| Mandibular discontinuity defects are most often due to tumor,trauma,which lead to bad results of patient's contour,phonic and chew function.So the effective reconstruction of the mandible is necessary and important.Bone grafts,metal implant and metal mesh with bone grafts are the most popular ways of mandibular reconstruction.Reconstruction plates and bone grafts were popularly applied to bridge mandibular defects following mandibular resection.The esthetics and continuity of the mandible with segmental defects was rehabilitated successfully.However,the function is difficult to reconstructed using these reconstruction methods.Furthermore,skin or mucosal perforation,plate fracture and loss of screw retention were the serious complications resulting in reconstruction failure and leading to poor life quality of the patients.This study is designed to reconstruct shape and function of the mandible with the lateral mandibular defects.It'll be helpful to solve the problems above.Objective:From the angle of anatomy and biomechanics,this study would use the CAD technique to make a integration design of prosthesis,including structure design with dental abutments and screw holes,and fixation design,would establish the model of Bio—Mechanical Mandible(BMM)for two—staged animal experimental study and clinical application in future.To evaluate the feasibility of designing and fabricating customized titanium bone substitutes to restore Lateral Defects of Mandible using Reverse Engineering and Rapid Prototyping techniques.Materals and Methods:1.After routine cephalometrics analysis to 40 adults (Male 21,Female)with normal occlusion from Shanghai area of China,the average height of mandibular base and mandibular alveolar in the position of caine and the first molar underwent measurement.Meanwhile,the mandibular SCT data of a candidate(Male) release as DICOM file was obtained,upon which GET—POINT software for contour extraction and the seamless technique between FEA and CAD were used for 3—DFEMs development of cortical bone and cancellous bone as well as teeth.Simulating central articulation on mandibular 3—DFEM,the law of biological stress distribution in normal mandible underwent study.2.Simplified 3—DFEMs of prosthesis were established by ANSYS parameter design language(APDL)for comparison and calculation of stress distribution in 4 kinds of hollow—out design for lateral mandibular reconstruction(3—7)were developed,on which comparison and analysis of biological stress distribution were also performed under vertical loading condition.Simulating lateral mandibular defect,3—DFEMs of BMM with two types of fixation that were double abutments with lag screws style and reconstruction plate style were established,on which the stress distribution in prosthesis and fixation units as well as surrounding bone underwent comparison and analysis under 4 loading conditions.3.Nine healthy minipigs were used to this study.Beforehand Spiral CT of the mandibular region was proformed.The data collected was related to the computer.Then a 5cm mandibular body defect were designed accordingly.Furthermore,Titanium mandibular prostheses were manufactured according to the above defect.Surgical operation was performed and the prosthesis were implanted into the aimed defects and internal fixation was used.Autogenic bone graft was introduced into the holes of the angle prosthesis.Radiographic(X—ray,CT,3D—CT and MICRO—CT)examination was used to evaluate the healing of prostheses every month after operation.1.2.3.6 months later,the animal was sacrificed and the mandible specimens were sectioned for histological and biomechanical evaluation.Results:1.Biological stress on the normal mandible was distributed alone lateral and medial oblique crests and inferior borders of ramus,which consisted of so called dental force tract and muscle force tract individually.Between them was so called "zero stress line".The maximum stresses of cortical bone and cancellous bone 105MPa and 12.9 MPa respectively.2.The stress distribution of prosthesis without hollow—out design was similar to that of normal mandible mainly.Study found that the hollow—out diameter had almost no influence to the prosthetic stress magnitude.However,1 mm hollow—out space could lead to stress concentration conspicuously,2—3 mm space had no great effect to stress distribution on the prosthesis,and the horizontal space had a more great influence to stress distribution,than the vertical space.3.The stress of BMM with lag screws fixation was reasonable and well distributed,where the maximum stress located to the angle area of mandible was 223.775 MPa.There was no statistical difference in stress distribution between fixation units(F =1.42,P =0.2997＞0.01).The cancellous bone surrounding inferior abutment underwent the maximum stress over the other fixation units.However,there existed no difference in maximum stress distribution of the cancellous bone surrounding inferior abutment against the normal(T=0.0849,P =0.9377＞0.05).4.Similar to BMM with lag screws fixation,The stress distribution of BMM with plate fixation was reasonable also.The maximum stress was 129.180 MPa. But there existed no statistical difference in stress distribution between fixation units(F =2.87,P =0.0704＞0.05).4.The titanium BMM was well fitted in all minipigs.The mandible reconstructed contour and the facial symmetry were judged to be good in the minipigs based on comparisons of preoperative,postoperative photographs,X—ray,CT, 3D—CT and MICRO—CT(1m.2m.3m.6m)studies.The radiological and clinical results for these three minipigs after reconstruction of mandible bone defects with CAD—RP titanium tray were good.Complications were not observed.These minipigs do eat nomally.The stability of the postoperative mandibular bone was perfect.The 6 mm (upper),2 mm(below)width,20 mm height titanium BMM with a weight 13.7024g withstands functional forces and follows the original contour of the mandible.The other three minipigs's complications were observed,but the connection between the prosthesis and the mandible is still stable.Radiographic examination revealed that there was presence of osseointegration in the connection area between prosthesis and mandibular.Conclusion:1.Referring to the normal values of the height of mandibular base and mandibular alveolar,the height of BMM was 20 mm,the upper width was 6 mm and the below width was 2 mm.2.The law of normal biological stress distribution provided a forceful evidence for BMM design which included buttress structure along mandibular stress tract and hollow—out structure along zero stress line.3.The stress distribution of prosthesis without hollow—out design gave a evidence to BMM design.Based on the optium design results of hollow—out structure,the horizontal space of BMM was not less than 3 mm and the vertical space was not less than 2 mm.4.The integration design of BMM with lag screws fixation could satisfied the strength demands to prosthesis,also fixation design conformed the biomechanical principle of mandible.5.The BMM with plate fixation was reasonable in the aspect of strength design also,even which had more great superiority than the BMM with lag screws fixation.However,its fixation design could resulted in the disadvantsge of stress concentration.6.The model of animal experiment was manufactured by R-E software and RP machine.7.CAD/CAM generated Titanium mandibular prosthesis is applicable in human model.The vitality of this experiment revealed that a tight closure plays an important role in the healing by osseointegration.If there was any exposure,the osseointegration will be failed.After all, CT imaging,rapid prototyping and computer modeling have improved the surgery planning and the manufacturing of titanium BMM and have also achieved efficient immediate reconstructurion.The use of these techniques leads to reduced operating time and eliminates operation errors,precise fit,and high stability after screw fixation. |
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