Direct Retainers Part 2 Flashcards
- Suprabulge
(2)
– Retentive arm approaches undercut from above the survey line
– Circumferential, Akers, Circlet
* Synonyms
- Infrabulge
(2)
– Retentive arm approaches undercut from below survey line
– Bar clasps
Bar Clasps
* Indicated:
(2)
– Distal extension RPD
– Tooth-supported RPD, esthetics factor
Bar Clasps
* Most common
(2)
– I-bar clasp
– ½ (modified) T-bar clasp
Bar Clasps
* Advantages
(4)
– More esthetic
– More flexible
– Less conducive to caries (less tooth coverage)
– Wider range of undercut adaptability
* MF, Mid-F (I-bar)
* DF ( ½ T-bar)
Bar Clasps
* Disadvantages
(4)
– Less bracing
– Possible tissue impingement
– Food impaction
– More difficult to repair or replace
Bar Clasps Rules
* Approach arm
(5)
– Must not impinge on soft tissue
– Tapered
* (Flexible minor connector: exception to rule)
– Cross gingival margin at 90 degrees
– Extend onto abutment tooth to the height of
contour
– Not positioned over soft tissue undercut: food
trap, tissue irritation
Selection of RPD Clasps
* Tooth-supported RPDs :
* Tooth-tissue supported RPDs :
– Class III RPD, short-span Class IV
– Class I & II RPD, long-span Class IV
Tooth-supported RPD Clasps
* Circumferential (Circlet)
(3)
– Most common
– Retentive undercut remote from edentulous
area
– Reciprocal clasp
Tooth-supported RPD Clasps
* I-bar
(3)
– Mesiofacial, Midfacial undercut
– Reciprocal clasp
– Tissue undercut prevents use
Tooth-supported RPD Clasps
* ½ T-bar
(4)
–Retentive undercut adjacent
to edentulous area
–Distal facial undercut
–Reciprocal clasp
–Tissue undercut prevents use
Tooth-supported RPD Clasps
* Reverse Circlet (Hairpin)
(3)
–Retentive undercut adjacent
to edentulous area
–Used when tissue undercut present
* ½ T or I-bar cannot be used
–Excess tooth coverage, disadvantage
Tooth-supported RPD Clasps
* Embrasure clasp
(4)
– Used when retainer is
necessary in a dentate area
–Used in Class IV & Class III arch with no
modification space
–Also used in Class II tooth-tissue
supported RPD, when no modification
space exists
Fulcrum Line
Glossary of Prosthodontic Terms
- an imaginary line, connecting occlusal rests,
around which a partial removable dental
prosthesis tends to rotate under masticatory
forces. The determinants for the fulcrum line
are usually the cross arch occlusal rests
located adjacent to the tissue borne
components
Tooth-tissue supported RPD Clasp Selection
* Rotation of the RPD toward the residual
ridge around the horizontal fulcrum line
(3)
– Factor in Class I & II extension RPDs
– RPD potentially functions as a lever
– Can result in detrimental clasp
forces on the abutment teeth
Tooth-tissue supported RPD Clasp Selection
* Why rotation of the RPD toward the
residual ridge around the horizontal
fulcrum line?
(3)
– Difference in potential movement of abutment
& mucoperiosteum over residual ridge
– 0.25mm periodontal ligament, 2mm
mucoperiosteum
– 0.25mm = 0.01” (related to retentive clasp)
Distal Extension RPD Clasp Selection
* Distal abutment retentive clasp should
NOT be anterior to terminal rest fulcrum
line
(2)
– Distal rest/Distal guide plane
- Mesial rest/Distal guide plane
– Distal rest/Distal guide plane
(2)
- Class I lever
- Torquing force on abutment
– Mesial rest/Distal guide plane
(3)
- Class II lever
- Retentive tip disengages abutment
- Stress-release clasp design
Anterior placement of the rest helps
direct the forces more — onto
the tissues of the residual ridge
vertically
Tooth-tissue supported
(Distal Extension)RPD Clasp Selection
* Stress-release clasps used on Class I & II
RPDs distal abutments
(5)
– RPI clasp assembly
– RPC (A) clasp assembly
– Combination clasp
– ½ T clasp
– Reverse circumferential clasp
RPI Clasp assembly
(3)
- Rest (mesial), Proximal Plate, I-bar
- Mesiofacial or Mid-facial retentive undercut
- Reciprocation
– ML Minor connector
– Distal proximal plate
RPI Clasp assembly
* Prepared Guiding Planes:
– Distal Guiding plane: Occlusal 1/3-1/2
(3)
- 2-3 mm
- Guide plate contacts ~1-1.5mm of plane
- Undercut is necessary below the guide plane
RPI Clasp assembly
* Prepared Guiding Planes:
– ML Minor Connector GP: Occlusal 1/3
(1)
- 1-2 mm
RPI Clasp assembly
* Structural Details:
(4)
– I-bar 2mm contact at retentive undercut
– Approach arm
– Distal proximal plate width
– Major connector
RPI clasp assembly
– Approach arm
(2)
- ~3mm from gingival margin
- Crosses gingival margin 90°
RPI Clasp assembly
– Distal proximal plate width
– Line angle to line angle
RPI Clasp assembly
– Major connector
* — from gingival margin: mandible
* —- from gingival margin: maxilla
3mm
6mm
RPI Clasp assembly
* Stress release:
(3)
– Rotation around mesial rest/fulcrum
– Distal guide plate & I-bar move mesiogingivally
* I-bar disengages from tooth (Stress release)
– ML minor connector rotates
mesio-occlusally
RPI Clasp assembly
* Stress release design criteria:
(2)
– Relief below distal plate, Undercut below guide
plane
* Allow proximal plate movement
– ML minor connector not contact adjacent
tooth to allow rotation
RPI Clasp assembly
* Design criteria
* Consideration:
need 5mm separation between
vertical minor connector
* If not 5mm, then should use lingual plate
RPI Clasp RPI Clasp assembly
* Contraindications
(3)
– Excess tissue undercut
– Shallow vestibule
* Tissue impingement
– Abutment excess facial inclination
* High survey line
* Retentive undercut too high
– Not in gingival 1/3
RPC Clasp assembly (RPA)
(4)
- Rest (mesial), Proximal Plate,
Circumferential (Akers) clasp - Mesiofacial retentive undercut
- Circumferential retentive clasp
– Cast round for Kennedy I w/o modification and
periodontally compromised teeth
RPC Clasp assembly (RPA)
* Reciprocation:
(2)
– ML minor connector
– Distal proximal plate
* Wrap to lingual
RPC Clasp assembly (RPA)* Structural Details:
(3)
– Hybrid clasp: RPI and circumferential concepts
– Similar criteria to Circumferential
Retentive Clasp
* Proximal 2/3
– Middle 1/3 of tooth
– On the survey line
* Terminal retentive 1/3
– Gingival 1/3, Below survey line
– Guide planes same as RPI
* Same criteria
RPC Clasp assembly (RPA)
* Stress release:
(2)
– Rotation around mesial rest/fulcrum
– Circumferential clasp & distal guide plate move
mesiogingivally
* Clasp disengages from tooth
* ML minor connector rotates
mesio-occlusally
RPC Clasp assembly (RPA)
* Stress release design criteria
(3)
– Relief below distal guide plate, undercut below plane
– Proximal 2/3 of retentive
clasp exactly on survey line
* Relief is provided below proximal 2/3
* Allow clasp to disengage during rotation
– Failure to follow criteria
* Fulcrum moves to proximal
area of clasp
* Class I lever with detrimental force
Combination Clasp
(2)
- Distal rest, Wrought wire
retentive clasp, Cast reciprocal clasp - Mesiofacial undercut
Combination Clasp
* Indications
(2)
– Mesial-inclination of abutment distal surface
* No undercut below guide plane
* RPC, RPI no stress release
– Mesial rest cannot be used
* RPC, RPI no stress release
Combination Clasp
* Stress release:
(2)
– Rotation occurs at distal rest minor connector
– Clasp moves occlusally
* Class I lever occurs
* Distal torquing force occurs
* Flexibility of wrought wire limits
torque
Modified (1/2) T Clasp
- Mesial rest, Distal Proximal plate, ½ T-bar
clasp
Modified (1/2) T Clasp
Indication:
* Reciprocation:
– Distofacial retentive undercut
ML minor connector,
Distal plate
Modified (1/2) T Clasp
* Contraindications
(3)
– Tissue undercut
– Shallow vestibule
– Abutment excess facial inclination
* High survey line
* Retentive undercut too high
– Not in gingival 1/3
Reverse Circumferential Clasp
- Mesial rest, Distal guide plate, Reverse cast
circumferential retentive clasp
Reverse Circumferential Clasp
* Indications:
(2)
* Reciprocation:
(1)
– Distofacial retentive undercut
– ½ T clasp contraindicated
– ML minor connector, Distal plate
Clasp Selection:
Abutment Anterior to Horizontal Fulcrum
* Class I lever occurs in Class II RPD with
posterior modification
Limit torquing forces by:
(4)
- No clasp
- Clasp in less undercut
(i.e. MB tooth #6) - Non-retentive clasp (tip not
in undercut) - Wrought wire clasp
