Crowns

Dental Crowns

A crown is a type of dental restoration which completely caps or encircles a tooth or dental implant and is typically bonded to the tooth using dental cement. It can be a precious metal (gold) crown, a metal ceramic crown (inside metal to give strength and outside ceramic to mimic tooth color), or non-metal ceramic crown (such as Empress ceramic crown which is weaker but more translucent and life-like or Zirconia crown which is not as translucent but stronger).

More on crowns: What types of crown are available nowadays?

Dental crowns are more commonly known as caps. They really look like caps that put on teeth. Here is an important question: “When are the situations that just putting a filling (or dentists call a direct restoration) is not enough and then you should consider having a crown on that tooth?” Let me list some guidelines for you:

When the tooth has extensive cavity that after removing the cavity, not much tooth structure is left for the filling to add on.

When the fillings are discolored or the tooth has discolored or the patient does not like the look of the tooth (I am talking about the front teeth), then for cosmetic reasons.

When the tooth has extensive filling that has the risk of fracturing the tooth because tooth with big filling may not be able to withstand heavy biting force in your mouth.

When the tooth has had root canal therapy. Why? First, you have to ask why the tooth requires root canal therapy. Most of the time it is because the tooth has had deep or extensive cavity that it involves the pulp of the tooth. When dentist removes the cavity on that tooth, you can imagine that not much tooth structure will be left. How does dentist perform root canal therapy? He/she will drill a hole on the top surface of the tooth into the inside core of the tooth where the pulp or nerve or blood vessels are located. You can imagine that after this procedure, more tooth structure will be gone. So a crown is needed exactly for the same reason as in point 2 above: the tooth becomes too weak to stand for your heavy biting force because not much tooth structure is left on the tooth.

Controversy scenario:

Then you may ask how about a tooth that has had root canal therapy but still has quite a lot of tooth structure left? Many dentists believe that once a tooth has root canal therapy, the tooth will become more brittle than teeth without root canal therapy, hence has a higher risk to fracture if not protected by crowns. In fact, research studies did not show such a case, at least not to the extent that every tooth has had root canal therapy must have a crown on it. In some situation, tooth requires root canal therapy may not have extensive decay. Maybe the tooth has had trauma, maybe the decay is quite small but very deep so that it involves the pulp, hence requires root canal therapy. In this case, root canal therapy can be performed with just drilling away the top surface away to get into the pulp. All the side-walls of the tooth is still intact, what we call only an occlusal surface is involved, then the tooth will not require a crown. There is research showing that if only the top surface is involved on a root canal treated tooth, the strength of that tooth only reduce by 5% while if the tooth is missing at least two surfaces, let’s say the top surface and one of the side surface are with filling and the tooth has had root canal treated, the tooth will have reduced strength by 45 to 50%. But how often we hear dentists tell their patients that every tooth that has root canal therapy must have a crown on it or it will break or fracture eventually. Why? Because crown procedure is expensive and lucrative!

How much a crown is worth?

I hope you are not mistaken that any dentist tells you that you need a crown for a tooth is trying to ripe you off. Crown treatment is the premium dental service you can get. It is expensive because it is very techniques sensitive and the materials that are involved in the procedure are very expensive. However, it is very cost effective and gives you a long lasting result. Researches show that a direct filling normally last for 5-7 years while crowns last for 12-15 years in average. Most of the crowns which fail are due to recurrent decay around the crown margins. This can be caused by your poor oral hygiene or the quality of the crowns made (more of this later). And so often I see crowns in many patients’mouths are still in good shape after 30 or 40 years of service in your mouth, 24 hours a day! Think about how long your car will last if you use it 24 hours a day! Don’t tell me you are not using your teeth while you’re sleeping. Every movement you do such as swallowing, breathing etc you do involve your mouth and your teeth! A crown also make your oral hygiene maintenance much easier to implement because either the gold alloy or the porcelain material is very smooth and does not attract plaques in your mouth compared to decayed surfaces or surfaces of direct filling materials. As mentioned earlier, crown protects your tooth from fracturing. Also one important advantage of crowns is that it can change the shape and color of the tooth and gives it a completely new look that is so esthetic that you will love it. Think about the beautiful white teeth in the models of the cover pages of many magazines. Many of them actually have crowns or veneers in their front teeth so that they can have such beautiful attractive smiles!

What kind of crowns are available?

Now let’s talk about the types of crown available nowadays. In the old times, full gold crowns (they are not 100% gold, the alloy used to make gold crown can range from quite low the gold content to as high as 80% gold content) are the standard crowns of choice. In poor countries, you can see stainless steel crown or some cheap alloy crowns as well. My discussion here does not include those stainless steel crowns or alike because they are not up to the North American standard requirements for crowns. – The main advantage of full gold crowns is its unparalleled strength. A well-made full gold crown is rarely fractured or broken into pieces. In addition, dentist needs to reduce the size of the tooth before placing a crown on it. Because gold alloy is stronger, crown from this material can be made thinner and still provide enough strength to withstand the biting forces of your mouth. In other words, when a dentist is delivering a full gold crown to a tooth, he will reduce the least tooth structure. In comparison, if the dentist is giving you a all-porcelain crown, because the porcelain is easier to break and more brittle, the crown made of this material has to be thicker in order to give enough strength to withstand the biting forces. In other words, the dentist has to reduce more tooth structure in order to fit the crown. If they don’t reduce the tooth small enough, but the crown has to have certain thickness, what will happen? The crown will become very bulky! Think about a very big cap fitting onto a small head!
This can create gum or periodontal problems because food and bacteria trapped underneath these overhangs cannot be easily removed.

Gold crown

Another advantage of full gold crown is that gold alloy is moldable and not brittle (as in the case of porcelain). Full gold crowns can be easily made to fit well at the margins where the crowns sit onto the teeth. Fortunately, with nowadays technology, all porcelain crowns can be fabricated to fit the margins precisely as well.

This is a very important factor to consider. If the margins do not fit well when a crown is placed onto the tooth, creating what dentists call “open margins,” this tooth will become very susceptible to decay again at the margins as food and bacteria will trap at these open margins. So when you are told that your existing crown which was made only 3 years ago needs to be replaced because of some recurrent decay at the crown margins, you may as well suspect that the crown delivered at that time may have an open margin already (even though your dentist will likely blame your poor oral hygiene for it, especially if he/she is the one who delivered that crown). So you may blame the dentist who delivered that crown to you!

Of course, nothing is perfect. The main disadvantage of full gold crown is its poor esthetics, at least according to nowadays North American Beauty standard. However, Beauty is always in the eye of the beholder. I have had at least two patients requested for full gold crowns in the front teeth! For most people, if the tooth that needs a crown is located deep inside the mouth where the teeth are rarely visible to other people, they may not mind having a full gold crown to benefit from all its advantages.

Porcelain fused to metal crowns:

Can you benefit all the advantages of full gold crowns and still have an esthetic tooth-colored crown? The answer is yes! Here comes what dentists call the porcelain-fused-to-metal crowns (PFM crowns or CMC crowns as they are also known as ceramo-metal crowns). They are crowns with gold alloy underneath but with porcelain wrapping around most, if not all, the metal so that the crown is tooth-colored when seated on a tooth. Wow, what an innovation! What is the catch, you may wonder? The main disadvantages of PFM are that dentists have to reduce more tooth structure than full gold crown (but still less than all-porcelain crowns) and because the crown has metal underneath the porcelain, it is not as transparent and life-like compared to all porcelain crowns and to the natural teeth. Have you ever noticed some friends of yours when they smile you can see some of their front teeth have a grayish margins close to the gum or the teeth somehow look a bit grayish or dark or not natural. Those teeth probably have PFM crowns. Therefore the demand from the public for a more esthetic solution has driven the dental business to develop a better product: all ceramic or all porcelain crown.

All-ceramic crowns:

In other words, if you expect paramount esthetics, a Hollywood smile, all-porcelain crowns is your choice of crowns. Because there is no metal underneath the porcelain, light can shine through it, giving it very life-like appearance. The disadvantage of porcelain is that it is quite brittle and weaker than metal, so crown fractures can happen quite frequently if your dentist just do all porcelain crowns on every patient indiscriminately. As a general rule, I do all porcelain crowns mainly in the front teeth only. Why? The front teeth are the visible teeth when you smile, so esthetics is very important. Front teeth normally do not have much biting force on them as compared to the posterior teeth. How about if patients insist to have only all porcelain crowns even in their posterior teeth, will I do it? I will explain to patients about the likelihood of fracturing or chipping of all porcelain crowns and if they still want to go ahead, by all mean! I will respect patients’ choice as far as they are well informed of what they are going for. I find it quite irresponsible for dentist who talks patients into all porcelain crowns without letting them know the fact that all porcelain crowns do fracture, and at a much higher rate than PFM crowns! I know most dentist will redo the crown for free if that happen within 1 or 2 years. But how about if the crown cracks in 5 years do you think your dentist will redo it for free? What they will then tell you? “Mr/Mrs so and so, because your bite is so strong (or because you grind your teeth so often), now we have to make a new crown for you. But fortunately, your insurance will cover some of it because the crown is over 5 years old!” Of course they will blame your bite rather than blaming himself/herself! But I believe as a health care giver, your dentist should deliver something suitable for you. If he/she cannot diagnoses in the first place that you have a heavy bite and prescribed an all-ceramic crown for you, sooner or later, you will crack that crown! Do you want to worry about cracking your crown when one day you bite on some hard stuff? I heard of at least two persons who have had the crown redone for 2 times and still cracked it the third time! Fortunately, by the time I am writing this paragraph, the dental market already has some all-porcelain crowns available which are very strong to withstand your heavy bite. Some examples of these strong crowns are Zirconia crowns However, most dentists find them not transparent enough to give that life-like appearance and prefer what is very transparent but quite weak (e.g. Empress crowns) over them. I myself do quite a few Empress crowns because they are really very esthetics. But I will never do them in the posterior teeth! So next time when your dentist suggests to you to have all-porcelain crowns in the posterior tooth, make sure you ask which one he/she chooses for you.

Teeth from opposing arches grind on each other all the time. If they are of same material, as in the case of two natural teeth touching together, they wear in the similar rate. But let’s say the bottom tooth has a crown and the top surface of that crown is of porcelain, which touches the top opposing tooth surface, which is enamel, the enamel surface from the top tooth will wear faster than the bottom tooth porcelain surface because porcelain is very abrasive. In other words, if you have a very heavy bite, porcelain crowns will grind away your natural teeth surfaces in the opposing arch very aggressively, whereas gold alloy is found to have a closer abrasiveness to our natural enamel than porcelain does. This is one more plus to have full gold crowns rather than porcelain crowns.