Quick answer
Teeth drift back toward their old positions after braces or Invisalign because the fibres holding each tooth take months to remodel and faces keep changing throughout adult life. Retainers hold the result: removable plates or clear retainers are typically worn nightly, full-time at first, and a fixed (bonded) wire behind the front teeth works constantly without depending on memory. Plan on retention indefinitely, checked at routine visits. A broken, lost or tight retainer should be dealt with within days, not months, because drift starts quickly.
Why teeth drift back
Each tooth sits in a hammock of elastic fibres. Move the tooth and the fibres stretch, then spend months slowly remodelling; until they do, they pull the tooth back like a rubber band. Add the fact that faces and jaws keep changing subtly through adult life, and unretained teeth drift, sometimes slowly, occasionally with surprising speed. Retention isn't an optional epilogue; it's the second half of treatment.
The two tools
- Removable retainers (clear trays or plates): worn as prescribed, usually a full-time phase then nightly. Their strength is full control of every tooth; their weakness is depending on human memory and surviving school bags and dogs, who genuinely love them.
- Fixed retainers (a thin wire bonded behind the front teeth): on duty around the clock with nothing to remember. They guard the front six teeth, need daily cleaning with floss threaders, and want a periodic check that the bond is intact.
Many patients have both: a fixed wire plus a nightly clear retainer over everything. Your plan says which and why, and both retainers are included in Align's written orthodontic quotes.
How long? Honest answer: indefinitely
The old "two years and done" advice produced a generation of adults with re-crowded lower front teeth. Current practice: retention continues indefinitely, tapering to nights, with the retainer checked at your routine dental visits. A nightly retainer is a five-second habit protecting a five-figure result.