During post-insertion review of a three-unit ceramometal fixed partial denture, one retainer shows chipping of the porcelain at the ceramometal junction. To have avoided this, the dentist should have:

Correct answer: D. Kept the porcelain-metal junction away from centric contacts

• A. Uniform porcelain thickness IS a genuine principle — abrupt changes in thickness create stress concentrations, and porcelain should generally not exceed about 2 mm — but the specific failure described, at the JUNCTION, is a junction-placement problem.
• B. Thinning the metal coping below its recommended minimum (about 0.3-0.5 mm for a noble alloy) makes the coping FLEX under load, and porcelain fractures because its substrate deforms beneath it. Thinning the metal makes the problem worse, not better.
• C. Placing all contacts on porcelain is the opposite of good practice. Where the situation allows, contacts on METAL are the most durable option; porcelain contacts must at least be well supported by an adequately designed metal framework.
• D. CORRECT. The porcelain-metal INTERFACE is the most vulnerable point of a PFM restoration. Placing a centric occlusal contact directly on that junction concentrates load precisely at the interface between two materials of very different elastic modulus, generating shear stress that propagates a crack and chips the porcelain. The design rule is to position the junction at least 1-1.5 mm AWAY from any centric stop, on a surface that will not receive functional load.

Note: Other contributors to PFM porcelain fracture worth reviewing: coefficient of thermal expansion mismatch between alloy and ceramic, contamination or improper degassing of the metal surface, sharp internal line angles in the framework, inadequate framework support ('unsupported porcelain'), overheating during adjustment, and parafunction.