A zirconia crown that keeps coming off is a tell-tale sign of it having poor retention and the only way to fix it is by redoing it with better retentive features.
Table of Contents:
Reasons for poor retention
If the zirconia crown is not staying on the tooth despite being glued on, it most likely has problems with its retention.
Causes of poor retention:
Short clinical crown
Over tapered axial walls
Poor cement choice
Heavy lateral excursive forces
Short clinical crown
A very short tooth will make it incredibly difficult for your zirconia crown to stay on. The reason is because the retention of the restoration is directly proportional to the surface that it has in contact with.
A bigger tooth will have a larger surface area for the crown to grab on to.
A smaller tooth will have a smaller surface area for it to grab on to.
Basically the larger the surface area the more surface friction the glue will have to hold the crown on. You lose a lot of this benefit when the tooth is short.
Video demonstration
If it's hard for you to visualize using words, we've created a video to show what we mean by having a short clinical crown.
Essentially, a longer or larger tooth will provide better retention. That means the crown will be less likely to pop off.
Over tapered axial walls
Crowns have the best retention when the axial walls are less tapered. If they are over tapered and have a high degree of convergence, the retention will consequently suffer.
Studies have shown that the ideal degree of convergence is 5 degrees. Once the taper starts increasing beyond that point, the crown will have a harder time staying on the tooth. In other words, it will come off more easily.
Wrong cement choice
The most popular type of dental cement that is used for zirconia crowns is resin modified glass ionomer (RMGI). It is versatile and works extremely well for most situations.
However some providers mistakenly believe that a "stronger" cement would be better such as a self adhesive resin or resin cement. While these are stronger, they may cause more retention issues.
Heavy lateral excursive forces
Zirconia tooth caps can get knocked off the tooth if there are heavy lateral excursive forces. This type of force isn't the up and down biting that you do when you chew. What we mean by this is the side to side motions.
If there are a lot of side to side interferences, it only increases the chances of it popping off. This may happen if your dentist didn't check the side to side occlusion.
How to increase zirconia crown retention
All hope is not lost because there are ways to increase the retention of your zirconia tooth cap. These all require a trip back to your dentist to implement it.
Ways to increase zirconia crown retention:
Adding retention slots
Using an appropriate cement
Decrease taper of axial walls
Adjust the occlusion
Add retention slots
An often glossed over technique to improve crown retention is to add slots to the crown preparation. Basically your dentist will drill a small rectangular box into the buccal (cheek side) and lingual (tongue side) of the tooth. These additional slots will add retention to the crown and help it stay on.
This technique will increase the surface area that the crown has in contact with the tooth. It will also increase the effectiveness of the dental cement.
Appropriate cement
If your dentist used a stronger cement like a resin one, you may want to try switching to RMGI which has less flex and less retention issues for zirconia.
Decrease taper of axial walls
If the lack of retention from your zirconia crown was due to over tapered walls, your dentist can decrease the taper by reprepping it. Basically they will redrill around the tooth and make the axial walls straighter.
Adjust occlusion
After cementing the crown back on, it would be prudent for your dentist to check the occlusion. Make sure it is not being occluded heavily on nor are there lateral interferences.
If you're hitting on that tooth hard when you chew, it'll only increase the likelihood of it popping off again. These high spots will need to be shaved down.
Alternatives
If none of the recommendations above work and the zirconia crown continues to come off, you should consider alternative options. Perhaps zirconium dioxide is not the best choice of dental material for your particular situation.
Switch to emax crown
Your dentist could switch ceramic materials to emax instead of zirconia. The biggest benefit of using an emax crown is the ability to bond it onto the tooth. This type of ceramic material is more similar to a veneer where it can be bonded. Zirconia on the other hand lacks the capability to be bonded because it can only be cemented.
Dental implant
If no crown can stay on, perhaps you should consider getting a dental implant. Of course you would need the tooth to be extracted first but afterwards you can get a zirconia implant crown that is screwed in.
For a screwed in implant crown you don't have to worry about the glue coming loose. The retention will instead rely on the screw.
Takeaway
Crowns coming off is not an uncommon complication from getting a dental crown. The challenges are increased if you opt for zirconia instead of emax since it cannot be bonded.
Nonetheless there are a couple of things which you can try to help improve the retention. We recommend discussing with your dentist about what your options are and what you should do.
