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Background: Stress shielding in the cemented hip prosthesis occurs due to the mismatching in the mechanical properties of metallic stem and bone. This mismatching in properties is considered as one of the main reasons for implant loosening. Therefore, a new stem material in orthopedic surgery is still required. In the present study, 3D finite element modeling is used for evaluating the artificial hip joint stem that is made of Function Graded (FG) material in terms of joint stress distributions and stem length. Method: 3D finite element models of different stems made of two types of FG materials and traditional stems made of Cobalt Chromium alloy (CoCrMo) and Titanium alloy (Ti) were developed using the ANSYS Code. The effects on the total artificial hip joint stresses (Shear stress and Von Mises stresses at bone cement, Von Mises stresses at bone and stem) due to using the proposed FG materials stems were investigated. The effects on the total artificial hip joint system stresses due to using different stem lengths were investigated. Results: Using FG stem (with low stiffness at stem distal end and high stiffness at its proximal end) resulted in a significant reduction in shear stress at the bone cement/ stem interface. Also, the Von Mises stresses at the bone cement and stem decrease significantly when using FG material instead of CoCrMo and Ti alloy. The stresses' distribution along the bone cement length when using FG material was found to be more uniform along the whole bone cement compared with other stem materials. These more uniform stresses will help in the reduction of the artificial hip joint loosening rate and improve its short and long term performance. Conclusion: FE results showed that using FG stem increases the resultant stresses at the femur bone (reduces stress shielding) compared to metallic stem. The results showed that the stem length has significant effects on the resultant shear and Von Mises stresses at bone, stem and bone cement for all types of stem materials. [ABSTRACT FROM AUTHOR]

Abstract: In previous work by Fouad (Medical Engineering and Physics 2010 ), 3D finite element (FE) models for fractured bones with function-graded (FG) bone-plates and traditional bone-plates made of stainless steel (SS) and titanium (Ti) alloy were examined under compressive loading conditions using the ABAQUS Code. In this study, the effects of the presence of the torsional load in addition to the compressive load on the predicted stresses of the fracture fixation bone-plate system are examined at different healing stages. The effects on the stress on the fracture site when using contacted and non-contacted bone-plate systems are also studied. The FE modelling results indicate that the torsional load has significant effects on the resultant stress on the fracture fixation bone-plate system, which should be taken into consideration during the design and the analysis. The results also show that the stress shielding at the fracture site decreases significantly when using FG bone-plates compared to Ti alloy or SS bone-plates. The presence of a gap between the bone and the plate results in a remarkable reduction in bone stress shielding at the fracture site. Therefore, the significant effects of using an FG bone-plate with a gap and the presence of torsional load on the resultant stress on the fracture fixation bone-plate system should be taken into consideration. [Copyright &y& Elsevier]

Abstract: In the present study, 3D finite element models for fractured bones with function-graded (FG) bone-plates and traditional bone-plates made of stainless steel (SS) or titanium (Ti) alloy are generated using the ABAOUS code. The predicted Von Mises stresses at the fracture site and underlying bone-plate are examined at different healing stages. The effects on the predicted Von Mises stresses at the fracture site of the presence of a gap between the plate and fractured bone are also studied. Based on the analytical results, it is found that the stress shielding at the fracture site at the fully healed stages decreases when using FG bone-plates compared to Ti alloy or SS bone-plates. In the initial healing stages, the Von Mises stresses at the fracture site increase (stress shielding decreases) by 17% and 11% when using FG bone-plates as compared to SS bone-plates for contacted and non-contacted bone-plate system, respectively. The significant effects of using an FG bone-plate with a gap on the resultant Von Mises stresses on the bone underneath the plate and on the bone stress shielding should be taken into consideration during fractured bone fixation. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, both experimental work and Finite Element Modeling (FEM) have been used to study the effects of notch geometry on the stress/strain behaviour and fracture of the ultra-high molecular weight polyethylene (UHMWPE) at quasi static conditions. The effects of notch profile radii on UHMWPE fracture behaviour, predicted stress/strain distribution, triaxial state of stress and hydrostatic pressure across the neck have been studied. Different techniques have been used to study the effects of notch geometry on the UHMWPE properties. Differential Scanning Calorimetry (DSC) has been used to study the changes in the tested material crystallinity. The universal testing machine has been used to measure the changes in the tested material mechanical properties. The SEM has been used to examine the change in the tested material fracture surface. Finally, the Finite Element Code (ANSYS10) has been used to investigate the effects of notch geometry on the predicted stress/strain distribution across the neck. The results show a notch strengthening behaviour for the tested material, where the axial yield properties increased significantly with the reduction of notch radii, while the axial ultimate properties decreased significantly for the notched specimens compared with plain ones. Also the predicted stress/strain distribution and stress triaxiality show a strong dependence on the notch geometry. The remarkable effects of notch geometry on the predicted stress/strains distributions across the neck show the importance of careful design of UHMWPE artificial joint components with the aim to eliminate the presence of stress risers such as undercuts and fillets. [Copyright &y& Elsevier]

A two-dimensional finite element based overland flow model was developed and used to study the accuracy and stability of three numerical schemes and watershed parameter aggregation error. The conventional consistent finite element scheme results in oscillations for certain time step ranges. The lumped and the upwind finite element schemes are tested as alternatives to the consistent scheme. The upwind scheme did not improve on the stability or the accuracy of the solution, while the lumped scheme provided stable and accurate solutions for time steps twice the size of time steps needed for the consistent scheme. A new accuracy based dynamic time step estimate for the two-dimensional overland flow kinematic wave solution is developed for the lumped scheme. The newly developed dynamic time step estimates are functions of the mesh size, and time of concentration of the watershed hydrograph. Due to lack of analytical solutions, the time step was developed by comparing numerical solutions of various levels of discretization to a reference solution using a very fine mesh and a very small time step. The time step criteria were tested on a different set of problems and proved to be adequate for accurate and stable solutions. A sensitivity analysis for the watershed slope, Manning’s roughness coefficient and excess rainfall rate was conducted in order to test the effect of parameter aggregation on the stability and accuracy of the solution. The results of this analysis show that aggregation of the slope data resulted in the highest error. The roughness coefficient had a smaller effect on the solution while the rainfall intensity did not show any significant effect on the flow rate solution for the range of rainfall intensity used. This work pioneers the challenge of providing guidelines for accurate and stable numerical solutions of the two-dimensional kinematic wave equations for overland flow. [Copyright &y& Elsevier]

Solving the kinematic wave equations for overland flow using the conventional consistent Galerkin finite element scheme is known to result in numerical oscillations due to the non-symmetric first spatial derivative terms in the kinematic wave equations. In this paper the lumped and the upwind finite element schemes are evaluated as alternatives to the consistent Galerkin finite element scheme. Stability analysis of the upwind scheme shows that the damping effect, that could reduce the oscillations, is small for the high Courant numbers encountered in overland flow problems. The upwind scheme, using upwind factors of 0.1 and 1.0, did not provide any improvement to the stability of the lumped and the consistent schemes. The lumped scheme considerably reduces oscillations without significant reduction in the overall solution accuracy. No analytical guidelines for time-step criteria that will insure stability and accuracy were provided by the stability analysis performed for the three schemes. Problem specific accuracy-based dynamic time-step criteria was developed and evaluated for the lumped scheme. These time-steps were found to be on average, double the size of the dynamic time-steps for the consistent scheme. [Copyright &y& Elsevier]