Gold has been used in dental restorations, such as crowns and inlays, for centuries due to its unique properties. It is highly biocompatible and resistant to the corrosive environment inside the mouth, making it a reliable material for long-lasting dental work. However, 24-karat gold is too soft and malleable to withstand the significant forces of chewing. Therefore, the specific purity, measured by the karat system, is carefully selected and mixed with other metals. This creates an alloy with the necessary strength and durability required for oral performance.
Understanding the Karat System in Dentistry
The karat system measures the purity of gold within an alloy, defining the proportion of gold compared to other metals. This system divides the whole into 24 parts, where 24 karats (24K) represents pure gold. To calculate the gold percentage, the karat number is simply divided by 24. For example, 18-karat gold contains 18 parts of gold out of 24, translating to 75% pure gold. Since dental restorations require gold to be combined with non-gold metals for strength, the resulting karat number is always lower than 24K.
The Specific Gold Purity Used in Dental Alloys
Dental gold alloys typically fall within a range of 16K to 20K, balancing strength and corrosion resistance. The gold content in a standard crown and bridge alloy often ranges from 62% to 78%. The American Dental Association (ADA) classifies these alloys based on their total noble metal content, which includes gold, palladium, and platinum. The most common category is the “High Noble Alloy,” which must contain a minimum of 60% noble metal content by weight. At least 40% of that total must be gold to meet this standard. Alloys with lower noble metal percentages are classified as “Noble Alloy” (at least 25% noble metals) or “Predominantly Base Metal” (less than 25% noble metals).
Why Alloying Metals are Essential
Alloying gold with other metals increases its hardness, strength, and wear resistance, which are necessary for a long-lasting restoration. This modification of physical properties ensures the final restoration can withstand the harsh oral environment. Copper is a common addition that significantly increases the alloy’s hardness and ultimate strength. Palladium and platinum are also incorporated to enhance strength, increase corrosion resistance, and raise the alloy’s melting point. Silver is often added to adjust the color of the alloy and can also influence its melting characteristics and corrosion resistance. These non-gold elements ensure the alloy is hard enough to resist wear but remains compatible with the opposing natural tooth enamel.