Is it fine to wear a cracked Invisalign retainer or should I get it replaced?

Hi,

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You should always clean off the last set of aligners and keep them somewhere safe when you move on to your next set. These can be used to maintain your progress if you break or lose your current set. If the crack has not split the tray, and if the tray is holding together pretty well, then you can continue wearing the tray.

Be careful while putting the aligner on and off, as the crack can get worse when removing your aligners. Use a flat edge stick to pop off the aligners and reduce bending while taking them off. When putting them back, press evenly and firmly to avoid adding additional stress to the aligners.

If the split is bad, and if the aligner cannot hold its shape, stop wearing the aligner. You will not get any benefits from wearing the tray. Go back to the previous tray to maintain your progress until you have a replacement aligner.