For information regarding how aligners should fit, refer to the following:

• Overview
• Fitting Issues
• Troubleshooting for Fitting Issues

Overview

What should a properly fitting aligner look like?

A properly fitting aligner should cover the teeth and fit snugly against the patient’s teeth. It will probably feel a little tight at first.

• The aligner should usually cover the gingival by at least 0–2 millimeters. (Aligners may be trimmed differently to compensate for undercuts, black triangles, recession, bridges, etc.)
• The incisal edges of the teeth should fit flush against the aligner without any gaps.
• The aligner should fit snugly over the distal surfaces of the rearmost molars, if the aligner extends that far back.

Fitting issues

Fitting issues can occasionally occur during treatment. There are a few possible causes for fitting problems:

• If the first step doesn't fit, the most likely cause is an inaccurate 3D model. This can be caused by distortions in the initial impressions or scans.
• If the fit of the aligners gets worse over time, the teeth may not be "tracking"—in other words, the aligners are progressing as originally planned in the treatment setup, but the actual teeth aren't keeping up due to lack of space or insufficient pressure.
• In rare circumstances, an aligner may be distorted due to physical damage or a manufacturing defect.

Troubleshooting Fitting issues

For detailed troubleshooting for fitting issues, refer to the following links: 

• Incisal gaps
• Aligners that are too big or too small
• Aligners that fit on only one side