Direct And Indirect Retainers in Preclinical Prosthodontics

Direct And Indirect Retainers

Retainers

Retainer provides retention to removable partial denture. There are various types of retainer designs; the operator selects one according to the requirement of partially edentulous condition. Retainer is also known as clasp assembly. Retention of removable partial dentures can also be achieved by precision attachments. This chapter presents a brief review of various clasp assembly design and their indications.

Direct Retainer

Definition:

A direct retainer is a component of a removable partial denture, which provides retention to the prosthesis and prevents its displacement against the path of insertion.

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Types:

There are two types of retainer based on their design and mechanism of achieving retention

Requirements/Objectives of Clasp Design:

Retention:

It is resistance offered by prosthesis to dislodgement forces acting against its path of

insertion. It is an inherent property of the retainer. It prevents dislodgement of the prosthesis. It holds a prosthesis to the foundation area. Retention is provided by the retentive terminal of the clasp by engaging the undercut present on the tooth. Retention decreases as the flexibility of the clasp increases.

Support:

Support is provided by the rest of the clasp assembly. It is characteristic of the prosthesis to resist movement of the prosthesis toward the foundation.

Stability:

This is the quality of the prosthesis to resist its movements against lateral or horizontal forces applied to it. Rigid components of clasp assembly provide stability. Rest and minor connectors also contribute to stability.

Encirclement:

Each clasp assembly or retainer should be designed in such a way that it encircles the abutment tooth more than 180°. This is important to offer retention and stability by retainer.

Reciprocation:

The retentive terminal of the clasp engages the undercut and it applies forces when activated. These forces are balanced by (neutralizing) the reciprocal arm of the clasp. This is important to prevent orthodontic tooth movement due to forces generated by the retentive terminal.

Passivity:

All components/parts of the clasp assembly should be passive. They should be active when dislodging forces are applied. For example, the retentive terminal of the clasp should be passive but it (becomes active) offers resistance to dislodgement when forces are applied to dislodge the prosthesis.

Parts of Direct Retainer (Clasp Assembly):

Parts of Circumferential Clasp:

The six parts of the circumferential clasp are as follows:

1. Rest
2. Body
3. Shoulder
4. Reciprocal arm
5. Retentive arm
6. Minor connector.

Parts of Bar Clasp:

The three parts of the bar clasp are as follows:

1. Retentive terminal (clasp terminal).
2. Approach arm (minor connector).
3. Proximal plate.

Factors Affecting Flexibility of Clasp:

• Length of the clasp:
  • Increase in length.
  • Increases in flexibility.
• Diameter of clasp arm:
  • Increase in diameter.
  • Decrease in flexibility.
• Cross-section form of clasp arm:
  • Round cross-section: More flexibility (wrought wire).
  • Half-round section: Less flexibility (cast clasp/circumferential).
• Materials used:
  • Cast gold alloys are more flexible than Cr-Co alloys.
  • Wrought alloys are more flexible than cast alloys.

Circumferential Clasp:

Simple Circlet Clasp:

Indication: Indicated for tooth-supported removable partial denture.

Advantages:

• Easy to construct/fabricate.
• Most versatile clasp design.
• Easy to repair.

Disadvantages:

• Covering more amount of tooth surface.
• Changes the tooth morphology due to more coverage.
• Not esthetically accepted on anterior teeth.
• Adjustment in occlusogingival direction is not possible.

Ring Clasp:

Indication: Tilted or tipped molars.

Advantages:

• Good coverage of tooth surface.
• Occlusal rest prevents tooth movements.

Disadvantages:

• A large amount of tooth surface is covered by metal.
• Unesthetic.

Embrasure Clasp:

Indications:

• For class II and class III conditions.
• Given on arch side opposite to edentulous space.

Advantages:

• Better retention.
• Use single embrasure for two clasps.

Disadvantage: More susceptible to fracture.

Fish-hook or Hairpin Clasp:

Indication: Used when retentive undercut is on the lingual side.

Advantage: Good option to use in lingual undercut.

Disadvantages:

• Need more amount of tooth structure.
• Cannot be used on smaller crowns.
• Aesthetically not acceptable.

Onlay Clasp:

Indication: When the occlusal surface of abutment teeth is below the occlusal plane.

Advantage: Used to increase the occlusal height of the abutment tooth.

Disadvantages:

• The tooth should be caries-resistant.
• Can abrade opposite teeth.

Combination Clasp:

The retentive arm is made up of wrought wire and the reciprocal arm is made up of cast metal.

Indications:

• Distal extension cases (mainly long-span class I cases).
• On abutment near to edentulous space.

Advantages:

• Minimum surface contact.
• Wrought wire combined with a cast reciprocating arm, acts as a stress equalizer.
• Helps to dissipate torquing forces.

Disadvantages:

• Difficult fabrication.
• Easy to fracture.
• Easy distortion.

Bar Clasp:

I-bar:

Indications:

• In esthetic region (on anterior teeth).
• Distal extension cases.

Advantages:

• Covers minimum tooth surface.
• Less visibility of metal parts.
• Esthetic.

Disadvantage: Encirclement is compromised.

Y-bar:

Indication: When the height of the contour on the facial side is higher on the mesial than the distal side.

Advantage: Minimum tooth coverage.

Disadvantages:

• Aesthetically superior to Aker clasp.
• Reduces torquing forces.

T-bar:

Indications:

• Used in tooth-supported dentures where undercut is located adjacent to edentulous space.
• Undercut on the distobuccal surface of the abutment.

Advantages:

• Minimum tooth coverage.
• Esthetically superior.

Disadvantages:

• Cannot be used over soft tissue undercut.
• Not used on terminal abutment.

Modified T-bar:

Indication: In esthetic zone.

Advantage: Least tooth coverage.

Disadvantages:

• Lesser encirclement.
• Cannot be used over soft tissue undercut.

Indirect Retainers

Retention in removable partial dentures is provided by direct retainers. In distal extension cases, the posterior termination is edentulous and not restricted by posterior natural teeth (Kennedy’s class I and class II). In such a situation, some additional retentive features are required to prevent the movement of the partial denture. These additional components are indirect retainers.

Definition:

A part of a removable partial denture that prevents rotational displacement of the denture around principal abutment teeth.

Fulcrum Line:

It is an imaginary line passing through the two most distal abutments located on either side of the arch. Vertical movements of distal extension bases can occur in two directions. In distal extension, the denture moves away from the ridge tissues via either displacement of the entire denture (resisted by the direct retainers), or a rotational movement around an axis. This axis passes through the most posterior abutment teeth and is called the fulcrum line.

Forms of Indirect Retainers:

• Auxiliary occlusal rest
• Canine rest
• Canine extension from occlusal rest
• Cingulum bars
• Linguoplate major connector Rugae area.