direct retainers Flashcards
RPD Retention, resists? accomplished with?
resist dislodging forces
accomplished with primary and secondary retention
primary retention
– Mechanical from retaining elements on abutment
teeth
secondary retention accomplished with?
– Intimate contact of minor connector with guide planes
– Intimate contact of denture base & maxillary major
connector with underlying tissues
what does a direct retainer do
RPD component used to retain and prevent
dislodgement
types of direct retainers location relative to crown
intra and extracoronal
intracoronal direct retainer
– Internal precision attachment: key/keyway
– Most esthetic
– Disadvantages: require crown,
complex fabrication, & high maintenance
extracoronal direct retainer
– Clasp assembly
– More commonly used
clasp types
supra and infrabulge
suprabulge clasp, names/types
– Retentive arm approaches undercut from above the
survey line
– Circumferential, Akers, Circlet
infrabulge clasp
– Retentive arm approaches undercut from below survey
line
– Bar clasps
A properly constructed suprabulge or infrabulge clasp assembly must incorporate the following components:
a rest (A),
a retentive clasp (B),
a reciprocal element (C),
and one or more minor connectors (D).
Specific design features of the various components include vertical and horizontal approach arms, clasp
termini, clasp bodies, and clasp shoulders.
rest
Rest: lies on occlusal, lingual, incisal surface
– Prevent tissueward movement of clasp
reciprocal components
flexible?
location relative to contour?
types?
– Rigid component, no flex
– Above height of contour
– Reciprocal clasp, Lingual plate, Proximal plate, Guide plate (minor connectors)
Retentive clasp arm/ retentive terminal
Retentive clasp arm
• Proximal 2/3
• Rigid portion, above height of contour
Retentive terminal
• Distal 1/3
• Flexible
• Below height of contour
• Provides direct retention
Minor Connector
Minor Connector
• Joins body of clasp to framework
• Rigid
Approach arm
•
infrabulge assemblies
• Component of bar clasp
• Connects clasp to framework
• Flexible minor connector- exception to rule of minor connectors
required qualities of clasp assemblies
- Support
- Stability
- encirclement
- reciporcation
- passivity
- retention
support of clasp assemblies
Resist vertical movement toward tissue
– Rests
stability of clasp assemblies resist?
what areas?
what portions of RPD?
– Resist horizontal displacement
– Areas above the survey line
– All rigid components of direct retainer
encirclement of clasp assemblies
– May be continuous or broken
– Minimum of 3 points used
– Prevents tooth from moving away from the clasp
Reciprocation of clasp assemblies
when does these contact tooth during insertion?
how about when retentive clasp passes over HOC?
what do these resist?
requires what type of POI?
– Reciprocal component contact tooth at same time or sooner than retentive clasp
– Remain in contact while retentive tip passes over height of contour
– Resist retentive tip lateral forces
– Requires parallel surface to path of insertion, possible preparation of tooth to accomplish
passivity of clasp assemblies
components of direct retainer exert no force when fully seated
retention of clasp assemblies
– Resists displacement of RPD away from supporting tissues
– Area below survey line: ideally in gingival 1/3
• Clasp arm tip: Terminal 1/3 below survey line
retention portion must be where in relation to POI? why?
Retention portion of tooth must be undercut in relation to path of prosthesis insertion
Retention amount minimum necessary to resist reasonable dislodging forces
Retention Factors
• Angle of convergence of the undercut
• Depth of undercut
• Flexibility of the retentive clasp
undercut depth required?
requires proper depth for cusp arm retention
when are retentive clasps active
ONLY when disclosing forces are applied to them.
retentive clasp ability to provide retention based on
Their ability to provide retention is based on
the resistance of the metal to deformation.
retentive clasps flexibilty factors
length
diameter
taper
cross sectional form
material
retentive clasp length and flex
– Flexibility= length^3
• Double length, 8x more flexible
– Increase length, curve clasp
retentive clasp diameter and flex
– Flexibility = 1/diameter^3
• Double diameter, 8x less flexible
taper and flex of rententive clasp
In 2 dimensions
– Clasp terminal ½ as thick as origin
• 1mm to 0.5 mm
– Clasp terminal ½ as wide as origin
• ~2mm to 1mm
cross sectional form and flex
– Round clasp more flexible than half-round
• Round flexes in all spatial planes
• Half-round flexes in one plane
material and flexibilty, depth?
Alloy used in casting the framework, pro?
‣ Usually Ticonium (NiCr) or Vitallium (CoCr)
‣ Easy to fabricate
Wrought wire
Produce by drawing metal through dies of smaller diameter
‣ Most flexible of all clasps
desinged length of retentive clasp, how much should we use?
should use maximum length possible
circumferential retentive qualties
approaches undercut from?
how does it resist displacement?
– Approaches undercut from above the survey line
– “Pulls” toward occlusal to resist displacement
Bar clasp retentive qualities
how does it approach the survey line?
how does it resist displacement?
seating and removal compared to circumferential clasp?
– Approaches undercut from below survey line
– “Pushes” toward occlusal to resist displacement
– Easier to seat, more difficult to remove than circumferential clasp
reciporacal clasp; rigid? shape? taper?
• Must be rigid
• Shaped different than flexible retentive clasp
• Not tapered
Reciprocation definedm
Reciprocation is the means by which one part of the
framework opposes the action of the retainer in
function.
Reciprocation may be achieved by what components?
• Rigid plating (LINGUAL PLATE)
• Minor connectors
• Guide planes extended around the vertical line of abutments
• Reciprocal clasp arms
• Contact areas of proximal teeth
Circumferential Clasps; most logical for what class? advantages?
• Most logical clasp for tooth-supported RPD (Class III)
• Advantages
– Excellent stabilization, support, bracing
– Easiest to design & construct
– Easier to repair
Circumferential Clasps Disadvantages
– Covers more tooth surface: more conducive to caries by more plaque retention
– Increases tooth circumference
• May cause greater occlusal force exerted on tooth
– More difficult to adjust
Circumferential Clasp Configuration; clasp kept where? why? survey line too low/high?
Circumferential Clasp Configuration
• Clasp kept as low on crown as survey line permits
– Less torquing action on tooth
– Less interference with occlusion
– If survey line is too high, enamel surface must be recontoured to lower the survey line
– Survey line too low, Surveyed crown required
Circumferential Clasp Configuration of Retentive clasp
shape? (circle or 1/2 round)
– Cast ½ Round
– Proximal 2/3
• In middle 1/3 of tooth
• Above the survey line
-Terminal retentive 1/3
• In gingival 1/3- Below survey line
• Tip curves to occlusal
– Tapers from origin (2.0mm) to tip (1.0 mm)
Circumferential Clasp Configuration of Reciprocal clasp
shape?
location?
taper?
– Cast ½ round
– Entire clasp in middle 1/3
– Above the survey line
• If survey line too high, must recontour
• Prepare surface parallel to path of insertion
– No taper: Uniform width 1.5 –2.0 mm
Types of Circumferential Clasps
- Circlet
- Embrasure clasp
- Reverse Circlet (hairpin) clasp
- Ring clasp
- Reverse Circlet clasp
- Combination Clasp
- RPC clasp
Circumferential Clasps: Circlet
used with which class RPD?
engage undercut from where?
• Circumferential clasp of choice Class III RPD
• Engage undercut remote from edentulous
area
Circumferential Clasps: Embrasure Clasp
used when? classes?
requires?
inadequate prep?
• 2 circlet clasps joined at body
• Used when retainer is necessary in a dentate area
– Class III with no modification space
– Class II with no modification space
– Class IV
• Requires heavy reduction of abutment teeth for
adequate clasp bulk
– Inadequate preparation, clasp fracture common
Circumferential Clasps: Reverse Circlet (Hairpin Clasp)
disadvantages?
indication?
• Engage undercut adjacent to edentulous area Class III RPD
• Covers extensive tooth surface, increased caries risk
• Difficult to fabricate
• Indication: Mesially-tipped mandibular molar with mesiofacial undercut
– ½ T preferred for this situation, if no tissue undercut at approach arm
• “Last choice” for mesially-tipped molars
Circumferential Clasps: Ring Clasp
indications?
engage what kind of undercut?
reciprocation from?
what additional component prevents further tilt?
when to avoid this?
• Indication: Mesiolingually-tipped mandibular molars Class III RPD
• Engage mesiolingual undercut
• Auxillary bracing arm for reciprocity
• Additional occlusal rest:
– support, prevent further mesial movement
• Avoid if possible
• Incorrect Ring clasp: No reciprocation, No support
Circumferential Clasps: Reverse Circlet Clasp
indication?
kennedy class(es)?
what is preferred? what can prevent this?
difficult to obtain?
avoid?
• Indication: Retentive undercut adjacent to edentulous space
– Class III RPD or Tooth-tissue supported RPD (Class I or II distal extension)
– ½ T bar clasp preferred, if no tissue undercut at approach arm
• Difficult to obtain adequate occlusal clearance
• Avoid, if possible
Circumferential Clasps: Combination Clasp
indications?
undercuts?
rest?
clasps?
• Wrought round wire retentive clasp
• Cast ½ round reciprocal clasp
• Indication: Distal extension RPD (Class I & II)
– Mesial rest not possible, distal rest must be used
– Mesiofacial undercut
• Wrought wire soldered to framework
RPC Clasp (RPA)
indications?
engage what undercuts?
clasp form?
reciprocation from?
• Rest (mesial), Proximal Plate, Circumferential (Akers) clasp
• Indication:
– Class I, II RPDs
– Mesiofacial retentive undercut
• Circumferential retentive clasp
– Cast ½ round (ALWAYS) except in periodontally compromised teeth, cast round.
• Reciprocation:
– Distal proximal plate
– ML minor connector
