Indirect Retainers Contents Introduction Definitions Stress consideration in

Indirect Retainers

Contents § § § § § Introduction Definitions. Stress consideration in a partial denture Rotation of denture bases around various axis Mechanical principles applicable in Removable Prosthodontics. Principle of Indirect Retainers Factors Influencing Indirect Retainers Auxiliary Functions Forms of Indirect Retainers

l Introduction

Definition “The component of a removable partial denture that assists the direct retainer 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. ”-GPT 8

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

Stress consideration in a partial denture The stresses can be divided as: Ø Vertical Displacing stresses Dislodging stresses Ø Horizontal Ø Torsional

Vertical stress Displacing stresses : Those forces which are the result of downward stresses along the long axis of the teeth in a crown to apex direction and the relatively vertical stresses on the ridge mucosa.

Dislodging stresses : These are the forces which tend to lift the partial denture from it’s rest position.

Horizontal stress They originate as a component of rhythmic chewing stroke. These forces are effective in mesio-distal and buccolingual direction. These lateral stresses are most damaging.

Torsional stress It is a twisting rotational type of force. It’s a combination of vertical and horizontal force.

l Rotation of denture bases around various axes

Rotation Around a Transverse Axis

Rotation Around A Longitudinal axis

Rotation Around A vertical Axis

l l l Resisted by the stabilizing components such as reciprocal clasp arms and minor connectors that are in contact with the vertical tooth surface.

Mechanical principles applicable in Removable Prosthodontics

l Lever principle l Inclined plane l Snowshoe principle l L beam effect

l Lever: A simple machine consisting of a rigid bar pivoted on a fixed point and used to transmit force, as in raising or moving a weight at one end by pushing down on the other. Three classes of levers (based on location of fulcrum, resistance and direction of effort (force). Class III

Class I lever Class I Fulcrum lies in the centre, Resistance is at one end and force at the other.

Class II lever Class II Fulcrum is at one end effort at the opposite end and resistance in the centre.

Class III lever Class III Fulcrum is at one end, resistance at opposite end and effort is in the centre.

Inclined plane Forces against an inclined plane may result in deflection of that which is applying the forces or may result in movement to the inclined plane, neither of these is desirable.

Snowshoe principle This principle is based on distribution of forces to as large an area as possible. Like in a snowshoe which is designed to distribute forces on the entire base area of the shoe, a partial denture should cover maximum area possible within the physiologic limits so as to distribute the forces over a larger area.

l L beam effect : This principle is applicable to the antero posterior palatal bar or strap major connector. In this component there are two bars /strap lying perpendicular to each other. The anterior and posterior bars are joined by flat longitudinal elements on each side of the lateral slopes of the palate. The two bars lying in two different planes produce a structurally strong L beam effect that gives excellent rigidity to the prosthesis.

Principle of Indirect Retainer

Rotational movement away from the tissue or ridge, The tip of the retentive clasp will act as fulcrum thus lifting the distal extension base

l l Sticky foods, tissues adjacent to the borders of the denture, such as tongue or buccinator muscle may lift the denture base from the ridge, when they are activated by speech chewing or swallowing. Gravity exerts an unseating force on maxillary prosthesis.

• If a additional rest is introduced anteriorly then it will act as fulcrum thus converting the class I lever to class II lever situation. • Now when the distal extension base is lifted the auxillary rest will prevent this movement as it will enhance the retention property of the clasp

l As the rest is moved anteriorly (MO), it moves the point of rotation anteriorly as compared to the conventionally placed disto-occlusal rest. It also transmits forces to the edentulous ridge in a more nearly vertical direction

l l An indirect retainer consists of one or more rests and the supporting minor connectors, The proximal plates adjacent to the edentulous areas also provide indirect retention.

l l l An indirect retainer should be placed as far from the distal extension base as possible, Most effective location of an indirect retainer is commonly in the vicinity of an incisor tooth, But incisor tooth may not be strong enough and may have steep inclines, -not favourable to support a rest,

l l In such situation, nearest canine or mesioocclusal surface of the first premolar – best location. Two indirect retainers closer to the fulcrum line are used.

Auxiliary functions of indirect retainers

l l 1. It tends to reduce anteroposterior-tilting leverages on the principal abutments, ---this is particularly important when an isolated tooth is being used as an abutment. Proximal contact with the adjacent tooth prevents such tilting of an abutment as the base lifts away from the tissues.

ü 2. Contact of its minor connector with axial tooth surfaces ü Contributes to support and stability of the partial denture counteracts horizontal forces applied to the denture. ü

l 3. Anterior teeth supporting indirect retainers are stabilized against the lingual movements.

l l 4. It may act as an auxiliary rest to support a portion of major connector facilitating stress distribution. eq. lingual bar.

l l Long span mandibular lingual bar major connector IR provides additional support and rigidity for lingual bar. Prevents impingement of the lingual bar on the mucosa during function.

l l 5. provide the first visual indications for the need to reline an extension base partial denture. Deficiencies in basal seat support are manifested by the dislodgement of indirect retainers from their prepared rest seats when the denture base is depressed and rotation occurs around the fulcrum.

l l The indirect retainer also acts as a third point of contacting tooth structure to ensure accurate repositioning of the frame work on the teeth during relining and rebasing, Occlusal rests on the terminal abutment teeth acts as the other two of the three points of the tripod.

l Factors influencing effectiveness of indirect retainers

Effectiveness of the direct retainer – l l The principal occlusal rests on the primary abutment teeth must be reasonably held in their seats by the retentive arms of the direct retainers. otherwise the partial denture will be dislodged.

l l In all class I and class II cases, the rest seat preparation must be saucer shaped completely devoid of any sharp angles or ledges. As forces are applied the rests must be free to move within the rest seat prepared to dissipate the forced. . which would otherwise be transferred to the tooth.

For better action of the indirect retainers the distance from it to the primary rest should be more than the distal extension base. l The efficiency of an indirect retainer is affected by the relative lengths of the distance from the end of the denture base to the clasp (AB) and from end of the rest to the clasp (BC).

Distance from the fulcrum line l l Location of the fulcrum line, How far beyond the fulcrum line the indirect retainer is placed.

Location of the fulcrum line, In class 1 arch Passes through most posterior abutments.

In class 11 arch fulcrum line is diagonal passing through the abutment on distal extension side and most posterior abutment on opposite side

If the abutment tooth anterior to the mod space lies far enough it may be used for indirect retainer.

Class iv arch fulcrum line passes through abutment adjacent to single edentulous space

Class iii arch with posterior tooth on the right side (poor prognosis. )

How far beyond the fulcrum line the indirect retainer is placed. Greater the distance between fulcrum line and IR more effective Because the mechanical advantage increases proportionately

Advantages of mesial rests for removable partial dentures. Gene A. Zach, Jpd 1975 ; 33: 32 -35

l l In distal rest, when distal extension move towards the tissue the distal rest pulls over the tooth thus applying stress on the abutment , whereas in mesial rest case , it allows the distal extension base to move towards the tissue and clasp to move more towards the retentive area of the tooth , thus applying less stress to the abutment tooth.

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

EFFECTIVENESS OF THE SUPPORTING TOOTH SURFACE l The indirect retainer must be placed on a definite rest seat on which slippage or tooth movement will not occur

The quality of support of the indirect retainer. An indirect retainers effectiveness is reported to decrease when it is placed on an inclined plane because the tooth moves and the retainer slips. (JPD 1975; 34; 2430)

Thus the choice of indirect retainer location should be made mostly on the basis of 1. Abutment tooth support, 2. A crown form favoring adequate rest seat preparation, 3. and the patients esthetic desires.

Forms of indirect retainers

Auxiliary occlusal rest l Consists of a minor connecter and a rest placed in a rest seat

l l l Most commonly used indirect retainer, Class I mandibular arch this location is usually mesial marginal ridge of the first premolar, Bilateral rests on the first premolar is quite effective,

l l l Maxillary class I same principle is used. Weaker single rooted teeth is not used, Interference with tongue is less, -minor connector is placed In the embrasure between the canine and premolar.

l For class ii – indirect retainers are placed on the marginal ridge of the first premolar tooth on the opposite side of the arch from the distal extension base.

Preparation of Rest Seats Occlusal rest seat: l Rounded triangle l Base at the marginal ridge and apex at the centre of the tooth l One-half the intercuspal distance and one-third/onehalf the mesiodistal width l Base must be less than 900 to long axis of tooth. l Depth 0. 5 at the thinnest portion and 1. 5 mm at the marginal ridge.

Canine Rests l Used when premolar is too close to fulcrum line l Teeth are overlapped so that the fulcrum line is not accessible. Canine rest may be made more effective by placing the minor connector in the embrasure anterior to the canine. l

Canine Extensions from Occlusal Rests

To effect indirect retention by increasing the distance from fulcrum line, Applicable when a first premolar must serve as a primary abutment. Should always be used with terminal rests on the mesial marginal ridge of the premolar tooth.

l Lingual Rest Lingual rests are primarily given on maxillary canines. If a canine is not present, multiple rests are spread over several incisor teeth instead of a single tooth. Lingual rest seat preparations on enamel should never be attempted on mandibular anterior teeth. This is because of lack of thickness of enamel , However, a lingual rest on cast restoration may be used on any anterior tooth, either maxillary or mandibular

l l l Lingual or cingulam rest is preferred over incisal rest because of its mechanical advantage, Incisal rest represents a long lever arm that concentrates stresses transmitted to partial denture on incisal edge of abutment teeth, This magnifies forces beyond that would take place if lingual rest were used.

l l Closer to centre of rotation, more esthetic, less prone to breakage. Rest seat is V-shaped and half moon shaped in configuration.

Cingulum Bar (Continuous Bar) and Linguo Plate They rest on unprepared lingual inclines. Aid the terminal rests in indirect retention. Splints of weak anterior teeth

l l In class I and 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 orthodontic movement is prevented.

Rugae area Coverage of rugae area –the rugae area is thought to be firm and will also help in providing support. Not as effective as positive tooth support. Usually as palatal arms, palatal bearing connectors

l The use of rugae support for indirect retention is usually part of a palatal horse shoe design, because posterior retention is inadequate.