Tooth preparation For All Ceramic Crowns

Tooth preparation For All Ceramic Crowns

Illustrated with diagrams, the tooth preparation on the right upper central incisor for receiving a porcelain jacket crown. Add a note on the advantages, indications, and contraindications of all-porcelain jacket crown.

Steps in Tooth Preparation for All-ceramic Restorations

• Placement of depth-orientation grooves.
• Incisal reduction.
• Facial reduction.
• Lingual reduction.
• Axial reduction.
• Lingual axial reduction.
• Marginal development and refinement.

Body Fluids	Muscle Physiology	Digestive System
Endocrinology	Face Anatomy	Neck Anatomy
Lower Limb	Upper Limb	Nervous System

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Armamentarium

• Mouth mirror.
• Periodontal probe.
• Explorer.
• Chisels and hatchets.
• High- and low-speed handpieces.
• Thin tapering fissure diamonds.
• Narrow round-tipped tapered diamonds, regular and coarse grit.
• Flat-end tapered diamond, regular grit.
• Football-shaped diamond.
• Finishing stones and carbides.

Placing of depth orientation grooves

• Three depth orientation grooves, 1.0 mm deep, are placed:
  One in the middle of the facial wall and one each in the mesiofacial and distofacial transitional line angles in the incisal edge.
• Two more depth orientation grooves of 2.0 mm depth are placed on the incisal half.
• 2 mm deep grooves are placed on the incisal edge for incisal reduction.

Incisal reduction

Instrument used

Flat-end tapered diamond.
The completed incisal reduction should provide 1.5–2 mm of clearance for porcelain in all excursive movements of the mandible.

Facial reduction

Done in two stages:

First step: Incisal half reduction

• A coarse, flat-ended diamond or No. 700 carbide bur is used to plane away tooth structure between the depth of orientation grooves, on the incisal half, at a 45° angle to the long axis of the tooth in a normal occlusal relationship.
• The reduction is done parallel to the original contour of the tooth to provide uniform porcelain thickness and good esthetics.

Second step

• A gingival portion of the labial surface is reduced with a flat-end tapered diamond in a flat plane perpendicular to the long axis of the tooth to a depth of 1.0 mm.
• The reduction is carried out with a cervical component parallel to the proposed path of withdrawal.
• This reduction extends to the labial proximal line angles.
• The reduction is done on half of the facial surface at a time.

Lingual reduction

• Depth orientation grooves of 0.8 mm depth are placed.
• Football-shaped diamond/Small wheel-diamond is used for lingual reduction.
• Reduction is carried out until a clearance of 1 mm in all mandibular excursive movements is obtained.

Axial reduction

• A thin tapered fissure diamond is used to break the contact point with the adjacent tooth. While breaking the contact point, the adjacent tooth should not be abraded.
• The mesial and distal areas are first reduced to a 2°–5° taper without establishing a shoulder at this time.

Lingual–axial reduction

Instrument used

• Flat-end tapered diamond
• The same path of withdrawal as that of the facial preparation is followed with lingual axial reduction
• A depth groove is placed in the middle of the cingulum wall
• The preparation of 2°–5° taper is done from the center of the cingulum wall until the lingual shoulder meets the facial shoulder
• A 0.75 mm cingulum shoulder is placed with a flat-ended tapered diamond.

Marginal development and refinement

Instruments used

• An end-cutting bur held perpendicular to the shoulder can be used for lowering margins
• A sharp chisel is used to remove undermined enamel and finish the shoulder
• The axial walls are smoothed and all the sharp line angles and point angles are rounded. An acceptable emergence profile needs to be created for good aesthetics.

Variation in margin preparation

• Instead of shoulder preparation a heavy chamfer can also be prepared
• For subgingival margin placement, a gingival retraction is done before the preparation for good access and less trauma to gingival tissue
• Initial preparation is followed by refinement with chisels
• The completed shoulder should be 1 mm wide, smooth, continuous, and without any irregularities.

All-ceramic Crowns

• Advantages
  • Superior esthetics
  • Good translucency as to that of natural tooth
  • Good biocompatibility
  • Can select the appropriate shade for a luting agent.
• Disadvantages
  • Reduced strength of the restoration if metal reinforcing substructure is not given
  • Significant tooth reduction on proximal and lingual aspects
  • Less conservative than metal-ceramic crown
  • Difficulties in obtaining a well-fitting margin
  • The success of the restoration depends on proper preparation design
  • An extensively damaged tooth cannot be restored with an all-ceramic crown
  • Cannot be used as retainers in long-span FPD
  • Large cross-sectional dimension connectors need to be incorporated for all-ceramic restorations to have the bulk of the material
  • Due to large connectors, impingement on the interdental papilla can lead to periodontal failure
  • Wear on the functional surfaces of opposing natural teeth.

All Ceramic Crowns Indication

• Where a high-esthetic requirement exists with sound tooth structure present.

All Ceramic Crowns Contraindications

• Where a more conservative restoration can be used
• Not recommended for molar teeth
• Where increased occlusal loads are present
• When adequate support cannot be provided
• When an even shoulder width cannot be prepared.