Orthogonal Digital Radiographs - A Novel Template for a Paediatric Femur Finite Element Model Development

Abstract

Surgical treatment options of paediatric femur fracture include flexible or rigid intramedullary nails and submuscular plate fixation. Current computational technology has enabled virtual testing of fracture fixation implants using finite element analysis (FEA) models. Unlike FEA models based on adult femur, limited literature is available for paediatric femur. The aim of the study was to develop and validate a FEA model based on simplified geometry of composite paediatric femur using digital radiographs as a template. The model consisted of two cylinders which intersected at 130 degrees. The first hollow cylinder represented the shaft whilst the second solid cylinder represented head, neck and trochanteric region. Material properties of a composite femur (compressive strength = 157 MPa, compressive modulus = 16.7 GPa, yield strength = 93 MPa, Poisson’s ratio = 0.26) were used. Simulation testing was performed in SolidWorks to estimate axial, four-point bending and torsional stiffness (k_FEA). An experimental study was undertaken on 4 femur specimens (k_Exp). FEA model predicted axial stiffness (k_{FEA Axial} = 704.83 N/mm). In comparison mean axial stiffness of the femur specimens measured k_{Exp Axial} = 667.39 N/mm (+/- 73.49). Four-point bending stiffness of FEA model measured (k_{FEA Bending} = 369.1 N/mm) whereas the mean four-point bending stiffness of femur specimens was k_{Exp Bending} = 353.49 N/mm (+/- 16.05). Torsional stiffness in external and internal rotation of FEA model was noted at k_{FEA Torsion ER} = 3.49 N m/deg and k_{FEA Torsion IR} = 3.49 N m/deg respectively. Mean torsional stiffness of femur specimens measured k_{Exp Torsion ER} = 3.58(+/- 0.05) N m/deg and k_{Exp Torsion IR} = 3.48 (+/- 0.14) N m/deg. Orthogonal digital radiographs can be used as a template to develop a simplified finite element model of paediatric femur. FEA model based on simplified geometry may be used for evaluation of routine stiffness parameters of paediatric femur.