The most common reason for an implant crown to keep loosening is if it is cement retained with a very short crown. Two possible solutions for this condition would be using a stronger cement or switching the crown to a screw retained one.
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Why it keeps coming loose
Personally, I've rarely ever seen screw retained implant crowns having retention issues. The only type that I do see coming loose more frequently would be the cement retained ones. As you may have guessed, it has to do with the cement and not having enough retention.
Cement retained implant crown
You may not be aware but it is standard protocol to glue in implant crowns with only temporary cement. Research studies have shown that while it may be a weaker cement, it is usually sufficient in holding the crown in place.
The retention is typically adequate with temporary cement because the taper of the abutment walls are made to be close to parallel. This is achievable since the abutment is made by the dental lab. This is usually not as readily achievable for a regular crown because it is being done on a live patient which makes working conditions more difficult.
Basically the less tapered the walls, the greater the retention of the tooth cap will be. Since the implant will be nearly straight, a temporary glue is more than adequate.
However due to other reasons, sometimes the temporary cement may not be sufficient. If your implant crown keeps coming loose, perhaps the temporary nature of the cement may be inadequate.
Short clinical crown
If you don't have enough interocclusal distance, you may end up with a very short tooth. If there isn't enough space in between the opposing arches, your dentist will be forced to make a very short tooth.
Shorter teeth tend to have retention issues for their crowns. The reason is that there isn't enough surface area for the glue to hold the cap to the tooth. Although in this case, there isn't enough surface area for the cement to hold the crown to the abutment.
The images above show different heights of various abutments. As you can imagine, it is easier to dislodge and tip over a crown if the abutment is very short.
It is similar to stacking cups into one another. The deeper the cup the more difficult it is to dislodge it. However if you have two very shallow cups stacking into each other, they are easier to remove. The same principle applies to implants with short crowns.
How to fix it
There are two solutions which you may try to fix an implant crown that keeps coming loose. You can either use a stronger dental cement or switch the cement retained crown to a screw retained one.
Stronger cement
Since most implant crowns use a temporary cement, you can try switching to a permanent one for better retention. A stronger glue will provide increased retention and thus make it less likely to come loose.
The reason that dentists don't use permanent cement is because of retrievability and ease of cement clean up afterwards.
Retrievability. If something happens to the implant, the temporary glue makes it easier for your dentist to take off the crown and fix whatever the problem is. If the cap is glued on permanently, they may not be able to remove it without damaging it.
Cement clean up. Temporary cement is easy to clean up afterwards, your dentist can easily flick it off. Permanent glue doesn't come off as easily and that is a problem. Residual implant cement is a risk factor for implant failure. That is much more catastrophic in comparison to just a loose crown.
Screw retained implant crown
If the crown keeps coming loose because it is glued in and has a short crown, you can bypass it by switching to a screw retained crown. The screw retained one does not need dental cement to hold it in because the entire prosthetic is screwed in.
After the cap is screwed in, your dentist will also torque it to make sure it is in tight. The chances of it loosening are very slim. In fact, we rarely if ever see a loose screwed retained crown. Most of the complications that we see are from the cemented ones.
Which solution is right for me?
We recommend starting with the most conservative option first which is to use a stronger dental cement. The reason is because it is easier and it is less expensive.
The fee to recement the implant crown with a permanent cement is a fraction of the cost of replacing the crown with a screw retained one. The entire process also takes about 5-10 minutes total.
However if the stronger glue doesn't work then you'll be forced to switch to a screwed in crown. That will definitely solve your problem but you'd have to pay for a brand new implant crown and you know how much that costs. The average cost of an implant crown is $2632.05 so that may make you think twice about jumping straight to switching.
Takeaway
An implant crown that keeps coming loose is most likely from a short cement retained crown. The first thing you can try to fix it is by switching to a stronger dental glue. If that doesn't work you can switch to a screwed in crown.
The best thing to do is to consult with your dentist. If you're in the Long Island City area, our dental office can help you decide.
