Direct and indirect retainers
Vinay Pavan Kumar .K 2nd year P G student
Dept of ProsthodonticsAECS Maaruti College of Dental Sciences
Direct retainers
Indirect retainers
Definition
Classification
Principles of design
Extracoronal retainers
Types
Definition
Factors affecting
types of indirect retainersRequirements
Direct retainer: It is that component of a
removable partial denture that is used to retain
and prevent dislodgment, consisting of a clasp
assembly or a precision attachment (GPT 8)
Intracoronal retainers
In 1906 the principle of the internal attachment was first formulated by Dr. Herman E.S. Chayes
Clasps mainly divided 2 types
Occlusally approaching which approach the undercut
from the occlusal area and gingivally approaching which
enter the undercut crossing the gingival margin.
Clasp Assembly
The part of a removable dental prosthesis that acts as a
direct retainer and/or stabilizer for a prosthesis by
partially encompassing or contacting an abutment tooth.
Components of the clasp assembly include the clasp,
the reciprocal element, the cingulum, incisal or occlusal
rest, and the minor connector.
Parts of clasp assembly
Circumferential Clasp(Retentive Arm)
Reciprocating(Bracing) Arm
DistalOcclusalRest Seat Proximal
Plate
Principles of Clasp Design
1. Encirclement- more than 180 degrees in greatest circumference if
the tooth engaged by the clasp assembly
2. Occlusal rest - to prevent the movement of the clasp arms
cervically.
3. Each retentive terminal should be opposed by a reciprocal
component
4. Clasp retainers on abutment teeth adjacent to distal extension bases should be designed to avoid direct transmission of forces to the abutment
5.The amount of retention should always be the minimum necessary to resist reasonable dislodging forces.
6. Reciprocal elements – junction of gingival and middle third
Terminal retentive arm – gingival third
Functional requirements of the clasp
Retention
Support
Stability
Reciprocation
Encirclement
Passivity
Retention
Is obtained by the incorporation of a flexible element of
the clasp into the undercut.
Sufficient undercut to be engaged to obtain desired
retention.
Force from the clasp arm on flexing must be within the
tolerance of the PDL and must be less to prevent
deformation of the clasp arm itself.
Factors affecting retention
◦Tooth factors: Size of the angle of cervical convergence
◦How far the clasp terminal is placed into the angle of cervical convergence
Length of the clasp
•The longer the clasp arm the more flexible.
• Flexibility is directly proportional to the cube of its length.
•By increasing the length, the horizontal stresses imparted to the abutment during placing and removal is reduced
Cross section: round > half round
Modulus of elasticity: more the modulus - less flexibilityDiameter of clasp: flexure inversely proportional to the
diameter.Alloy: wrought > cast
Support Support is the quality of the clasp assembly to resist
displacement of the prosthesis in the apical direction.
a rest must contact the surface of the abutment tooth at
a properly prepared surface- rest seat
A properly prepared rest will prevent the
tissueward movement of the prosthesis.
maintains the position of the clasp assembly in
relation to the abutment.
Transmits forces along the long axis of the
abutments
Reciprocation
Counteracts lateral displacement of an abutment when retentive clasp terminus passes over the height of contour
Passivity
Prevent the transmission of the adverse forces
to the associated abutment
Be passive until a dislodging force is applied
Classification of extra-coronal retainers
Supra bulge clasps (occlusally approaching, circumferential clasps)
Infrabulge clasps (gingivally approaching, projection or bar clasps)
Combination clasps
Circumferential clasps
The cast circumferential clasp design was introduced by
Dr N B Nesbitt in 1916.
Simple, easy to construct- excellent support, bracing,
retentive properties.
Close adaptation to tooth therefore minimises food
entrapment
Disadvantage- covers large amount of tooth surface
Circlet clasp.
Reverse circlet
Multiple circlet clasp
Embrasure clasp.
Reverse action or hair pin clasp
Ring clasp.
Back action and reverse back action clasp
Simple Circlet clasp
Tooth support RPD
Undercut remote from
edentulous area
Half round
Disadvantages
- Increase tooth coverage
- compromised esthetics
Variations of circlet clasp
Back action clasp
Reverse back action
Ring clasp
C clasp or hair-pin clasp
Reverse circlet clasp
Undercut located adjacent to edentulous area
Kennedy class I ,II
Disadvantages
- Lack of rest adjacent to edentulous area
- Poor esthetics
Multiple circlet design
2 simple circlet clasp joined at the terminal aspect of
their reciprocal elements
Principle abutment is periodontal compromised and the
forces are distributed between multiple abutment teeth
Embrasure clasp
2 simple circlet joined at bodies
Used on side of the arch where there is no edentulous
space
Can be used only when adequate tooth preparation is
possible
C-clasp design
Fish hook” or “Hairpin” claspSimple circlet clasp with loop back retentive armSufficient crown height
Disadvantages - Insufficient flexibility - Tooth coverage - Esthetics compromised
Combination claspCast metal reciprocal arm and wrought wire retentive
arm abutment adjacent to Kennedy class I and II area Advantage• kinder to the tooth can engage greater undercut
Disadvantage• more prone to breakage than cast• minimal stabilizing
Gingivally approaching clasps /Bar/Roach type
Approach the undercut gingivally and have a push type of retention.
Approach arm
• It is a minor connector that connect the retentive tip to the denture base.
• It crosses the gingival margin at right angle and it is the only flexible minor connector.
• Flexibility of the clasp is controlled by the taper and length of the approach arm
• More esthetic
T-claspKennedy class I and IIUndercut locate adjacent edentulous area
Contraindication - Severe soft tissue undercut - Height of contour locate near occlusal surface
Modified T-clasp
No retentive horizontal projectionKennedy class I and IIUndercut locate near adjacent edentulous areaCanine and premolarAdvantage - esthetics
Y-clasp
Equivalent to T-clasp
Approach arm terminates in the cervical third
Mesial and distal projection terminate near occlusal
surface
A comparative study on Co-Cr and Acetal resin clasps; Pal .H etal TPDI • January 2014, Vol. 5, No. 1 pg 9- 13
• Acetal resin clasps are esthetic and are available in sixteen different shades.
• To evaluate the effect of cast Co-Cr and acetal resin clasp on the surface of tooth.
• The retentive force of cast Co-Cr clasp showed a decrease from 12.4 N to 8.1 N.
• The retentive force of acetal resin clasp, reduced from 5.2 N to 4.03 N at the completion of experiment.
• Acetal resin clasps do not abrade the surface of tooth and maintain retention
Implants as direct retainers
• Eliminates a visible clasp
• placement of an implant within a modification space to the advantage of retentive needs requires consideration of anterior, mid, or distal placement
• retainers utilizing teeth have always been restricted to tooth locations at either end of a span
Indirect retainerResists rotational displacement of an extension base
from the supporting tissueKennedy class I, II and IV
Factors determining indirectretainer
Occlusal rests must be held in rest seats by direct
retainer
Distance from fulcrum line
Placed on definite rest seat to prevent slippage
Rigidity
Auxiliary functions
Reduce A-P tilting of abutments
Stabilization – auxiliary guide planes
Anterior teeth stabilized
Auxiliary rest – stress distribution
Visual indication for reline
Forms of indirect retainer
Auxiliary occlusal rest
Canine extension from occlusal rest
Continuous bar retainer & lingual plate
Rugae Support
References Carr AB, Mc Givney GP, Brown DT, McCracken’s
Removable Partial Prosthodontics, 12th edition , Canada, Elsevier Publishers, 2005 , pp:68-102
Stewart, Phoenix, Cagna, De Freest, Clinical Removable Partial Prosthodontics, 3rd edition, 2001, USA, Quintessence publishers, pp:53-96
Grant AA, Johnson W, An Introduction to Removable Denture Prosthetics, 1st edition, USA, Churchill livingstone, 1983, pp: 96-101
Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Retention ,Brit Dent J 2000;189(12):646-657
Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P, Bracing and reciprocation ,Brit Dent J 2001;190(1):10-14
Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P, Clasp design , Brit Dent J 2001;190(2):71-81
Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P, Indirect Retention ,Brit DentJ 2001;190(2):128-132