Dental Restorations Part 5: Metal-Ceramics or All-Ceramics

An area where looks, quality, and reliability combine is in a metal and ceramic blend called a Porcelain Fused to Metal (PFM) restoration, or a Metal Ceramic Restoration (MCR) — two terms for the same thing.

Many are surprised to learn both crown sets above have metal in them. PFMs (MCRs) can be a best of both worlds option. The metal provides strength, while the ceramic provides appearance. These metal-ceramic crowns can also use a softer type of ceramic that is then strengthened by the metal underneath - reducing the risk for this ceramic to wear down opposing teeth. The cement they use allows them to adhere very well to a tooth, and they are simple to remove if replacement is needed because the metal is discernible underneath, and the material is soft to a dental bur. These restorations also have a nice backup function, in that the metal remains if some of the outer ceramic fractures. That means there is no chance for a total through-fracture, which exists with all ceramic options. Often times, the fracture can be polished away and the restoration continues working.

PFM (MCR) restorations are common, although they are being replaced by all-ceramic alternatives. All-ceramic restorations are the newest version of inlays, onlays, crowns, and bridges that used to be dominated by metal, or PFM/MCR options. These ceramic restorations have been growing rapidly in popularity over the last 15-20 years. This is driven by a number of reasons, including people desiring completely tooth-colored restorations, ceramic options having the ability to be produced in greater number and at lower costs due to the use of milling machines, and increases in ceramic strength that previously were not possible. While PFM (MCR) crowns need physical impressions and typically two appointments to make, scanned, pure ceramic options are increasingly possible in a day (otherwise, these need two or more appointments as well). Although appealing, there are more things to consider when it comes to these restorations.

Depending on the different types of ceramic, the strength can vary. Ceramics in the middle range are common for inlays, onlays, veneers, and some crowns. All ceramic restorations may have a greater risk for fracture. As a result, more tooth structure may need to be removed to increase the ceramic thickness. This leads to a greater risk for the tooth becoming sensitive and potentially needing a root canal. High strength ceramics used in dentistry can avoid this, and involve a material known as zirconia. This is similar to what artificial diamonds (cubic zirconia) are made from. Dental zirconia-based restorations are heated at different temperatures, and have different materials — however the strength translates. You can hit some types of zirconia restorations with a hammer and they will nail into wood as opposed to breaking. The strength of zirconia lets it perform similar to a metal, while retaining a white tooth color. In fact, some zirconia types can also stop cracks. As a result, where PFM (MCR) restorations may fracture and break, certain zirconia crowns can almost or completely eliminate this risk - making them an exceptional standout from many limitations in dental history. 

It is important to note, just because a crown may be called "zirconia", does not mean it is in fact a zirconia with the strength properties just mentioned. Zirconia strength is based on the percent of ingredients in the mixture which makes it, particularly an ingredient called Yittria. Yittria is placed in varying amounts depending on how lifelike the ceramic is to look. The more yittria, the more lifelike, however the less strong the zirconia. 1.5% yittria is the only amount which allows full strength, full power zirconia. as it goes higher to 3%, 4%, 6%, and beyond, the fragility increases.

If a ceramic is part zirconia, it could be described as "zirconia". More significantly, if the dentist does not know to specify this quantity, the laboratory making the crown may decide, and they often favor the most "lifelike" material so the crown looks the best on the try-in date, thus setting everyone up for success. What more, the more fragile the zirconia, the easier it is to clean out of a milling machine - so it is a double benefit for the lab. The problem arises in dentistry when oversight or quality control fall, such as in offices which may not take the time to audit the type of zirconia crowns they are giving to patients. As a result, some research studies are finding up to a 20% failure rate of "zirconia" crowns by year 10.

True zirconia ceramic crowns/bridges that can stop cracks and perform with durability similar to a metal must be made with the utmost attention to detail in all aspects of production, from treatment to design. When done properly, you can expect long-term performance with no theoretical end date. Working with a dentist who distinguishes between authentic powerful zirconia, and alternatives, is invaluable to success. 

While strength is superior in zirconia, any completely tooth-colored dental ceramic has the ability to be traditionally cemented to a tooth similar to MCR/PFM or all-metal option, however the cement may have difficulty holding the restoration on the tooth if the design is not ideal. This is not a problem with MCR/PFM or metal options because the metal can be roughened on the inside, and the cement gets great retention. If you roughen a ceramic, you make microscopic cracks that increase the risk of fracturing. So the inside of the ceramic can only be altered so much, if at all. It depends on the ceramic. As a result, all-ceramic restorations may have a greater risk of coming off the tooth. To prevent these problems, some ceramics are adhesively bonded to a tooth - basically using a very hard, strong glue - which is an advantage and a disadvantage. In the same way bonded composites can be affected, so too can these adhesive agents. Acid foods (such as tomatoes, oranges, lemons, etc), drinks (coffee, soda, orange juice, etc), and plaque all cause these adhesives to be particularly vulnerable. Metabolism of the bonding agent from enzymes in the tooth will also occur over the restoration's lifespan. So a tooth with a proper design enabling many retentive features, coupled with a traditional cement and a new, state-of-the-art ceramic restoration can be one of the most superior designs favoring the best of all generations of science. 

Furthermore, for manufacturing all-ceramic options, a digital, 3-D manufacturing machine is typically used to make these restorations, rather than a human who physically sculpts the material, thus adding the risk for imperfection. As a result, ceramic restorations can be produced efficiently and rapidly, allowing a lower cost for dental care. This works very well with third party insurance that allows lower cost dental care. All-ceramic options can have low-cost and expensive versions, with the latter further mitigating some risks mentioned. It all depends on who makes it and how they do it. With the highest standards, you can get decade-spanning results. 

In the current world with large businesses involved in dentistry, cost-cutting, and other factors striving for increased efficiency, you have to make sure your restoration is made well. Very low-cost, or cheap dental options can open doorways for these to be made with lesser-grade materials or oversight, and these restorations can be particularly easy for labs in other countries to make and import for an additionally low cost. Desiring to get 10 years or more from a low-grade ceramic crown is a good overall range in this author's opinion, 30 years and beyond for superior quality. The varying fit, cementation abilities, and chances for a fracture all contribute to how these restorations perform through that timeframe and beyond.

With advances in material science, ceramics have become more predictable with clinical results that make many happy. While more variables can affect these, the science now allows them to look great, retain long-term performance, and offer new benefits that contribute to their overall success. With more ceramic material being employed in dentistry, esthetics, durability, and affordable treatment is becoming increasingly possible.