Prosthodontics & Occlusion · single question
What should be done in a lithium disilicate (e.max) crown preparation to prevent fracture?
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Correct answer: C. Round internal angles
✓ C) Round internal angles is correct.
Sharp internal line angles concentrate stress in ceramic restorations, promoting crack propagation and fracture. Rounding all internal angles distributes stress evenly. This is critical for lithium disilicate and all ceramic crowns.
✗ Divergent walls would reduce retention. A shoulder margin is used but alone doesn't prevent fracture — rounded angles at the axio-pulpal line angle are the key factor.
Sharp internal line angles concentrate stress in ceramic restorations, promoting crack propagation and fracture. Rounding all internal angles distributes stress evenly. This is critical for lithium disilicate and all ceramic crowns.
✗ Divergent walls would reduce retention. A shoulder margin is used but alone doesn't prevent fracture — rounded angles at the axio-pulpal line angle are the key factor.
- A. Sharp margins create stress concentration — the opposite of what is needed. Rounded internal angles are required.
- B. For lithium disilicate (e.max) preparations, ROUNDED INTERNAL LINE ANGLES are critical to prevent STRESS CONCENTRATION and ceramic fracture. Sharp internal angles create stress risers that concentrate forces and initiate crack propagation through the ceramic. Additional requirements: adequate occlusal reduction (1.5-2mm), smooth preparation without undercuts.
- C. Divergent walls reduce crown retention and create stress concentrations at margins — NOT recommended for lithium disilicate preparations.
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