The effect of hydrogen ion concentration on the force-degradation rate of orthodontic polyurethane chain elastics.

The effect of pH on the force-degradation rates of seven commercial orthodontic polyurethane chain elastics was evaluated in an in vitro study. The pH values of 4.95 and 7.26 were selected for testing because they represent values close to the reported extremes of plaque and saliva pH. Seven test elastic products were extended to (1) equal distances and (2) equal initial force levels, and the force-degradation rates were recorded over 4 weeks. All the test products yielded a significantly greater force-decay rate in the basic (pH 7.26) solution than in the acidic (pH 4.95) solution over 4 weeks. A hypothesis is presented that the decay rate of orthodontic polyurethane chain elastics is inversely proportional to the oral pH, with a corollary that basic pH levels are most hostile to polyurethane chain elastics.