GENERAL CONSIDERATIONS

Technique Tips

  • Ensure even reduction of the anatomic form.
  • Provide enough room for the ceramic to allow adequate strength.
  • Sharp transitions and sharp internal edges/line angles or undercuts MUST be avoided.
  • Margins should have a pronounced chamfer or shoulder with butt joint margins. Avoid tapered margins, feathered edges or bevelled shoulders.
  • Ensure there is an adequate path of insertion.

Suggested Margin Preparation

 
  • Shoulder with rounded axio-cervical line angle, uniform circular ablation.
  
  • Chamfer preparation (with reduced mechanical support of the crown). Particularly used with reduced substructure (e.g., after repeated crowning).

Common Preparation Errors

  1. Insufficiently defined and finished preparation margins.
  • Uneven preparation limit ("gutter" preparation, vertical unevenness).
  • Irregular marginal reduction of layers (horizontal unevenness).
  • Wrong shape of preparation limit.
  • Unnecessarily deep subgingival preparation.
  • Preparation in root dentin.
  1. Poorly controlled tooth reduction.
  • Excessive reduction, especially in the upper anterior teeth (vestibular) and premolars.
  • Excessive incisal/occlusal reduction causing reduced retention and stability.
  • Insufficient reduction at the palatal side of the upper anterior teeth (malfunction occlusion).
  • Excessive taper.
 
  • Incorrect preparation of the labial surface: Preparation in one plane; therefore insufficient reduction of substance. Fracture of the crown may result due to insufficient wall thickness of the crown.
  
  • Incorrect preparation of the labial surface, risk of damage to the pulp.

ANTERIOR CROWN PREPARATION



  • Reduce lingual surface 1.5 mm at the area of centric contact (not less than 1.0 mm) and 0.8 to 1.0 mm along the remaining lingual surface.





  • Incorporate taper of 8 to 10 degrees (never more than 12º)





  • Tooth preparation length should reflect a 1.5 to 2.0 mm occlusal (incisal) reduction.

  • Original position of teeth.
   
  • Preferred final position of teeth.
   
  • Actual preparation required to achieve preferred final position of teeth.

VENEER PREPARATION

Technique Tips

  • On average, allow for a minimum of 1 mm of porcelain. A reduction greater than 1 mm may be required in areas masking severely discolored enamel.
  • Allow for veneer thickness of approximately 0.8 mm labially (with incisal covering).
  • Proximal margins are completed labial to contact point. Preserve contacts wherever possible. It may be necessary to prepare through the proximal contact point when clinical conditions dictate (such as slice prep), interproximally to obtain proper emergence profile.
  • Provide a definite finish line for the technician with a chamfer lingual preparation and a 0.5 mm labial gingival finishing area.
  • Use medium grit, round-ended diamond bur to remove uniform thickness of facial enamel by joining the depth-cut grooves.

Incisal Preparation:

  • Overlap of incisal edge is recommended.
  • Long bevel lingual preparation.
  • Lingual butt margin provides greatest strength as butt shoulder.
  • 0.5 mm labial gingival finishing line.
  • All line angles rounded, preparation smooth.





Interproximal Extension:

  • Using round-ended diamond bur, extend the interproximal preparation from the gingival chamfer into the interproximal space. NOTE: Eliminate all discolored enamel into the interproximal (i.e., elbow prep).





Lingual Reduction to Increase Length:

  • Reduce tooth surface in stress bearing areas to provide bulk of porcelain for function.
  • Angle diamond bur to eliminate undercuts and establish deep chamfer on lingual aspect of preparation.

POSTERIOR CROWN PREPARATION



INLAYS/ONLAYS

Inlay Preparation

Technique Tips

  • Ensure all walls end in a butt margin. No flared or feather edge margins.
  • Enamel surfaces created by parallel preparation are generally sufficient for acid etching with flat cusps; a diverging preparation helps optimize acid etching of the enamel.

 
  • Avoid sharp internal line angles and undercuts.
  • Smooth prep walls and trim excess lining material with finishing diamond or bur
  • Do not include undercuts
  • Create a 5º to 15º divergence in the proximal walls from the floor to the occlusal margin of the preparation.




Onlay Preparation
  • Avoid sharp internal line angles and undercuts.


  • With onlays, cuspal reduction of a minimum of 1.5 mm is required.


  • Smooth prep walls and trim excess lining material with finishing diamond or bur.


  • Do not include undercuts.


  • Create a 5º to 15º divergence in the proximal walls from the floor to the occlusal margin of the preparation.


  • Ensure all walls end in a butt margin.


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