To confirm a crown open margin, first prove the crown is fully seated, then check the margin with a sharp explorer, magnification, and a bitewing where the margin is interproximal. Most crowns that look open at try-in are actually sitting high on a tight contact, an internal binding spot, or leftover provisional cement. If the gap is still there after full seating, do not cement over it: document it, recapture the prep, and send the lab the evidence it needs to find the cause.

This guide is a chairside checklist for that moment. It covers how to tell a seating problem from a true margin problem, what each finding usually means, and what to send back so the next crown fits without another round trip.

Reviewed by Dr. Kellen McWhorter, prosthodontist and chief clinician at Peak Dental Studio.

What Counts as an Open Margin on a Crown

An open margin is a gap between the crown edge and the prepared finish line that you can detect clinically, either by an explorer catch, by direct vision, or on a radiograph. It is different from an overhang, where the crown extends past the finish line, and from a short margin, where the crown stops before the finish line and leaves prepared tooth exposed.

The distinction matters because each one has a different cause:

• Open margin: the crown is not reaching the tooth. The cause is often incomplete seating or an internal fit problem.
• Short margin: the crown reaches the tooth but stops early. The cause is often a finish line the technician could not read on the scan or die.
• Overhang: the crown runs past the finish line. The cause is often a traced margin placed beyond the real prep, or tissue captured as tooth.

Name the problem precisely before you decide what to do. “Open on the distal” and “short on the distal” send the lab looking in two different places.

Step 1: Prove the Crown Is Fully Seated

Before you judge the margin, rule out the three things that hold a well made crown off the prep. A crown that is open all the way around is usually a seating problem, not a margin problem.

Work through these in order:

1. Clean the prep. Remove every trace of provisional cement and debris. A thin film on one wall is enough to lift the crown.
2. Check the contacts. Seat the crown and pass floss through each contact. If floss will not pass, or snaps through hard, the contact may be holding the crown up. Adjust and polish the contact in small steps, then reseat and recheck the margin.
3. Check internal fit. Load the crown with a low-viscosity fit-checking silicone, seat it with firm pressure, and remove it. Show-through spots mark internal binding. Relieve only those spots, conservatively, and repeat.
4. Check the tissue. On subgingival margins, swollen or displaced tissue can block the crown or hide the margin. Note it rather than forcing the seat.

Only after the crown seats against firm, even resistance should you evaluate the margin itself. If the gap closes after a contact adjustment, the margin was never the problem.

Step 2: Check the Margin the Right Way

Use more than one method, because each one sees a different part of the margin. An explorer, magnification, and a radiograph together give you a reliable read.

• Explorer. Run a sharp explorer from crown to tooth, then from tooth to crown, around the full circumference. A catch going crown to tooth suggests an open or short margin. A catch going tooth to crown suggests an overhang.
• Magnification. Loupes or a microscope, with good light and a dry field, show color and contour changes the explorer misses, especially on facial margins.
• Bitewing radiograph. Useful for interproximal margins on radiopaque materials such as zirconia and metal ceramic. It cannot show facial or lingual margins, and lithium disilicate is less radiopaque, so read it with that in mind.

How much gap is acceptable? Published research has long cited roughly 120 microns as a clinically acceptable marginal gap, which is far finer than you can judge by eye. In practice, many clinicians treat any catch they can feel with a sharp explorer as a margin worth stopping for.

Step 3: Read the Pattern Before You Decide

Where the gap shows up tells you a lot about why it happened. Map the open area before you remove the crown.

• Open all the way around: almost always incomplete seating. Recheck contacts, internal binding, and debris.
• Open on one surface, contacts fine: often a capture problem at that surface. Tissue, blood, or saliva may have covered the finish line, or the scan may have stitched poorly there.
• Open on one side, tight on the other: the crown may be rocking on an internal high spot or a tight contact on the tight side.
• Short margin on one area: the finish line was likely unreadable on the scan or die, so the technician had to estimate where it ended.
• Margin fit on the model but not in the mouth: suspect a change between capture and try-in. The prep may have been refined after the scan, or the provisional may have shifted the tissue or the tooth.

Our guide to crown margins and prep design covers which finish lines read cleanly on a scan and which ones force the lab to guess.

Step 4: Decide Whether to Adjust, Recapture, or Remake

If the crown is fully seated and the margin is still open, the safe answer is not to cement it. Cement and bonding agents are not designed to close a marginal gap, and an open margin is a place for plaque, recurrent decay, and tissue irritation to start.

Your realistic options are:

• Adjust and reseat when the cause was contacts, debris, or internal binding and the margin closes once the crown seats.
• Recapture and remake when the crown seats fully but the margin is still open or short. A new scan or impression with the finish line clearly visible is the fix.
• Talk to your technician first when the cause is not obvious. A short call can tell you whether the issue traces to the scan, the die, the margin trace, or the design, and what the new capture needs to show.

A useful rule: do not send the patient home with a crown you would not want to remove in five years. A second try-in costs an appointment. Recurrent decay at an open margin costs the tooth’s future.

What to Send the Lab With a Returned Crown

Send evidence, not just the crown and a note that says “open margin.” The more precisely the technician can see what you saw, the faster they find the cause and the less likely the second crown repeats it.

Include:

• Location and type. Which surface, and whether it was open, short, or an overhang.
• What you already ruled out. Note whether you adjusted contacts, relieved internal spots, and cleaned the prep, and what changed after each step.
• Photos. A magnified photo of the seated crown at the problem margin, and a photo of the fit-checking silicone if you used it.
• A fresh capture. A new scan or impression with the margin fully retracted and visible all the way around. If you refined the prep, say so, and send a new bite record and opposing scan.
• Retraction notes. What you used to expose the margin this time. Our article on cord packing and retraction technique covers the options for subgingival margins.
• The original crown, and the model if the case was analog, so the technician can compare the old fit against the new capture.

This is where direct access to the technician who built the case pays off. One person who knows the original design can compare it against the new scan and tell you exactly what changed.

How to Prevent the Next Open Margin

Most open margins are decided at capture, before the case reaches the lab. A readable finish line and an honest chairside check of the scan remove most of the guesswork.

The habits that matter most:

• Retract and dry before you scan, especially on subgingival margins.
• Zoom in on the scan before the patient leaves and rescan any segment where the margin breaks up.
• Flag any margin you are unsure of in the prescription, so the technician can treat it with extra care.
• Keep the provisional well adapted so the tissue and the tooth are where they were at capture.

For the full prevention side, see our checklist on how to reduce crown remakes, which walks through margin capture, reduction, shade, and the prescription.

Frequently Asked Questions

Can you cement a crown with a small open margin?

It is not recommended. Cement is not designed to fill a marginal gap, and the exposed cement line is where plaque and recurrent decay tend to start. If a sharp explorer catches at the margin after the crown is fully seated, recapture and remake is the predictable path.

How do you tell an open margin from a crown that is not fully seated?

Look at the pattern. A gap all the way around usually means the crown is sitting high on a tight contact, an internal binding spot, or debris. A gap on one surface with good contacts and even internal fit points to a true margin or capture problem at that surface.

Will a bitewing show an open crown margin?

A bitewing can show interproximal open margins on radiopaque materials such as zirconia and metal ceramic. It cannot show facial or lingual margins, and lithium disilicate is less radiopaque, so pair the radiograph with an explorer and magnification.

What causes a crown open margin if the prep looked good?

The usual causes are a finish line covered by tissue, blood, or saliva at capture, a scan that stitched poorly at one surface, a prep refined after the scan, or a provisional that let the tissue or tooth shift. Sending photos and a fresh capture helps the lab pinpoint which one it was.

Send Your Next Crown Case

If you want a crown and bridge lab where one technician owns your case and you can talk to that person directly when a margin needs a second look, send your first case to Peak. You can see what we make on our crown and bridge services page.