Dental materials and radiology basics: metal-ceramic thermal compatibility: practice question

A Dentistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Dentistry · Dental materials and radiology basics: metal-ceramic thermal compatibility

Sample item

A metal-ceramic crown cracks in the porcelain on cooling after firing. Which relationship between coefficients of thermal contraction is correct for a compatible system?

APorcelain higher than alloy, leaving the porcelain in tensionBAlloy slightly higher than porcelain, leaving the porcelain in compressionCIdentical values, since no stress can be allowedDPorcelain much lower than alloy, giving large residual stress

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Alloy slightly higher than porcelain, leaving the porcelain in compression

Ceramic is strong in compression and weak in tension. A slightly higher contraction in the metal leaves the porcelain under mild compression and prevents cracking.

Option by option

A. Porcelain higher than alloy, leaving the porcelain in tension
Porcelain contracting more leaves it in tension and it cracks.
B. Alloy slightly higher than porcelain, leaving the porcelain in compression (correct)
Correct. A slight mismatch puts the porcelain in compression.
C. Identical values, since no stress can be allowed
A small compressive mismatch is actually desirable, so equal values are not required.
D. Porcelain much lower than alloy, giving large residual stress
A large mismatch causes stress, bowing or crazing.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Dentistry question

What is the answer to this Dentistry question?

The best answer is B, Alloy slightly higher than porcelain, leaving the porcelain in compression. Ceramic is strong in compression and weak in tension.

Why is "Porcelain higher than alloy, leaving the porcelain in tension" not the answer?

Porcelain contracting more leaves it in tension and it cracks.

Why is "Identical values, since no stress can be allowed" not the answer?

A small compressive mismatch is actually desirable, so equal values are not required.

Why is "Porcelain much lower than alloy, giving large residual stress" not the answer?

A large mismatch causes stress, bowing or crazing.

What concept does this question test?

It tests Dental materials and radiology basics: metal-ceramic thermal compatibility in Dentistry. A slightly higher contraction in the metal leaves the porcelain under mild compression and prevents cracking.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Dental materials and radiology basics: metal-ceramic thermal compatibility?

Learn the dental formula, eruption ages and tooth structure once, as tables. Then use the flashcards and the other Dentistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.