INDIRECT RETAINERS

DR. ABDUL RAZAK

PROF AND HEAD

DEPT. OF PROSTHODONTICS AND CROWN & BRIDGE

MES DENTAL COLLEGE AND HOSPITAL, PERINTHALMANNA

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DEFINITION

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the component of a removable partial denture that assists the direct retainer(s) in preventing displacement of the distal-extension denture base by functioning through lever action on the opposite side of the fulcrum line when the denture base attempts to move away from the tissues in pure rotation around the fulcrum line

Beginning Of Indirect Retention

• Kennedy wrote that Cummer advanced the idea of indirect retention and had revolutionized partial denture designing. Cummer reasoned that the rationale of indirect retention was a modification of a Class 1 lever.
• Indirect retention was a valuable contribution to partial denture design. However, it is a much better example of a Class 2 lever than a Class 1 lever.

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Lever is a rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object.

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PRINCIPLE OF LEVERS

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FIRST ORDER LEVER

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SECOND ORDER LEVER

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THIRD ORDER LEVER

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Indirect retainer principle. A, Beams are supported at various points.

B, Lifting force will displace entire beam in absence of retainers.

C, with direct retainers (dr) at fulcrum, lifting force will depress one end of beam and elevate other end.

D, With both direct and indirect retainers (ir) functioning, lifting force will not displace the beam. The farther the indirect retainer is from the fulcrum, the more efficiently, it should control movement.

This saddle has an occlusal rest and a clasp on the abutment tooth, and the connector is a sublingual bar. Although normally a mesial rest might well be preferred, a distal rest has been used in this example to simplify the explanation which follows. When sticky foods displace the saddle in an occlusal direction the tips of the retentive clasps engaging the undercuts on the abutment teeth provide the only mechanical resistance to the movement. The saddle thus pivots about the clasp tips.

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In the maxilla this movement of the saddle away from the ridge may also be caused by gravity.

If the design is modified by placing a rest on an anterior tooth, this rest (indirect retainer) becomes the fulcrum of movement of the saddle in an occlusal direction causing the clasp to move up the tooth, engage the undercut and thus resist the tendency for the denture to pivot.

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F = Fulcrum – indirect retainer, a component which obtains support.

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R = Resistance – retention generated by the clasp.

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E = Effort – displacing force, eg a bolus of sticky food.

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It can thus be seen that to obtain indirect retention the clasp must always be placed between the saddle and the indirect retainer.

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Indirect retainers do not prevent displacement towards the ridge. This movement is resisted by the occlusal rest on the abutment tooth and by full extension of the saddle to gain maximum support from the residual ridge. In addition, it may be necessary to compensate for the compressibility of the denture-bearing mucosa by using the altered cast impression technique (A Clinical Guide to Removable Partial Dentures, Chapter 19).

In order to understand the way in which indirect retainers are located it is necessary to consider the possible movement of the denture around an axis formed by the clasps. This clasp axis is defined as the line drawn between the retentive tips of a pair of clasps on opposite sides of the arch.

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Where there is more than one clasp axis, as in this Kennedy Class III denture, it is the clasps on the axis closer to the saddle in question which make the major contribution to indirect retention.

INDIRECT RETENTION AND THE CLASS 2 LEVER

• Indirect retention is an application of a Class 2 lever.
• The tooth surface on which the indirect retention rests is the fulcrum.
• The retentive part of the direct retainer clasp becomes the resistance, and the power is represented by any force that tends to move the denture base away from the underlying tissue.

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• The resistance arm length, then, is the distance between the retentive part of the direct retainer clasp and the pivotal edge of the indirect retention.
• The potential length of the power arm is the distance between the free end of the denture base and the pivotal edge of the indirect retention.

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FACTORS THAT INFLUENCE THE EFFECTIVENESS OF INDIRECT RETENTION

• The effectiveness of the direct retention (the resistance).
• The effectiveness of the tooth and its rest seat (the fulcrum) on which the indirect retention rests or pivots.
• The rigidity of the major and the minor connectors (the lever).
• The mechanical advantage of the lever.

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WAYS TO REDUCE THE MECHANICAL ADVANTAGE

Shorten the power arm at the power end of the lever. If the power arm is shortened at the fulcrum end, the mechanical advantage will be increased because the resistance arm is shortened the same amount but proportionately more.

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To lengthen the resistance arm at either of its ends. The optimum length of the resistance arm can be attained on extension base partial dentures by placing the indirect retention as far from the denture base as feasible and by placing the retentive clasp tips of the direct retainer as near to the denture base as possible.

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Fulcrum Lines and Indirect Retention

• an imaginary line, connecting occlusal rests, around which a removable partial denture tends to rotate under masticatory forces; the determinants for the fulcrum line are usually the cross-arch occlusal rests on the most distally located abutments

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FULCRUM LINE

• An imaginary line, connecting occlusal rests, around which a partial removable dental prosthesis tends to rotate under masticatory forces.
• A theoretical line around which a removable tends to rotate.
• The determinants for the fulcrum line are usually the cross arch occlusal rests located adjacent to the tissue borne components.

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• The primary fulcrum line on distal extension partial dentures, then, is an imaginary line passing through the most distal rest seat (of a tooth) on each side of the arch. If the denture base extends mesially, the primary fulcrum line passes through the most mesial rest seat on each side of the arch.

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• Secondary fulcrum line:

A secondary fulcrum line is the line established when a partial denture includes indirect retention.

When a second fulcrum line is produced by indirect retention, the prosthesis rotates about the primary fulcrum line if the denture base is forced tissueward, and about the secondary fulcrum line if the denture base is lifted off the tissue.

• However, if a second fulcrum line is not produced by indirect retention, the prosthesis rotates about the primary fulcrum line whether forces move the denture base toward or away from the tissue.

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A and B, In Class I arch, fulcrum line passes through the most posterior abutments, provided some rigid component of framework is occlusal to abutment's heights of contour.

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C, In Class II arch, fulcrum line is diagonal, passing through abutment on distal extension side and the most posterior abutment on opposite side.

D, If abutment tooth anterior to modification space lies far enough removed from fulcrum line, it may be used effectively for support of indirect retainer.

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E and F, In Class IV arch, fulcrum line passes through two abutments adjacent to single edentulous space.

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G, In Class III arch with posterior tooth on right side, which has a poor prognosis and will eventually be lost, fulcrum line is considered the same as though posterior tooth were not present. Thus its future loss may not necessitate altering original design of the removable partial denture framework.

H, In Class III arch with non supporting anterior teeth, adjacent edentulous area is considered to be tissue-supported end, with diagonal fulcrum line passing through two principal abutments as in Class II arch.

Indirect Retention as Major Connector Stabilizers

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• Major connector stabilizers transmit forces from the major connector to the teeth and other tissues, and vice versa.
• They serve many functions.
• The indirect retainer does more than offer indirect retention. It helps to stabilize the major connector against vertical and lateral forces, and decreases stresses on the principal abutment teeth by transmitting forces to other teeth in the arch. Thus, the so called indirect retainer serves many functions and is really a major connector stabilizer that offers indirect retention as its primary function.

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• Any minor connector plus its rest(s) incisal, cingulum, ledge, or occlusal on tooth surface(s) is a major connector stabilizer.
• Clasps for stabilization and retention may be included.

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FACTORS INFLUENCING EFFECTIVENESS OF INDIRECT RETAINERS

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The principal occlusal rests on the primary abutment teeth must be reasonably held in their seats by the retentive arms of the direct retainers.

If rests are held in their seats, rotation about an axis should occur, which activates the indirect retainers.

If total displacement of the rests occurs, there would be no rotation about the fulcrum, and the indirect retainers would not be activated.

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Distance from the fulcrum line. The following three areas must be considered:

a. Length of the distal extension base

b. Location of the fulcrum line

c. How far beyond the fulcrum line the indirect retainer is placed

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• Rigidity of the connectors supporting the indirect retainer. All connectors must be rigid if the indirect retainer is to function as intended.
• Effectiveness of the supporting tooth surface. The indirect retainer must be placed on a definite rest seat on which slippage or tooth movement will not occur.
• Tooth inclines and weak teeth should never be used to support indirect retainers.

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FORMS OF INDIRECT RETAINERS

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Auxiliary Occlusal Rest

• An occlusal rest seat can best be prepared on an occlusal surface so that forces transmitted to it can be directed apically through the long axis of the tooth.

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Occlusal rest seats can be prepared

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1)In enamel

2)As part of a new cast metal restoration

3)On the surface of an existing cast metal restoration

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Occlusal rest and rest seats in enamel

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• A rest should never be placed on a tooth that has not been adequately prepared
• The outline form of an occlusal rest seat should be rounded triangular, with the base of the triangle located at the marginal ridge and the rounded apex directed toward the center of the tooth

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• The rest seat should occupy one third to one half the mesiodistal diameter of the tooth and approximately one half the buccolingual width of the tooth measured from cusp tip to cusp tip

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• An occlusal rest seat may be prepared using a variety of rotary instruments.

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• Either round diamond burs, or burs with rounded ends and tapering sides are preferred

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• Finishing procedures are performed using a green stone in a low-speed handpiece.The green stone is intended to gently round sharp angles and eliminate scratches produced by the diamond bur.

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• Polishing is performed using a small, carborundum-impregnated rubber point in a low-speed handpiece . Polishing procedures are intended to provide smooth surfaces that will not retain plaque and debris

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Canine Rests�

When the mesial marginal ridge of the first premolar is too close to the fulcrum line or when the teeth are overlapped so that the fulcrum line is not accessible, a rest may be used on the adjacent canine tooth.

Such a rest may be made more effective by placing the minor connector in the embrasure anterior to the canine, either curving back onto a prepared lingual restseat or extending to a mesio incisal rest.

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The shape is half moon shape. The same types of canine rests as those previously outlined, which are the lingual or incisal rests, may be used

Canine Extensions From Occlusal Rests

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• Occasionally a finger extension from a premolar rest is placed on the prepared lingual slope of the adjacent canine tooth. Such an extension is used to effect indirect retention by increasing the distance of a resisting element from the fulcrum line.
• This method is particularly applicable when a first premolar must serve as a primary abutment.
• The distance anterior to the fulcrum line is only the distance between the mesio occlusal rest and the anterior terminal of the finger extension.

Cingulum Bars �(Continuous Bars) and Linguoplates

• In Class I and Class II partial dentures, a cingulum bar or linguoplate may extend the effectiveness of the indirect retainer if it is used with a terminal rest at each end. In tooth-supported partial dentures, a cingulum bar or linguoplate is placed for other reasons but always with terminal rests

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• In Class I and Class II partial dentures especially, a continuous bar retainer or the superior border of the linguoplate should never be placed above the middle third of the teeth so that orthodontic movement is prevented during the rotation of a distal extension denture.
• This guideline is not as important when the six anterior teeth are in nearly a straight line, but when the arch is narrow and tapering, a cingulum bar or linguoplate on anterior teeth extends well beyond the terminal rests and orthodontic movement of those teeth is more likely. Although these are intended primarily to stabilize weak anterior teeth, they may have the opposite effect if not used with discretion.

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• If the occlusal rest on the secondary abutment lies far enough from the fulcrum line, it may serve adequately as an indirect retainer.
• Its dual function then is tooth support for one end of the modification area and support for an indirect retainer. The most typical example is a distal occlusal rest on a first premolar when a second premolar and first molar are missing and the second molar serves as one of the primary abutments. The longest perpendicular to the fulcrum line falls in the vicinity of the first premolar, making the location of the indirect retainer nearly ideal.

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AUXILIARY FUNCTIONS OF INDIRECT RETAINERS

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• It tends to reduce antero-posterior-tilting leverages on the principal abutments. This is particularly important when an isolated tooth is being used as an abutment, a situation that should be avoided whenever possible.
• Ordinarily, proximal contact with the adjacent tooth prevents such tilting of an abutment as the base lifts away from the tissue.

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• Contact of its minor connector with axial tooth surfaces aids in stabilization against horizontal movement of the denture. Such tooth surfaces, when made parallel to the path of placement, may also act as auxiliary guiding planes.
• Anterior teeth supporting indirect retainers are stabilized against lingual movement.

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• It may act as an auxiliary rest to support a portion of the major connector facilitating stress distribution.
• For example, a lingual bar may be supported against settling into the tissue by the indirect retainer acting as an auxiliary rest.
• Some auxiliary rests are added solely to provide rest support to a segment of the denture and should not be confused with indirect retention.

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• It may provide the first visual indications for the need to reline an extension base partial denture.
• Deficiencies in basal seat support are manifested by the dislodgment of indirect retainers from their prepared rest seats when the denture base is depressed and rotation occurs around the fulcrum.

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Edentulous Situations

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Class I

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• It must always be used and positioned as far anteriorly as possible.

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Class II�

• Its use is not as critical as in class I
• If no modification space exists-an abutment tooth suitable for clasping should be selected as far anterior on the tooth-supported side as possible.
• This rest and clasp assembly, may serve as the indirect retainer if it is located far enough anterior to the fulcrum line.

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• Two indirect retainers should generally be used in a class I design, where as one placed on the side opposite the distal extension base may be adequate in a class II design.

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• If modification space exists- most anterior abutment on the tooth supported side, with its rest and clasp assembly, may be located far enough anterior to the fulcrum line to serve as the indirect retainer.
• A definite rest seat positioned even farther anterior,if possible,may increase the effectiveness of the indirect retention.

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Class III

• Indirect retention is not ordinarily used.
• Auxillary rests may be needed to provide additional vertical support for a lingual plate major connector, lingual bar major connector or an extensive palatal major connector.

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• If the contours of the posterior abutment teeth in class II or III partial denture are not suitable for retention.
• Non retentive stabilizing clasp are designed for posterior teeth and anterior indirect retention is a must.

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Class IV

• The indirect retainer must be located as far posterior as possible.

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Summary

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If the design is modified by placing a rest on an anterior tooth, this rest (indirect retainer) becomes the fulcrum of movement of the saddle in an occlusal direction causing the clasp to move up the tooth, engage the undercut and thus resist the tendency for the denture to

pivot.

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It can thus be seen that to obtain indirect retention the clasp must

always be placed between the saddle and the indirect retainer.

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Indirect retainers do not prevent displacement towards the ridge.

This movement is resisted by the occlusal rest on the abutment tooth and by full extension of the saddle to gain maximum support from the residual ridge.

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In order to understand the way in which indirect retainers are

located it is necessary to consider the possible movement of the

denture around an axis formed by the clasps. This clasp axis is defined as the line drawn between the retentive tips of a pair of clasps on opposite sides of the arch.

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Where there is more than one clasp axis, as in this Kennedy Class III denture, it is the clasps on the axis closer to the saddle in question which make the major contribution to indirect retention.

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Mechanical disadvantage of the denture design

The clasp is always nearer to the indirect retainer (fulcrum) than is the displacing force. The clasp is therefore working at a mechanical disadvantage relative to the displacing force.

The RPD design should strive to reduce the mechanical advantage of the displacing force by placing the clasp axis as close as possible to the saddle and by placing the indirect retainers as far as possible from the saddle.

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In this RPD design the indirect retainers (the rests on the molar teeth) are inefficient because they are placed too close to the clasp axis.

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If the clasp axis is moved closer to the saddle the effectiveness of the indirect retention is improved.

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Support for the indirect retainer

1. When possible, the indirect retainer should rest on a surface at right angles to its potential path of movement.

(2) If it rests on an inclined tooth surface, movement of the tooth might occur with resulting loss of support for the indirect retainer.

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RPD designs which include indirect retention

Kennedy I: Indirect retention in this design is provided by incisal rests on 43 and 33.

The part of the saddle susceptible to displacement in an occlusal direction is indicated by an asterisk.

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RPD designs which include indirect retention

Kennedy II: Indirect retention in this instance is provided primarily by rests on 44 and 43 as they are furthest from the clasp axis.

The rests on 35, 46 and 47 are close to the clasp axis and therefore contribute little to the indirect retention.

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Kennedy III:

In the case of a bounded saddle there is the potential for

direct retention from both abutments. When this can be achieved, as

for the saddle replacing 16 and 15, indirect retention is not required. However, it is not uncommon for only one of the

abutments to be suitable for clasping. In this design a clasp on 23 has been omitted for aesthetic reasons. Under such

circumstances indirect retention can be employed, the major contribution being made by the rest on 17.

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Kennedy IV:

In a maxillary denture it is sometimes difficult to achieve

much separation of the clasp axis and indirect retainers. In this

example, clasps engage the mesiobuccal undercuts on 16 and

26 and indirect retention has been achieved by placing the

rests on 17 and 27 as far posteriorly as possible.

BIBILIOGRAPHY

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1.McCracken’s Removable partial prosthodontics, 11^th Ed., 2005

2.Grasso & Miller Removable partial prosthodontics,3^rd Ed 1991.

3.Osborne. J & Lammie G.A. : Partial Dentures, 4^th Ed 1985

4. Stewart, Rudd & keubker : Clinical Removable Partial prosthodontics,3^rd Ed 2003

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