Direct Retainers Part 1

Direct Retainers Part 1

Direct Retainers Part 1

Dr. Akshi GvalaniPG. Dept. of ProsthodonticsTerna Dental College Nerul, Navi Mumbai

RETENTIONThat quality inherent in the dental prosthesis acting to resist the forces of dislodgment along the path of placementContents Retention in an RPD Direct retainer terminologyReview of literature Types of direct retainersPrinciple of clasp functionAttributes of a direct retainer Clasp selection Types of clasps

Adhesion Cohesion Atmospheric pressure Plastic molding of tissues on polished surfaces Gravity Mechanical

Direct retainer TerminologyDirect retainerThat unit of an RPD used to retain and prevent dislodgement away from the basal seat tissues consisting of a clasp assembly or precision attachment GPT

Parts of a clasp

Rest Body Arms (2) Shoulder Minor connector (truss,tail,tang,upright,approach arm )

Review of literature Dr. Herman S. Chayes 1906 first formulated the principle of the internal attachment Prothero in 1916 gave the Cone theory Kennedy coined the term Height of Contour DeVan coined the terms infrabulge and suprabulge areas Nesbit in 1916 introduced the cast circumferential clasp FE Roach in 1930 gave the term Roach claspBlatterfein in 1951 gave a classification of survey lines Dr. O C Applegate in 1965 introduced the combination claspMiller in 1972 gave the cingulum claspKratochvil and Krol developed the RPI bar clasp concept in order to deal with the stresses on abutment teeth in a distal extension partial denture Grasso developed the VRHR concept Eliason in 1983 gave the RPA clasp Goodman developed the equipoise clasp systemClassification Intra coronal Extra coronal Manufactured Flexible clips / rings Clasp type Circumferential / Akers / Suprabulge / occlusally approaching / Neys type IBar -type / Roachs / Infrabulge / Vertical projection / gingivally approaching / Neys type II

GINGIVALLY APPROACHING CLASPSInfra BulgePush ClaspRoach ClaspThis bar clasp approaches the retentive undercut in a gingival direction resulting in a push type of retention. This push type of retention is more effective than pull type retention characteristic of circumferential claspTYPES OF CLASP RETAINERS

Circumferential clasps/ occlusally approaching Circumferential clasp Embrasure clasp. Ring clasp. Back action clasp. Reverse action / hair pin clasp. Multiple clasps. Half-and-half clasp. Combination clasp. Onlay clasp.Bar/Roach clasps / gingivally approaching clasps T-clasp Modified T-clasp Y-clasp I-clasp RPI concept.Other clasp designs RPA clasp. VRHR clasp. Clasps utilizing proximal undercuts -Mesiodistal clasp -Devan clasp Cingulum clasp

Types of clasps construction Cast grain structure crystalline Wrought grain structure more fibrous Combination

Principles of clasp function Extracoronal Resistance of metal to deformation Undercut at a single path of placement and removal Frictional resistance

Intracoronal Consists of a receptacle created in the crown or onlay on the abutment tooth into which a projection in the cast framework fits snugly. Wedging Frictional resistance

Attributes of a direct retainer Retention Stability Support Passivity Encirclement Reciprocation Retention 1. The size of the angle of the cervical convergence.2. How far into the cervical convergence the clasp terminal is placed.3. Flexibility of the clasp arm.

Flexibility/resiliency of the clasp arm:1. The length of the clasp arm.2. The diameter of the clasp arm.3. The taper of the clasp arm4. The cross-sectional form.5. The type of metal used.6. The kind of alloy used 7. The heat treatment Size of the angle of cervical convergence

Guiding planes determine the path of placement and removal of a partial denture.

Degree of UndercutRetentive clasp arms must be located so that they lie in the same approximate degree of undercut on each abutment tooth, despite the variation in the distance below the height of contour. The measurement of the degree of undercut by mechanical means is achieved by the help of an undercut gauge attached to a dental surveyor.

Flexibility of clasp arms Length of the clasp arm

CircumferentialBar type

Length (inches)Flexibility (inches)Length (inches)Flexibility (inches)

0-0.30.0040-0.70.004

0.3-0.60.0080.7-0.90.008

0.6-0.80.0120.9-1.00.012

Diameter of clasp arm Inversely proportional Midway between its origin and its terminal end. Thickness of the clasp arm in the buccolingual direction is to be considered rather than the width in the occluso-gingival direction.

Cross-sectional form Round cross-sectional form enables the clasp to be flexible in all directions whereas the half-round form limits the flexibility to only one direction. Cast clasps are half round in form and they flex away from the tooth, but edgewise flexing is limited.

If the cross-sectional area of clasp is doubled, the stiffness will be increased 4 times and the flexibility reduced 4 times.Material used for clasp arm The modulus of elasticity of Cobalt-chrome alloys cast gold wrought gold wires.

Structure of the alloy Wrought wires have greater flexibility than a cast structure due to its grain structure being fibrous. The tensile strength of a wrought structure is at least 25% greater than that of the cast alloy from which it was made. Wrought forms can be used in smaller diameters to enhance the flexibility and they offer minimum friction and can have a stress breaking effect.

Stability is the resistance to horizontal displacement of a prosthesis. All clasp terminal except the retentive clasp terminals contribute to this property in varying degree. SupportProperty of the clasp that resist the displacement of the clasp in gingival direction.Primary support units of a clasp are occlusal, lingual or incisal rest

EncirclementMore than 1800 of greatest circumference of the tooth must be included passing from diverging axial surface to converging axial surface.

Passivity A clasp in place should be completely passive. The retentive function is activated only when dislodging forces are applied to the partial denture and during removal of the denture.

Reciprocation

The retentive arm exerts a fleeting, unreciprocated, thrust (whiplash) against the tooth each time the prosthesis is inserted or removed from the mouth.Reciprocation:The means by which the effect of the retentive clasp arm on the abutment tooth is countered by the action of the non-retentive clasp arm. Located at the junction of the gingival and middle 3rd of the crowns of an abutment teeth.

Bilateral Opposition: Buccal retention on one side should be opposed by buccal on the other or lingual on one side opposed by lingual on the other. In class II situation, the 3rd abutment may have either buccal or lingual retention. In class III retention may be either bilaterally or diameterically opposed.

Stress Breaking:Clasp retainers on abutment teeth adjacent to distal extension bases should be designed so that they will avoid direct transmission of tipping and rotational forces on abutments.

SURVEY LINEBlatterfein in 1951 put forth a simple and comprehensive classification of survey line with suggestion on clasp selection. He described four kinds of survey line:

Typical survey line or medium Atypical A or Diagonal Atypical B or High Atypical C or Low

Typical or medium:Extends from the occlusogingival midpoint in the near zone to the junction between the occlusal two third and cervical one third in the far zone. Clasps suggested for use where such a survey line exists include the occlusally approaching and gingivally approaching clasps.

Atypical A or diagonal:This runs diagonally across the tooth surface from a high position in the near zone to a low position in the far zone. - A reverse action or hairpin clasp is recommended.- Gingivally approaching clasp may also be used.

Atypical B or High Survey line:This type of survey line is parallel to the occlusal surface and lies close to it.A wrought occlusally approaching clasp arm may be used.If accompanied by a low survey line on the opposite side of the tooth, a ring or a back action have been recommended.

Atypical C or Low Survey line:The low survey line is parallel to occlusal surface but has just above the level of the gingival margin.This type of survey line contraindicate the placement of a retentive clasp arm on the tooth surface concerned, as the arm would need to be placed too close to the gingival margin for safe application. When all the usable surface of a tooth present the survey lines of this type, retention may be obtained by Placement of a crown on the tooth to artificially develop undercuts. Placement of a class V gold inlay a dimple is in the inlay and a ball head on a gingivally approach arm is positioned to engage the dimple. An extended clasp may be used where the tooth offers favorable conditions for retention. Undercut may be developed by recontouring the tooth.

Ney 3 basic survey lines with an appropriate clasp form.Class I: Survey line runs diagonally across the tooth surface from a low position on the side of the rest to a high position on the other proximal side.A cast occlusally approaching arm or its variants, back action, reverse action and ring clasps are recommended called as Ney class I clasps.Class II: Similar to Blatterfein atypical A or Diagonal survey line. Here gingivally approaching is recommended and termed as Ney class II clasp.Class III:It is the same as the Blatterfein Atypical B or High survey line. The wrought wire arm is used and termed as the Ney class III clasp.

Clasp selection criteria Survey line location and degree of undercut. Root condition Oral hygiene Undercuts (soft/hard ) Esthetic requirements Selection of clasp based on requirement of retention & stabilityA large number of edentulous area, bilaterally placed would mean more number of clasps and guide planes, thus the entire prosthesis has greater retention and stability.Cast circumferential clasp properly designed shows greater retentive and bracing properties than a bar clasp used in a similar situation.Ideal amount of retention is that which will retain the removable partial denture against reasonable dislodging forces without placing the undue strain on the abutment teeth. Thus minimum retention and maximum stability is the ideal.Root size and form of the tooth: Abutment teeth with short, conical roots, bone loss, and periodontal ligament, mild periodontal problem may not be able to withstand lateral forces that would be within physiologic limits of a healthy sound tooth. Such tooth must be preserved.Oral hygiene and cariesHigh caries index. Contraindicate the use of clasp with an unprotected tooth surface as with a ring clasp.Bar clasp has minimum tooth structure contact and less interference with natural cleansing action.CLINICAL SITUATIONSCIRCUMFERENTIAL CLASPThis clasp is the most universally used clasp today as it fulfills almost all the requirements of an ideal clasp

No edentulous space on opposite side of archTooth supported partial dentureLone standing abutmentTilted molarPeriodontally weak teethLingually inclined premolarsAnterior retentionDiastemaOcclusal cariesAbutment tooth without undercutProximal undercutsDEPD Mesial undercut Distal undercut KENNEDYS CLASS II, III, IV EMBRASURE CLASPBonwill/ Crib/ Double Akers/ Back to back

TOOTH SUPPORTED PARTIAL DENTURE MULTIPLE CLASP

LONE STANDING ABUTMENT/TILTED ABUTMENTRING CLASP

DISTAL EXTENSION PARTIAL DENTUREMESIAL UNDERCUTCOMBINATION CLASP (tissue undercut)

Advantages and disadvantages of combination clasp Combines the resiliency of wrought clasps with the stabilizing features of cast clasps

Enables better neutralization of the torsional forces exerted on the abutment teeth Less fatigue failure Minimum tooth contact 1. Wrought arm reduces the horizontal stabilization of the combination clasp

2. Increased base movement thus subjects residual ridges to a greater degree of lateral stress3. Steps of fabrication

DISTAL UNDERCUTBAR CLASP

BACK ACTION CLASP HAIRPIN CLASP (tissue undercut)

MESIALLY TILTED ABUTMENTHAIRPIN CLASP (ring contraindicated)PERIODONTALLY WEAK ABUTMENT MULTIPLE CLASP

LINGUALLY INCLINED PREMOLARSHALF AND HALF CLASP

OCCLUSAL CARIESONLAY CLASP

ABUTMENT TEETH WITHOUT USABLE UNDERCUTSEXTENDED CLASP

PROXIMAL UNDERCUTSDEVAN CLASP

ANTERIOR RETENTIONBAR CLASP CINGULUM CLASP

DIASTEMA MESIODISTAL GRIP CLASP

EQUIPOISE CLASP Based on the principles of the back action clasp

Clasp selection criteria Survey line location and degree of undercut. Root condition Oral hygiene Undercuts (soft/hard ) Esthetic requirements

REFERENCES Applegate O.C.: Text book of Removable Partial Prosthodontics, St. Louis, CV Mosby Co. Cecconi B.T., Asgar K., Dootz F. The effect of partial denture clasp design on abutment tooth movement. J. Prosthet. Dent., 1971; 25: 44-55. Davenport J.C., Baskar R.M., Heath J.R., Ralph J.P. A color atlas of RPD, Wolfe Medical Publications Ltd., 1988. Henderson D., McGivney G.P., Castleberry D.J.: McCrackens removable partial prosthodontics, 8th Edn. St. Louis; CV Mosby Co. Miller E.L., The cingulum clasp. J. Prosthet. Dent., 1972; 28: 369-372. Miller E.L. Text book of Removable Partial Prosthodontics. St. Louis, CV Mosby Co. Osborne J., Lammie G.A. Partial dentures, 4th Ed CBS publishers and Distributors, Delhi, India. Stewart,Rudd and Kuebker: Clinical removable Partial Prosthodontics ,2nd Ed, Euro America Inc., publishers Tokyo, 1997

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