Key Characteristics of Ceramic Crown Preparation

1. Finish Line: Use a deep chamfer or rounded shoulder finish line, flat and smooth with a 1.5 mm thickness.
2. Cavo-Surface Angle: Achieve a 90-degree angle with the external surface to ensure stability.
3. Internal Angles: Keep internal angles rounded to avoid stress points in the ceramic.
4. Axial Wall Taper: Aim for an 8-10 degree taper between the axial walls for retention.
5. Space for Ceramic: Allow approximately 2 mm of space in the cusp area (1.5 mm minimum) for ceramic thickness.
6. Occlusal Surface: Create convexities and concavities that mirror the tooth’s natural anatomy to ensure uniform ceramic thickness.

Modern ceramic crown systems offer flexibility, with varying preparation requirements based on the ceramic type. Common ceramic systems include injection-molded ceramics like Empress, CAD/CAM systems like CEREC, and materials with strengthened copings, such as lithium disilicate or zirconia.

read this guide about Anterior Crown Preparation: step by step

Three Main Stages in Crown Preparation

The preparation involves three main stages, each applied to three main areas of the tooth:

Stages

1. Cutting
2. Refinement
3. Finishing & Polishing

Areas

1. Free Surfaces (Buccal & Lingual)
2. Occlusal Surface
3. Proximal Surfaces (Mesial & Distal)

Step-by-Step Ceramic Crown Preparation

1. Cutting

Start by dividing the tooth into three main areas and proceed as follows:

• Free Surfaces (Buccal & Lingual):

  
  
  
  • Use a spherical bur angled at 45 degrees to create a cervical groove, maintaining a distance from the gingiva.
  • Follow with a cylindrical bur to prepare longitudinal grooves, which align with the inclination of the tooth’s middle and occlusal thirds.
    
    
    

• Occlusal Surface:

  
  
  
  • Use the cylindrical bur to deepen grooves, following the slopes of the cusps to preserve anatomical integrity.

• Proximal Surfaces (Mesial & Distal):

  
  
  
  • Place a metal matrix to protect adjacent teeth.
  • Create a slice with a thin tapered bur to separate the prepared tooth.
  • Use the cylindrical bur to shape the mesial half, achieving a consistent reduction that allows space for ceramic.
    
    
    

2. Refinement

After initial cutting, refine each area to ensure proper angulation and depth:

• Free Surfaces:

  • Use a bulky, tapered bur to adjust wall angulation and refine the cervical margin near the gingiva.
  • The diameter of the bur should be approximately twice the desired reduction for precision.

• Proximal Surfaces:

  • Employ a narrower, rounded-end tapered bur to create near-parallel walls.

• Occlusal Surface:

  • Smooth and round the transition angles from one surface to another to enhance fit and strength.

3. Finishing & Polishing

In this stage, polish the preparation using fine and extra-fine grits to achieve a smooth surface:

• Cervical Margins:

  • Manually refine the cervical margins with a hand instrument to eliminate irregularities, ensuring a smooth transition.

• Final Polishing:

  
  
  
  • Use customized abrasive rubber points for final polishing, enhancing surface smoothness and aiding in impression-taking and temporary restoration fitting.

Final Checks and Occlusion Evaluation

Once the preparation is completed, evaluate the space in both static and dynamic occlusion:

• Ensure that there’s sufficient space in maximum intercuspal position as well as during protrusive and lateral movements to accommodate the ceramic without interference.

read this intro about dental occlusion

Conclusion

Careful preparation following these steps ensures the longevity and success of ceramic crowns on posterior teeth. By focusing on precision in the cutting, refinement, and polishing stages, dentists can achieve a smooth, perfectly tapered crown preparation that meets all functional and aesthetic requirements for posterior teeth.