All Ceramic Crown Preparation in Fixed Prosthodontics Lecture Quiz

Preparation of All Ceramic Crowns

• General guidelines:
  • Smooth preparation and rounding of all line and point angles to avoid stress concentration and fracture
  • Uniform sufficient reduction to give sufficient bulk for the ceramic for resistance and retention and structural durability

Axial Taper and Convergence

• Axial taper: 6-10 degree occlusal convergence to give common path of insertion
• Avoid over-convergence, which is inversely proportional to retention and resistance

Finish Line

• Type: Deep chamfer or shoulder with round internal angle to avoid stress concentration
• Thickness: 1mm
• Location: Supra-gingival
• Should be smooth and continuous for marginal integrity, structural durability, and periodontal preservation

Preparation of Anterior Teeth

• Make putty index before preparation to check the amount of reduction and make provisional restorations
• Diamond stones needed for proper preparations:
  • Needle stone
  • Tapered stone with round end for axial and incisal reductions
  • Flame stone or football for palatal fossa
  • Wheel stone for incisal reductions
  • Finishing stones for rounding line and point angles of the preparation

Incisal Reduction

• Amount of reduction: 1.5-2 mm, giving bulk for translucency of the incisal restoration
• Direction: Incisal reduction is done with slight inclination 45° toward the lingo-gingival to oppose the forces perpendicularly on the incisal edge

Labial Reduction

• Two sets of depth grooves are made cervically and incisally in a two-plane reduction using a tapered with flat end diamond stone instrument of known diameter
• Depth of reduction: 1-1.5 mm (1.2 mm as an extra 0.3 mm will be added during the finishing process)
• Finish Line: 90-degree rounded shoulder or deep chamfer 1-1.5 mm
• Groove directions: Cervical 1/3 (parallel to the long axis of the tooth), incisal 2/3 (parallel to the contour of the labial surface)

Proximal Reduction

• Direction: Opposing proximal walls should converge incisally 6-10 degrees
• Instrument: Tapered stone with flat end
• Depth of reduction: 1-1.5 mm

Palatal Reduction

• A. Palatal Fossa:
  • A sloping surface offers little resistance to tipping
  • The curvature of the palatal fossa should be maintained, following the original concavity, while providing a 1mm clearance in all excursive movements
  • Horizontal surface at the junction of the cingulum and fossa should be preserved
  • Instrument: Wheel diamond stone or football stone
• B. Cingulum:
  • Reduced to have minimal convergence with the cervical portion of the labial surface
  • Minimal taper is required to achieve maximum resistance, while mesio-distally the convexity is preserved
  • Instrument: Tapered stone with flat end
  • Depth of reduction: 1 mm

Finishing of the Preparation

• All sharp line and point angles are rounded to avoid stress concentration
• End-cutting stone may be used to safely place the finish line subgingivally to improve aesthetics or retention of short abutments

Advantages and Disadvantages of All Ceramic Crowns

• Advantages:
  • Excellent esthetics, translucency, and color stability
  • Highly compatible with soft tissues, even if sub-gingival finish line, due to high smooth surface
  • More conservative than metal ceramic
  • Can be bonded to tooth structure → better retention & less marginal leakage
• Disadvantages:
  • High brittleness and liability to fracture
  • More expensive than full metal & PFM
  • Extensive amount of reduction compared to partial coverage, full metal
  • Pulp vitality can't be done
  • Radiographic examination can't be done to detect caries under the restoration

Indications and Limitations of All Ceramic Crowns

• Indications:
  • As a single restoration in cases with high esthetic demands
  • As a retainer for all ceramic anterior bridge when esthetic is important
  • In case of bad oral hygiene, root canal treated teeth, malposed teeth, and badly decayed teeth
  • Fractured anterior teeth that cannot be restored with another more conservative restoration
  • Proximal or facial caries that cannot be restored with another restoration
  • Favorable occlusal forces
• Limitations:
  • Young patient with large pulp size & high epithelial attachment
  • When more conservative treatment can be done
  • Unfavorable occlusal loads