An underbite is a common term used to describe the lower front teeth being further forwards and in front of the upper front teeth. Though the front teeth give us a measure of how much correction is necessary, there are other reference points that your dentist/orthodontist can use to help correct your bite.
The most important consideration is your facial profile, how your jaw position looks like and what are the treatment goals for correcting the bite. I will use the landmarks of the jawbones to determine what are the movements required to achieve a better facial profile and plan the treatment accordingly.
If the goal is not to change the facial profile and only correct the teeth position, I will use the bite of your other teeth as a reference to determine how much movement is necessary.
Teeth fit each other like a jigsaw puzzle, hence the back teeth bite gives me a clue on how to position the front teeth. I can also use the canine position as a reference for where the ideal bite should be. Your dentist can also do a diagnostic mock-up (i.e a set up of the final outcome with the missing teeth replaced in its ideal location) plan the treatment based on this setup.
