Take a look here for a detailed explanation: nous nous sommes brossé les cheveux
I hope this is helpful.
Bonne journée !
This type of query has been answered many times in the past but is very tricky to get right.
The point to understand is just what does the reflexive pronoun represent?
Does it represent a direct object or an indirect object? We are brushing the hair of ourselves.
I write it in this way to point out that there is the preposition "of" before "ourselves" - (second reflexive pronoun nous).
Therefore there is no past participle agreement because the reflexive pronoun (nous) represents an indirect object, not a direct object.
Similarly with your "teeth" example -- it is the teeth of themselves therefore no past participle agreement.
Hope this helps to explain for you.
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