Dental crowns are tooth-shaped caps that fit over damaged or broken teeth. They improve the look of the teeth and often relieve pain. The process for a crown typically involves multiple visits to the dentist.
Your dentist will numb the tooth and gum tissue and then reshape it to make room for the crown. This may require a bevel in the functional cusps.
Porcelain
Porcelain dental crowns are the gold standard in beauty and durability. However, they can be more costly than other options.
The best candidate for porcelain crowns is a patient who is not prone to grinding or clenching of the teeth (bruxism), as these habits can cause the new porcelain crown to fracture. The dentist can determine if you have this issue during a comprehensive examination of your mouth and teeth.
If you are a candidate for porcelain crowns, your dentist will recommend the right material for your treatment plan. This will depend on the location of the tooth that needs to be crowned, its function, and your budget.
There are four main types of crown materials. These include porcelain fused to metal (or a combination of durable metallic structures with ceramic on top), porcelain-only crowns, and metal crowns. Metal crowns are often recommended for molars as they tend to wear down less quickly than front teeth and do not break as easily as porcelain. They are also corrosion-resistant. The newest type of all-ceramic crowns, known as E Max, are very strong and beautiful but can be opaque and unattractive. They are a good choice for patients who want the strength of a metal crown but prefer the aesthetics of porcelain. These are available in one or two layer versions.
Zirconia
Zirconia is a newer material that’s rapidly gaining popularity in restorative dentistry. The material is stronger than porcelain and some metal alloys, allowing it to withstand the bite forces that occur during chewing and bruxism. It also resists the wear and tear that occurs over time from abrasions. Zirconia is more resistant to staining than other materials, making it a good choice for patients who have stained teeth.
Zirconium is similar to titanium, and like titanium, it’s biocompatible with the human body. This means that unlike traditional metal crowns, zirconia doesn’t trigger an allergic reaction in patients with sensitivities to metal.
As an added benefit, zirconia doesn’t expand or contract with changing temperatures, which can affect the integrity of the natural tooth under a traditional metal crown. The material also looks more like a natural tooth, thanks to its translucent nature and ability to reflect light.
Zirconia can be fabricated in one solid block, ensuring that the final crown is seamless and matches your natural smile. The dental lab will custom color your crown so that it blends in with the surrounding teeth. They can then use resin ionomer cement to bond the zirconia crown to your natural tooth structure. This prevents the crown from coming off or cracking in the future, as it will be mechanically retained by the tooth.
Non-noble Alloys
The noble metals used for crowns are too soft to be practical by themselves. To make them stronger and less expensive, they are mixed with other sturdier metals to form alloys. Stainless steel is one example of this. While these metals are very strong, they are not as aesthetically pleasing as other types of dental crowns. Also, they can wear down the teeth they bite against.
In contrast, porcelain-fused-to-metal (PFM) crowns have a porcelain core with a metal base. The metal provides strength, while the porcelain gives the crown its natural tooth appearance. Because of the combination of materials, PFM crowns are generally considered more durable than porcelain-only crowns. However, the metal base can show through the porcelain at the gum line and some patients have reported sensitivity to the material.
All-ceramic crowns are typically reserved for front teeth due to their better color blending and translucency. However, their brittle nature means they can chip or break more easily than other crowns. Additionally, ceramic crowns require the dentist to remove more tooth structure for a proper fit.
Another type of all-ceramic crown is the’monolithic’ crown, which is computer designed and milled from a single uniform block on a CAD/CAM machine. This type of restoration is strong, long lasting, and can withstand biting pressures without breaking. However, it can have issues with shade matching and can cause sensitivity to hot or cold liquids.
All Ceramic
With high translucency and natural vitality, all ceramic crowns are a popular option for anterior teeth (incisors and canines) as they provide the most reliable aesthetics. They also wear less than molars and can resist impact, which may be important for those who enjoy contact sports and are concerned about fracturing their teeth.
A ‘bi-layered’ all ceramic crown consists of a zirconia core veneered with a glass-based ceramic (Fig. 2). This system has a good clinical track record. However, it is important to ensure compatibility between the core and veneer materials (Holden et al., 2009).
Alternatively, a ‘monolithic’ all ceramic crown can be computer designed and milled from a single uniform block of material on a CAD/CAM machine. This system has a better esthetic than the bi-layered system but is prone to delamination and fracture of the veneers.
Both monolithic and bi-layered all ceramic crowns are supported by a metal alloy base (typically titanium but some patients are allergic to this metal) which is bonded to the zirconia or porcelain. Typical alloys are nickel, cobalt and chrome with a higher level of nickel free alternatives available. The marginal adaptation of both types is typically similar and a combination of underlying substrate colour, tooth preparation geometry, margin location and cementation system will allow optimum outcomes. A recent study of leucite-reinforced glass-ceramic restorations showed low failure rates and excellent esthetics after 11 years (Esquivel-Upshaw, 2013). All-ceramic crowns require a high standard of care as the margins can be vulnerable to fractures, delamination and discoloration.