Having missing teeth does not have to get in the way of your having a straight smile. Indeed, depending on the reason you have missing teeth, it may even be medically necessary for you to get orthodontic treatment in order to prevent further teeth from falling out or becoming unusable.
Yes, really. Sometimes, with cases of extreme crowding, dentists may even pull a few teeth out. From the point of view of orthodontics, it is not that different than from having a few teeth missing in the first place, depending on the reason that the teeth went missing in the first place.
After all, orthodontic appliances work by gently guiding teeth through alveolar tissue, by exerting force and pushing, pulling, or rotating them into a position in which they can work better. Orthodontics only work with the existing number of teeth that you actually have, and seeing as your orthodontic treatment should be tailored to you and your teeth, this should not be a problem.
What About Crowns?
No problem as long as the root is real. Crowns, you see, are on top of either existing teeth or tooth stubs, which can be moved about as they are real teeth anchored in real alveolar tissue or on top of dental implants. Dental implants cannot be moved about; they are drilled into your jawbone, and you do not want them to move at all. Bad idea. But if your crown is on top of natural teeth, they can be realigned just like all other teeth, so no problems there.
There are a few things to consider, though. If you already have missing teeth, and crowding elsewhere, this is an ideal situation. This way, your crowding will be gone and your teeth will have space to move into. What you should consider though is how much will your teeth move, and if that will leave gaps that are more problematic than the original spaces.
The other consideration if you already have missing teeth is how you are going to replace them. If you have too many teeth missing, you can start having tooth loss become a permanent condition, in which case you may start losing all of your teeth one by one. If you do choose to replace your missing teeth with a dental implant as is recommended, then you should tell your dentist that you want braces before getting them, this way you can know if you should get the dental implants before or after getting orthodontic treatment.