Invisalign wear time and lost aligners: the 20-hour rule in practice

Why 20 hours

An aligner only moves teeth while it is on them. Bone remodelling needs sustained pressure; when the tray is out for long stretches, the ligament and bone start to stabilize the tooth where it is, and the next tray, which assumes the previous movement happened, no longer fits. A slightly higher figure was traditionally quoted; 20 is the standard used at the recommended clinic and the practical floor. More is better, and an extra hour or two a day adds up over a year.

Counting honestly

Most people overestimate their wear time, and it is rarely meals that cost them it. Trays come out for every drink that is not water, and a coffee here and a soft drink there add up faster than the three meals do. Grazing through the day is the other common drain: each small snack means the trays come out, teeth are cleaned and the trays go back in.

The habit that fixes it is simply getting the trays back in as soon as you have finished and cleaned your teeth, rather than leaving them out until the next time you think of them. A timer app, or noting the time the trays come out, keeps the count honest. Trays out at a long dinner with friends is fine once; every night, it is not. If you are unsure whether your wear time is where it needs to be, your orthodontist can tell from how the trays are tracking.

Changing trays

• Change on the schedule set by the orthodontist, in days counted from the last change. Weekly is common; some movements are prescribed longer. The old two-week rule is not universal.
• Change at night, so the tightest hours are spent asleep.
• Use chewies for five minutes after inserting a new tray to seat it fully.
• If the current tray still feels tight on change day after a poor-wear week, wear it two or three more days before moving on. Do not skip trays to catch up.
• Keep the previous tray until the next one has been worn a few days. Then keep the most recent one as the standing backup.

Tracking problems

"Tracking" means the teeth are following the plan. A tray that seats fully with no gap between its edge and the gum line is tracking. A visible gap at the biting edge of a tooth, especially a canine or a lateral incisor, means that tooth is lagging. Chewies and more hours often fix it within a week. If not, tell the clinic; a refinement scan or an attachment tweak may be needed. Ignoring a lagging tooth for several trays makes the problem bigger.

Lost, cracked or ill-fitting

• The current tray is lost
  • The previous tray is available: Put it in now. Call the clinic the same day. Depending on where you are in the tray's schedule, they may advise moving to the next tray early or ordering a replacement.
  • The previous tray is gone too: Put in the next tray if you have it and it seats reasonably; call the same day. If nothing fits, call; teeth can drift within days.
• The current tray is cracked
  • It still seats fully: Keep wearing it, or move to the next tray if the crack is spreading and the clinic agrees. Call.
  • It no longer seats: Treat as lost.
• A tray no longer seats fully after a lapse in wear
  • Any: Do not force it. Go back to the last tray that seats fully and wear it full-time. Call; a new scan may be needed. A few days of lapse is usually recoverable; a few weeks usually means a refinement.
• A tray has a sharp edge
  • Yes: Smooth it with a clean emery board or a nail file, or put wax on it. Not urgent.

Questions people ask