Agarwal M, et al. - The long-term impacts of 7 different cleaning methods on light transmittance, surface roughness, and flexural modulus of a polyurethane retainer material were studied by the experts. Out of the 7 cleaning methods, Invisalign cleaning crystals, Polident, and Listerine were noted to produce the least amount of change in light transmittance values for the polyurethane specimens over 6 months. No effect on surface roughness and flexural modulus values were determined.