| BackgroundSagittal split ramus osteotomy (SSRO) is one kind of intraoral approach orthognathic surgery to plastic the deformity of mandible, which has been put forward earliest and most common used in clinic nowadays. It is mainly used to plastic the micromaxillary deformity, mandibular protrusion or complicated jaw deformity combining with other surgical methods. The advantages of SSRO are as follows:the intraoral approach could avoid leaving over the operation scar on the face; bodily movement of the mandible and the denture could preserve the integrity; major contact surface of the bone would be beneficial to promote the healing of the bone. Thus SSRO has been identified as the milestone of the phylogeny of the orthognathic surgery. However, the short of sagittal split ramus osteotomy is that there are some complications during or after the surgery, such as inferior alveolar nerve irreversible damage, fracture of the mandible and recurrence mandible deformity, etc. SSRO is not only to plastic the maxillofacial deformity and improve the occluding relationship, but also to change the biomechanics of the muscle tissue and bone tissue of the maxillofacial region, consequently to influence the fixed stability after the SSRO. In addition, the complication of the displacement of proximal bone with condyle process leading to temporomandibular disorders may also happen often. After the fixed conception of rigid internal fixation (RIF) put forward, there have been many fixing means of SSRO appeared in clinic. Although a lot of scholars domestic and overseas have researched the different fixed forms of SSRO from different ways, until now there is still no final conclusion about with the least complications and the most stable fixed method.At present, the clinical review, animal experiment and computer-assisted simulation experiment have been used for the stability research methods of SSRO. But the clinical experiment, animal model and in vitro model study would hardly copy the complicated biomechanics environment of the maxillofacial region. Since the application of the three-dimensional finite element method (FEM) developed rapidly, it laid a solid foundation for the study about the stability of SSRO. Through finite element method, we could rebuild the mandible3D model by CT data, simulate the SSRO surgery with different fixed ways, and we also could analysis the biomechanics characteristics of the mandible after the loading of the occlusal force. It can not only authentically reproduce the geometrical morphology of the mandible and simulate the force environment, and also avoid the limitation of the mechanics measurement in clinical trial or in vitro experiment.ObjectiveIn this study, we used the three dimensional reconstruction software and finite element calculation software to rebuild the3D models of the mandible and the fixed system. And the SSRO was done on the models with three different fixed methods which frequently-used in clinic to fix the bone stump. To built the3D finite element models of SSRO and provide a favorable research foundation for the biomechanical study of the sagittal split ramus osteotomy. And it would offer a convenient and efficient simulation method and the data support of the evaluation about expected effect. On the base of this study, to conduct the occlusal force loading on the3D finite element models of SSRO and analyze the stability of the three different fixed forms. It can supply a theoretical support for the selection and improvement of the fixed forms of SSRO and reducing the postoperative complications in clinical.Materials and MethodsPart I The establishment of digital model of the bilateral sagittal split ramus osteotomy1. Object of study:a healthy adult female volunteer,25years old, complete dentition and normal occluding relationship, without disease or paramorphia of mandible or temporal mandibular joint.2. Scanning mode of sample:64-slice computed tomography was used to conduct the continuous horizontal scanning on the head. The multi-slice CT scanner is SOMATOM Definition CT, the product of SIEMENS Company of Germany. The scanning parameters:the slice thickness was1mm, the tube current was200mA, and the voltage was120kV. The object of study was lying on her back on the CT scanister. the cross positioning lightbeam was as the criterion. The frankfort plane was vertical with the horizon. And the time base was parallel with the frankfort plane. The scanned area was from the frontal sinus to the soft tissue of chin. The image data was saved as the form of DICOM.3. Establishment of the3D entity model of the preoperative mandible:the head image data of CT was input into the three dimensional reconstruction software Mimics. The3D model of the cortical bone, the cancellous bone and the teeth were rebuilt respectively. After a series of image processing as smoothing, polishing and denoising, the three dimensional figure was generated as IGES file format.4. Establishment of the3D entity models of the internal fixation system:we used the vernier caliper to measure the three dimensional data of the four-hole plat,8mm unicortical screw and bicortical screw (Wlorenz Company, the States). The rebuilt of the screws used the simplified cylinder models. The plate and screws were rebuilt by the three-dimensional CAD software Solidworks.5. Establishment of the3D entity models of the sagittal split ramus osteotomy: importing the data of the preoperative mandible into ANSYS Workbench, the Workbench was used to conduct the Boolean operation of the components and split the bilateral mandible ramus, prepose the distal bone segment.6. Establishment of the SSRO model with three different internal fixed methods: inputting the data of the3D models of the plate and the screws into the SSRO model, they were fitting together with the mandible according to the fixed points of three frequently-used fixed methods in clinical.Pattern I:One four-hole plate was put on the midcourt line of the vertical osteotomy line and perpendicular to the vertical osteotomy line, fixed by four unicortical screws; Pattern Ⅱ:One four-hole plate was put on the midcourt line of the vertical osteotomy line and perpendicular to the vertical osteotomy line, fixed by four unicortical screws, one bicortical screw fixed under3mm from the alveolar border far-end of the second molar; PatternⅢ:One bicortical screw fixed under3mm from the alveolar border far-end of the first molar and second molar respectively, and another bicortical screw fixed under20mm from the screw under the alveolar border of the first molar, which was parallel to the vertical osteotomy line, the three screws were arranged as inverted L.7. Finite element mesh generation of the3D solid model:the whole mandible model and fixed system were all used Solid187entity unit tolerant in Workbench. The unit dimension of the mandible model took2mm-5mm, and which of the fixed system took2mm. Then we got the SSRO three dimensional finite element models with three different internal fixed ways.PartⅡ Biomechanical analysis of the sagittal split ramus osteotomy with three different fixed methods1. Boundary conditions:Fixed and restrained the bilateral condyle processes, and conducted the finite element analysis of static mechanics. Selected the condylar surface which needed to be constrained in the Workbench, we set the boundary conditions of the model.2. Contact settings:The mandibular cortical bone was completely fixed with the cancellous bone. The teeth contacted with the cortical bone as bonding, so did the adjacent teeth. And the titanium plate and the titanium screws were set to bond with the bone after installing into it, too. The contact between the splitting surfaces was set to be no separation.3. Occlusal force loading and the biomechanics analysis of the mandibular bone: Simulated the static load on both sides of the first molars of132N and proceeded the calculation and analysis of the equivalent stress and the displacement about the normal mandible and sagittal split models with three different faxed methods. And the same sites were marked at the bone junction to compare the different biomechanical characteristics of different fixed ways.4. Biomechanics analysis of fixed system:After occlusal loading, extracted the stress distribution, stress concentration and displacement of the titanium plate and screws under three different fixed ways and analyzed the biomechanics characteristics. We compared the three kinds of fixation stability of the fixed way combining with bone biomechanics analysis.Results1. Through the precision CT scanning of the healthy adult volunteer’s mandible, we used3D reconstruction software Mimics successfully rebuilt the normal mandible three-dimensional entity model including complete denture, cortical bone and cancellous bone, with good geometrical similarity.2. We reversely reconstructed the3D model of fixed system for the experiment (8mm unicortical screw,12mm bicortical screw, and four-hole titanium plate). In order to avoid the local distortion of larger stress concentration leads to the result distortion, the two kinds of titanium screws adopted simplified cylindrical model.3. On the base of the3D solid model of the normal mandible, we did the sagittal split ramus osteotomy with fitting the fixation appliances into the bone according to the fixed points of three different ways. We got the3D solid model of SSRO with three kinds of fixed way. After finite element mesh generation, the three dimensional finite element models were established. 4. The biomechanical characteristics of the normal mandible and SSRO with three different fixed methods under the occlusal loading on both sides of the first molars:According to the von mises distribution, the stress concentration scope were larger in Pattern Ⅰ and Pattern Ⅱ, which concentrated mainly on the mandibular ramus trailing edge, condyle process, sigmoid notch and mandibular ramus eading edge, and stress concentration was also obvious at the fixed region. The stress distribution of PatternⅢ was more homogeneous than the other two ways, and the maximum principal stress was also lesser than Pattern Ⅰ and Pattern Ⅱ, which was close to the stress distribution of the normal mandible. The maximum von mises in three fixed methods:Pattern Ⅲ<Pattern Ⅰ<Pattern Ⅱ. Therefore, the stress concentration in the front two fixed ways might affect early stage bone healing at the fixed region. The stress of PatternⅢ was uniform and it was the minimum one at the bone joint area, beneficial to the early stage healing. Otherwise, compared with the normal mandible, there was markedly increased scope of maximum von mises near the condyle process of all the three fixed methods.According to the distribution of displacement, the maximum displacement appeared at the mandibular forepart. Through the dot deformation marked near the splitting bone area can be obtained:the maximum displacement value PatternⅢ<Pattern Ⅱ<Pattern Ⅰ. The maximum displacement of PatternⅢ was the least one than the other two methods, as well as the same dot deformation marked. The equivalent displacement distribution of PatternⅢ at the field of operation was relatively thinner than the other two and the value was lesser too. Consequently, the fixed effect of bicortical screws as "reversed L" fixation way was the most stable, which could best resist the rotary force of the occlusal force loading, also in favor of the fixation of the fixed device, to avoid the screws loosing.5. Stress and displacement analysis of fixed system:the maximum displacement of Pattern Ⅰ was at the front end of the plate and the front screw, the maximum stress was at the leading edge of the hole of the proximal side near the splitting line and the corresponding screws contacted with it. The distribution situation of Pattern II was like Pattern I, but the maximal displacement value is less than the former; the maximum stress value is mainly located at the bicortical screw close to the superior border of mandible, its distribution is more uniform compared with Pattern I. The maximum displacement of PatternⅢ located at the screw close to the lower edge of mandible, the maximum stress was at the contacted area of the screw which close to the superior border of mandible, distal bone segment. However, the overall distribution if relatively uniform and the maximum stress was significantly less than the former two.Conclusions1. Though3D reconstruction software Mimics combined with three-dimentional CT scanning data, the3D entity mandibular model we rebuilt had excellent geometric similarity, after processing we could separately set up the complete denture which composed of a single tooth, the cortical bone and cancellous bone. Using reverse engineering software Geomagic to conduct smoothing, grinding, denoising and a series of image processing, the three-dimensional entity model was more close to the reality, which laid a good foundation for the further mandibular biomechanical characteristics analysis.2. We used the3D CAD design software Solidworks to reconstruct the faxed system, which conducive to fix the fractured bone after mandibular sagittal splitting, then the three-dimensional entity model of SSRO was set up and biomechanical analysis was done.3. Through the function of ANSYS Workbench, the mandibular sagittal split ramus osteotomy was successfully simulated in the3D entity model of normal mandible. With fitting the3D entity model of fixed system, we got the3D entity model and3D finite element model of SSRO with three different fixed ways. It provided the effective data supporting of convenient means of simulation and the evaluation of expected effect for the individualized treatment of mandibular sagittal split ramus osteotomy in clinical. 4. Through the stress and displacement deformation of the mandible and the fixed system for target as the evaluation basis, this experiment respectively confirmed that the stress distribution of reversed L fixed method of sagittal split ramus osteotomy was the most uniform, and the distribution at the splitting region was minimum without obvious stress concentration, which could best effectively resist all the rotary force from each direction generated by occlusal force loading. It was the most stable fixed than the other two ways, which was closest to the biomechanical characteristics of the normal mandible. It was more beneficial to promote early healing of surgical area, to avoid future recurrence of deformities. This investigation provided the theoretical support for the selection and improvement of fixed mode, reduction of intraoperative complications of mandibular sagittal split ramus osteotomy in clinical. |
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