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RETENTION OF REMOVABLE PARTIAL DENTURES

RETENTION OF REMOVABLE PARTIAL DENTURES:

RETENTION OF REMOVABLE PARTIAL DENTURES Dr. Amal Fathy Kaddah Professor of Prosthodontic, Faculty of Oral &Dental Medicine, Cairo University

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DIRECT RETENTION

Intracoronal attachments:

Intracoronal attachments

Extracoronal attachment:

Extracoronal attachment

Extracoronal attachment:

Extracoronal attachment

Clasp retainer :

Clasp retainer Retentive arm Bracing arm Occlusal rest Minor connector

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DESIRABLE UNDERCUT NON DESIRABLE UNDERCUT

REQUIREMENTS OF CLASP DESIGN.:

REQUIREMENTS OF CLASP DESIGN. Bracing arm better located In the apical portion of the Middle 1/3 Retentive arm better located In the gingival 1/3 for better esthetics & mechanics

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UNDERCUT IS BETTER BE FOUND WITHIN THE GINGIVAL1/3 For mechanical reasons Post Is More Readily Removed by Application of Force Near Its Top Than by Applying Same Force Nearer Ground Level

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180 ENCIRCLING TOOTH continuous intermittent Tooth can't move horizontally away from the clasp

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Each retentive terminal should be reciprocated as retentive arm exerts some orthodontic movement during placement and removal as it flexes about the height of contour

Amount of retention depends on:

Amount of retention depends on The greater the distance we go into the undercut the greater will be the retention A- depth of undercut

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B- flexibility of clasp arm Increasing arm length increases the flexibility ,thus decreasing the retention

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Round clasps have universal flexibility thus lowers the retention than ½ round and flat clasps shape of the clasp

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Degree of taper , retentive arm should be tapered in two dimensions

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C- Angle of cervical convergence

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Occlusally approaching clasp Gingivally approaching clasp Clasps according to way of approach to undercut

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Occlusally Approaching Clasps

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1-Akers clasp (circlet) 0.01 of an inch

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2-DOUBLE AKER (embrasure)

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3- Circumferential `C` Clasp

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4-R.P.A

5- Multiple Clasp:

5- Multiple Clasp

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6- Extended arm clasp Abutment has no undercut Adjacent tooth has a reasonable undercut

7- Half & Half clasp:

7- Half & Half clasp 2 M.C., 2 O.R. & 2 arms. Mainly indicated for dual retention commonly in unilateral cases.

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7-RING CLASP Provides unilateral bracing. Used for single tilted molars More flexible than aker because it is one arm clasp.

8-Back action clasp:

8-Back action clasp Single arm clasp Minor connector starts mesio-lingually It engages mesio-buccal undercut. O.R. is located distally. Used in free end saddle.

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9-Reverse Back action clasp M.C. originating Mesio-buccally Retentive arm engage Mesio-lingual undercut

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Reverse Back action clasp Back action clasp

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10-Mesio-Distal clasp Used only in anterior teeth Depends on frictional resistance for retention

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Gingivally Approaching Clasps

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A retentive clasp arm A rigid reciprocal clasp arm An occlusal rest and a minor connector The tip of the retentive arm may be in the form I , T, U , C or Y. One 1- The I-bar clasp (Roach clasp arm): The I - bar clasp consists of

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T clasp Modified T clasp Different forms of gingivally approaching clasps I clasp

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R.P.I The base of the I bar Should be 3mm away From the gingival margin Provides unilateral bracing. Commonly used for tooth mucosa borne partial dentures. A retentive clasp arm

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Contraindications for the use of gingivally approaching clasps

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Devan clasp More esthetic due to interproximal position No distortion due to it`s proximity to denture border

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COMBINATION CLASPS

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Combination clasp Gingivally app . Retentive arm (buccally) & Occlusally app . Bracing arm (lingually)

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Buccal wrought wire retentive arm soldered to the base Lingual casted bracing arm Another type of combination clasp

Clasps with splinting action:

Clasps with splinting action Double Aker clasp Extended arm clasp Multiple Aker clasp

Clasps with stress breaking action :

Clasps with stress breaking action All gingivally approaching clasps except Devan clasp Occlusally approaching clasps Reverse Aker clasp Back action clasp Reverse back action clasp R.P.A. Combination clasps

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Types of Survey Line Near the saddle Away from the saddle

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A-MEDUIM SURVEY LINE Midway between Occlusal surface & Gingival margin in the Near zone Nearer to gingival margin In the far zone We can use occlusally or gingivally approaching clasps

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B-DIAGONAL SURVEY LINE Near occlusal surface In the near zone Near gingival margin In the far zone Or Or Or Back action & Reverse back C clasp Ging.app. With T bar Ging.app. With L bar

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C-HIGH SURVEY LINE Near to the occlusal surface Wrought wire occlusally app. Back action or reverse back action commonly in inclined teeth Bracing Arm

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D- LOW SURVEY LINE Near the gingival margin Extended arm clasp Devan clasp engaging proximal undercut Crowning of the tooth

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Tripping action is attributed to clasp arms that engage the undercut directly from a gingival direction . Not all bar clasp arms have tripping action , since the retentive terminal may actually engage the undercut from an occlusal direction as is true with the "T" bar or modified "T" bar.

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Occlusally Gingivally Retention due to tripping action Bracing 2 Arms above survey line provide bracing Esthetics less visible due to gingival position Tolerance Gingivally app.clasp arm relieved from gingiva creating space accumulating food and causing discomfort Caries More tooth coverage increasing the risk for caries Gingival health Trauma may occur due to distortion or inadequate relief

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Special Considerations for Clasp design in Free End Saddle Cases

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MOVEMENT OF CLASP WITH DISTAL O.R. Circlet clasp during function: Loading force (F) causes clasp to rotate at (R). Where minor connector breaks contact with tooth. Retentive clasp arm tip moves occlusally during function and directs a distal torquing force to the tooth.

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In function, rotational forces take place around distal occlusal rest. Cast circumferential clasp places an extremely destructive distal tipping or torquing force on abutment tooth (arrows).

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when force is directed against unsupported end of beam cantilever can act as first class lever Torque on the abutment tooth F

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A cantilever design allows excessive vertical movement toward the residual ridge

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Reverse Aker Clasp F Class ? Lever

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Improperly designed RPA clasp located above survey line. Under occlusal force A, rigid portion of retentive arm cannot move gingivally. This moves center of rotation distally. Mesial rest lifts out of its rest seats. Retentive portion of clasp arm engages mesial undercut, torquing tooth distally

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Properly designed RPA clasp showing movement from occlusal forces. Proximal plate, C. drops gingivally and slightly mesially as rotation occurs around mesial rest with approximate center of rotation, B. Rigid portion of retentive arm contacts tooth only along survey line, A. and moves gingivally and mesially. Retentive end of clasp arm moves mesially and slightly gingivally

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Laboratory blockout of refractory cast for RPA clasp. Rigid portion of retentive clasp arm is waxed for casting over blocked-out distofacial surface, E, only anterior border will be in contact with tooth. Cast under retentive end of clasp arm is not blocked out

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Placement of the I bar at the greatest prominence (2) or to the mesial (3) permits the I bar under function to move freely away from the tooth. The I bar placed on the distal (1) cannot move freely away from the tooth thereby producing a torquing action

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Combination Ackers and modified “T” bar clasp with distal rest and distal undercut

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P.P minor connector should contact approximately 1 mm of the gingival portion of the g. p. in distal extension cases

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Changing the position of the guiding plates changes the center of rotation ( . ) indicates center of rotation

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MOVEMENT OF CLASP WITH MESIAL O.R. The cicumferential clasp arm and proximal plate move in mesiogingival direction disengaging from the tooth

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WROUGHT WIRE CLASP DURING FUNCTION Wrought wire (WW combination) clasp during function. Loading force (F) causes clasp to rotate, where minor connector breaks contact with tooth. WW clasp arm tip moves occlusally and directs a distal torquing force to the tooth. Flexibility of WW arm limits torquing.

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Placing the occlusal rest away from the distal extension base has the following advantage 1- By changing the direction of torque on the abutment from the distal to the mesial side of the tooth, the resistance to torque action will be applied from the neighboring teeth

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Reverse Aker Clasp F Class ? Lever

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Depression of the base does not exert a distal stress (the pump handle effect) on the abutment but rather a mesial one 2- Clasp disengagement from the tooth 1- Class II Lever

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4 - lever arm, represented by distance from rest to denture base, is increased. This increase in length makes rotational action caused by up-and down movement of denture base in function more vertical. A vertical force in better tolerated by ridge than is a horizontal oblique force

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5- As rest is moved anteriorly more of residual ridge will resist rotation

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THANK YOU

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