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
