Insight 85 — Why Does an Implant Crown Cemented with Temporary Cement Keep Falling Off?

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Clinical Explanation

I delivered an implant crown with temporary cement two days ago, and the next day the patient called to say the crown had come off. I cemented it again with temporary cement, and today it has come off again. Is the problem the fit of the crown?

Answer

Before we think about remaking anything, we have to find the cause, and for that a few checks are carried out in order.

The first thing is to take the abutment out of the mouth and assess the stability of the crown on it outside the mouth. Seat the crown on the abutment and see whether it turns with a rotational movement, and whether it rocks when moved buccolingually and mesiodistally. Rotational stability depends on the abutment having a proper anti-rotation feature and on that feature having been correctly transferred to the crown. A crown that turns or rocks in this test does not have a correct internal fit with the abutment, and no temporary cement can compensate for that defect.

If the crown is stable outside the mouth, tighten the abutment in the mouth and check the seating of the crown. As with any other indirect restoration, this check starts with the proximal contacts, because a tight contact with the adjacent tooth does not let the crown go down, and until that problem is corrected, checking anything else is meaningless. Once the contacts are right and the crown still does not seat completely, we move on to other factors, including remnants of the previous cement inside the crown, or gingiva that has covered the abutment margin. This can be present from the start, for example when the abutment margin sits one to one and a half millimeters below the gingiva, or, in an abutment that has been in the mouth for a long time, the gingiva may have gradually grown up over it.

Once the crown seats completely, it is the turn of the occlusion, which must be checked both in centric contact and in lateral and protrusive movements. Incorrect occlusion puts extra force on the crown, and that same force can lift a crown seated with temporary cement.

A point to keep in mind independently of these checks is that a short abutment, or an abutment with a lot of taper, does not give reliable retention with temporary cement, even with a good fit. Shortness is, of course, acceptable to some extent in certain cases such as Ti-bases, because these abutments have features that improve retention, but a Ti-base should not be cemented with temporary cement; on sandblasting it, there is no definitive, agreed opinion either. On conventional abutments, sandblasting the abutment and the intaglio surface of the crown, or using a stronger cement, can increase retention.

Question

Should the crown be remade, or the abutment too?

Answer

With the checks explained above, you can determine for yourself where the problem is and decide on that basis. If the crown is not stable outside the mouth, the problem may lie with the crown or with the abutment, for example an abutment that is poorly designed, is very short, has a lot of taper, or lacks a proper anti-rotation feature; whichever one is causing the problem is the one that gets remade. If the crown is stable outside the mouth, neither the crown nor the abutment needs to be remade; in that case you remove whatever prevented the crown from seating, or correct the occlusion, and cement the crown again.

Question

The crown seated completely, but because the finish line was slightly below the gingiva, the gingiva blanched when the crown was placed and recovered after a few minutes. In this situation, should the abutment be changed so that the finish line comes up higher?

Answer

No. On a conventional abutment onto which the crown is cemented, as long as the finish line is at most one to one and a half millimeters below the gingiva, there is no need to change the abutment. Transient blanching of the gingiva at this depth is normal, and a small amount of gingiva in that area can be released so the crown seats more easily. Where there is enough keratinized gingiva, this can be done with a ceramic bur. This range, however, applies to cement-retained restorations; in screw-retained restorations, because removing excess cement from the sulcus is not an issue, a greater depth is acceptable.

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