Finite Element Analysis Of Medial Malleolus Fracture Fixed By Hollow Screws

Background : In recent years,with the continuous development of computer science and medical imaging technology,as a new biomechanics research method three-dimensional finite element analysis has been widely applied in the field of foot and ankle surgery.Finite element technology has been playing an important role in the analysis of the occurrence of fracture mechanism,the stability of internal fixation and bone structure change.Three-dimensional finite element technology and traditional biomechanical experiment complement each other,in which way provides a rich theoretical basis for the diagnosis and treatment for orthopaedic diseases.Objective: Vertical shear fractures of the medial malleolus occur through supination – adduction loading of the talus onto the articular surface.Techniques available for fixation of these fractures mainly include hollow screws and antiglide plates.Construct three kinds of the medial malleolus fractures finite element models with different angles(60°?75°?90°)and all fractures are fixed by two hollow screws.Apply an offset axial load simulating supination–adduction loading.The major observation indexes include the maximal displacement and the stress distribution nephogram.Explore the changing rule of the fixation strength and provide biomechanical basis for the fixation techniques choice of the grade ? supination-adduction ankle fractures.Methods:Choose a healthy male volunteer and obtain the ankle CT scan images.Import the scan data in DICOM format into Mimics13.0 software.Use thresholding adjustment tool to extract the skeleton outline,define the threshold value in 230-3020 Hu,distinguish the bone and soft tissue.Generate the ankle bone model by 3D technology.Use 3D Region Growing tool to select hot spots and segment the distal tibia model.After simply smoothing the surface,the distal tibia 3D model will be eventually generated.Use the same method to establish the distal tibia cancellous bone 3D model.Use Boolean operation to generate the distal tibia cortex 3D model.Import the models in STL(Standard Template Library)format into the reverse engineering software Geogamic9.0 for Simplification,denoising,smoothing and three-dimensional curved surface configuration.Built 60 °?75 ° and 90 ° of medial malleolus fracture models fixed by hollow screws by means of Sketch cutting and Boolean operation in Mimics13.0 software.Mesh the models and save them in CDB format.Input the data into the FE modules of ANSYS software.Mesh the models again and the three-dimensional finite element models of medial malleolus fracture fixed by hollow screws will be finally created.Apply an offset axial load from 100 N to 300 N on the articular surface of the medial malleolus simulating the working condition of the talus varus crushing the medial malleolus in the supination-adduction ankle injury.Record the degree of fracture displacement and bone stress distribution.We respectively use 5.7 MP,3.9 MP and 2.2 MP to differentiate different yield strength of bone mineral density of cancellous bone,in order to describe bone cutting in the stress nephogram.Results:1 Displacement: In the same model when the loading gradually increases,the maximum fracture displacement will gradually increase.In the same load when the fracture angle gradually increases,the maximal fracture displacement will also gradually increase.When the shear loading is 200 N and 300 N,the maximal displacement of the 90 ° medial malleolus fracture model will reach to more than 2 mm.2 Stress distribution: One of the stress concentration area is the contact region of hollow screw thread with cancellous bone.When import the three shear loading into 90 °medial malleolus fracture model,a range of thread area stress suffered by the cancellous bone is more than 5.7 Mp(normal cancellous bone yield strength),which indicates that the holding force of the hollow screw tails is reduced and the unstability of the internal fixation.Conclusion:1 Fixed requirements can always be achieved by using hollow screws to fix the medial malleolus fractures when within a reasonable load.However with the increase of angle,maximum displacement of fracture presents the form that the more close to vertical direction,the more obvious.So we suggest that the vertical medial malleolus fracture should be fixed by antiglide plates.2 When the talus varus crushing the medial malleolus by using hollow screws fixing medial malleolus fracture,the stress is mainly concentrated in area of the head and thread zone of the screws.For osteoporosis patients,the bone yield strength is lower than normal adults,bone cutting seems to be more easily to appear when only using hollow screws to fix the fractures.In order to avoid the failure of internal fixation,antiglide plates seem to be more advised.3 Vertical medial malleolus fracture patients should avoid ankle rotating moment in early functional exercise,which can lead to fracture displacement.4 Medial malleolus fracture model can be established by finite element technology and orthopaedic biomechanics analysis can be taken under different experimental conditions.The operation is simple and the data is reliable.Finite element technology and traditional biomechanical analysis can verity with each other to some extent.We can suppose that finite element technology will have a good prospect.