A dental crown is a custom-made, tooth-shaped cap used to restore a damaged, broken, or severely worn-down tooth, fitting over the entire visible portion of the tooth. Dentists use crowns to protect a weak tooth from fracture, cover a large filling, or restore a tooth after a root canal procedure. Metal crowns have historically been favored for their exceptional strength and longevity. However, introducing any foreign material prompts concerns about the biocompatibility and long-term health effects of the metal alloys used.
The Materials Used in Metal Crowns
The term “metal crown” is a broad category that covers various alloys, and the safety profile is highly dependent on the specific metallic content. Dental alloys are generally classified based on their noble metal content, which includes elements like gold, platinum, and palladium. Noble metals are chemically inert, meaning they resist corrosion and chemical reactions within the mouth.
High-Noble and Noble Alloys
High-Noble Alloys are considered the most biocompatible and must contain at least 60% noble metal, with a minimum of 40% gold content. These alloys exhibit excellent resistance to tarnish and corrosion, minimizing the release of metal ions. Noble Alloys contain a minimum of 25% noble metal content, offering a balance between performance and cost.
Base Metal Alloys
Base Metal Alloys contain less than 25% noble metals and are primarily composed of non-noble elements such as nickel, cobalt, and chromium. These alloys are known for their extreme hardness and strength, allowing them to be fabricated into thinner crowns. While highly durable, these base metals are chemically more reactive and are associated with the majority of biological safety concerns, particularly involving nickel.
Addressing Biological Reactions
The primary biological concern with metal crowns is the potential for an adverse reaction in the surrounding tissues. Metal sensitivity, particularly to nickel, is the most common allergic reaction associated with dental metals, affecting up to 20% of the population. A nickel allergy can manifest as localized inflammation, redness, or a rash-like reaction in the soft tissues of the mouth.
Another safety aspect involves the slow release of metal ions, known as leaching, which occurs as the metal alloy corrodes in the presence of saliva and oral fluids. While High-Noble alloys are highly stable and release negligible amounts of ions, Base Metal Alloys can release trace amounts of elements like nickel and chromium over time. Regulatory bodies ensure these materials meet strict biocompatibility standards, meaning the levels of ion release are generally considered low-risk for the majority of patients.
However, in individuals with a known metal sensitivity or a compromised immune system, even low-level exposure to base metal ions can potentially trigger a response. Dental professionals often recommend screening patients for known metal allergies before placing a base metal restoration. Selecting a High-Noble or all-ceramic option mitigates these risks, as these materials have a significantly lower potential for causing allergic or toxic reactions.
Safety Considerations Beyond Biology
Beyond the body’s internal reaction, other factors relate to the physical and electrical safety of metal crowns.
MRI Interference
One concern is the interaction between metal crowns and Magnetic Resonance Imaging (MRI) machines, which use powerful magnetic fields. Ferromagnetic metals can be attracted to the strong magnet, posing a theoretical risk of heating or movement, although this is extremely rare with modern restorations. A more common issue is the creation of imaging artifacts, where the metal crown distorts the MRI signal, leading to blurred images of the head and neck region. Most modern dental alloys are non-ferromagnetic and considered safe for MRI, but patients should always inform the technician of any metal restorations.
Galvanic Corrosion and Durability
Galvanic corrosion occurs when two different metallic restorations, such as a metal crown and an old amalgam filling, are in contact within the mouth. This can create a minor electrical current, sometimes causing a metallic taste or a mild, transient shock. The superior structural strength of metal crowns also contributes to long-term functional safety by reducing the risk of restoration failure. Because they are highly resistant to chewing forces, metal crowns minimize the chances of the restoration cracking or the underlying tooth structure fracturing.
Comparing Metal and Non-Metal Crowns
Metal crowns are often compared to all-ceramic options like porcelain or zirconia, which have different safety and performance trade-offs. Ceramic crowns are entirely metal-free, making them the safest option for patients with known metal sensitivities or allergies, eliminating the risk of nickel-related reactions. They also do not interfere with MRI scans.
However, ceramic crowns, particularly traditional porcelain, can be more brittle than metal, increasing the risk of chipping or fracture under heavy chewing loads. Metal crowns, especially those made from gold alloys, are softer than some ceramic materials, which is beneficial because they wear down at a rate similar to natural tooth enamel. Conversely, some harder ceramic materials, like certain types of zirconia, can cause excessive wear on the opposing natural teeth. Ultimately, the choice involves balancing the high durability and minimal tooth preparation of metal against the superior aesthetics and zero allergic risk of all-ceramic options.