How many kinds of dental appliances are there?

There are many kinds of materials for filling teeth. There are four kinds of braces at present. The first type is ordinary metal braces, which are relatively compact and durable. The second is ceramic braces, which are similar in appearance to real teeth. The third is self-locking braces, which are improved on more traditional appliances. The fourth one is invisible braces. It's very convenient to take off braces at ordinary times.