Porcelain-fused-to-metal (PFM) crowns are a type of dental crown that combines a metal base with a porcelain exterior. They have been a popular choice in restorative dentistry due to their strength and aesthetic appearance. The process of making, preparing, and placing PFM crowns involves several steps:

1. Tooth Preparation:

- Similar to other crowns, the first step involves preparing the tooth. This includes removing decay, old fillings, or damaged tooth structure and reshaping the tooth to allow space for the crown.

2. Taking Impressions and Scans:

- An impression of the prepared tooth and surrounding teeth is taken. This can be done using traditional dental impression materials or digitally with an intraoral scanner.

- The impressions are sent to a dental laboratory for the crown fabrication.

3. Fabrication of the PFM Crown:

- In the lab, a metal alloy substructure is first created to fit the prepared tooth precisely. The metal used is typically a high noble alloy (like gold, platinum, and palladium), a noble alloy (with less noble metal content), or a base metal alloy (like nickel or chromium).

- Porcelain is then applied in layers over the metal substructure and fired in a high-temperature oven. This process is repeated several times to build up the porcelain to the correct shape, color, and size.

- The final layer of porcelain is often tinted to match the color of the natural teeth.

4. Fitting the Crown:

- Once the crown is returned to the dentist's office, the dentist checks the fit of the crown on the prepared tooth, ensuring it aligns correctly with your bite and adjacent teeth.

- Adjustments are made as necessary for fit, comfort, and aesthetics.

5. Cementing the Crown:**

- The crown is then cemented onto the prepared tooth with dental cement.

6. Importance of Precious Metal Use:**

- The type of metal alloy used in the crown's substructure can impact its biocompatibility, strength, and durability.

- High noble alloys, which include gold, are often preferred for their excellent biocompatibility, reduced likelihood of corrosion, and minimal wear on opposing teeth.

- However, crowns with high noble metal content tend to be more expensive. Non-precious alloys are less costly but can sometimes cause allergic reactions or be less aesthetically pleasing due to their darker color.

PFM crowns have been widely used for many years and offer a good balance between strength (due to the metal component) and aesthetics (due to the porcelain exterior). However, with advancements in dental materials, all-ceramic and zirconia crowns have become more popular, especially for front teeth, due to their superior aesthetics and comparable strength. The choice between PFM and other types of crowns will depend on various factors, including the location of the tooth, aesthetic considerations, cost, and individual patient needs.