3.3 Removable Prosthodontics
Key Takeaways
- Kennedy Class I (bilateral free-end saddles) and Class II (unilateral free-end saddle) are tooth-and-tissue supported and require stress-breaking clasps (RPI/RPA) and indirect retainers; Class III (bounded saddle) is tooth-supported and the most stable
- The RPI clasp (Rest, Proximal plate, I-bar) is designed for distal-extension saddles: the mesial rest and proximal plate direct forces along the long axis of the abutment, and the gingivally approaching I-bar disengages under functional load
- A rest transfers occlusal load along the tooth's long axis (support); a clasp arm provides retention against dislodgement; an indirect retainer resists rotation of the saddle away from the ridge
- The copy denture technique preserves the polished surface and palatal fit of a successful existing denture when fabricating a replacement, avoiding the common problem of a new denture that the patient cannot tolerate
- Post-dam creates a peripheral seal at the posterior border of an upper complete denture by displacing the soft tissues over the vibrating line; without it, the denture breaks suction and drops
Kennedy Classification
The Kennedy classification (Edward Kennedy, 1923) is the most widely accepted system for classifying partially edentulous arches. Applegate's rules govern its application.
| Class | Description | Support | Stability |
|---|---|---|---|
| I | Bilateral edentulous areas posterior to all remaining teeth (bilateral free-end saddles) | Tooth-and-tissue | Least stable; rotates in function |
| II | Unilateral edentulous area posterior to all remaining teeth (single free-end saddle) | Tooth-and-tissue | Less stable than III |
| III | Unilateral bounded edentulous area (teeth present anteriorly and posteriorly) | Tooth | Most stable; minimal rotation |
| IV | Single bilateral (crossing the midline) edentulous area anterior to remaining teeth | Tooth | Variable; requires indirect retention |
Applegate's Rules (selected)
- Rule 2: A missing third molar that will not be replaced is not considered.
- Rule 3: A present third molar used as an abutment is considered.
- Rule 5: The most posterior edentulous area determines the class.
- Rule 6: Additional edentulous areas are modification spaces, designated by number (e.g., Class I Mod 2).
- Rule 7: Only the number of modification spaces matters, not their size.
- Rule 8: Class IV has no modification spaces; any additional space reclassifies the case.
Components of a Partial Denture
Every partial denture has five main components, each with a distinct function:
| Component | Function | Notes |
|---|---|---|
| Direct retainer (clasp) | Retention against dislodgement | Engages undercut on the abutment |
| Rest | Support; transfers occlusal load along the long axis of the tooth | Placed on prepared rest seats |
| Major connector | Unites the components; distributes load across the arch | Lingual bar, palatal bar/strap, lingual plate |
| Minor connector | Connects clasps, rests, and saddles to the major connector | |
| Denture base and artificial teeth | Restores aesthetics and occlusion; transmits load to the ridge | Acrylic or cobalt-chromium |
| Indirect retainer | Resists rotation of a free-end saddle away from the ridge | Required in Class I and II; placed far from the fulcrum line |
Rest vs Retention vs Support
- Support is resistance to vertical movement towards the tissues. Provided by rests (occlusal, cingulum, incisal) seated on tooth rest seats, and by the denture base on the ridge.
- Retention is resistance to movement away from the tissues. Provided by clasps engaging undercuts, by adhesion and cohesion (complete dentures), and by muscular control (polished surface).
- Stability is resistance to lateral (horizontal) movement. Provided by broad adaptation of the base, tooth contact (rests, guiding planes), and balanced occlusion.
Clasp Design
Clasp Materials and Types
- Cast cobalt-chromium (Co-Cr): Rigid; used where undercut is 0.25 mm or less. Suitable for gingivally approaching or circumferential designs. Strong and durable; the most common material for cast partial dentures.
- Wrought wire (stainless steel or gold): More flexible than cast; used where the clasp arm must flex into a deeper undercut without permanent deformation. The undercut depths conventionally taught in the UK are 0.25 mm for cast cobalt-chromium, 0.5 mm for cast gold and 0.75 mm for wrought stainless steel. Commonly used on premolars and canines.
- Acrylic resin: Used in acrylic partial dentures (transitional); less retentive and less durable.
Clasp Designs for Distal-Extension Saddles (Class I and II)
| Design | Components | Action |
|---|---|---|
| RPI | Rest (mesial), Proximal plate, I-bar (gingivally approaching, mesiobuccal) | Mesial rest directs force along the long axis; under functional load the saddle rotates and the I-bar disengages, reducing stress on the abutment |
| RPA | Rest (mesial), Proximal plate, Aker (circumferential cast clasp) | Used when a gingivally approaching I-bar is contraindicated (e.g., severe buccal undercut, shallow vestibule); less stress-breaking than RPI |
| Combination clasp | Cast Co-Cr retainer + wrought wire clasp arm | Wrought wire provides flexibility for deeper undercuts; commonly used when a cast clasp would be too rigid |
Circumferential (Aker) Clasp
- Used in Class III (tooth-supported) saddles where the saddle does not rotate.
- Engages a mesiobuccal or mesiolingual undercut (0.25 mm typical).
- Requires a buccal undercut and an occlusal rest; provides excellent retention for bounded saddles.
Altered Cast Technique
For Class I and II distal-extension saddles, the saddle is tooth-supported at one end and tissue-supported at the other. A single impression cannot capture both accurately. The altered cast (altered impression) technique modifies the working cast so the free-end saddle area reflects functional support of the mucosa:
- Make a preliminary impression and cast.
- Construct the metal framework on the preliminary cast.
- Take a secondary impression of the free-end saddle area using the framework as an impression tray, with a mucostatic or functional material (e.g., zinc oxide eugenol or silicone with pressure).
- Modify the working cast by replacing the saddle area with the new impression.
This distributes load between the abutment tooth and the ridge more favourably, reducing abutment stress and improving stability.
Complete Dentures
Border Seal and Post-Dam
Retention of a complete denture depends on a peripheral (border) seal preventing air ingress beneath the denture. The Pound's or neutral zone and polished surface contours harness cheek, lip, and tongue musculature to aid retention.
- Post-dam: A raised groove along the posterior border of an upper denture that displaces the soft tissues over the vibrating line (ah-line), creating a peripheral seal. Without a post-dam, the upper denture breaks suction and drops. It is cut into the working cast or pressed into the finished denture.
- Border moulding: The impression borders are shaped to fill the sulcus functionally, capturing the depth and width of the vestibule during movement.
- POIP (Polished Surface of the Impression): Refers to the external surface of the impression that becomes the polished surface of the denture; it must be contoured to be compatible with surrounding musculature.
Copy Denture Technique
The copy denture technique is used when an existing denture is functionally successful but worn or stained. It preserves the polished surface, palatal fit, and occlusal relationship that the patient has adapted to. Steps:
- Take an impression of the existing denture (or mould a copy).
- Pour a copy in acrylic or investment.
- Use the copy as a base for a new impression of the underlying tissues (wash impression).
- Process the new denture maintaining the established polished surface and tooth positions.
Advantages: faster adaptation, avoids the common problem of a new denture the patient cannot tolerate. Indicated for elderly or frail patients who have worn the same denture for many years.
Immediate, Transitional, and Overdentures
| Type | Description | Indication |
|---|---|---|
| Immediate denture | Inserted immediately after extraction of remaining teeth | Maintains aesthetics and function during healing; alveolar resorption requires early reline or replacement |
| Transitional (interim) partial denture | Temporary; replaces teeth while definitive treatment is planned | Post-extraction healing, waiting for implant placement, or stabilising occlusion |
| Definitive partial denture | Long-term; usually cast Co-Cr | Stable dentition and healed ridges |
| Overdenture | Complete or partial denture over retained roots or implants | Retained roots preserve alveolar bone and provide sensory feedback; attachments (studs, bars) aid retention |
Common Problems and Adjustments
- Pain / sore spots: From over-extension, premature contacts, or sharp acrylic. Relieve the area; recheck occlusion.
- Looseness: From poor border seal, inadequate undercut engagement, or unfavourable muscle forces. Re-border, adjust clasps, or improve polished surface contour.
- Gagging: From posterior over-extension of the upper denture or psychological factors. Reduce posterior palatal extension; ensure post-dam is correctly placed.
- Retained teeth with a new denture: If teeth are not accounted for in the design, clasps may not engage correctly.
- Denture stomatitis: Candida albicans under the denture. Treat with antifungal (miconazole gel or nystatin), improve denture hygiene, and advise leaving the denture out at night.
Patient Instructions
- Wear the denture; remove at night where possible to rest tissues.
- Clean over a basin of water to avoid fracture if dropped.
- Use a soft brush and soap or denture paste (not toothpaste, which is abrasive to acrylic).
- Return for review after 24-48 hours for adjustments; expect a settling-in period of several weeks.
A patient is missing the upper right first and second molars and the upper left first and second molars; all premolars and anterior teeth are present. What is the Kennedy classification, and which design features are essential?
In a Class I lower partial denture using an RPI clasp on a premolar abutment, what is the function of the mesial rest and the I-bar under functional load?
A patient has worn the same upper complete denture successfully for 15 years. It is stained and the teeth are worn, but the fit and the patient's adaptation are excellent. What is the most appropriate technique to replace the denture while preserving the features the patient has adapted to?
A patient returns with a new upper complete denture that drops when the patient speaks and yawns. On examination, the posterior border sits 2 mm short of the vibrating line and there is no post-dam. What is the cause and the appropriate correction?
A patient presents with a Kennedy Class II lower partial denture (one free-end saddle on the right). Which component is specifically required to resist rotation of the saddle away from the ridge, and where should it be placed?