Direct Retainers Part 2 Flashcards
what is the clasp types for suprabuldge and where is it located
- circumferential, akers, circlet ( synonyms)
- retentive arm approaches undercut from above the survey line
what is the clasp type for infrabuldge and where is it located
- retentive arm approaches undercut from below survey line
- bar clasps
when are bar clasps indicate
- distal extension RPD
- tooth supported RPD, esthetics facor
what are the most common bar clasps
- I bar clasp
- 1/2 T bar clasp
what are the advantages and disadvantages to bar clasps
- A: more esthetic, more flexible, less conducive to caries bc less tooth coverage, wider range of undercut adaptability (MF, mid -F : Ibar) (DF 1/2T bar)
- D: less bracing, possible tissue impingement, food impaction, more difficult to repair or replace
what are the bar clasp rules of the approach arm
- 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
what falls under the category of tooth supported RPDs
- class III RPD, short span class IV
what falls under the category of tooth tissue supported RPDs
-class I and II RPD, long span Class IV
what are the tooth supported RPD clasps in order of most favorable to least
- circumferential
- I bar
- 1/2 T bar
- reverse circlet (hairpin)
- embrasure clasp
when is circumferential clasp used
- most common
- retentive undercut remote from edentulous area
- used as reciprocal clasp
when is I bar used and what prevents use
- Mesiofacial, midfacial undercut
- used as reciprocal clasp
- tissue undercut prevents use
when is 1/2 T bar used and what prevents its use
- retentive undercut adjacent to edentulous area
- distal facial undercut
- reciprocal clasp
- tissue undercut prevents use
when do you use reverse circlet (hairpin) and what is its disadvantage
- retentive undercut adjacent to edentulous area
- used when tissue undercut present and I/2 T bar or I bar cant be used
- excess tooth coverage is disadvantage
when is embrasure clasp used-
- when retainer is necessary in a dentate area
- used in class IV and class III arch with no modification space
- also used in class II tooth tissue supported RPD when no modification space exists
what is the fulcrum line
an imaginary line connecting occlusal rests around which a partial removable dental prosthesis tends to rotate under masticatory forces
what are the determinants for the fulcrum line
the cross arch occlusal rests located adjacent to the tissue borne components
describe what happens in rotation of the RPD toward the residual ridge around the horizontal fulcrum line
- factor in class I and II RPDs
- RPD functions as a lever
- can result in detrimental clasp forces on the abutment teeth
why rotation of the RPD towards the residual ridge around the horizontal fulcrum line
- difference in potential movement of abutment and mucoperiosteum over residual ridge
- 0.25mm periodontal ligament, 2 mm mucoperiosteum
- 0.25mm = 0.01’
where should distal abutment retentive clasp not be
anterior to terminal rest fulcrum line
describe a distal rest/ distal guide plane-
- class I lever
-torquing force on abutment
describe mesial rest/ distal guide plane
- class II lever
- retentive tip disengages abutment
- stress release clasp design
what does anterior placement of the rests help
direct the forces more verticaally onto the tissues of the residual ridge
when are stress release clasps used
on class I and II RPDs distal abutmens
what are the types of stress release clasps in order of most favorable to least
- RPI clasp assembly
- RPC class assembly
- combination clasp
- 1/2 T clasp
- reverse circumferential clasp
describe the RPI clasp assembly
- Rest (mesial), Proximal plate (next to edentulous area, I bar (reciprocal clasp type)
- mesiofacial or mid facial retentive undercut
- reciprocation: ML minor connector, distal proximal plate
what are the prepared guiding planes in RPI clasp assembly
- distal guiding plane: occlusal 1/3-1/2
- 2-3 mm reduction needed
- guide plate contacts 1-1.5mm of plane
- undercut is necessary below the guide plane
- ML minor connector GP: occlusal 1/3
-1-2mm reduction needed
what are the structural details of the RPI clasp assembly
- I bar 2mm contact at retentive undercut
- approach arm: 3mm from gingival margin, crosses gingival margin at 90 degreees
- distal proximal plate width: line angle to line angle
- major connector: 3mm from gingival margin on mandible, 6mm from gingival margin on maxilla
what is the stress release on RPI clasp assembly
- rotation around mesial rest/ fulcrum
- distal guide plate and I bar move mesiogingivally: I bar disengages from tooth
- ML minor connector rotates mesio occlusally
what is the stress relief design criteria for RPI clasp assembly
- relief below distal plate, undercut below guide plane: allow proximal plate movement
- ML minor connector not contact adjacent tooth to allow rotation
what is the design criteria for RPI clasp assembly
- need 5mm separation between vertical minor connector
- if not 5mm then should use lingal plate
when is RPI clasp assembly contraindicated
-excess tissue undercut
- shallow vestibule bc tissue impingement
- abutment excess facial inclination: high survey line, retentive undercut too high ( not in gingival 1/3)
describe RPC clasp assembly
-Rest (mesial), proximal plate, circumferential clasp
- MF undercut
- circumferntial retentive clasp: cast round for Class I without modification and periodontally compromised teeth
- reciprocation: ML minor connector, distal proximal plate wraps to lingual
describe the structural details of RPC clasp assembly
- hybrid clasp: RPI and circumferential concepts
- similar criteria to circumferential retentive class: 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
describe the stress release on RPC clasp assembly
- rotation around mesial rest/fulcrum
- circumferential clasp and distal guide plate move mesiogingivally
- clasp disengages from tooth
- ML minor connector rotates mesio occlusally
what is the stress release design criteria for RPC clasp assembly
- relief below distal guide palte, 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
what happens in failure to meet stress release design criteria in RPC clasp assembly
-fulcrum moves to proximal area of clasp
- class I lever with detrimental force
describe the combination clasp
- distal rest: wrought wire retentive clasp, cast reciprocal clasp
- mesiofacial undercut
when is combination clasp indicated
mesial inclination of abutment distal surface
- no undercut below guide plane
- RPC, RPI no stress release
- mesial rest cannot be used
what is the stress release for combination clasp
- rotation occurs at distal rest minor connector
- clasp moves occlusally
- class I lever
- distal torquing force occurs
- flexibility of wrought wire limits torque
describe the modified 1/2 T clasp
- mesial rest, distal proximal plate, 1/2 T bar clasp
- reciprocation: ML minor connector, distal plate
when is modified 1/2 T clasp indicated
DF retentive undercut
when is modified 1/2 T clasp contraindicated
- tissue undercut
- shallow vestibule
- abutment excess facial inclination
- high survey line
- retentive undercut too high - not in gingival 1/3
describe the reverse circumferential clasp
- Mesial rest, distal guide plate, reverse cast circumferential retentive clasp
- reciprocaton: ML minor connector, distal plate
when is reverse circumferential clasp indicated
-DF retentive undercut
- 1/2 T clasp is contraindicated
describe general tooth supported RPD clasp selection
- rests adjacent to edentulous area
- clasp by convenience: no fulcrum lines, no rotation, determine by site of retentive undercut, tissue undercut
- reciprocation rules still apply
describe general tooth tissue supported RPD clasp selection
- mesial rests preferred
- stress release clasps used to accomodate for rotation toward residual ridge around horizontal fulcrum
- reciprocation still needed: often ML minor connector and distal plate rather than circumferential reciprocal clasp
