WHY DID MY DENTAL CROWN BREAK?

WHY DID MY DENTAL CROWN BREAK?

March 3, 2020

WHY DID MY DENTAL CROWN FAIL?

There are generally 2 ways that crowns fail: they either fracture and/or break – or – they dislodge and fall off.

As a continuation of our last blog: How long does a dental crown last?, let’s look at these two situations separately. Considering the reasons for crown failure helps to inform us when we are trying to do everything that we can to make our crowns last as long as humanly possible.

THE BREAKING OR FRACTURING OF A DENTAL CROWN IS DIRECTLY RELATED TO WHAT THE CROWN IS MADE OF.

First, let’s consider the situation when a crown fractures and/or breaks.

When a tooth is prepared for a crown it has to first be shaved down (we call it “prepped”) to remove decayed or damaged parts of the tooth. We also need to make sufficient room for the materials that will be used to construct the crown.

There are quite a few different choices for the materials with which the crown is constructed. Each material has its own specifications for its required thickness. This is so that the crown can be strong enough to withstand the stress of chewing and biting on it. It stands to reason that if the dentist uses a type of porcelain which needs to be 2mm thick in order to achieve enough strength, then reducing the tooth by only 1mm would place the finished crown at risk of possible breakage from chewing or biting. The crown would be strong enough for chewing most things but may not be strong enough to withstand the sudden stress of encountering a cherry pit in a cherry pie, or some other unexpected hard substance in your food.

Some types of materials are strong even when they are very thin. Gold is a good example and it turns out that gold was used very extensively in the earlier days when the price of gold wasn’t as high as it is now. The reason is that gold is very ductile. It will bend and stretch before it will ever fracture. This allows the dentist to make a gold crown extremely thin and never worry that it will break. The problem then is that over a number of years you can wear through a very thin gold crown because the same properties that make it bend and stretch also make it less resistant to abrasion. It is easy to wear holes through the thin parts of a crown made of gold.

Traditionally, porcelains were prized for their beauty and translucence but suffered the problem of being very brittle. This prompted the development of the porcelain fused to metal crown. These crowns started with a very thin layer of metal, usually a metal with a high gold content, and then a much thicker layer of porcelain that is bonded over the top of it. This gives us the beauty and translucence of porcelain with the strength of metal. These crowns have both high tensile strength (from the metal) and high compressive strength (from the porcelain). The porcelain fused to metal crown was the most common crown to be made and placed from the 1960’s to well into the 2000’s.

Most people over 40 yrs of age today have a porcelain fused to metal crown in their mouth, if not quite a few of them. What happens when these crowns break is that the porcelain fractures off and leaves all or a portion of the metal substructure still on the tooth. Generally, the tooth doesn’t hurt because it is still covered by the metal part of the crown. People complain because the crown now has sharp edges, it doesn’t quite bite against the opposite tooth where the porcelain breaks off, or the exposed metal looks objectionable and unesthetic.

Today, we have Zirconia porcelains which are made from the same material as Zirconia (artificial) diamond. These porcelains are almost impossible to break. It is said that you can drive a truck over one of these crowns without breaking it. Sounds great doesn’t it? Problem is the colors and translucence of Zirconia is not as good as the traditional porcelain crowns. They tend to look kind of fake if they are used for the front teeth. This is why Zirconia crowns are most often used for the back teeth.

picture of a broken steel crownThe takeaway here is that there are many choices. Each offer their own advantages and disadvantages. The dentist must become an expert in the science of dental materials so that he or she can help advise the perfect choice of materials to be used in each specific case.

The other takeaway is that it is very important for the patient to have these types of conversations with their dentists before their tooth is worked on.

“What are my material choices for this crown?” and “which type do you recommend for me and why?”

CROWNS THAT DISLODGE AND FALL OFF:

Next, let’s consider the crown that comes off.

As a dentist, I’ve been handed crowns many times where you look in the mouth and see where the tooth has fractured down to the gumline leaving me to wonder what the crown was holding onto in the mouth?

The answer comes when I look into the crown and see the tooth structure that the crown was attached to is still cemented solidly in the crown. The tooth has broken off below the crown margin. This obviously cannot be simply cemented back onto the tooth.

HOW DOES A CROWN STAY ON A TOOTH TO BEGIN WITH?

Roughly speaking, when a tooth is “prepped” for a crown, it is shaved all the way around until it takes the form of a short cylinder with some material shaved off the top (biting surface) to allow for the replacement of the biting surface. If a crown were made to fit a perfect cylinder it would have a natural retention when forced onto the cylinder which would be directly proportional to the height of the cylindrical walls. I’ve seen crowns that were milled by the dentist to such perfect cylinders that we couldn’t get them off after we placed them on the teeth just to try the fit. For this reason the dentist tries to mill the tooth (crown prep) so that it is slightly less than a perfect cylinder, somewhat more like a slightly tapered cylinder or very slight cone shape to it. The more the cone shape the less the frictional retention.

There are obviously going to be some differences in the technical skills of each dentist but if we disregard that, the other important consideration is this; the taller the parallel walls of the prepared tooth, the greater the frictional retention and vice versa.

The skilled and knowledgeable dentist will size up the tooth before he or she starts to drill and assess whether the prepped tooth (cylinder) will be tall enough (have long enough parallel or nearly parallel walls) to have sufficient frictional retention so that it will be able to withstand the anticipated forces of chewing on it. If the prep looks like a pancake with almost no height to the parallel walls, then we’re going to have a problem with retention, or in other words – the crown staying on the tooth! When a dentist notices that insufficient tooth structure is present for the preparation of a retentive crown prep, they may advise the patient to consider a “crown lengthening” procedure which would give them more height to work with.

However, what really holds the crown in place is the cement that is used to cement (glue) the crown in place. To date there are no cements (glues) that are strong enough to hold a flat crown onto a flat tooth prep. All crowns need some initial frictional retention first. The cement is simply an adjunct to ensure that the crown doesn’t come off.

No matter how good the initial fit and retention of a crown, over time the cement will eventually start to wash out from under the crown. If the crown doesn’t have good frictional retention before the cement goes into it, it will start to experience micro-movements while chewing after only a small amount of the cement has washed out. As soon as these small movements start they promote more cement to wash out. Before long the crown comes loose and falls out.

Lastly, the fit of the margins of the crown to the tooth are most important to the longevity of the crown. Why? If there is a gap between the crown margins/edges, then leakage can occur and bacterial plaque can grow and be difficult to clean out.

The result of this is that tooth decay can begin and quickly spread within the tooth structure under the crown. Once tooth decay begins under the crown there’s no way to stop it short of cutting the crown off or waiting until enough of the tooth decays so that the crown falls off.

You may wonder – “well, if there is a gap at the margin, doesn’t it get filled in with cement when the crown is cemented on?”

This is true but – the wider the cement line, the more the cement is exposed to the harsh oral environment that subjects it to washout (dissolving away). It doesn’t take much cement to wash out before there is an actual gap formed. There, bacterial plaque can grow and cannot be accessed for cleaning. The perfect crown margin would be where the fit of the crown to the tooth at the edges is so perfect that there is essentially no gap at all. The fit would be so perfect that there would be no cement line subject to washing out and so tight that there would be no fluid leakage. Of course this is impossible. Although, the message is that in order to place crowns that won’t fail as dentist’s, we are always looking at the crown margins and striving for the perfect fit.

As you can see there are many reasons why a crown might fail. Whether the material it was made with is brittle, wasn’t fabricated to the right specs, the metal has worn through, the tooth underneath has suffered from tooth decay, the cement has washed out, or many other possible reasons: it is not uncommon for dental crowns to fail.

If you are experiencing pain in a tooth that has a dental crown, your crown has broken, or if it has become dislodged and/or completely fallen off, please give our office a call and visit one of our expert dentists for an exam. We will be happy to assess the reasons for the crowns failure, replace the crown if possible and will provide a road map for the best next steps to restoring your smile.

Blog written by Dr. Adrian Pawlowski DDS, MSD, in-house periodontist at Bell Harbour Dental – PerioInnovations

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