Direct Retainers Flashcards
What is primary retention?
Mechanical from retaining elements on abutment teeth
What is secondary retention?
– Intimate contact of minor connector with guide planes
– Intimate contact of denture base & maxillary major connector with underlying tissues
What is a direct retainer?
RPD component used to retain and prevent dislodgement
What are the two clasp types?
- suprabulge
- infrabulge
What is a suprabulge clasp?
Retentive arm approaches undercut from above the survey line
What is the infrabulge clasp?
Retentive arm approaches undercut from below survey line
What are synonyms for suprabulge clasps?
Circumferential, Akers, Circlet
What is another name for an infrabulge clasp?
bar clasps
A properly constructed suprabulge or infrabulge clasp assembly must incorporate the following components:
- a rest
- a retentive clasp
- a reciprocal element
- one or more minor connectors
What is the rest component of the clasp?
lies on occlusal, lingual, incisal surface
– Prevent tissueward movement of clasp
What is the reciprocal component of the clasp?
– Rigid component
– Above height of contour
What are the different parts of the reciprocal component?
Reciprocal clasp, Lingual plate, Proximal plate, Guide plate
On what part of the tooth is the retentive clasp arm?
proximal 2/3
- rigid part, above the height of contour
On what part of the tooth is the retentive terminal?
distal 1/3
- flexible, below the height of contour
What provides direct retention: the retentive clasp arm or the retentive terminal?
retentive terminal
What is the minor connector?
- Joins body of clasp to framework
- Rigid
What is the approach arm?
- Component of bar clasp
- Connects clasp to framework
- Flexible minor connector
Minor connectors are usually rigid. What part of the clasp is an exeption to this rule?
approach arm
- is flexible rather than rigid
What are the requirements of clasp assembly?
- support
- stability
- encirclement of more than 180 degrees
- reciprocation
- passivity
- retention
Why does the clasp need to encircle more than 180 degrees?
Prevents tooth from moving away from the clasp
How many points do you need for a clasp asembly?
minimum of 3
Does the clasp encirclement of 180 degrees need to be continous?
– May be continuous or broken
What allows for the clasp to have stability?
– Areas above the survey line
What is reciprocation of the clasp?
– Resist retentive tip lateral forces
– Requires parallel surface to path of insertion
___________ component contact tooth at same time or sooner than retentive clasp
Reciprocal
What is passivity of the clasp?
– Components of direct retainer exert no force when fully seated
Where on the tooth does the clasp allow for retention?
Area below survey line: ideally in gingival 1/3
Retention portion of tooth must be ___________ in relation to path of prosthesis insertion
undercut
What determines the amoutn of retention?
- Angle of convergence of the undercut
- Depth of undercut
- Flexibility of the retentive clasp
Retentive clasps should become active ONLY when…
disclosing forces are applied to them
The ability of the retentive clasp to provide retention is based on…
the resistance of the metal to deformation
What are the flexibility factors for the retentive clasp?
- length
- diameter
- taper
- cross-sectional form
- material
How is flexibility related to length of the retentive clasp?
Flexibility = length^3
- Double length, 8x more flexible
How is diameter related to flexibility of the retentive clasp?
Flexibility = 1/diameter^3
- Double diameter, 8x LESS flexible
The clasp terminal needs to taper in how many dimensions?
2
– Clasp terminal ½ as thick as origin
* 1mm to 0.5 mm
– Clasp terminal ½ as wide as origin
* ~2mm to 1mm
How thick and how wide shoudl the clasp terminal be compared to the origin?
– Clasp terminal ½ as thick as origin
* 1mm to 0.5 mm
– Clasp terminal ½ as wide as origin
* ~2mm to 1mm
Round clasp _______ flexible than half-round
more
Round clasp flexes in _____ planes
Half-round flexes in ____ planes
- Round flexes in all spatial planes
- Half-round flexes in one plane
Half-round clasps are almost always…
circumferential clasps
Half-round clasps flexes in how many directions?
1
Round clasps are usually…
wrought wire clasps
Round clasps flex in how many directions?
omnidirectionally
‣ Provides maximum flexibility
Cast chromium alloys are ______ flexible than cast gold alloys
less
Wrought alloys ______ flexible than cast alloy
more
With increasing clasp flexibility, must use _______ undercut to obtain equal retention
deeper
What are the undercuts for the differently alloys?
- 0.010”: Cast Chrome alloy
- 0.015”: Cast Gold alloy
- 0.020”: Wrought wire
What alloy is used in casting the framework?
‣ Usually Ticonium (NiCr) or Vitallium (CoCr)
‣ Easy to fabricate
How do you produce a wrought wire clasp?
Produce by drawing metal through dies of smaller diameter
What is the most flexible of all clasps?
wrought wire
What are the different options for clasp material?
wrought wire
- also stainless steel and type IV gold
The design of a retentive clasp arm should incorporate maximum clasp length for…
optimal flexibility
What is a circumferential clasp?
– Approaches undercut from above the survey line
– “Pulls” toward occlusal to resist displacement
What is the bar clasp?
– Approaches undercut from below survey line
– “Pushes” toward occlusal to resist displacement
Which clasp is easier to seat but more difficult to remove?
bar clasp
Should the reciprocal clasp be rigid or flexible?
rigid
- not tapered
What is the definition of reciprocation?
the means by which one part of the framework opposes the action of the retainer in function
Reciprocation may be achieved by…
- Rigid plating (LINGUAL PLATE)
- Minor connectors
- Guide planes extended around the vertical line of abutments
- Reciprocal clasp arms
- Contact areas of proximal teeth
What is the most logical clasp for tooth-supported RPD (kennedy class III)?
Circumferential Clasps
What are the advantages of the circumferential clasps?
– Excellent stabilization, support, bracing
– Easiest to design & construct
– Easier to repair
What are the disadvantages of the circumferential clasps?
– Covers more tooth surface: more conducive to caries by more plaque retention
– Increases tooth circumference/greater occlusal force exerted on tooth
– More difficult to adjust
What is the ideal configuration of the circumferential clasp?
- Clasp kept as low on crown as survey line permits
- Retentive clasp
- Reciprocal clasp
Why should a circumferentail clasp be kept as low as the survey line permits?
– Less torquing action on tooth
– Less interference with occlusion
If survey line is too high for a circumferential clasp…
enamel surface must be recontoured to lower the survey line
If survey line is too low for a circumferential clasp…
surveyed crown required
What is the ideal configuration of the retentive clasp for the circumferential clasp?
– Cast ½ Round
– Proximal 2/3 (above survey line)
– Terminal retentive 1/3 (below survey line)
– Tapers from origin (2.0mm)
to tip (1.0 mm)
What is the ideal configuration of the reciprocal clasp for the circumferential clasp?
– Cast ½ round
– Entire clasp in middle 1/3
– Above the survey line
– No taper: Uniform width 1.5 – 2.0 mm
What are the different types of circumferential clasps?
- Circlet
- Embrasure clasp
- Ring clasp
- Combination Clasp
What is the best type of circumferential clasp for class III RPD?
circlet
What is an embrasure clasp?
2 circlet clasps joined at body
When do you use an embrasure type circumferential clasp?
- Used when retainer is necessary in a dentate area
– Class III with no modification space
– Class II with no modification space
– Class IV
When should you use a ring type circumferential clasp?
Mesiolingually-tipped mandibular
molars Class III RPD
What does a ring type circumferential clasp need?
Additional occlusal rest: support, prevent further mesial movement
* Incorrect Ring clasp: No reciprocation, No support
When should you use a reverse circlet type circumferential clasp?
Retentive undercut adjacent to edentulous space
– Class III RPD
– Tooth-tissue supported RPD (Class I or II distal extension)
– ½ T bar clasp preferred, if no tissue undercut at approach arm
What two types of circumferential clasps should you avoid if possible?
- ring clasp
- reverse circlet clasp
What is a combination type circumferential clasp?
- Wrought round wire retentive clasp
- Cast ½ round reciprocal clasp
When should you use a combination type circumferential clasp?
Distal extension RPD (Class I & II)
– Mesial rest not possible, distal rest must be used
– Mesiofacial undercut
What is the RPC class assembly (RPA)?
Rest (mesial), Proximal Plate, Circumferential (Akers) clasp
When should you use the RPC class assembly (RPA)?
– Class I, II RPDs
– Mesiofacial retentive undercut
The circumferential retentive clasp in the RPC class assembly (RPA) should be…
– Cast ½ round (ALWAYS)
– except in periodontally compromised teeth and Kennedy I w/o modification (use cast round for these)
