stress breakers in prosthodontics

stress breakers in prosthodontics

1. Stress breakers in prosthodontics Presented by Sirisha.G II year PG student Lenora Institute of Dental sciences

2. Contents Introduction Aims of stress breaking Applications Distal extension RPD Philosophies of stress distribution Stress breakers in FPD Tooth-Implant supported FPD Review of literature Conclusion References 2

3. Introduction Stress: (GPT 8) F orce per unit area.(perpendicular cross sectional area over which the force is applied.) The deformation caused in a body by such a force. An internal force that resists an externally applied load or force. 3

4. Stress breakers: stress directors: (GPT 8) A device or system that relieves specific dental structures of part or all of the occlusal forces and redirects those forces to other bearing structures or regions. A stress breaker is something like a hinge joint placed within the denture framework, which allows the two parts of the framework on either side of the joint to move freely. (Mc Cracken ) Nonrigid or resilient attachment Intracoronal / extracoronal 4

5. 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 early 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 is distributed as evenly as possible over the whole mucosal surface. To provide greater comfort to the patient. 5

6. Applications Distal extension removable partial dentures FPD with pier abutment FPD with tilted abutments Tooth – Implant supported prosthesis 6

7. Distal extension RPD Removable partial dentures are not rigidly connected to the teeth or tissues - movement Movements  Stress  Damage Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial Prosthodontics . 11 th ed , Elsevier publications, Mosby Company, Delhi. P.25 7

8. Movement …. ??? Teeth  efficient support  limite prosthesis movement. The reaction of the ridge tissue to functional forces can be highly variable. This disparity leads to variable amounts of prosthesis movement. 8

9. 9 Tooth support Tissue support

10. Problem As the tissues are more compressible, the amount of stress acting on the abutments is increased. In order to protect the abutment from such conditions, stress breakers are added to the denture. A stress breaker is something like a hinge joint placed within the denture framework, which allows the two parts of the framework on either side of the joint to move freely. Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial Prosthodontics . 11 th ed , Elsevier publications, Mosby Company, Delhi. P.145 10

11. Some dentists strongly believe that a stress-breaker is the best means of preventing leverage from being transmitted to the abutment teeth. Others believe just as strongly that a wrought-wire or bar-type retentive arm more effectively accomplishes this purpose with greater simplicity and ease of application. Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial Prosthodontics . 11 th ed , Elsevier publications, Mosby Company, Delhi. P.149 11

12. Guidelines: Rule1: if the teeth are strong and the ridge is poor, flat, knife edged or narrow – RIGID Rule 2: if the teeth are weak with + or more mobility and the ridge is strong – STRESS BREAKER Burns D.R,Ward J.E. A review of attachments for removable partial denture design: part 2. Treatment planning and attachment selection. Int J Prosthodont 1990;3:169-74. 12

13. Types of stress breakers TYPE 1 Hinge Sleeve Cylinder Ball and socket 13

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15. TYPE 2: flexible conncection between direct retainer and denture base wrought wire connectors split major conncetors Hidden lock partial dentures Disjunct partial dentures 15

16. Wrought wire connector: The 12 guaze chrome wire stress breaker: 16

17. The 12 gauge wire is adapted to the refractory cast. The wire is coated with die lubricant and the wax up is completed. The wax must not go beyond the maximum convexity of the wire. The wire is removed and the casting is completed. 17

18. After recovering the casting, the wire is welded or soldered. Then the connection between the denture base and the main major connector is separated to activate 12 gauge chrome wire. 18

19. Advantages: 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 and are also simple. 19

20. Split bar major connector: Split is provided between the denture base area and the major connector . When occlusal forces are applied they are transferred more towards the tissue supported base and then they are transmitted to the abutment teeth. 20

21. Hidden lock partial denture: This is a two piece casting, the top half, which is the major connector supporting the direct retainers and other rigid components, is cast first. The bottom half, which is the connector between the denture bases, is cast to the major connector next. Cecconi B.T, Kaiser G, Rahe A.L. Stress breakers and the removable partial denture. J Prosthet Dent 1975;34:147-51 21

22. The hidden lock is created by mechanical means, and the split between the two connectors is made possible by the thin oxide shell that forms during the making of the two sections. What appears to be a conventional lingual bar or linguoplate actually is two bars connected by a movable joint at the midline Disadvantages More prone to collect debris and become un hygienic. And also there may be chances of tissue trap at the junction between the two parts. 22

23. Disjunct Removable partial denture: Tooth borne & mucosa-borne parts of denture are disjoined. Tooth borne part providing splinting of remaining teeth & only retention for mucosa borne part. Geissler P. R, and Watt D. M. Disjunct dentures for patients with teeth of poor prognosis. Dent Pratt 1965;15:421-23 23

24. Structural details: The tooth borne part is a lingual plate and thus provides stabilization for the remaining teeth. The tissue borne part is a lingual bar which consists of denture bases along with the teeth at its terminals. 24

25. Advantages: Independent movement between the tooth supported and tissue supported parts decreases the forces on periodontally weakened remaining teeth. Disadvantages : It is technically difficult to fabricate Patient may complains of rattling of the framework during mastication. 25

26. Philosophies of design These philosophies are based upon three approaches to force distribution. Stress equalization / broken stress philosophy Physiologic basing Broad stress distribution Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. pp. 234 26

27. Stress equalization Proponents believe that rigid connections between denture bases and direct retainers are damaging, and that stress directors are essential to protect the abutments Articulated prosthesis Hinge – most common Ball and socket Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. pp. 233-240 27

28. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. Pp. 512 28

29. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. pp. 512 29

30. Advantages Disadvantages Minimize the tipping forces on abutment teeth, thereby limiting bone resorption . comparatively fragile Minimal direct retention because the denture bases operate more independently than do those used in conventional removable partial denture applications Costly Constant maintenance Difficult / impossible to repair Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. pp. 233-240 30

31. Physiologic basing Proponents - Equalization can best be accomplished by recording the anatomy of the edentulous ridge in its functional form and ensuring that the associated denture base accurately reflects this anatomy. Depressing the mucosa during impression Relining the denture base after it has been constructed. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. pp. 233-240 31

32. Denture bases formed over compressed tissues will show an increased ability to withstand vertical forces. The prosthetic teeth and occlusal rests will be positioned above the existing occlusal plane when the prosthesis is not in function - 32

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34. Advantages Disadvantages physiologically stimulating effect on the tissues of the residual ridges  promotes tissue health and reduces the necessity for frequent relining or rebasing procedures. Premature contacts between the opposing teeth and the prosthesis during closure. The minimal retention requirements  lightweight prostheses  minimal maintenance and repair. Difficult to produce effective indirect retention because of the vertical movement of the denture and the minimal retention provided by the direct retainers. 34

35. Broad stress distribution Advocates - distributing forces over as many teeth and as much of the soft tissue area as possible – prevents trauma Additional rests and clasp assemblies and broad coverage of denture bases Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. pp. 233-240. 35

36. Advantages Disadvantages Wider force distribution Increased coverage – patient acceptance Minimised lateral forces Oral hygiene maintenance Multiple clasp assemblies  added horizontal stability  as like removable splinting  retaining periodontally weak abutments for longer time. Preventive dental programs No added retentive components Rigid – excellent horizontal stability No need for relining and rebasing Easy and inexpensive 36

37. Stress breakers in FPD FPD with pier and malaligned abutments The connection between the pontic and retainer / within the pontic Non rigid connectors 37

38. Pier abutments An edentulous space on both sides of a tooth, creates a lone freestanding pier abutment 38

39. Physiologic tooth movement, arch position of the abutments , and a disparity in the retentive capacity of the retainers Studies in periodontometry have shown that the faciolingual movement ranges from 56 to 108 microns and intrusion is 28 microns. stresses in a long-span prosthesis Standlee and Caputo suggest that tension between the terminal retainers and their respective abutments, rather than a pier fulcrum, is the mechanism of failure. Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J AM Dent Ass 1973;87:1195-99 39

40. Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J AM Dent Ass 1973;87:1195-99 40

41. Non rigid connectors GPT 8 A connector that permits limited movement between otherwise independent parts of a Fixed partial denture. The nonrigid connector is a broken-stress mechanical union of retainer and pontic Internal connector: a non rigid connector of varying geometric design using a matrix to unite the members of an FPD Subocclusal connector: an interproximal non rigid connector positioned apical to and not in communication with the occlusal plane 41

42. Indications: Malaligned abutments Pier abutments Longspan FPD Distal abutments with questionable prognosis Contraindications: Teeth with latge pulp chambers Abutment with reduced clinical occlusogingival height 42

43. Size of the connectors Shape of the connectors Position of the connectors 43

44. Key and keyway/ Dovetails Split pontics / tapered pins Cross pin and wing 44

45. Keyway position..? Nearly 98% of posterior teeth tilt mesially when subjected to occlusal forces. If the keyway of the connector is placed on the distal side of the pier abutment, mesial movement seats the key into the keyway more solidly. Placement of the keyway on the mesial side, however, causes the key to be unseated during its mesial movements. Shillingberg H.T, Fisher D.W. Non rigid connectors for Fixed partial dentures. J AM Dent Ass 1973;87:1195-99 45

46. Dovetail It is necessary to align the path of insertion of the keyway with that of the distal abutment. This technique is best suited for relieving stress at midspan on long pontics . 46

47. Procedure: The wax pattern for the retainer on the pier abutment is fabricated on the working cast. A deep box form is carved into the distal surface of the wax pattern to create space for the placement of the plastic keyway pattern. Place the working cast, with the wax pattern seated, on the table of a surveyor. Assemble the key and keyway portions of the connector, and lock the mandrel that extends from the top of the key portion of the pattern into the vertical spindle of the surveying instrument. 47

48. Manipulate the surveyor table until the mandrel and attachments are parallel with the path of insertion of the distal preparation. Then lower the plastic pattern to the middle retainer wax pattern and lute it in place with sticky wax Remove the key portion and complete the middle retainer wax pattern by blending the distal surlace with the keyway. The pattern is then invested, burned out, and cast. After the casting has been cleaned and air abraded, carefully cut off any part of the keyway portion of the attachment that protrudes above the occlusal surface. 48

49. Place the casting on the working cast, and place the prefabricated plastic pattern for the key into the keyway. At this point the pontic wax pattern is attached to the plastic key. The pontic pattern is completed, removed from the working cast, invested, burned out, and cast. After the casting is recovered from the investment, the mandrel and any excess on the top portion of the key are carefully reduced so the key and keyway are flush. 49

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52. Split Pontic It is particularly useful in tilted abutment cases The wax pattern for the anterior three-unit segment ( mesial retainer- pontic -pier retainer) is fabricated first, with a distal arm attached to the pier retainer. The underside of the arm is shaped like the tissue-contacting area of a pontic . A surveyor is used to position either the key or the keyway segment 52

53. Procedure: Invest, burn out, and cast the mesial three-and-a- halfunit segment. After preliminary finishing, seat the cast segment on the working cast. Place the plastic pattern down into it (if the keyway is in the casting), or down onto it (if the key was left facing upward on the pontic base). Wax the distal retainer and the disto-occlusal two-thirds of the pontic pattern. The pontic can be metal-ceramic, but there should be a thin collar of metal around the periphery of the ceramic section. Try it on the prepared teeth in the mouth, making adjustments as necessary. Cement the mesial segment first, followed immediately by the distal segment. No cement should be placed between the two segments of the pontic . 53

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55. Cross-pin and Wing The cross-pin and wing are the working elements of a two-piece pontic system that allows the two segments to be rigidly fixed after the retainers have been cemented on their respective abutment preparations. Accommodating abutment teeth with disparate long axes. The path of insertion of each tooth preparation is made to parallel the long axis of that tooth. 55

56. Attach a vertical wing, cut out of a piece of baseplate wax, to the mesial surface of the distal retainer wax pattern. The wing should parallel the path of insertion of the mesial abutment preparation, extend out 3.0 mm mesially from the distal retainer, have a 1.0-mm thickness faciolingually , be 1 0 mm short of the occlusal surface, and have an undersurface that follows the intended contour of the underside of the pontic . Invest, burn out, and cast the distal retainer, with wing. Seat the retainer on the cast, and drill a 0.7-mm hole through the wing with a twist drill in a handpiece . Place a 0 7-mm-diameter pencil lead through the hole and build the wax pattern around the lead and the wing. Remove the lead, withdraw the retainer- pontic wax pattern, and replace the 0.7-mm lead in the hole in the pontic pattern to maintain the patency of the hole during investing and casting 56

57. Assemble the two parts of the fixed partial denture on the working cast. Use a tapered 8/0 machinist reamer to ream a smooth, tapered hole through the pontic and wing, following the pilot hole produced by the 0.7-mm pencil lead. Fabricate a pin of the same alloy used for the fixed partial denture casting. A mold can be made by drilling a hole in a piece of aluminum with the machinist reamer and filling the hole with autopolymerizing resin. An impression of the reamer can be made with polyvinyl siloxane impression material and filled with resin or molten wax. Invest, burn out, and cast it It must be long enough to extend all the way through the pontic -wing assembly. Try the pin for fit in the components on the cast. 57

58. Cement the retainer with the wing first, followed by the retainer- pontic segment . Seat the pin in the hole with a punch and mallet. Remove excess length from the pin both facially and lingually . If it is ever necessary to remove part of this fixed partial denture, the pin can be tapped out and the parts dealt with separately. This technique requires no special patterns and does allow for a completely rigid prosthesis when completed. 58

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60. Tooth – Implant supported FPD Implant supported fixed dental prosthesis has been proven as an efficient modality of treatment. However, tooth and implant…? 60

61. Advantages: splinting of a natural tooth to an implant Increased mechanoreception Additional support for the total load on the dentition. Connecting teeth with implant broadens treatment possibilities for the restorative dentist Reduces the cost for teeth replacement and Avoids the use of cantilevers. sadvantages : Higher need for maintenance and repair with such conncections . 61

62. Problem: The natural tooth and the osseointegrated implants have dissimilar mobility patterns and this may subject the implant to excessive stresses. Numerous studies have reported pronounced marginal bone loss or failure of implant to osseointegrate . This led to the controversy of whether connecting implant to the natural teeth is a viable option. Various complications like, intrusion of the teeth,mechanical failure, caries and loss of occlusal contacts have been reported in the literature associated with this treatment approach. 62

63. Mismatch in tooth and implant movement: The natural teeth are attached to the alveolar bone by means of periodontal ligament fibers ; whereas osseointegrated implant is rigidly anchored The tooth exhibits normal physiological movement in vertical, horizontal and rotational direction to the bone Osseointegrated implants exhibit only linear movement during the entire loading cycle in proportion to the applied load without initial rapid movement due to lack of periodontal ligament – viscoelastic nature 63

64. A healthy natural tooth can move 200 μ in response to a 0.1 N force while an implant can be displaced 10 μ or less. The ratio of the amount of movement of the tooth in a healthy periodontium to that of an implant has been estimated to be 10:1 and 100:1 64

65. Physiologic movement of the natural tooth causes the prosthesis to act as a cantilever generating maximum resultant load up to two times the applied load on the implant. Implant would receive higher amount of loads in function and could lead to potential complications. 65

66. Types of connection Rigid connection: The tooth is rigidly connected to the implant with a fixed dental prosthesis. Non rigid connection: The tooth is non‑ rigidly connected to the implant by means of precision attachments, non‑ precision attachments and telescopic restorations. It acts as a stress breaking element. Resilient connection: It incorporates a flexible component that simulates the periodontal ligament. It acts as a stress absorbing element. 66

67. Different types of non rigid connectors are described with most common being key and key way. The placement of the key way on the natural teeth seems to be beneficial as it would allow for physiological tooth movement under function. 67

68. Biomechanical studies demonstrate that a shift of force distribution from the superstructure to the supporting teeth occurs when non‑ rigid connectors are used and tooth intrusion was considered as potential complication of non‑ rigid connection with frequent emergency appointments. Non‑ rigid connectors should be used with caution as it increases unfavorable stresses on the abutment. 68

69. Becker et al., suggested to splint implant to two teeth when non‑ rigid connectors are considered. 69

70. Complications associated with tooth implant supported prosthesis Biologic complications: Gradual bone resorption around the implant neck Bone fracture Loss of osseointegration Peri‑implantitis Endodontic problems - caries after cement dissolution Root fracture. 70

71. Technical complications: Mechanical damage to the teeth or implant and includes fatigue Induced implant fracture Fracture of abutment screw Loosening of abutment screw Loss of prosthesis cement bond to tooth or abutment Abutment fracture Teeth or root fracture Tooth intrusion Fatigue induced prosthesis fracture. 71

72. Guidelines Guideline 1: Splint implants to natural teeth only when the teeth need support: Teeth do not stabilize implants. Guideline 2 : Do not end the fixed prosthesis on the weakest splinted abutment. Guideline 3: Regardless of the connection teeth must be cemented using definitive cement Guideline 4: For a natural pier abutment between two implants a stress breaker is not indicated Guideline 5: Design of the prosthesis should allow minimal movement in a buccolingual direction Shenoy V.K, Rodrigue S.J, Prashanti . E, saldanha S.J.R. Tooth Implant supported Prosthesis: A Literature review. J Inter Discip Dent 2013;3:143-150 72

73. Connecting implant to natural teeth is accompanied by various adverse sequelae . It is paramount to formulate a treatment plan for predictable treatment outcome. A risk benefit analysis and anticipated complications should be presented to the patient and appropriate consent obtained before the treatment plan is finalized. The main focus should be to reduce the risk of intrusion of the tooth and of overloading the implant. Shenoy V.K, Rodrigue S.J, Prashanti . E, saldanha S.J.R. Tooth Implant supported Prosthesis: A Literature review. J Inter Discip Dent 2013;3:143-150 73

74. R eview of Literature

75. Mc Leod N.S (1977) made a theoretical analysis of the mechanics of the Thompson dowel semiprecision intracoronal retainer. The analysis locates the center of rotation during function and identifies the factors that affect its position. The degree to which the dowel should be relieved to permit unrestricted rotation has been established 75

76. Arunkumar G et al (2011) Three-dimensional finite element analysis of the stress distribution around the implant and tooth in tooth implant-supported fixed prosthesis designs in order to suggest a design, which transmits less stress to the bone. From the study, it could be suggested that if natural teeth and implants are used as support for fixed prosthesis, the NRC should be placed on the implantsupported site to reduce the load on the implant and natural teeth. 76

77. Conclusion Stress breakers may not be in regular use. However, mandatory usage is needed in specific conditions. The patient should be educated about the maintenance Regardless of design, most stress breakers effectively dissipate vertical forces which is the purpose for which they are used 77

78. References Carr A. B, Mc Givney G. P, Brown D. T. Mc Cracken’s Removable Partial Prosthodontics . 11 th ed , Elsevier publications, Mosby Company, Delhi. P.25 Shenoy V.K, Rodrigue S.J, Prashanti . E, saldanha S.J.R. Tooth Implant supported Prosthesis: A Literature review. J Inter Discip Dent 2013;3:143-150 Shillingberg H.t . Fundamentals of fixed Prosthodontics , 3 rd ed. Quintessence Books, India. Phoenix R.D, Cagna D.R, Defreest C. F. Stewart’s Clinical Removable Partial Prosthodontics , 3 rd ed. Quintessence books, India. 78

79. Akulwar R. F, Kodgi A. Non rigid connector in managing pier abutment in FPD. J Cli Diag Res 2014;8:12-14 Bevilacqua M et al. The influence of cantilever length and implant inclination on stress distribution in maxillary implantsupported fixed dentures. J Prosthet Dent 2010;105: 5-13 Steffel V.L, Columbus, Ohio. Fundamental Principles involved in partial denture design. J Am Dent Ass 1951;42:534-545. 79

80. 8. Cecconi B.T, Kaiser G, Rahe A.L. Stress breakers and the removable partial denture. J Prosthet Dent 1975;34:147-51 9. Burns D.R,Ward J.E. A review of attachments for removable partial denture design: part 2. Treatment planning and attachment selection. Int J Prosthodont 1990;3:169-74. 80