Direct Retainers Part 1 Flashcards
Intracoronal Direct Retainers
Advtantages:
+Internal attachment
+Most esthetic
Disadvantages:
- Costly
- Requires Crown
- Complex fabrication (not all dental labs can process)
- High Maintenance
Extracoronal Direct Retainers
Clasp Assembly
More commonly used
Extracoronal Clasp Types
Suprabulge
&
Infrabulge
Suprebulge
Top portion above the contour of the abutment type
Retentive arm approaches undercut from above the height of the contour (above the survey line)
Circumferential, Akers, Circlet
Infrabulge
The retentive arm approaches the undercut from BELOW the survey line
Has Bar Clasps
******Where are these located: Rest Reciprocal Clasp Retentive Clasp Proximal Plate
go over this question again
Rest ~ on the occlusal
Reciprocal Clasp - on the lingual
Retentive Clasp ~ on the buccal
Proximal Plate (guide plate) ~ type of minor connector
Clasp Assembly
Lies on the:
occlusal, lingual, or incisal surface
~Prevent tissue ward movement of clasp
Reciprocal Components
~Rigid Component
~Above height of contour
~Reciprocal Clasp, Lingual Plate, Proximal Plate, Guide Plate (minor connectors)
Retentive Clasp Arm
~Proximal 2/3 of tooth (which is rigid)
~Rigid portion, above the height of the contour
Retentive Terminal
~Distal 1/3
~Is Flexible
~Below the height of contour
~Provides a direct retention
Minor Connector
~Joins the body of the clasp to the framework
~Is Rigid
Approach Arm
~Is a component of the bar clasp
~Connects the clasp to the framework
~Flexible minor connector
~Exception to the rule
What are the requirements of Clasp Assembly?
- Support
- Stability
- Encirclement of more than 180 degrees
- Reciprocation
- Passivity
- Retention
What is Support for Clasp Assmebly?
~Resists vertical movement toward tissues
~Rests
What is Stability for Clasp Assembly?
~Resists horizontal displacement
~Areas above the survey line
~All rigid components of direct retainer
What is Encirclement?
– May be continuous or broken
– Minimum of 3 points used
– Prevents tooth from moving away from the
clasp
What is Reciprocation?
– 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
What is Passivity?
– Components of direct retainer exert no force
when fully seated
What is Retention?
– 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 of tooth must be _____ in relation
to path of prosthesis insertion
undercut
Retention amount minimum necessary to resist
reasonable ____
dislodging forces
Undercut must always
come from below the height of the contour
1mm from the gingival margin on the apical 1/3
What are the 3 Retention Factors?
• Angle of convergence of the undercut
• Depth of undercut
– 0.010”, 0.020”, 0.030”
(undercuts we are looking for)
• Flexibility of the retentive clasp
~Retention should not be too much or too little~
Retentive Clasp
• Retentive clasps should become active ONLY
when disclosing forces are applied to them.
• Their ability to provide retention is based on
the resistance of the metal to deformation
Longer Cusp arm =
more flexible
Retentive Clasp Flexibility Factors
- Length
- Diameter
- Taper
- Cross-sectional form
- Material
Retentive Clasp Flexibility Factors
Length
– Flexibility = length3
• Double length,
8x more flexible
– Increase length, curve clasp
Retentive Clasp Flexibility Factors
Diameter
– Flexibility = 1/diameter3
• Double diameter,
8x less flexible
Retentive Clasp Flexibility Factors
Taper
– Clasp terminal ½ as thick as origin
• 1mm to 0.5 mm
– Clasp terminal ½ as wide as origin
• ~2mm to 1mm
Retentive Clasp Flexibility Factors
Cross-sectional form
– Round clasp more flexible than halfround
• Round flexes in all spatial planes
• Half-round flexes in one plane
Retentive Clasp Flexibility Factors
Material
– Cast chromium alloys less flexible than cast gold
alloys
– Wrought alloys more flexible than cast alloy
– Cast Chromium
What alloy is used in casting the Clasp Framework?
Usually Ticonium (NiCr) or Vitallium (CoCr)
Is easy to fabricate
How is Wrought wire produced?
‣ Produce by drawing metal through dies of smaller
diameter
Wrought wire is ….
the most flexible of all clasps
What are other Clasp Materials?
Stainless steel, type IV gold
The design of a retentive clasp arm should be ____
‣ Produce by drawing metal through dies of smaller
diameter
Circumferential Clasp should ____
– Approach undercut from above the survey line
– “Pull” toward occlusal
to resist displacement
The Bar Clasp ____
– Approaches undercut from below survey line
– “Pushes” toward occlusal to resist displacement
– Easier to seat, more difficult to remove than
circumferential clasp
The Reciprocal Clasp must be ____
Rigid
The Reciprocal Clasp is
is shaped different than the flexible retentive clasp
and Not tapered
What is Reciprocation?
Reciprocation is the means by which one part of the
framework opposes the action of the retainer in
function.
How can Reciprocation be achieved?
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
Circumferential Clasps are the most logical clasp for
tooth-supported RPD (CLASS III)
What are the advantages of Circumferential CLasps?
~Excellent Stabilization, support and bracing
~Is the easiest to design and construct
~Is the easiest to repair
What are the disadvantages of Circumferential Clasps?
~Covers more tooth surface; is 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 is kept ____
as low on the crown as the survey line permits
because of:
~Less torquing action on tooth
~Less interference with occlusion
~If the survey line is too high, the enamel surface must be recontoured to lower the survey line
~the survey line being too low, so surveyed crown will be required
Describe a Retentive Circumferential Clasp
– 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)
Describe a Reciprocal Circumferential Clasp
• Reciprocal clasp
– 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
What are the types of Circumferential Clasps?
- Circlet
- Embrasure clasp
- Reverse Circlet (hairpin) clasp
- Ring clasp
- Reverse Circlet clasp
- Combination Clasp
- RPC clasp
What is Circlet Circumferential Clasp?
• Circumferential clasp of choice Class III RPD
• Engage undercut remote from edentulous
area
What is Embrasure Circumferential Clasp?
• 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
What is Reverse Circlet (Hairpin) Circumferential Clasp?
• 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
What is Ring Circumferential Clasp?
• 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
What is a Reverse Circlet Circumferential Clasp?
• Indication: 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
• Difficult to obtain adequate occlusal clearance
• Avoid, if possible
What is Combination Circumferential Clasp?
• 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
What is RPC Clasp (RPA)?
• 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
