What is Dental Crown

What is "Crown".
Crown or cap is a dental restoration of a tooth that has extensive damage or decayed,  which cannot be restored by fillings to a functional and desired appearance state.

The usage of crown are:
  • Re-establish the original tooth structure after losing significant tooth structure due to dental caries or chipped tooth
  • Create a new tooth structure for cosmetics purpose
  • Cover the implant
  • Support false teeth in the dental bridge
  • Protect the weak tooth such as the tooth that has root canal treatment or fractured tooth
How is a crown made

In general, to place a new crown requires two dental visits; the first visit involves the first to the fifth step described in the next articles. The second visit includes the final step.
   
The first step, the dentist takes an x-ray of the tooth that is a candidate for a crown. The x-ray helps identify any infection, abnormality of supporting structures and roots.  The tooth and the supporting structures must be strong and in a healthy condition prior to crown fabrication.

The second step, the dentist takes an impression prior to the tooth preparation.  This impression is used to make a temporary crown.  The dentist will skip this step if a patient chooses Cerec crown.

Cerec crown is made by using the cad/cam milling machine in the dental office.  The whole process requires one to two hours.

The third step, if using porcelain or resin materials, the color (shade) of a new crown is selected to the exact or closest to the color of the adjacent teeth.  In metal crown, this step is not necessary.

The fourth step, the dentist, anesthetizes the tooth, reduces the size of the tooth to a particular shape (see illustration - crown prep) to create space for a new crown.

The amount of tooth reduction depends on the type of material used to make a crown.

   Long-section tooth         Crown prep    Crown cemented

The Fifth step, the dentist, takes a final impression of the prepared tooth along with the opposing teeth and send to the dental laboratory. The dental laboratory technician creates a mold from the impression and uses it to fabricate a new crown.  While waiting for the crown, the dentist makes a temporary crown for protecting the tooth and aesthetic purpose.

The final step, the dentist, removes the temporary crown, compare the color of a new crown with the rest of the teeth and evaluate the accuracy of the fitting.  Before cementation, the dentist will let the patient see the dental crown and approves the crown appearance.  Any color alteration or adjustment to the crown can be achieved prior to cementation. 

Once the dentist and the patient accept the crown, the new crown is cemented to the prepared tooth. In the event that the patients want to have their teeth bleached, they should have bleaching completed prior to having a crown.  


Taking impression

Impression
Plaster mold

Materials used to fabricate a crown:
  1. High noble alloy (precious metal), e.g., gold crown
  2. Semi-precious metal crown
  3. Non-precious metal crown
  4. Porcelain fused to metal crown
  5. All ceramic (porcelain) crowns (Lithium Discilicate, Zirconia, etc.)
Gold Crown (precious, high noble metals) - gold-based alloys
Gold crown is a preferred choice for many clinicians for thousands of years due to its superior physical and mechanical properties (longevity, durability, functionality), and bio-compatibility.

Gold in a pure state is too soft, therefore, to improve the strength, gold need to mix with other metals to become an alloy. This alloy has proper physical properties essential for making a dental crown. Typical metals added to gold are:
  1. Copper - the hardening agent of the gold alloys
  2. Platinum - add strength and toughness to gold alloys, make the alloys resist tarnishing or corrosion by saliva
  3. Palladium - add strength and toughness to gold alloys and raise the melting range of gold alloys, allowing the use of higher grade solders
Gold alloys used for a dental purpose divide into four types (depending on the amount of gold content):

Type 1 is the softest and has the highest gold content; used for non-stress bearing areas, e.g., small inlays.

Type 2 is for larger inlays and crown

Type 3 is for crown and bridge

Type 4 is for cast gold partial dentures

Advantages of using gold-based alloys crown:
  • Minimal tooth reduction
  • Less wearing off of the opposing tooth
  • Long lasting
  • Thinner margin, less food residual trapped at the margin of the crown
  • Less likely to cause an allergic reaction
  • Doesn't chip and seldom crack, less repair is needed
  • Doesn't change color 
Disadvantages of using gold-based alloys crown:
  • The alloys transmit heat and cold; patient with sensitive tooth may experience sensitivity when eating hot, cold food
  • Color doesn't match with natural tooth
Semi-precious metal crown - This group of alloys has over 25% noble metal content (gold, platinum, palladium)

Non-precious metal crown has
Iron-base - 60% iron, 6% nickel
Nickel-base  - 80% nickel, 3% or less beryllium

Both semi-precious and non-precious have similar advantages as in gold except:
  •  Lower in cost to fabricate
  •  Greater strength
Disadvantages when use semi-precious or non-precious metal to gold are:
  • Thicker margin than gold alloy due to less malleable ability
  • Report of hypersensitivity
  • Toxicity concern from the potential formation of extremely toxic nickel carbonyls and beryllium content when use nickel-base alloy
  • Higher ability to wear off opposing teeth
Porcelain fused to Metal Crown
Porcelain fused to metal improves aesthetic to the crown by adding tooth-colored porcelain on top of the metal substrate framework which can be high noble metal, semi-precious metal, or non-precious metal.

Advantages:
  • A better match color with adjacent teeth than metal crowns
  • Do not transmit heat and cold 
Disadvantage:
  • Requires more tooth reduction
  • Wear off opposing tooth 
  • Porcelain can chip off from the metal framework
  • Unsightly metal margin occurs at the gum line
  • Loss the natural tooth translucency due to the added opaque layer needed to mask the metal color
All ceramic crown (Porcelain)

Advantages:

  • Biocompatibility, resistant to dissolution in the mouth
  • Better aesthetic than any other types of crown
  • Maintain the translucency of natural teeth
  • Durability, wear resistance
  • Less tooth reduction in comparison to porcelain fused to metal.  Newer porcelains have improved strength that requires minimal tooth reduction
  • Color stability
Disadvantages:
  • Some types of porcelain tend to wear off opposing teeth, e.g., Zirconia
  • Require higher skill to accomplish good results to prevent fracturing
  • Hard to remove once cemented
  • More expensive
  • Fracturing can occur

Porcelain fused to Metal Crowns

All Ceramic - Lithium Discilicate Crowns