Why does my crown keep falling out

One of the dental problems that annoys and causes most upset to  patients is when their crown (or cap/false tooth) keeps coming out for no apparent reason. The result is that it leaves a massive gap which is really unsightly especially when the gap is at the front of the mouth for everyone to notice. Even if the gap is at the back of the mouth, food will tend to get trapped where the crown was, causing halitosis. The result is embarrassment and a damn nuisance especially if it is at an inconvenient time when you can’t get to your dentist quickly.

In theory all crowns can come out at any point but we are talking about a crown that repeatedly dislodges within a short period of time. Most patients never get to find out why their crown repeatedly dislodges. The best time to get a proper answer from your dentist is when a crown comes out the second time because as mentioned above, any crown can come off just the once.

There are several reasons why your crown keeps coming off so let’s go through each one in turn.

Decay

Decay under a crown is perhaps the most common reason why your crown came off and why it keeps dislodging. When there is decay inside the remaining tooth, the glue or cement that is used to keep the crown in place will not adhere to decay. This reduces the surface area of the underlying tooth to hold the crown securely in place. Until all the decay is totally removed, the crown will keep on coming out. Often, you won’t even be eating anything particularly hard or sticky and out comes the crown again. So firstly ask your dentist if the underlying tooth is healthy.

If there is decay under the crown, the only remedy to this problem is usually to clean out any decay inside your  tooth and then make a new crown because the old crown will no longer match the shape of what your tooth originally was like.

One thing to bear in mind is that often early decay or caries cannot be easily picked up on your routine x-rays or by visual probing so during this time, the decay becomes worse until the only solution is eventually to have the tooth extracted.

Lack of Retention

A crown relies on 2 factors to keep it in place. Firstly, the strength of the luting cement or glue that the dentist uses to put the crown on in the first place.  Nowadays, the dental glue or cement is quite consistent compared to years ago when the cement was hand mixed and this could lead to discrepancies in consistency and quality. If you had a crown placed many years ago and it has now come off, it can be that the cement has partly washed away and deteriorated. The older type of dental cements were based on Zinc phosphate or Zinc Polycarboxylate. The newer modern types of dental luting cements for crowns are based on Glass Ionomer or resin based.

However, the luting cement is not the only factor which holds the crown securely in place. The other factor is the shape of the underlying remaining tooth over which the crown fits. If you recall to when you first had your crown made, the dentist shaped your tooth first. The shape of the tooth over which  your crown goes over is called the “crown preparation” or crown prep. There are certain rules for an ideal crown preparation such as having near parallel walls with a small taper and adequate tooth height. These factors will all provide the retention for your permanent crown and therefore if your crown keeps on coming out, it may be that the crown preparation lacks retention.

Changes to the bite

You may not think so but even tiny changes or alteration to your bite ( sometimes referred to as the occlusion) will dislodge your crown. The answer is to make sure that your crown is not subject to heavy unbalanced forces on its biting surface. The bite or occlusion is probably the most commonly overlooked factor when trying to see why your crown keeps falling out. When a crown is put back in having fallen out, the bite should feel absolutely exactly the same as before it fell out. If your bite feels odd then the chances are that much increased that your crown will fall out again. Even, once your crown has been recemented and your bite seems comfortable, it is critical that your dentist rechecks the bite to make sure it is balanced because changes in occlusion can occur over a period of time due to reasons such as loss of a tooth/teeth elsewhere, a new filling, or the bite changing as a result of a clenching/ teeth grinding habit ( bruxism) or other factors. How does a dentist check your occlusion? Even a highly trained dentist just cannot assess your bite from just looking at your teeth so he uses a thin narrow strip of special paper called articulating paper. Articulating paper is very thin and has a coloured dye coating which transfers marks onto your teeth. The articulating paper is placed between your upper and lower teeth and you are instructed to tap your teeth up and down a couple of times and also to grind from side to side and backwards and forwards. In this way, the dentist can assess how your teeth meet in function i.e in a grinding type of motion which is how your teeth connect together during eating. The dentist then takes the articulating paper out of your mouth and looks where the dye has marked areas on your teeth and looks for premature contacts. Premature contacts are specific points where the occlusion in unbalanced. Where premature contacts exist, it is important to adjust the patient’s bite so that the bite is balanced again. It is the case that  crowns that keep falling out often have premature contacts which need to be removed otherwise the same problem will reoccur. This is why sometimes the biting surface of a crown needs to be adjusted in order to create an even bite again. If this is not done, then you are increasing the like hood of the crown falling out again.  An important point to bear in mind is that as soon as your crown has come off, you should make arrangements to have it recemented again before there is any chance of opposing or adjacent teeth moving hence changing the occlusion. I have seen changes in occlusion in less than 24 hours so it’s imperative to have your crown recemented as soon as it has dislodged and to prevent the bite from altering.

The fit is no longer tight.

When your crown has come off for the very first time, in order for your dentist to put it back onto your tooth again, it is necessary to clean out the old luting cement first. This is performed by drilling out the old cement which is still stuck to the inside of the crown. The problem that can arise here is when the luting cement is drilled off,  the crown material itself can also in the process be slightly drilled away as well hence creating a less tight fit compared to the original crown. From this point onwards, your crown will never have the same fit as when it was constructed originally and this will naturally increase the chances of the crown coming off again in the future. If this is the problem, then the only solution is to re-make a new crown from scratch again.

Other Reasons

Fracture of underlying root

A fracture of the underlying root occurs when a tooth has been root filled or the tooth has become dead also known as the tooth becoming non vital.

 A root treated tooth or a non-vital tooth is always much weaker than a tooth which has its nerve and blood supply still intact.

 The tooth which has fractured may also involve a special type of Crown called a post crown.

 Root fractures involving are more common in the upper and lower anterior incisors/ canines but do occur in molars as well.

 When a dentist sees the tooth where the crown has come out, it will be immediately obvious that a root fracture has occurred.

 There is no treatment for root fractures.  In the past, however at our practice we have managed to “have a go” at trying to put a crown back in which has sustained a fracture and they have lasted a considerable length of time.  The patient obviously has to be very careful not to put any direct pressure but on the whole, once a root has fractured, that crown will keep on coming out.

 The treatment options here are therefore an extraction and a replacement tooth using either a denture, a bridge or an implant.

Fracture of post or pin

Many crowns have additional retention such as using a pin or a post in order to stay in properly.

 When there is a fracture of the post or the pin, there is now nothing for the crown to hold on to and as a result the Crown will keep on coming off.

 Sometimes, it is possible to take the fractured post or pin out of the root of the tooth, and then insert a new post or pin.

 However, in the vast majority of cases, the only option unfortunately is to have the tooth taken out.  Once the tooth has been taken out, it can be replaced if necessary with a denture, a bridge or a dental implant.

Fracture of underlying tooth

When your crown keeps on coming out, you may have noticed that a part of the underlying tooth has also broken off. The underlying tooth may break off partially or extensively.

 If it has broken off partially and the dentist feels the tooth can still be saved, it will be necessary to have the tooth built backup and a new crown constructed.

 If the brake is extensive, it depends on whether the fracture is above or below the gumline.

 If the fracture is above the gum line, the tooth can often still be saved by cleaning out any decay present, and then building the tooth back up with a pin or a post. Finally a new crown can be constructed.

 If the fracture is below the gum line, it then becomes extremely difficult or impossible to salvage the tooth. In a few cases, you may be lucky to have gum surgery and repair to the broken tooth carried out.   More often than not, you will be advised to have the tooth taken out and have a replacement as necessary such as a denture, Bridge or a dental implant.

What to do in an emergency if your crown keeps coming out

 The crown always keeps on coming out during the most inconvenient of times. These times always just seem to happen to be in an evening on a weekend and during a long bank holiday. Or if you are away on holday, of course.   Panic sets in.  Depending on which area you live in, you may be able to access an emergency dentist in your area.  The Out of hours NHS service is there but a crown coming off is not classified as a dental emergency so you won’t be seen.

  If you need to keep your crown in your mouth to save dignity, super glue is not the best of methods.  If you go to any supermarket or a chemist, you can buy a simple emergency dental kit which will have some temporary glue that you can use. Another option is to buy a tube of denture adhesive. Finally, if your crown keeps on coming off and you haven’t decided on a more permanent option, you should always have some temporary dental adhesive with you at all times as you never know when that crown will come off again without warning.

Conclusion

So in conclusion, when a crown keeps falling out or becoming loose, it’s not simply just to fix it back over your tooth again but to find out the reason why it may have dislodged and to address the causes. You should always be prepared to have further treatment such as a new crown altogether. A new crown is a two visit procedure where impressions are taken followed by the fitting appointment.

 

 

 

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