Using a No. 245 bur held perpendicular to the occlusal surface, what depth of cut corresponds to the ideal pulpal floor depth for an occlusal amalgam preparation?

Correct answer: B. Half the head length, approximately 1.5 mm

• A. 6 mm would perforate into the pulp chamber of virtually any posterior tooth.
• B. CORRECT. The ideal pulpal floor depth for an occlusal amalgam is 1.5 mm measured from the CENTRAL FISSURE, which places it about 0.2–0.5 mm into dentine. Since the 245 bur head is 3 mm long, sinking exactly HALF the head gives that depth directly, with no measuring instrument required.
• C. Burying the full 3 mm head would take the preparation dangerously close to the pulp.
• D. 0.75 mm would leave the floor within enamel, giving inadequate bulk for amalgam, which requires at least 1.5–2 mm to resist fracture.

Note: The related dimensions worth holding: 1.5 mm pulpal floor depth, 0.2–0.5 mm into dentine, a 90° cavosurface margin, and an axial wall depth of 0.2–0.5 mm inside the enamel-dentine junction for a proximal box.