Role of Stress Breakers in removable partial denture

Class I, Class II And Class IV Are Greater Subjected Stresses To Than Class III because support for a prosthesis must be derived from both teeth and soft tissue. Soft tissue being compressible, vertical and movements. permits rotational

CLASS III

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 Mc. Cracken • Stress breakers are those elements of a partial denture which are interposed in a connector system in order to introduce a controlled and intentional degree of flexibility into the structure. [Alan A grant and Wesley Johnson]

• Stress equalizer is an integral part of a partial denture that will enable the operator to limit the movements between the clasps on the abutment teeth and the free end saddle to such a degree that the movement will be within the physiologic tolerance of the underlying tissues and equalize the stress between the abutment teeth and edentulous area. [Herman levitch]

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

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

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.

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 posteriorslot. 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 breaking. stress

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.

Mesial placement of occlusal rests

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.

Split palate stress breaker

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.

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.

Quadrilateral Configuration The quadrilateral configuration is indicated most often for Class III arches particularly when there is a modification space on the opposite side of the arch. A retentive clasp should be positioned on each abutment tooth adjacent to the edentulous spaces. This results in the denture being confined within the outline of the four clasps, and leverage on the denture is effectively neutralized.

Quadrilateral Configuration

For a Class III arch where no modification space exists, the goal should be to place one clasp as far posterior on the dentulous side as possible and one as far anterior as space and esthetics permit. This retains the quadrilateral concept and is the most effective way to control stress.

Tripod Configuration Tripod clasping is used primarily for Class II arches. If there is a modification space on the dentulous side, the teeth anterior and posterior to the space are clasped to bring about the tripod configuration. If a modification space is not present, one clasp on the dentulous side of the arch should be positioned as far posterior as possible, and the other, as far anterior.

TRIPOD CONFIGURATION

Bilateral Configuration In case of bilateral distal extension group the single retentive clasp arm on each side of the arch should be located near the centre of the dental arch or the denture bearing area. In the bilateral configuration the clasps exert little neutralizing effect on the leverage induced stresses generated by the denture base.

Precision attachment Introduction The precision attachment is a special type of direct retainer used in partial denture construction. It consists of a closely fitting key/keyway mechanism, one part of which is attached to the abutment tooth and the other to the metal framework. When the two units are fitted together they provide direct retention by means of a combination of friction and spring action.

Frequently used synonyms are • Internal attachment • Frictional attachment • Slotted attachment • Key/keyway attachment • Parallel attachment

In 1960, Schatzmann introduced an attachment where retention was enhanced by a spring loaded plunger, in the male unit engaging a slot in the female unit. Some of the attachments that has come to the market recently include: • Ney – Chayes attachments • Stern Goldsmith attachments • Baker’s attachments

Precision and Semi precision Attachments Precision attachments may be prefabricated by a manufacturer or they may be fabricated in the dental laboratory. The former is called precision and the latter semi precision attachment. The manufactured type of attachment is made of precious metal and thus is more precise in construction than is the typical laboratory fabricated attachment. The male portion most often takes the shape of a "T" or "H, " which fits an appropriately shaped slot.

The female attachment is fitted into the restoration in the tooth either by casting the gold to it or by placing it in a prepared receptacle in the restoration and by attaching the two together with solder. The semi precision attachment is also referred to as the "precision rest, " the "milled rest, " or the "internal rest. " This type of retainer takes the form of a dovetail shaped keyway built into the proximal surface of a wax pattern of a gold crown. The stud or male portion is then made as an integral part of the metal framework.

Advantages • They can provide excellent transfer of vertical and horizontal loads from the denture saddles to the abutment teeth on which they are placed. • The precision attachment is less stressful to the abutment tooth than is the conventional clasp. The reason is that it is located deep within the confines of the tooth, all stress is directed along the long axis of the tooth, thus being resisted by virtually all of the fibers of the periodontal ligament. Stress directed in this manner is concentrated nearer to the center of rotation of the tooth than is the case with a conventional clasp.

• When four strategically located teeth (in all four quadrants of the mouth) are available, it must be conceded that masticatory stresses are almost ideally controlled with precision attachments. • Good aesthetics are obtainable, with no clasp arms being visible, for instance. • They may be more hygienic, with minimal external components present to trap food and accumulate plaque. • They are generally well tolerated by patients & as their form normally avoids irritation of the tongue, lips or cheeks.

Disadvantages • Their application is usually much more time consuming in both the clinical and laboratory areas. In addition, the intrinsic cost of many of the attachments is high. Thus their use may need to be limited by economic considerations. • Placement of an attachment on an abutment tooth may necessitate the removal of a considerable amount of sound tooth substance.

• Special problems can arise where a precision attachment is to be used to provide retention of a free end saddle. If an attachment is used which provides a rigid link between the saddle and the abutment tooth, then destructive overload of the periodontal attachment of the abutment tooth can occur. Instead, it is usually recommended that an attachment of a more complex (and hence expensive) type should be used which provides some form of flexible link between the saddle and the abutment tooth, as well as providing direct retention. Attachments of this type are referred to as stress broken attachments.

Limitations of the Precision Type of Attachment • precision type of attachment will not be successful when used with the tooth that has either a short or a very small crown. • Another factor limiting the use of the precision device is the size of the pulp, because of the danger of encroachment on this sensitive organ.

Classification (Alan A grant , Wesley) A primary classification of precision attachments based on the site of attachment to the abutment tooth: • Class 1 Coronal attachments • Class 2 Root face attachments

• For Class 1, sub classification may be made on the basis of the site at which retention is achieved: Class IA Extra coronal attachments Class IB Intra coronal attachments • Class 2, sub classification may be made on form of the attachment: Class 2 A Stud type Class 2 B Bar type

Classification [G. E. Ray] • Intracoronal • • • Extracoronal Conjunctors Anchors Bars Accessory components

Classification according to rigid or movable articulations [G. E. Ray] • Rigid articulations Group I: Attachments used principally with vital teeth Group II: Anchorage used principally with pulp less teeth • Movable articulations Group I: Conjunctors used principally with vital teeth Group II: Conjunctors used principally with pulpless teeth

• Extracoronal attachments The retentive element lies external to the crown of the abutment tooth. The extra coronal type of retainer often has built into a movable joint of one type or another (a stress breaker) that permits the base to move independently of the retainer. Three Groups Of Extra Coronal Precision Attachments: 1. Those attachments that project from the crown of a tooth to which prosthesis is attached. E. g. Dalbo 669. The advantage is that, there is minimal preparation of the abutment tooth. Also where there is insufficient bucco lingual width. disadvantage It may cause gingival irritation.

2. Connecting Rotating Attachment: These do not anchor a prosthesis to the tooth as such, they merely provide a joint which may allow some movement between abutment tooth and denture. e. g. : Steiger and Boitel rotation joint 3. Combined Units: These consist of a hinge type of connecting element outside the tooth which is joined directly to an attachment. e. g : Crismani combined unit [spring controlled] stern stress breaking unit [simple hinge]

Intracoronal attachments These are generally much more complicated in form than the extra coronal attachments. The female unit of the attachment is set within an abutment inlay or crown and presents a dovetailed slot. The male unit is attached to the denture saddle. When the two units slide into each other they provide frictional grip retention, which may be augmented in some types by a spring action. These are of two types: • Retention by friction • Retention enhanced by mechanical lock

Types Of Intracoronal Attachments • Earlier designs introduced by Chayes had 'T' shaped flanges. Later 'H' shaped flanges came to the market. These flanges help increase the frictional surface area and hence the bracing action and retention. Retention is adjusted by opening the base of the male unit with a special instrument.

Application Of Intra coronal Attachment 1. R. P. D Retainers: to retain bilateral bound saddle and free end saddle partial denture. Unilateral bound saddle denture 2. Connectors: Sections of a fixed prothesis may be joined in groups by intracoronal attachments. In case of long space edentulous cases, it also provides stress breakage.

3 Free end Saddle Removable Partial Denture: Precision attachment may be the choice of attachment in such situations. A strong attachment should be selected as the attachments used here are subjected to considerable forces.

Ball and socket /stud type attachment • As a preliminary to the use of such attachments, root treatment of the abutment tooth is carried out and the crown is cut off near to the level of the gingival margin. They are particularly indicated for use where the periodontal status of the root is adequate but the condition of the crown is judged to be unsuitable to allow placement of a coronal attachment. A cast metal post and diaphragm are prepared for the root, the diaphragm offering retention for the base unit of the stud.

BALL AND SOCKET /STUD TYPE ATTACHMENT

The male unit is attached to the root face diaphragm and carries a ball shaped head. The female unit is placed within the denture saddle and has a hemi spherical cavity to provide frictional fit on the ball head of the male unit. Vertical slits in the female unit allow a degree of flexibility and provide grip action as the two units are fitted together. These are used in a nearly edentulous conditions where only 2 or 3 teeth are remaining. e. g. 1) Gerber attachment 2) Dalbo system 3) Rothermann attachment Adjustments for wear is done either by unscrewing or compressing the sleeves.

Bar type attachments. As in the use of attachments of the Class 2 A type, abutment teeth are root filled and the crowns removed near to gingival level. Posts and diaphragms are again prepared. The attachment takes the form of a bar which extends between and is attached to each of two diaphragms. eg. , dolbar

Swing lock denture The term swing lock describes a special design of partial denture which can be used in any situation in the mouth but is particularly useful where there has been loss of a considerable number of teeth. The appliance is designed with a rigid lingual or palatal plate connector to which is attached, by means of a hinge, a labial or buccal bar. This bar is placed in the sulcus and has struts or fingers, extending to contact the labial or buccal surfaces of the natural teeth below the survey line, so providing retention.

Swing lock denture