Biomaterials 24 (2003) 2113–2120 Delayed fracture of beta titanium orthodontic wire in fluoride aqueous solutions Kazuyuki Kaneko a , Ken’ichi Yokoyama b, *, Keiji Moriyama a , Kenzo Asaoka b , Jun’ichi Sakai c , Michihiko Nagumo c a Department of Orthodontics, School of Dentistry, The University of Tokushima, 3-18-15 Kuramoto-cho, Tokushima 770-8504, Japan b Department of Dental Engineering, School of Dentistry, The University of Tokushima, 3-18-15 Kuramoto-cho, Tokushima 770-8504, Japan c Department of Materials Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 10 October 2002; accepted 13 December 2002 Abstract Hydrogen embrittlement of a beta titanium orthodontic wire has been examined by means of a delayed-fracture test in acid and neutral fluoride aqueous solutions and hydrogen thermal desorption analysis. The time to fracture increased with decreasing applied stress in 2.0% and 0.2% acidulated phosphate fluoride (APF) solutions. The fracture mode changed from ductile to brittle when the applied stress was lower than 500 MPa in 2.0% APF solution. On the other hand, the delayed fracture did not occur within 1000 h in neutral NaF solutions, although general corrosion was also observed similar to that in APF solutions. Hydrogen desorption of the delayed-fracture-tested specimens was observed with a peak at approximately 5001C. The amount of absorbed hydrogen was 5000– 6500 mass ppm under an applied stress in 2.0% APF solution for 24 h. It is concluded that the immersion in fluoride solutions leads to the degradation of the mechanical properties and fracture of beta titanium alloy associated with hydrogen absorption. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Beta titanium; TMA; Orthodontic wire; Delayed fracture; Hydrogen embrittlement; Fluoride 1. Introduction A beta titanium alloy for orthodontic wire was first introduced by Goldberg and Burstone in 1979 [1]. The alloy is now marketed under the brand name TMA (Ormco Corporation, Glendora, CA), which stands for ‘‘titanium–molybdenum alloy’’. The beta titanium alloy exhibits excellent properties, including low elastic modulus, high springback, high formability and high weldability compared to conventional stainless steel and cobalt–chromium–nickel orthodontic wires [2–9]. More- over, the advantage of this alloy over other orthodontic wires is that it does not contain nickel. However, the alloy has a known tendency to fracture during clinical use. If this tendency is eliminated, the alloy will be used more widely in the future. We have proposed that one of the reasons for the fracture of titanium and its alloys is hydrogen embrit- tlement in the oral cavity [10–15]. The fracture or degradation of mechanical properties caused by hydro- gen is generally termed hydrogen embrittlement. This hydrogen embrittlement is often represented as a reduction in both fracture strain and area, and is accompanied by a change in the fracture mode. Hydrogen embrittlement of titanium alloys such as Ni–Ti superelastic alloy in the oral cavity sometimes occurs in the presence of fluoride [15]. Fluoride is added in toothpaste, prophylactic agents, and dental rinse, because of its cariostatic effect. Caries-preventing prophylactics generally contain 100–10000 ppm F, with pH between about 3.5 and neutral. The effects of fluoride on titanium and its alloys have been investi- gated from the viewpoint of corrosion or discoloration by several workers [16–29]. It was clarified that the corrosion resistance of titanium and its alloys markedly decreases in fluoride solutions. However, the hydrogen embrittlement of the beta titanium alloy in fluoride *Corresponding author. Tel.: +81-88-633-7334; fax: +81-88-633- 9125. E-mail address: yokken@dent.tokushima-u.ac.jp (K. Yokoyama). 0142-9612/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9612(02)00642-7