Direct Retainers Part 2 Flashcards
1
Q
what is the clasp types for suprabuldge and where is it located
A
- circumferential, akers, circlet ( synonyms)
- retentive arm approaches undercut from above the survey line
2
Q
what is the clasp type for infrabuldge and where is it located
A
- retentive arm approaches undercut from below survey line
- bar clasps
3
Q
when are bar clasps indicate
A
- distal extension RPD
- tooth supported RPD, esthetics facor
4
Q
what are the most common bar clasps
A
- I bar clasp
- 1/2 T bar clasp
5
Q
what are the advantages and disadvantages to bar clasps
A
- 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
6
Q
what are the bar clasp rules of the approach arm
A
- 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
7
Q
what falls under the category of tooth supported RPDs
A
- class III RPD, short span class IV
8
Q
what falls under the category of tooth tissue supported RPDs
A
-class I and II RPD, long span Class IV
9
Q
what are the tooth supported RPD clasps in order of most favorable to least
A
- circumferential
- I bar
- 1/2 T bar
- reverse circlet (hairpin)
- embrasure clasp
10
Q
when is circumferential clasp used
A
- most common
- retentive undercut remote from edentulous area
- used as reciprocal clasp
11
Q
when is I bar used and what prevents use
A
- Mesiofacial, midfacial undercut
- used as reciprocal clasp
- tissue undercut prevents use
12
Q
when is 1/2 T bar used and what prevents its use
A
- retentive undercut adjacent to edentulous area
- distal facial undercut
- reciprocal clasp
- tissue undercut prevents use
13
Q
when do you use reverse circlet (hairpin) and what is its disadvantage
A
- 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
14
Q
when is embrasure clasp used-
A
- 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
15
Q
what is the fulcrum line
A
an imaginary line connecting occlusal rests around which a partial removable dental prosthesis tends to rotate under masticatory forces
16
Q
what are the determinants for the fulcrum line
A
the cross arch occlusal rests located adjacent to the tissue borne components
17
Q
describe what happens in rotation of the RPD toward the residual ridge around the horizontal fulcrum line
A
- factor in class I and II RPDs
- RPD functions as a lever
- can result in detrimental clasp forces on the abutment teeth
18
Q
why rotation of the RPD towards the residual ridge around the horizontal fulcrum line
A
- difference in potential movement of abutment and mucoperiosteum over residual ridge
- 0.25mm periodontal ligament, 2 mm mucoperiosteum
- 0.25mm = 0.01’
19
Q
where should distal abutment retentive clasp not be
A
anterior to terminal rest fulcrum line
20
Q
describe a distal rest/ distal guide plane-
A
- class I lever
-torquing force on abutment
21
Q
describe mesial rest/ distal guide plane
A
- class II lever
- retentive tip disengages abutment
- stress release clasp design
22
Q
what does anterior placement of the rests help
A
direct the forces more verticaally onto the tissues of the residual ridge
23
Q
when are stress release clasps used
A
on class I and II RPDs distal abutmens
24
Q
what are the types of stress release clasps in order of most favorable to least
A
- RPI clasp assembly
- RPC class assembly
- combination clasp
- 1/2 T clasp
- reverse circumferential clasp
25
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
26
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
27
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
28
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
29
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
30
what is the design criteria for RPI clasp assembly
- need 5mm separation between vertical minor connector
- if not 5mm then should use lingal plate
31
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)
32
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
33
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
34
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
35
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
36
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
37
describe the combination clasp
- distal rest: wrought wire retentive clasp, cast reciprocal clasp
- mesiofacial undercut
38
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
39
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
40
describe the modified 1/2 T clasp
- mesial rest, distal proximal plate, 1/2 T bar clasp
- reciprocation: ML minor connector, distal plate
41
when is modified 1/2 T clasp indicated
DF retentive undercut
42
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
43
describe the reverse circumferential clasp
- Mesial rest, distal guide plate, reverse cast circumferential retentive clasp
- reciprocaton: ML minor connector, distal plate
44
when is reverse circumferential clasp indicated
-DF retentive undercut
- 1/2 T clasp is contraindicated
45
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
46
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
