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Tooth Preparation for Crown Restorations

Dental preparations must be carried out according to previously established scientific principles, which are: mechanical, biological and aesthetic. This constitutes one of the most important stages in the construction of a dental prosthesis, either as an individual crown or a fixed dental prosthesis retainer.

Principles of tooth preparation

A good preparation ensures that subsequent techniques (e.g., interim restoration, impression making, pouring of dies and casts, waxing) can be accomplished.

The design and preparation of a tooth for a cast metal or porcelain restoration are governed by five principles:

A) Preservation of tooth structure

B) Retention and resistance form

C) Structural durability of the restoration

D) Marginal integrity

E) Preservation of the periodontium

Successful tooth preparation and subsequent restoration depend on simultaneous consideration of all these factors. Improvement in one area often adversely affects and may lead to failure in another area.

A) Preservation of tooth structure

Excessive removal of tooth structure can have many ill effects. If a tooth is over-tapered or shortened too much, there will be an unnecessary sacrifice of retention and resistance.

TIPS:

  • Avoid excessive destruction
  • Design restoration to reinforce and protect remaining enamel and dentin
  • Overtapered or shortened too much : no retention and reistance , thermal hypersensitivity , pulpal inflammation , necrosis

B) Retention and Resistance

The feature of a tooth preparation that resists dislodgement of a crown in a vertical direction or along the path of placement is known as retention.

The features of a tooth preparation that enhance the stability of a restoration and resist dislodgement along an axis other than the path of placement is known as resistance.

Retention and Resistance are interrelated and inseperable qualities.

Factor of retention and resistance

  • Occlusogingival length
  • Greater the surface area of preparation (longer preparation), the greater its retention (preparations on large teeth are more retentive than preparations on small teeth)
    • Cement creats mechanical interlocks between inner surface of restorationand axial wall of preparation
    • Surface area can be increased by adding boxes and grooves
  • The shorter the wall the more important its inclination-shorter preparations: as little taper a possible to increase resistance
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C) Structural Durability

Sufficient tooth structure must be removed to create space for an adequate bulk of restorative material to accomplish this.

Occlusal reduction

Enough tooth reduction must be removed from the occlusal surface of the preparation, so that metal will be thick enough to prevent wearing or distorting.

A flat occlusal surface is undesirable, because metal in the area of the grooves will be too thin, with a risk of perforation.

  • Functional cusp:buccal of mandibular molars and premolars
  • Nonfunctional cusp: lingual of mandibular molars and premolars
    • Full metal: 1.5 mm functional cusp , 1.0 mm nonfunctional cusp
    • Metal-ceramic: 1.5-2.0 mm functional cusp , 1.0-1.5 mm nonfunctional cusp
    • All-ceramic: 2.0 mm of clearance: Provides space for a cast restoration of adequate thickness in an area of heavy occlusal contact
      • Prevents thin casting and perforation
      • Prevents overcontoured restoration and poor occlusion
      • Prevents overinclination (destroy tooth structure, less retention)

Axial reduction

  • Space for adequate thickness of restorative material
  • Prevent thin walls and weak restoration
  • Prevent overcontoured restoration

D) Marginal Integrity

Margins of restoration must be closely adapted to finish line of preparation.

Finishing line: is the junction between a cemented restoration and the tooth.

Finish line configurations : knife edge, slice, shoulder (radial shoulder, shoulder with a bevel), chamfer (heavy chamfer, chamfer with a bevel).

Knife-edge and slice finishing lines: are more conservative to tooth structure, but they are not recommended because they do not provide sufficient bulk and the location of the margin is difficult to locate.

Chamfer finishing line: has distinct margin, adequate bulk. It is used in full metal crowns, lingual margin (if unveneered) of ceramo-metal crowns.

Shoulder finishing line: provides bulk of restorative material. It is used in facial margin (veneered) of ceramo-metal crowns, and all-ceramic crowns.

E) Preservation of the periodontium

  • Margins as smooth as possible
  • Kept clean by the patient
  • Duplicated by the impression
  • Placed in enamel if possible
  • Supragingival placement of finish lines whenever possible!
  • Subgingival finish lines 2.0 mm from alveolar crest (combined dimension of epithelial and connective tissue attachments)

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