Stress breakers /prosthodontic courses/ oral surgery courses

St ress
Breakers
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

Contents
• Introduction
• Definition
• Types of stresses created on the abutment teeth
• Concept
• Aims of stress breaking
• Guidelines of stress breaking
• Classification
• Advantages and disadvantages
• Factors that influence the magnitude of stress that is
transmitted to the abutment tooth
• Design considerations in the control of stress

Introduction
Stress may be defined as force per unit area within a
structure subjected to an external force or pressure.
Stress exerted against the teeth and their attachment
apparatuses by occlusal forces may be within the
adaptive capacities of the tissues or else the tissues may
not be capable of compensation and adaptation and the
result is tissue destruction.

Various approaches are been made to distribute the
stress or forces acting on a partial denture between the
soft tissue and teeth one among them is stress breaker.
The stress breaker is suggested as a means to allow
the base to move slightly with less strain to the
abutments . www.indiandentalacademy.com

Definition
• A stress breaker is a device that allows some
movement between the denture base or its
supporting framework and the direct
retainers [whether they are intracoronal or
extracoronal] by McCracken
• Stress breaker is a flexible or a movable joint
between the teeth and the metal framework,
so that the denture base can move
independently of the clasp. [Joseph.E.
Grasso]

Types of stresses created on the abutment teeth
In general there are three types of stresses on the
abutment teeth
• Vertical
• Lateral
• Oblique/anteroposterior

A fundamental fact is that the periodontal ligament is not
designed by nature to provide a cushioning effect for the
tooth but is a suspensory ligament by means of which
the tooth is suspended in its alveolus thus, the horizontal
stress applied against the tooth will be resisted by fewer
than half of the periodontal membrane fibres, whereas a
vertical stress will be resisted by all of the fibres with the
exception of those at the apex.

The forces that act on the tooth in a direction along its
long axis are transferred by the periodontal ligament to
the bone as tension, which is tolerated quite well. In
contrast to this, the transverse or torsional stresses that
are transmitted to the tooth are transferred to the
periodontal ligament and to the bone as pressure, which
is not well tolerated. Depending on the magnitude and
the duration of the stress, the result may be crushing of
the periodontal ligament, or even necrosis and bone
resorption.

Thus vertical stress results from a lack of distal tooth
support.
lateral stress results from a horizontal movement of the
denture .
anteroposterior stress is a result of a combination of the
first two.
In all types of stress, the abutment becomes the fulcrum.
To control these stresses and to distribute them between
mucosa and the adjacent teeth requires a careful
consideration of:
1] The condition of the teeth and mucosa
2] The impression techniques
3] The denture design
4] The distribution of stress between the mucosa and as
many supporting teeth as possible

Concept
A partial denture base that is unsupported at one end
may move on its displaceable foundation when
masticatory loads are applied, the prospect of this
movement will transmit torsional stress to the
abutment through the direct retainer which may affect
the health and longevity of the abutment tooth. The
resiliency of the average tooth under load is 0.1
mm,whereas the alveolar ridge which is 4 to 20 times
as displaceable as a natural tooth, may be displaced
between 0.4 to 2mm.

These have led to the concept that the abutment tooth
should be relieved of this load and that part of this
burden should be placed on the residual ridge. The
transfer of stress is accomplished either by the
employment of specially designed device interposed
between the denture base and the clasps or the frame
work design that permits movement of the base
independently of the clasp .The direction and extend of
the movement that the base is permitted to make
depends on the design and the construction of the
particular stress breaker device being used.

According to Kennedy a well designed round wire clasp
is itself a stress breaker and allows enough saddle
movement to prevent excessive strain on the abutment
teeth.
The basic requisite for an ideal stress breaker is to
equalize the stress placed on the abutment teeth and
edentulous areas so that the work load for each will
approach their physiologic tolerance.

Aims of stress breaking
• To direct occlusal forces in the long axis of the
abutment teeth.
• To prevent harmful loads being applied to the remaining
natural teeth.
• To share load as evenly as possible between the natural
teeth and saddle areas according to the ability of these
different tissues to accept the loads.
• To ensure that part of the load applied to the saddle area
is distributed as evenly as possible over the whole
mucosal surface.
• To provide greater comfort to the patient.

Guidelines for stress breaker
To decide whether to use a stress breaker or a rigid
design
Rule 1: If the teeth are strong and the ridge is poor
flat, knife edged, or narrow- use a rigid design.
Rule 2: If the teeth are weak e.g., loss of supporting
tissues so the mobility is plus or more and the ridge
is strong, use a stress breaker.

Classification
Stress-breakers can be classified according to their mode
of action:
• Type 1 - Those utilizing a hinge or moveable joint.
( moveable joint between direct retainer and the denture
base )
• Type 2 - Those utilizing flexible connection.

Type 1 Stress-breakers
• These can be used in association with either precision
attachments or clasp units as tooth-bearing direct
retainers. In this group fall the hinges, sleeves and
cylinders, and ball and-socket devices.
• The hinge is usually of a rigid design, the soft tissue
absorbs a minimum of load adjacent to the hinge and a
maximum of load toward the distal of the ridge. The base
is permitted movement in a vertical plane only. The
movement may be unrestricted, or it may be controlled
within definite limits by a stop arrangement built into the
device.

• This serves to prevent some direct transmission of
tipping forces to the abutment teeth as the base moves
tissue ward under function. The hinge type of device
spares the tooth from all stresses that results from
vertical movement of the base, but it is still subjected to
all the lateral and torsional stresses .

• An example of this
group are the various
hinges, the Swiss
made Dalbo
attachment and the
Crismani attachment.
• If the device works on
a ball and socket
principle, movement
of the base is allowed
in all planes and the
tooth is relieved of all
stress.

Type 2
Second group include those design having a flexible
connection between the direct retainer and the denture
base including wrought wire connectors, divided major
connectors and other flexible devices for permitting
movement of the distal extension base also included in
this group are those using a moveable 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.
These are normally used in association with clasp units
as direct retainers .

Various forms which are commonly applied are:
1. Torsion bars/split bar major connectors
Used in the design of a lower partial denture
carrying bilateral free- end saddles. Bars extend
anteriorly from the clasp units on each side to
join a lingual bar near the midline.
Flexibility can be controlled by varying the
cross-section of the torsion bars, the method of
construction (cast or wrought) and the material
of construction (normally gold alloys or cobalt
chromium alloys).

Disadvantages are
• In a torsion bar structure in that the double
bar system is liable to trap food and cause
irritation to the tongue.
• Some split connectors used as stress
breakers have been known to pinch the
underlying soft tissues or the tongue as
they open and close under function.

Torsion bar stress-
breaker used in a
lower partial
denture.

2. Partial division of connectors
This principle can be applied in both upper and lower
dentures. For example, in a lower denture, a lingual plate
may be partly divided by an antero-posterior slot. The
upper portion of the plate is attached to the retainer unit
on the abutment tooth and the lower portion is attached
to the saddle a degree of flexibility between the retainer
unit and the saddle is so developed.

A lower partial
denture framework
with partial division
of a lingual plate to
achieve stress
breaking.

3. Mesial placement of occlusal rests
This offers the simplest available approach to
stress-breaking. The degree of stress-breaking
achieved is though, much less than that
available where more complex devices are
employed. It may be used in the design of either
upper or lower dentures.
By positioning the rest of the clasp unit on the
mesial instead of on the distal fossa of the
abutment tooth and by using a minor connector
to link the rest to a major connector some
flexibility may be introduced into the clasp
unit/saddle link .www.indiandentalacademy.com

Other types of stress breakers
12-gauge chrome wire stress breaker
The advantages :
• The extension base moves vertically immediately and
the resiliency of the wire quickly returns the base to its
original position
• The rigidity of the 12 gauge wire avoids overloading the
mucosa.
• The mucosa is also more evenly loaded.
• It is easy to splint teeth with this design.
• The fabrication is relatively simple .Repairs are rarely
needed.

Split palate stress breaker
A stress breaker for a maxillary partial denture is often
not necessary, has there is more alveolar ridge for
support .
In case of Kennedy class V, partial denture may be
difficult to design, as the placement of a rest on the weak
lateral incisor is not considered desirable. The left
second molar would be required to absorb most of the
load of mastication on the left side. In such cases a split
palate stress breaker was designed.

The amount of movement is tested by seating the casting
in the mouth and creating vertical movement using finger
pressure on the area of the extension base. The isolated
molar is now splinted to the right side by the rigid major
connectors and the use of multiple clasps. During
loading, the denture base will move vertically with equal
force on the soft tissues.

Advantages
1. Since the horizontal forces acting on the abutment
teeth are minimized, the alveolar support of these
teeth is preserved.
2. By careful choice of the type of flexible connector, it is
possible to obtain a balance of stress between the
abutment teeth and the residual ridge.

3 Intermittent pressure of the denture bases massages
the mucosa, thus providing physiologic stimulation,
which prevents bone resorption and eliminates the
need for relining.
4 If relining is needed but not done, the abutment teeth
are not damaged as quickly.
5 Splinting of weak teeth by the denture is made
possible despite the movement of a distal extension
base. www.indiandentalacademy.com

Disadvantages
1. The broken stress denture is usually more difficult to
fabricate and therefore more costly.
2. Vertical and horizontal forces are concentrated on the
residual ridge, resulting in increased ridge resorption.
Many stress breakers designs are not well stabilized
against horizontal forces.
3. If relining is not done when needed, excessive
resorption of the residual ridge may result.

4. The effectiveness of indirect retainers is reduced or
eliminated altogether.
5. The more complicated the prosthesis ,the less it may
be tolerated by the patient. Spaces between
components are sometimes opened up in function,
thus trapping food.
6. Flexible connectors may be bent and distorted by
careless handling. Even a slightly distorted connector
may bring more stress to bear on the abutment
7. Repair and maintenance of any stress breaker is
difficult, costly, and frequently required.

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