Choosing the correct materials for your patients can be a challenge. Use this post to help you select between full cast crowns and bridges, PFMs, and...
PORCELAIN-FUSED-TO-METAL CROWN: Alloys used and potential limitations of choosing PFM
Traditional porcelain-fused-to-metal (PFM) crowns have been a popular choice for decades. Read about alloys used to fabricate PFMs and potential limitations of choosing PFM.
Traditional porcelain-fused-to-metal (PFM) crowns have been a popular choice for decades, being used to restore damaged teeth on many patients across the country. These crowns are suitable for posterior and anterior crowns, providing clinicians and patients with reliable and aesthetically-pleasing restorations that will last for years. PFM crowns have an acceptable biocompatibility that helps promote reasonable periodontal health. In addition, they also have good mechanical qualities. These crowns may have a substructure made from non-precious, semi-precious, or high-noble metals. With the proper maintenance, such crowns can last for many years, in some cases patients can successfully maintain their PFM crowns for decades. With sub-gingival margin preparation and good oral hygiene; PFM restoration can even be a good cosmetic choice.
Alloys Used to Fabricate PFMs
When high-noble is chosen, the product will have a composition of over 60% noble metal that could be gold, platinum, or palladium, that must be more than 40% gold. The remaining metals in this alloy can consist of silver and copper with smaller amounts of iridium, indium, zinc or rhenium. The colour of the alloy can be silver, white or yellow.
Semi-precious metal alloys or noble alloys have at least 25% noble metal content and may include tin, zinc, indium, cobalt, ruthenium or gallium. The colour of semi-precious alloys can be white or silver.
Non-precious alloys are sometimes called base metals. In these alloys, the noble metal content is less than 25%. Some non-precious alloys may not have any noble metal content at all and instead will contain large percentages of chromium or beryllium, as well as nickel and cobalt. Other elements that may be included in these dental alloys include tungsten, manganese, iron, silicon, carbon, and molybdenum. Non-precious alloys are silver coloured.
The exact composition of any alloy is dependant on the manufacturer. Your dental laboratory will be able to provide a complete list of elements and their percentages for each available alloy.
Potential Limitations of Choosing PFMs
Despite the many attributes and reliability of PFM crowns, they do have some limitations. One problem with using PFMs to restore anterior teeth is that sometimes the margin can have or will develop a dark metal edge, especially when gum recession occurs. Porcelain margins, however, may offer a way to overcome this issue. Another thing to consider is that sometimes PFMs may appear quite opaque, lacking the natural translucency and liveliness found in natural teeth and in all-ceramic restorations. If proper reduction is not provided. This is because natural light is not transmitted through the crown. If a patient has all-ceramic crowns on adjacent teeth, it can be extremely difficult to get an exact shade match when using PFMs. Gum tissue sensitivity is another potential problem, especially when non-precious alloys are used. Some patients experience gum irritation which may result in gingival recession. While precious metal alloys may produce less irritation, they can sometimes prove costlier depending on the current price of gold. Patients who have any sensitivities to alloys may be better advised to choose all-ceramic crowns, depending on the alloy chosen.
Even though the technologies used to produce PFMs have been available for decades, these restorations are often still a good choice. The introduction of modern porcelains allows skilled technicians to create lifelike crowns where sufficient room is available to correctly layer the porcelain. While all-ceramic crowns and the use of CAD/CAM technology to create metal-free crowns is becoming more common, many of the procedures used to create and fit these restorations are sensitive to technique. When correctly performed, it’s possible to achieve excellent results, but there is little room for error. In comparison, PFMs are a more forgiving restoration.
Please be reminded that our experienced technical team is here to assist you should you wish to discuss a case in more detail.