Keiichi Sasaki Osamu Suzuki - Interface Oral Health Science 2016

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Mitsuo Niinomi

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To inhibit stress shielding, which is caused by the mismatch in Youngs modulus between an implant and bone and leads to bone absorption and poor bone remodeling, low-Youngs modulus titanium (Ti) alloys have been or are being developed for implant devices such as artificial hip joints, bone plates, and spinal fixation devices.

A novel -type Ti alloy composed of nontoxic and allergy-free elements (Ti29Nb13Ta4.6Zr mass%, hereafter abbreviated as TNTZ) is a low-Youngs modulus Ti alloy developed by Niinomi et al. based on the d-electron design method []. The amount of springback is determined by both the strength and Youngs modulus. Given the same strength, a rod with the higher Youngs modulus will exhibit less springback, that is, a high Youngs modulus is preferable to suppress springback from the viewpoint of surgeons. There is therefore a conflicting requirement concerning the Youngs modulus from the viewpoint of patients and surgeons, which cannot be fully satisfied by TNTZ.

Recently, many researchers [] have focused on the development of new -type Ti alloys to solve this problem, such as TiMo and TiCr alloys. These alloys possess a novel property called changeable Youngs modulus or Youngs modulus self-adjustability, in which the deformed region of the material exhibits a high Youngs modulus, whereas the Youngs modulus of the undeformed region remains low.

Therefore, changeable-Youngs modulus Ti alloys suitable for the rods of spinal fixation devices are introduced with a focus on TiMo and TiCr alloys.

The springback of the deformed rods of spinal fixation device can be reduced by increasing the Youngs modulus in the deformed region. This reduction can be achieved if a secondary phase with a high Youngs modulus is induced by deformation. In Ti alloys, metastable phases such as martensite with hcp structure, martensite with orthorhombic structure, and -phase with hexagonal or trigonal structure can form between the stable - and -phases based on the amount of -stabilizing element, as depicted in the schematic phase diagram of Ti alloys in Fig.. Therefore, various phases such as -, -, and -phases can be induced by deformation. -phase precipitation is known to increase the Youngs modulus of the alloys, whereas - and -phase precipitations result in a decrease. Therefore, if the -phase stability is controlled for the -phase to be induced by deformation, the Youngs modulus of the alloys is partially increased in the deformed region, leading to a decrease in springback of the alloy and thereby maintaining the deformed shape of the rod.

The addition of molybdenum (Mo) results in -stabilizing properties in Ti alloys [] reported that TiMo alloys exhibit excellent corrosion resistance.

Hanada et al. reported that the -phase forms during the cold working of as-quenched TiCr alloys within the composition range of 811.5 mass% Cr [].

Thus, Mo and Cr are suitable alloying elements to develop Ti-based biomaterials with Youngs modulus adjustability.

However, as depicted in the schematic illustration of the relationship between temperature and alloying element content in Fig. ], with each appearing phase, deformation-induced -phase precipitation occurs, and the lowest Youngs modulus before deformation is highly expected in the relatively high stability region.

To optimize the chemical compositions of TiMo and TiCr alloys exhibiting the greatest increase in Youngs modulus after deformation (the best Youngs modulus adjustability), the Mo equivalent, Moeq, was considered, and 1 mass%Cr is equivalent to 1.25 mass%Mo. Then, the TiMo and TiCr alloys, whose nominal chemical compositions are listed in Table ], were examined. The Youngs moduli, microstructures, and deformation behaviors of both alloy systems were examined before and after deformation; deformation was simulated by cold rolling with a reduction of 10 %.