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What are the preparation requirements for a PFM crown, and why do they matter?

Sound preparation is what allows a PFM to combine strength and esthetics. Because the restoration stacks a metal coping, an opaque layer, and veneering porcelain, the preparation must create enough room for all three while protecting the margin and the long-term seal.

Occlusal clearance

Provide a minimum of 1.5–2.0 mm of occlusal clearance. This range is what accommodates the metal coping plus the opaque and body porcelain without forcing the technician to thin the porcelain (which invites chipping) or overbuild the contour (which compromises occlusion and esthetics). Verify clearance in both centric and excursive positions, since interferences that appear only in lateral movements are a common cause of porcelain fracture later.

Margin placement

Place margins equigingival to subgingival for the best esthetics. A slightly subgingival margin helps conceal the metal collar and the opaque transition so the restoration blends with the gingival tissue. Where esthetics are less critical, an equigingival margin simplifies impression-making and cleanup.

Why clean, dry margins are essential

A successful PFM depends on a precisely captured margin. Blood or fluid around the preparation during impression-taking or scanning can produce open or short margins in the final restoration. Manage tissue and moisture so the margin is clearly visible and dry before you capture records (the records and impressions article covers retraction and capture technique in detail).

Clinical rule of thumb: Adequate, evenly distributed reduction is the single best insurance against veneering-porcelain chips. If you cannot achieve 1.5–2.0 mm of clearance, reconsider the material or plan for metal occlusal coverage rather than thinning the porcelain.

ROE's CAD/CAM design automatically computes cement space and detects the margin, but it can only work with what the preparation and records provide. Crisp, well-defined margins and adequate clearance let the digital coping design deliver the marginal precision and internal fit the process is built for.

 

Additional Resources

For more information concerning the Porcelain Fused Metal (PFM):

Contact Information

For help with a Locator Fixed solution for your next case, contact ROE Dental Laboratory: