2In the distal extension RPD support is derived from both the residual ridge and abutment teeth. The variation in consistency providing this support allows some rotational movements of the free-end base towards the soft tissues
3Harmful Tissue-ward movements of distal extension under occlusal load are transmitted to the abutment teeth resulting in loosening of these teeth
4This principle is fulfilled by stress breaker design Strain on the abutment teeth is minimized through:Broad tissue coverage,The use of functional basing.Use of narrow teeth and harmonious occ.Placing the artificial teeth on the anterior two-thirds of the base.Correct choice of direct retainer.This principle is fulfilled by stress breaker design
5It is also called "Stress director" or "Stress equalizer". The stress breakerIt is a device or design, which allows movement between the denture base and the retainer to reduce lateral and tipping forces on abutment teeth.It is also called "Stress director" or "Stress equalizer".
6Types of Stress Breakers 1-RPD having a movable joint between the direct retainer and the denture base2-Partial dentures having a flexible connection between the direct retainer and the denture basea- Split major connectorsb-Wrought wire connectors soldered to the main major connectorc-Clasps having a stress breaking effect
7Types of Stress Breakers 1-RPD having a movable joint between the direct retainer and the denture baseThis joint may be in the form of hinges, ball and socket devices or sleeves and cylindersHinged type stress breakers allows vertical and hinge movement of the base
8B- Chrisman intra-coronal retainer A- Dalbo extra-coronal precision attachment deviceHinged type stress breakers allows vertical and hinge movement of the base to prevent direct transmission of tipping forces to the abutment
92-Partial dentures having a flexible connection between the direct retainer and the denture base(a)Split major connectorsA lower partial denture framework with partial division of a lingual plate to achieve stress breaking action
10(b)Wrought wire connectors soldered to the main major connector
11c-Clasps having a stress breaking effect 1- R.P.I.2- Reverse Aker Clasp3- R.P.A.4- WROUGHT WIRE CLASP
12Indications for the use of stress breakers: 1- In distal extension RPDs2- Cases exhibiting well formed residual ridges and weak abutment teeth3- If internal attachments are used to retain a distal-extension base
13Distribution of stress between abutment teeth and the residual ridge. Advantages of Stress BreakersDistribution of stress between abutment teeth and the residual ridge.The alveolar support of abutment teeth is preserved because horizontal forces acting on them are minimized.Physiologic stimulation of the mucosa helps to minimize bone resorption due to the intermittent pressure caused by movement of the denture base.If relining of the denture base is needed but not done immediately for any reason, the abutment teeth are not damaged.
14Disadvantages of Stress Breakers Difficult to construct.If the intermittent pressure are beyond the physiologic tolerance; ridge resorption results.Easily distorted and difficult to repair.Reducing the effectiveness of I.R.Expensive.Less tolerated by patients.Allow food trapping and occasionally tissue-trapping.If relining of the denture base is needed but neglected for a long time , excessive ridge resorption results.