Finite element analysis of external fixator combined with limit internal fixation and open reduction and internal fixation for Sanders type II calcaneal fracture

Abstract

Objective To compare the stability and stress distribution of external fixator combined with limit internal fixation (EFLIF) and open reduction and internal fixation (ORIF) for Sanders type Ⅱ calcaneal fractures by finite element analysis. Methods A LightSpeed 16-slice spiral CT of the Imaging Center was used to scan the volunteer’s right foot, then the scanned data was imported into Mimics 15.01 software in DICOM format, and to reconstruct the three dimensional structure of the bones of foot and ankle and surrounding soft tissue after threshold segmentation. Mimics, Patran, Solidworks were used to establish a complete finite element model of foot and ankle. According Sanders type Ⅱ calcaneal fractures, while establishing Sanders typeⅡ calcaneal fractures finite element model fixed with EFLIF and ORIF. Abaqus software was used to loading a 700 N longitudinal load stress and a 160 N maximum tensile stress from calcaneal tendon in non-weight bearing state, record the relative displacement between the fracture fragments, and stress distribution of on external fixator, screws and plate. Results When loading 700 N on the vertical, fixed with EFLIF and ORIF, the maximum relative displacement on the model are 1.462 mm and 2.174 mm, the maximum relative displacement between the fracture fragments are 0.0144 8 mm and 0.326 2 mm, both of the two maximum relative displacement between the fracture fragments are less than 1 mm. Maximum stress distribution on the EFLIF model was 102.2 mPa, less than the yield strength of the external fixator, stress distribution was mainly focused on the connection between external fixator and bone. Maximum stress distribution on the ORIF model was 2 491 mPa, bigger than the yield strength of the screw, stress distribution was mainly focused on the sustentaculum tali screw near the fracture line. When loading 160 N from the Achilles’s tendon, fixed with EFLIF and ORIF, the maximum relative displacement on the model are 1.462 mm and 2.174 mm, the maximum relative displacement between the fracture fragments are 0.333 5 mm and 0.180 8 mm, both of the two maximum relative displacement between the fracture fragments are less than 1 mm. Maximum stress distribution on the EFLIF model was 47.2 mPa, stress distribution was mainly focused on the schanz screw. Maximum stress distribution on the ORIF model was 141.9 mPa, stress distribution was mainly focused on the connection between the plate and screw. Both of them are less than the yield strength of medical devices. Conclusion The external fixator combined with internal fixation is essential for fixation of type calcaneal fracture to achieve the biomechanical stability and can be used in clinical practice to promote. Both EFLIF and ORIF could provide sufficient biomechanical stability for Sanders type Ⅱ calcaneal fractures, relative displacement in EFLIF was lesser, and the stress distribution was more reasonable. Stress concentration was occurred on the ORIF model, the screw may be out of work when loading 700 N. Key words: External fixator; Open reduction and internal fixation; Calcaneal fracture; Finite element analysis