A removable partial denture does not seat fully and moves horizontally while the patient chews. Which design quality is deficient?

Correct answer: A. Stability

• A. Correct. STABILITY is resistance to HORIZONTAL and rotational displacement during function. A prosthesis that shifts, rocks or slides laterally while chewing has a stability failure — most often from inadequate rigid major/minor connectors, insufficient guide plane contact, poor extension, or an ill-fitting denture base. The fact that it also fails to seat fully points to a fit or path-of-insertion problem undermining that horizontal bracing.
• B. Incorrect. Reciprocation is the specific action of a rigid clasp arm neutralising the force of the retentive arm as it passes over the height of contour — a component-level concept, not the overall quality described.
• C. Incorrect. Support is resistance to VERTICAL displacement TOWARD the tissues, provided by rests on abutments and by the denture-bearing mucosa. A support failure presents as the prosthesis sinking into the tissues, causing soreness.
• D. Incorrect. Retention is resistance to VERTICAL displacement AWAY from the tissues — a retention failure means the denture lifts or drops out, not that it slides sideways during chewing.

Note: The three quickly: SUPPORT = vertical, toward tissue. RETENTION = vertical, away from tissue. STABILITY = horizontal/rotational.