direct retainers pt. 2 the sequel Flashcards
Clasp Types relative to HOC?
supra and infrabulge
other names?
- Suprabulge
– Retentive arm approaches undercut from above the survey line
– Circumferential, Akers, Circlet= Synonyms
infrabulge
retentive cusp from below survey line
bar clasps
when are bar clasps indicated
distal extension RPD
tooth supported RPD (no distal extensions), esthetics
most common bar clasps
I bar and half modified T bar
esthetics? flex? caries? undercut adaptability/location for I/T bars?
bar clasp advantages
– 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)
bracing? tissue? food? repair?
bar clasp disadvantages
– Less bracing (less encirclment)
– Possible tissue impingement
– Food impaction
– More difficult to repair or replace
Bar Clasps Rules for Approach arm
impinge soft tissue?
taper? why?
crossing gingival margin?
extending onto abutment tooth?
not positioned over?
– 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
Tooth-supported RPDs
Tooth-supported RPDs
– Class III RPD, short-span Class IV
Tooth-tissue supported RPDs
Tooth-tissue supported RPDs
– Class I & II RPD, long-span Class IV
Tooth-supported RPD Clasps most common type/ preferred?
undercuts location relative to edentulous area?
where is reciporacal clasp?
Circumferential (Circlet)
– Most common
– Retentive undercut away from edentulous area
– Reciprocal clasp on lingual
I bar with tooth supported RPD
where is undercut?
what prevents it use?
second choice behind circumferential
Mesiofacial, Midfacial undercuts
– Reciprocal clasp on lingual
– Tissue undercut prevents use
half t bar with tooth supported cases
undercut location relative to endentulous area?
reciprocal clasp located?
what prevents its use?
–Retentive undercut adjacent
to edentulous area
–Reciprocal clasp on lingual
–Tissue undercut prevents use
tooth supported RPD and hairpin/ reverse circlet clasp
rententive undercut location?
used when?
disadvantage?
–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
used when? examples?
– Used when retainer is
necessary in a dentate area
–Class IV & Class III arch with no modification space
–Class II tooth-tissue supported RPD, when no modification space exists
Fulcrum Line
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
rotation around fulcrum line
• Rotation of the RPD toward the residual ridge around the horizontal fulcrum line while in function
– Factor in Class I & II extension RPDs
– RPD potentially functions as a lever
– Can result in detrimental clasp forces on the abutment teeth
Why rotation of the RPD toward the
residual ridge around the horizontal
fulcrum line?
– Difference in potential movement of abutment
& mucoperiosteum over residual ridge
– 0.25mm periodontal ligament, 2mm at mucoperiosteum
– 0.25mm = 0.01” (related to retentive clasp)
Distal Extension RPD Clasp Selection
class I vs II levers
where to put rests and guide planes
Distal abutment retentive clasp should NOT be anterior to terminal rest fulcrum line
– Distal rest/Distal guide plane
• Class I lever = Torquing force on abutment
– Mesial rest/Distal guide plane
• Class II lever
• Retentive tip disengages abutment
• Stress-release clasp design
Anterior placement of the rest helps to?
Anterior placement of the rest helps direct the forces more vertically onto the tissues of the residual ridge.
wider arc of rotation
- Stress-release clasps types/ when used?
- Stress-release clasps used on Class I & II RPDs distal abutments
– RPI clasp assembly
– RPC (A) clasp assembly
– Combination clasp
– ½ T clasp
– Reverse circumferential clasp
RPI Clasp assembly
parts?
rest location?
undercuts location?
reciprocation from?
- Rest (mesial), Proximal Plate, I-bar
- Mesiofacial or Mid-facial retentive undercut
- Reciprocation from:
– ML Minor connector
– Distal proximal plate
prepping RPI guide planes
M or D?
where on tooth?
demensions?
how much is contacted by guide plate?
undercut necessary?
– Distal Guiding plane: Occlusal 1/3-1/2
* 2-3 mm
* Guide plate contacts ~1-1.5mm of plane, not whole plane
* Undercut is necessary below the guide plane
– ML Minor Connector GP: Occlusal 1/3
* 1-2 mm
RPI Clasp assembly
* Structural Details:
I bar?
approach arm?
D PP width?
major connector?
– I-bar 2mm contact at retentive undercut
– Approach arm : ~3mm from gingival margin, Crosses gingival margin 90°
– Distal proximal plate width: Line angle to line angle
– Major connector: 3mm from gingival margin: mandible OR 6mm from gingival margin: maxilla
RPI Clasp assembly Stress release:
– 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
Disengage abutment tooth= release
RPI Clasp assembly Stressm release design criteria:
relief is below?
undercut below?
allow what component to move?
ML minor conn shouldnt contact what?
– Relief below distal plate, Undercut below guide plane
* Allow proximal plate movement
– ML minor connector not contact adjacent tooth to allow rotation
Need ____ separation between
vertical minor connector with RPI
* If not?
Need 5mm separation between
vertical minor connector with RPI
* If not 5mm, then should use lingual plate
RPI Clasp RPI Clasp assembly
• Contraindications
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)
components?
undercut where?
type of retentive clasp?
• Rest (mesial), Proximal Plate,
Circumferential (Akers) clasp (not I bar)
• Mesiofacial retentive undercut
• Circumferential retentive clasp
RPC(A) Reciprocation:
Reciprocation:
– ML minor connector
– Distal proximal plate
* Wrap to lingual
RPC Clasp assembly (RPA)• Structural Details:
– 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:
– 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
relief where?
prox 2/3 retentive clasp where?
relief provided by?
failure to follow these criteria?
– 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
parts?
undercut?
indications
Distal rest, Wrought wire
retentive clasp, Cast reciprocal clasp
* Mesiofacial undercut
- Indications
– 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
comes from the flexibility of matieral
– Rotation occurs at distal rest minor connector
– Clasp moves occlusally
• Class I lever occurs
• Distal torquing force occurs
• Flexibility of wrought wire limits torque
RP? indication? reciprocation?
Modified (1/2) T Clasp
- Mesial rest, Distal Proximal plate, ½ T-bar clasp
- Indication:
– Distofacial retentive undercut - Reciprocation: ML minor connector, Distal plate
Stress release:Modified (1/2) T Clasp
Modified (1/2) T Clasp
• Contraindications
– Tissue undercut
– Shallow vestibule
– Abutment excess facial inclination
• High survey line
• Retentive undercut too high
– Not in gingival 1/3
Clasp Selection: Abutment Anterior to Horizontal Fulcrum
Class I lever occurs in Class II RPD with posterior modification
Limit torquing forces by:
1. No clasp
2. Clasp in less undercut
(i.e. MB tooth #6)
3. Non-retentive clasp (tip not
in undercut)
4. Wrought wire clasp
Overview Tooth-supported RPD Clasp
Selection
• Rests ?
• Clasp by?
• Reciprocation rules?
• Rests adjacent to edentulous areas
• Clasp by convenience
– No fulcrum lines, no rotation
– Determine by site of retentive undercut
– Tissue undercut
• Reciprocation rules still apply
– Often Cast circumferential clasp
Tooth-supported RPD Clasp Selection table:
RPI, 1/2 T, circlet, reverse, embrasure
where is undercut?
indications/contra?
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Overview Tooth-Tissue Supported
(Distal Extension) RPD Clasp Selection
rests preferred?
stress release?
reciprocation?
• Mesial rests preferred
• Stress-release clasps used
– Accommodate for rotation toward residual ridge
around horizontal fulcrum
• Reciprocation still needed
– Often ML minor connector & Distal plate rather
than circumferential reciprocal clasp
RPI? RPC? combo clasp? 1/2 t bar? Reverse?
undercuts? when used/not
Distal Extension RPD Stress-Release Clasp Selection: table
prefered clasp assembly for dista; extensions
RPI
why must the retentive clasp be posterior to occlusal rests in distal extension cases
must be posterior to fulcrum line to prevent dislodging forces on the tooth
I bar will disengage with function
where is the reciprocation with RPI
rest minor connector and prox plate
how long should the RPI approach arm be?
1 PM
deminsions of the approach arm for RPI
top: 3mm from margin
bottom: 6mm
what if its too big?
area of I bar touching tooth, deminsions
the pod
1.5- 2mm in width and 2-3 mm tall
can impinge on gingival margin if too large
i bar contraindications
tissue undercut greater than 1.5mm at the approach arm
high frenum (impingment)
when beading the maxillary cast when should it fade out? why?
6mm from margin to avoid excessive pressure on tissues
Reverse Circumferential Clasp
* components?
* Indications:
* Reciprocation:
- Mesial rest, Distal guide plate, Reverse cast
circumferential retentive clasp - Indications:
– Distofacial retentive undercut
– ½ T clasp contraindicated - Reciprocation:
– ML minor connector, Distal plate
Reverse Circumferential Clasp stress release
similar to others
