FE and experimental study on how the cortex material properties of synthetic femurs affect strain levels.

The primary aim of this work was to validate the "numerical" cortex material properties (transversely isotropic) of synthetic femurs and to evaluate how the strain level of the cancellous bone can be affected by the FE modeling of the material's behavior. Sensitivity analysis was performed to find out if the parameters of the cortex material affect global strain results more than the Polyurethane (PU) foam used to simulate cancellous bone. Standard 4th generation composite femurs were made with 0.32g/cm3 solid PU foam to model healthy cancellous bone, while 0.2g/cm3 cellular PU was used to model unhealthy cancellous bone. Longitudinal and transversal Young's moduli of cortical bone were defined according the manufacturer data, while shear modulus and Poisson's ratios were defined from the literature. All femurs were instrumented with rosette strain gauges and loaded according to ISO7206 standards, simulating a one-legged stance. The experimental results were then compared with those from finite element analysis. When cortical bone was modelled as transversely isotropic, an overall FE/experimental error of 11% was obtained. However, with isotropic material the error rose to 20%. Strain field distributions predicted inside the two bone models were similar, but the strain state of a healthy cancellous bone was much more a compression state than that of unhealthy bone, the compression state decreased about 90%. Strain magnitudes show that average strain-levels of cancellous bone can be significantly affected by the properties of the cortical bone material and, therefore, simulations of femur-implanted systems must account for the composite behavior of the cortex, since small shear strains would develop near isotropic cancellous bone-implant interfaces. Moreover, the authors suggest that changing the volume fraction of glass fibers used to manufacture the cortical bone would allow a more realistic osteoporotic synthetic femurs to be produced.

External fixator configurations in tibia fractures: 1D optimization and 3D analysis comparison.

The use of external fixation devices in orthopedic surgery is very common in open tibial fractures. A properly applied fixator may improve the healing process while one improperly applied might delay the healing process. The several external fixator systems used in clinical today, can be categorized into uniplanar-unilateral, uniplanar-bilateral, biplanar and multiplanar. The stability on the fracture focus and, therefore, the fracture healing process, is related with the type of external fixator configuration that is selected. The aim of this study is to discuss the principles for the successful application of unilateral-uniplanar external fixation, the assembly of its components, for the case of a transverse fractures using computational models. In this context, the fixation stiffness characteristics are evaluated using a simplified 1D finite element model for the tibia and external fixator. The beams are modeled with realistic cross-sectional geometry and material properties instead of a simplified model. The VABS (the Variational Asymptotic Beam Section analysis) methodology is used to compute the cross-sectional model for the generalized Timoshenko model, which was embedded in the finite element solver FEAP. The use of Timoshenko beam theory allows accounting for several kinds of loads, including torsion moments. Optimal design is performed with respect to the assembly of fixator components using a genetic algorithm. The optimization procedure is based on the evaluation of an objective function, which is dependent on the displacement at the fracture focus. The initial and optimal results are compared by performing a 3D analysis, for which different three-dimensional finite element models are created. The geometrical model of a tibia is created on the basis of data acquired by CAT scan, made for a healthy tibia of a 22 year old male. The 3D comparison of the 1D optimal results show a clear improvement on the objective function for the several load cases and, therefore, it is shown that appropriate selection of the external fixator geometrical features can lead to an improvement on the stability of the external fixator. The results obtained show that the optimal position of the side beam and the first pin should be as close as possible to the bone interface and as close as possible to the fracture focus, respectively. Concerning the second pin, it should be placed away from the first pin in case of flexion loads, to axial and torsion loads the second pin should be placed near the first pin.