Dental Crowns
 

What are the Risks of having a Crown? What Problems can you get with Crowns?

We were told at Dental School never to call anything “permanent” because it is unlikely to last forever and will at some point in the future, probably need to be replaced.

A crown is the most long-term solution available for repairing an extensively damaged tooth. That said, there are risks that you should be aware of and problems that can arise. Lets look at them together:

It is even more important that you have good hygiene following a crown, as plaque can collect at the margin where the tooth and crown meet. Whilst the crown itself will not decay, the tooth still can, so it is important to make sure this doesn’t happen.

Good brushing and flossing will do the trick. If caries does happen fluoride and improved cleaning can stop it in its tracks if it is detected in the very early stages- (the importance of seeing a dentist regularly!) If it has progressed but remains small, it is possible that a filling can be placed (assuming it is in an accessible area) but once underneath the crown- the crown itself will need to be taken off(often this requires cutting), the decay removed, the core replaced or repaired and a new crown made. The longer this decay is left to fester underneath without being treated, the more the core is being compromised for the new crown- so don't put it off if this is happening, or you may end up losing the tooth altogether. Since the cost of a crown is quite expensive, It just makes sense to make the effort in the first place to look after it and stop what is a largely preventable problem from occurring.

Plaque (the root of all evil) around the crown can also cause gingivitis and if left, eventually this may progress to periodontal disease and the loss of support for your tooth.

  • The crown comes off
Your crown can come off for three main reasons:

NOTE. It is crucially important that you keep the crown and take it with you to the dentist, otherwise the dentist simply has no choice but to make you a new crown. They cannot stick it back in if you leave it at home!

(i) The core fails. If your crown comes off, take a look inside it… If you can see what looks like filling material and some tooth, then it is likely that the core is inside. The prognosis in this circumstance is not very good. It is possible that the tooth may be saved with a root canal treatment and a post crown. However, if it has fractured a little below the gum, then either crown lengthening is required first or a decision made to extract the tooth and plan a replacement such as an implant or a dental bridge. This is why it is important to place a crown when sufficient strength and tooth remains for a good core and not wait until the last minute when the core will be weak!

(ii) The cement fails. On the other hand (and keep your fingers crossed that this is the case), take a look inside again...if the crown looks just like crown material on the inside with a hollowed out appearance ( and there is still some tooth and filling in your mouth, resembling a stump, this suggests the core is still present and assuming it is ok, it can just be cleaned up and the crown simply stuck back in. Sometimes decay underneath has caused this to happen, and if the crown no-longer fits when this has been cleaned out, then a new one will need to be made. The new generation cements that we are currently using are very strong and failure from the cement is becoming less and less likely. In fact they are so strong, that when a crown needs to be replaced, especially (if it has been bonded to the tooth) the dentist, more often than not, needs to completely cut away the old crown. Crowns cemented in years gone by with weaker cements are much more prone to this type of failure.

(iii) A post crown comes out. If this happens you will see a large post sticking out the end of the crown. This could be because of the cement failing, as a result of a short ,or too tapered post, or because of a root fracture.

In the first situation it can often be just re-cemented, though unless the post is made longer or changed then it is likely to come out again at some point. That said, the materials we have for bonding in crowns are improving all the time and this improved bond can help.

A root fracture however will often mean the tooth cannot be saved and must be extracted. So keep your fingers crossed this doesn't happen! Unfortunately, it's a very common occurrence with the lateral upper incisor because this tooth has such a thin root.

Just to mention that crowns don't necessary come completely out. You may find that your crown becomes loose but is still hanging in there. The reasons for the looseness are the same, so get yourself along to the dentist quickly and be smart!

  • Chip or fracture
Porcelain itself is a brittle material and fracturing, whilst fairly rare, is one of the more common problems we see with crowns. The fracture can be part of the porcelain overlying the metal core in a porcelain fused to metal crown- often, the seal or functionality of the crown will not be affected- only the look. Depending where it is in the mouth, this may or may not be a problem.

Porcelain crowns are more likely to fail completely, rather than chip (unless of course, you bang your tooth). In such circumstances the porcelain may have been weaker, (not enough thickness for strength or fired incorrectly in the lab) and therefore subject to very high stress such as grinding.

If this happens the tooth underneath may need to be re-prepared to give more strength and a different crown material might be selected or not. Small chips can often be replaced with a composite filling material- this has varying results and can sometimes be prone to falling off!

  • The colour or shape is not right
The dentist (particularly if the crown is in the front of your mouth) should sit you up and check you are happy before cementing the crown or crowns in. Have a look at both the shape and the colour. It is important that the contact points are good between the teeth on both sides, or food packing may become a problem.

Crowns are often done to stop this problem which can occur with very large fillings but if they are over adjusted or made poorly, food can still get stuck. Some teeth that have lost bone support as a result of periodontal disease may drift and cause the contacts to change- unfortunately this is largely out of anybody's control.

  • A dark line at the gum
This, as we have mentioned is a problem with porcelain crowns that have a metal sub structure (VMK or PFM) when the gum has receded. Recession occurs naturally as we grow older, but may be accelerated with gum disease or toothbrush abrasion.

The metal underneath that was once hidden becomes visible. This is often difficult to mask with white filling material and attempts still show some greyness. If it is a problem for you in the front of your mouth, the crown will need to be replaced.

  • Nerve problems
Despite all precautions and investigations, certain teeth will play up and require a root canal treatment or re-root canal treatment through the crown. In general 1-15% of teeth that are crowned lose vitality and need a root canal.

Root canal in this circumstance can be a little more difficult. To begin with, finding the nerve tends to harder and the visual access to the canals slightly more limited. The crown may not align with the roots as you may expect and this can end up with the dentist needing to take away more tooth, reducing the strength of the core.

Going through the porcelain will reduce the strength of the crown and for VMKs, the bond between porcelain and metal can be weakened which sometimes leads to porcelain fracturing away. The vibration and process itself can also weaken the cement that holds the crown in and this may cause the crown to come off.

After finishing the root canal, the access that was created can often just be filled with composite or amalgam. Though it would be more ideal, given the discussion above, if the crown was actually replaced with a new one.

If your tooth already has a root canal and this decides to play up- a re-root treatment can be attempted or you may need an apicectomy. The only other option unfortunately is extraction. This is why it is important to do as many investigations as possible, to determine if a root canal is necessary or if an existing one needs to be replaced.

  • Wear of opposing teeth
Adjustment of the porcelain surface of your crown to get the bite correct, causes the porcelain to lose its glaze and if not properly polished by the dentist, it can feel quite abrasive against the natural teeth that it bites on to. As we have discussed some crown materials are more harsh and likely to cause more wear than others. Your bite will also be a factor in the amount of wear that is seen. (