Biomechanical Analysis Of Mandibular Symphyseal Curved Defects Reconstruction By Distraction Osteogenesis

TDDO is praised as endogenous tissue engineering technology and brings a newmeans for mandibular segmental defects reconstruction. But symphyseal curved defectbrings challenges to TDDO application. The treatment success is affected by many factors.Among them, biomechanical factors closely related to therapeutic outcome. The questionsare:(1) Can the distractor-mandibular complex afford functional load, to ensure thestability of fixation?(2) How the stresses distribute at the center of the bone formationafter the traction force applied? Can produce a mechanical micro-environment conduciveto bone formation?(3) What kind of impacts to the temporomandiublar joint would behappen undertaking such distraction?(4)How the tooth on the transport disc tends tomovement during the distraction? These issues are urgently to be explained. Not yetaccess to the relevant literature to answer these questions. The purpose and significance ofthis study is to investigate the biomechanical parameters involved in the distractionprocess, trying to optimize the structure of the distractor and to explore the biomechanical law when TDDO technology is used to repair the mandibular symphyseal defects.The computer aided design (CAD) method was used to establish three-dimensionaldigital model including defective mandible, distractor, transport disk, bone formationcentre, articular disc and tooth. And then, biomechanical data was analyzed with differentdistraction rate and different forms of distractor model by finite element method.Based on biomechanical analysis of16different rails, the results show that:(1) thedistributions of stress at the rail were mainly at the rail itself and the junction of the fixedarm and the rail.(2) The rail height and thickness increasing can afford more stress andreduce displacement value of mandibular stump, but all the maximum stress values werebeyond the yield strength of titanium.(3) The stress distribution on the condylar surface isbilateral symmetry with various specifications rail fixation. Compared with intactmandible, the stress distribution patterns was changed when height*thickness of the railwas4*4mm2and6*4mm2. With the rail height and thickness increasing, stressdistribution patterns were similar to the intact mandible. And the stress value is constantlyclosed to be normal.(4) When the height and thickness are over8mm, the efficacy ofenhancing rail performance by increasing cross-sectional area should be reduced.(5) Byusing auxiliary lingual bracket, it can effectively reduce stress values, reduce displacementvalues of mandibular stump, and improve the mechanical environment of the joint area.Based on analysis fixation arm morphology and distribution of screws, the resultsindicated that:(1) by using I type fixation arm, stability and load carrying capacity are theworst, stress values in the cortical bone around the screws are higher, and stressconcentration is around only two screws.(2) by using T type fixation arm, stability andload carrying capacity are better, stress values in the cortical bone around the screws arelower than I type, and stress concentration is around three screws.(3) by using C typefixation arm, stability and load carrying capacity are the best, stress concentration isaround the mesial and distal, upper and lower four screws, and the maximum stress valuesdecreased nearly doubled.Based on stress distribution analysis of transport disk-fixation arm complexes, thedata showed that:(1) by using工type fixation arm, the arm elastic deformation occurs when the distraction amplitudes are0.1mm or0.2mm per time. The arm plasticdeformation occurs when the distraction amplitudes are0.3mm or0.4mm per time. Themaximum stress value exceeds the ultimate strength of the titanium material when thedistraction amplitude is0.5mm per time.(2) by using I type fixation arm, the arm elasticdeformation occurs when the distraction amplitude is0.1mm per time. The arm plasticdeformation occurs when the distraction amplitudes are0.2mm or0.3mm per time. Themaximum stress value exceeds the ultimate strength of the titanium material when thedistraction amplitudes are0.4mm or0.5mm per time.When use工type fixation arm, the stress concentrated around the upper distal screw,no significant stress concentration around the upper mesial and lower two screws. Whenuse I type f fixation arm, the stress concentration is around both two screws.Under the distraction force, the strain gradient is founded in generation center. Thevalues of strain are higher in the buccal and inferior border of mandible, and are lower inthe lingual and the alveolar crest. The changes of distraction amplitude and the fixationarm only affect the strain values, without changing the gradient rule.When distraction amplitude is0.1mm per time, regardless of工type or I type, thestrain value is less than300μstrain in the lingual side of generation centre, which is notconducive to osteoblast differentiation. When distraction amplitude is0.2mm per time,both工type and I type, the strain value is conducive to osteoblast differentiation. Whendistraction amplitude is0.3mm per time, the strain is feasible by using工type, but byusing I type, the strain value was3011μstrain in inferior border of mandible, which mayleads to fibroblasts differentiation. When distraction amplitude is0.4mm and0.5mm pertime, regardless of工type or I type, the strain value beyond3000μstrain, which mayleads to fibroblasts differentiation.The results of tooth movement analysis show that: When distraction amplitude is0.5mm per time, the tooth on the transport disk tends to rotate towards distal direction.The maximum stress (60KPa) in the periodontal ligament locate in the distal cervical area,and distribution following distal area>mesial area, cervical area>apical area, lingual side>buccal side. After150g antagonistic force application, displacement contour is almost parallel to the long axis of the tooth, which manifested as tooth parallel shift to thedistraction direction. Although antagonistic force application did not change the stressdistribution law in the periodontal ligament, the maximum stress value dropped to0.117KPa.The conclusions of this study are:(1) The rail and the fixation arm is the mainload-bearing structure. By increasing the height and thickness, the distractor’s carryingcapacity and fixation stability can be increased, and the mechanical environment of thecondyle can be improved. However, the risk of breakage, the fistula occurrence anddistractor exposure cannot be reduced. And the harmful mechanical factors which lead toinduce TMD cannot be eliminated.(2) When the height and thickness are over8mm, theefficacy of enhancing rail performance by increasing cross-sectional area should bereduced. It is an effective way that the auxiliary lingual bracket is adapted.(3) Theregulation of the fixation stability and carrying capacity of fixation arm and screwsfollowing I type（2screws）<T type（3screws，triangle distribution）<C type（6screws，rectangle distribution）. So, C type is better meeting the requirements of the biomechanics.(4) When use工type fixation arm,0.2mm or0.3mm per time distraction amplitude canbe chosen. When use I type f fixation arm,0.2mm per time should be chosen. Otherwise,the micro-environment is not conductive to bone formation when the distractionamplitudes are set as0.1mm,0.4mm or0.5mm per time.(5) If there is tooth on thetransport disk, it should be applied an antagonistic force to the tooth, preventing therotation of the teeth in the opposite direction.Combined with a review of the literature and experimental data, this study provide anumber of forward-looking recommendations on TDDO technology applied to repair themandibular symphyseal curved defect, such as the distractor structure optimization design,the use of the auxiliary lingual bracket to enhance fixation stability, the suitable distractionamplitude and antagonistic force is applied to the teeth. And from biomechanical aspects,the present study try to explain some puzzled problems which were found in animalexperiments and clinical, such as distractor breakage, fistula and bracket exposures, TMD,osteodysgenesis and tooth movement towards the opposite direction.