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Indirect Retainers

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

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Indirect Retainers

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  1. Indirect Retainers

  2. 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

  3. Introduction

  4. 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.”-GPT8

  5. 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

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

  7. 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.

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

  9. 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.

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

  11. Rotation of denture bases around various axes

  12. Rotation Around a Transverse Axis

  13. Rotation Around A Longitudinal axis

  14. Rotation Around A vertical Axis

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

  16. Mechanical principles applicable in Removable Prosthodontics

  17. Lever principle • Inclined plane • Snowshoe principle • L beam effect

  18. 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 I Class II Class III

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

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

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

  22. 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.

  23. 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.

  24. 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.

  25. Principle of Indirect Retainer

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

  27. 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.

  28. 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

  29. 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

  30. 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.

  31. 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,

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

  33. Auxiliary functions of indirect retainers

  34. 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.

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

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

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

  38. 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.

  39. 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.

  40. 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.

  41. Factors influencing effectiveness of indirect retainers

  42. Effectiveness of the direct retainer – • 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.

  43. 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.

  44. For better action of the indirect retainers the distance from it to the primary rest should be more than the distal extension base . • 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).

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

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

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

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

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