In which situation is it acceptable to leave unsupported enamel?

Correct answer: B. The occlusal wall of a Class V preparation

• A. The occlusal margin of a Class I preparation bears the full force of mastication; unsupported enamel there will fracture.
• B. CORRECT. Unsupported enamel is left only where it will not bear occlusal load. The gingival third of a facial or lingual surface — the site of a Class V — is NOT a stress-bearing area. Removing unsupported enamel there would needlessly extend the preparation apically toward the pulp and toward the gingival margin for no mechanical benefit.
• C. The marginal ridge is one of the most highly stressed regions of a posterior tooth.
• D. The functional cusp bears the heaviest loading of all and is precisely why it receives cuspal coverage in an onlay.

Note: General rule: remove unsupported enamel in load-bearing areas, retain it in non-load-bearing areas. Bonded restorations relax this further, since adhesive resin can support thin enamel that amalgam cannot.