Direct And Indirect Retainers
Retainers
Retainer provides retention to removable partial denture. There are various types of retainer designs; operator selects one according to the requirement of partially edentulous condition. Retainer is also known as clasp assembly. Retention of removable partial denture can also be achieved by precision attachments. This chapter presents brief review about various clasp assembly design and their indications.
Table of Contents
Direct Retainer
Definition:
Direct retainer is component of removable partial denture, which provides retention to prosthesis and prevents its displacement against path of insertion.
Read and Learn More: Preclinical Prosthodontics Notes
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 inherent property of retainer. It prevents dislodgement of prosthesis. It holds prosthesis to the foundation area. Retention is provided by retentive terminal of clasp by engaging the undercut present on tooth. Retention decreases as the flexibility of clasp increases.
Support:
Support is provided by rest of clasp assembly. It is the characteristic of prosthesis to resist movement of prosthesis toward foundation.
Stability:
This is the quality of prosthesis to resist its movements against lateral or horizontal forces applied to it. Rigid components of clasp assembly provide stability. Rest and minor connector also contribute to stability.
Encirclement:
Each clasp assembly or retainer should be designed in such a way that it encircles abutment tooth more than 180°. This is important to offer retention and stability by retainer.
Reciprocation:
Retentive terminal of clasp engages undercut and it apply forces when get activated. These forces are balanced by (neutralize) reciprocal arm of clasp. This is important to prevent orthodontic tooth movement due to forces generated by retentive terminal.
Passivity:
All component/parts of clasp assembly should be passive. They should be active when dislodging forces are applied. For example, the retentive terminal of clasp should be passive but it (become 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 circumferential clasp are as follows:
- Rest
- Body
- Shoulder
- Reciprocal arm
- Retentive arm
- Minor connector.
Parts of Bar Clasp:
The three parts of bar clasp are as follows:
- Retentive terminal (clasp terminal).
- Approach arm (minor connector).
- 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:
- Large amount of tooth surface 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 lingual side.
Advantage: Good option to use in lingual undercut.
Disadvantages:
- Need more amount of tooth structure.
- Cannot be used on smaller crowns.
- Esthetically not acceptable.
Onlay Clasp:
Indication: When occlusal surface of abutment teeth is below occlusal plane.
Advantage: Used to increase occlusal height of abutment tooth.
Disadvantages:
- Tooth should be caries resistance.
- Can abrade opposite teeth.
Combination Clasp:
Retentive arm is made up of wrought wire and 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 combines with cast reciprocating arm, act as 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 part.
- Esthetic.
Disadvantage: Encirclement is compromised.
Y-bar:
Indication: When height of contour on facial side is higher on mesial than distal side.
Advantage: Minimum tooth coverage.
Disadvantages:
- Esthetically superior to Aker clasp.
- Reduces torquing forces.
T-bar:
Indications:
- Used in toothsupported denture where undercut is located adjacent to edentulous space.
- Undercut on the distobuccal surface of 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 denture 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 situation, some additional retentive features required to prevent movement of partial denture. These additional components are indirect retainer.
Definition:
A part of a removable partial denture that prevents rotational displacement of the denture around principal abutment teeth.
Fulcrum Line:
It is imaginary line passing through the two most distal abutments located either side of the arch. Vertical movements of distal extension bases can occur in two directions. In distal extension, 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.
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