There are a lot of misconceptions out there about teeth. One of the most common ones that I come across is that teeth stay straight as we get older. That simply isn't true (at least for most people).
I'll start with the 800-pound gorilla in the room - The best studies that we have about teeth staying straight tell us that they don't stay straight. I wish they did! It would make my job as an orthodontist so much easier if I could line teeth up and they would stay there. But, the reality is, for most people, that isn't the case. Don't believe me? Just check out the research, especially the articles by Dr. Robert Little. Even when we extracted teeth on most of our patients, that still didn't lead to long-term stability. That's why most orthodontists don't extract permanent teeth on the majority of their patients anymore.
So, we know that teeth get more crowded with age, whether we do braces or not. Quite frankly, we, as orthodontists, haven't done a terrific job in the past of telling this to our patients. For this reason, it's fairly common to see adults back in either braces or Invisalign that have had orthodontic treatment earlier in life. I have patients going through treatment for their third and fourth times... because their teeth didn't stay put. That's frustrating for me but way more frustrating for them as they repeatedly pay for the same thing.
What do we do about this? Well, each orthodontist has their own answer to that question. For an increasing number of us, it seems that answer is permanent (or bonded) retainers. Permanent probably isn't the best name for them... because, like anything that we dentists put in the mouth, you can break them. But, the name does at least give people the idea that they are for life. Bonded retainers are glued to the back of the upper and lower front teeth with tooth-colored glue. It is the same material that is used for white fillings. Just like those fillings, it tends to break down with time - but it is easily replaceable. Nowadays, there are enough of us orthodontists placing bonded retainers that it shouldn't be difficult to find someone to repair it if when something happens.
Because I am paranoid about teeth moving after orthodontic treatment, I also give my patients upper and lower clear retainers - the bottom picture in the previous link. These serve two main purposes: 1 - to hold the back teeth... especially for patients who have had a lot of expansion of the back teeth, and 2 - to serve as a backup if the bonded retainers are ever broken. I ask my patients to wear their clear retainers at night time only after their braces are removed. For the first year, I tell them to wear them every night. After that, it gets a little tricky. Most people can put them in once every week or two, and they will still fit fine. However, some people's teeth still shift quickly, even after a year... so they may have to wear them more often. It varies from person to person. The safe answer is to wear them nightly for the rest of your life. That's the thing that no orthodontist actually wants to tell their patients... but for some people, it's just the truth. One upside to wearing the clear retainers is that they can be used as whitening trays. The gel can be purchased from most general dentists and isn't very expensive.
I hope that helps clear things up about retainers. If you have any questions, feel free to let us know.