Teeth moved with braces need time after the braces are removed to become stable.
First, teeth are moved over a relative short time. The way they bite with teeth in the opposing arch changes. Teeth need to wear into their new position and bite. This process can be sped up with a secondary treatment called occlusal equilibration. This is the selective reshaping of the biting surfaces of the teeth so they fit better together.
Second, the bone and ligaments that hold the teeth need to re-form. The bone needs to increase density and the ligament needs to re-organize it’s fibers.
These processes take time. Just as a broken bone takes time to reach full strength, so does the bone surrounding your teeth after they are moved with braces.
The ligaments become stretched. This is especially problematic if a tooth is turned or rotated. The fibers of the attachment ligament can re-rotate the tooth if they do not re-position. These fibers can be forced to re-position by cutting them and allowing them to re-attach. This is an uncomplicated procedure done with local anesthetic.
Third, it is normal and natural for teeth to move over time. It is un-natural for them to stay perfectly straight like at the end of orthodontic treatment. To counter this natural ageing process, retainers must be worn long term. They can be used like any other tool to maintain good health and a great smile.