5.3 RPD Design Sequence, Mouth Preparation, Impressions, Try-In, Delivery and Maintenance
Key Takeaways
RPD design follows a fixed order: classify the arch, plan support (rests), then guide planes and retention, then the major connector, indirect retainers and finally the denture bases.
Mouth preparation proceeds from surgery and periodontal therapy to restorations and surveyed crowns, and finally enamel recontouring: guide planes are prepared before rest seats.
The altered (corrected) cast technique records the edentulous ridge of a mandibular distal extension under functional load to improve support and reduce base rotation.
A distal extension base that has lost tissue contact rocks around the fulcrum line; when the indirect retainer lifts as the base is pressed, the base needs relining.
Framework try-in uses disclosing media to remove binding points so the framework seats fully before jaw relations and tooth arrangement are recorded.
Kennedy classes, rests, connectors and clasps (earlier sections) are the vocabulary of RPD design. This section covers how a dentist turns that vocabulary into a prosthesis: the design order, the preparatory treatment, the impression for distal extensions, and the try-in, delivery and maintenance visits that the SDLE blueprint lists under removable partial prosthodontics.
Case Selection and Diagnosis
An RPD is indicated when spans are too long or abutments too weak for a fixed bridge, when a distal extension lacks a posterior abutment, when bone and soft tissue must be replaced for esthetics, or when cost or medical status rules out implants. Evaluate caries risk, periodontal health, abutment mobility, the opposing occlusion, available inter-arch space, and the patient's hygiene, because RPDs increase plaque accumulation on abutment teeth.
The Design Sequence on the Surveyed Cast
- Classify the arch (Kennedy-Applegate) and decide whether support is tooth-borne or tooth-and-tissue-borne.
- Support: place rests adjacent to edentulous spaces; for distal extensions use mesial rests on terminal abutments.
- Guide planes and path of insertion: survey the cast, tilt it to create parallel guide planes and acceptable retentive undercuts.
- Direct retention: select clasps by tooth-borne or distal-extension situation (Akers for tooth-borne spaces; RPI, RPA or combination clasps for distal extensions).
- Major connector: choose the most rigid connector the anatomy allows.
- Indirect retention for Kennedy Class I and II: rests as far anterior to the fulcrum line as possible.
- Minor connectors and denture bases: maximum coverage of the primary stress-bearing areas for distal extension bases.
Mouth Preparation Sequence
| Phase | Typical procedures |
|---|---|
| Relief of pain and infection | Extractions, endodontics, treatment of abscesses |
| Surgical | Removal of tori that interfere with connectors, retained roots, hyperplastic tissue |
| Periodontal | Scaling, root planing, control of inflammation before impressions |
| Restorative | Caries control, surveyed crowns with built-in guide planes, rest seats and undercuts |
| Enamel modification | Guide planes first, then rest seats, then recontouring heights of contour to create or reduce undercuts |
Guide planes are prepared before rest seats because guide-plane reduction can lower the marginal ridge and change the rest seat outline. Rest seats on surveyed crowns are built into the wax pattern rather than cut afterwards.
Impressions and the Altered Cast Technique
- Tooth-borne RPDs (Class III): a single anatomic impression in irreversible hydrocolloid or elastomer, poured promptly, is adequate.
- Distal extension RPDs (Class I and II), especially mandibular: the ridge is displaceable while the teeth are not. The altered cast (corrected cast) technique first makes the framework from an anatomic impression; then acrylic trays attached to the framework record the edentulous ridges with a functional (selective-pressure) impression while the framework is held seated on the rests. The edentulous areas of the master cast are cut away and re-poured from this impression, giving the base better support and less rotation.
Framework Try-In
- Inspect the casting for porosity, nodules and sharp edges.
- Apply disclosing media (pressure-indicating paste or disclosing wax) and seat gently; adjust only binding points revealed as burn-through areas until the framework seats completely.
- Confirm that rests seat fully, clasps are passive at rest, connectors have correct relief, and indirect retainers contact their rest seats.
- Check occlusion of metal components against the opposing teeth.
Jaw Relations, Tooth Try-In and Delivery
- Record jaw relations with record bases on the framework: an interocclusal record in maximum intercuspation when enough opposing contacts remain, or an occlusion rim at the established vertical dimension when they do not.
- Try in the wax set-up for esthetics, phonetics and occlusion.
- At delivery, use pressure-indicating paste under the bases to relieve pressure spots, adjust clasps if needed, refine occlusion, and teach insertion and removal along the path of insertion (never biting the RPD into place).
Instructions and Maintenance
- Remove the RPD at night, clean it and the abutment teeth daily, and use fluoride toothpaste on abutments.
- Recall at about 6-month intervals to check abutment health, occlusion and base fit.
- Relining is needed when a distal extension base no longer contacts the ridge. Test by pressing on the base: if the indirect retainer lifts from its rest seat or the base rocks around the fulcrum line, reline the base with the framework held seated on its rests.
- Rebasing replaces the whole acrylic base when the resin is degraded.
Common Problems After RPD Delivery
| Complaint | Likely cause | Management |
|---|---|---|
| Sore abutment tooth | Active clasp, interference at the rest, occlusal prematurity on metal | Adjust clasp or occlusion; check rest seating |
| Ulcer under the base or major connector | Inadequate relief, overextension, rough tissue surface | Relieve with pressure-indicating paste |
| Food trapping under the base | Poor base adaptation or no tissue contact | Reline the base |
| Clasp fracture | Work hardening from repeated adjustment, deep undercut for a cast clasp | Replace with a wrought-wire clasp or new framework |
| Poor retention | Clasp tips not in undercut, distorted clasps | Adjust or replace clasps; recheck undercut depth |
Exam Traps
- A rest seat cut before guide planes may need re-preparation.
- Relining a distal extension with the patient biting in occlusion can unseat the framework; hold the framework on its rests.
- Clasps that are active at rest move abutments orthodontically.
During mouth preparation for a mandibular Kennedy Class II RPD, in which order should the enamel modifications on abutment tooth 34 be performed?
All enamel modifications are made after the framework has been cast and tried
Rest seat first, because guide planes are cut only after the framework fits
Recontour the height of contour first, then rest seat, then the guide plane
Guide plane first, then rest seat, then recontouring of the height of contour
A mandibular Kennedy Class I RPD framework has been tried in and fits. Which impression approach gives the best support for the distal extension bases?
No further impression, because the anatomic cast already records the ridges
An altered cast impression of the ridges using trays on the seated framework
A second irreversible hydrocolloid impression taken with the framework removed
An impression of the ridges made while the patient bites firmly in occlusion
Two years after delivery of a maxillary Class I RPD, pressing on the left distal extension base makes the anterior indirect retainer lift from its rest seat. What does this indicate?
An over-tight clasp on the terminal abutment that needs to be loosened
Normal physiological movement that requires no clinical intervention
Loss of tissue support under the base, so the base should be relined
A fractured major connector that must be soldered or remade at once
Sections you finish are checked off in the contents.