Stress breakers

Stress breakers in removable partial denture

Introduction

The typical removable partial denture is a one-piece casting with clasps, a lingual bar, or in the case of an upper restoration, either a horseshoe type or a palatal bar rigidly connected with the saddle.

It is generally recognized that as long as the denture saddles remain in contact with the tissue, moderate occlusal forces can be absorbed without causing damage to the abutment teeth.

The damage occurs when the ridge resorption takes place. On a partial prosthesis where all or most of the posterior teeth are missing, the restoration, in effect, becomes a Class I lever with the occlusal rests as fulcrum points.

Furthermore, ridge resorption, due to the patient's chewing habits of favoring one side of the arch over the other, is very seldom equal, causing a further strain on the abutment teeth.

When an occlusal load is applied in a tooth-tissue supported partial denture, the denture becomes instable due to difference between compressibility of abutment teeth and soft tissues.

As the tissues are more compressible, the amount of stress acting on the abutments is increased, resulting in harmful effects on abutment teeth.

So to protect the abutment from the harmful effects, stress breakers are incorporated in the denture.

class I lever effect on abutment teeth

When uneven resorption occurs, the rest of the right abutment becomes the fulcrum point.

If resorption under one saddle is severe enough, the tendency would be for the right abutment to drift posteriorly with a corresponding drift of the left bicuspid abutment.

Stress director

Definition: A device or system that relieves specific dental structures of part or all of the occlusal forces and redirects those forces to other bearing structures or regions. (GPT 8)

Since edentulous ridge has to bear some part of the masticatory load, ridge resorption is likely to occur. At the same time the abutment tooth is subjected to torques in both antero-posterior and bucco-lingual directions, which are liable to cause the premature breakdown of its supporting bony tissues.

Stress equalization

According to this concept compressibility & resiliency of tooth, periodontal ligament & mucosal tissue is different

It causes an unequal distribution of stress on abutment teeth & alveolar ridge resulting in weakening of abutment teeth

So it advocates use of Stress Director Or Stress Equalizer

Types of stress breakers

Stress breakers can be divided into two groups:

1. Those having a movable joint between the direct retainer and the saddle.

2. Those having a flexible connection between the direct retainer and the saddle

Type 1

First group are necessary when a precision attachment is used.

Allow independent movement of the denture base (or its supporting framework) and the direct retainers. This form of stress-breaker, also referred to as a stress equalizer, has been used as a means to compensate for inappropriately designed removable partial dentures.

Hinges in upper RPD.

Distal sress equalizers (DSE) hinges

Used on bilateral clasp retained free end removable partial dentures.

Reduce loading or torquing of abutments.

The small size is easy to work with and eliminates multiple inventory requirements. The unique design provides for easy freeing after casting and provides total lateral stability.

For patients, it allows patient comfort and abutment protection by allowing independent unilateral function eliminating torquing leverage on the abutments on the nonfunctioning side.

DSE hinges

Kennedy Class II, modification 1, removable partial

denture using hinge stress-breaker of Baca design

Type 2

In the second group the flexible connector may be a wrought gold wire or a bar of cast metal of thinner section than the lingual bar.

The tapered, round wrought-wire clasp arm acts somewhat as a stress-breaker between the denture base and the abutment tooth by reducing the effect of the denture base movement on the tooth through its flexibility.

Existing removable partial denture with cast connector and acrylic teeth and saddles.

Early type of stress equalizer. The direct retainers (clasps) are connected by 1 6-gauge round wrought wire.

Advantages of wrought wire over cast connector

1. A more flexible union is obtained between the retainer unit and the saddle.

2. The round section of the wire connector allows equal freedom of movement in all directions radial to sections whereas flatter cast connector allows movement best where stress acts across the thinnest section.

3. The metallurgical structure of wrought wire makes it less liable to fracture as a result of intermittent stressing.

Disadvantages of wrought wire over cast connector

1. The wrought wire is always soldered to the saddle or retainers. These soldered joints may fracture as a result of fatigue.

2. Since soldering to chrome cobalt alloys is difficult, wrought connectors are usually limited to the gold dentures.

3. The gold wires are more liable to be deformed accidentally than the thicker cast connectors.

4. The accurate relationship between saddles and retainer unit is liable to be lost when soldering.

Advantages of stress breakers

Alveolar support of the abutment teeth is preserved.

Equal distribution of stress between abutment teeth and residual ridge.

Abutment teeth are not damaged.

Avoid frequent need of relining and rebasing.

Disadvantages of stress breakers

Design is complicated and expensive.

The assembly is very weak and tends to fracture easily.

Difficult to repair.

Reduced stability against horizontal forces.

Both vertical and horizontal forces are concentrated on the ridge leading to excessive ridge resorption.

Conclusion

It is concluded that harmful horizontal torques acting on the abutment teeth are reduced by stress breaking and that, in consequence, their supporting structures are less liable to break down.

However, the edentulous ridge is called upon to accept more vertical and horizontal stress and as a result tends to resorb quickly.

Summing up, decreased stresses are applied to the standing teeth and increased stresses are applied to the edentulous ridge areas.

Stress breakers in fixed partial dentures

Introduction

A pier abutment is a single tooth with two edentulous spaces on either side. In this case the single tooth will have to act as an abutment for both the edentulous spaces in the Dental Bridge.

Since a single abutment supports two edentulous spaces, these sorts of Dental Bridges are subjected to unbalanced forces which can lead to trauma of the periodontium.

The forces acting on one end of the Dental Bridges will tend to lift the other end of prosthesis and thus there will be lever forces and thus the life time of Dental Bridges will be reduced.

Because the pier abutment used in rigid FPDs can act as a fulcrum, the restoration of a terminal abutment may become loose.

Pier abutment acting as fulcrum (blue represents

anterior loading, and red represents posterior loading situations).

In order to prevent trauma to the abutment a stress breaker is provided with in this sort of Dental Bridges near the pier abutment.

The stress breaker is a non-rigid connector with a key in key-way.

If the pier abutment is mobile then a rigid connector should be used instead of non-rigid connector

The stress breaker is a key in a keyway.

The keyway is usually placed on the distal surface of the pier abutment.

The male component or the key is attached to the mesial surface of mesial pontic of distal edentulous space.

During occlusion molar tend to tip mesially and hence excessive stress is transferred through the connector to pier abutment.

Forces acting on the distal end of the distal edentulous space will disengage the connector and prevent stress to the pier abutment.

Non rigid connector

Incorporated in the wax pattern stage

Consists of :

1. A mortise (also referred to as the female component) prepared within the contours of the retainer

2. A tenon (also referred to as male component) attached to the pontic.

The mortise is usually placed on the distal aspect of the anterior retainer.

Mortise must parallel the path of withdrawal of the distal retainer.

Contraindication to non rigid connector

In case of a mobile pier abutment a rigid connector is used.

If the posterior abutment does not have any opposing tooth, it may supra-erupt and unseat the key.

Conclusion

The area of minimum stress concentration occurs in pier abutment when a nonrigid connector is located at the distal region of the pier abutment.

References

McCracken’s removable partial denture 11th ed.

Partial dentures John Osborne, George Alexander Lammie 4th ed.

Clinical removable partial prosthodontics Stewart, Rudd, Kuebker 2nd ed.

Contemporary fixed prosthodontics Rosensteil, Land and Fujimoto 3rd ed.