Why Does My Crown Keep Coming Off?
A crown stays on because there is enough sound tooth under it for the cement to grip, and because the bite is not pushing it sideways. When one keeps coming off, it is almost always one of four things: decay under the margin, not enough tooth left to hold it, a bite loading it in a direction it cannot take, or a crack in the tooth underneath. Recementing without working out which one restarts the same clock.
What actually holds a crown on
A crown is a shell. What keeps it in place is the shape of the tooth prepared underneath — the taller and more parallel the remaining walls, the more grip there is — plus a cement filling the microscopic gap, plus a bite that pushes the crown down onto the tooth rather than sideways off it.
Take away any one of those and the crown becomes a countdown rather than a repair. That is why the question is never really “which cement did you use”, which is what people usually expect to be asked.
The four usual explanations
Decay under the margin. The commonest. The join between crown and tooth is where bacteria get in, and once the tooth softens underneath, the crown is gripping something that is no longer solid. It often looks perfectly fine from outside, which is why it surprises people.
Not enough tooth left. A tooth that was heavily filled before it was crowned may have very little height to grip. Sometimes the answer is to rebuild the core, sometimes to lengthen the visible tooth surgically, and occasionally to accept that the tooth has given what it has to give.
The bite. A crown taking a sideways shove every time you chew, or grinding at night, is being levered rather than pressed. Adjusting how the teeth meet often matters more than anything done to the crown itself.
A crack underneath. A cracked root or a split tooth flexes under load, and nothing cemented to it will stay put for long. This is the one with the least comfortable answer, and it is better found early than after a third recementing.
What we do before cementing anything
Take the crown out of the picture and look at the tooth. We check for decay at and under the margin, measure how much sound tooth is left above the gum, check the bite with the crown out and in, take a radiograph, and look for cracks under magnification.
That examination is the whole value of the visit. It is what decides whether this crown goes back on, a new one is made, the tooth is restored a different way, or the tooth is beyond saving — and knowing which is worth far more than another few months of cement.
Can the same crown go back on?
Often, yes, and it is the cheapest good outcome. If the crown is undamaged, the fit is still accurate and the tooth under it is sound, recementing it properly after cleaning both surfaces is entirely reasonable.
What changes the answer is anything that has altered the shape of the tooth. Once decay has been removed, the crown no longer fits what is left, and a crown that does not fit will leak whatever it is stuck down with.
Bring the crown with you
Keep it, and bring it, even if it is in two pieces. It tells us how it failed, whether there is tooth stuck inside it (which answers several questions at once), and whether it can be reused. Do not try to glue it back yourself — household adhesives are not removable, and a crown cemented at a slight angle can do more damage than a missing one.
In the meantime the exposed tooth may be sensitive to cold and air. It is usually comfortable enough to wait a day or two, and worth telling us if it is not.
Questions
Is a crown coming off an emergency?
Usually not, though it is worth being seen promptly. The tooth underneath is unprotected and can be sensitive, and the teeth either side can drift slightly if it is left for weeks. If there is pain rather than sensitivity, that is a different matter and should be looked at quickly.
Can I glue it back myself?
Please do not. Household glues are not designed to come out again, and if the crown seats even slightly off, the bite will be wrong and the tooth underneath can be damaged. Pharmacy temporary cement is acceptable for a day or two if you are travelling.
Will I need a new crown?
Not always. If the crown is intact and the tooth under it is sound, the same one can often be recemented. A new one is needed when decay has had to be removed, when the fit has been lost, or when the crown itself is damaged.
What if there is tooth stuck inside the crown?
That tells us the tooth fractured rather than the cement failing, and it changes the plan. Depending on how much came away, the tooth may be rebuilt with a core, or may need a different kind of restoration. It is one of the main reasons to bring the crown with you.
Does it mean the original work was bad?
Almost never. Crowns have a working life, and most that come loose have been doing their job for many years. Decay restarting at a margin, years of grinding and the gum receding past the join are all ordinary biology rather than anyone's mistake.