Finite Element Analysis Of Calcaneal Sanders TypeⅡb Fracture Models With Double-parallel Screws At Different Angles

ObjectiveTo build calcaneal Sanders typeⅡB fracture models with double-parallel screws the screws angle to the long axis of the calcaneus and parallel to the long axis of the calcaneus.Moreover,to perform a finite element analysis on these two models,analyze and compare the biomechanics of fracture block and internal fixation between two double-parallel screws internal fixation methods,to provide a theoretical basis for the treatment of Sanders typeⅡB fractures of the calcaneus.MethodThe ankle CT data of a healthy adult male are imported into Mimics software for the establishment of calcaneal mesh model.The mesh model of the calcaneus is transformed into a solid model using the Geomagic software.The solid model is imported into Solid Work software to establish a calcaneus Sanders type Ⅱ B fracture model and two double-parallel screws internal fixation methods,the screws angle to the long axis of the calcaneus and parallel to the long axis of the calcaneus.To explore the displacement degree,stress distribution of the fracture block and the deformation degree and stress distribution of the internal fixation in the two internal fixation methods,both models are applied with a force of 600 N vertically downward and subjected to finite element analysis by ANSYS 17.0software.ResultsSeveral kinds of simulation models are established in this study,including:Calcaneal model,calcaneal Sanders TypeⅡB fracture model,calcaneal Sanders typeⅡB fracture models with double-parallel screws the screws angle to the long axis of the calcaneus and parallel to the long axis of the calcaneus.A complete finite element analysis condition is set for calcaneal fracture internal fixation model.Under different angle of the double-parallel internal fixation treatments,the bones’ peak deformation of the two models that the screws angle to the long axis of the calcaneus and parallel to the long axis of the calcaneus are 0.19955 mm and0.17815 mm,respectively.The maximum displacement points of the two fixation methods were both located at the calcaneo-cubic joint,and the maximum displacement is less than 1mm.Under different angle of the double-parallel internal fixation treatments,the bones’ peak stress of the two models that the screws angle to the long axis of the calcaneus and parallel to the long axis of the calcaneus are 98.618 Mpa and79.382 Mpa,respectively.They are located on the calcaneus set to be fixed to the ground.The stress peaks on the fracture surface are 19.424 MPa and 20.994 MPa,respectively.Both the high stress range of the overall and fracture surface are extremely small,both stress are uniform,and most of the stress is small.Under different angle of the double-parallel internal fixation treatments,the screws’ peak deformation of the two models that the screws angle to the long axis of the calcaneus and parallel to the long axis of the calcaneus are 0.12938 mm and0.16293 mm,respectively.Both the upper screws of two group get smaller deformation than lower screw.The maximum value of the double-screw deformation is located at the top of the lower screw near the calcaneal joint surface,which is at the small fracture.In the model that screws angle to long axis of the calcaneus,the deformation of the upper and lower screws are similar.While in the model that screws parallel to long axis of the calcaneus,the deformation of the lower screw head is greater,compare to entire screw,and the deformation of the lower screw is much greater than that of the upper screw.Under different angle of the double-parallel internal fixation treatments,the screws’ peak stress of the two models that the screws angled to the long axis of the calcaneus and parallel to the long axis of the calcaneus are 39.596 MPa and47.489 MPa,respectively.The peak stress of the junction of the screws and the large fracture piece’s surface are 8.124 MPa and 7.7081 MPa,respectively.The peak stress of the junction of the screws and the small fracture piece’s surface are4.343 MPa and 4.6912 MPa,respectively.Both groups show that the stress on the back of the foot near the pressured surface is high,and the stress decreases toward both sides.In the model that screws angle to the long axis of the calcaneus,the stress at the junctions of the upper screw and two fractures surfaces is more uniform than another model,the high stress area at the junction is found only at the junction between the lower screw and the fracture surface.In the model that screws parallel to the long axis of the calcaneus,both the upper and lower screws have high stress areas at the junction with the fracture surface.ConclusionIn the treatment of Sanders typeⅡB calcaneal fractures with double parallel screws,whether the screws are parallel to the long axis of the calcaneus or at an angle with the long axis of the calcaneus,the reduction effect can be effectively maintained and the fracture healing can be promoted.Under the condition of standing still on one foot after operation,the method of entering the screw at an angle with the long axis of calcaneus makes the stress distribution of the screw more uniform and the peak value smaller,resulting in uniform overall deformation and smaller maximum deformation,which has less risk of nail withdrawal and breakage,and better stability and safety of the internal fixation.Using finite element analysis method to study the biomechanics of fracture model has repeatability,good simulation and enough credibility,which can provide biomechanical reference for the selection of surgical methods in clinic.