How does a tooth with a crack typically behave under transillumination?

Correct answer: B. The light stops at the crack, so only part of the tooth illuminates

• A. Complete failure to illuminate suggests a heavily restored tooth, an opaque restoration in the light path, or that the light is not properly coupled to the tooth.
• B. CORRECT. A fibre-optic light held against the tooth transmits through intact tooth structure but is BLOCKED at the interface of a crack because of the change in refractive index across the fracture. The result is a sharply demarcated pattern in which the segment nearest the light glows and the segment beyond the crack stays dark — PARTIAL illumination. This is the classic finding and is the reason transillumination is such a useful chairside test.
• C. Cracks do not fluoresce; that describes caries detection with certain fluorescence devices, not transillumination of a crack.
• D. Uniform illumination across the whole tooth indicates intact structure with no crack propagating through it.

Note: Combine tests for cracked tooth syndrome: transillumination, a bite test on individual cusps (pain on RELEASE is characteristic), staining with methylene blue, and removal of existing restorations for direct inspection.