Degradation and fracture of Ni-Ti superelastic wire in an oral cavity

Superelastic Ni-Ti wire is widely used in orthodontic clinics, but delayed fracture in the oral cavity has been observed. Because hydrogen embrittleme nt is known to cause damage to Ti alloy systems, orthodontic wires were cha rged with hydrogen using an electro-chemical system in saline. Tensile test s were carried out, and fracture surfaces were observed after hydrogen char ging. The strength of the Co-Cr alloy and stainless steel used in orthodont ic treatment, was not affected by the hydrogen charging. However, Ni-Ti wir e showed significant decreases in strength. The critical stress of martensi te transformation was increased with increasing hydrogen charging, and the alloy was embrittled. The Fractured surface of the alloys with severe hydro gen charging exhibited dimple patterns similar to those in the alloys from patients. In view of the galvanic current in the mouth, the fracture of the Ni-Ti alloys might be attributed to the degradation of the mechanical prop erties due to hydrogen absorption. (C) 2001 Elsevier Science Ltd. All right s reserved.