Jean Geringer

Two-dimensional finite element simulation of fracture and fatigue behaviours of alumina microstructures for hip prosthesis.

This paper describes a two-dimensional (2D) finite element simulation for fracture and fatigue behaviours of pure alumina microstructures such as those found at hip prostheses. Finite element models are developed using actual Al2O3 microstructures and a bilinear cohesive zone law. Simulation conditions are similar to those found at a slip zone in a dry contact between a femoral head and an acetabular cup of hip prosthesis. Contact stresses are imposed to generate cracks in the models. Magnitudes of imposed stresses are higher than those found at the microscopic scale. Effects of microstructures and contact stresses are investigated in terms of crack formation. In addition, fatigue behaviour of the microstructure is determined by performing simulations under cyclic loading conditions. It is shown that crack density observed in a microstructure increases with increasing magnitude of applied contact stress. Moreover, crack density increases linearly with respect to the number of fatigue cycles within a given contact stress range. Meanwhile, as applied contact stress increases, number of cycles to failure decreases gradually. Finally, this proposed finite element simulation offers an effective method for identifying fracture and fatigue behaviours of a microstructure provided that microstructure images are available.

Degradation of alumina and zirconia toughened alumina (ZTA) hip prostheses tested under microseparation conditions in a shock device

This paper considers the degradation of alumina and zirconia toughened alumina vs. alumina for hip implants. The materials are as assumed to be load bearing surfaces subjected to shocks in wet conditions. The load is a peak of force; 9 kN was applied over 15 ms at 2 Hz for 800,000 cycles. The volumetric wear and roughness are lower for ZTA than for alumina. The long ZTA ageing did not seem to have a direct influence on the roughness. The ageing increased the wear volumes of ZTA and it was found to have a higher wear resistance compared to alumina.

Finite Element Modelling of Shock-Induced Damages on Ceramic Hip Prostheses

The aim of this work was to simulate the behaviour of hip prostheses under mechanical shocks. When hip joint is replaced by prosthesis, during the swing phase of the leg, a microseparation between the prosthetic head and the cup could occur. Two different sizes of femoral heads were studied: 28 and 32 mm diameter, made, respectively, in alumina and zirconia. The shock-induced stress was determined numerically using finite element analysis (FEA), Abaqus software. The influence of inclination, force, material, and microseparation was studied. In addition, an algorithm was developed from a probabilistic model, Todinovs approach, to predict lifetime of head and cup. Simulations showed maximum tensile stresses were reached on the cups surfaces near to rim. The worst case was the cup-head mounted at 30∘. All simulations and tests showed bulk zirconia had a greater resistance to shocks than bulk alumina. The probability of failure could be bigger than 0.9 when a porosity greater than 0.7% vol. is present in the material. Simulating results showed good agreement with experimental results. The tests and simulations are promising for predicting the lifetime of ceramic prostheses.

Synergism effects during friction and fretting corrosion experiments – focusing on biomaterials used as orthopedic implants

Abstract: This chapter is focused on the specific phenomenon of wear (friction and fretting) and corrosion related to orthopedic implants. Both fields are closely linked through mechanical (wear) and chemical (corrosion) factors, and by mutual synergism. After defining differences between wear corrosion and fretting corrosion, we focus our attention on the synergism that exists between mechanical wear and corrosion. Careful characterization of synergistic effects has proved to be the key factor in determining the rates of tribological degradation of metals and alloys. Indeed, an insulating material, such as poly(methylmethacrylate) (PMMA), which is not susceptible to corrosion, is subjected to mechanical damage during wear or fretting. However, in the case of a metal fretting against a soft material like PMMA, the surface does not suffer purely mechanical degradation, as it is too hard and its mechanical properties (Young’s modulus, yield strength, etc.) are far superior to those of PMMA. Thus, metals suffer degradation under these circumstances when in contact with an aqueous solution, because of the conjoint action of corrosion and mechanics. Thus, this chapter is mainly focused on describing wear and fretting in a corrosive medium, their differences, and determining the effects of corrosion, mechanics and the synergistic interactions between them.

Fretting corrosion processes and wear mechanisms in medical implants

Two materials (one is metal) under slight relative motion in a liquid medium are submitted to fretting corrosion. This chapter is dedicated on studying fretting corrosion of implants. After describing the most significant implants submitted to fretting, fretting corrosion is defined. Fretting corrosion is a particular mechanism of degradations; it highlights the key role of passive film, crevice corrosion, etc. For understanding the electrochemical effect of the fretting corrosion of metal, some investigations are presented at free corrosion potential and at applied potential in order to measure the specific current density. Moreover the role of proteins is investigated because they constitute the biological environment. Thus they play a significant role in the fretting corrosion processes. Finally results from Atomic Force Microscopy (AFM) show the particular debris, size about 100 nm. Problem about debris influence is discussed

Wear analysis of hip explants, dual mobility concept: Comparison of quantitative and qualitative analyses

Total hip replacement fails mainly because of wear. It is of interest to analyse wear to be able to increase the longevity of the hip implants. One way to achieve it is to use instruments on explants but the most suitable depends on the application. This article aims at comparing several methods of surface analysis in the particular application of wear determination in a series of dual mobility explants. Wear measurement could help understand the wear mechanism only partially known. A coordinate measuring machine is used to get three-dimensional points representing the explants, then Pro/Engineer® and Matlab® are used to calculate wear. A mechanical (SOMICRONIC®) and an optical profilometer (Bruker nanoscope Wyko® NT 9100, ex. Veeco) are used to access roughness parameters. The comparisons of the two software showed similar results for wear calculation except in a few cases where differences are due to the theoretical volumes calculation. The comparison of the two profiling techniques resulted in similar results particularly for Sa and Sdr. The comparison of the results showed that wear is present for four explants; it is relevant with the observed characteristics. The mechanical profilometer showed better accuracy than the optical one which enable to conclude that it must not be neglected for that particular application, even if measurements need more time.

Crack simulation of nano-bioceramic composite microstructures with cohesive failure law: effects of sintering, loads and time.

Crack behaviour of zirconia toughened alumina (ZTA) microstructures are simulated with a two-dimensional finite element simulation. Finite element models are developed using actual microstructure images of zirconia toughened alumina and a bilinear cohesive zone law. Simulation conditions are similar to those found at frictional contact between a femoral head and an acetabular cup of hip prosthesis. Effects of microstructures and contact stresses are investigated in terms of crack generation. Moreover, fatigue behaviour of a microstructure is determined by performing simulations under cyclic loading conditions. It is identified that total crack length observed in a microstructure increases with increasing the magnitude of applied contact stress. Cyclic simulation results show that progressive crack growth occurs with respect to number of fatigue cycles. In addition, it is demonstrated that zirconia grains resist crack growth in microstructures.

Ageing, Shocks and Wear Mechanisms in ZTA and the Long-Term Performance of Hip Joint Materials

The surface morphologies and microstructures of Zirconia Toughened Alumina (ZTA) femoral heads were analyzed following in vitro tests aiming to simulate in vivo degradation. Three phenomena potentially leading to degradation were investigated: shocks, friction and hydrothermal ageing. Shocks due to micro-separation created the main damage with the formation of wear stripes on the femoral head surfaces. Atomic Force Microscopy (AFM) images suggested the release of wear debris of various shapes and sizes through inter- and intra-granular cracks; some debris may have a size lower than 100 nm. A decrease in hardness and Young’s modulus was measured within the wear stripes by nanoindentation technique and was attributed to the presence of surface and sub-surface micro-cracks. Such micro-cracks mechanically triggered the zirconia phase transformation in those worn areas, which in return presumably reduced further crack propagation. In comparison with shocks, friction caused little wear degradation as observed from AFM images by scarce pullout of grains. The long-term resistance of the ZTA composite material against hydrothermal ageing is confirmed by the present observations.

From acid etching treatments to tribocorrosive properties of dental implants: do some experimental results on surface treatments have an influence on the tribocorrosion behaviour of dental implants?

Surface treatments of dental implants aim at promoting osseointegration, i.e. the anchorage of the metallic part. Titanium-, grade II‐V, based material is used as a bulk material for dental implants. For promoting the anchorage of this metallic biomaterial in human jaw, some strategies have been applied for improving the surface state, i.e. roughness, topography and coatings. A case study, experimental study, is described with the method of acid etching on titanium grade 4, CpTi. The main goal is to find the right proportion in a mixture of two acids in order to obtain the best surface state. Finally, a pure theoretical prediction is quite impossible and some experimental investigations are necessary to improve the surface state. The described acid etching is compared with some other acid etching treatments and some coatings available on dental implants. Thus, the discussion is focused on the tribocorrosion behaviour of titanium-based materials. The purpose of the coating is that the lifetime under tribocorrosion is limited. Moreover, the surgery related to the implantation has a huge impact on the stability of dental implants. Thus, the performance of dental implants depends on factors related to surgery (implantation) that are difficult to predict from the biomaterial characteristics. From the tribocorrosion point of view, i.e. during the mastication step, the titanium material is submitted to some deleterious factors that cause the performance of dental implants to decrease. (Some figures may appear in colour only in the online journal)

Fretting corrosion in biomedical implants

Abstract: Fretting corrosion is a phenomenon which occurs when two materials are submitted to friction under a slight relative motion. This chapter will focus on fretting corrosion degradations in the field of implants. After describing the most significant implants subject to fretting, fretting corrosion is defined. Laboratory investigations highlight particular degradation mechanisms such as the key role of passive film stability and crevice corrosion. Free corrosion and applied potential conditions are investigated in order to understand tribocorrosion. Finally, the role of proteins in tribological behaviour is investigated, since the fretting corrosion degradations of biomaterials such as implants occur in a biological environment and proteins play a role in the tribocorrosion processes.