Retainers Flashcards
What is the function of retainers?
Retainers are part of an RPD that engage the abutments and resist dislodging forces.
Provide resistance to displacement by deformation of the retentive arm.
What causes displacement of RPDs?
Can occur through gravity, sticky foods, or functional jaw movements
What are the types of direct retainers?
Intracoronal (Precision attachments)
Extracoronal (Retentive clasp assemblies and extracoronal attachments)
What are the disadvantages of clasp-retained partial dentures?
Strain on the abutment teeth often is caused by improper tooth preparation or clasp design.
Clasps can be unaesthetic, particularly when placed on visible tooth surfaces.
Caries may develop beneath clasp components, especially if patient fails to keep the prosthesis and the abutments clean.
What are the components of a clasp assembly?
Rest
Retentive arm
Reciprocating arm
Clasp body: Proximal plate
Minor connector
What are the types of extracoronal retainers?
Retentive clasp assemblies
Extracoronal attachments
What are the types of retentive clasp assemblies?
Suprabulge assemblies
Infrabulge assemblies
What is the difference between suprabulge and infrabulge assemblies?
Suprabulge: Retentive arm approach the region of the abutment from the occlusal direction.
Infrabulge: Retentive arm approach the undercut region of the abutment from the apical direction
What is the difference between a direct and indirect retainer?
Direct: Adjacent to edentulous area
Indirect: Away from edentulous area
How are retainers classified?
Based on retention (direct/indirect)
Based on construction (Adapted, wrought wire, cast)
What are the requirements of a retentive unit?
Support (Mainly by the rest distributing load through the abutment protecting soft tissues and periodontium)
Stabilization (Bracing effects resist horizontal forces evenly through all abutment teeth)
Retention (Resisting occlusal displacement)
Reciprocity (Insertion force of retention arm must be opposed by reciprocating arms and/or other components of the RPD)
Engagement (Components must encircle more than 180 degrees of the tooth to prevent movement of the abutment out of the assembly)
Passivity (When fully seated the clasp assembly exerts no force on the tooth)
Which parts of the retainer are responsible for stabilization of RPD?
Reciprocating arm
Proximal plate (Minor connector)
Initial third of the retentive arm
What components of the retainer provides retention of RPD?
Terminal third of the retentive arm
Should be distributed throughout the arch
Depth of the undercut should be determined by the flexibility of the alloy.
Where is the correct location of a retention arm?
Middle and/or gingival third (3 sections shoulder midsection and terminal third)
The terminal third should be flexible and should engage below the height of contour.
What is the ideal shape of the retention arm?
Tapered uniformly
Cross-sectional form: Half-round form
How can flexibility of the retention arm be modified?
Length of the arm (Longer = more flexible)
Degree of taper (Lengthwise and widthwise)
Cross-sectional form (greater diameter = less flexible)
Material type used
How does kind of material affect the flexibility of the retention arm?
All cast gold alloys show some degree of flexibility
Cast gold alloys have greater resiliency than Cr-Co alloys.
Cast alloys flexibility is inversely proportional to bulk
Wrought wire clasp has higher tensile strength than cast clasps. For this reason it can be used in smaller diameter to provide greater flexibility without fatigue and fracture.
What is the function of the reciprocal arm?
To resist movement in response to deformation of the retentive arm.
Must be in contact during entire period of retentive clasp deformation
Stabilization of framework against horizontal movements during deformation of retentive arm
What are the features of the reciprocating arm?
Rigid
Half-round form
Average diameter greater than the retentive arm opposing it
No taper or taper in one dimension (lengthwise)
Above height of contour
What is the function of the clasp body?
Unites the arms and rest with the framework.
What are the features of the clasp body?
Above height of the contour
Rigid
It will be in contact with guiding planes (frictional retention)
What are guiding planes and what is their function?
Flat parallel surfaces prepared on abutment teeth (occlual one/two thirds)
Guide planes provide a single path of insertion
GP are used to control and limit directions of movement of RPD during insertion removal or function
Components of the denture start contacting the abutments at the guiding plane area.
How are guiding planes made most effective?
Parallel
Include more than one common axial surface (proximal and lingual surfaces)
Placed on several teeth
Cover a large surface area (long and/or broad)
What are the types of suprabulge assemblies?
Circumferencial clasp types include:
Simple
Reverse
Embrasure
What are the features of a simple circumferential clasps made?
The most simple and versatile clasp
Clasp assembly has one retentive arm opposed by a reciprocal arm originating from the rest
What are the indications of a simple circumferential clasp?
Molars and premolars
Intercalated edentulous area
Canines with no aesthetic restriction
What limits ability to use the simple circumferential clasp?
Tilting of the tooth towards the edentulous space
What are the features of reverse circumferential design?
Rest and body are opposite to the edentulous area
The arms run towards the edentulous space
A proximal plate is needed
What are the limitations of a reverse circumferential design?
Short clinical crown
No space for the connector
What are the features of the ring clasp design?
Mesial and distal rests
Encircles almost all of the tooth
What are the indications for ring clasps?
Intercalated edentulous area
Molars tipped in a mesiolingual direction
When should a ring clasp not be used?
Free end saddle
What are the types of infrabulge clasps?
T-bar clasp
Modified T-clasp
I-clasp
RPI clasp
What are the features of a T-bar clasp?
Approach arm originates from components in the edentulous area
Retention arm cross gingival margin at 90 degrees
Two horizontal projections
What are the indications of a T-clasp design?
Intercalated or free end edentulous area
What are the contraindications for a T-clasp design?
Interference with frenulum
Height of contour near the occlusal surface
Severe soft tissue undercut (metal can irritate the soft tissue and can cause food entrapment)
What is the modified T-clasp design used for?
Avoiding the significant soft tissue undercuts
What are I clasp designs used for?
Both intercalated and free end saddle.
What are the features of I-clasp designs?
Crosses perpendicular to the gingival margin
Should be placed mesially to the midfacial prominence of the abutment
What are the limitations of the I-clasp designs?
Same as other bar clasp designs:
Interference with frenulum
Height of contour near the occlusal surface
Severe soft tissue undercut (metal can irritate the soft tissue and can cause food entrapment)
What are the components of an RPI clasp?
Rest, proximal plate, and I bar
I bar is located in the mesio-buccal undercut
What is needed for an RPI clasp design?
Guiding planes
When is an RPI clasp used?
For Kennedy class I and class II designs.
What are the limitations of RPI clasp designs?
Need to reduce tooth structure
Tipped teeth require severe axial reduction
What are the features of the proximal plate in an RPI?
Wide minor connector touching the distal guiding plane
Junction of the occlusal and middle third of the guiding plane
1 mm thick
Joins the framework at a 90 degree angle.
What are the benefits of the proximal plate in an RPI clasp design?
Improves stabilization of the prosthesis
Improves retentiveness by limiting the path of insertion and removal
Provides reciprocation
Reduces food impaction if the guiding planes are done properly
What are telescopic crown retained removable partial dentures?
Parallel-sided inner and outer crowns that are rigidly connected to abutment teeth. The denture is slotted into these crowns for stability.
What are the limitations of telescopic crowns?
High treatment costs
Increased risk for tooth fracture
Treatment and laboratory procedures are highly complex
