Direct retainers

Direct retainers- Part 1 Covering Retentive Clasp Assemblies

1. Dr. Kriti Trehan MDS II Direct Retainers 1

2. Contents Introduction Definitions Classification Basic part of clasp assembly Basic Principles of clasp design Cast circumferential clasp Bar clasp Other types of clasp Conclusion 2

3. Introduction Retention is that quality of a removable partial denture (RPD) which resists the forces of gravity, the adhesiveness of foods, and the forces associated with opening the jaws. A removable partial denture must include components that prevent displacement of the prosthesis from the patient’s mouth during function. The components of a removable partial denture that engage abutments and resist dislodging forces are called direct retainers. 3

4. This ensures effective prosthesis retention and minimizes the transmission of detrimental forces to the associated abutments and supporting tissues. In general, there are two types of direct retainers: intracoronal direct retainers and extracoronal direct retainers. 4

5. DEFINITIONS Direct retainer : That component of a removable partial denture used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment Direct retention : R etention obtained in a removable partial denture by the use of clasps or attachments that resist removal from the abutment teeth 5

6. Classification Of Direct Retainers 6

7. Extracoronal direct retainers They serve to retain and stabilize removable partial dentures when dislodging forces are encountered. Extracoronal direct retainers may be divided into two distinct subcategories: extracoronal attachments and retentive clasp assemblies. 7

8. Extracoronal attachment First proposed by Henry H Boos 1900 later modified by Ewing F Roach in 1908. Located outside the teeth. It derives their retention from closely fitting components termed matrices and patrices . Permit vertical movement during occlusal loading,minimize potentially damaging forces to abutment. This concept has led to “stress breaking” or “stress directing” theories of removable partial denture design. 8

9. Retentive clasp assemblies It first appeared in the dental literature with Dr W. G. A. Bonwill’s description in 1899. The retentive element of an individual clasp assembly is a metal clasp arm that displays a limited amount of flexibility. A clasp terminus designed to contact the abutment surface apical to the height of contour will resist displacement in an occlusal direction. Resistance to displacement is encountered because the clasp arm must undergo “deflection” or “bending” to pass over the height of contour 9

10. Therefore, retention of the removable partial denture is determined, in part, by the location of the clasp terminus relative to the height of contour. The chosen path of insertion and removal will always define the height of contour and the associated areas of undercut. 10

11. Prothero’s concept Proposed “cone theory” of clinical crown in 1916 that provides conceptual basis of mechanical retention. Contours of clinical crown resembles two cones sharing a common base. The line formed at the junction of this base represents the greatest diameter of the tooth. This greatest diameter is called height of contour or point of maximum convexity. 11

12. M.M DEVAN introduced the terms Suprabulge and Infrabulge . These terms explain the coronal form, clasp form and clasp placement. According to him : Suprabulge : It is that portion of an abutment that converges towards the occlusal or incisal surface and is considered as suprabulge aspect of the abutment. Infrabulge : It is that portion of the clinical crown that converges apically from the height of contour and is considered the infrabulge aspect of the abutment. 12

13. As an extension of DeVan’s concepts, professionals have defined two basic categories of retentive clasp assemblies: Circumferential or suprabulge direct retainers Vertical projection, bar-type, or infrabulge direct retainers 13

14. The basic parts of a clasp assembly : 14

15. Rest : It is the part of the clasp that lies on the occlusal, lingual or incisal surface of a tooth and resist tissue ward movement of the clasp. Body of the clasp : It is the part of the clasp that connects the rest and shoulder of the clasp to the minor connector. It must be rigid and above the height of contour. 15

16. Shoulder : It is the part of the clasp that connects the body to the clasp terminals. It must lie above the height of contour and provide some stabilisation against horizontal displacement of the prosthesis. Reciprocal arm : A rigid clasp arm placed above the height of contour on the side of the tooth, opposing the retentive clasp arm . 16

17. Retentive arm : It is the part of the clasp comprising the shoulder which is not flexible and is located above the height of the contour . Retentive terminal: It is the terminal end of the retentive clasp arm. It is the only component of the removable partial denture that lies on the tooth surface cervical to the height of the contour. It possesses a certain degree of flexibility and offers the property of direct retention. 17

18. Minor connector : It is the part of the clasp that joins the body of the clasp to the remainder of the framework and must be rigid. Approach arm : It is a component of the bar clasp. It is a minor connector that projects from the framework, runs along the mucosa and turns to cross the gingival margin of the abutment tooth to approach the undercut from a gingival direction. 18

19. Retention Support Stability Reciprocation Encirclement Passivity Requirements of a Clasp assembly 19

20. Retention is the quality of the clasp assembly that resists forces acting to dislodge components away from the supporting tissues. It is not only the direct retainer which makes a RPD retentive but the effective design and its accurate construction . Only the retentive terminus should engage the prescribed undercut . The accompanying rest must provide support so the clasp terminus is maintained in an optimal location. The minor connector must be sufficiently rigid to ensure proper stability. Retention: 20

21. 4.The reciprocal element must contact the abutment slightly before the retentive element contacts the tooth, and it must maintain contact until the prosthesis is fully seated to protect the abutment from potentially destructive lateral forces. 5.Components must provide sufficient encirclement to prevent movement of the abutment away from the associated clasp assembly, otherwise retention will be lost. 6. Indirect retainers must resist forces acting to dislodge the prosthesis from its fully seated position 21

22. Factors affecting the amount of retention provided by a clasp assembly : T ype of clasp used Flexibility of the retentive arm Location of the retentive terminal in the prescribed undercut 22

23. Type of clasp used : Infrabulge Vs Suprabulge Infrabulge clasp requires that the retentive arm be "pushed" over the height of contour. Suprabulge clasp is "pulled" over the height of contour . Push type of retention better than pull type. 23

24. Flexibility Dr K. J. Anusavice defines maximum flexibility as the strain occurring when a material is stressed to its proportional limit. Maximum flexibility of a retentive clasp arm may be defined as the greatest amount of displacement that can occur without causing permanent deformation of the clasp arm. 24

25. Flexibility of the clasp arm depends on its Length Cross-sectional form Cross-sectional diameter Longitudinal taper Clasp curvature Metallurgical characteristics of the alloy. By increasing the flexibility of the clasp, the horizontal stresses imparted to an abutment during placement, withdrawal, and movement of the prosthesis may be reduced. 25

26. Length Flexibility  as Length  Flexibility = L3 Increasing the length and thus the flexibility reduces the horizontal forces on the abutment tooth during insertion and withdrawal. 26

27. Cross sectional diameter Flexibility as diameter . The average diameter to be considered is at a point midway between its origin and its terminal end. Thickness of the clasp arm in the buccolingual direction is to be considered rather than the width in the occluso -gingival direction. 27

28. Taper: A uniformly tapered clasp is more flexible than a nontapered clasp of the same proximal dimensions. The taper should be consistent for both width and thickness. The cross-sectional dimensions at the shoulder of the clasp should be twice the crosssectional dimensions at the clasp terminus. 28

29. A circular cross-sectional clasp form imparts omni directional flexure, while a half-round form allows only bidirectional flexure . Consequently, a clasp exhibiting a circular cross-sectional form may permit dissipation of detrimental forces in all spatial planes. Cross-sectional form 29

30. Curvature of a clasp in more than one spatial plane reduces the clasp's flexibility. When contouring a wrought-wire clasp arm, the operator should ensure that the clasp arm remains in a single plane of space. Multiple bends, especially those placing the clasp arm in multiple planes of space, may produce permanent deformation of the microstructure and lead to increased rigidity. Curvature of a clasp 30

31. Material used for clasp arm Gold alloy : Greater flexibility than chrome alloys Disadvantage of cast gold alloys : Bulk of the prosthesis. Costly Chrome alloys: Have a higher modulus of elasticity & therefore less flexibility. So in less undercut areas CoCr alloy can be used but in cases of deep undercut wrought metal can be used. 31

32. Two dimensions of an undercut: Mediolateral or horizontal dimension Occluso -apical or vertical dimension. Location of retentive clasp terminus 32

33. Greater the horizontal undercut engaged, greater will be the retention. 33

34. When the analyzing rod is positioned in the surveyor and placed against the tooth surface, an angular space is formed apical to the height of contour . This angular space represents a mechanical undercut. Placement of a clasp terminus into a mechanical undercut forms the basis for removable partial denture design, construction, and service. The angle formed between the analyzing rod and the tooth surface apical to the height of contour is called the angle of cervical convergence. 34

35. 35 As the angle of cervical convergence becomes greater, the force required to remove the retentive clasp from the abutment also becomes greater. Vertical distance from the height of contour is also very important. Less the distance from the survey line steeper the inclined plane and so greater force required to remove the clasp out of the undercut.

36. Support is the quality of a clasp assembly that resists displacement of a prosthesis in an apical direction. Components for vertical support: Rest Elements that contact the abutment occlusal to the height of contour ( eg , a reciprocal element or shoulder of retentive clasp) Support 36

37. Stability is the quality of a clasp assembly that resists displacement of a prosthesis in a horizontal direction. All framework components that are rigid and contact vertically oriented hard and soft tissues may contribute to the stability of a prosthesis. Components such as reciprocal element, rigid portion of retentive clasp arm, minor connectors and guide plates contribute to bracing. Stability 37

38. Reciprocation is the quality of a clasp assembly that counteracts lateral displacement of an abutment when the retentive clasp terminus passes over the height of contour. This component is known as a reciprocal element. Reciprocation 38

39. 39 The reciprocal element may be a cast clasp, lingual plating, or a combination of mesial and distal minor connectors.

40. Encirclement is the characteristic of a clasp assembly that prevents movement of an abutment away from the associated clasp assembly. Each clasp assembly must be designed to provide direct contact over at least 180 degrees of the tooth’s circumference. Encirclement 40

41. A clasp assembly should be passive when fully seated. The retentive arm should be activated only when dislodging forces are applied to RPD. If the clasp assembly is not fully seated, the retentive terminus will not be positioned in its intended location. As a result, the clasp assembly will apply non-axial ( ie , lateral) forces to the abutment. Passivity 41

42. Criteria for clasp selection Survey line Requirements of retention and stability depending on the number, configuration of edentulous areas. Nature of support Root size and form Esthetics Presence of excessive tissue undercut Oral hygiene and patient awareness 42

43. SUPRABULGE INFRABULGE 43

44. Types of clasps Combin - Ation clasp Onlay clasp Ring clasp Embrasure Clasp Multiple circlet Fish-hook Hair pin Reverse circlet Simple circlet Cast Circumfere - ntial Suprabulge 44

45. Suprabulge or Cast circumferential or Akers clasp Introduced by Dr.N.B.Nesbitt in 1916. Design : The clasp approaches undercut from edentulous area and engages undercut opposite to edentulous space. Indication : Tooth supported partial denture. Contraindication : They cannot be used for distal extension cases as they engage the mesiobuccal undercut 45

46. Advantages Simplicity of design Easy construction Provides excellent support, bracing and retentive properties Close adaptation to tooth surface so no food entrapment. Disadvantages Large amount of tooth covered, so underlying enamel prone to decalcification. The display of metal. 46

47. Design rules: The clasp should arise from the main body of the clasp assembly above the height of contour. All the components of the C clasp should be present above the height of contour except the retentive tip. The retentive terminus should always be directed towards the occlusal surface never towards the gingiva . It should always terminate at the mesial or distal line angle never at midfacial or midlingual surface. 47

48. Simple circlet design The simple circlet clasp design is versatile and widely used. Indication : Tooth-supported removable partial dentures. 48

49. Advantages : Fulfills the design requirements. E asy to construct Simple to repair. Disadvantages: Adjustment is difficult. The clasp assembly tends to increase the circumference of the clinical crown which may interfere with the elimination of food from the occlusal table and may deprive the adjacent gingival tissues of essential physiologic stimulation. Increased tooth coverage may promote decalcification and compromise dental esthetics. 49

50. Reverse circlet clasp Used when undercut is located at the facial or lingual line angle adjacent to an edentulous space. Design : It consists of a mesial occlusal rest, a horizontal reciprocal arm, and a retentive arm engaging the distobuccal undercut adjacent to the edentulous area. 50

51. Disadvantages:- Weak clasp if sufficient preparation is not done. Poor aesthetics if used in premolars and cuspids . 51

52. Multiple circlet clasp A multiple circlet clasp design involves two simple circlet clasps joined at the terminal aspects of their reciprocal elements. 52

53. Embrasure clasp Two circlet clasps joined at the body. This design is most frequently used on the side of the arch where there is no edentulous space. Occlusal rests must be used to support the embrasure portions of the clasp. 53

54. 54 Breakage due to inadequate occlusal clearance and concomitant lack of metal thickness. The problem can be avoided by adequate rest seat preparation and by careful preparation where the clasp assembly enters and exits the rest area.

55. Wedging of the abutments can also occur if rest seat preparations are omitted or inadequate. The rest seat preparations must be deeper toward the center of the occlusal surface than at the marginal ridges. 55

56. Ring clasp Indication – tipped mandibular molars. Contraindication - limited vestibular depth precludes placement of an auxiliary bracing arm. 56

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58. Advantages:- Provides adequate encirclement. Excellent retention with adequate flexibility due to increased length of clasp arm Disadvantages:- O ral hygiene becomes more complicated. Decalcification of teeth. Increased occlusal table. Poor structure of clasp. Susceptible to distortion and fracture. 58

59. A simple circlet clasp in which the retentive arm loops back to engage an undercut apical to the point of origin. Used when a distofacial undercut is present adjacent to the edentulous space. C-clasp (reverse action, hairpin or fishhook clasp) 59

60. Retentive arm has two horizontal components The occlusal portion - minor connector and must be rigid. The apical portion -pass over the height of contour to engage the desired undercut. Consideration:- Sufficient clinical crown height. Space between occlusal and apical arm. Occlusal arm shouldn’t interfere within the occlusion. This clasp is indicated when the: 1. S oft tissue contour precludes use of a bar-type clasp and 2. When the reverse circlet cannot be considered because of a lack of occlusal clearance. 60

61. DISADVANTAGES : A C-clasp design generally yields inadequate flexibility as a result, the abutment may be subjected to harmful non-axial forces. The C-clasp also results in considerable coverage of the abutment surface. The excessive display of metal associated with this clasp often renders the C-clasp esthetically unacceptable. 61

62. Onlay clasp design An onlay clasp consists of a rest that covers the entire occlusal surface and serves as the origin for buccal and lingual clasp arms. This clasp design is indicated when the occlusal surface of the abutment lies noticeably apical to the occlusal plane. The onlay rest serves as a vertical stop and also aids in the establishment of an acceptable occlusal plane. 62

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64. Wrought-wire circumferential clasp Introduced by Dr. O.C.Applegate in 1965. The combination clasp consists of an occlusal rest, a cast metal reciprocal arm, and a wrought-wire retentive arm. Indication : Kennedy Class I or Class II posterior edentulous area when the usable undercut is located at the mesiofacial line angle of the most posterior abutment. 64

65. The wrought-wire component is circular in cross section, thereby permitting flexure in all directions. This omnidirectional flexure allows the clasp to flex in all planes and can minimize the transfer of potentially harmful forces to the abutment. 65

66. Advantages: 1.Increased flexibility allows the clasp tips to be placed in deeper undercuts and are more esthetic. 2.Allows for greater adjustability, because of their round form, they can be adjusted in any spatial plane. Disadvantages: 1.Additional laboratory procedure 2.Increased potential for permanent deformation by the patient. 66

67. Infrabulge clasp (Vertical projection or roach or bar type clasp) Popularized by Ewing Roach in 1930 called it the Bar Clasp. An infrabulge clasp approaches the undercut region of an abutment from an apical direction. Push type retention. Flexibility of clasp from length and taper. More aesthetic than C clasp. Ex :- Y clasp, T clasp, I clasp 67

68. Design rules for Infrabulge Clasps: Approach arm should not impinge the soft tissues. Tissue surface smooth and well polished. Approach arm should cross the free gingival margin at 90 degrees. To optimize flexibility the approach arm should be uniformly tapered from its origin to the clasp terminus. Clasp terminus should be positioned as far apically on the abutment as is practical. 68

69. Types of I nfrabulge clasps: Y- clasp Modified T – clasp T – clasp I - clasp 69

70. T – Clasp Indications : Kennedy Class I or Class II partially edentulous situations when an undercut is located adjacent to the edentulous area. Contraindication : Available undercut is located on the mesiofacial aspect of the most posterior abutment. If the approach arm must cross over an area of severe soft tissue undercut. When the height of contour is located near the occlusal surface. 70

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72. Modified T-clasp design The modified T-clasp is essentially a T-clasp that lacks the nonretentive , horizontal projection. Uses : Used on canines and premolars for aesthetics Disadvantage : 180 ° coverage is not present which compromises bracing and reciprocation. 72

73. Y – Clasp A Y- clasp is formed when the approach arm terminates in the cervical third of the abutment , while the mesial and distal projections are positioned near the occlusal surface. When height of contour high on the mesial & distal line angle but lower in centre. 73

74. I- bar clasp In 1963, Kratochvil introduced I-bar design philosophy. Consists of A mesial rest, Long distal proximal plate I- bar retentive element . I clasp lack the horizontal retentive arms but only a horizontal retentive tip. The retentive tip contacts the abutment surface only at the undercut region. The amount of contact is about 2 to 3mm in height and 1.5 mm in width. 74

75. KROL’S RPI SYSTEM This concept of clasping abutment teeth is a modification of the concept presented by Kratochvil . The components of the clasp are a rest with its minor connector, proximal plate, and an I bar clasp. 75

76. Mesiocclusal rest The abutment tooth contains the mesiocclusal rest with the minor connector placed into the mesiolingual embrasure but not contacting the adjacent tooth . It acts as the point of rotation and exerts a mesial force on the tooth rather than a distal displacing force. 76

77. Proximal Plate The superior edge of the proximal plate is located at the bottom of the prepared guide plane which should be at the junction of occlusal 1/3 and middle 1/3 of the proximal surface. The remainder of the proximal plate lies below the guide plane. The proximal plate extends lingually just far enough so that the distance between the minor connector and proximal plate is less than the mesiodistal width of the tooth . 77

78. I-bar The approach arm of the I bar extends from the framework so as to remain at least 3 mm from the gingival margin and then crosses the gingival margin at right angles Approximately 2 mm of the I bar contacts the tooth surface, usually at the gingival one third of the tooth. The I-bar terminus assumes a more mesial position to achieve more efficient reciprocation from the small Proximal Plate. 78

79. CONTRAINDICATIONS FOR RPI CLASP Insufficient depth of vestibule to permit the approach arm of the I bar to be located at least 3 mm from gingival margin. A tooth which has a severe lingual tilt and no labial or buccal undercut. Tissue undercut so severe that the approach arm of the I bar would be too far away from the tissue and act as a food trap or irritate the mucosa of the lip or cheek. Teeth which are severely flared labially or bucally . A tooth which has only a distobuccal retention undercut and does not require a restoration. A mouth with a high floor in which a lingual plate is used on the lingual surface of the abutment tooth. 79

80. RPA clasp (rest-proximal plate- akers clasp): The rest-proximal plate- Aker’s clasp was developed and described by Eliason in 1983. It consists of a mesial occlusal rest, proximal plate and a circumferential clasp arm, which arises from the superior portion of the proximal plate and extends around the tooth to engage the mesial undercut. 80

81. 81 Advantages : The circumferential-type retentive arm is easier to grasp for removal of the prosthesis. The clasp is simple in design with few variations among patients and thus can be easily and consistently fabricated by dental laboratories. The circumferential retentive arm avoids the tissue problems around abutment teeth and allows the RPA clasp to be used in many situations where the RPI clasp is contraindicated.

82. RII CLASP Consists of a rest, lingual I-bar (usually rigid)and buccal I-bar (usually flexible and retentive) Used on molar abutments of maxillary tooth -supported segments. The lingual I-bar is located at the distolingual line angle and provides for bracing. The buccal retentive l-bar is usually located at the distal portion of the facial surface. 82

83. The RII assembly is purported to be more hygienic than the more conventional circumferential clasp assembly. Tilted molars, particularly mandibular molars, present a difficult clasping problem . Mesiolingual l-bar projecting from the inferior distal border of the major connector. Because any stress created by the l-bar is opposed by the natural mesiolingual drift of the abutment, a buccal arm may not be necessary. However, if additional bracing is desired, a buccal arm should be included. 83

84. Estheti clasp The Estheti clasp is recommended for patients with required abutment teeth in the esthetic zone (incisors and bicuspids). Advantages : Optimum esthetics, No attachment maintenance cost. It utilizes the proximal undercuts and encircles the tooth by 181°. 84

85. Estheti clasp may be in the form of Lclasp or C-clasp. L-clasp : The design consists of the clasp arm extending from lingual minor connector with an independent reciprocal rest. The L-clasp has greater rigidity than the C-clasp. C-clasp : It consists of a modified back-action clasp with rest incorporated in clasp. 85

86. Saddle lock The saddle-lock system eliminates facial clasps by using the available mesial /distal concave surfaces of the abutment teeth for retention instead of the buccal undercuts. 86

87. The benefits of saddle lock : Superior esthetics, without visible clasps. Improved retention with little or no adjustment. Easy vertical insertion that protects abutments. Applicable in most partial denture cases. Simple preparation procedures for less chair time. 87

88. Equipoise RPD system Proposed by J. J. Goodman, it is an esthetic retentive concept for distal extension situations. The equipoise clasp is a lingual backaction clasp that is fully reciprocated and extremely esthetic with no facial clasp displays. Rests are placed away from edentulous span. Vertical inter-proximal reduction of 1 mm between abutment and adjacent tooth is done. 88

89. The unique Class II lever design protects, preserves and strengthens abutment teeth while directing all forces down the long axis of the abutment tooth. This concept works very well on premolar abutment. The occlusal rest provides excellent reciprocation for the clasp and the clasp tip can engage an undercut in the distobuccal line angle. 89

90. Proflex clear wire clasps Clear wire is an excellent new way to fabricate clear, strong, flexible clasps in minutes. This new material and technique can be used to make T-bars, I-bars, Roaches, Acers, and most other types of clasps. It can also be used to add or repair clasps in an existing partial denture. 90

91. Metal-free clasps The metal-free materials available now are ideal for flexibility and esthetics thus allowing esthetic functional care in the true sense. Opti•flex invisible clasp partials: With the Opti•Flex acetyl resin clasps, metal-free, lightweight partial dentures that provide natural esthetics and a comfortable fit can be designed. 91

92. Flexite plus cast thermoplastic Flexite Plus ‘Flexible’ partial dentures eliminate the use of metal, providing patients with a metal partial denture alternative. Flexite Plus is fabricated from a flexible thermoplastic material that is available in three tissue shades. The material is monomer-free, virtually unbreakable, lightweight, and impervious to oral fluids. Flexite Plus may also be combined with a metal framework to eliminate the display of metal labial clasps. 92

93. The Use of a Modified Poly-Ether-Ether- Ketone (PEEK) as an Alternative Framework Material for Removable Dental Prostheses. Traditional removable dental prostheses (RDP) with chromecobalt frameworks and clasps have been an inexpensive and predictable treatment option for the rehabilitation of partially edentulous patients. The esthetically unacceptable display of metal clasps, the increased weight of the prosthesis, the potential for metallic taste, and allergic reactions to metals led to the introduction of a number of thermoplastic materials in clinical practice such as nylon and acetal resins 93

94. Advantages of nylon : Provide improved esthetics Reduction of rotational forces on the abutment teeth due to their low elastic modulus. Disadvantages of a nylon RDP The inability for a reline procedure Lack of occlusal rests as well as rigid frameworks, that could lead to occlusal instability and sinking, especially in Kennedy class I and II cases. On the other hand, acetal 94

95. Acetal resins present adequate mechanical strength to form a framework more rigid than nylon with retentive clasps, connectors, and supportive element. However, the acetal resin material lacks natural translucency and vitality. An alternative restoration material (poly-ether-ether- ketone [PEEK]) can be used for construcion of RPD. A modified PEEK material containing 20% ceramic fillers is a high performance polymer ( BioHPP ; Bredent GmbH, Senden , Germany),. 95

96. Advantages : High biocompatibility Good mechanical properties High temperature resistance Chemical stability. Due to a 4 GPa modulus of elasticity, it is as elastic as bone and can reduce stresses transferred to the abutment teeth. Furthermore, the white color of BioHPP frameworks provides a different esthetic approach than the conventional metal framework display does. Elimination of allergic reactions and metallic taste High polishing qualities Low plaque affinity Good wear resistance. 96