James Shih-Shyn Wu

The computer simulation of wear behavior appearing in total hip prosthesis

Computer algorithms are proposed for the estimation of wear appearing in artificial hip joints using finite element analysis based on the modified Archards wear law, contact features and an analogue wear process. A pin-on-disk plate experiment is reconstructed to assess the efficiency and validity of the algorithms proposed here. Through the successful verification of wear depth and volume loss of the pin-on-disk plate as well as the artificial hip joint, the current algorithms provide significant agreement with experiments, clinical measurements and numerical calculations and are shown to be both valid and feasible. Further investigation into the effect of femoral heads with various sizes suggests that the larger femoral head may induce larger wear volume but gives a smaller wear depth and that wear depth and volume loss are apparently nonlinearly related to the femoral head diameter. It is shown that the current algorithms are useful and helpful in understanding wear behavior for alternative or new designs of artificial hip joints and even for other analogous structures.

Computer simulation on fatigue behavior of cemented hip prostheses: a physiological model.

This paper is concerned with the investigation on the fatigue failure of implant fixation by numerical approaches. A computer algorithm based on finite element analysis and continuum damage mechanics was proposed to quantify the fatigue damage rate of cement mantle under physiological conditions. In examining the interfacial debonding effect, the interface elements were introduced at cement-stem interfaces and calibrated with the increase of loading cycles. Current results reveal that the major sites for failure initiation are in the proximal anterior-medial regions and at the distal prosthesis tip, which clearly demonstrate the same failure scenario as observed in clinical studies. Such fatigue failures not only result in the corruption of cement-stem interfaces, but also greatly affect the cement stress distribution and the damage rate in subsequent loading cycles. Another significant result is that the predicted damage rate increases steadily with gait cycles. This trend in damage development is consistent with the findings obtained from fatigue tests available in literature. It is anticipated that presented methodology can serve as a pre-clinical validation of cemented hip prostheses.

The effect of contact interface on dynamic characteristics of composite structures

In this project, nonlinear characteristics on the rolling interface of a linear guide were studied by the finite element analysis and experimental verification. Contact of the ball/surface rolling interface in the rolling guides was simulated as a three-dimensional membrane element without thickness. By introducing Hertzian contact theory and applying proper normal/shear stiffness to such contact elements in the overall finite element model, dynamic behaviors of linear guides affected by preload were thus investigated. In the finite element procedure, three contact models, 1D point-to-point, 2D point-to-point and 3D surface-to-surface, were sequentially introduced for purpose of verification with experiments. As a validation in this project, vibrational experiments on linear guides with different preloads were conducted and related frequency spectrums were derived. Both the finite element and the experimental results reveal that the natural frequency of a linear guide increases with the increment of the preload. In addition, the dynamic characteristics predicted by finite element analysis agree well with those measured from instrumental experiments. The proposal of current study may provide an alternate and reliable way for understanding of the dynamic characteristic of the rolling contact components in machine design field.

Evaluating the accuracy of wear formulae for acetabular cup liners.

This study proposes two methods for exploring the wear volume of a worn liner. The first method is a numerical method, in which SolidWorks® software is used to create models of the worn out regions of liners at various wear directions and depths. The second method is an experimental one, in which a machining center is used to mill polyoxymethylene to manufacture worn and unworn liner models, then the volumes of the models are measured. The results show that the SolidWorks® software is a good tool for presenting the wear pattern and volume of a worn liner. The formula provided by Ilchmann is the most suitable for computing liner volume loss, but is not accurate enough. This study suggests that a more accurate wear formula is required. This is crucial for accurate evaluation of the performance of hip components implanted in patients, as well as for designing new hip components.

Measurement of polyethylene wear — a new three-dimensional methodology

Studies in the field of polyethylene wear of the acetabular cup have been generally discussed using a two-dimensional assumption on the coronal plane; significant errors thus appear. The present study proposes a new, advanced methodology in order to accurately estimate the polyethylene wear. Through the usage of the distance between X-ray focus and film and only one follow-up, anteroposterior (AP) radiograph, a three-dimensional (3D) algorithm is introduced here. Here, 91 primary total hip joint replacements in 67 patients have been examined and three findings are obtained. Results show that the mean rate of 3D linear wear of the polyethylene estimated by the current method is 0.230+/-0.036 mm per year, that of the 2D linear wear is 0.148+/-0.028 mm per year. Moreover, the wear depth of the femoral head on the sagittal plane is 0.173+/-0.043 mm per year by the current method. This study also shows that the adoption of only one AP radiograph in the evaluation of the 3D penetration of femoral head is possible. Furthermore, the methodology proposed here is more convenient than others.

Wear patterns of, and wear volume formulae for, cylindrically elongated acetabular cup liners

This study analyzed the wear patterns of, and wear volume formulae for, cylindrically elongated acetabular cup liners. The geometric patterns of the wear surface were first classified, then wear volume formulae were derived by integral calculus. SolidWorks® software or published formulae were used to verify the accuracy of the proposed formulae. The analytical results showed that the wear shape of the liner can be categorized into seven wear patterns, including the special case of wear at 90°, and the seven corresponding wear formulae were derived. In addition, wear of the cylindrical elongation might add considerably to the volume loss of the liner, depending on the height and shape of the elongation and the depth and direction of the linear penetration, being maximally 21% in the investigated model. The proposed wear formulae and patterns will be useful for more accurate performance evaluation of existing hip components implanted in patients and for the designing of new hip components.

A COMPARATIVE STUDY ON WEAR BEHAVIOR OF HIP PROSTHESIS BY FINITE ELEMENT SIMULATION

A numerical approach was proposed to investigate the wear behavior occurred in the artificial hip joints in this paper. In the numerical simulations, the wear coefficients taken from pin-on-disk tests were introduced into the wear analysis model to assess the wear rates of polyethylene acetabular cups against metallic or ceramic femoral heads. For the established material combinations, different values of polyethylene wear rates were obtained respectively, which were not necessarily the realistic one as expected in vivo but could be confirmed after further discussion on the wear mechanism involved in wear tests. Current results indicated that the polyethylene/ceramic couples represented better wear performances than the polyethylene/metal couples. Furthermore, the ratio of wear rates for polyethylene cups against alumina and the metallic femoral heads was 0.5, which agreed well with that deduced from clinical studies or laboratory hip simulators. It is obvious that these comparable wear behaviors observed from clinics or laboratory studies also can be found by means of the numerical simulation.

A Finite Element Approach by Contact Transformation for the Prediction of Structural Wear

Understanding of the wear behaviors between mechanical components is a significant task in engineering design. Finite Element (FE) simulation may offer valuable wear information. However, longer computational time, poor data precision and possible divergence of results are unavoidable in repetitive procedures, especially for large FE structures. Thus, research is necessary to improve FE wear simulations. However, recent proposals for improvement of FE wear simulations are inadequate. To address these issues, the current method proposes a hypothesis that the strain energy is completely transferred through the contact regions of components; further that only variables on the contact surface are involved in the solution procedure, which is the most costly repetitive steps during simulation. The derivation of the related formulations is described herein. Our qualitative comparison demonstrates that the formulations in the current study are valid, offering significant implications for further application. Furthermore, interface implementation to any commercialized FE packages with/without the multi-thread systems may be easily accessed.

Effects of interfacial debonding on fatigue damage of cemented hip prostheses

Abstract Clinical studies on retrieved cement mantles have pointed out that the cemented hip prostheses failed after long‐term use due to debonding at the cement‐stem interface and local fractures in the cement mantles. These were linked to fatigue damages of cement mantles proved by fatigue experiments. In this paper, a numerical approach based on finite element analysis and continuum damage mechanics is proposed to investigate the fatigue behavior of cement mantles during gait cycles. Results reveal that the major sites for failure initiation are at the proximal medial regions and at the distal prostheses tip. Such fatigue failures not only result in the corruption of cementstem interfaces, but also greatly affect the stress distribution and damage rate of the proximal cement mantles in subsequent loading cycles. The interfacial debonding rate increases from 2.5% to 15% with gait loadings from five to twenty million cycles. Meanwhile, owing to the partial debonding of interface, the cement stresses on the remaining regions increase by 91% to 871% when compared with those generated with a fully bonded interface, which in turn accelerates the fatigue damage accumulation rate of the cement mantle from 5.99 % to 21.5%.

The study of wear behaviors on abducted hip joint prostheses by an alternate finite element approach

An acetabular cup with larger abduction angles is able to affect the normal function of the cup seriously that may cause early failure of the total hip replacement (THR). Complexity of the finite element (FE) simulation in the wear analysis of the THR is usually concerned with the contact status, the computational effort, and the possible divergence of results, which become more difficult on THRs with larger cup abduction angles. In the study, we propose a FE approach with contact transformation that offers less computational effort. Related procedures, such as Lagrangian Multiplier, partitioned matrix inversion, detection of contact forces, continuity of contact surface, nodal area estimation, etc. are explained in this report. Through the transformed methodology, the computer round-off error is tremendously reduced and the embedded repetitive procedure can be processed precisely and quickly. Here, wear behaviors of THR with various abduction angles are investigated. The most commonly used combination, i.e., metal-on-polyethylene, is adopted in the current study where a cobalt-chromium femoral head is paired with an Ultra High Molecular Weight Polyethylene (UHMWPE) cup. In all illustrations, wear coefficients are estimated by self-averaging strategy with available experimental datum reported elsewhere. The results reveal that the THR with larger abduction angles may produce deeper depth of wear but the volume of wear presents an opposite tendency; these results are comparable with clinical and experimental reports. The current approach can be widely applied easily to fields such as the study of the wear behaviors on ante-version, impingement, and time-dependent behaviors of prostheses etc.