Why are my crowns not fitting?

First let me say that most of my crowns end up fitting nicely, but most do not drop into the moth without the need for any adjustment. I am writing this post for primarily for dentists who are having problems with the fit of their gold or porcelain fused to  crowns and are looking for help with this problem.

There are a number of reasons for crowns not to fit and these include over extensions, poor preparation, over tight contacts, poor impressions and poor laboratory work are just a few reasons, but let's assume for the moment that you are  reasonably happy with your laboratory , feel confident with your preparation and impressions and your crown is still not fitting.

Since I ditch my own dies over extension is not usually a problem and I use small dental laboratories where the same technicians perform my work and usually my preparations are carefully done. Even so, sometimes crowns do not fit on my intitial try in. I also almost always use a bevel or a finishing line with at least a .5mm clearly readable width. When one of my crowns doesn't fit there are usually several possible common reasons and I have a standard way of trouble shooting my crowns.


First I check for an over tight contact. Improperly tight contacts can keep crowns from seating. In order to check a contact, after I have initially seated a crown,  I use unwaxed dental floss and see if it can be moved without a great deal of effort through both contacts. If it breaks or shreds I usually assume the contacts need adjusting. Commonly I paint Accufilm marking liquid {Parkell}(Occlude spray can also be used) on the contact areas of the crown and seat it again and look for where the ink gets removed.  This is usually the spot that needs adjustment. I do this repeatedly and make small adjustments until my crown can be flossed easily with unwaxed floss. I also find that a product called Contact Eaze is helpful as well. Its a tiny extremely fine short lightening strip mounted on a plastic handle that is easily used to adjust contacts while a crown is seated on its pindexed model.

After I determine if my contacts are OK I routinely use GC fit checker to check the internal fit of my castings. To do this I mix a small amount on a paper pad with a plastic instrument , apply it in a thin coat to the walls of the dried crown and then seat it. After 90 seconds I remove the crown and examine the inside carefully. If I can't see an extremely thin coat at the margin I look carefully for any metal poking through the silicone . Usually these tiny metal "drag" marks are due to tiny undercuts on the crown preparation that have not been blocked out by the lab technician. On my crowns I commonly find these on the proximal surfaces of my preparations near the occlusal aspect. I relieve these tiny spots with a small latch type round bur run at low speed and then sandblast the area. (Another approach that I sometimes use  is to paint the inside of the casting with some Accufilm marking liquid and re-seat the crown. The Accufilm can adhere to the tooth and show me where the tiny under cuts are located). I do the fit checking repeatedly until I am happy with the fit of each casting. I know that I have a well fitting casting when the bevel area looks like it has an extremely thin even coat of fit checker on it and most times the film thickness on the bevel is thin enough that the grey from the casting is apparent through the set fit checker.

Another thing that the fit checker can help detect is tissue impingement around the preparation that may not allow a crown to fully seat. In that case the casting may have a thick blob of set white fit checker at one part of the margin. It is always a good idea to make sure that the margins are cleaned of tissue before re-seating the crown with the next round of fit checker . Over extensions also may become apparent with the fit checker. These are areas at the margin of a casting that have no fit checker. If I find this denuded area repeatedly on a margin it either indicates an over extension or a binding spot at the margin that needs slight relief.

The number of times that I apply fit checker could be as little as once to as many as ten times. If I have to use many fit checker applications, I am not a happy camper. Most times after this process I am happy with the fit and cement in my patients crown, but if I am not, I will take a new impression and ask my laboratory to redo my crown. If I find that my crowns are needing too much adjustment in a given month, I definitely will have a "pow-wow" my lab. In the last year or so I have started dividing my work between two high quality dental labs. Both are aware that I split my work between them and it seems to have the overall effect of improving the quality of their restorations.