The amount of adjustment possible on a BASE METAL clasp arm of a removable partial denture is limited by which mechanical property?

Correct answer: B. Elongation (ductility)

• A. Toughness is the total energy absorbed to fracture (the area under the stress-strain curve). It is related, but the specific property describing permissible permanent deformation is elongation.
• B. CORRECT. ELONGATION is the percentage of permanent (plastic) deformation a material can undergo before it fractures, and it is the direct measure of DUCTILITY. Base metal cobalt-chromium alloys have a LOW percentage elongation — typically only around 1-2%, compared with 8-10% or more for a gold-based partial denture alloy. That is precisely why a Co-Cr clasp tolerates very little bending before it work-hardens and snaps, and why chairside adjustment of cast base metal clasps must be minimal and done gradually.
• C. Hardness is resistance to indentation and scratching. Base metal alloys are considerably harder than gold, which makes them harder to finish and polish, but hardness is not the property that governs how far a clasp can be bent.
• D. Malleability describes deformation under COMPRESSION into a sheet. The relevant property for bending a wire or clasp arm in tension is ductility, quantified as percentage elongation.

Note: The clinical trade-off in a nutshell: base metal Co-Cr has roughly twice the modulus of elasticity of gold, so a clasp of the same dimensions is about twice as STIFF — it can be made thinner and lighter, but it flexes less and adjusts poorly. Wrought wire clasps are chosen where a flexible, adjustable retentive arm is needed on a distal extension abutment.