Degradation of modular hip joint implants

Angela Bermudez Castañeda
EPFL, 2018
Thèse EPFL

Résumé

Total hip joint arthroplasty is a medical procedure to replace a natural failed joint by an artificial joint to improve the quality of life of the patient. In spite of its broad success, there is a concern about the relation between the presence of modular hip joint implants and health complications. In parallel, several degradation mechanisms such as are pitting, galvanic corrosion, hydrogen embrittlement, crevice-corrosion and fretting-corrosion have been proposed to explain what occurs in the modular joints. However, no consensus except for the recognized role of fretting-corrosion has been reached. Based on the required conditions for these mechanisms to happen, the two more likely occurring phenomena are fretting-corrosion and crevice-corrosion. Even though, several contradictions subsist about the effective incidence of crevice-corrosion on biomedical Ti and CoCr alloys. Accordingly, the general goal of this work is to clarify and shed more light on the mechanisms responsible for modular implant degradation, identifying the occurrence of crevice-corrosion in modular hip joint configuration and to rationalize through a predictive model the different mechanical and chemical mechanisms responsible for fretting corrosion degradation. The strategy taken here consists in the combination of an experimental and theoretical approach. The study of crevice-corrosion of Ti6Al4V and CoCr alloy was carried out in a configuration mimicking the geometrical crevice found in modular hip trunnions in contact with ceramic heads, under different pH conditions and several potential applied in NaCl 0.9% wt. No potential drop or evidence of degradation was found when testing CoCr and Ti biomedical alloys up to a duration of one month. In addition, an existing IR-Voltage model was used to predict the occurrence of crevice-corrosion based on the experimental results and data from the literature. Results from the experiments and the model indicate that crevice corrosion can occur only under the unrealistic condition that the electrical conductivity of the body fluid surrounding the implant approaches the one of drinking water. According to these results, the attention was addressed towards fretting-corrosion phenomena as the main responsible for modular hip joint implants deterioration. A predictive model of fretting corrosion wear integrating mechanical and chemical phenomena has been developed. A set of experiments was carried out in NaCl 0.9%wt under cathodic and anodic potential; in order to study the degradation mechanisms of the modular hip joint implants under different electrochemical and mechanical conditions. The properties of the third body formed depend on the prevailing electrochemical conditions in the contact, thus determining the wear rate. Considering these observations and taking into account mechanical and chemical parameters, this work proposes a fretting wear model inspired from an existing tribocorrosion model based on the mass balances between the bulk material, the third body and the electrolyte. The model allows to determine wear based on crucial chemical and mechanical parameters and constitutes a first step towards the development of theoretical tools scaled to modular hip joint configuration.

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https://infoscience.epfl.ch/record/255304?ln=fr

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Angela Bermudez Castañeda
EPFL, 2018
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/255304?ln=fr

À propos de ce résultat

Concepts associés (11)

Concepts associés

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Publications associées (2)

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High volume fraction particulate and interpenetrating phase composites (IPC)

Ghodratollah Roudini

The role of reinforcing phase contiguity in high volume fraction particulate composites is investigated using model composites of pure aluminium reinforced with α-Al2O3 particles. To produce the composites, alumina particle (5µm) preforms were either loosely packed or, alternatively, packed and CIPed; these were then sintered, varying the sintering time and temperature, and infiltrated with pure liquid aluminium using gas pressure infiltration. The microstructure of the resulting composites was characterized using image analysis in terms of reinforcement phase contiguity (β), defined as the ratio of particle surface that is in contact with other particles to that in contact with the matrix. The influence of this parameter on several characteristics of the composites and the preforms they were made from was then studied, using theoretical models in the literature to isolate the influence of this parameter from that of other variables, such as the volume fraction ceramic. To this end, the thermal conductivity of the sintered preforms, basic composite mechanical properties (hardness, tensile strength, and damage evolution during straining) and the thermal expansion of the composites were characterized and compared with theory; furthermore, quenching and neutron diffraction were used to probe in more depth the thermal expansion behaviour of the composites. The results show that the thermal conductivity of the preform increases, as expected, upon increasing the relative density (volume fraction) and the contiguity of alumina particle after sintering. Comparison of data with theory shows that, while experimental values for the CIPed samples agree with the prediction by Argento and Bouvard for the thermal conductivity of powder preforms as a function of densification, neither this model, nor that of Montes et al. , fits all experimental values. Rather, when pooled the data show that the thermal conductivity of powder preforms, normalized for relative density using the differential effective medium model, obeys relatively well a simple linear relation between contiguity and conductivity. The hardness of the composite increases with increasing relative density and increasing contiguity. Here again, after normalization for the dependence of hardness on ceramic volume fraction using current theory, a nearly linear relation is found between hardness and contiguity. Tensile testing results show that the strength of the composite increases with increasing fraction ceramic and increasing contiguity while the elongation decreases, composites with an interconnected ceramic phase rapidly becoming quite brittle, with elongations falling below half a percent. No clear relationship is found between contiguity and Young's modulus after normalization for the volume fraction ceramic. Measurements of internal damage evolution show that damage increases very rapidly with deformation in the present interconnected composites, in agreement with their brittle character. The thermal expansion results show that the CTE of the present composites often varies with temperature at a rate higher than that predicted to result from physical parameter variations by existing theoretical models. With regard to their thermal expansion, the composites can be separated into three classes that are characterized by the level of contiguity of the low expansion phase: low contiguity (i.e. β ≤ 0.045), medium contiguity (i.e. 0.05 ≤ β < 0.074) and high contiguity (i.e. β ≤ 0.074 ). The CTE values during heating for the low contiguity composites lie between Schapery's upper bound and the rule of mixtures, starting upon heat-up from the Schapery upper bound at low temperature and transiting above roughly 150°C to the rule of mixture prediction; upon cool-down the same behaviour is observed in reverse. The CTEs of the composites with medium contiguity at low temperatures are somewhat below the Schapery upper bound, increasing somewhat faster than predicted for this bound with increasing temperature to reach Shapery's upper bound at around 200°C. For the composites with high contiguity, the CTEs lie between elastic and plastic bounds predicted using the self-consistent model, the latter model being obtained by assigning a zero shear stiffness to the matrix. Physical reasoning and comparison of data with theory indicate that these transitions in expansivity of the composites are mostly a result of matrix yield following fully elastic deformation past a certain thermal excursion, this transition being prevented by increased ceramic phase contiguity. This interpretation is confirmed by neutron diffraction results. Increased reinforcement contiguity in the composites also affects the amplitude of strain hysteresis during thermal cycling, the magnitude of hysteresis decreasing in nearly linear fashion with increasing contiguity, reaching zero near β = 15%. Samples that were quenched into liquid nitrogen after a relaxation heat treatment and reheated to room temperature yielded data globally similar to those of as-processed samples. Nitrogen-quenched samples with higher contiguity (β ≥ 0.05) exhibit furthermore signs of ratcheting upon cycling, which was not observed in the composites with low contiguity.

EPFL2008

Chargement

State-of-the-art acoustic models for Automatic Speech Recognition (ASR) are based on Hidden Markov Models (HMM) and Deep Neural Networks (DNN) and often require thousands of hours of transcribed speech data during training. Therefore, building multilingual ASR systems or systems on a language with few resources is a challenging task. Multilingual training and cross-lingual adaptation are potential solutions. However, context-dependent states modeling creates difficulties for multilingual and cross-lingual ASR because of the large increase in context dependent labels arising from the phone set mismatch. The goal of this thesis is to improve current state-of-the-art acoustic modeling techniques in general for ASR, with a particular focus on multilingual ASR and cross-lingual adaptation. We systematically exploited new training frameworks, from Maximum Likelihood Estimation, Connectionist Temporal Classification to Maximum Mutual Information, in the context of phoneme-based multilingual training. In order to minimize the negative effects of data impurity arising from language mismatch, we investigated language adaptive training approaches which help further improve the multilingual ASR performance. Through comprehensive experimental comparison we demonstrated that phoneme-based multilingual models are easily extensible to unseen phonemes of new languages, from which the cross-lingual adaptation yields significant improvement over traditional approaches on limited data. Finally, we proposed a semi-supervised training approach based on dropout to boost the performance in low-resourced languages using untranscribed data. In the other part of the thesis, we conducted more theoretical analysis of techniques found to be useful in sequential multilingual training. More specifically, we revisited the recurrent architecture based on Bayesâs theorem. This leads to a Bayesian recurrent unit dictated by the probabilistic formulation and naturally support a backward recursion. Experiments show that the proposed architecture exceeds the performance of conventional recurrent network. Together, this thesis constitutes a thorough analysis of the current field. Through theoretical and experimental comparisons, the proposed approaches are shown to yield significant improvement over the conventional hybrid systems on multilingual speech recognition.

EPFL2020

Chargement

EPFL2020

High volume fraction particulate and interpenetrating phase composites (IPC)

Ghodratollah Roudini

EPFL2008