Of all the options available, full gold crowns are probably the best choice for a patient. They require the least reduction of underlying tooth they are the kindest to opposing teeth and do not break. The truth is that most of my patients, in my New York Dental practice, will not allow me to place one anterior to their second molars. Instead most people (myself included) desire ‘natural looking crowns”. For premolars and molars, porcelain fused to metal crowns are still the best choice. These crowns have stood the test of time and when properly made function with the fewest problems. They are made with a bevel at their gingival margin and this leaves the neck of the tooth relatively strong, since it is not overly prepared. Teeth do not only receive vertical compressive forces. They also receive lateral forces that can cause flexing of the underlying preparation and can eventually cause loosening of a bonded crown.
Also many teeth can require crowns due to the presence of vertical cracks. These cracks cannot be removed and a crown is used to protect them from occlusal forces that might cause a crack’s further propagation. Porcelain fused to metal crowns, with their bevels, probably affords more protection for these teeth than do the newer ‘metal free’ crowns.
The newer crowns, emax,Lava,and Procera, are usually made with more relief inside than a standard porcelain fused to metal crown. They are also made with a chamfer or shoulder finish line that requires more tooth reduction than a traditional bevel does. This can result in flexing of the prepared tooth under a crown when it is heavily loaded during the chewing of dense foods. If a patient’s tooth flexes he or she will feel pain. The newer ‘metal free’ crowns are usually held in with bonded compomer or composite luting cements and although these are supposed to be stronger than traditional dental cements, in fact they can absorb some water and can become a weaker luting material over time. All composite materials are part plastic and have some porosity that allows water to be absorbed. When this happens the composite can ‘grow’ and can actually cause a separation of the cement and crown from a tooth. This can result in a crown separating, becoming loose or painful.
Since porcelain fused to metal crowns fit more intimately and because they usually have a retentive bevel , traditional dental cements can be used to lute them to the underlying prepared teeth. Traditional dental cements are actually salts that are the result of an acid –base reaction and are relatively inert over time. In effect they work by filling up the tiny gap between the crown and the tooth with salts and make it difficult for a crown to accidently separate from underlying tooth structure. They can ‘fail’ if there is insufficient tooth structure to provide a retentive preparation or if they are not properly fitted. Also they tend to be slightly water soluble. This last factor can pose a problem for crowns that have a large gap at the margin. Gaps between the casting and the underlying tooth can allow washout of these dental cements. To make these crowns work well , the dentist and the lab must make sure that the margins are well sealed and have a small gap (less than the thickness of a hair).
Porcelain fused to metal crowns are in my opinion, still the best choice for restoring posterior teeth, when esthetics is a concern. I do fabricate some of the newer crowns (Lava and Procera) but I prefer porcelain fused crowns because I feel they are still offer a more predictable long term result. The new crowns will most likely eventually supplant the pfm crowns since they can be fabricated using optically generated impressions and cad cam technology. As this technology gradually improves it will most likely become the treatment of choice, but for the time being, Porcelain fused to metal crowns still remain my clear favorite for the esthetic restoration of posterior teeth.