There are many reasons for repeated dislodgment of a crown. In order to address it, the underlying factor(s) must be identified and dealt with:
- A short clinical crown height
Some teeth, especially 2nd molars, can have a pre-existing short clinical crown height. This will make it easier for the crown to come off. Retentive features like slots or grooves may be incorporated into the prep design to overcome this. Surgical crown lengthening or orthodontic extrusion of the tooth may be needed to access more tooth structure.
- The excessive taper of the crown preparation
If the axial walls of the tooth are too convergent after preparation, the crown can come off the tooth more easily. Avoiding this during the preparation phase is key, but if this has already occurred, retentive features like slots or grooves may be incorporated into the prep design.
- Cement failure
If the crown has been cemented in wet conditions (saliva contamination, crown margins under the gum) then the cement may not set properly. The cement that does not set properly may not be as strong.