Tooth Preparation for Dental Crowns

Biomechanical Principles of Tooth Preparation

• Tooth preparation involves removing precise amounts of tooth and filling material to prepare for a dental crown.
• Goals of tooth preparation:
  • Remove enough dental tissue to be replaced by the restoration.
  • Eliminate undercuts to seat the crown in place.

Biologic Considerations

• Preservation of tooth structure:
  • Conservation of tooth structure during preparation.
  • Protection against tooth fracture.
  • Adjacent teeth preservation.
• Preservation of periodontium:
  • Avoidance of overcontouring.
  • Location of the margins or finish lines.
• Preservation of the TMJ: Harmonious occlusion.

Preservation of Tooth Structure

• Conservation of tooth structure:
  • Avoid injury to tooth pulp by not overheating.
  • Use enough water cooling.
  • Do not use old burs.
  • Decrease pressure during cutting.
• Amount of reduction: as minimal as possible without interfering with other principles.
• Age and pulp size: as age increases, pulp size decreases.
• Chemical irritation: avoid opening dentinal tubules.

Protection Against Tooth Fracture

• Keep enough tooth structure to avoid fracture.
• Include parts of the tooth that could fracture during preparation.

Protection of Adjacent Teeth

• Use a metal matrix band around adjacent teeth.
• Leave an enamel lip between teeth during preparation.

Preservation of Periodontium

• Periodontal tissues are essential for oral health.
• General guidelines:
  • Use biocompatible materials for restorations.
  • Final restorations should be highly polished.
  • Care should be taken during procedures to avoid harming periodontal tissues.
• Avoid overcontoured and overhanging restorations.
• Location of margins: supragingival > subgingival.

Biologic Width

• Fixed distance between the crest of alveolar bone and the gingival sulcus.
• Depth: 1.5-2 mm.
• If biologic width is not respected, it can lead to bone loss and recession.

Retention and Resistance

• Restoration material and luting material:
  • Luting materials: zinc phosphate, zinc polycarboxylate, glass ionomer, resin cements.
  • Cementation: zinc phosphate > glass ionomer > zinc polycarboxylate.
  • Bonding/adhesion: resin cements are most retentive, especially with all-ceramic restorations.

Structural Durability

• Ability of the restoration to withstand forces of mastication without fracture or distortion.
• Factors to consider:
  • Occlusal reduction.
  • Axial reduction.
  • Functional cusp bevel.
  • Reinforcing struts.

Occlusal Reduction

• Enough bulk of material should be present on the occlusal surface to withstand forces of mastication.
• Gold: 1.5 mm functional cusp, 1 mm non-functional cusp.
• PFM: 1.5-2 mm functional cusp, 1-1.5 mm non-functional cusp.
• All Ceramic: 1.5 mm all occlusal surface.

Functional Cusp Bevel

• Provides bulk of restoration at an area of heavy occlusal contact.

Axial Reduction

• Enough axial reduction should be made to prevent thin restorations and overcontouring.

Reinforcing Struts

• Important in different designs of restorations.
• Made to provide bulk of material in weak areas or areas withstanding high occlusal load.

Esthetic Considerations

• Treatment planning and smile analysis are crucial for successful esthetics.
• In anterior restorations, esthetics take precedence.
• In posterior restorations, function is more important.
• General rules for esthetics:
  • Color: enough reduction needed to hide opaque layer or tooth structure.
  • Shape: final restoration must mimic normal tooth shape and emergence profiles.