Dental Crown Preparation Principles

Tooth Preparation

• There are 4 crown categories: all-metal, metal-ceramic, ceramic, and composite.
• Different crown materials have different requirements regarding space and margin configuration.
• Preparation for a cast restoration follows 5 principles: Tooth structure preservation, retention and resistance, structural durability, marginal integrity, preservation of the periodontium.

Tooth Structure Preservation

• The goal is to avoid weakening the tooth unnecessarily and compromising the pulp.
• The preparation should only remove what is necessary.
• Less destructive preparation options include:
  • Inlays and onlays instead of crowns.
  • Partial veneer crowns instead of full veneer crowns.
  • Supra-gingival margins instead of sub-gingival margins.
  • Chamfer margins instead of shoulder margins.
• Minimizing the taper of the preparation is important to avoid excessive tooth structure loss.

Retention and Resistance

• Retention prevents the restoration's removal along its insertion path or the tooth preparation's long axis.
• Resistance prevents dislodgement of the restoration by forces directed apically or obliquely and prevents movement under occlusal forces.
• Retention is achieved with two opposing vertical surfaces, either on the external surfaces (sleeve retention) or internal surfaces (wedge retention).
• Factors that improve retention and resistance include minimizing taper, limiting freedom of displacement, maximizing length, substituting internal features, and ensuring a single path of insertion.
• Preparations with near-parallel axial walls and grooves maximize retention.
• Over-tapered, short, and unprepared grooves lack retention.

Structural Durability

• The preparation must withstand occlusal forces.
• Consider these factors for structural durability:
  • Occlusal reduction: 1.5mm clearance for functional cusps and 1mm clearance for nonfunctional cusps with gold alloys.
  • Functional cusp bevel: provides space for an adequate metal bulk in areas of heavy occlusal contact.
  • Axial reduction: ensures sufficient space for restorative material thickness.
• Inadequate occlusal reduction weakens the restoration, can cause thin areas, hinders functional morphology, and may lead to perforation during finishing procedures.
• Inadequate axial reduction results in thin restoration walls that distort and cause over contouring of the axial surface, potentially triggering periodontal issues.

Marginal Integrity

• The restoration margin should closely adapt to the preparation's cavosurface finish line for its survival in the oral cavity.
• Different finish line configurations include:
  • Chamfer
  • Heavy chamfer
  • Shoulder
  • Radial shoulder
  • Shoulder with bevel
  • Knife edge
• Chamfer is the preferred gingival finish line for full veneer metal restorations as it exhibits the least stress on the underlying cement.

Preservation of the Periodontium

• The placement of finish lines impacts the ease of fabrication and restoration success.
• Margins should be as smooth as possible for optimal cleaning and success.
• Ideally, finish lines should be placed in accessible areas, making finishing by the dentist and patient cleaning easy.
• The impression should capture the finish lines without tearing or deformation.
• Finish lines should be placed in enamel whenever possible.
• Sub-gingival margins are sometimes necessary due to:
  • Caries presence
  • Increased retention requirements
  • Esthetic concerns
  • Extension of previous restorations
  • Trauma