Construction And Analysis Of A Finite Element Model Of Patella Layered Fracture Fixed With Mini-plate

Objective: The incidence of patella fracture is higher among the patients with knee injury.For the treatment of patella fracture,most of scholars give priority to tension band,especially patella transverse or distal polar fracture,although they have proposed a wide range of treatment options.However,during clinical practice,we often encounter many patella layered fractures which are still lack of uniform treatment strategies,rarely reported about their classifications and treatments.In this regard,we put forward a new therapy for patella layered fracture fixed with mini-plate and have achieved preliminary clinical curative.Now,in order to validate its feasibility,we construct a finite element model of patella layered fracture fixed with mini-plate,by further analyzing its stress distribution and displacement,and provide reliable theory basis for this new clinical treatment scheme.Methods: This research should construct 3D model of patella?plates and screws,and perform finite element analysis by ANSYS 13.0,Mimics 17.0,3-matic 9.0,Geomagic Studio 12.0 and PTC Creo Parametric 3.0.Making 3D models: After putting the CT scan images of the knee into Mimics software,we can create patella and cancellous bone 3D model by thresholding,region growing and edit masking functions.And in Geomagic Studio,we modify the image by denoising,relaxation,packaging,fast smoothing,surfacing.Then,input the image by the STL format into 3-matic in which cortical bone model was generated by boolean subtraction operation.We design mini-plates and screws plane sketches according to the actual size by PTC Creo Parametric software,and generate one piece of 8-hole T plate and 5-hole straight plate and 11 piece of locking screws.After this,we should assemble Patella fixed model,design fracture line,and generate surface and volume mesh.Finite element parameter setting: We should import finite element model of normal patella and patella fracture into ANSYS software with CDB format,create the skin and the finite element model of geometry,and set model material properties,including Youngs' s modulus and poisson's ratio.According to the experimental requirements we can set contact area,contact relationship and redrew tetrahedral mesh.Before calculating,patella upper and lower pole should be constrained,and we should respectively apply 4.4MP,3.5MP,2.0MP stress to upon 1/3 the middle and lower area of patella articular surface to simulate the forces of the patella when knee flexion 90°,45°,20°.Image post-processing: After solved each model result in different situations,we should insert the equivalent stress and displacement distribution cloud picture,and the images should be cut in order to observe the distribution of stress and displacement on each section.Results:1 Stress distribution of normal patella model: In three kinds of knee bending angle,the ligament constraint area of patella upper and lower pole have varying degrees of stress concentration.In addition,every patellar lower pole area have stress concentration and it is positively correlated with the degree of flexion.With the increasing of the degree of flexion,the patella overall stress is gradually closer to the proximal end.2 Stress distribution of patella fracture model: In different bending angle,the ligament constraint area also have varying degrees of stress concentration.In addition,every patellar lower pole area have stress concentration and it is also positively correlated with the degree of flexion.Near the transverse fracture line,the plates have the most stress which is positively correlated with the degree of flexion,and the stress gradually extend to the ends.The stress of proximal and distal screw is also positively correlated with the degree of flexion.The greater degree of flexion,the more stress is concentrated in the proximal screws.Between both ends of transverse fracture,there are pressures which is positively correlation with the degree of flexion and is showing uneven distribution-even-uneven trends.Similarly,between both ends of layered fracture,there are also pressures which is positively correlation with the degree of flexion and is showing from even distribution to uneven trends.3 Displacement distribution of patella fracture model: In different degrees of flexion,patella and plate show similar displacement law.In the Y-axis direction(the horizontal direction),from the middle to the upper and lower poles of the patella,and patella and plate displacement decrease gradually.In the Z-axis direction(the vertical direction),from the heads of the screws to ends,the displacement increase gradually.The proximal screws show a vertical downward displacement and the distal screws show a vertical upward displacement.In the Y-axis,near the transverse fracture line,patella and plate displacements are positively correlated with the degree of flexion,and the maximum displacement is about 0.3mm when 90 ° flexion,while the maximum displacement of the layered fracture is about 0.9mm when 45°flexion.In the Z-axis,proximal screw displacements are positively correlated with degree of flexion,and the maximum displacement is about 0.2mm,while the maximum displacement of the distal screw is about 0.5mm when 45°flexion.Conclusions:1 In different degrees of flexion,patellar lower pole stress is relatively concentrated.2 Mini-plate is placed on the tension side of patella,fixed firmly,which have different levels of pressure effects for transverse and layered fracture.3 The pressure generated by mini-plate is positively correlated with the degree of flexion,which is beneficial to early postoperative activities.4 Mini-plate has significant biomechanical advantage for treatment of patella fracture,especially for patella layered fracture.