One of the most frustrating things in my practice is when old restorations come loose. The first thing that I wonder when I see “recement crown” on the schedule is what the odds are that I will actually be able to simply recement. Actually the first thing I wonder is whether I did the restoration, then I wonder about the condition of the tooth and how the patient will react if the restoration can not simply be cemented back into place.
Typically the patient comes in with the crown and post in a Ziploc bag, or with a complaint that the crown feels loose. My exam reveals recurrent decay at the crown margin, and often extending into the post space. Occasionally the root is fractured, but the most common feature of these restorations is very little natural tooth structure remaining.
There are many factors we have to consider when restoring severely compromised teeth. One of those factors is the long-term success of the restoration, and a critical factor is our ability to create adequate ferrule effect. Ferrule effect is the amount of remaining tooth structure where we can create opposing parallel walls that are at least 1mm thick, this creates retention form and resistance form. A key point is the presence of opposing parallel walls, so we can not simply measure from the margin up depending on the shape of the preparation. A few other things to think about with ferrule effect is that opposing buccal and lingual walls are critical, the interproximal plays a less significant role. Additionally the walls need to have adequate thickness (1mm at a minimum) to prevent fracture and flex of this remaining tooth structure.
When I evaluate my ability to create ferrule effect I try to err on the side of being conservative and want to see at least 2mm of opposing buccal and lingual tooth that is 1.5mm in thickness. If I have less tooth structure then this the patient and I need to discuss our options for improving the restorability of the tooth, like crown lengthening or orthodontic extrusion, or removing the tooth and replacing it. When the clinical situation presents with adequate tooth structure, I still try to minimize risk by gaining as much wall height as possible, even if I have to place margins subgingivally, bonding both the post and the final restoration and helping the patient have a realistic understanding of the lifespan of the restoration.