Orthodontic polyurethane elastomeric ligatures 1,2 may suffer compositional changes under the effect of enzymes, water and humidity present in the oral cavity 1 being also subject to pH and temperature changes as well as bacterial activity. 2,3 However, the exact polyurethane composition is proprietary to each manufacturer.Complaints relative to colour instability of aesthetic elastomeric ligatures have become a rather frequent event. 1,2,[4][5][6][7][8] They may also be related to physical properties alterations, in particular the loss