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Questions about Crowns
- What IS a crown?
- Are there different types of crowns?
- What are the materials used for crowns?
- If I get a porcelain crown, are there different ways to make it?
- Why is a crown so expensive?
- Are there any disadvantages to getting a porcelain crown in my mouth?
- Are there different kinds of gold crowns?
- Are there gold crowns that cannot be seen conversationally?
- What is the expected lifetime of a porcelain crown as opposed to a porcelain crown?
- What is the most conservative type of crown I can get in my mouth?
- How does the dentist get the shade of porcelain to match my other teeth?
- I’ve noticed that some older people have teeth that don’t match each other – is this because some are crowns? Did they ever match?
- Why is it that crowns sometimes fall off?
Answers about Crowns:
- What IS a crown? The basic principle of a crown is that it is designed to protect parts of the tooth that would not be protected if a simple filling were done. Perhaps there was enough decay so that one or more of the cusps are undermined structurally, or a filling would have to be so large that the remaining tooth structure would be rather thin in some areas, liable to breakage. Many people have had the experience of having a cusp break off from outside a filling. I’ve had that happen a couple of times, in movie theaters no less. Popcorn can be the culprit – you are watching an exciting movie while chomping on handfuls of popcorn and one of those little unpopped kernels goes in and is banged down on the cusp – cusp joins the rest of the popcorn. When some material is placed over the biting, occlusal, surface of the tooth it can protect the weakened cusps against this kind of destructive force by spreading the force over the entire surface. Crowns of all types are made OUTSIDE the mouth and then cemented in. They may not cover the entire occlusal surface, but usually do. They need not cover the outside surfaces of the tooth, facial, lingual and those surfaces touching the adjacent teeth, but usually do. The tooth needs to be prepared so that the cemented crown stays on for many years, it needs to have retention – but that can be achieved in many ways…………………BACK to questions
- Are there different types of crowns? Certainly! Gold crowns can readily be made to cover only parts of the tooth, not all of the outside walls and not necessarily all of the occlusal, biting, surface. They require less reduction of the healthy tooth to hold the crown in place and are cemented with non-adhesive cements. And there are ceramic crowns that require more reduction of healthy tooth structure and are cemented with adhesive resin cements instead of relying on mechanical retention. Depending on the material used, the tooth can be prepared for the crown in various ways, saving more or less of the healthy tooth structure while other parts are being protected. Another difference we need to consider, is that crowns for the back teeth and front teeth are very different – meeting completely different needs. ALSO – there are crowns that are BOTH metal and porcelain – the ceramometal or porcelain-fused-to-metal, PFM, crown. For these there is a cast metal coping or substructure that covers the prepared tooth, and OVER that is placed a porcelain layer for the appearance…………………BACK to questions
- What are the materials used for crowns? There are a variety of different materials that can be used – but the most general two are gold and ceramic. There are various ceramics that can be used. Ceramic crowns can be made of several materials including porcelain, zirconia and lithium disilicate glasses. These ceramic crowns need to be thicker than gold so they don’t fracture, and hence they require more reduction of tooth structure. All of the all-ceramic crowns are cemented with adhesive resin cements, instead of mechanically held in place as are gold crowns. PFM crowns use a special novel allow over the prepared tooth with porcelain over that. These crowns are also mechanically retained and do not require cements with the adhesive properties………………… BACK to questions
- If I get a porcelain crown, are there different ways to make it? Yes. ALL-porcelain crowns can be made BY HAND by a skilled laboratory technician. In this instance the lab tech can build the crown using various shades and transparencies of porcelain, so the appearance matches the adjacent teeth as much as possible. There is definitely an ART to the hand fabrication of crowns, and for the front teeth, this is an art that can be greatly appreciated. Alternatively, they can be cut by a computer-driven milling machine. Great advances have been made in the esthetics of CAD/CAM crowns these days (computer aided design/computer aided machining). In this later instance there are blocks of porcelain that are cut by machine to form the final crown. If the dentist buys the expensive blocks that have different colored layers, the computer can decide where in the block to cut the crown, achieving some customizing of the appearance. When done well, a satisfactory result can be obtained. The main advantage of the CAD/CAM crowns is they can be made in one visit – but the disadvantage is that the esthetics is not as certain as handcrafted crowns. For PFM crowns, porcelain-fused-to-metal, the metal part over the prepared tooth is made by a casting process, so the margins are well adapted, and the porcelain is handcrafted by the laboratory technician………………… BACK to questions
- Why is a crown so expensive? There are a lot of steps and a lot of technology and often artistic judgments, and these take time. Whether CAD/CAM of handcrafted, one appointment or two, the fabrication of a crown is a pains-taking process when done well. Part of the fee, of course, is to pay the laboratory technician for his services, but for CAD/CAM part of the fee goes toward paying the lease on the equipment. A dentist may pay $4000 per month just to have this machine in his office to do crowns…………………BACK to questions
- Are there any disadvantages to getting a porcelain crown in my mouth? There can be! The main disadvantage to ALL ceramics is the wear they cause to the opposing teeth. This can be substantial and significant if you tend to grind your teeth already. I’ve seen people that have ground there lower front teeth in HALF once they had porcelain placed on the upper teeth! If you have a ceramic crown opposing another ceramic crown, there is not a significant problem with opposing wear. But ceramic against tooth or gold can be catastrophic. Your dentist will advise you on what is a reasonable choice for a crown and how best to avoid the wear problem, which always must be considered. The other problems with porcelain crowns, as compared with gold crowns, is the fit of the crown to the tooth at the margins is far better for gold, leading to a longer lifetime because decay is less likely to start around the edges…………………BACK to questions
- Are there different types of gold crowns? There are different designs. They all tend to use the same alloy of gold, but the amount of tooth structure that the crown covers can be quite conservative. In fact, many times we can do gold crowns that do not cover the facial, or more visible, wall of the tooth. In this case they are conversationally invisible. A ¾ crown, for example, covers the occlusal or biting surface, and three of the four side walls, generally leaving uncovered the facial wall. So the esthetics of the crown are determined by the original enamel seen in the mouth, and all of the gold is hiding from view. These ESTHETIC GOLD CROWNS offer the longevity of gold, the more conservative cutting of healthy tooth structure, natural appearance (because all you see is natural tooth), and NO wear to the opposing teeth!………………..BACK to questions
- Are there gold crowns that cannot be seen conversationally? Yes indeed. Any gold crown that is made such that the facial wall of the tooth is left natural, will tend to be invisible conversationally. Even lower crowns that cover the biting surface are not visible unless the wearer smiles very broadly with an open mouth. But under normal circumstances the crown will not be seen, but this is especially true for the upper teeth. Of course, we’re talking about the back, posterior, teeth. ………………..BACK to questions
- What is the expected lifetime of a porcelain crown as opposed to a gold crown? Often a porcelain crown must be replaced because it is doing too much damage to the opposing tooth. But also crowns need to be replaced if they get decay around the edges (margins). The marginal adaptation of gold is far better, so it will last longer both from the wear perspective AND the marginal integrity perspective. If the gold crown is made so some margins are accessible, the gold can be burnished (rubbed against) to the margins, giving an even better adaptation, very resistant to recurrent decay. While the expectations vary wildly from patient to patient and dentist to dentist – a porcelain or ceramic crown can be expected to last in the average case around 15 years, while a gold crown can last the rest of your life………………… BACK to questions
- What is the most conservative type of crown I can get in my mouth? The most conservative crown in terms of the amount of healthy tooth structure that must be cut to prepare the tooth for the crown is the gold onlay. It only covers the occlusal surface and the proximal walls, that touch the adjacent teeth. In general, the onlay uses areas of the tooth that are already missing for retention – so little more needs to be cut. More of the margin is accessible for burnishing, so the marginal adaptation is great, and gold doesn’t have to be anywhere near as thick as ceramics to be strong and safe from damage…………………BACK to questions
- How does the dentist get the shade of porcelain to match my other teeth? Good question because they don’t always. It can be very challenging for the front, anterior, teeth, particularly for older people whose enamel is more transparent and who tend to have more variations in shade in the mouth. The dentist who is very familiar with porcelain materials can specify which materials of various levels of transparency should be used in different areas so the lab tech can have a pretty clear idea of what to do. The dentist can diagram the tooth and indicate what porcelains and shades should be used where. This is a skill that only dentists who do a lot of cosmetic dentistry on older people have – and it is a skill taught to them by their laboratory technician by working together on quite a few patients. For simpler cases, younger people for instance where there is not so much transparency, a SHADE GUIDE is used. These are porcelain tabs of different standardized hues, values and saturations that can be matched to the patient’s teeth. The shade guide allows a dentist to communicate to ANY lab technician what he would like done. They do not deal with transparency at all…………………BACK to questions
- I’ve noticed that some older people have teeth that don’t match each other – is this because some are crowns? Did they ever match? Most of the time a person’s front, anterior, teeth do not match because they have had numerous restorations, either with composite or porcelain. The composite or porcelain restorations may not have matched well when they were done, but as people age their natural teeth change while the artificial materials tend not to. So the natural teeth look different from the artificial materials, and the composite and porcelain restorations may not match each other either. The result is not a good look. ………………..BACK to questions
- Why do crowns sometimes fall off? Gold crowns tend to fall off very rarely – they are mechanically retained and use cements that don’t depend on chemical adhesion. If the tooth is well prepared and tall enough for the crown to get a good “grip”, it will be there for a long time. Ceramic crowns are retained on the tooth only by chemical adhesion. While we have the technology and materials to effect a very good bond between porcelain, for example, and dentin, there are abbreviated ways to do it, with fewer steps, which compromise the bond. I’ve had two porcelain crowns in my mouth, and one fell off in a week and the other stayed in for about 5 years before falling off. This is one area where dentists need to pay careful attention and take their time…………………BACK to questions