Dental gold describes the metal alloys used to create durable dental restorations such as crowns, bridges, and inlays. It is rarely pure gold, which is too soft and malleable for the immense chewing forces exerted in the mouth. Instead, dental gold is a carefully engineered alloy designed to balance color, mechanical strength, and chemical stability in the mouth. The specific combination of metals determines the alloy’s properties, cost, and classification.
Defining the Metallic Components
The composition of dental gold alloys is complex and varies widely, often containing less than 50% gold by weight, despite the common name. Gold (Au) is included primarily for its resistance to corrosion and tarnish, which prevents the release of metal ions into the body. It also provides the characteristic yellow color and enhances workability during fabrication.
To achieve the necessary hardness and strength, other platinum-group noble metals are incorporated. Platinum (Pt) and Palladium (Pd) increase the alloy’s melting temperature and contribute to its mechanical strength and rigidity. Palladium is often used to produce “white gold” alloys, as it helps bleach the yellow color imparted by the gold.
The remaining components are typically base metals, such as Copper (Cu), Silver (Ag), and Zinc (Zn), each added to fine-tune specific properties. Copper increases strength and hardness, though high concentrations can reduce corrosion resistance. Silver modifies the color and adjusts strength, and small amounts of Zinc serve as a scavenger to remove oxygen during casting, ensuring a denser final restoration.
Classification by Noble Metal Content
Dental alloys are formally categorized based on the proportion of noble metals they contain, a system established by standards like those from the American Dental Association (ADA). Noble metals are elements that resist chemical action, corrosion, and oxidation, including Gold, Platinum, and Palladium. This categorization predicts an alloy’s performance and cost.
High Noble Alloy
The highest quality material is the High Noble Alloy, which must contain at least 60% noble metals by weight, with a minimum of 40% being gold. These alloys offer the best corrosion resistance and biocompatibility, making them the most expensive choice for demanding restorations.
Noble Alloy
The next group is the Noble Alloy, defined by having a total noble metal content of at least 25% by weight, but with no minimum gold requirement. These alloys offer a balance of performance and cost, often relying more heavily on Palladium for their noble properties.
Predominantly Base Metal Alloys
This final category contains less than 25% noble metal content. These materials, which include alloys like Nickel-Chromium and Cobalt-Chromium, are much harder and stronger than noble alloys and are significantly less expensive. However, lacking protective noble metals, their long-term corrosion resistance is lower, and some base metals like Nickel carry a risk of allergic reaction in some patients.
Functional Requirements of Dental Alloys
The specific blend of metals in dental gold alloys is dictated by the harsh environment of the mouth and the need for long-term functional success.
Biocompatibility
One primary concern is Biocompatibility, meaning the material must not cause toxic effects or allergic reactions when in constant contact with oral tissues. The corrosion resistance provided by noble metals is closely linked to this, as corrosion can release metal ions that trigger adverse tissue responses.
Corrosion Resistance
Corrosion Resistance is paramount, as saliva, temperature changes, and acidic byproducts create a highly aggressive chemical environment. Noble elements resist this chemical attack, ensuring the restoration maintains its structural integrity and polish over decades.
Mechanical Strength and Wear Resistance
The alloy must also possess sufficient Mechanical Strength and Wear Resistance to handle the repetitive, high-impact forces of chewing without deforming or fracturing. Alloys are formulated to be hard enough to resist wear but not so hard that they excessively abrade the opposing natural teeth. This balance is achieved through the inclusion of metals like Palladium and Copper, which transform the soft, pure gold into a strong, resilient material.