Why Crowns Don't Seat, and How to Prevent It

A crown that will not seat costs chair time and patience on both sides. When we look back at the cases that came back for adjustment, the cause usually traces to a small handful of things. Most are preventable when the office and the lab each do their part. Here is what we see and what helps.

Unclear margins

If the margin is not clearly captured, we have to interpret where it is. A margin we read as slightly long can hold the crown up. One we read as short can leave an open edge.

Prevention:

• Use retraction and control moisture before scanning or taking the impression
• Check that the margin is visible all the way around
• Rescan or retake any area that is unclear rather than hoping we can read it

When we receive a case with a margin we cannot read with confidence, we call before we design.

Scan or impression distortion

Distortion can creep in at several points. In a scan, stitching errors, missing data and motion can affect accuracy. In a physical impression, pulls, voids, tears and early removal all affect the result.

Prevention:

• Review the scan before you send it, especially the prep and contacts
• Inspect impressions for voids and pulls before they leave the office
• Follow the impression material manufacturer's instructions for setting and handling

Our guide to sending digital scans to a dental lab covers what we need in a digital case.

Bite record issues

A bite record that is incomplete, distorted or taken in the wrong position will mount the models incorrectly. The crown may seat but need occlusal adjustment.

Prevention:

• Capture the bite in the position you want restored
• Make sure the record does not prevent full closure
• Include enough teeth on both sides for a stable relationship

Tight contacts

A contact that is too tight can keep a crown from seating even when the margins and internal fit are right. This is one of the most common reasons for a crown that rocks or stops short.

We check every contact on the model before the case leaves the lab. If you find a contact tight at try-in, check it with floss before you assume the margin or internal fit is the problem.

Undercuts in the prep

An undercut on the prep blocks the path of insertion. The lab blocks it out on the model, which leaves space under the crown in that area. If the undercut is large, the crown may not seat on the path you expect.

Prevention:

• Check the prep for draw from the planned path of insertion
• Review the scan for undercuts before you send it
• Tell us the intended path of insertion when it is not obvious

Die and model issues

On our side, die trimming, spacing and model accuracy all affect fit. We trim each die to the margin you captured, check the model against the scan or impression, and inspect every case personally before it leaves the lab.

Provisionals and tooth movement

Teeth can move while the patient waits for the final crown. A provisional that is loose, short or out of contact can let adjacent or opposing teeth drift. The crown fits the model but no longer fits the mouth.

Prevention:

• Make sure provisionals hold the contacts and the occlusion
• Ask the patient to call if the provisional comes off
• Keep the time between prep and seat reasonable

Most of our cases return in 8 to 9 business days, with rush often possible, which helps limit movement.

Working together on fit

When a crown does not seat, call us. We would rather look at the case together and learn from it than guess. A complete prescription form and a clear scan prevent most of the problems above.

To send your next case, call us at 781.853.0777 or submit a case online.