DOCTOR ยท CLINICAL GUIDE
The art of impression taking
Great aligner results start with great records. This guide covers the principles behind accurate impressions and scans for clear aligner treatment, tray selection and technique, the digital scanning workflow, the failure modes that get records rejected, and exactly what we need to see before a case moves into planning.
WHY IT MATTERS
Why accuracy matters
Your impression or scan is the foundation your entire case is built on. Distortions, voids or missing margins lead to ill-fitting aligners and avoidable refinements, so getting records right the first time saves time for everyone. Not checking impressions before sending wastes time and creates duplicate costs, which is why a few minutes of review at the chair is the cheapest insurance in the whole workflow.
PRINCIPLES
Principles of a good record
Capture the full arch with clean margins and accurate occlusion. Keep the field dry, work methodically, and review your capture before the patient leaves the chair. Whichever method you use, the rules are the same. We need every tooth present, clear gingival margin, no holes or distortion, and a true bite with the patient in centric occlusion.
Full arch, all anatomy
Capture every tooth, including at least half of the 8’s where they are present, with nothing cut off at the ends of the arch.
Clear gingival margin
Show 2 to 4mm of gingival tissue around the teeth so the margin reads cleanly and tooth and gum stay distinct.
A true bite
Register an accurate bite with the patient in centric occlusion. Do not let the patient bite edge to edge.
PVS IMPRESSIONS
Tray selection and PVS technique
If you are taking physical impressions, use PVS material. Alginate impressions are rejected because they distort too much. Choose the correct tray size so the rear molars are fully covered, and let the material overflow both ends of the tray to capture the distal of the second molars, and the 8’s if they are present. If 8’s are present we need to see at least half of them. Capture 2 to 4mm of gingival margin all the way round, and where you can, provide a good bite registration for digitising.
Before you arrange a collection, inspect the impression carefully. Check that all anatomy is present and look for air bubbles, double impressions, missing anatomy, drag, push-through, or blended tooth and gingiva. A record that reads cleanly here is a record that moves straight into planning.
- Alginate impressions are rejected, they distort too much
- Use the correct tray size, with the rear molars covered
- Overflow both ends of the tray to capture the distal of the second molars, and 8’s if present
- If 8’s are present, we need at least half of them
- We need to see 2 to 4mm of gingival margin
- If possible, provide a good bite registration for digitising
DIGITAL SCANS
Scanning vs impressions
A digital scan removes many of the variables that affect physical impressions and integrates directly into the digital workflow. Whichever you use, technique and attention to detail are what make the record dependable. For a scan, make sure all anatomy is visible, including at least half the 8’s if they are visible, and that there are no holes in the scan. Capture 2 to 4mm of gingival tissue, and register an accurate bite with the patient in centric occlusion, never edge to edge.
Watch for debris. If debris is picked up during the scan, make sure it is not attached to any anatomy, otherwise the scan is rejected and you will need to rescan. A clean, complete, hole-free scan with a true bite is accepted first time.
- Make sure all anatomy is visible, including at least half the 8’s if they are visible
- Make sure there are no holes in the scan
- If debris is picked up during the scan, ensure it is not attached to any anatomy, otherwise the scan is rejected and you will need to rescan
- Capture 2 to 4mm of gingival tissue
- Accurate bite with the patient in centric occlusion, do not let the patient bite edge to edge
SCAN STRATEGY
Why scan strategy matters
Scan strategy directly affects the accuracy and dimensional stability of a full-arch scan, because it determines how individual images are stitched together in the scanning software. Think of drawing a 5-metre line using only a 15cm ruler. Small errors accumulate over each segment, and over a longer distance the deviation grows. Full-arch scanning is similar. As the scanner head moves across a surface, images are stitched rapidly, and on longer spans errors add up and cause deformation. Sufficiently deformed upper and lower arch scans also make it hard for the software to superimpose the buccal bite scan and find the best fit, which is a common cause of unreliable bite registration. A guided, sequence-based scan strategy is what avoids this.
For a worked example, our Dentsply Sirona scan strategy guide covers the recommended Omnicam and Primescan sequences in full, working distances, the four-part occlusal, buccal, lingual and approximal approach, buccal bite registration, and how to export to the lab.
FAILURE MODES
What gets a record rejected
Most rejections come down to a short list of avoidable faults. Knowing what they look like means you can catch them at the chair rather than after a collection has been arranged. Inspect every record against the points below before you send it.
On a PVS impression
- Air bubbles and voids in the material
- Double impressions
- Missing anatomy or teeth cut off at the ends
- Drag and push-through
- Blended tooth and gingiva
- Alginate material, which is always rejected
On a digital scan
- Holes in the scan
- Missing anatomy or 8’s not captured
- Debris attached to anatomy
- Too little gingival margin captured
- An edge-to-edge or inaccurate bite
- Deformation from a poor scan sequence
SUBMISSION
Submitting your records
Once your record reads cleanly, submit a clean, full-arch digital scan, or accurate physical impressions, capturing clear margins and a true bite. Pair the record with a complete prescription stating your treatment goals and any specific instructions, the standard clinical photo set, and signed patient consent. The clearer the brief, the closer the proposed plan will be to what you want. If you are sending physical impressions, include the impression collection slip so your submission is logged and routed correctly.
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