Role of Stress Breakers in removable partial denture

1. Role of Stress Breakers in removable partial denture

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

3. CLASS III

4. 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 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]

5. 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]

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

7. 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

8. 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.

9. 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.

10. 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.

11. 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.

12. 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 .

13. 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.

15. 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.

16. 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).

17. 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.

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

19. 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 antero­posterior 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.

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

21. 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 .

22. Mesial placement of occlusal rests

23. 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.

24. 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.

25. Split palate stress breaker

26. 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.

27. Advantages • Since the horizontal forces acting on the abutment teeth are minimized, the alveolar support of these teeth is preserved. • 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.

28. 3 Intermittent pressure of the denture bases massages the mucosa, thus providing physiologic stimulation, which prevents bone resorption and eliminates the need for relining. • If relining is needed but not done, the abutment teeth are not damaged as quickly. • Splinting of weak teeth by the denture is madepossible despite the movement of a distal extension base.

29. Disadvantages • The broken stress denture is usually more difficult to fabricate and therefore more costly. • 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. • If relining is not done when needed, excessive resorption of the residual ridge may result. .

30. The effectiveness of indirect retainers is reduced or eliminated altogether. • 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. • 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.

31. 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.

32. Quadrilateral Configuration

33. 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.

34. 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.

35. TRIPOD CONFIGURATION

36. 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.

37. 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.

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

39. 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

40. 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.

41. 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.

42. 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.

43. 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.

44. 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.

45. 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.

46. 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 thissensitive organ.

47. 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

48. 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 2A - Stud type Class 2B - Bar type

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

50. 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