Crowns or Tooth Caps
by Liverpool Dentist Peter Smyth
Teeth that have been weakened by large repairs or root fillings in the past can be prepared for crowns in order to strengthen them.
Porcelain and gold crowns (porcelain bonded, PBC, metal ceramic)
The majority of crowns are gold thimbles covered with tooth coloured porcelain to make them look like natural teeth. These are one of the most successful repairs that restorative dentists can provide. They should last over 10 years if you look after them carefully (e.g. regular check ups, good home care, tooth brushing, flossing and regular visits to the hygienist for professional tooth cleaning).
Crowns made entirely out of precious gold are the most durable crown you can have. They are mainly used on back teeth that are not visible in full smile. However, some cultures are happy to display gold crowns on front teeth as a sign of affluence or a fashion statement.
Gold is a rare metallic element, its chemical symbol Au is short for the Latin word aurum which means glowing dawn. It has several properties that make it ideal for dental crowns, it is non-toxic, it does not react with water or oxygen and it can be cast and polished.
Is it pure Gold?
Gold has been used in Dentistry since 7 BC, for example gold wire and then gold foil, bridges, inlays and crowns.
High quality gold alloys in modern dentistry are a mixture of gold and the noble metals platinum, palladium or silver plus copper and zinc. The gold alloy is easy for the dentist and dental technician to manipulate but is strong, stiff, durable and resistant to tarnish and corrosion.
The gold content of the alloy for a typical crown and bridge alloy may contain 60% to 80% gold. This is slightly higher than 18 carat gold used in jewellery. Japan, Germany and the United States account for almost 70% of all dental gold alloy manufacture.
The high cost of gold in the 70’s and 80’s led to an increase in use of inferior, low gold content alloys in dentistry. In some cases gold contents as low as 12% were used.
Dentists prefer gold!
If a practicing dentist is asked what type of material they would prefer themselves for dental restorations with few exceptions the answer is always gold.
Is it possible to be allergic to gold?
Yes, dentists and patients can develop contact allergies to gold but this is very rare.
Metal free crowns
There are many types of crowns in this category. Traditional all porcelain crowns (porcelain jacket, PJC) can look superb and indistinguishable from natural teeth. These are recommended on front teeth when an aesthetic result is desirable.
Empress and Procera crowns use modern technology to make very strong and aesthetic crowns for front teeth. They involve scanning the impression of the tooth preparation. This is emailed to a remote location. A thimble of ceramic is made to fit over the tooth stump by casting a crown from an ingot of glass. The crown can be cut back and porcelain added to make beautiful crowns.
How are teeth prepared for a crown?
For example, a molar tooth that has been root filled and built up ready for a crown will require tooth preparation. The tooth is shaped using diamond drills and water spray. This is a painless procedure with dental anaesthetics to numb the teeth.
After tooth preparation special wool is placed around the tooth to hold the gum out of the way. An impression can then be made of the tooth stump. A temporary crown may be made and cemented in place. The impression is sent to the laboratory technician who casts the impression with dental plaster. The technician makes a wax crown, casts this in precious gold and then adds dental porcelain and fires this in an oven. The final porcelain and gold crown is polished and returned to the dentist.
The dentist checks the crown is a perfect fit and adjusts the biting surface. The crown is then cemented in place to protect the underlying tooth.
Natural looking crowns
The following aspects help dentists and their technicians reproduce crowns that have a natural looking appearance:
Tooth shade – some teeth have are a solid block of colour, these are easier to copy that natural teeth with translucent edges.
Surface texture – youthful teeth tend to have more surface texture than older teeth that have become shiny and smooth after repeated tooth brushing.
Communicating with photographs of teeth
Digital photographs help communicate the appearance of your teeth to the dental technician as they are building up crowns with porcelain. It is often worth a trip to sit for the technician when they finalise your crown work to enable them to compare their work directly with your natural teeth.