Why is my Zirconium crown lacking retention.?

 

retentive crown preparation for all zirconium crown

I was asked this question by a frustrated dental student. She explained that she tried in a zirconium crown and it had no real retention. She asked me why this had happened. I asked her to show me the crown and the model it was made on.

Now truth be told, zirconium crowns can have more relief between the prep and the crown than some porcelain fused to metal or full metal crowns, but all types of crowns can have a problem with being non retentive due to a number of circumstances.

I explained the following reasons for a poorly retentive crowns to result from our efforts:

1) the degree of taper. In theory crowns can be fabricated on preparations with miniumal taper (3 degrees), but many practitioners are unable to make this sort of preparation without producing undercuts. In their effort to make a preparation without undercuts, it is not uncommon to end up with preparations with greater than a 10 degree taper. These "tepee" preparations tend to be completely dependent on the cement used to lute them in and can result in a crown that is more likely  to become de-cemented later on.

2) The length of the preparation is another important factor since short preparations (less than 3 mm) are more likely to result in crowns that are not particularly retentive. Surface area of a preparation is also a factor since the greater the surface area, the more tooth structure that is available for luting cements to adhere to.

3) undercuts in the preparation are likely to reduce any crowns retention, since the preparations need to be blocked out either digitally( Increased digital relief) or manually ( with block out material placed on a die). This definitely can reduce the retention of a zirconium crown since the technician will likely use digital relief to  increase amount of relief between the preparation and the entire milled crown.

To avoid these issues dentists should spend more time with their preparations to minimize fit problems. In the case of a "short prep" it helps to make it longer by either building it up or dropping the shoulder closer to the gingiva. Sometimes crown lengthening ( careful removal of supporting bone) is advisable in order to create a longer preparation. Also with short teeth, making a minimal taper without any significant  undercuts so the resulting  zirconim  that is significantly retentive.