Optimal Design And Biomechanical Study Of 3D-Printed Titanium Plates In The Reconstruction Of Mandibular Defects With Fibula

Background:Tumors,inflammation,and trauma are the primary causes of segmental defects of the mandible,which have a serious impact on oral function and facial appearance.In order to restore the continuity of the mandible,guarantee oral function,and maintain facial appearance,the anatomical structure of the mandible must be reconstructed.Vascularized bone grafting,which requires titanium reconstruction plates for fixation and mechanical support for strong internal fixation,is the gold standard for the reconstruction of segmental mandibular defects.Commercial titanium reconstruction plates encounter problems such as stress concentration,stress fatigue,erroneous profile bending,and longer operation times,when they are being bent to fit the mandibular profile.3D-printed titanium plates can avoid above problems,which are primarily produced using the electron beam melting(EBM)and selective laser melting(SLM)technologies.While SLM technology has greater forming accuracy and a wider range of print sizes than EBM technology,its high-energy laser scanning melting and lamination process will quickly melt and solidify the titanium alloy powder,and the abrupt drop in temperature will easily introduce local stress concentration and other defects,making it challenging to achieve the theoretical yield strength of 847 MPa for medical grade V titanium(TC4)materials.The safety of clinical use cannot be ensured due to the lack of a systematic evaluation of the mechanical properties of SLM titanium reconstruction plates.Moreover,thickness is a crucial structural factor influencing the reconstruction effect of titanium reconstruction plates.Several areas of the mandible are susceptible to various forces under occlusal circumstances,necessitating various mechanical supports of 3D-printed titanium plates.In a word,the safety and long-term regression of mandibular repair and reconstruction depend on how to demonstrate a safe range of 3D-printed titanium plate thickness to reduce its stress shielding effect on the fibula,the occurrence of mechanical complications,and the risk of postoperative exposure.Objective:(1)To evaluate the mechanical properties and the security of using SLM titanium reconstruction plates in clinical situations;(2)To assess the safe range of titanium reconstruction plate thickness for fibular reconstruction in different types of mandibular defects and to clarify the mapping relationship between the stress-strain of titanium reconstruction plates and fibula and the thickness in different types of mandibular segmental defects;(3)After thickness topology optimization,to investigate the effects of volume reduction on the mechanical safety of titanium plates for clinical applications.Methods:(1)With reference to commercial titanium plate parameters,five sets of standard titanium plates with thicknesses of 1.1 mm,1.4 mm,1.7 mm,2.0 mm,and 2.3 mm were designed and manufactured through SLM.To forecast failure region of titanium plates,finite element analysis(FEA)was performed in Ansys Workbench software.A universal testing machine was used to conduct tensile and bending tests and plot the stress-strain curve.A scanning electron microscope was used to inspect the fracture layer of a failed titanium plate,and explore the reason of the fracture.(2)Three major types of mandibular defects were established using the six-stage fibula reconstruction method:mandibular angle defects(T1),mandibular angle and condyle defects(T2),and defects not involving the mandibular angle and condyle(T3).Each major type of defects was further subdivided into four subclassifications based on the occlusal condition and the extent of the defects,which were recorded as Cl-4.Titanium reconstruction plates of five different thicknesses—1.1 mm,1.4 mm,1.7 mm,2.0 mm,and 2.3 mm—were used to fix each group.The results were utilized to evaluate the clinical safety.(3)The fibular reconstruction model(T3C3)with the defect involving one molar region and bilateral anterior areas was selected,and it was fixed with the clinically used thickest 2.8 mm titanium reconstruction plate.The topology optimization of the model was conducted in Workbench software.The constraints were an upper limit of 800 MPa and to retain 60%of the titanium plate volume.The scope of action should include all titanium plate volumes except the bone-plate contact surface.Based on the topology optimization results,in areas where the thickness of the titanium plate had a low contribution to the strength of the titanium plate,the thickness of the titanium plate should be replaced with 1.1 mm,1.4 mm,and 1.7 mm,which correspond to type A,type B,and type C composite titanium plates,respectively.In order to examine the fixing effect and the maximum stress-strain between the titanium plate and the fibula in seven sets of fibula reconstruction models,finite element analysis was done on 1.1 mm,1.4 mm,1.7 mm,2.8 mm titanium plates,and three composite titanium plates.Results:(1)The fracture area of the titanium plate in tensile test was concentrated in the region of the screw hole,which was consistent with the stress concentration point in finite element analysis.The mechanical strength of different titanium plate thicknesses varied statistically,while the yield strength of SLM titanium plate was 330.21±10.20 MPa,the tensile strength was 406.64± 11.80 MPa,and the flexural strength was 838.42±60.03 MPa.The microscopic morphology of the titanium plate section that failed the tensile test exhibited overall ductile tough nest fracture mechanical characteristics,step cracks in the 1.7 mm and pore-like structures in the 1.4 mm sections,and partial melting of the titanium powder in some sections,according to SEM results.(2)Titanium plate thicknesses with a maximum stress exceeding 800 MPa were 1.1 mm for T1C2 and T1C4,1.1 mm and 1.4 mm for T2C2,1.1 mm,1.4 mm,and 1.7 mm for T2C4,1.1 mm titanium plates for T3C2,1.1 mm and 1.4 mm for T3C3,and 1.1 mm titanium plates for T3C4.The maximum stress-strain decreased with increasing thickness,the stress concentration area of titanium plate coincided with the maximum strain region,and these regions were the mandibular angle,the fibula-mandibular joint,and the fibula-fibula joint.The sequence of the stresses from greatest to smallest was the mandibular angle area,fibula-mandibular joint area,and fibula-fibula joint area.High strain values that could cause bone damage were observed at the mandibular angle region around the fixation nail holes and at the extremities of the fibular segment,with T1C2,T2C2,and T2C3 showing higher strains of bone damage than the other types.Based on the findings of these analyses,3D-printed titanium plates with the proper thickness were chosen in the light of the stresses in patients with fibular reconstruction of mandibular defects.At a 6-month postoperative follow-up,all cases showed no complications related to titanium plates,demonstrating good bone healing between the fibula and the fibula and between the fibula and the mandible.(3)The highest stresses were 1045.884 MPa,1063.09 MPa,679.605 MPa,and 459.206 MPa for titanium plates with thicknesses of 1.1 mm,1.4 mm,1.7 mm,and 2.8 mm,respectively.Maximum stresses for types A,B,and C were 522.874 MPa,517.248 MPa,and 611.037 MPa,respectively.These values were 13.9%,12.6%,and 33.1%higher than those for 2.8mm titanium plate,and 51.1%,51.3%,and 10.1%lower than those for 1.1mm,1.4mm,and 1.7mm titanium plate,respectively.However,they were all less than the yield strength of 847 MPa in the ideal situation of TC4 material.The volume of the type A,type B,and type C plates were reduced by 15.21%,13.21%,and 9.43%,respectively,compared to the 2.8 mm thickness titanium plate,while the maximum peroneal strain was decreased by 48.1%,35.2%,and 25.4%for the type A,type B,and type C plates,respectively,compared to the 1.1 mm,1.4 mm,and 1.7 mm titanium plates.Conclusion:(1)SLM titanium reconstruction plates have superior mechanical qualities to commercial titanium plates while less than TC4 material,and could therefore satisfy the requirements of clinical application.(2)The mechanical characteristics of the 3D-printed titanium reconstruction plate and their effect on the physiological environment of fibula should be considered while implementing mandibular reconstruction.The thinnest titanium plate can be chosen in conjunction with the stress-strain value of the titanium plate when there is no obvious bone damage strain in the fibula after reconstruction;it is safer to choose a 2.3 mm thick titanium plate when there is bone damage strain in the fibula.This was based on the idea that the titanium plate must have adequate mechanical properties.(3)The composite titanium plate had mechanically secure features in terms of stress-strain that was comparable to those of a 2.8 mm titanium plate.This represented a new sensible approach for the creation of 3D-printed titanium plates.