This is a question we get asked many times and I think the answer is simple, but has often not been discussed. A crown is placed to structurally support a tooth when it has been broken, had root canal treatment, or simply has too much filling or decay to function with a smaller restoration. The crown is made to fit very closely to the shape of the prepared tooth and is held in place by very strong bonding agents or cement.
I think people hear “crown” (or also “cap”) and believe it is a synthetic material which prevents the tooth from decaying. The truth is the tooth under the crown is always susceptible to decay- especially when you consider the damage already done to a tooth to necessitate a crown. When a crown is placed, the micro-margin where it meets the tooth becomes an easily forgotten home for bacteria. If the crown is not tight to the tooth or the cement is not the correct type, bacteria can slowly eat their way under the crown and begin the decay process in a protected environment. The results of this can be catastrophic and dramatic. Most of our dental materials do not allow us to see “through” them with x-rays, just around and below. So having the margins of the crown visually checked at cleaning visits and taking advantage of the views x-rays will allow are important preventive steps.
At home, you have to pay close attention to brushing thoroughly (not roughly- you’re not scrubbing paint here!) at the margins of your crown and to always floss well where the crown contacts the adjacent teeth. Don’t underestimate how quickly bacteria can damage the remaining tooth under the crown.
Keep in mind that a crown still has to function in an environment which favors the growth of bacteria, so you have to take the steps to protect it. With some diligent care and regular check-ups, the goal is for your crown to last many years.