If the speaker meant device/equipment, it's likely that an article would have been used (as in "Eu uso um aparelho"). If the defined article is used (i.e. "Eu uso o aparelho"), we would also expect some kind of context explaining which device/equipment we are talking about.
Without any article, I think the sentence can really be braces only. (However, I'm not a native.)
Thank you, Duolingo, for offering us quirky phrases what are not always obvious to someone learning the language. Every language has this, and even the quirks of the United States are not shared by the English. I can use a lift in a hotel in London and take a lift on a road in Wyoming.
The quirks of Americans aren't shared all that much by Canadians, either. There's even some specific differences between their English and ours. (for example, in some circumstances their English allows for "...you and me") Generally, I find that most Americans feel their English is 'proper' English, far more than English spoken even by the English themselves. The word 'compensation' comes to mind, for some reason...
Because it's "a device on your teeth", I suppose :-) Whereas 'braces' are plural in English, it's not necessarily that way in other languages. In Norwegian, e.g. (which is my native tongue), the word would be "regulering", which is also singular and actually means 'regulation' (meaning that you are 'regulating' the position of your teeth).