direct retainers part 1 Flashcards
resist disloding forces
RPD retention
mechanical from retaining elements on abutment teeth
primary retention
-intimate contact of minor connector with guide planes
-intimate contact of denture base and maxillary major connector with underlying tissues
secondary retention
RPD component used to retain and prevent dislodgement
direct retainer
two types of direct retainers
intracoronal
and
extracoronal
-internal precision attachment: key/keyway
-most esthetic
(type of direct retainer)
intracoronal direct retainer
disadvantages of intracoronal direct retainers
require crown
complex fabrication
high maintenance
-clasp assembly
-more commonly used
(type of direct retainer)
extracoronal
retentive arm approaches undercut from above the survey line
suprabuldge
type of suprabulge clasps:
type of infrabulge clasps:
suprabulge:
circumferential, akers, circlet
infrabulge:
bar clasps
retentive arm approach undercut from below survey line
infrabulge
properly constructed suprabulge or infrabulge clasp assembly must incorporate the following components:
- rest
- retentive clasp
- reciprocal clasp
- and one or more minor connector
6 clasp assembly components
- rest
- reciprocal components
- retentive clasp arm
- retentive terminal
- minor connector
- approach arm
this component lies on occlusal, lingual, incisal surface
-prevents tissueward movement of clasp
rest
4 types of rests
- occlusal rest
- cingulum chevron rest
- lingual ball rest
- incisal rest
-rigid component
-above height of contour
-reciprocal clasp, lingual plate, proximal plate, guide plate (minor connectors)
reciprocal component of clasp assembly
what part of the retentive clasp is in proximal 2/3, rigid portion, and above height of contour
retentive clasp arm
what part of the retentive clasp is in distal 1/3, flexible, below height of contour, and provides direct retention
retentive terminal (clasp tip)
part of clasp assembly that joins body of clasp to framework; rigid
minor connector
what clasp assembly component joins body of clasp to framework; rigid
minor connector
what clasp assembly component of bar clasp connects clasp to framework and is flexible minor connector (exception to the rule)
approach arm
6 requirements of clasp assembly
- support
- stability
- encirclement of more than 180 degrees
- reciprocation
- passivity
- retention
this requirement of clasp assembly resist vertical movement toward tissue
(rests)
support
this requirement of clasp assembly resists horizontal displacement, areas above the survey line, all rigid components of direct retainer
stability
this requirement of clasp assembly may be continuous or broken, minimum of 3 points used, prevents tooth from moving away from the clasp
encirclement of more than 190 degrees
this requirement of clasp assembly
-reciprocal component contact tooth at the 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
reciprocation
this requirement of clasp assembly components of direct retainer exert no force when fully seated
passivity
this requirement of clasp assembly 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
this portion of the tooth must be undercut in relation to path of prosthesis insertion
retention portion
retention factors:
- angle of convergence of the undercut
- depth of undercut
- flexibility of the retentive clasp
when should the retentive clasps should become active
ONLY when disclosing forces are applied to them
retentive clasp ability to provide retention is based on
the resistance of the metal to deformation
retentive clasp flexibility factors
1
2
3
4
5
- length
double length, 8x MORE flexible [increase length, curve clasp] - diameter
double diameter, 8x LESS flexible - taper
-clasp terminal 1/2 as thick as origin (1mm to 0.5mm)
-clasp terminal 1/2 as wide as origin (2mm to 1mm) - cross-sectional form
round clasp more flexible than half-round
-round flexes in all spatial planes
-half round flexes in one plane - material
cast chromium<cast gold<wrought
cross sectional shapes
1. this almost always as in circumferential clasps= flexes in only one direction
2. as in wrought wire clasps=flexes omnidirectionally and provides max flexibility
- half round
- round
with increasing clasp flexibility, must use______ undercuts to obtain equal retention
deeper
0.010 cast chrome
0.015 cast gold
0.020 wrought
what is the alloy used in casting the framework
usually ticonium (NiCr) or vitallium (CoCr)
-easy to fabricate
this is produced by drawing metal through dies of smaller diameter and most flexible of all clasps
wrought wire
the design of a retentive clasp arm should incorporate maximum clasp _____for optimal flexibility
length
-approaches undercut from above the survey line
-Pulls toward occlusal to resist displacement
(retentive qualities for what type of clasp)
circumferential clasp
-approaches undercut from below survey line
-pushes towards occlusal to resist displacement
-easier to seat, more difficult to remove than circumferential clasp
(retentive qualities for what type of clasp)
bar clasp
this clasp must be rigid, shaped different than flexible retentive clasp, and not tapered
reciprocal clasp
the means by which one part of the framework opposes the action of the retainer in function
reciprocation
how may the reciprocation be achieved by
-rigid plating (lingual plate)
-minor connectors
-guid planes extended around the vertical line of abutments
-reciprocal clasp arms
-contact areas of proximal teeth
this is the most logical clasp for tooth-supported RPD (kennedy class III)
-excellent stabilization, support bracing
-easiest to design and construct
-easier to repair
circumferential clasps
what are the disadvantages of circumferential clasps
-covers more tooth surfaces: more conducive to caries
-increases tooth circumference: cause greater occlusal force exerted on teeth
-more difficult to adjust
why keep the circumferential clasp as low on crown as survey line permits
-less torquing action on tooth
-less interference with occlusion
-if survey line too high, enamel surface must be recontoured to lower survey line
if the survey line is too low
surveyed crown required
this clasp is cast 1/2 round
-proximal 2/3 in middle 1/3 above survey line
-terminal 1/3 in gingival 1/3 below survey lines and tip curves to occlusal
*tapers from origin 2mm to tip 1mm
retentive clasp
this clasp is cast 1/2 round
-entire clasp in middle 1/3 above survey line
-no taper: uniform width 1.5-2mm
reciprocal clasp
what are the 4 types of circumferential clasps
- circlet
- embrasure clasp
- ring clasp
- combination clasp
this circumferential clasp
-engage undercut remote from edentulous area
-choice of class III RPD
circlet
this circumferential clasp
-2 circlet clasps joined at body
-used when retainer is necessary in dental 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
embrasure clasp
this circumferential clasp
indication: mesiolingually-tipped mandibular molar class III RPD
-engages mesiolingual undercut
-bracing arm for reciprocity
additional occlusal rest
-avoid if possible
ring clasp
this circumferential clasp
indication: retentive undercut adjacent to edentulous space
-class III RPD
-tooth-tissue supported RPD (class I or II distal extension)
-1/2 T bar clasp preferred, if no tissie undercut at approach arm
-avoid if possible
reverse circlet clasp
this circumferential clasp
-wrought round wire retentive clasp
-cast 1/2 round reciprocal clasp
combination clasp
this circumferential clasp
indications: distal extension RPD (class I or II)
-mesial rest not possible, distal rest must be used
-mesiofacial undercut
-wrought wire soldered to framework
combination clasp
RPC clasp assembly (RPA)
Rest (mesial), Proximal Plate, Circumferential (akers) clasp
what are the indications for RPC clasp assembly
-class I or II RPDS
-mesiofacial retentive undercut
for RPCs, the circumferential retentive clasp is ALWAYS:
except in periodontally compromised teeth and Kennedy I w/o modification: cast round
cast 1/2 round
what are the reciprocations in RPCs
distal proximal plate and ML minor connector
