What is the optimal depth of a pinhole into dentine for a pin-retained amalgam restoration?

Correct answer: A. 2 mm

• A. CORRECT. The optimal pinhole depth into DENTINE is 2 mm, with roughly 2 mm of pin projecting into the restoration. Shallower placement gives inadequate retention; deeper placement risks perforating either into the pulp chamber or through the external root surface into the periodontal ligament.
• B. 4 mm substantially increases the risk of pulpal or periodontal perforation.
• C. 0.5 mm gives insufficient engagement to retain the restoration.
• D. 6 mm would almost certainly perforate.

Note: Pin placement rules: locate the pin 0.5–1 mm inside the enamel-dentine junction, parallel to the external root surface, and place pins at line angles where dentine is thickest. Every pin introduces microcracks in dentine, so use the fewest that will do the job — the modern preference is for bonded amalgam or a cuspal-coverage restoration instead.