What can be done about failing crowns and bridges?
Old crowns and bridges can fail through decay at the margin, fracture, loss of the supporting tooth, or a change in the bite around them. Treatment starts by finding why the restoration failed. Replacing it without addressing the cause tends to produce the same result a second time.
The usual reasons
Recurrent decay at the margin is the most common. The junction between the crown and the tooth is where plaque collects, and once it becomes exposed as the gum recedes, decay can progress under the restoration with very little to see from the outside. Fracture is next, either of the ceramic surface or of the tooth underneath, and a root-treated tooth carrying a post is more vulnerable than most. Cement can also wash out, which lets the crown leak and sometimes come off intact.
Bridges add their own failure modes. A bridge relies on the teeth at each end, so losing one of them takes the whole span with it, and the load carried by those teeth is higher than it would be if they stood alone.
Why the cause has to be found first
A crown that decayed because the margin was impossible to clean, a crown that fractured because of grinding, and a crown that failed because the tooth beneath it cracked are three different problems with three different plans. Remaking the same restoration in the same conditions gives it the same conditions to fail in. Working out the cause often involves removing the old restoration to see what is underneath, which is why the definitive plan is sometimes only settled once the tooth is exposed.
What the options usually are
Where enough sound tooth remains, replacement with a new crown is straightforward. Where decay has reached too far below the gum, the choices narrow to crown lengthening surgery, orthodontic extrusion of the root, or extraction. For a failed bridge, the alternatives are a new bridge if the remaining abutment teeth are sound, an implant to replace the missing tooth without loading its neighbors, or a removable partial denture.
Where several restorations of a similar age are failing at once, treating them as one case is often more predictable and less disruptive than replacing them individually as each one gives way.
Reducing the chance of a repeat
Margin hygiene, a night guard where grinding is a factor, treating any active gum disease, and reviewing the design of the original restoration all reduce the risk. So does honesty about service life. Restorations do not last indefinitely, and a plan that acknowledges that from the start is easier to maintain than one that assumes it is finished.