INDIRECT RETAINERS

CONTENTS Introduction Definitions Biomechanical considerations Possible movements of the RPD Factors influencing effectiveness of indirect retainers Auxiliary functions of indirect retainers Forms of indirect retainers Support for indirect retainers Summary References

INTRODUCTIONThe goal To provide useful, functional removable partial denture prostheses by striving to understand how to maximize every opportunity for providing and maintaining a stable prosthesis

Prosthesis not firmly attached to teeth

Need to control potential movement under functional load

Ensures stability

Occlusally directed dislodging forces

Denture base moves away from the supporting tissue

Anterior segment of major connector impinges upon underlying soft tissue

Potentially destructive forces to hard and soft tissues of dental arch

This stress should not exceed the level of physiological tolerance which is a range of mechanical stimulus that a system can resist without disruption or traumatic consequences

Tooth supported partial dentures effectively use teeth to control movement away from the tissue Tooth tissue supported partial dentures do not have this capability since once end of the prosthesis is free to move away from the tissues

The concept of indirect retainers as a means of resisting rotational displacement was originally advanced by Dr. W. E. Cummers

Definitions The component of a partial removable dental prosthesis 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 moves away from the tissues in pure rotation around the fulcrum line

Glossary of prosthodontic terms. J Prosthet Dent 2005; 94(1):10-92

The framework component that resists rotational displacement of an extension base away from the supporting tissue is called an indirect retainer

Stewarts Clinical Removable partial prosthodontics

Simple machines

Levers The lever is a rigid bar supported somewhere along its length It may rest on a support or be supported from above The support point of a lever is known as the fulcrum If weight is applied at a point on the rigid bar, rotation or movement will occur around the support or the fulcrum

Types of levers first class

R

F E

Second class

Third class

Cummer reasoned that the rationale of indirect retention was a modification of a Class 1 lever. However, it is a much better example of a Class 2 lever than a Class 1 lever

Possible movements of dentureFRONTAL SAGITAL

HORIZONTAL

Horizontal About a longitudinal axis as the distal extension base moves in a rotary direction about the residual ridge Resisted by Rigidity of major and minor connector

If this rigidity is lacking: Applies undue stress to the sides of the residual ridge Causes horizontal shifting of the base

Frontal About an imaginary vertical axis located near the center of the dental arch This movement occurs under function as diagonal and horizontal occlusal forces act on the denture. Resisted by stabilizing components: reciprocal clasp arms minor connectors

SAGITTAL Rotation about an axis through the most posterior abutmentsFULCRUM LINE

Through occlusal rest or any other rigid component of the direct retainer located above the height of contour

Fulcrum lines serves as the center of rotation as the distal extension base moves towards the supporting tissues when occlusal load is applied

Indirect retainers do not prevent displacement towards the ridge. Resisted by the occlusal rest By full extension of the saddle By compensating for the compressibility of the denture-bearing mucosa by using the altered cast impression technique

Displacement away from tissue Sticky foods Gravitational Pull on Maxillary Appliances precautions may be taken to lessen this disadvantage: reduce the weight of the prosthesis Resin teeth would be of less weight.

Displacement Forces from Cheek and Tongue In a maxilla strong buccal contact of the cheek During involuntary movements (such as yawning) In mandible if the lower extension base is overextended, the tongue

Sticky foods Gravity Muscular action

Principle use of indirect retainers

Primary and secondary fulcrum lines The primary fulcrum line on distal extension partial dentures is an imaginary line passing through the most distal rest seat 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.

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

Types of Fulcrum Lines Class 1 arch

Class 2 arch

Class 3 archs

Class 4 archs

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.

The indirect retention is located in relation to the retentive part of the direct retainer clasps, not to the primary fulcrum line. Indirect retention prevents the retentive part of the direct retainer clasps from becoming a fulcrum or point of rotation about which the prosthesis could rotate when forces move the denture base away from the underlying tissue

When rotation about the clasp tips is prevented by any unit of the partial denture, the denture base cannot move away from the tissue until the resistance to deformation of the retentive clasps is overcome. Indirectly then, partial denture units preventing this rotation retain the denture bases in contact with the underlying tissue, hence the name indirect retention.

Indirect retainer principle

Mechanical advantage of the lever The lower the mechanical advantage, the more lifting force is necessary on the extension base to overcome the resistance of the direct retention and move the base off its seat Aim Decrease the mechanical advantage of the lever and increase the effectiveness of the indirect retention

Mechanical advantage = length of effort arm length of resistance arm Two ways to reduce the mechanical advantage of a class 2 lever To shorten the power arm at the power end of the lever To lengthen the resistance arm at either of its ends

Shorten power arm 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.

The mesiodistal span of these teeth should be kept to a minimum to keep the effective length of the power arm to a minimum. Use smaller teeth The denture bases should not be shortened because the base itself contributes physiologic retention

Lengthen the resistance arm The optimum length of the resistance arm can be attained 1. By placing the indirect retention as far from the denture base as feasible 2. By placing the retentive clasp tips of the direct retainer as near to the denture base as possible

Factors influencing effectiveness of indirect retainers1. Effectiveness of the direct retainer Indirect retention cannot be more effective than the direct retention of the clasps.

Indirect retention does not begin to function until the physiologic retention contributed by the extension base is overcome and, simultaneously, the resistance offered by the direct retention is challenged.

2. Distance from the fulcrum line. The following three areas must be considered:1. Length of the distal extension base effort arm 2. Location of the fulcrum line 3. How far beyond the fulcrum line the indirect retainer is placed resistance arm

3. Rigidity of the connectors supporting the indirect retainer. All connectors must be rigid if the indirect retainer is to function as intended

4. 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

Frank and Nicholls conducted a study to measure the effectiveness of an indirect retainer and to determine the effect of the other components of a removable partial denture on displacement of the denture base

An investigation of the effectiveness of indirect retainers Frank R P, Nicholls J I. J Prosthet Dent November 1977;38(5):494-506

The type of clasp used has the greatest influence on the amount of denture base displacement. Indirect retainers have little effect upon retention of the distal-extension partial denture base. Guiding planes on the proximal surfaces of terminal abutment teeth are important in preventing denture base lifting. Use of a mesial instead of a distal rest on the terminal abutment tooth does not decrease indirect retention.

Thus the choice of indirect retainer location should be made mostly on the basis of abutment tooth support, a crown form favoring adequate rest seat preparation, and the patients esthetic desires.

Auxillary functions of indirect retainers1. Tends to reduce antero-posterior tilting leverage on principle abutments 2. Contact of its minor connectors with axial tooth surfaces aids in stabilization against horizontal movement of the denture

3. Anterior teeth supporting indirect retainers are stabilized against lingual movement

4. It may act as an auxillary rest to support a portion of the major connector facilitating stress distribution. 5. It may provide the first visual indications for the need to reline an extension base partial denture.

Force transmission by indirect retainers McDowell used a photo elastic model to determine the presence and effect of forces transmitted to the indirect retainer abutment when a unilateral vertical load was applied in the first molar region of a distal extension removable partial denture.1. 2. 3. The stress at the apex of the alveolus for the indirect retainer abutment tooth on the loaded side is twice as great when indirect retainer ispresent. The presence of an indirect retainer reduces the stresses at the apices of the alveoli of the primary abutment teeth bilaterally as well as on the crest of the residual ridge on the nonloaded side. The presence of an indirect retainer reduces the stresses on the lingual shelf of the residual ridge bilaterally and on the lingual shelf adjacent to the primary abutment tooth on the nonloaded side.

Force transmission by indirect retainers during unilateral loading. Glen C. McDowell, J Prosthet Dent June 1978;39(6): 616-621

Frechette et al demonstrated that removal of the indirect retainers from a RPD results in the application of more pressure to the direct abutment teeth. The indirect retainer probably is effective in distributing forces to teeth other than direct abutments, and in preventing denture base lifting

The influence of partial denture design on distribution of force to abutment teeth J Prosthet Dent 1956;6:195-212

Forms of indirect retainers according to Osborne and LammiePositionAnterior

Tissues on which retainers restTooth

Types of indirect retainerCummers arms Anteriorly placed incisal or occlusal rest Continuous bar and its modification Palatal arm Anterior palatal bar

Palate

Posterior

Tooth Palate

Occlusal rest Posterior palatal bar or extension of palatal plate

According to Grant and Johnson: They can be divided into two groups depending on position: GROUP A: Those that are positioned on the standing teeth and can be used in either upper and lower arch:1. 2. 3. 4. 5. Rests Tooth bearing direct retainers Continuous bar Cummers arms Tooth bearing connectors

GROUP B: Those that are positioned on the mucous membrane. should only be used in upper arch.1. 2. Palatal arms Palatal bearing connectors

Auxiliary occlusal rests Mesial marginal ridge of first pre molar Ideal region of central incisors but too week lingual surfaces too perpendicular to support rest Consists of a minor connecter and a rest placed in a rest seat

Bilaterally placed in class I Class II - on the marginal ridge of the first premolar tooth on the opposite side of the arch from the distal extension base

Form of occlusal rest The outline form - rounded triangular shape with the apex toward the center of the occlusal surface Should be as long as it is wide,

Base of the triangular shape - at least 2.5 mm Rest seats of smaller dimensions do not provide an adequate bulk of metal for rests

a reduction of the marginal ridge of approximately 1.5 mm is usually necessary. The floor of the occlusal rest seat should be apical to the marginal ridge and the occlusal surface and should be concave, or spoon shaped

An inclined plane effect

Canine rests When mesial marginal ridge is too close to fulcrum line more effective by placing the minor connector in the embrasure anterior to the canine, either curving back onto a prepared lingual rest seat or extending to a mesioincisal rest

If an anterior tooth is sound and the lingual slope is gradual rather than perpendicular, a lingual rest may sometimes be placed in an enamel seat at the cingulum or just incisally to the cingulum Avoid on mandibular canineMaxilla Mandible

Form of a cingulum rest V shaped in cross section Relatively upright lingual wall When viewed lingually, it should be cresent shaped

Incisal rest Incisal edge of anteriors

According to Grant and Johnson

Form 1

Form 2

Form 3

Form4

Comparison between cingulum and incisal rest A lingual rest is preferable to an incisal rest rests Less tipping force More esthetic Less subject to breakage and distortion

Rotational centre

Canine extension from occlusal rest A finger extension from a premolar rest is placed on the prepared lingual slope of the adjacent canine tooth. Increases 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.

Although the extension rests on a prepared surface, it is used in conjunction with a terminal rest on the mesial marginal ridge of the premolar

The continuous clasp A metal band passing continuously over the cingula of teeth from saddle to saddle Its shape is fluted so it may rest on the cingulum Technically they are not indirect retainers as they rest on unprepared lingual inclines of the anterior teeth. They aid the terminal rests in indirect retention by providing resistance to rotatory displacement of free end saddle

Advantage1. Helps in distribution of the load 2. When splinting of anterior teeth is required 3. Acts as an auxiliary connector of saddles

Contraindications1. Not tolerated well due to feeling of the wire behind the teeth 2. Difficulty in phonetics especially d and t 3. Not possible when insufficient space exists 4. On palatally or lingually inclined teeth 5. On short lingual crowns 6. In spaced anteriors use modification of continuous clasp

Lingual plate Major connector that serves as an indirect reatiner if rest seats are prepared onto the cingulum Covers cingulum as well as marginal gingiva

Intended primarily to stabilize weak anterior teeth Superior border of the linguoplate should never be placed above the middle third of the teeth so that orthodontic movement during the rotation of a distal extension denture is avoided.

Cummer arm Consist of a metal arm extending from suitable component of a denture to terminate on the unprepared palatal/lingual surface of anterior teeth. Used bilaterally on periodontally sound canines only

Advantage Can be used when insufficient space exists for continuous clasp

Disadavantage:

Since it uses a single tooth, excessive load may cause tooth movement Long cummer arms may get distorted

Less effective than continuous bar as is closer to axis of rotation

Palatal arm Rests on rugae Used when inadequate space exists for continuous clasp Not as effective as positive tooth support

Can me made with metal, acrylic Broader than cummer arms so less chance of distortion Disadvantage gives rise to irritation to tongue interferes with phonetics

Anterior Palatal Bar Formed by the union of two palatal arm in the mid line. Disadvantage: Covering of the rugae area difficulty in phonetics Irritability

Posterior palatal bar Only form of posterior retainer which bears on the hard palate Sometimes only means for indirect retention in long Class IV Used along with posteriorly placed occlusal rests

Direct-indirect retention In the maxillary arch where only anterior teeth remain, full palatal coverage is usually necessary, the added retention & lesser bulk of the cast metal is usually adequate to prevent lifting of the base away from the tissues & provide direct-indirect retention.

Modification areas The primary abutments in a Class II, modification 1, partial denture are the abutment adjacent to the distal extension base and the most distal abutment on the tooth-supported side. The fulcrum line is a diagonal axis between the two terminal abutments

The anterior abutment on the tooth supported side is a secondary abutment, serving to support and retain one end of the tooth supported segment, as well as adding horizontal stabilization to the denture If the occlusal rest on the secondary abutment lies far enough from the fulcrum line, it may serve adequately as an indirect retainer.

Tooth bearing direct retainers Clasp units and precision attachments may also function as indirect retainers Mostly in cases of Kennedy's class III mod 1 scenario

Rugae support Some clinicians consider coverage of the rugae area of the maxillary arch as a means of indirect retention as: rugae area is firm usually well situated to provide indirect retention for a Class I removable partial denture.

But, tissue support is less effective than positive tooth support Rugae coverage is undesirable if it can be avoided. The use of rugae support for indirect retention is usually part of a palatal horseshoe design

Support for the indirect retainer Tooth support is preferable to mucosal support because the compressibility of mucosa allows movement of the denture to occur. If there is no alternative to mucosal support the indirect retainer should cover a sufficiently wide area to spread the load and avoid mucosal injury.

This consideration effectively limits mucosally supported indirect retainers to the maxilla where the load can be distributed over the hard palate

When the saddle is first displaced, mucosal compression beneath the indirect retainer allows the denture to rotate around the clasp axis (fulcrum). The path of movement of the indirect retainer is thus directed obliquely, rather than at right angles, to the mucosal surface. This combination of oblique approach and mucosal compression may allow a significant degree of movement of the denture in function.

When possible, the indirect retainer should rest on a surface at right angles to its potential path of movement. If it rests on an inclined tooth surface, movement of the tooth might occur with resulting loss of support for the indirect retainer

Summary The principal of indirect retention is shown to be an application of class 2 lever Indirect retainers do not play a role in vertical movement towards tissues They only facilitate the direct retainers in retention and prevent the retentive clasp tip from becoming a fulcrum about which the prostheses could rotate It is also a major connector stabilizer They act by moving the fulcrum line anterior to the force

Proper design and placement of the component parts of a removable partial denture will not prevent but at least minimize the displacement of the partial denture and thus preserve the oral structures

Refernces A.A.Grant and W.Johnson:Removable denture Prosthodontics, 2nd edition .1992,Churchill livingstone Inc. Davenport.J.C, Basker.R.M,Heath.J.R,Ralph.J.P,Glantz.PO,Hammond.P: Indirect retention,B Dent J:19(3);128132,2001 Ernest L.Miller and Joseph E.Grasso: Removable Partial Prosthodontics, ed 2. 1979,Williams and Willkins. Glen McDowell and Richard Fisher: Force transmission by indirect retainers when unilateral force is applied. J Prosthet Dent :47(4);360-365,1982 Glen P. Mcgivney, Alan B. Carr.Mccrakens : Removable Partial Prosthodontics, ed10. St. Louis ;2000 ,The C.V. Mosby Co. G A Lammie and W R E Laird. Osborne and Lammies Partial Dentures. 5th edition. Blackwell