In this article we will discuss how to improve the appearance and strength of your teeth with the use of crowns.
What is a crown?
A crown is a thin cover made of either metal, porcelain, zirconia (or a combination of these materials) that is cemented or bonded onto a tooth or an implant.
When would you require a crown?
Crowning a tooth can improve its appearance. It can strengthen a tooth that has been weakened by large fillings or root canal treatment. Crowns can help rebuild broken-down teeth and can strengthen teeth that have cracks.
Fig 2. Steps in crowning a tooth.
The tooth need to be made shorter and narrower
to provide space for
the crown material.
What is involved in having a crown made?
First appointment: We remove any old filling material and decay and refill the tooth with a bonded filling.
Then we remove a small amount of tooth material around the entire tooth and from the top. This creates room for the crown material. (Approx 1 mm of tooth all around and 1mm to 2 mm of tooth material from the top of the tooth).
We make an impression or mold of the tooth. This impression is then sent to a dental lab that makes the crown.
A digital photo is taken of the tooth and is sent to the lab so they know the colour they need to make the crown in order to match it to the neighbouring teeth.
We cement a temporary crown on your tooth. The dental lab will make the permanent crown and return it to our office in one to two weeks.
Second Appointment: The temporary crown is removed. The permanent crown is tried on and I check its fit and appearance.mmWe then either cement or bond the crown to the tooth.
Different Kinds of Crowns
1. Porcelain bonded to metal crown. (PBM)
Outer layer of porcelain baked onto an inner metal core.
- Strength: The hard metal inner core gives strength to the PBM crown The metal core in almost unbreakable. This is important if you grind or clench your teeth during sleep or in athletes who are involved in sports with a greater chance of getting hit in the mouth.
Can be used to replace any tooth in the mouth or this reason (front and back teeth).
- Appearance: The porcelain layer gives the tooth a very life like appearance. Very good cosmetic results can be achieved but the kind of underlying metal can determine best results.
For a tooth that is very visible in the smile, gold or a semi-precious metal should be used, which will add to the cost of the crown.
- Strong Bond to Tooth: If the PBM crown is chemically bonded to the tooth it has a much greater chance of holding to the tooth permanently. This can become important when the tooth you are crowning is a relatively short tooth.
- Appearance: For the best cosmetic result an All Porcelain Crown will look more life-like
- Porcelain chipping: This is not common but it can occur. If an area of porcelain chips off the crown the may in some cases be repaired without removing the crown.
- Tooth reduction: You need to reduce the size of a tooth to provide sufficient space for both metal and porcelain. More tooth reduction is required for a PFM crown than for other crown options. This can be a problem when you are placing a crown on a tooth that is somewhat short to begin with.
2. All-Porcelain Crown
As the name suggests this crown is pure porcelain. There is no metal core.
- Most natural looking crown
- can be bonded to the tooth to decrease the chance of the crown slipping off in future
- lower strength than a PBM crown; more likely to crack especially if you grind or clench your teeth during sleep
- should only be used to replace teeth toward in the front of the mouth ie the front six top and bottom. Biting forces get stronger the further back you go.
- An All-Porcelain crown placed on a molar or premolar has a higher likelihood of cracking or breaking.
3. Zirconia Crown
These crowns can be made out of all Zirconia or Zirconia with a layer of porcelain bonded onto its outer surface.
A Zirconia crown looks much like an All-Porcelain crown.
Zirconia is composed of a white metallic powder known as zirconium oxide.
Strength: Zirconia crowns are stronger (more fracture resistant) than All-Porcelain Crowns so they can be used anywher in the mouth.
Appearance: Because they do not have a metallic looking core they can be more esthetic than a PBM crown.
We will discuss some of their limitations below.
4. The fourth kind of crown we use in dentistry is the all-metal crown, which is made of either gold, a semi-precious or non-precious metal.
We will discuss advantages and disadvantages of these different kinds of crowns in a moment, but first:
If you need a crown what kind should be placed?
Usually patients prefer a crown that looks like a natural tooth so that usually means using a porcelain, porcelain-metal or zirconia crown.
Most people do not want an all-metal or gold crown placed on their teeth however, sometimes patients will choose a metal crown for an upper back second molar which, in many patients, is completely hidden from view when they talk or smile.
Metal crowns are virtually unbreakable and require removal of less tooth material than any other type of crown.
This brings up the issue of all-porcelain crowns being made by dentists right in their offices on a computerized machine. Some dentists advertise “crowns made in one appointment while you wait”. In this procedure the dentist reduces the size of your tooth the way I described earlier. The dentist then scans the tooth and sends image of the tooth into a computerized milling machine. This machine then carves a crown out of a block of porcelain while you wait.
This crown can then be cemented or bonded on your tooth in one appointment. An all porcelain crown made on a computerized milling machine in a dentist's office will not be as strong as a porcelain bonded to metal crown or a zirconia crown. .
Cementing or Bonding Crowns to a Tooth
Bonding the crown means chemically attaching the crown to the tooth surface. When you bond a crown to a tooth the crown is fastened much tighter to the tooth than if it were merely cemented.
One such situation is when you are placing a crown on a relatively short tooth. Bonding a crown to a short tooth can make the difference between success and failure of the crown.
All-porelain crowns as well as porcelain bonded to metal crowns can be chemically bonded to teeth.
Zirconia crowns do not bond as well to teeth. In fact, the strength of the bond between a zirconia crown and a tooth is 40% weaker than the strength of the attachment between a porcelain or metal crown and a tooth. I have had several patients come in over the last few years with zirconia crowns made by other dentists that have fallen out.
Crowns made from Zirconia appeal to patients who say they don’t want any metal their body. This is ironic as Zirconia is referred to as a ceramic but is, as I stated earlier, a metal.
Chemically, zirconium is similar to titanium which is a metal used in medicine and dentistry and is well tolerated by the body, as is titanium . In fact, Zirconium has been a component in artificial hips for the past several years.
For patients who are sensitive to certain metals they may be less likely to react less to a zirconia based crown that they would to a porcelain bonded to metal crown metal.
Zirconia crowns are not made in dental offices. They have to be in a dental lab with specialized equipment that many labs do not have. The process is also time consuming.
They are being markete for patients who grind their teeth heavily. This is a relatively small subset of the patients the typical dentist will see.
The thought is that if you have a patient who grinds their teeth so severely that they break the porcelain off their crowns, an all zirconia crown will offer a crown that looks like porcelain but is as strong as an all-metal because it zirconia is a metal .
In the past, severe tooth grinders would best be given crowns with a gold or metal biting surface to eliminate the chance of porcelain fracture. Time will tell how all-zirconia crowns perform as they are newer to dentistry.
One final thought if you are considering a crown:
Patients who habitually grind and/or clench their teeth or who compete in contact sports should think twice about choosing crowns made entirely of porcelain, due to their increased risk of fracture as compared to porcelain bonded to metal crowns.