Anterior Teeth – Crown Preparations

anterior crown preps

In Part III of this series, "Solving Esthetic Problems - Indirect Veneer Preparations," the discussion focused on the rational for the three depths of facial reduction for indirect veneer preparations. A similar thought process will be followed when making a decision on which of three types (depths) of anterior crown preparation is best suited to accomplish the esthetic goal of treatment. 

The depth of tooth reduction is based on the degree and location of the discoloration (esthetic dimension problem), the expectation of the outcome (degree of change required), and the space requirements of the restorative material. 

Crowns are indicated:

  • When the tooth was previously restored with a crown
  • When the tooth is structurally compromised
  • To mask severe discoloration
  • To change the morphology, including the palatal/lingual aspect of the tooth beyond the cingulum
    • May include functional reasons such as changing the morphology to accommodate a change in the angle of disclusion or an additive process to establish opposing tooth contact

The extent of facial reduction will be determined by assessing the current appearance of the tooth and determining the changes required to achieve the desired outcome. The depth of tooth reduction needs to take into consideration the restorative material selection, the laboratory fabrication technique (monolithic versus laminated ceramic), the degree of masking that is required to achieve the esthetic outcome, and whether it is an additive versus subtractive restorative process.

The recommendations for materials for each type of crown preparation are dependent on the technician fabricating the restoration  

Minimal Crown

anterior crowns minimal crown

  1. Indications: 0 – 3 shade/value changes
  2. Facial depth: 0.8/1.0/1.2 mm taper depth from gingival to incisal third of tooth
  3. Bur: RW Ext .8/1.0/1.2 (Brasseler USA)
  4. Material choices:
    1. Leucite reinforced glass ceramic (ex: Ivoclar Empress)
      1. Monolithic or 0.5 mm layer of enamel glass ceramic in the incisal half of the restoration
    2. Lithium disilicate (ex: Ivoclar e.max Press or CAD)
      1. Monolithic or 0.5 mm layer of enamel glass ceramic (e.max Ceram) in the incisal half of the restoration
    3. Translucent monolithic zirconia (ex: Glidewell BruxZir Anterior - 650 MPa, Ivoclar Zenostar MT – 550 MPa)

The facial reduction and restoration thickness for the minimal crown is identical to the extensive veneer. Therefore, the same esthetic challenges exist if there will be three shade/value changes. If there is no discoloration of the preparation (0 - 1 shade/value change), a relatively translucent ceramic can be used to fabricate the crown. If there are 2 - 3 shade/value changes, this implies there is now moderate masking of the tooth preparation that requires a more opacified ceramic. If a monolithic restoration is fabricated, this opacity comes to the surface of the restoration. It is more difficult to simulate a natural translucent appearance, especially in the incisal third. The violet and blue characteristics that are used to simulate translucency are applied on the surface of the ceramic. The outcome will appear more artificial compared to a layered ceramic restoration. 

Conventional Crown

anterior crowns conventional crown

  1. Indications: 3 - 4 shade/value changes, significant masking of the preparation is required
  2. Facial depth: 1.0/1.2/1.5 mm tapered depth from gingival to incisal third
  3. Bur: WDL 1.0 gingival, WDL 1.5 incisal third (Brasseler USA)
  4. Material choices:
    1. Monolithic
      1. Leucite reinforced glass ceramic
      2. Lithium disilicate - e.max Press or CAD low translucency (LT) or possibly e.max Press medium opacity (MO)

If a monolithic restoration is fabricated, the esthetic outcome is highly dependent on how much masking is required to make the necessary shade/value changes. The restoration will be significantly more opacified compared to natural dentition.

  1. Bi-layered
    1. Leucite reinforced glass ceramic
    2. Lithium disilicate (e.max Press MO)
    3. PF A (porcelain fused to alumina)
    4. PF Zr (porcelain fused to zirconia)
    5. PF M (porcelain fused to metal)

My recommendation is to avoid using PF A, PF Zr and PF M for conventional crowns. It will be easier for the technician to manage these materials at the recommended facial reduction for an extensive crown. With a conventional facial reduction, applying a layer of enamel ceramic allows the technician to build back the necessary translucency to simulate a natural tooth. The characteristics to simulate mamelons, craze lines and other effects are placed internally. The internal effects will not change over time, which is a significant advantage over surface characterized restorations. Surface stains do wear off over time, changing the esthetic appearance of a restoration.

My preference for materials and technique for a conventional reduction is lithium disilicate e.max medium opacity (MO) base, layered with dentin and enamel (e.max Ceram). The required minimum thickness of the e.max Press material relative to the overall thickness of the restoration is 50 percent. I would recommend reserving the more opaque substrates for the extensive crown restorations. The e.max Press high opacity (HO) must be at least 0.8-mm thick to totally mask the underlying preparation. For a conventional crown reduction, this results in a minimum thickness of material to layer in the desired shade/value and translucency.  

Extensive Crown

anterior crowns extensive crowns

  1. Indications:
    1. Masking of severely discolored tooth preparation
    2. Cases when the treatment plan is to use metal, zirconia, alumina or e.max Press HO as the substrate (base) of the restoration
  2. Facial depth reduction: 1.2/1.5/1.7 tapered depth from gingival to incisal third
  3. Bur: WS8M 1.2 (Brasseler USA). This bur has a 6Ëš taper so at its widest diameter of diamonds at the shaft, it measures 1.8 mm.
  4. Material choices:
    1. Monolithic restorations are contraindicated
    2. Bi-layered
      1. Lithium disilicate e.max Press HO (possibly e.max Press MO)
      2. PF A
      3. PF Zr
      4. PF M

The increased depth of tooth reduction creates 0.2 mm of additional space for the opaque substrates. These substrates are reduced in thickness in the incisal third in order to create space for the dentin and enamel layers. You will notice that the metal and opaque are the same thickness as the zirconia substrate. The metal will totally mask the underlying preparation. The same thickness of zirconia is not totally opaque, so there will be some negative influence from the preparation if it is severely discolored. 

This article outlined the rational for three types of facial reduction to complete a crown preparation, and suggestions for materials and fabrication techniques for each.  Following the guidelines set forth in this series of articles gives the dentist and technician a methodology to create predictable restorative outcomes.