Stress breakers /certified fixed orthodontic courses by Indian dental academy


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Stress breakers /certified fixed orthodontic courses by Indian dental academy

  1. 1. INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. Role of St r ess Br eaker s in r emovable par t ial dent ur e
  3. 3. 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 Precision attachments Swing lock denture Review of literature Summary & conclusion References
  4. 4. 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.
  5. 5. Of all partial dentures an all tooth supported or class III partial denture can best resist forces because it, like the fixed partial denture, is supported by abutment teeth on either side of the edentulous space, however there may be only limited movements and the prosthesis may lift in function .These movements create stress that need to be controlled by additional teeth, soft tissues or components of the denture .
  6. 6. Classes I ,II,IV removable partial dentures are subjected to greater stresses because their support is a combination of tooth and soft tissue. Forces must be controlled by maximum coverage of soft tissue by proper use of direct retainers , by placement of the components in the most advantageous position no matter how the impression is made there will be more movement when the prosthesis is supported by soft tissue that is an extension base.
  7. 7. 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 .
  8. 8. 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. being permits Soft tissue compressible, vertical and rotational movements.
  9. 9. CLASS III
  10. 10. 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] • 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]
  11. 11. • 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]
  12. 12. Types of stresses created on the abutment teeth In general there are three types of stresses on the abutment teeth • Vertical • Lateral • Oblique/anteroposterior
  13. 13. 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.
  14. 14. 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.
  15. 15. 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
  16. 16.
  17. 17. 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. According to Steiger and boitel 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.
  18. 18. 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.
  19. 19. 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.
  20. 20. 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.
  21. 21. 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.
  22. 22. 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.
  23. 23. 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 .
  24. 24. • 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.
  25. 25.
  26. 26. 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. These are generally fabricated by the laboratory with a dual casting technique. 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 .
  27. 27. 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 crosssection of the torsion bars, the method of construction (cast or wrought) and the material of construction (normally gold alloys or cobalt chromium alloys).
  28. 28. 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.
  29. 29. Torsion bar stressbreaker used in a lower partial denture.
  30. 30. 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.
  31. 31. A lower partial denture framework with partial division of a lingual plate to achieve breaking. stress
  32. 32. 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 .
  33. 33. Mesial placement of occlusal rests
  34. 34. 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.
  35. 35. Fabrication A master cast with wax block outs and reliefs A 12- gauge wire is adapted to the refractory cast Finished stress breaker The wire is coated with die lubricant and the wax up is completed
  36. 36. 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.
  37. 37. Split palate stress breaker
  38. 38. 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.
  39. 39. 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.
  40. 40. 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.
  41. 41. 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.
  42. 42. 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.
  43. 43. Factors influencing the magnitude of stresses transmitted to the abutment teeth The amount of stress that is transmitted to the abutment tooth, by means of the clasp in a distal extension base type of partial denture, depends on a number of factors Length of the span Longer the edentulous span, the longer will be the denture base and the greater will be the force transmitted to the abutment teeth .
  44. 44. Quality of ridge Large well formed ridges are capable of absorbing greater amounts of stress than are small, thin, or knife edged ridges. Broad ridges with parallel sides permit the use of longer flanges on the denture bases which help stabilize the denture against lateral forces. Type of mucosal covering A healthy mucoperiosteum is capable of bearing a greater functional load than a thin atrophic mucosa .Soft, flabby, displaceable tissue contributes little to the vertical support of the denture and nothing to the lateral stability of the denture base. This type of tissue allows excessive movement of the denture, with resultant transmission of stress to the adjacent abutment tooth .
  45. 45. Clasp as a factor in stress The type, design and construction of the partial denture clasp can greatly affect the severity of the stress that is transmitted to the abutment tooth. Type of clasp The more flexible the retentive arm of the clasp, the less stress is transmitted to the abutment tooth. A flexible clasp arm contributes less resistance to the more destructive horizontal stresses.
  46. 46. As the flexibility of the clasp increases both the lateral and vertical stress transmitted to the residual ridge increase. If the periodontal support of the abutment is good, a less flexible clasp such as a vertical projection clasp would be indicated because the tooth would more likely be able to withstand a greater amount of stress. If the periodontal support has been has been weakened, a more flexible clasp with a wrought wire retentive arm should be used so that the residual ridge would share more of the resistance to horizontal forces acting on the partial denture.
  47. 47. Clasp design • A clasp that is designed so that it is passive when it is completely seated on the abutment tooth will exert less stress on the tooth than one that is not passive. • Only when the framework is completely seated will the retentive clasp arms be passive. • During insertion or removal of the prosthesis the reciprocal arm contacts the tooth before the retentive tip passes over the greatest bulge of the abutment tooth. This will stabilize or neutralize the stress to which the abutment tooth is subjected as the retentive terminal passes over the greatest bulge of the tooth.
  48. 48. Length of clasp Flexibility can be increased by lengthening the clasp. Doubling the length of the clasp will increase its flexibility five times. Clasp length may be increased by using a curved rather than a straight course on an abutment tooth. Material used in clasp construction A clasp made of chromium-cobalt alloy exerts more stress on the abutment tooth than with a gold clasp .A retentive clasp arm made of wrought alloy is more resilient than one made of cast alloy and thus will transmit less stress to the abutment.
  49. 49. Amount of clasp surface in contact with the tooth The greater tooth to metal contact between the clasp and the tooth, the more will be the stress exerted on the tooth. Type of abutment tooth surface A gold surface offers more frictional resistance to movement of the clasp than will enamel, thus more stress will be exerted against the tooth that has been restored with a gold casting
  50. 50. Occlusal harmony A disharmonious occlusion ,one in which deflective occlusion contacts between opposing teeth are present generates horizontal forces that ,when magnified by the factor of leverage can transmit destructive forces to both the abutment teeth and the residual ridges .The type of opposing occlusion can play a role in determining the amt of stress generated by occlusion .
  51. 51. A partial denture constructed to oppose a complete denture will be subjected to much less occlusal stress than one opposed by natural dentition .That is because some individuals with natural teeth can exert a closing force of 300 pounds per square inch, whereas the closing force of a person wearing complete denture may not exceed 30 pounds per square inch.
  52. 52. The area of the denture base against which the occlusal load is applied influences the amount of stress transmitted to the abutment teeth and ridge .If the occlusal load is applied to the base adjacent to the abutment tooth, there will be less movement of the denture base and less stress transmission than if the load is applied at the distal end of the denture base. Ideally the occlusal load should be applied in the center of the denture bearing area both anteroposteriorly and buccolingually.
  53. 53. Design considerations in the control of stress No removable partial denture can be designed or constructed that will not be destructive in the mouth. However Long term clinical observation enables the partial denture planner to use a combination of design and construction principles that will distribute the functional stresses equally between the hard and soft tissues. So that the effect of leverage is minimized and neither structure is stressed beyond its physiologic tolerance.
  54. 54. Retention as a method of stress control The retentive clasp is the element of the partial denture that is responsible for transmitting most of the destructive forces to the abutment teeth . A removable partial denture should always be designed to keep clasp retention to a minimum yet provide adequate retention to prevent dislodgement of the denture by unseating forces. Any retention supplied by units of the prosthesis other than the clasps has the effect of reducing the amount of retention that the clasps are required to provide, which in turn diminishes the stress that must be borne by the abutment teeth.
  55. 55. • Clasp position Leverages can be controlled entirely by means of clasps, if there are sufficient abutment teeth and they are distributed strategically in the dental arch .Usually the position or the relation of the retentive clasp to the height of the contour is more important in retention and in controlling stress than is the number of clasps .
  56. 56. 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.
  57. 57. Quadrilateral Configuration
  58. 58. 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.
  59. 59. 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.
  61. 61. 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.
  62. 62. Clasp design as a method of stress control The conventional circum-ferential cast clasp originating from a distal occlusal rest on the terminal abutment tooth and engaging a mesiobuccal retentive undercut should not be used on a distal extension removable partial denture. The terminal of this clasp reacts to movement of the denture base toward the tissue by placing a distal tipping or torquing forces on the abutment tooth. This particular force is the most destructive force a retentive clasp can exert.
  63. 63. A reverse circlet clasp, approaching a distobuccal undercut from mesial occlusal surface, may be acceptable for a distal extension partial denture. As the denture base moves towards the tissue, a retentive clasp tip will tend to move into an area of greater undercut .This action releases torquing forces that can damage an abutment tooth.
  64. 64. A bar clasp is used on the terminal abutment tooth on a distal extension partial denture when the retentive undercut is located on the distobuccal surface .As the denture base is loaded toward the tissue, the retentive tip of the T clasp rotates gingivally to release the stress being transmitted to the abutment tooth the bar clasp does not produce the wedging force. The clasp that subjects the abutment tooth to the least unfavorable torque is the T clasp with a distal occlusal rest and a rigid circumferential reciprocating clasp
  65. 65. A combination clasp When a mesiobuccal undercut exists on an abutment tooth adjacent to the distal extension edentulous ridge, the combination clasp can be employed to reduce the stress transmitted to the abutment tooth. Wrought alloy wire, by virtue of its internal structure, is more flexible than a cast clasp. It can flex in any spatial plane, whereas a cast clasp flexes in the horizontal plane only. The wrought wire retentive arm has a stress-breaking action that can absorb torsional stress in both the vertical and horizontal planes.
  66. 66. Splinting of Abutment Teeth Adjacent teeth may be splinted by means of crowns to control stress transmitted to a weak abutment tooth. Splinting two or more teeth will increase the periodontal ligament attachment area and distributes the stress over a larger area of support. Splinting by means of crowns also has the effect of stabilizing the abutment teeth in a mesiodistal or anteroposterior direction. Splinting is also indicated when the proposed abutment tooth has either a tapered root or short roots such that there is not an acceptable amount of periodontal ligament attachment present. The tying together of two such teeth by crowns will in effect produce an acceptable multirooted abutment tooth .
  67. 67. Occlusion A smoothly functioning occlusion that is in harmony with the movements of both the temporomandibular joints and the neuromusculature will minimize the stress transferred to the abutment teeth and residual ridge. The occlusal surfaces of the artificial teeth can transmit various amounts of stress to the supporting structures.
  68. 68. A large or broad occlusal surface delivers more stress than does one that has been reduced in buccolingual width. The number of teeth being replaced may also be reduced to decrease the stress. Steep cuspal inclines on the artificial teeth should be avoided because they tend to introduce horizontal forces that can produce torsional stresses on the abutment teeth. Artificial posterior teeth should possess sharp cutting surfaces and sluiceways for the escape of food between the teeth to be as efficient as possible and to relieve some of the unnecessary force in mastication.
  69. 69. Denture Base The denture base should be designed to cover as extensive an area of supporting tissue as possible. The stress created by the partial denture in function will thus be distributed over a large area, so no single area will be subjected to stress beyond its physiologic limit. The denture base flanges should be made as long as possible to help stabilize the denture against horizontal movements.
  70. 70. The distal extension denture bases must always extend onto the retromolar pad area of the mandible and cover the entire tuberosity in the maxilla. Equal care must be taken not to overextend the borders of the denture base. Interference with the functional movements of the surrounding tissues by an overextension will produce and transmit significant stresses to the remaining teeth.
  71. 71. The more accurate the adaptation of the denture base to the residual ridge, the better will be the retention, in part because of the forces of adhesion and cohesion. There will be fewer tendencies for the denture base to move in function, and as a result less stress will be transmitted to the abutment teeth.
  72. 72. The type of impression used to record the mucoperiosteum of the residual ridge will influence the amount of stress on the residual ridge and the abutment tooth. If the ridge is recorded in its functional state rather than its resting form when the denture base is actually subjected to occlusal loading, the tissue wouldn't be displaced to any great extent. The magnitude of the stress transmitted to the abutment teeth would be minimal.
  73. 73. The contours of the polished surface of the denture base need to be developed in order to assist in retention of the denture. If done properly, this will reduce movement of the partial denture and thereby decrease the stress transferred to the abutment teeth and supporting tissues.
  74. 74. Major Connector In the mandibular arch the lingual plate major connector that is properly supported by rests can aid in the distribution of functional stresses to the remaining teeth. It is particularly effective in supporting periodontally weakened anterior teeth. The lingual plate also adds rigidity to the major connector. The added rigidity contributes to the effectiveness of cross-arch stabilization. Stresses created on one side of the arch are transmitted through the major connector to the teeth on the opposite side, thus reducing the stress applied to any single portion of the arch.
  75. 75.
  76. 76. In the maxillary arch the use of a broad palatal major connector that contacts several of the remaining natural teeth through lingual plating can distribute stress over a large area. • The hard palate often provides a valuable area for support. A maxillary major connector that uses maximum coverage of this area can contribute greatly to the support, stability, and retention of the prosthesis. This in turn substantially reduces the stress that ordinarily would be transferred to the abutment teeth.
  77. 77. Rests • Properly prepared rest seats help control stress by directing forces transmitted to abutment teeth down the long axis of those teeth. The periodontal ligament is capable of withstanding vertical forces of far greater magnitude than horizontal or torsional forces • In all Class I and II partial dentures the rest seat preparation must be saucer-shaped, completely devoid of any sharp angles or ledges.
  78. 78. • As forces are applied to the partial denture, the rest must be free to move within the rest seat to release stresses that would otherwise be transferred to the tooth. This movement of the rest within the rest seat is similar to the action of a ball-and-socket joint. If ledges or walls are developed in the rest seat, the releasing action cannot take place and undesirable forces are concentrated on the abutment tooth. • The number of abutment teeth influences the amount of force each tooth must absorb. The more teeth that bear rest seats, the less will be the stress placed on each individual tooth.
  79. 79. 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.
  80. 80. Frequently used synonyms are • Internal attachment • Frictional attachment • Slotted attachment • Key/keyway attachment • Parallel attachment
  81. 81. History • The principle of modern day precision attachment was first formulated by Dr.Herman E.S.Chayes in 1906, the modifications of which are still in use and are called the Chayes attachments. There is a male and a female part having a 'T' shape in cross section. In the early stages, the female part was placed within the abutment tooth and the male one in the prosthesis.
  82. 82. • Brown suggested a modification to the Chayes design and this was called the "Brown Attachment". It had a dumbell shaped cross section. • Bayler suggested another modification where the male component was having a dove tail like preparation which was tapering.
  83. 83. 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
  84. 84. 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.
  85. 85. 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 of the metal framework.
  86. 86. 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.
  87. 87. • 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.
  88. 88. 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.
  89. 89. • Special problems can arise where a precision attachment is to be used to provide retention of a freeend 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.
  90. 90. 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.
  91. 91. 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
  92. 92. • 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
  93. 93. Classification [G.E.Ray] • • • • • • Intracoronal Extracoronal Conjunctors Anchors Bars Accessory components
  94. 94. 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
  95. 95. • 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.
  96. 96. 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]
  97. 97.
  98. 98.
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  100. 100. 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
  101. 101. 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.
  102. 102. Factors To Be Considered Before Choosing Intracoronal attachment designs: • Bulk - A retentive device requiring a large female element is not a good indicator for intracoronal attachment. • Adjustment- Adjustment for compensating the large frictional wear should be simple and straight forward and there should be easy access.
  103. 103. • Breakage - A sturdy attachment should be selected. The springs if present, should be protected from food impaction. • Provision For Trimming The Attachment - It may be necessary to shorten an attachment to accommodate it within the tooth. If it engages near the occlusal surface, it will be damaged as soon as attachment is shortened .
  104. 104. 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.
  105. 105. 3 Free end Saddle Removable Partial Denture: Precision attachment may be the choice of attachment in such attachment should situations. be selected A strong as the attachments used here are subjected to considerable forces.
  106. 106. 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 .
  108. 108. 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 attachment2) Dalbo system3) Rothermann attachment Adjustments for wear is done either by unscrewing or compressing the sleeves.
  109. 109. Bar type attachments. As in the use of attachments of the Class 2A 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 bar
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  111. 111. 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.
  112. 112. Swing lock denture
  113. 113. The labial bar or acrylic veneer is designed to move on its hinge in the manner of a gate. The denture is inserted with the 'gate' open and when it is closed, it is fastened to the main framework by means of a latch or lock positioned on the opposite side of the arch from the hinge.
  114. 114. The principle behind this type of denture is that it allows wide distribution of stress during function to all the remaining teeth and residual ridges rather than to just a few abutments. When the appliance is inserted and locked into position, the enclosed natural teeth are held rigidly in fixed position. This acts as a splinting or stabilizing action on these teeth which may have a poor periodontal condition. The splinting, therefore, spreads the applied stress allowing it to be shaped and also resists tipping and rotation of the appliance.
  115. 115. This denture is, therefore, suitable for use in compromised clinical situations and particularly useful where only a few teeth remain and load distribution may be a problem due to mobility or inadequate periodontal support .
  116. 116. Review of literature Meyer 1936 stated that in the act of mastication, the denture is forced against tissues without any apparent stress on the supporting teeth. When the pressure on the teeth has been relieved the stress breakers automatically allow the denture to re-assume its rest position in relation to the tissues. This connective pressure followed by relaxation, massages the gum tissues and should tend to keep them in a healthier condition.
  117. 117. Van Hinden(1943)suggested in his article that we must see that the abutments shall bear only the vertical and lateral stresses that properly should fall on them during mastication. As a result of the equitable distribution of the masticatory stress over the entire alveolar ridge the need of rebasing is reduced.
  118. 118. NEUROHR (1944) FAVOURS STRESS BREAKING BY STATING THAT IT IS A MISTAKE TO TAKE IMPRESSIONS OF THE SADDLE AREAS UNDER PRESSURE . When such sustained pressure is incorporated in a applaince, we invite periodontal lesions and abnormal recession of the alveolar process .impressions should be taken without pressure with the tissue in a passive state. The restoration should be designed such that the masticatory forces are distributed evenly over the largest area possible
  119. 119. Capt. Arthur R.Frechette (1951) divided stress distribution in partial denture under three headings of bracing, support and retention. He advocated the use of rigid connectors whereby, force imparted to any point on the denture is transmitted through out the appliance and delivered to all the teeth and the edentulous ridges. For poor ridges with weak teeth, it is imperative to obtain optimum ridge coverage and to utilize additional teeth for bracing. When extensive saddles are used with rigid connection and properly selected clasps, effective bracing is obtained, torque is reduced and horizontal forces are controlled.
  120. 120. • Luzerne G.Jordan, (1952) in his article on designing RPD, wrote about stress breakers designed with the primary purpose to decrease the occlusal load on the abutment and thus divert it to the mucosa. He wrote that their application should be to those who have only a few teeth remaining in the arch for such cases where it is considered absolutely essential where the edentulous ridge would offer no retention for the complete denture or for cleft palate patients.
  121. 121. Lamie and Osbourne (1954) in his article on “Bilateral free end saddle lower dentures” classified stress breakers as 1. Those that have a movable joint between the saddle and the direct retainer. 2. Those that have a flexible connection between the direct retainer and the saddle. He outlined the advantages and disadvantages of each type.
  122. 122. George W. Hindels (1957) states that the load as received by the artificial teeth during mastication is transmitted through the denture base and the various parts of the clasps to the contacted anatomic structures. This resulted in 2 types of stress. 1.Unaltered stresses – These were stresses which were transmitted by the partial denture to the supporting anatomic structures in the same direction as received. 2. Altered stresses – These were altered in magnitude and direction by the contacts established between different parts of the appliance and the anatomic structures.
  123. 123. E.Hirschtritt (1957) gave a design for partial dentures with the stress broken basis. He kept strict differentiation between the tooth and the tissue bearing areas of the mouth. The tooth borne unit with its splitting and supporting ability, protects the teeth against overstress, in spite of the fact that it receives a fair amount of the masticatory forces transmitted from extension bases. The firm but flexible stress breaking bar keeps the base in a well balanced position. Therefore, an independent movement of the base becomes possible and a better protection of the ridges is achieved.
  124. 124. • Charles V. Caldarone (1957) stated that it is advantageous to use a precision attachment as retainers where there is great amount of force and stress, or where the periodontal condition of the abutment tooth is questionable. • R.I.Nairn (1966) in his article on problems of denture bases states that partial dentures incorporating a stress breaking feature do attempt to provide a more even pattern of load distribution on the teeth and the displaceable soft tissues. There are circumstances where the preservation of some remaining natural teeth is of paramount importance and it is here that such design should be seriously considered.
  125. 125. • Cecconi, Kaiser (1975) did a study to determine the effect of 2 types of partial dentures stress beakers on 1.Abutment tooth movement and 2.Ridge displacement. These movements were measured when stress breakers were both active and non-active (rigid). They used the pin dalbo precision attachment stress breaker and the ticonium hidden lock stress breaker. Results showed that ridge displacement was significantly decreased when the dalbo was made rigid and tooth movement was significantly decreased when the hidden lock was made rigid.
  126. 126. • P.V.Reitz(1985) studied to determine whether there was a reduction in stresses to abutments from loads applied to the removable partial denture with a mandibular split lingual bar as major connector. They concluded that there were minimal differences in stress generated by the rigid and the short split framework. When the split extended to the midline, the stress on distalextension abutment was in a more vertical direction and decreased in magnitude .
  127. 127. • H. Itoh (1998) studied the effects of periodontal sopport and fixed splinting on load transfer by removable partial dentures. They came to the conclusion that under the same load conditions the highest stresses developed in the model with the largest osseous defect .increasing the number of splinted teeth did not provide a proportional decrease in maximum stress levels . The more severe the osseous defect, the greater assistance was provided by splinting to periodontally sound teeth . This study suggests that routine cross arch splinting may not be appropriate.
  128. 128. • Ray S Krug (2003) in his article said that in case of distal extension R.P.D. a clasp assembly that creates minimal stress is desirable . The combination clasp ,which uses an 18-gauge Paladium based wrought wire has been used extensively as a gentle yet retentive clasp assembly that reduces stress to the abutment tooth .
  129. 129. Summary & Conclusion All RPD, regardless of the method employed for their support and retention should restore esthetics and the function of mastication while contributing to the preservation of the remaining dentition and its supporting structures. The decision can be influenced by factors like length of the saddle, condition of the abutment and their periodontal health, compressibility of the mucous membrane covering the ridge, economical consideration. The methods to restore a free end saddle should attempt to control the load delivered to the teeth and residual ridges. Many attachments allow a certain degree of movement between the components, they are known as “stress breakers”.
  130. 130. Though the stress breaker is a device which would not be used indiscriminately, it should be used according to the case with constant recall and monitoring. The patient should be educated and advised on its maintenance and importance of periodic check-up visits for the long term use of the restoration.
  131. 131. Thank you For more details please visit