Cosmetic Dentistry Blog Cosmetic and General Dentistry Questions Answered

July 9, 2016

Why does my crown keep falling off?


We thank our advertisers who help fund this site.

Dr. Hall,
I have had a crown put on by a dentist and it was done same day. No temporary. Not sure what type of material he used for the crown. It fell out three times in less than a year. I went to a different dentist on the fourth time it fell out and he replaced the setting with a new one stating the other dentist didn’t create a good setting. This new crown is Zirconia Porcelain and it has fallen out twice in a month. He is suggesting a full porcelain crown on the same setting as he states the cement is adhering to my tooth but not the crown. He says the all porcelain has a more rough underside to adhere to the cement better. Any suggestions on what my next move should be?
– Stephanie from North Carolina

Stephanie,
Your same-day crown was a CEREC crown or a similar type that is milled by a computer in the dental office while you wait. But when properly prepared and bonded on, it will stay on permanently. It’s not the material your crown is made of. Gold crowns, porcelain crowns, CEREC crowns, zirconia crowns–all of them can be made to stay on solidly, permanently. I have most of those types of crowns in my own mouth and they stay on just fine.

There are two main factors for retention of a dental crown–the bonding strength of the cement, and the shape of the tooth preparation. Of those two, the shape of the tooth preparation is far more important. If the tooth is prepared with only a slight taper, a crown can be cemented with a very weak cement and it will still stay on. If it is prepared with a lot of taper, some of the strongest cements will not hold it on.

I’m not saying that getting a good bond between the crown and your tooth wouldn’t solve your problem. The strength of the cement is a factor. And I know very little about your tooth and the techniques these dentists used. But I do know that a tooth prepared with good retention form will not have a crown falling off three times in less than a year, regardless of the cement used. So even though I know very little about your tooth, I’m pretty confident that it was prepared with inadequate retention form.

So what should your next move be? I would find a dentist who knows how to do crowns that stay on. In 23 years of dental practice, I never had a crown fall off of any that I did for my patients, so I know it can be done.

For further understanding of this principle, of creating a crown that stays on, see my blog post from about nine months ago, “The main reason your crown probably fell off.”

I hope this is helpful.
Dr. Hall

Do you have a comment or a question or anything else to add? We’d love to hear from you. Enter your comment below.

Click here to ask Dr. Hall a question of your own.

About David A. Hall

Dr. David Hall was one of the first 40 accredited cosmetic dentists in the world. He practiced cosmetic dentistry in Iowa, and in 1990 earned his accreditation with the American Academy of Cosmetic Dentistry. He is now president of Infinity Dental Web, a company in Mesa, Arizona that does complete Internet marketing for dentists.

October 8, 2015

The main reason your crown probably fell off

We thank our advertisers who help fund this site.
.

I happened across a website the other day, Oral Answers, that had a post, “Six Reasons Why Your Crown Fell Off.” As I read it, in my mind it highlighted the difference between this website and other dental websites. The dentist writing this gave six “politically correct” possible answers that pretty nearly absolve dentists of any responsibility in this mishap and skirt the main cause.

The author of the website appears to be a nice guy. He is a dentist and identifies himself only as Tom. And it’s not that the information is incorrect, it’s just sanitized. It seems to adhere to the philosophy I’ve had preached at me from other dentists, that we professionals need to stick together and defend each other. And so Tom, rather than frankly explain what he was taught in dental school about crowns falling off, instead tries to portray dentists as nearly infallible.

For me, I can’t bring myself to do that. I need to tell it like it is and let the chips fall where they may. My professional duty, as I see it, isn’t to make other dentists look good but to help the patient.

Here are the six reasons Tom gives, with my comments on each one. Again, they’re all true. It’s just that he leaves off the main reason, which is what he and I were taught in dental school. So after I list his six reasons, I’ll let you in on a dental school lecture about the main reason crowns actually come off.

1 – The tooth underneath the crown got decayed.
This will definitely cause the crown to come off. However, if you are having regular checkups, your dentist will spot this decay well before it is extensive enough to cause the crown to fall off.

2 – The cement holding the crown on wasn’t strong enough. This could be due to contamination of the cement while it was being prepared or any number of other reasons. If this is the case, your dentist can usually re-cement the crown back onto your tooth.
This is the closest he comes to faulting the dentist. But he presents this as so easy to fix that it makes the dental mistake easy to forgive. In reality, I would attribute this cause to fewer than 1% of crowns falling off. I don’t think I ever saw a case like this in my practice.

3 – You ate too many chewy foods.
I was never taught in dental school to advise my patients with permanently cemented crowns not to eat chewy or sticky foods, and I never told them that. I will add that in 23 years of practice, I never had a crown that I put on just fall off, and my patients ate jelly bellies, taffy, whatever. I also have eight crowns myself and don’t observe any diet restrictions because of them. But if you do have a borderline crown or a temporary crown, yes, sticky foods will give you problems.

4 – The crown broke.
Again, this will do it, but it rarely happens. I’m trying to remember if I ever saw a broken crown in my practice. I may have seen one or two over my 23 years. In the old days, some pure porcelain crowns would break if they were used too far back in the mouth.

5 – You abused your crown.
He mentions grinding your teeth or using your teeth as tools. Yes, he is correct, this will stress your crown, but if the crown is done well it won’t be enough to loosen it so that it comes off.

6 – There wasn’t enough tooth structure to hold onto the crown.
Here he’s getting close to the main reason that crowns come off. This happens most often with short teeth. However, we dentists are taught how to address short teeth and get a crown to stay on. There are retentive grooves and other preparation features we can put into the crown preparation to deal with a short tooth.

The Main Reason

Keeping a crown from falling out

Now let’s go to our dental school training. Here are a couple of slides from a dental school lecture. Dr. Al Amri is teaching his students crown retention and resistance forms, the two principles involved in crown preparation that keep the crown on the tooth.

He explains to the students that the greater the taper in the crown preparation, the less likely the crown is to stay on. In his graph, which I have enlarged below, he teaches that by tightening up the taper,Taper the prep to keep the crown from falling off the dentist can increase the retention dramatically.

This seems like a simple solution and you may wonder, then, why every dentist doesn’t just prepare the teeth with minimum taper. There are a couple of reasons. First, to prepare a tooth with minimum taper requires a lot of precision. What if the dentist is off just a little bit? He or she could end up with what is called an undercut. I’ve illustrated that in this small diagram below. crown retention graph In an undercut, the tooth preparation is narrower crown-retention-graph-edited at the bottom than at the top. Having even the slightest undercut makes it impossible to make a crown to fit—it simply won’t slide on the tooth. If there is a small undercut in just one small part of the preparation, the crown won’t work. And the less taper there is in the crown, the more time and effort it takes to tell if you have an undercut.

The second reason is that the tighter taper makes it harder to seat the crown. The tighter taper, which makes it hard to dislodge the crown once it’s on, makes it harder to get on during the seating appointment. Any irregularity in the surface of the tooth or the internal surface of the crown needs to be resolved. There is less room for error.

A dentist can avoid these issues by simply creating more taper in the crown. And some dentists do just that. They may not be intentionally taking shortcuts—they may just lack confidence in their ability to create a perfect crown, so they just shoot for acceptable, with the result that they have some crowns that end up falling off. In my practice, I always prepared crowns with as little taper as I possibly could, and in 23 years I never had a crown fall off. However, I did see crowns done by other dentists that did come off, and most often these were crowns with inadequate retention form—too much taper.

We dentists, when we talk among ourselves, will talk about this issue of crown retention, and we fully understand that we are pretty much in control. But many dentists, when they are talking to patients, don’t feel the inclination to be quite so honest, for fear it will tarnish the profession. That’s their mindset. But it’s not mine.

Do you have a comment? We’d love to hear from you. Enter your comment below.

Click here to ask Dr. Hall a question of your own.

About David A. Hall

Dr. David Hall was one of the first 40 accredited cosmetic dentists in the world. He practiced cosmetic dentistry in Iowa, and in 1990 earned his accreditation with the American Academy of Cosmetic Dentistry. He is now president of Infinity Dental Web, a company in Mesa, Arizona that does complete Internet marketing for dentists.

Powered by WordPress

Follow this blog

Get every new post delivered right to your inbox.


Categories