What is a dental crown?
The (natural clinical) crown of a tooth is the portion of the tooth which is covered with tooth enamel and projects through the gums into the mouth. It is the part of the tooth you can see, as compared to the tooth root which is generally below the gumline. When a dentist mentions a “crown” as an option for repairing a tooth, they are talking about a prosthetic crown.
If a tooth has been extensively decayed, chipped or cracked, and cannot be restored to its proper shape, function and appearance another way, the dentist may recommend placement of a prosthetic crown.
Crowns are most often made in a laboratory by a skilled dental technician, and the process is very detailed. Use of computerized milling devices to make ceramic crowns is becoming more common, but they may not be appropriate in all applications.
The tooth is first prepared for a crown by removing any decay, and filling in any voids. It is then reduced in shape to a tapered stump, which the new crown will slip down over.
Crowns for baby (primary) teeth differ significantly from crowns for permanent teeth, from the shape of the prepared tooth stump, to the way the crowns are made and the materials they’re made of.
The process of creating a dental crown
Before the procedure
When you have invasive dental procedures like crowns done, the dentist will review your health history. If you have replacement joints (for example, total knee replacement, hip replacement, etc.), you may be pre-medicated with antibiotics for the procedure. If you have certain types of heart murmurs or replacement heart valves, you may also need to take an antibiotic pre-medication prior to the procedure.
If you are anxious about dental procedures, your dentist may recommend sedating you for the procedure. There are several methods of relaxing patients for dental treatment, including oral anti-anxiety pills; inhaled anti-anxiety medication like nitrous oxide; and intravenous anti-anxiety medication.
The following describes the typical crown preparation and laboratory process (if applicable) in detail. Your procedure may vary a bit from the procedure described.
The tooth to be crowned is usually numbed by injecting local anesthetic around the nerve(s) that supply sensation to the tooth. Discomfort from the injection can be minimized by use of a topical numbing gel for a minute or two prior to the injection.
Frequently, a preliminary impression (mold) is made of the teeth before they are altered. The material used most for crown impressions is polyvinyl siloxane, a dimensionally stable and extremely accurate elastomer (meaning it’s stretchy, but returns to the shape it takes when it cures after a minute or two). Other materials may be used. The preliminary impression can be made of silicone or other elastic materials. It can be used to make a temporary crown for the tooth while the final crown is being made in a laboratory, a process that can take a couple of weeks.
If the tooth is to be crowned with a tooth-colored crown, a shade matching guide will be used to determine the shade of your natural teeth. The shade is generally matched in natural lighting, also called “full spectrum” lighting. Fluorescent lights can make teeth appear blue to grey; Incandescent lights can make them appear too yellow. Dental porcelains and resins available today can produce a stunningly precise match for the shade and optical properties of your natural tooth enamel, allowing a single crown to be made that matches your teeth nearly imperceptibly.
The tooth is isolated from mouth structures like the tongue and cheeks to prevent injuries from instrumentation used to prepare the tooth. An isolation barrier known as a rubber dam or dental dam is frequently used, but there are other retraction devices in use. Some dentists may simply use cotton rolls and cheek shields.
The tooth is prepared by removing old restorative materials (if necessary), removing any decay, and (if necessary) filling in any deep holes or missing corners of the tooth. It may be necessary to place small metal (normally titanium) pins in the tooth to rebuild the portion of the tooth that will be covered by a crown. The dentist may use any of a variety of filling materials to rebuild the tooth, including composite resin, glass ionomer, and silver amalgam.
Core buildups are considered a separate billable procedure from crowns. A distinction is made between buildups that involve all or a substantial portion of the core of the tooth, and those that require only a small amount of filling material in the core to build out a minor chip (called “basing to proper contour”). It’s a good idea to know in advance whether a complete core build-up is likely to be required, to avoid unexpected costs.
The outer surface of the tooth is reduced in all dimensions (biting surface and sides) by 0.75mm to 3mm to make room for the crown that will be placed on the tooth. The walls of the preparation are tapered to allow the crown to be slipped down over the tooth. A ledge (margin) is created around the circumference of the tooth against which the crown will be tightly sealed.
Impression of the prepared tooth
An impression of the prepared tooth and the teeth that bite against it is made, taking care to gently reflect the gum tissue away from the prepared tooth. Often, the dentist will place retraction cord in the trough between the gums and prepared tooth prior to making the impression. The cord will be removed after the impression is made. This technique allows the crisp outline of the prepared tooth to be recorded in the impression, so that a precise fit can be achieved between the crown and tooth.
Temporary crown (if needed)
If the crown is not going to be made by a computerized milling device in the office (often the case), the impression that was made prior to preparing the tooth can now be used to make a temporary crown. This is done by filling the pre-impression with a gooey tooth-colored resin material, having the consistency of thick syrup, and placing it over the prepared tooth. The resin material will gel in about a minute (in the shape of the un-prepared tooth), and the impression can be removed from the mouth. The temporary crown completely hardens in another minute or two. Once hardened, it will be trimmed to proper fit, polished, and cemented onto the tooth with temporary cement.
Lab work and final crown seat
Although some dentists now have computer-controlled milling machines for making ceramic crowns in their offices, a more common scenario is that the crown will be made in a dental prosthetics laboratory. Essentially, the lab work involves the following:
The impression of the prepared tooth and the teeth that oppose it is poured with lab plaster and allowed to harden.
The stone casts of the teeth are assembled into a hinged jaw simulation device known as an “articulator” in their proper bite relationship.
Die Preparation and fabrication of a wax pattern
The cast of the prepared tooth (working die) is inspected closely for undercuts and any other irregularities. The technician will create a crown from wax using sculpture techniques. This wax pattern will be used in any of a variety of ways to produce the final crown, depending on whether the crown is to be made of ceramic, metal, or a combination of those.
Production of the actual crown
Depending on which type of crown is to be fabricated, the crown may be cast from a variety of metal alloys, pressed from ceramic, or made of ceramic fused to the metal alloy.
Depending on which type of crown is made, it may require metal finishing and polishing, or other staining and glazing procedures (tooth colored crowns) to make the tooth match the patient’s natural teeth as closely as possible.
Delivery of final crown
When your final crown is ready, a second visit is necessary to remove the temporary crown and replace it with the permanent crown. About half of the time, a patient will ask to be anesthetized for the second visit, to avoid any discomfort associated with removing the temporary crown and cleaning the temporary cement from the prepared tooth. Teeth which have been endodontically treated (i.e. root canal) generally do not need to be anesthetized for the delivery of a permanent crown, although many dentists prefer the patient to be numb for the crown preparation due to the potential for
discomfort associated with soft tissue management (gum retraction, etc.).
Adjust and Polish
The occlusion (i.e. functional biting relationship) of the crown may need to be adjusted slightly, and the crown re-polished. This should take just a minute or two under normal circumstances.
Some advantages and benefits of dental crowns
- Since they are made in a laboratory (or computerized milling machine) and not your mouth, it is easier to rebuild the ideal contours of the natural tooth than it is with large fillings.
- Crowns can prevent cracks from spreading through a tooth.
- Porcelain crowns can restore the tooth to its natural contour, function, and appearance.
- Metal crowns will not generally chip, and provide the tooth with a surface that is durable.
- Porcelain fused to metal crowns offer a good combination of durability and esthetics.
- Porcelain fused to zirconium crowns arguably offer the best combination of durability and esthetics.
Potential disadvantages and risks of dental crowns
- Compared to fillings, onlays and crowns are relatively expensive. However, they generally afford better protection against tooth fractures, and can make it more predictable to achieve contact with adjacent teeth (versus leaving a gap). It may also be easier to sculpt an ideal tooth shape working outside the mouth.
- Traditionally, onlays and crowns require two visits to complete. Often, this means being “numb” at both appointments. Computer-milled inlays, onlays and crowns, which are delivered the same day the tooth is prepared, eliminate the need for a second appointment; however, these may not be appropriate for all teeth, and are still not widely available.
- Preparing a tooth for an onlay or crown involves reducing the sides of a tooth and its biting surface. Depending on how thick the remaining walls of the prepared natural stump are, the tooth itself can be weakened, reducing its long-term prognosis.
- Inflammation and/or infection from the preparation procedures can occur, and may be more likely with ceramic onlays and crowns, because it is necessary to remove more natural tooth structure than for metal ones. This is because porcelain must be thicker than metal to have comparable strength. Studies aiming to quantify the risk of tooth abscess following crown preparation have shown an abscess rate of about 10%.
- If the tooth being prepared is alive (vital), it will normally be numbed with local anesthetic. Local anesthetic has disadvantages of its own, which are generally considered minor compared to the advantages of having the procedure performed comfortably.
- Fillings, inlays, onlays and crowns can fall out and be lost, requiring replacement. Modern bonding adhesives and cements used in dentistry have significantly reduced this risk.
- Ceramic and composite resin inlays, onlays and crowns can chip, requiring repair or replacement. This is particularly true in patients who grind (brux) their teeth, or use them inappropriately (e.g. chewing ice, popcorn kernels, etc.).
- Leakage can occur along the interface between an inlay, onlay or crown over time, and the prepared tooth (margin), leading to sensitivity and decay. Poor oral hygiene is often a significant factor in such cases.
- All dental procedures can produce lip dryness, chapping and cracking. Some patients develop cold sores following dental treatment. Some degree of post-operative discomfort frequently accompanies tooth preparation, including tenderness to biting and cold which is normally temporary. Some studies have shown prolonged sensitivity over a year in certain patients. Such cases are relatively rare.
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