Hi there! In addition to Dr Sylvia's answer, I would like to add the following tips:
1) Avoid eating and drinking with your clear retainers. Food stains from curry, coffee, beetroot etc will penetrate your clear retainers and are very difficult to clean away.
2) Consistency is the Key. Food and Plaque deposits that are left on clear retainers over a long period of time hardens into calculus. This looks like chalky, white substances that are hard to rub off without professional dental help.
3) Do not try to sterilise clear retainers using hot water as this permanently distorts and warps them!
Good job wearing your retainers 4 years after finishing braces treatment!
Cheers,
Dr Poon Kee Hwang