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Crown types and options

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How to decide on what type of crown?

 

In pre-clinical lab, I learned three types of crown preps: 1) all metal, 2) porcelain fused to metal and 3) all ceramic. We essentially learned that the differences of these crown types had to do with how much tooth structure we needed to reduce (0.5mm, 1.5mm etc) and what type of margin we needed to prepare (chamfer, shoulder, etc).  We never discussed why you would pick one type of crown over another. We certainly never discussed, once you decide on which type of crown, which type of material to use. Now that I am treating patients in clinic and having to write lab prescriptions on a daily basis for crowns and bridges, I have come to realize that this is crucial information that should be understood thoroughly before picking up a hand piece to begin a crown preparation.

 

All-metal crowns are used mainly in the posterior. The advantages of such a tooth are strength, longevity, less wear on opposing dentition and less tooth reduction is necessary. The only disadvantage of this crown is the un-esthetic metallic appearance.  Once you have decided on an all-metal crown, you must now choose which type of alloy to use: 1) High- noble 2) noble or 3) non-noble. The most preferable choice is the high noble, which is at least 60% noble metal, of which at least 40% must be gold. Because of the content of noble metals, the high noble crown is also the most expensive. The non-noble option is the least expensive, but it also posses an allergy risk to about 15% of the population due to its nickel and chrome content.

 

Porcelain fused to metal crowns (PFMs) are advantageous because they combine the strength of a metal crown with the esthetic appearance of a porcelain crown. A disadvantage of the PFM is that porcelain can cause excessive wear on the opposing dentition. This is especially important for people with a bruxing habit. Additionally, a greater amount of tooth reduction is required to make room for both the porcelain and the metal.

 

All-ceramic crowns and the most esthetic crown option because more of the crown is made of translucent materials, like porcelain, that mimic a natural tooth’s coloring.  Another advantage is that all-ceramic crowns cause no more wear on the opposing dentition than actual enamel would cause.  In certain dental offices that have milling units, all-ceramic crowns can be fabricated “same-day in office.” There are machines that can fabricate crowns out of ceramic blocks in less than an hour. A disadvantage to all-ceramic crowns is that they have inferior physical characteristics, such as reduced strength, reduced hardness and reduced resistance to fracture. Fore this reason, all-ceramic crowns are more commonly placed in the anterior.

 

Once you have chosen an all-ceramic crown for your restoration, you must decide which type of ceramic to go with. There are several main categories: 1) Feldspathic Porcelain 2) Reinforced Glass-based ceramics- IPS Empress 3) Reinforced Glass based ceramics- IPS E.Max, 4) Zirconia based

 

The Feldspathic is a powder-liquid porcelain. They can be layered and therefore can have multiple opacities. It must be bonded to teeth instead of cemented. Marginal integrity is a disadvantage, as chipping at the margin is common.

 

IPS Empress porcelain is leucite reinforced, which makes it twice as strong as traditional porcelain. The must be bonded instead of cemented as well. The marginal integrity is superior to Feldspathic because of the method of fabrication. It should not be used with dark preparations because of its high level of translucency.

 

IPS E.Max is reinforced with lithium disilicate, is twice as strong as IPS Empress and it can be cemented as long as there is 2.0mm thickness of ceramic material. Due to the increased strength, it can be used in short span bridges, but not for molar replacement.

 

Zirconia- based restorations are very strong, much stronger than the other all-ceramic options, which are glass based. Zirconia can be used as an alternative to metal options because of its strength. It can also be used in posterior bridges that replace molars and long-spanning anterior bridges.

 

I hope that this brief overview helps new dental students out when thinking about crown types for specific teeth. There is a wealth of information on specific brands, metals, ceramics and designs online. I encourage everyone to do some research before your next crown preparation!