Dental crown

Modern materials can return structural integrity to ‘at risk’ broken down teeth and keep them serviceable for considerably longer than any other process.
The results are most often stunningly lifelike.

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A crown is a device that is custom made in a laboratory by a skilled dental technician.
Prior to the laboratory process the tooth to be crowned has been prepared by a dentist who conservatively reshapes the tooth to allow for the crown to completely encase and protect all remaining weakened structure.
When returned from the laboratory, the crown is cemented or bonded in place.
To be functionally stable over a long period, a crown has to be extremely strong and capable of withstanding great forces.

Why place a crown?

Dental crowns will most often be used to restore ‘at risk’ teeth. It’s rare that they will be placed for purely aesthetic purposes as porcelain veneers are a far more conservative option for a tooth in good condition. Below are the common reasons we place crowns.

Limited remaining structure
Crowns are generally considered when less than 40% of your tooth structure remains.
When only this much structure remains there is not enough tooth structure to hold a filling or an onlay.

Each time a filling is replaced, a little more structure is lost.
When limited structure remains it is almost always possible to place a filling instead of a crown. However, when this restoration fails, there may not be enough structure left even to place a crown.

A crown covers the whole tooth and acts to splint and protect it from breaking or fracturing.
The following are common causes of excessive tooth structure loss:

Caries
If left untreated, tooth decay can destroy a lot of structure leaving the remaining structure weakened and prone to fracture.
It may also end up very close to the pulp (central nerves and blood vessels). In this instance it is preferable to place the longest lasting restoration possible as if only a filling is placed and it fails, the next decay episode often quickly tracks to this deep area and again threatens the pulp.

Failed large restorations
Large restorations often fail because the weakened structure around them fractures off.
The larger the void left to fill after an old filling is removed for replacement, the harder it is technically to get an ideal result.
Correct contours and contacts with surrounding teeth can be impossible to get right. The filling will function but will encourage food trapping leading to its early failure.

Tooth wear
Teeth will wear when there is grinding, erosion (from gastric reflux or having too much acid in your diet) or most commonly from a combination of these factors. Wear can continue and extend past the tough outer enamel surface to the softer dentine below and eventually place the pulp (central nerves and blood vessels) at risk.
Wear will often effect multiple teeth and the only long term solution to protect such teeth is to place crowns.
Even crowns can be difficult to place if the wear remains unchecked as they require at least 3mm of tooth height above the gums to be successful.

Unstable structure remaining
Sometimes the amount of remaining tooth structure is reasonable, but its quality is poor.

After RCT
Root canal treatment removes an infected pulp (central nerves and blood vessels). Healthy teeth are actually slightly flexible, but with the loss of internal blood vessels, the tooth looses a water supply and its this dehydration that causes it to become more brittle. A crown will brace this weakened structure an make future fracture more unlikely.The sooner this process is carried out after root canal treatment completion, the better.

Deep fractures
Fractures are evident in the surface of all teeth as tooth enamel is ceramic like in nature, is supported by the more flexible dentine, and is constantly asked to withstand heavy forces. These fractures are mostly shallow and are not cause for concern.
Sometimes these fractures can extend deeper and if left unchecked can extend into the pulp (central nerves and blood vessels) or extend down a root surface so far that they cannot be repaired.
A crown will brace this weakened structure and attempt to prevent this crack from propagating any further.

Aesthetics
While a crown can readily change the colour and shape of a tooth, its rare that it would be chosen to simply improve the look of a tooth. Porcelain veneers are far more conservative of tooth structure and would be the treatment of choice most times.
Very dark discoloured teeth may sometimes be better served with a crown. Porcelain veneers are generally not thick enough to mask a dark underlying colour (particularly in the third of the tooth closest to the gums).

To replace a failed crown
If a crown fails, the only option to replace it is another crown.

As part of a dental bridge
A traditional dental bridge involves placing crowns on teeth either side of a gap.
These crowns are splinted together by a fake tooth attached in the middle.

To restore a dental implant
A dental implant only actually replaces the root of the tooth – the part of the tooth that was buried in the jawbone.
A crown is required to be attached to the top of this implant to restore the visible part of the tooth and make it look and function like a tooth again.

Types of crowns produced at Melbourne Dentist

At Melbourne Dentist crowns are produced by skilled dental technicians at The Dental Solutions Australia, a large dental laboratory in Melbourne’s CBD. The types of crowns they produce for us are:

All Ceramic Crowns
All ceramic crowns are far superior to the ones produced in the past and are now the most common type of crown we place.
Without a metal substructure (like that found in the more traditional porcelain fused to gold crown) they have excellent aesthetics. They are also now strong in quite thin sections and can be produced to conserve more tooth structure.
There are two types:

e.max (IPS e.max CAD)
This is the leading brand of biocompatible lithium disilicate glass-ceramic.
They are very strong while offering true-to-nature aesthetic results.
Their fitting surface can be treated so that a chemical bond can form between the crown, cement and tooth. This increases their strength, durability and longevity but also increases the time and expertise required to ‘bond’ them into place – hence they are slightly more expensive than other crowns we produce.

Zirconia (IPS e.max ZirCAD)
Zirconia is extremely strong in thin sections and is great in situations where the forces are very high.
In the past they could be a little opaque in appearance but the newer crowns are far more translucent and natural tooth coloured. Therefore, little characterising porcelain needs to be placed over the surface to make them look tooth like.
We do not use them at the front of the mouth as the aesthetics of e.max is always superior.
The result is an ultra strong, aesthetic crown that is quite conservative of remaining tooth structure.
At Melbourne Dentist, these crowns are slightly cheaper than e.max crowns. This is due to a slight decrease in ‘chair time’ – specifically, they are easier and quicker to insert as they are cemented rather than bonded into place.

Gold Crowns
Gold is still the most durable, predictable and conservative material available. It’s only issue is its colour.
At Melbourne Dentist we use only high percentage gold.
We don’t use semi precious or non precious metals as they do not fit as well and do not last as long.
Only one or two grams of gold is generally used and this amount really doesn’t add significantly to the laboratory cost to justify not using it.

Porcelain fused to gold crowns (PFM’s)
This is the traditional ‘aesthetic’ crown that has been used for many years. They look reasonable, and they have long term data to back their longevity.
We no longer place many PFM’s because many times you must remove a little more tooth structure to make them look reasonable, they are never as aesthetic as the all ceramic crowns and many people are resisting the use of metals in their mouths in all forms.
They do not let light pass through them so overtime a dark line can appear at their junction with the tooth. This can appear a little unsightly. This junction is often hidden under the gum line when the crown is made, but this gum line will always move to eventually show this junction.
In contrast, All ceramic crown junctions do not always have to be hidden in this fashion so they may be less irritant to surrounding gums, and wont fail aesthetically when gums do naturally move.

The production process.

Before the crown is prepared

There may be a few steps involved before getting a dental crown fitted to ensure an ideal, long-lasting result. These steps involve assessment of the teeth to ensure that the remaining structure is sound and that the internal nerves and blood vessels are healthy. If the tooth has been previously root filled (root canal treatment), it is important to assess that this has healed adequately and that there is no sign of persisting infection or root fracture. Steps we may have to undertake include:

X-rays
X-rays may hint at infections involving the pulp (central nerves and blood vessels) as well as those involving the surrounding supporting bone. A tooth is not a good candidate to support a crown until such conditions are resolved. A further highly accurate CBCT scan may be required in some instances as some conditions are difficult to assess adequately with x-rays.

Vitality test
One in ten of all large restorations (including crowns) will suffer pulp death. If this occurs, root canal treatment will be required to clear the internal infection before the crown is placed (otherwise the tooth will need to be extracted).
A cold test (Vitality test) is one way to assess the vitality of the pulp (central nerves and blood vessels).

Core placement
Any old filling material involved with the supporting tooth is removed, along with decay. Fractures that are present within remaining tooth structure are assessed for their depth, and for the possibility that they may extend into deeper areas of the root surface or into the pulp.
This is considered a separate billable procedure from the crown procedure.

Root canal treatment
Any time a tooth is prepared for a crown (including core placement) or any large restoration for that matter, there is a chance the pulp tissues may die. This is because all tooth preparation leads to inflammation of the pulp (internal blood vessels and nerves). If the drilling is deep, or the pulp was previously compromised, pulp death may be the end result of such internal swelling.
Some studies have estimated that the number of teeth requiring root canal treatment following large fillings and crown preparations is around 10%.
These internal infections can develop very slowly so the tooth may need root canal treatment at any time, including a long time after the crown is placed. However, most such infections show up around the time of core placement.
If discovered in this way, it’s best to apply root canal treatment to the tooth before the crown is inserted.

Crown lengthening surgery
Sometimes, a defect in a tooth leaves very little structure remaining above gum level. Crown lengthening can be applied to reposition the gums and underlying bone in this area. More tooth structure will now protrude through the gums in this area and the finish line of the crown will be made to fit onto this newly exposed surface. This will provide more ideal support for the crown and a more hygienically manageable junction. Sometimes, orthodontics can be used to move a tooth to similarly expose more tooth root surface that can then be used to support a crown.

Visit 1

Anaesthetic
The tooth and surrounding areas are made numb. A specially formulated topical numbing gel is used prior to anaesthetic placement. This, plus other techniques learned over the last 30 years, makes this process more comfortable than ever.

“Bite” impression
A quick impression is taken of the tooth to be crowned, along with a couple of teeth either side of it. This will be used to produce a temporary crown at the end of the appointment.

Isolation of the tooth
Sometimes, the tooth is isolated from the rest of the mouth to protect surrounding structures by the use of an isolation structure called a rubber dam or dental dam.

Tooth preparation
The outer surface of a tooth, and any filling present, is removed by somewhere between 0.5 to 1.5mm to make room for the crown. The walls of the tooth are converted from convex to near-parallel sides. A finish line, or ledge, is produced circumferentially around the tooth to allow for a tight seal at the junction between the crown and the tooth.

Gum retraction and impressions
The gums are gently reflected away from the finish line with retraction cord.
This very fine piece of specialised “string” is placed in the trough between the gums and the preparation finish line. It’s removed just before the impression is taken, so impression material with a very fluid consistency can be applied to this area. A material of thicker consistency is then placed over this to record all teeth shapes in the area. Incredible detail is thus recorded of the finish line and the root surface just below this. This process is one of the most important elements in obtaining an accurately fitting crown and can take 15 minutes of time to apply correctly.

Temporary crown production
A temporary crown is produced and cemented over the preparation with temporary cement. This temporary crown is produced from the simple impression taken of the tooth before any work was done. Therefore, it most commonly feels exactly the same shape as the tooth did prior to preparation, and thus is readily tolerated by the tongue and cheeks for the two weeks that it takes for the crown to be produced by the laboratory.

Shade selection
A shade is taken using the surrounding teeth as a guide. If the tooth is a front tooth, we prefer that you visit the laboratory that will produce the crown. There you will meet with expert technicians that will be involved in the crown production who will take detailed notes on the shade, shape and texture of surrounding teeth to ensure the crown matches your teeth nearly imperceptibly.

Visit 2

Anaesthetic
Little actual tooth preparation is carried out at this visit but the tooth needs to be thoroughly cleaned and polished after the temporary crown is removed, and this process will normally require the tooth to be numb.

Crown fit-check
While some clinics now produce crowns in house, we still believe that the most ideal result will be achieved if the crown is produced in a dental laboratory. Sure it takes a little longer, but it will be designed by an actual technician (rather than dental support staff), and it can be ‘re-marginated’ for ideal fit after milling, characterized for realism and glazed for smoothness and beauty in a way that can not be replicated within most all dental practices.
Before the crown is cemented into place, it must be critically assessed for fit to your tooth, and its correct relationship to the teeth it surrounds and opposes.

Gum retraction and impressions
The gums are gently reflected away from the finish line with retraction cord to ensure they are completely out of the way of the edges of the crown.

Cementation
The tooth surface is prepared, along with the fitting surface of the crown, to accept permanent cement and the crown is then gently pushed into place. Once the cement has set, excess cement is cleaned away and the crown is again assessed.
Sometimes there can be mild sensitivity to hot and cold temperatures for a few weeks after cementation. This is generally temporary and should resolve on its own.
If the crown feels a little odd in the bite after the first 48 hours, you will likely need to return to our practice to have your bite checked and adjusted – a simple 5 minute procedure.

Maintaining a dental crown

The tooth that supports a dental crown is never as strong or easy to clean as a completely intact tooth. Proper care is needed to ensure this device provides you long trouble free service.

The following information may help you take care of your new dental crown.

Immediately after placement

  • Take care not to damage areas of your mouth that remain numb. Don’t eat or drink anything too hot until sensation completely returns
  • There may be initial sensitivity of the supporting tooth to hot or cold. This should resolve by itself. If the tooth involved slowly becoming more sensitive, or a little tender to bite on, please give us a call. It’s likely that the crown biting surface needs a simple adjustment.
  • It can take a little while to get used to the feel of a new crown in your mouth. Sometimes your tongue may get in the way and be bitten, but it will soon learn to stay out of the way. In the mean time you may have to take a little care and eat more slowly.

Oral hygiene measures

  • While the surface of a crown is highly polished and glazed to prevent plaque build up or staining, it has a weak spot – the ‘margins’. The margin of a crown is where it meets the tooth surface. We try our utmost to ensure this junction is as close fitting as possible to allow for good daily maintenance. However, at a microscopic level, this area is not smooth and therefore it requires particular attention during cleaning.
  • It is preferable to use an electric toothbrush to maintain a bridge.
  • Floss or interdental brushes should be used to clean between the teeth at either end of the bridge
  • Floss threaders or ‘Superfloss’ can be used to pass floss underneath the false tooth to allow cleaning in this area

Controlling bite forces

  • Hard biting, clenching or grinding will hasten damage to the bridge and force its early replacement. Sometimes, clenching or grinding will occur during the day, particularly during stressful periods of your life. Some sports or exercise (eg. weightlifting) can cause heavy clenching. Most people who grind or clench do so at night while asleep, and if this is suspected, an ‘Occlusal Splint’ should be considered.  READ MORE
  • Beware of habits that may place undue stress on the porcelain. Although very strong, porcelain can chip, particularly if it comes up against a similarly hard and brittle object. Chewing on very hard foods (like bones, stone fruit pips, crab shells and ice) should be avoided. Chewing on pens or fingernails will also lead to damage as will using the bridge as a tool (eg opening bottles, cracking nuts).

The average bridge should last from 10 – 15 years (depending on local forces and materials used).
As with your natural teeth, the longevity of a bridge depends on good daily dental care. Its life expectancy will also increase with regular continuing care appointments.