Crown Failures - Problematic Margins
There are a lot of reasons crowns fail. There are problems with fit of crowns, the bite or occlusion on crowns, the cement used, the labs used, the impression taken by the dentist and cleansability problems. I will go over each of the problems, causes and solutions one at a time.
First, the fit of crowns can be problematic. If a dentist uses an inexpensive lab to save a few bucks in making the crown it won't have the time and attention to detail required of an intimate fit. Crowns can have openings between the crown tooth interface of 20-200 microns.
A dental explorer has an average tip diameter of 70 microns so a marginal gap of 100 microns or less is hard for a dentist to distinguish. In our practice our laboratory uses a microscope for all steps in fabrication of a crown with a measured gap of less than 50 microns for all restorations performed, yet many labs don't utilize these techniques.
While luting cements fill in these micro-gaps, over time they will leach out due to saliva, diet, wear and provide a gap condusive to bacterial infiltration leading to decay at the margin and failure of the crown.
These microscopic gaps can harbor bacteria, lead to inflammation of gum tissues, gingivitis, periodontal disease and further erosion of cement and concommitant decay. So while marginal gap can't be the only reason crowns fail it certainly makes sense that the larger the pool of cement, the faster the crown may fail.
Average Crown Life?
Studies have shown that average life of dental crowns is five (5) years. It is suggested here however that in our experience these crowns never fit precisely and when the cement sealer began to fail the crown failed.
I have been practicing for 17 plus years and on recall to date crowns placed at my office 17 years ago are intact and beautiful in patients with good oral hygiene and recare. In fact some crowns done by my father over forty (40) years old are still in wonderful shape.
Occlusal Factors
The bite can affect the fit of crowns. If a tooth is hitting too hard, prematurely or is subjected to unusual forces it can become dislodged more easily leading to premature failure, sensitivity, or fracture.
Understandably then, the dentist should insure a great fit and even, harmonious biting to insure longevity of the crown. In fact, even the contacts to adjacent teeth can affect whether or not a crown fails. If the contact is too loose, it can lead to food impaction, calculus, pus and periodontal problems.
These conditions will increase the bacterial load at the cement interface leading to bacterial infiltration, decay under the crown and failure of this crown. Conversely, contacts that are too tight, will keep the patient from flossing, decreasing hygiene and leading to failure for periodontal reasons as well!
Idealized Crown Preparation
Cements are great but can't overcome tee-pee preparations. The dentist is responsible for insuring that the crown prep is retentive. If a tooth looks like an Indian tee-pee no amount of cement will overcome this poor crown design. Dentists not only need to insure a 3-5 degree taper to the preparation to insure the crown will go down and stay on, but they also need to make sure the height of the crown prep is sufficient.
In fact, if the tooth is too short, even beautiful tapers aren't enough to keep a crown on. In these cases the dentist may need to do crown lengthening procedures to make the tooth longer. Picture a thimble (the crown)... if the thimble is made longer it can fit better - a longer wall for the cement to adhere to.
Proper Selection of Crown Cements
The dentist must choose a cement that will compliment their preparation design, adhere to tooth and crown material and not adversely affect the fit. Any mistakes in design, cement choice or contamination of the tooth during cementation can spell doom for the tooth leading to marginal decay, and crown loss.
Laboratories are probably the single most important part of insuring against marginal decay. Using the finest materials, taking time while doing every step of making a crown can give beautiful looks, fit, contacts, occlusion, and contours that all optimize tissue health, occlusal health, and insure proper function for a crown or bridge.
Quality Control - Fabrication Process
Our laboratory uses a quality control system to verify that the five (5) hours it takes to fabricate each crown will insure that all of my criteria are met. Using certified and registered dental technicians can insure that crowns aren't being shipped overseas to unlicensed labs that won't insure the careful attention to detail that provides long term success.
Critical Attention to Detail
Lastly, the dentist must pay attention to detail. Crowns can't be buried under the gum too deep or the gum tissue won't have a home. We call this violating biological width, and failure to give this gum tissue a home will result in blue gums, bleeding, pufffiness, pus and failure of a crown.
If the dentist doesn't obtain a good impression the crown won't fit. If there is tissue trapped under a crown when cementing the tissue will slough and die, and a huge gap under the crown will lead to marginal decay and crown failure in a short time.
Patching crowns is a horrible idea. You can't see if you got all the decay out. This bacteria can eat away tooth structure silently til there is no tooth left to restore. You can't even see this decay on an x-ray as crowns are metallic and can't be assessed via x-rays.
While this is certainly not an exhaustive article on all reasons we get decay at the margins, it is certainly a lot of information and can be used to identify why failures occur. These reasons apply to veneer failure, implant crown failure, bridge failure, etc.
Feel free to contact Dr. Winter if for any additional questions you have.
Richard Winter, DDS FAGD
5323 W. Hampton Avenue
Milwaukee, WI 53218
(414) 464-9021
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