Being rejected by your crown hurts, especially when it was recently made and cost some serious money. Why does it happen? Usually either it has to do with the shape of the prepared tooth holding it or because the crown is ill fitting.
For a crown to be retentive (resist the forces that may tend to remove it) the preparation has to be smooth and only SLIGHTLY tapered. It also has to be long enough to provide enough surface area to grab the cement. If the prepared tooth is too irregular, too short or too tapered, it may not have the proper shape to hold onto its crown.
Also, another possibility is that the crown can be cemented with a poor fit. In this case the crown does not hug the tooth well enough for the crown to stay put and since the crown is completely dependent on cement to stay. An analogy would be found in furniture making where most good furniture is made with dovetails, that increase the ability of the furniture to hold the wood together. Without dovetails, the pieces tend to work themselves loose more quickly.
The same can be true with crowns, since their success is dependent on having a properly shaped tooth holding them and an intimate contact with it. The space between the crown and the tooth should be minimal (20-40 microns) in order for the cement achieve maximum holding power.
Most dentists faced with a permanent crown that comes out will first try to re-cement it. In my practice, I upgrade to a different permanent cement with more holding power. Unfortunately these cements do not always offer a long term solution and I am faced with the option of making the crown over. When I do so I pay special attention to correcting any shortcomings in my tooth preparation and making sure that I take a perfect impression.
I should add a few additional reasons that crowns can come loose. If the bite is high or just unfavorable, crowns can be more likely to come loose. Crowns do better when most of the occlusion is experienced along a mostly vertical force vector and without much of a horizontal component. Anterior teeth with deep over bites can experience a lot of horizontal forces on them and can sometimes be challenging to crown. Also some patients do have habits that are just not that friendly to crowns such as eating stale tootsie rolls or very hard and sticky candies. When chewed often these sweets tend to loosen even well fitting crowns
Dr. Pankaj Malhotra is a Periodontist & Implantologist in DELHI NCR region of INDIA, who maintains a practice in Noida. For more information about his Dental Practice please visit his website at www.dentalclinicnoida.in