Bone response to machined cast titanium implants

The aim was to evaluate the bone response to machined cast titanium (Ti) implants. Commercially pure (c.p.) machined Ti implants served as controls. Analyses of the surface composition and topography by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) revealed no differences comparing the two materials. Cast screw-shaped and identical machined Ti implants were inserted in the tibial metaphysis of 6 rabbits. After 3 and 6 months, the amount of bone within threads and the degree of bone-implant contact were histomorphometrically evaluated. The bone area of cast Ti implants was 45% after 3 months and 62% after 6 months. The corresponding values for machined Ti implants were 51% and 58%, respectively. The total bone-implant contact for cast Ti implants was 19% (25% control implants) after 3 months and 45% (37% for control implants) 6 months after implantation. No statistically significant differences were observed between the two materials at any time interval. The present experimental results indicate that machined cast Ti implants integrate equally well in bone as machined c.p. Ti implants do.     

Fracture of titanium orthopaedic implants

At the Royal National Orthopaedic Hospital, in 1964, the first titanium-containing bone and joint replacement was inserted. By now over 250 titanium-containing endoprostheses of various kinds have been inserted. In 1972 Ti318 (Ti-6 wt% Al-4wt% V) was introduced and this alloy replaced the previously used commercially pure titanium. Over 200 Ti318 total hip replacements have been inserted, as have a variety of other titanium devices. This article is concerned with the fracture of these components in the body. The cause of fracture is discussed and the importance of good design and good surface finish is emphasized. The incidence of fracture is determined and on preliminary evidence Ti318 is found to be at least as satisfactory as other presently used implant alloys.     

High-temperature mechanical properties of nickel-based superalloys manufactured by additive manufacturing

Mechanical behaviour of Haynes 230 and Inconel 625 manufactured by selective laser melting (SLM) has been experimentally investigated in 870^oC. Significant embrittlement of SLM Inconel 625 was found, accompanied by hardening behaviour during the test. Moreover, due to inhomogeneous grain deformation, the hardness dispersion of the two SLM alloys increased significantly. Employing a systematic research with scanning electron microscopy and energy-dispersive spectrometer, intergranular cracking of two SLM alloys can be observed from the surface cracks and fracture appearances, respectively. In addition, carbides along grain boundaries of the two nickel-based superalloys were found and affected the strength of the grain boundary at 870^oC. Therefore, it resulted in different fracture mechanisms.     

The design and assessment of bio-inspired additive manufactured stab-resistant armour

The performance of modern fibre-based or polycarbonate armour has significantly improved since their introduction, providing protection against a range of low- and high-velocity threats. While this is so, users of such armour frequently report of issues relating to their operational suitability resulting in impaired performance and physiological effects. Recently, researchers have focused on how naturally occurring protective mechanisms could be utilised to enhance the protective and operational performance of wearers of engineered body armour. The research presented within this paper therefore utilises a series of key design characteristics exhibited within naturally occurring elasmoid scale armour, coupled with established laser sintering manufacturing parameters, for the realisation and assessment of a scale-based stab-resistant armoured structure to internationally recognised test standards.     

Quantitative bone tissue response to commercially pure titanium implants

Commercially pure titanium implants were inserted in rabbit tibia for 3, 6 and 12 weeks. Each rabbit had two implants inserted, one for removal torque measurements and the other for histomorphometrical analysis. Light microscopic observations revealed that there was a continuing bone remodelling, with new bone formation in the periosteal region after 3 and 6 weeks, which diminished with time, i.e. up to 12 weeks of follow-up. A higher removal torque was observed with increasing time of implant insertion. The removal torque values for the 12 week samples were converted to three different shear forces depending on three different theoretically calculated implant to bone attachment levels. The mean shear forces related to the entire length of the implant surface was 0.6 N mm^-2. If considering the length of the implant inside the cortical bone only, the mean shear force was 1.9 N mm^-2, and if the bone-metal contact length as related to an estimate of the bone-implant contact a mean shear force of 14.8 N mm^-2 was calculated. Histomorphometrical measurements revealed more bone-metal contact as well as a larger bone area in the threads with increasing time of insertion. Statistically significant differences were observed between all measurements of the 3 and 6 week samples and between the 3 and 12 week samples. For the 6 and 12 week sections a statistically eignificant difference could be demonstrated only when comparing bone areas in the three best consecutive threads located in the cortical region.     

Galvanic corrosion behavior of titanium implants coupled to dental alloys

The corrosion of five materials for implant suprastructures (cast-titanium, machined-titanium, gold alloy, silver-palladium alloy and chromium-nickel alloy), was investigated in vitro, the materials being galvanically coupled to a titanium implant. Various electrochemical parameters E_CORR, i_CORR Evans diagrams, polarization resistance and Tafel slopes) were analyzed. The microstructure of the different dental materials was observed before and after corrosion processes by optical and electron microscopy. Besides, the metallic ions released in the saliva environment were quantified during the corrosion process by means of inductively coupled plasma-mass spectrometry technique (ICP-MS). The cast and machined titanium had the most passive current density at a given potential and chromium-nickel alloy had the most active critical current density values. The high gold content alloys have excellent resistance corrosion, although this decreases when the gold content is lower in the alloy. The palladium alloy had a low critical current density due to the presence of gallium in this composition but a selective dissolution of copper-rich phases was observed through energy dispersive X-ray analysis. ©2000 Kluwer Academic Publishers     

Data-driven evaluation of fatigue performance of additive manufactured parts using miniature specimens

This overview firstly introduces the state-of-the-art research progress in length scale-related fatigue performance of conventionally-fabricated metals evaluated by miniature specimens. Some key factors for size effects sensitive to microstructures including the specimen thickness, grain size and a ratio between them are highlighted to summarize some general rules for size effects. Then, ongoing research progress and new challenges in evaluating the fatigue performance of additive manufactured parts controlled by location-specific defects, microstructure heterogeneities as well as mechanical anisotropy using miniature specimen testing technique are discussed and addressed. Finally, a potential roadmap to establish a data-driven evaluation platform based on a large number of miniature specimen-based experiment data, theoretical computations and the ‘big data’ analysis with machine learning is proposed. It is expected that this overview would provide a novel strategy for the realistic evaluation and fast qualification of fatigue properties of additive manufactured parts we have been facing to.     

Hierarchical microstructure of explosive joints: Example of titanium to steel cladding

The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength ≈ 0.5 mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size ≈ 100-200 μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe_9.64Ti_0.36. Most of the observed phases do not appear in the equilibrium Fe-Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100-300 nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound.     

Bone response to a titanium aluminium nitride coating on metallic implants

The design, surface characteristics and strength of metallic implants are dependant on their intended use and clinical application. Surface modifications of materials may enable reduction of the time taken for osseointegration and improve the biological response of bio-mechanically favourable metals and alloys. The influence of a titanium aluminium nitride (TAN) coating on the response of bone to commercially pure titanium and austenitic 18/8 stainless steel wire is reported. TAN coated and plain rods of stainless steel and commercially pure titanium were implanted into the mid-shaft of the femur of Wistar rats. The femurs were harvested at four weeks and processed for scanning electron and light microscopy. All implants exhibited a favourable response in bone with no evidence of fibrous encapsulation. There was no significant difference in the amount of new bone formed around the different rods (osseoconduction), however, there was a greater degree of shrinkage separation of bone from the coated rods than from the plain rods (p = 0.017 stainless steel and p = 0.0085 titanium). TAN coating may result in reduced osseointegration between bone and implant.     

Early tissue response to titanium implants inserted in rabbit cortical bone

The tissue response to screw-shaped implants of commercially pure titanium was studied 3–180 days after insertion in the rabbit tibia by means of transmission electron microscopy. Red blood cells and scattered macrophages predominated at the implant surface after 3 days. At day 7 and later intervals, multinuclear giant cells were the cell type found at the implant surface protruding into the bone marrow and in areas with no bone-titanium contact. Osteoblasts or mesenchymal cells were rarely seen at the implant surface at any time period. Two modes of mineralization could be distinguished in the interface. Firstly, the typical mineralization of osteoid seams produced by osteoblasts. Secondly, an accumulation of scattered hydroxyapatite crystals in the unmineralized collagen matrix in the interface. Mineralized tissue was observed close to the implants surface from day 14. However, the innermost 2–20 m were poorly mineralized although scattered hydroxyapatite crystals were present. The collagen fibrils did not reach the implant surface but were separated from it by an amorphous layer, being 0.3–0.5 m thick which did not decrease in width with time. An electron-dense lamina limitans-like line containing mineral was observed between the amorphous layer and the bone tissue.     

Bone tissue reactions to biomimetic ion-substituted apatite surfaces on titanium implants

The aim of this study was to evaluate the bone tissue response to strontium- and silicon-substituted apatite (Sr-HA and Si-HA) modified titanium (Ti) implants. Sr-HA, Si-HA and HA were grown on thermally oxidized Ti implants by a biomimetic process. Oxidized implants were used as controls. Surface properties, i.e. chemical composition, surface thickness, morphology/pore characteristics, crystal structure and roughness, were characterized with various analytical techniques. The implants were inserted in rat tibiae and block biopsies were prepared for histology, histomorphometry and scanning electron microscopy analysis. Histologically, new bone formed on all implant surfaces. The bone was deposited directly onto the Sr-HA and Si-HA implants without any intervening soft tissue. The statistical analysis showed significant higher amount of bone–implant contact (BIC) for the Si-doped HA modification (P = 0.030), whereas significant higher bone area (BA) for the Sr-doped HA modification (P = 0.034), when compared with the non-doped HA modification. The differences were most pronounced at the early time point. The healing time had a significant impact for both BA and BIC (P < 0.001). The present results show that biomimetically prepared Si-HA and Sr-HA on Ti implants provided bioactivity and promoted early bone formation.     

Early tissue response to titanium implants inserted in rabbit cortical bone

The tissue response to screw-shaped implants of commercially pure titanium was studied by light microscopy 3–180 days after insertion in the rabbit tibia. The implant site in the tibial metaphysis consisted mainly of cortical bone. Three days after implantation, osteoblasts, producing osteoid, were observed at the endosteal surface and elongated mesenchymal cells were present in the injury area. Some macrophages but rather few other inflammatory cells were identified. Multinuclear giant cells were in direct contact with the implant and formed an almost continuous layer along the surface from the 7th day. The number of giant cells decreased with time and with increased bone-titanium contact. Bone formation was never seen direct on the implant surface but was first observed at day 7 as a woven trabecular bone formed at the endosteal surface and extending towards the implant and as a solitary formation of woven bone close to the implant. The solitary bone matrix served as a base for surface osteoblasts which produced osteoid in a lamellar arrangement. With time the two types of newly formed bone fused and more bone filled the threads and became remodelled by bone remodelling units. Light microscopic morphometry in ground sections demonstrated that the bone/titanium contact and bone area in the threads increased with time up to 6 months after implantation     

Measurement and modelling of residual stress in wire-feed additively manufactured titanium

Residual stresses were characterised in a wire-feed additively manufactured titanium alloy component. A numerical simulation based on the inherent strain method was used to model residual stresses arising from the manufacturing process. The contour method was used to experimentally determine the residual stress field. High tensile residual stresses were seen at and around the interface of the substrate and the deposited metal. Compressive residual stresses were present in the substrate and at the top of the deposit. The satisfactory correlation was achieved between the results from the numerical simulation and the contour method, except for the location of the root of the deposit. The effect of pre-heating the sample substrate on the residual stress distribution is also discussed.     

Failure analysis of a titanium strut tank support in the space shuttle Columbia

A titanium midtank strut support from the Columbia space shuttle was submitted for failure analysis. Stereomicroscopy, light optical microscopy, and scanning electron microscopy with energy-dispersive spectroscopy were used to investigate the failed part. Evidence of melting was found on the fracture surface. Additionally, the high temperature associated with the melting was combined with cooling from the β-phase field, as indicated by the formation of a martensitic microstructure in the surrounding material. Craters were found on the surface of the part due to impact of foreign particles containing iron, nickel, and chromium. Failure of the titanium strut support was due to the severe high temperatures encountered as a result of the shuttle breakup and re-entry into the earth’s atmosphere.     

Fabrication of porous titanium implants with biomechanical compatibility

By using H₂O₂ as foaming reagent, porous titanium with open and interconnected pore morphology was obtained. The morphology, pore structure and elemental composition were observed by SEM–EDX. The mechanical property was determined by compressive test. The results show that the compressive strength and Young's modulus of porous titanium with 64% porosity were 102 ± 10 MPa and 3.3 ± 0.8 GPa, respectively, and for 76% porosity porous titanium, the values were 23 ± 10 MPa and 2.1 ± 0.5 GPa. These results suggest that the former has sufficient mechanical properties for clinical use under load-bearing conditions and the latter has the potential application for tissue engineering scaffolds.     

Microstructure and mechanical behavior of laser additive manufactured AISI 316 stainless steel stringers

Laser additive manufacturing of stainless steels is a promising process for near net shape fabrication of parts requiring good mechanical and corrosion properties with a minimal waste generation. This work focuses on high aspect ratio AISI 316 steel structures made by superposition of sequential layers. A special nozzle for precise powder delivery together with a monomode fiber laser allowed producing high quality steel stringers on AISI 316 steel substrates. Although the stringers average compositions were inside the austenite plus ferrite range, only austenite phase was verified. The cladded structure presented some internal pores and cracks, responsible by the low Young’s moduli. However, the tensile properties were similar to the base material and other literature results. The three-point flexural tests also produced good results in terms of formability. The fabricated structures proved to be useful for use in mechanical construction.     

Production of new titanium alloy for orthopedic implants

The beta titanium alloys is one of the most promising groups of the titanium alloys. This fact is due to the good formability, mechanical properties and potential applications; moreover, these alloys present the highest level of mechanical, fatigue and corrosion resistance. The beta titanium alloys present the lowest elastic modulus, an interesting property for orthopedic implants. A β alloy recently developed for this application is Ti–35Nb–7Zr–5Ta. In this work, the alloy was produced by powder metallurgy, unique available alternative for obtaining parts with porous structure (until 50% of porosity), that is one important characteristic for the osteointegration. The Ti–35Nb–7Zr–5Ta samples were manufactured by blended elemental method from a sequence of uniaxial and cold isostatic pressing with subsequent densification by sintering among 900 at 1700 °C, in vacuum. The objective of this work is the analysis of alloy microstructural evolution from the elemental powders dissolution under the increase of the sintering temperature. The alloy was characterized by scanning electron microscopy, X-ray diffraction and Vickers microhardness measurements. Density was measured by Archimedes method. The results show that a β-homogeneous microstructure is obtained in the whole sample with the increase of sintering temperature. With the beginning of the β-stabilizers (Nb and Ta) dissolution, at low sintering temperatures, there is the formation of an intermediary Widmanstätten (α+β) phase.