Removable Partial Dentures: Classification and Framework Design
Core concepts
RPD questions reward a disciplined sequence: classify the arch, survey the casts, then design a framework that protects teeth and residual ridges. Learn Kennedy Classes I–IV and Applegate's rules - the class is set by the most posterior edentulous area, missing third molars are not counted unless being replaced, and an edentulous area distal to a remaining tooth is included in the classification. Surveying establishes the path of insertion, guiding planes and undercuts. Rest seats (occlusal, lingual, cingulum) direct forces along the long axis of abutments; clasp assemblies (circumferential, RPI, RPA) provide retention and bracing; major connectors distribute load; indirect retainers resist rotation about the fulcrum line in Class I and II designs. Distal extension bases need an altered cast impression so the tissue-borne base is recorded under function.
Traps
Candidates lose marks by counting a missing third molar, misassigning a modification area to the wrong side, or forgetting that a free-end saddle has no distal abutment - so the clasping concept must change. Examiners also test whether you know a rigid major connector is contraindicated across a prominent torus, that a lingual bar needs adequate floor-of-mouth clearance, and that indirect retention is essential in Class I and II but not in a tooth-borne Class III.
Rule of thumb
Every rest seat should be a smooth, spoon-shaped preparation roughly 1–1.5 mm deep at its deepest point, with the floor directed so occlusal forces pass along the long axis of the abutment. If a design cannot seat without rocking, suspect a non-parallel guiding plane before blaming the impression.
Worked example
A patient is missing the maxillary left molars (free-end) and the maxillary right first premolar. The most posterior edentulous area is the molar span, so this is Kennedy Class II, modification 1. The left side needs a distal extension base with indirect retention on an anterior abutment; the right premolar space is a tooth-borne modification managed with a simple clasped unit, not a second stress-bearing base.