Definition:

Stress breakers can be designed in RPD’s by using connectors fitting between the retention unit and denture base. It is designed to relieve abutting teeth from excessive stress during chewing. Stress breakers benefits:

Horizontal forces transferred to teeth are minimised.
There is a balance of force between the teeth of the alveolar ridge.
The intermittent pressure results in a massaging effect to the alveolar ridge.
Re-lining is unnecessary and is claimed to save the abutment tooth.
Splinting of mobile teeth is still possible.

Stress breakers come under two categories:

- Type 1 stress breaker- utilizes a hinge or movable joint between the direct retainer and the denture base,

This joint may be in the form of hinges, ball and socket devices or sleeves and cylinders. Hinged type stress breakers allows vertical and hinge movement of the base.

The hinge is usually rigid. The soft tissue absorbs a minimum of the load adjacent to the hinge, and a maximum of load toward the distal of the ridge. The base aloows movement in a vertical plane only. The movement may be unrestricted, or it may be controlled by a stop arrangement built into the device. This serves to prevent some direct transmission of the tipping forces to the abutment teeth as the base moves toward the tissue under function. The hinge type of device spares the tooth from all stresses that results from vertical movement of the base.

- Type 2 stress breaker- utilizes a flexible connection between the direct retainer and the denture base.

This design allows movement of the distal extension base. Also included in this category are those using a movable joint between two major connectors. The earliest of such connectors were double lingual bars of wrought metal, one supporting the clasp and the other components and the other supporting and connecting the distal extension bases

More commonly there are:

Split major connectors-A lower RPD framework with partial division of a lingual plate to achieve stress breaking action. (split is provided between the denture base area and major connector) when occlusal forces are applied they’re transferred more towards the tissue supported base and then they are transmitted to the abutment teeth.
Wrought wire connectors soldered to the main major connector
Clasps having a stress breaking effect
Hidden lock partial dentures- two piece casting, the top half is cast with major connector supporting the direct retainers and then the bottom half (the connector between denture base) is cast to the major connector. The split between the two connectors is made by a thin oxide shell. The bottom half (connector) is two bars connected by the midline.
Disjunct partial dentures- tooth borne and mucosa borne parts of denture are disjoined. Provides independent movement between the tooth supported and tissue supported parts decreases the forces on periodontally weakened remaining teeth.

Stress breakers may be indicated if the patient has well formed residual ridges and weak abutment teeth. This is because the forces applied to the abutment teeth must be minimal in order to maintain the remaining health of the teeth. Too much force and the abutment teeth may be damaged. As the ridges are still healthy the stressbreaker is able to minimize the forces applied to the abutment teeth and direct it down into the tissues. If the ridges had resorbed the tissues would be left more compressible resulting in more stress acting on the abutment teeth.

Another indication is if there are distal extensions in the RPD. The longer the saddle the more torqueing force and rotation may occur, these forces will increase the stress applied to the abutment teeth.

Finally if internal attachments are used to retain a distal-extension base…..

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Explain the RPI clasp system in RPD construction.