There can be a variety of reasons why a dental crown might need to be replaced.
Some of them are:
1. The Formation of Tooth Decay
While a dental crown cannot decay the tooth on which the crown is cemented certainly can. If dental plaque is allowed to accumulate on a tooth in the region where the crown and tooth meet, a cavity can start.
While there can be a lot of variables with this type of situation, the worst case scenario for your dental crown is that in order for your dentist to be able to get at the decay, and subsequently restore your tooth properly, the crown will need to be removed and replaced with a new one.
2. The Dental Crown Becomes Excessively Worn.
Dental restorations are not necessarily more wear resistant than your own natural teeth, nor is it in your best interest that they should be. The ideal dental crown would be one made out of a material which has the same wear characteristics as your own teeth. This way neither the dental crown nor your teeth would wear each other excessively.
Dental crowns can wear out, especially in those cases where a person clenches and grinds their teeth excessively. A dentist will sometimes detect a small hole on the chewing surface of a dental crown in that area where it makes contact with an opposing tooth (meaning a tooth which touches on the crown when you bite). In these cases, since the crown no longer seals over your tooth your dentist will probably recommend that a new crown be made, before that point in time when
dental plaque has seeped under the crown and has been able to start a cavity
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3. The Dental Crown Has Broken.
Dental crowns can break, or more precisely the porcelain component of a dental crown can fracture. Some dental crowns are made in a fashion where their full thickness is porcelain. In these cases if the crown breaks it will most likely break through and through. Even if the broken piece of the crown doesn't come off either the esthetics, function, or the seal of the crown will most likely have been compromised and the crown will need to be replaced.
Other ceramic dental crowns are of the "porcelain fused to metal" variety. When this type of crown is fabricated the dental technician first makes a thin metal shell that fully covers over the tooth, a layer of porcelain is then fused to this metal so to give the crown a tooth-like color. In cases where this type of crown has broken it is the layer of porcelain that has fractured off, usually revealing the metal tooth covering underneath (which is often grey in appearance). While the function and esthetics of the crown may have been compromised, the crown's seal over the tooth may not have been affected.
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4. The esthetics of the crown have become objectionable.
Some dental crowns are replaced because, from a cosmetic standpoint, their appearance is no longer pleasing.
Also related to the cosmetic appearance of a dental crown, there can be times when, as years have elapsed, the color of the crown no longer closely matches the shade of its neighboring teeth. In these cases it is not the color of the porcelain used to make the dental crown that has changed but instead the neighboring teeth have stained and darkened.
There can be two solutions to this situation. The dental crown can be replaced with a new one that more closely matches the current color of its neighboring teeth. Another solution might be to use a teeth whitening gel to return the neighboring teeth closer to the color they were when the dental crown was originally placed.
This young man walked in with sensitivity in his molars. As evident the Amalgam fillings had marginal leakage that was the cause of his sensitivity. Also the second molar was beginning to chip-off at the distal. Keeping these facts in mind porcelain crowns were advised and the patient was extremely satisfied with the result symptomatically and esthetically.
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Dental Crowns procedures 
