Stress breakers / dentistry dental implants


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Stress breakers / dentistry dental implants

  1. 1. St ress BreakersINDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. 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
  3. 3. 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.
  4. 4. 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 .
  5. 5. 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]
  6. 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. 7. 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.
  8. 8. 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.
  9. 9. 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
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  11. 11. 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.
  12. 12. 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.
  13. 13. 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.
  14. 14. 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.
  15. 15. 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.
  16. 16. 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.
  17. 17. 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.
  18. 18. • 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 .
  19. 19. • 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.
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  21. 21. 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 .
  22. 22. 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).
  23. 23. 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.
  24. 24. Torsion bar stress- breaker used in a lower partial denture.
  25. 25. 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.
  26. 26. A lower partial denture framework with partial division of a lingual plate to achieve stress breaking.
  27. 27. 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
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  29. 29. 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.
  30. 30. 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.
  31. 31. 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.
  32. 32. 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.
  33. 33. 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.
  34. 34. 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.
  35. 35. 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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