Enhancing Functionalities of Metallic Materials by Controlling Phase Stability for Use in Orthopedic Implants
This chapter aims to review the recent trends pertaining to the enhanced functionalities, including low Young’s modulus, self-tunable Young’s modulus, and low magnetic susceptibility, of titanium and zirconium alloys for use in orthopedic implants. These value-added functionalities can be realized by controlling the type of crystal structure and their lattice structure stabilities, which are related to the phase stability of titanium and zirconium alloys.
KeywordsMagnetic susceptibility Metallic materials Orthopedic implant Phase stability Young’s modulus
One of the most important factors concerning the use of orthopedic implants is to ensure safety in usage, which is often associated with their mechanical reliability to endure physiologically cyclic loading and unexpected large loads during treatment. Given these considerations, metallic materials are advantageous over ceramic and polymeric materials for use as implantable materials. Therefore, more than 80 % of the implant devices used till date are made of metallic materials . Another important factor concerning the use of orthopedic implants is their toxicity toward living tissues. In general, the human body inherently resists any incoming toxic element. In other words, human body exhibits low permittivity to highly toxic elements eluted from orthopedic implants . That is, the toxicity of orthopedic implants depends not only on the nature of the metallic elements but also on the amount of them, which, in turn, strongly depends on the corrosion resistance of each metallic material. Therefore, in the human body, a metallic material with high corrosion resistance is highly imperative to ensure their safe usage as orthopedic implants.
Conventionally, industrial metallic materials with high corrosion resistance, such as stainless steels (SUS316L), titanium (Ti) alloys (CP Ti and Ti–6Al–4V ELI alloys), and cobalt (Co) alloys (Co–Cr alloys), have been widely used in biomedical applications . Among these materials, Ti alloys have recently attracted considerable attention because of the feasibility of imparting improved functionalities to orthopedic implants. For instance, Ti undergoes allotropic transformation at 1,155 K, which is considered to be very important in terms of phase stability to obtain various functions. In simple terms, Ti alloys can be tuned to perform special functions by adept control of phase stability by varying the chemical composition. In addition, zirconium (Zr), which is one of the congeners of Ti, has also received considerable attention, and new Zr alloys for orthopedic implants have been developed on the basis of phase stability.
In this chapter, we have reviewed the latest trends in the development of Ti and Zr alloys for orthopedic implants with special functionalities, especially those obtained by controlling phase stability.
7.2 Low Young’s Modulus
7.3 Wear Properties of Low Young’s Modulus Titanium Alloy
7.4 Self-Tunable Young’s Modulus
In case of spinal fixation devices, high rigidity can increase the risks of stress shielding effect and adjacent segment degeneration. Therefore, materials with low Young’s modulus are often preferred to realize healthy spine formation [28, 29]. However, these devices also require high Young’s modulus as they are subjected to bending during surgery to obtain the physiological curvature of the spine . In this case, the device must be bent within a limited space inside the patient’s body. Therefore, it is often difficult for the surgeon to make an intended curvature if the spring-back of the spinal fixation devices is relatively large . Furthermore, it has been reported that the bending tool used by a surgeon to bend the device often leads to scratches on the device surface during the surgery. This, in turn, decreases the mechanical reliability of the spinal fixation devices . Large spring-back leads to difficulty in bending, resulting in the repetition of contouring during operation. This increases the risk of failure of spinal fixation devices . The degree of spring-back depends on both the strength and Young’s modulus of spinal fixation devices. Given the same strength, it is the spinal fixation devices with higher Young’s modulus that will exhibit a smaller spring-back. That is, these devices are often preferred to suppress the spring-back . Therefore, there is a conflicting requirement in Young’s modulus from the viewpoint of patients and surgeons, which cannot be completely satisfied by β-type Ti alloys with low Young’s modulus . In order to overcome this issue, recent studies have proposed a novel concept using a deformation-induced ω-phase transformation in β-type Ti alloys , such as Ti–Cr , Ti–Mo , Ti–Zr–Mo , Ti–Zr–Mo–Cr , and Ti–Cr–O  alloys. These materials exhibit novel functionality, wherein the deformed material possesses high Young’s modulus, while the non-deformed part has low Young’s modulus. This is made possible by the phenomenon of deformation-induced ω-phase transformation localized within the deformed part of the material, which provides an opportunity to satisfy the conflicting requirement in terms of Young’s modulus.
7.5 Low Magnetic Susceptibility
Metallic materials used in orthopedic implants are required to have high mechanical reliability and corrosion resistance. In addition to these conventional properties, additional value-added functionalities are being considered beneficial for the successful use of metallic materials in orthopedic implants. Therefore, in this chapter, the authors have reviewed the recent topics pertaining to the improved functionalities (low Young’s modulus, self-tunable Young’s modulus, and low magnetic susceptibility) of titanium and zirconium alloys via controlling the phase stability, which imparts essential functionalities to the implants. This overview is expected to facilitate a better understanding of biomedical metallic materials in potential future applications.
This work was partly supported by the inter-university cooperative research program “Innovative Research for Biosis–Abiosis Intelligent Interface” from Ministry of Sports, Culture, and Education, Japan, the Industrial Technology Research Grant Program in 2009 from the New Energy and Industrial Technology Development Organization (NEDO), and Grant-in-Aid for Scientific Research (A), Young Scientists (A), and Challenging Exploratory Research from the Japan Society for the Promotion of Science (JSPS), Japan.
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