Fatigue endurance of Ti-6Al-4V alloy with electro-eroded surface for improved bone in-growth

Ti-6Al-4V hour-glass shaped rotating beam specimens with duplex microstructure were processed by electric discharge machining (EDM). A comparatively high peak current of 29A was utilized in order to increase surface roughness for improved osteointegration. High cycle fatigue (HCF) tests were performed in rotating beam loading (R=-1) on these EDM specimens and results were compared with electrolytically polished specimens serving as reference. As expected, the HCF performance of EDM specimens was inferior to the electrolytically polished specimens. A detailed study of fatigue crack nucleation and microcrack growth was carried out on failed specimens by SEM. The poor HCF strength of EDM specimens is explained by early crack nucleation due to the high notch sensitivity of Ti-6Al-4V. In addition, process-induced residual tensile stresses and microstructural effects may also account for the drastic loss in HCF performance relative to the electropolished baseline.     

The effect of surface treatments on the fretting behavior of Ti-6Al-4V alloy

Stem modularity in total hip replacement introduces an additional taper joint between Ti-6Al-4V stem components with the potential for fretting corrosion processes. One possible way to reduce the susceptibility of the Ti-6Al-4V/Ti-6Al-4V interface to fretting is the surface modification of the Ti-6Al-4V alloy. Among the tested, industrially available surface treatments, a combination of two deep anodic spark deposition treatments followed by barrel polishing resulted in a four times lower material release with respect to untreated, machined fretting pad surfaces. The fretting release has been quantified by means of radiotracers introduced in the alloy surface by proton irradiation. In a simple sphere on flat geometry, the semispherical fretting pads were pressed against flat, dog-bone shaped Ti-6Al-4V fatigue samples cyclically loaded at 4 Hz. In this way a cyclic displacement amplitude along the surfaces of 20 mum has been achieved. A further simplification consisted in the use of deionized water as lubricant. A comparison of the radiotracer results with an electrochemical material characterization after selected treatments by potentiostatic tests of modular stems in 0.9% NaCl at 40 degrees C for 10 days confirmed the benefit of deep anodic spark deposition and subsequent barrel polishing for improving the fretting behavior of Ti-6Al-4V.     

Functionally graded Co-Cr-Mo coating on Ti-6Al-4V alloy structures

Functionally graded, hard and wear-resistant Co–Cr–Mo alloy was coated on Ti–6Al–4V alloy with a metallurgically sound interface using Laser Engineering Net Shaping (LENS™). The addition of the Co–Cr–Mo alloy onto the surface of Ti–6Al–4V alloy significantly increased the surface hardness without any intermetallic phases in the transition region. A 100% Co–Cr–Mo transition from Ti–6Al–4V was difficult to produce due to cracking. However, using optimized LENS™ processing parameters, crack-free coatings containing up to 86% Co–Cr–Mo were deposited on Ti–6Al–4V alloy with excellent reproducibility. Human osteoblast cells were cultured to test in vitro biocompatibility of the coatings. Based on in vitro biocompatibility, increasing the Co–Cr–Mo concentration in the coating reduced the live cell numbers after 14 days of culture on the coating compared with base Ti–6Al–4V alloy. However, coated samples always showed better bone cell proliferation than 100% Co–Cr–Mo alloy. Producing near net shape components with graded compositions using LENS™ could potentially be a viable route for manufacturing unitized structures for metal-on-metal prosthetic devices to minimize the wear-induced osteolysis and aseptic loosening that are significant problems in current implant design.     

Chemical surface modification of high-strength porous Ti compacts by spark plasma sintering

The biological properties of a titanium (Ti) implant depend on its surface oxide film. The aims of the present study were to increase the specific surface oxide area on Ti using a porous structure and to study the relationship between the amount of apatite coating in simulated body fluid (SBF) and the actual surface area on titanium powders. Ti powders of 110 microm average diameter were sintered by spark plasma sintering. The sintered compacts had a porosity of 28%, a compressive elastic modulus of 7.9 GPa and an ultimate strength of 112 MPa. The compressive strength of the compacts was increased to 588 MPa by subsequent annealing in a vacuum furnace at 1000 degrees C for 24 h. The sintered compacts were treated with aqueous NaOH solution and subsequently heated at 600 degrees C. The pretreated compacts showed apatite crystal precipitation in SBF. The amounts of precipitates through the compacts were compared with those of the Ti plate substrates subjected to the same chemical pretreatment. It was confirmed that the amounts of precipitates through the compacts were more than one hundred times higher than those on the Ti plates. It was concluded that the metal porous compacts developed may be used as functional materials for immobilizing functional proteins and/or drugs, because the precipitated apatite can adsorbed these substances.     

Effect of different Ti-6Al-4V surface treatments on osteoblasts behaviour

The purpose of the present work was to examine the effect of different Ti-6Al-4V surface treatments on osteoblasts behaviour. Previous work in this laboratory has demonstrated that an ageing treatment reduces metal ion release from this alloy compared to standard passivation procedures. In this study. human osteosarcoma MG-63 were used in short-term in vitro tests to assay for cell viability and cell proliferation at 12, 24 and 72 h while SaOS-2 were used in long-term in vitro tests to assay for osteonectin, osteopontin, osteocalcin gene expression, total protein amount (TP). alkaline phosphatase activity (ALP) and fibronectin production (FN) for 1-4 weeks. Epifluorescence microscopy was used to observe SaOS-2 cell morphology. After 24h, there was no difference in MG-63 cell viability proliferation or in SaOS-2 cell morphology between the different surface treatments. For the long-term tests, the aged Ti-6Al4V induced significantly higher cell proliferation than the control Ti-6Al-4V at 72h. At week 1, no difference in the osteonectin, osteopontin, and osteocalcin gene expression was found between samples. The peak of ALP activity appeared earlier at week 2 for the control surface compared with the passivated and aged surfaces. The early increase in ALP activity for the control sample could be a compensatory effect of decreased osteoblasts proliferation. There was no difference in the expression of FN for the different surface treatments. Our present results showed that the different surface treatments, which induced different metal ion release kinetics and surface properties, influenced the cell proliferation and ALP activity of osteoblast cells. Aluminium ions release kinetics as well as presence of vanadium ions may play a major role in influencing the osteoblasts behaviour in the present study.     

Acoustic emission during fatigue of porous-coated Ti-6Al-4V implant alloy.

Acoustic emission (AE) events and event intensities (e.g., event amplitude, counts, duration, and energy counts) were recorded and analyzed during fatigue loading of uncoated and porous-coated Ti-6Al-4V. AE source location, spatial filtering, event, and event intensity distributions were used to detect, monitor, analyze, and predict failures. AE provides the ability to spatially and temporally locate multiple fatigue cracks, in real time. Fatigue of porous-coated Ti-6Al-4V is governed by a sequential, multimode fracture process of: transverse fracture in the porous coating; sphere/sphere and sphere/substrate debonding; substrate fatigue crack initiation; slow and rapid substrate fatigue crack propagation. Because of the porosity of the coating, the different stages of fracture within the coating occur in a discontinuous fashion. Therefore, the AE events generated are intermittent and the onset of each mode of fracture in the porous coating can be detected by increases in AE event rate. Changes in AE event rate also correspond to changes in crack extension rate, and may therefore be used to predict failure. AE offers two distinct advantages over conventional optical and microscopic methods of analyzing fatigue cracks--it is more sensitive and it can determine the time history of damage progression. The magnitude of the AE event intensities increased with increasing stress. Failure mechanisms are best differentiated by analyzing AE event amplitudes. Intergranular fracture and microvoid coalescence generated the highest AE event amplitudes (100 dB), whereas, plastic flow and friction generated the lowest AE event amplitudes (55-65 dB). Fractures in the porous coating were characterized by AE event amplitudes of less than 80 dB.     

Electrochemical investigation of chromium oxide-coated Ti-6Al-4V and Co-Cr-Mo alloy substrates

Hard coatings for articulating surfaces of total joint replacements may improve the overall wear resistance. However, any coating approach must take account of changes in corrosion behavior. This preliminary assessment analyzes the corrosion kinetics, impedance and mechanical-electrochemical stability of 100 μm thick plasma sprayed chromium oxide (Cr?O?) coatings on bearing surfaces in comparison to the native alloy oxide films on Co-Cr-Mo and Ti-6Al-6V. Cyclic potentiodynamic polarization, electrochemical impedance spectroscopy, and mechanical abrasion under potentiostatic conditions were performed on coated and substrate surfaces in physiological saline. SEM analysis characterized the coating morphology. The results showed that the corrosion current density values of chromium oxide coatings (0.4-1.2 μA/cm2) were of the same order of magnitude as Ti-6Al-4V alloy. Mechanical abrasion did not increase corrosion rates of chromium oxide coatings but did for uncoated Co-Cr-Mo and Ti-6Al-4V. The impedance response of chromium oxide coatings was very different than Co-Cr-Mo and Ti-6Al-4V native oxides characterized by a defected coating model. More of a frequency-independent purely resistive response was seen in mid-frequency range for the coatings (CPE(coat) : 40-280 nF/cm2 (rad/s)(1-α) , α: 0.67-0.83) whereas a more capacitive character is seen for Co-Cr-Mo and Ti-6Al-4V (CPE(ox) around 20 μF/cm2 (rad/s)(1-α) , α around 0.9). Pores, interparticle gaps and incomplete fusion typical for thermal spray coatings were present in these oxides which could have influenced corrosion resistance. The coating microstructure could have allowed some fluid penetration. Overall, these coatings appear to have suitable corrosion properties for wear surfaces.     

New surface modification for Ti-6Al-7Nb alloy: oxygen diffusion hardening (ODH)

PVD-TiN coating and N+ implantation of Ti-6Al-7Nb alloy resulted in surface hardening to a depth of less 3 microns. The new oxygen diffusion hardening (ODH) treatment increased the hardness gradually to 50 microns. PUD-TiN showed an improvement in the tribological properties, while N+ implantation increased the PE wear rate. The wear rate of the ultrahigh-molecular-weight as well as the friction coefficient were reduced to one-half of the values achieved with the combination of CoCrMo-alloy when paired against the ODH-treated surface. In pairing with ZrO2-containing bone cement the ODH-treated surface showed only a minimal reaction.     

Wear-accelerated corrosion of Ti-6Al-4V and nitrogen-ion-implanted Ti-6Al-4V: mechanisms and influence of fixed-stress magnitude

Wear-accelerated corrosion rates at constant anodic potentials were evaluated for unimplanted and nitrogen-ion-implanted surgical Ti-6Al-4V while wearing against ultrahigh-molecular-weight polyethylene at stress levels up to 6.90 MPa (1000 psi). The ion implantation processing was found to reduce the wear corrosion rates in both saline and serum solutions at all applied stress levels. During wear testing, all of the ion-implanted surfaces remained visually unchanged from the polished condition. However, many of the unimplanted surfaces developed damage zones characterized by wear tracks and black wear debris. A surface-damage mechanism is proposed and discussed which involves disruption of the Ti-6Al-4V protective oxide film, subsequent entrapment of oxide particles in the polyethylene, then self-perpetuating damage due to the abrasive action of the embedded particles.     

Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting

Selective electron beam melting (SEBM) was successfully used to fabricate novel cellular Ti-6Al-4V structures for orthopaedic applications. Micro computer tomography (muCT) analysis demonstrated the capability to fabricate three-dimensional structures with an interconnected porosity and pore sizes suitable for tissue ingrowth and vascularization. Mechanical properties, such as compressive strength and elastic modulus, of the tested structures were similar to those of human bone. Thus, stress-shielding effects after implantation might be avoided due to a reduced stiffness mismatch between implant and bone. A chemical surface modification using HCl and NaOH induced apatite formation during in vitro bioactivity tests in simulated body fluid under dynamic conditions. The modified bioactive surface is expected to enhance the fixation of the implant in the surrounding bone as well as to improve its long-term stability.     

In vitro biocorrosion of Ti-6Al-4V implant alloy by a mouse macrophage cell line

Corrosion of implant alloys releasing metal ions has the potential to cause adverse tissue reactions and implant failure. We hypothesized that macrophage cells and their released reactive chemical species (RCS) affect the alloy's corrosion properties. A custom cell culture corrosion box was used to evaluate how cell culture medium, macrophage cells and RCS altered the Ti-6Al-4V corrosion behaviors in 72 h and how corrosion products affected the cells. There was no difference in the charge transfer in the presence (75.2 +/- 17.7 mC) and absence (62.3 +/- 18.8 mC) of cells. The alloy had the lowest charge transfer (28.2 +/- 4.1 mC) and metal ion release (Ti < 10 ppb, V < 2 ppb) with activated cells (releasing RCS) compared with the other two conditions. This was attributed to an enhancement of the surface oxides by RCS. Metal ion release was very low (Ti < 20 ppb, V < 10 ppb) with nonactivated cells and did not change cell morphology, viability, and NO and ATP release compared with controls. However, IL-1beta released from the activated cells and the proliferation of nonactivated cells were greater on the alloy than the controls. In summary, macrophage cells and RCS reduced the corrosion of Ti-6Al-4V alloys as hypothesized. These data are important in understanding host tissue-material interactions.     

Fatigue properties of cast and heat treated Ti-6Al-4V alloy for anatomic hip prostheses

The intricate shape of Ti-6Al-4V anatomic hip prostheses necessitates the use of casting as a fabrication method, since neither machining, nor forging or powder metallurgial processing are reasonably acceptable. Cast and hot isostatically pressed Ti-6Al-4V, however, has a relatively low ductility and reduced fatigue properties. Appropriate thermal treatments can improve these properties. Specifically, treatments comprising a beta-solutionizing and an aging step yield ductilities similar to the forged and annealed condition, and fatigue properties 11.5% above the non-treated condition. Such treatments produce small alpha-beta platelet colonies with a ragged, tortuous morphology. The reduction of scatter in fatigue strength is the result of the formation of this well controlled microstructure.     

Nanometer-scale surface modification of Ti6Al4V alloy for orthopedic applications

This communication presents a novel technology to enhance the biocompatibility of bioinert Ti6Al4V alloy as implant materials for orthopaedic application. The surface of Ti6Al4V alloy was electrochemically activated in NaOH solution to create a porous structure with nanometer topographic features and an alkaline environment, thus promoting the formation of bone-like hydroxyapatite coating and enhancing the bonding strength of the coating. This innovative activation process was proved to be effective and essential. The activated surface was confirmed to be pure TiO2 and the formed coating was characterized of pure hydroxyapatite with a nanometer-scaled grain size structure by means of XPS, FESEM/SEM/EDX, XRD, and TEM techniques.     

The effect of microstructure on fatigue performance of Ti-6Al-4V alloy after EDM surface treatment for application in orthopaedics

Three different microstructures--equiaxed, bi-modal and coarse lamellar--are prepared from Ti-6Al-4V alloy. Electric discharge machining (EDM) with a high peak current (29 A) is performed in order to impose surface roughness and modify the chemical composition of the surface. Detailed scanning electron microscopy (SEM) investigation revealed a martensitic surface layer and subsurface heat affected zone (HAZ). EDX measurements showed carbon enriched remnants of the EDM process on the material surface. Rotating bending fatigue tests are undertaken for EDM processed samples for all three microstructures and also for electropolished-benchmark-samples. The fatigue performance is found to be rather poor and not particularly dependent on microstructure. The bi-modal microstructure shows a slightly superior high cycle fatigue performance. This performance can be further improved by a suitable heat treatment to an endurance limit of 200 MPa.     

Self-organized nanotubular oxide layers on Ti-6Al-7Nb and Ti-6Al-4V formed by anodization in NH4F solutions

The present work reports the fabrication of self-organized porous oxide-nanotube layers on the biomedical titanium alloys Ti-6Al-7Nb and Ti-6Al-4V by a simple electrochemical treatment. These two-phase alloys were anodized in 1M (NH(4))(2)SO(4) electrolytes containing 0.5 wt % of NH(4)F. The results show that under specific anodization conditions self-organized porous oxide structures can be grown on the alloy surface. SEM images revealed that the porous layers consist of arrays of single nanotubes with a diameter of 100 nm and a spacing of 150 nm. For the V-containing alloy enhanced etching of the beta phase is observed, leading to selective dissolution and an inhomogeneous pore formation. For the Nb-containing alloy an almost ideal coverage of both phases is obtained. According to XPS measurements the tubes are a mixed oxide with an almost stoichiometric oxide composition, and can be grown to thicknesses of several hundreds of nanometers. These findings represent a simple surface treatment for Ti alloys that has high potential for biomedical applications.     

Cell/surface interactions and adhesion on Ti-6Al-4V: effects of surface texture

This paper presents the results of an experimental study of the effects of surface texture on the interactions between human osteo-sarcoma (HOS) cells and Ti-6Al-4V. These include the Ti-6Al-4V with polished (smooth); Al(2)O(3) blasted (rough); and laser micro-grooved geometries with controlled spacings and depths. Immuno-fluorescence staining of adhesion proteins (actin and vinculin) was used to study the spreading and adhesion of HOS cells in 2 day culture experiments. Quantitative measures of adhesion were also obtained using an enzymatic detachment assay. The results are discussed within the context of existing theories of cell adhesion. The implications of the results are also examined for the design of textured surfaces in biomedical systems.     

Effects of surface treatment of Ti-6Al-4V titanium alloy on biocompatibility in cultured human umbilical vein endothelial cells

Among the titanium alloys employed as implant materials, the Ti-6Al-4V alloy is still widely used. Ti-6Al-4V titanium alloy samples, in untreated state and subjected to treatments in air by furnace or glow-discharge processes, were put in contact with human umbilical vein endothelial cells (HUVEC) in order to evaluate their effects on biocompatibility. In HUVEC kept for 48 h in the presence of the three sample types neither cell proliferation nor protein content nor lactate dehydrogenase release in the culture medium are affected, while apoptosis is induced after 48- and 96-h contact of the cells with the untreated sample type, and after 96-h contact with the plasma treated one, the furnace treated sample type being ineffective. The expression of two adhesion molecules, intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) was also studied. The incubation of HUVEC with the three sample types for 48 or 96 h induces a significant increase in ICAM-1 protein levels, in comparison with control cells, while VCAM-1 expression is not detectable. In the same way, TNF-alpha release in the culture medium, assayed after 48- and 96-h contact of the cells with the three sample types, is significantly higher, in comparison with control, even if the highest values are registered in the presence of the untreated samples. Taken together, these data indicate that, although Ti-6Al-4V alloy samples, and in particular the treated ones, show a good biocompatibility, attention must be given to the first signs of inflammation.     

用磁控溅射技术制备钛合金表面HA生物涂层

目的探讨磁控溅射法制备的HA生物涂层组织结构以及涂层与基体的界面结合性能. 方法利用射频磁控溅射技术在Ti-6Al-4V基体表面制备HA生物涂层,利用扫描电镜(SEM)观察HA生物涂层表面形貌和断面形貌,利用X射线衍射仪(XRD)分析涂层的相结构,利用能量分散谱仪(EDS)分析涂层的Ca/P比,采用环氧树脂E-7对接法测定HA涂层与基体的界面结合强度. 结果溅射HA生物涂层的Ca/P比为1.7, 后处理生物涂层中不存在其它钙磷杂质相,HA的晶化程度高,HA涂层与基体的界面结合强度为51.2MPa. 结论射频磁控溅射技术制备的HA生物涂层,表面形貌良好,涂层与基体的界面结合强度较高.