Removable partial dentures (RPDs) play a crucial role in modern restorative dentistry, offering a functional, aesthetic, and cost-effective solution for the replacement of missing teeth. While fixed prostheses and implant dentistry continue to advance rapidly, RPDs remain a mainstay of treatment—particularly in cases where multiple teeth are missing, implant placement is not feasible, or patient factors limit other prosthodontic options.
Understanding the Foundations of RPD Design
An RPD must function harmoniously with natural oral structures while resisting the displacing forces encountered during function. To achieve this, each component of the denture must be thoughtfully designed to provide support, retention, stability, and comfort. Below, the essential terms and biomechanics are expanded and contextualized.
1. Saddle
The saddle is the portion of the RPD that directly overlies the edentulous ridge and supports the artificial teeth and surrounding acrylic gumwork. A well-designed saddle distributes occlusal forces evenly across the residual ridge, preventing trauma and long-term bone resorption. Saddle design considerations include:
- The width and extension of the saddle
- Type of mucosa (firm vs. compressible)
- The relationship to undercuts
- Need for relief in areas with prominent bony structures
Saddles may be bounded (between natural teeth) or free-end (extending posteriorly without distal abutments), each presenting unique biomechanical challenges.
2. Connectors (Major and Minor)
Connectors join individual components—clasp assemblies, rests, and saddles—into a unified prosthesis.
Major Connectors
These unite one side of the arch to the other, providing rigidity and stability. A well-designed major connector:
- Prevents flexure
- Distributes forces evenly
- Avoids impingement on soft tissues
- Provides an efficient framework for retention and support
Minor Connectors
Minor connectors link small components (clasps, rests, proximal plates) to the major connector. They must:
- Be rigid
- Minimize food stagnation
- Preserve gingival health
Connector choice depends on anatomical constraints, patient comfort, and biomechanical requirements, all of which will be discussed in detail later.
3. Support
Support is the resistance of a denture to movement toward the mucosa. It is primarily derived from:
- Rests on abutment teeth
- The residual ridge under the saddle
- Intimate contact with the mucosa
Good support prevents tissue trauma, discomfort, and accelerated bone resorption.
4. Retainers
Retainers resist the displacement of the denture away from the tissues—typically during mastication or speaking. They include:
- Direct retainers: mechanical components (clasps, precision attachments)
- Indirect retainers: components that resist rotational displacement
Both types are essential for RPD stability.
5. Indirect Retention
Indirect retention prevents rotational movement of a distal extension saddle around a fulcrum line. It is provided by components placed opposite to the saddle, usually:
- Rests
- Proximal plates
- Minor connectors
Indirect retainers help stabilize the denture during sticky-food dislodgement and functional forces.
6. Fulcrum Axis
The fulcrum axis is an imaginary line around which a tooth- and mucosa-borne denture rotates when forces attempt to dislodge it. In distal extension cases, this axis typically passes through the most posterior rest seats.
Understanding this axis is critical for:
- Preventing tipping of the denture
- Correct placement of indirect retainers
- Functional stability
7. Bracing
Bracing (or stabilization) refers to resistance against horizontal or lateral movement. Effective bracing distributes functional forces across abutment teeth and soft tissues, preventing:
- Soreness
- Tooth mobility
- Instability during mastication
8. Reciprocation
Reciprocation counteracts the forces exerted by the retentive clasp arm as it flexes over the survey line. Proper reciprocation ensures:
- Protection of abutment teeth
- Comfort
- Prevention of orthodontic-like tooth movement
This is typically provided by:
- A reciprocal clasp arm
- Proximal plates
- Guiding planes
9. Guide Planes
Guide planes are two or more parallel surfaces on abutment teeth that help determine the path of insertion and removal. Benefits include:
- Increased stability
- Improved retention
- Enhanced bracing and reciprocation
Guide planes are often prepared by minimal enamel modification.
10. Survey Line
The survey line marks the greatest contour (height of contour) of a tooth in a specific path of insertion. It is essential for determining:
- Clasp design
- Clasp position
- Direction of withdrawal
Understanding survey lines allows the clinician to customize retention and stability per tooth.
11. Stress-Breaker
A stress-breaker is a mechanical device that allows limited movement between the saddle and the clasp assembly. It is useful when:
- Abutments are weak
- There is a long free-end saddle
- Ridge resilience varies between sides
While used less frequently today, the principle remains biomechanically important.
12. Gum-Stripper
This is a tissue-borne partial denture that sinks into the mucosa due to lack of proper support. Clinically, this leads to:
- Ulceration
- Bone resorption
- Instability
Good support is essential to avoid gum-stripping.
13. Swinglock Denture
A swinglock denture has a labial bar that opens and closes like a gate:
- Hinged on one side
- Locks on the other
It is useful in cases of missing multiple anterior teeth or when retention from conventional clasps is limited.
14. Sectional Denture
Sectional dentures are fabricated in components and joined intraorally. They are indicated when:
- There are severe undercuts
- Limited mouth opening
- Maxillofacial defects
Classification Systems for RPDs
1. Kennedy Classification
One of the most widely used systems, Kennedy classification describes patterns of partial edentulism.
Class I
Bilateral free-end saddles.
Class II
Unilateral free-end saddle.
Class III
Unilateral bounded saddle.
Class IV
A single, anterior bounded saddle crossing the midline.
Modifications
Additional edentulous spaces beyond the main classification.
Class IV cannot be modified.
This system is clinically valuable because it influences:
- Type of support
- Clasp design
- Connector choice
- Need for indirect retention
2. Craddock Classification
Based on how the denture is supported:
- Tooth-borne – bounded saddles
- Mucosa-borne – solely supported by tissue
- Tooth- and mucosa-borne – free-end saddles
This classification highlights the importance of distributing forces appropriately.
Acrylic vs. Metal Partial Dentures
Acrylic dentures are more common in the UK (approx. 75%), typically because of lower cost and easier fabrication. However, metal RPDs remain superior in most clinical situations.
Advantages of Metal Frameworks
- Better rigidity
- Improved hygiene
- Reduced bulk
- Less tissue coverage, improving comfort
- Superior long-term durability
When Acrylic Dentures Are Indicated
- Temporary replacement (e.g., following trauma or in children)
- When future modifications to the denture are expected
- When abutment support is inadequate for a metal framework
Preventing the Gum-Stripper Effect
When acrylic dentures are used as long-term prostheses, design modifications can reduce tissue damage:
- Wide mucosal coverage
- Avoiding extension over gingival margins
- Eliminating interproximal acrylic where possible
- Using labial flanges for retention
- Adding wrought stainless steel rests
These modifications improve stability and longevity.
Components of an RPD
1. Saddles
Saddles can be:
- All acrylic, or
- Metal subframework with acrylic overlay
The choice influences strength, hygiene, and long-term behavior.
2. Rests
Rests are crucial for support and preventing denture movement. They may be:
- Occlusal rests – on posterior teeth
- Cingulum rests – on anterior teeth
Functions of Rests
- Maintain vertical dimension of occlusion (VDO)
- Prevent drifting or over-eruption
- Transmit functional forces along the long axis of teeth
Types of Rests
- Cast rests – more rigid
- Wrought rests – more flexible
3. Clasps (Direct Retainers)
Clasps engage an undercut on the tooth to resist dislodgement.
Clasp Action
The retentive arm flexes over the survey line and engages the undercut. This movement must be:
- Balanced by reciprocation
- Protected from excessive force on abutment teeth
Types of Clasps
Clasps are classified by:
Approach:
Occlusally approaching
Gingivally approaching (e.g., RPI system)
Material:
Cast cobalt-chrome
Wrought stainless steel
Gold alloys
Cast Clasps
Advantages:
- Rigid
- Accurate fit
- Can be cast as part of framework
Limitations:
- Stiff (limited flexibility)
- Limited to 0.25 mm undercuts
- Risk of distortion
Wrought Clasps
Advantages:
- More flexible
- Greater engagement of undercuts (0.5–0.75 mm)
- Easily adjusted
Indications:
- Periodontally mobile teeth
- Saddle extension cases requiring flexibility
- Premolars with less enamel support
Factors Affecting Clasp Design
- Undercut depth
- Survey line classification
- Tooth shape
- Occlusion
- Sulcus anatomy
- Periodontal health
- Material used
Connectors and Their Clinical Uses
Major Connectors and Their Properties
Major connectors must be:
- Rigid
- Smooth
- Tissue-friendly
- Self-cleansing
Table 7.4 Summary (Expanded)
| Connector | Tolerance | Indirect Retention | Support | Use |
|---|---|---|---|---|
| Anterior bar | Low | Moderate | Low | Kennedy Class IV |
| Mid-palatal bar | Low | Moderate | Good | Short spans |
| Posterior palatal bar | High | Low | Very high | Deep palate |
| Ring | Low | Moderate | Low | Flabby ridges (muco-compression needed) |
| Full plate | High | Very good | Very good | Multiple saddles / poor abutments |
| Horseshoe | Good | Moderate | Moderate | Large tori, but less hygienic |
Mandibular Connectors (Expanded)
Lingual Bar
Most common lower connector. Requirements:
- ≥7 mm from gingival margin to floor of mouth
- Provides 3 mm clearance
- Rigid and hygienic
Lingual Plate
Indicated when:
- Not enough space for lingual bar
- Periodontal support is poor
- Teeth are mobile
Provides:
- Excellent indirect retention
- Good bracing
- Additional support
Sublingual Bar
Used when:
- Lingual sulcus is deep
- Incisors are retroclined
More rigid than lingual bar but may reduce comfort.
Continuous Clasp (Cingulum Bar)
Runs across the cingula of lower anterior teeth.
Disadvantages:
- Poor tolerance
- Risk of plaque accumulation
Dental Bar
Similar to continuous clasp but larger.
Useful when:
- Clinical crowns are long
- Additional support needed
Buccal/Labial Bar
Indicated when:
- Severe lingual inclination or retroclination
- Inadequate lingual space
Aesthetically unappealing but sometimes necessary.
Biomechanics of RPD Design
Understanding the forces acting on a partial denture is essential to prevent long-term damage to oral structures. Key factors include:
Vertical Forces
Managed by:
- Rests
- Saddle design
- Support from teeth and mucosa
Horizontal Forces
Controlled by:
- Bracing elements
- Reciprocal arms
- Guide planes
Rotational Forces
Prevented with:
- Indirect retainers
- Correct placement of rest seats
- Rigid major connectors
Flexure
A rigid metal framework minimizes flexure, protecting soft tissues and abutment teeth.
Clinical Application: Designing an Effective RPD
1. Assessment Phase
Before designing an RPD, clinicians must perform:
- Caries risk assessment
- Periodontal evaluation
- Occlusal analysis
- Ridge assessment
- Restorative planning
A mandibular arch with reduced periodontal support, for example, may necessitate:
- Wrought wire clasps
- Broad coverage for support
- A lingual plate for bracing
2. Surveying
Surveying determines:
- Path of insertion
- Undercut locations
- Required enamel modifications
- Appropriate clasp design
This ensures optimal retention and stability.
3. Selecting Clasp Type and Position
Factors include:
- Aesthetics
- Periodontal support
- Undercut availability
- Functional forces
- Opposing arch position
For example, anterior teeth benefit from gingivally approaching clasps due to reduced aesthetic interference.
4. Choosing the Major Connector
Should balance:
- Rigidity
- Comfort
- Anatomical limitations
- Hygiene
For example, a large palatal torus may necessitate a horseshoe connector—despite its reduced support.
5. Determining the Need for Indirect Retention
Indirect retainers are essential in distal extension saddles. They:
- Resist rotation
- Stabilize during dislodging forces
- Prevent tissue damage
Conclusion
Removable partial dentures continue to play a significant role in contemporary dental practice. A properly designed RPD can enhance function, restore aesthetics, preserve oral structures, and serve as an affordable long-term prosthetic option. Mastery of RPD design principles—support, retention, stability, and biomechanics—is essential for achieving successful clinical outcomes.
Understanding each component of the RPD, from the saddle and clasp assembly to the connectors and indirect retainers, enables clinicians to craft prostheses that distribute occlusal loads evenly, maintain periodontal health, and improve patient comfort.
As dentistry evolves, the demand for durable, comfortable, and aesthetically pleasing RPDs remains strong. Building a strong foundation in RPD principles not only enhances prosthodontic practice but also equips clinicians to provide comprehensive, patient-centered care.
References
- Zarb, G. A., Bolender, C. L., Eckert, S., Jacob, R., Fenton, A., & Mericske-Stern, R.
Prosthodontic Treatment for Edentulous Patients: Complete Dentures and Implant-Supported Prostheses. 13th ed. Mosby; 2012. - Carr, A. B., Brown, D. T.
McCracken’s Removable Partial Prosthodontics. 13th ed. Elsevier; 2016. - Stewart, K. L., Rudd, K. D., Kuebker, W. A.
Clinical Removable Partial Prosthodontics. 2nd ed. Ishiyaku EuroAmerica; 1983. - Phoenix, R. D., Cagna, D. R., DeFreest, C. F.
Stewart’s Clinical Removable Partial Prosthodontics. 4th ed. Quintessence Publishing; 2008. - O’Sullivan, M., & Walls, A. W. G.
Preprosthetic and Prosthetic Care. In: Clinical Dentistry Series. Churchill Livingstone; 2014. - Chan, M. F. W., et al.
“Removable partial dentures: A review of design principles.”
Dent Update. 1998;25:80–87. - Jacobson, T. E., & Krol, A. J.
“A contemporary review of the biologic aspects of RPD design.”
J Prosthet Dent. 1983;50:526–531. - Phoenix, R. D.
“Mechanical and clinical considerations of RPDs.”
J Prosthodont. 2010;19:79–85. - Kapur, K. K.
“A clinical evaluation of dentures supported by implant attachments vs. conventional means.”
J Prosthet Dent. 1992;68:799–805. - Mijiritsky, E.
“Tooth-supported vs. tooth-and-implant–supported RPDs: A review.”
J Prosthodont. 2007;16:361–365. - Academy of Prosthodontics.
Guidelines for the Design of Removable Partial Dentures. - British Society of Prosthodontics (BSSPD).
Prosthodontic Guidelines: Removable Partial Dentures. - American College of Prosthodontists.
Prosthodontic Diagnostic Index (PDI) for Partial Edentulism. - Kennedy, E.
“Partial Denture Construction.”
Dental Items of Interest. 1925. - Craddock, F. W.
“Classification of Partial Dentures According to Support.”
J Prosthet Dent. 1968. - Krol, A. J.
“RPD Design Fundamentals.”
J Prosthet Dent. 1973;29:26–35. - Sedgley, C., & Hullah, R.
Handbook of Clinical Dentistry. Wiley-Blackwell; 2020. - Johnson, W.
Essentials of Complete Denture Prosthodontics. Quintessence Publishing; 2019.