular retrusion before and after orthognathic surgeries can be established by synthetically used of Mimics,Geomagic Studio,Unigraphics NX and Workbench.Before and after orthognathic sugeries,condylar neck is always the mostconcentrated site of TMJ in stresses,the anterior bevel of condyle is the mainly functional surface.In some ranges,with the increasing of the mandible fronted distance,the size of the contact surface of condyle and the posterolateral region of articular fossa becomes larger and the stresses of the region become higher.T-he standard orthognathic surgery generally doesn't affect the condyle and articular fossa of the patient with skeletal mandibular retrusion negatively in the l-ong-term. |
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