Why won't a crown seat?

This post maybe too technical for my lay readers, but is a good one for dental students and young dentists, who sometimes aren't clear on how to fit a new crown. They can become frustrated when a crown doesn't easily insert and are not entirely clear about how to go about getting their crown to seat fully.

Patients crowns usually fit the stone model that the lab uses to fabricate a crown, but sometimes they require "tweaking" to actually seat fully in the mouth. When fitting a porcelain fused to metal crown and hopefully cementing it, I leave a generous amount of time booked in my schedule (one hour). Now if everything goes without a hitch, I can be finished early, but most times I need most of the booked time.


The first thing I check after trying to seat a new crown is the contact area pressure. If this is too tight it certainly will not allow a crown to seat fully. I try flossing the two contacts with unwaxed floss and if the floss rips or won't pass through, I adjust the contacts. I mark them with blue Accufilm IV (made by Parkell) and seat the crown. This indicating ink tends to rub off at the binding spot. Sometimes I am lucky and both contacts are perfect from the get go, but often they require several adjustments.


Once my contacts have been demonstrated to be perfect, I use Fit checker placed inside the crown. Since Porcelain fused to metal  and full crowns are made on stone dies, sometimes the stone is abraded and can cause crowns to fit too tightly and not full seat. Fit checker is a fast setting, low viscosity silicone mix that will coat the walls of the inside of the crown. Before it sets the crown must be pressed onto the prepped tooth and left in place for 60 seconds or more.  After removing the crown, the internal surface is checked for "binding spots". These are small areas of metal that are poking through the set Fit checker. These must be adjusted with a small round bur because they can be the cause of a crown not fully seating.When the crown fits it will exhibit an extremely thin coat of Fit checker near all the margins and no metal binding points from the rest of the preparation.

The last thing I do is take a bitewing x-ray to check whether the crown full covers prepped tooth. Sometimes a crown doesn't cover the entire preparation even when fully seated. If the crown turns out to be fine, I will check and adjust the occlusion and afterwards polish and cement the crown with permanent cement.



from Ask Dr. Spindel - http://lspindelnycdds.blogspot.com/2019/01/why-wont-crown-seat.html - http://lspindelnycdds.blogspot.com/

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