It's hard to say how many possibilities there are for arch expansion. The first one is that your upper dental arch is smaller than your lower dental arch, and the second one is that your dentition is crowded and you are required not to extract teeth. If you are the second type, the recurrence rate is very high. Also, if you want to see you protrude by 3 mm, you'd better give me your profile. 77515 109@qq.com, my email address.