Direct Retainers in removable partial denture prosthodontics

types of direct retainers in removable partial denture prosthodontics

1. DIRECT RETAINERS

2. INTRODUCTION An essential key to success in practice of removable partial denture is thorough planning of each structural detail.

3. INTRODUCTION A removable partial denture must have support derived from abutment teeth through use of rests and from residual ridge through will fitting base. It must be stabilized against horizontal movement through use of rigid connectors, indirect retainers and other stabilizing components. In addition, removable partial denture should have sufficient retention to resist reasonable dislodging forces. 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. The term retention refers to resistance to dislodgement of the prosthesis in an occlusal direction. 

4. TERMINOLOGIES :   RETENTION - that quality inherent in the dental prosthesis acting to resist forces of dislodgement along the path of placement. (GPT-9)   DIRECT RETENTION - retention obtained in a partial removable dental prosthesis by use of clasps or attachments that resist removal from the abutment teeth. (GPT 9)   DIRECT RETAINER - that component of a partial removable dental prosthesis used to retain or prevent dislodgement, consisting of a clasp assembly or precision attachment. (GPT 9)  The components of a removable partial denture that engage abutments and resist dislodging forces are called direct retainers. (Stewart)   A direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth to resist displacement of the prosthesis away from basal seat tissue. (McCracken)    

5. TYPES OF RETENTION : PRIMARY RETENTION Retention of removable partial denture is accomplished mechanically by placing retaining elements on abutment teeth. By the intimate relationship of denture bases and minor connectors with underlying tissues The latter is similar to retention, of complete dentures and proportionate to: Accuracy of Impression registration. Accuracy of fit of denture bases. Total area of contact involved. SECONDARY RETENTION

6. TYPES OF DIRECT RETAINERS :

7. INTRACORONAL DIRECT RETAINERS

8. It is cast or attached totally within the restored natural contours of an abutment tooth. Developed by Dr Herman E. S. Chayes in 1906. Two components- matrix n patrix 2 types: Semi precision attachment Precision attachment This type of retainer is composed of prefabricated machined key and keyway , with opposing vertical parallel walls. Parallelism results in well defined path of insertion and removal and in mechanical binding when off axis forces are applied to the prosthesis.  

9. PRECISION ATTACHMENT SEMI-PRECISION ATTACHMENT Fabricated in metal using high precision manufacturing techniques. Components originate as wax or plastic patterns which are subsequently cast in metal. The components exhibit long parallel walls. Gently Tapering walls. Exceptional surface adaptation. Less intimate fit.

10. Advantages More esthetic More effective cross bar stabilization Direct vertical forces along long axis of abutment tooth Disadvantages Require cast placement on abutment tooth Can’t be used on teeth with large pulp Tend to loosen with use Complicated clinical procedure Expensive & difficult to repair/ replace

11. EXTRACORONAL DIRECT RETAINERS

12. First proposed by Henry H Boos  1900 later modified by Ewing F Roach in 1908. Located outside the teeth. Derive their retention from closely fitting components named patrices and matrices. Retention from  mechanical resistance. Permit vertical movement during vertical loading. Minimize potentially damaging forces to abutment  Stress breaking or stress directing effects. 1. ATTACHMENTS

13. 2. CLASP ASSEMBLIES It’s the most common method for Extra coronal direct retention. It first appeared in the dental literature with Dr W.G.A. Bonwill’s description. Extracoronal direct retainer or clasp direct retainer operates on the principle of resistance of metal to deformation. When vertical dislodging force is applied the terminal end of clasp is forced to deform. It is this resistance to deformation that generates retention.

14. In 1916 – J.H. Prothero’s Cone Theory was introduced which is the basis of clasp retention. Shape of the crowns of PM and molar teeth can be considered as two cones sharing a common base. The line formed at the junction of these cones represents the greatest diameter of the tooth. Kennedy called it as Height of contour Cummer as Guideline De Van as Supra & infra bulge regions PROTHERO’S CONE THEORY : 

15. Vertical arm of surveyor contacts the tooth at the height of contour which determines areas for retention and support. Height of contour changes depending on position of teeth and to a given path of placement or removal.

16. TYPES OF CLASPS Circumferential/ Supra bulge/ Akers clasp: Has two clasp arms that encircle the abutment tooth. Clasp approaches undercut from above the height of contour 2) Bar Clasp/ Infra bulge clasp: Has an approach arm ending up in two clasp arms one placed above and the other placed below the height of contour. Clasp approaches undercut from below the height of contour.

17. PARTS OF A CLASP: Retentive arm Retentive terminal Reciprocal arm Rest Body Shoulder

18. 1. REST GPT: A rigid extension of removable partial denture which contacts a remaining tooth or teeth to dissipate vertical or horizontal forces. FUNCTIONS: It lies on the occlusal ,lingual or incisal edge Resists tissue ward movement of the clasp by acting like a vertical stop. ( gives SUPPORT ) Transmit functional forces along the long axis of the tooth.

19. 2.BODY Part of the clasp that connects the rest and shoulders of the clasp to the minor connectors.   It must be rigid and must be above the height of contours. It contacts the guide plane of the abutment during the insertion and removal Part of the body that contacts the guiding plane - proximal plate  

20. Part of the clasp that connects the body to the clasp terminal Lies above the height of contour Provides against horizontal displacement 3.SHOULDER

21. 4. RECIPROCAL ARM GPT: A component of the clasp assembly specifically designed to provide reciprocation by engaging a reciprocal guiding plane Rigid ,placed above the height of contour on the side of the tooth opposing the retentive clasp arm. Non-tapered

22. - It resists the tipping force generated by the retentive terminal as it passes over the height of contour -Contact the tooth before retentive clasp ,remain in contact while retentive clasp passes over height of contour Functions of reciprocal arm Stability & reciprocation against retentive arm Stability against horizontal movement May act as indirect retainer if placed on the opposite side of fulcrum

23. In the fully seated position the bracing arm lies above the height of contour line. Bracing arm stabilizes the tooth while the retentive arm is slipping over the height of the contour during insertion of the prosthesis Together with the clasp shoulder and retentive arm , the bracing arm completes the bodily encirclement of the abutment (270 º)

24. “A flexible segment of a removable partial denture which engages a under cut on an abutment which is designed to retain the denture” – GPT Terminal 3rd / retentive terminal Middle 3rd Proximal 3rd RETENTIVE ARM

25. Terminal 3rd / retentive terminal/ Terminus – It is flexible and should point towards occlusal surface – Only component to lie cervical to height of contour to provide direct retention. Middle 3 rd – Provides limited flexibility --Placed above the height of contour Proximal 3rd – Rigid – Placed above Retentive arm , not flexible and is located above the height of contour Retentive terminal is flexible and lies below the height of contour.

26. 6.MINOR CONNECTOR Part of the clasp that join the body of the clasp to the reminder of the framework. It should be rigid.

27. It projects from the framework, runs along the mucosa turns to cross-gingival margin of abutment tooth. • Body and retentive terminal attached to it. • It is slightly flexible 7. APPROACH ARM

28. POINTS TO REMEMBER:

29. REQUIREMENTS OF A CLASP ASSEMBLY

30. Retention: The path of escapement of each retentive clasp terminal should be other than path of removal of the prosthesis . The amount of retention always should be the minimum necessary to resist reasonable dislodging forces . Only the terminal third of an occlusally approaching clasp (stippled section) should engage the undercut. A gingivally approaching clasp contacts the tooth surface only at its tip.

31. Tooth Prosthesis Angle of cervical convergence Cross-section Diameter of clasp arm Length of clasp arm Material flexibility Factors producing Retention: Curvature of the clasp

32. Size and distance into the angle of cervical convergence - when surveyor blade contacts a tooth on cast at height of contour , triangle is formed -Apex- at point of contact of surveyor blade and tooth -Base- area of gingival tissues -This apical angle : Angle of cervical convergence Greater the angle of cervical convergence , greater is the force required to remove the clasp from the abutment.

33. Suprabulge height of contour infrabulge angle of cevical convergence areas cervical to height of contour may be used for placement of retentive clasp component -Areas above it for non-retentive components

34. 2. Flexibility D = 4PL 3 Ewt 3 Where, D= deflection P= applied force L= length E= Modulus of Elasticity W= beam width T= Thickness

35. Length of the clasp arm : F is directly proportional to L 3 The longer the clasp arm, the more flexible it is, all other factors being equal. The length of a clasp arm is measured from the point at which a uniform taper begins. A Co-Cr clasp arm engaging the same degree of undercut will have different flexibility and resistance to distortion on the molar and the premolar because of the difference in length.

36. Diameter of clasp arm F is inversely proportional to D 3 The diameter of a clasp arm is inversely proportional to its flexibility, all other factors being equal . The average diameter to be considered is at a point midway between its origin and its terminal end . The thickness of the clasp arm in the buccolingual direction is to be considered rather than the width in the occluso -gingival direction.

37. Taper and Curvature Uniform taper A clasp should be half as thick at its tip as at the origin (1:2) More the curvature of the clasp lesser is the flexibility.

38. Cross-section of the clasp arm Round cross-sectional form enables the clasp to be flexible in all directions (omnidirectional) whereas the half-round form limits the flexibility to only one direction. Tooth –borne: half-round circumferential clasp (flexes only during placement and removal) Distal –extension: round cricumferential clasp (flexes during placement and removal & during functinal movements)

39. -Chrome alloys high modulous of elasticity less flexible - smaller cross-sectional - less depth of undercut - wrought wire internal grain structure more flexibility So, for equal retention deeper undercut required Material Used:

40. According to Stewart For a 0.010” undercut - Cast chrome alloy 0.015” undercut - gold alloy 0.020” undercut - wrought wire

41. To summarise : Factors affecting the flexibility of the clasp arm- Increases with increase in length Increases as cross- sectional dimension decreases A uniformly taper ed clasp is more flexible than a non-tapered clasp of the same dimension. A circular cross-section al clasp imparts omnidirectional flexure, while a half round clasp allows only bidirectional flexure. Curvature of clasp in more than one spatial plane reduces the clasp’s flexibility Metallurgical properties of an alloy influence clasp flexibility

42. Stability is the resistance to horizontal displacement of a prosthesis. All clasp parts except the retentive clasp terminals contribute to this property in varying degree. Eg : Cast circumferential clasp offers great amount of stability, through the reciprocal element and also its shoulder is rigid which aids in stabilization. Vertically oriented minor connector also offers stabilization. The wrought wire clasp has flexible shoulder. Bar clasp does not have a shoulder hence, provide less stability. 2. Stability:

43. 3. Support: “It is the property of the clasp that resist displacement of the clasp in the gingival direction”. • it is provided by occlusal , lingual and incisal rests • It should transmit forces parallel to the long axis of the abutment.

44. 4.Encirclement: “Encirclement is the characteristic of a clasp assembly that prevents movement of an abutment away from the associated clasp assembly”. Clasp assembly  180 degrees contact.

45. The engagement can be in the form of continuous contact, such as circumferential clasp, or discontinuous contact, such as Bar clasp. Both provide tooth contact in at least 3 areas encircling the tooth- These are : occlusal rest area the retentive terminal area and reciprocal terminal area .

46. 5.Reciprocation: 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. Rigid and must contact the abutment tooth at or occlusal to the height of contour (junction of middle and gingival thirds) During insertion of the prosthesis, the reciprocal element should contact the abutments slightly before the retentive arm contacts the abutment. It should be positioned on the surface of a tooth that is reasonably parallel to the denture’s path of insertion and removal. If it is placed on the surface that is tapered occlusally , a slight movement of denture will cause the clasp to loose contact with the tooth and reciprocation and retention is lost.

47. The reciprocal element may be a Retentive arm of clasp Lingual plating , Combination of mesial and distal minor connectors.

48. 6.Passivity: “Passivity is the quality of a clasp assembly that prevents the transmission of adverse forces to the associated abutment when the prosthesis is completely seated.” When fully seated  it should be passive. Should be activated only when dislodging forces are applied.

49. Retentive clasp element should be placed on facial surface of tooth.(mainly in premolars) Only one retentive element should be used per tooth opposed by a reciprocal element. Clasp retainers on the abutment adjacent to distal extension bases should be designed so they will avoid direct transmission of tipping & rotational forces to the abutment. The path of escapement for each retentive clasp terminal must be other than parallel to the path of removal for the prosthesis to require clasp engagement with resistance to deformation. Balanced retention should be present i.e. if buccal retentive clasp is present on one side, the same should be present in the opposite side and vice-versa. OTHER PRINCIPLES:

50. Location of retentive clasp terminus Retentive clasp should be positioned on the facial surface of the abutment Facial placement permits increased length of the retentive arm and yields improved clasp flexibility In contrast, lingual placement of the retentive arm results in decreased clasp length and an accompanying decrease in flexibility

51. Lingual retentive elements can be used with molar abutments If retentive clasp on one side of the arch is positioned on the facial surface of an abutment , at least one retentive clasp on the contra lateral side of the arch should be located on the facial surface of an abutment.

52. COMBINATION BAR CIRCUMFERENTIAL CAST WROUGHT CLASSIFICATION OF CLASPS: ON THE BASIS OF MATERIAL BEING USED: ON THE BASIS OF DESIGN:

53. CAST CIRCUMFERENTIAL /SUPRABULGE/ OCCLUSALLY APPROACHING/AKER’S CLASP

54. By N.B.Nesbitt in 1916. They are popularly known as Aker’s clasp. They approach the undercut from the occlusal direction. ADVANTAGES: Easiest claps to make and repair. Less food retention. Best when applied in a tooth supported partial denture . Derives excellent support, bracing and retention.

55. DISADVANTAGES: Covers a large tooth surface which causes Alteration of buccolingual width of the tooth. Affects the normal food flow pattern leading to food accumulation. Decalcification of tooth structure. Damage to soft tissues due to lack of physiological stimulation. Due to its half round configuration it can be adjusted only in one plane ( buccolingual ) If these clasps are placesd more occlusally on the tooth the width of the food table increases leading to more occlusal forces. Not as esthetic

56. RULES FOR USE OF CAST CIRCUMFERENTIAL CLASP: a) The retentive clasp arm should originate above and its terminal third should be positioned below the height of contour . b) The retentive clasp should always point towards the occlusal surface . This help to produce a curved clasp results in greater flexibility.

57. c)The retentive terminal should terminate at the mesial or distal line angle of the abutment tooth, never at the midfacial or midlingual surfaces. d)Clasp arm should be kept as low on the tooth as possible without violating its relation with height of contour. f) A cast circumferential clasp should not be used to engage The distofacial surface of an abutment adjacent to an anterior edentulous space or The mesiofacial surface of an abutment adjacent to an extensive posterior edentulous space. e)Designing for an extension base RPD , special consideration must be given to potentially detrimental forces associated with cast circumferential clasps.

58. TYPES OF CAST CIRCUMFERENTIAL CLASP

59. Retentive terminal approaches the undercut of the tooth from above the survey lines. This type of retention is called as “pull type” of retention. Most widely used clasp. Is the design of choice for tooth supported rpd Advantages and disadvantages are same as cast circumferential clasp. SIMPLE CIRCLET CLASP

60. REVERSE CIRCLET CLASP Indicated when the available undercut is located at the facial or lingual line angle adjacent to an edentulous space. The clasp of choice in such type of an abutment is bar clasp. But in cases bar clasp is contraindicated like if mucosa has undercut area or due to buccoversion of the abutment tooth. This clasp is usually designed such a way that the clasp arises from the mesial side and ends on the distal under cut. In a distal extension partial denture the reverse approach circlet clasps helps control stresses transmitted to terminal abutment tooth on the edentulous side.

61. As a distal extension base is loaded, the posterior aspect of the prosthesis moves toward the underlying tissues. At the same time, the tip of the retentive clasp moves into an area of greater undercut. Therefore, torsion stresses on the abutment are minimized. Disadvantages: Strength would be compromised if one fails to achieve sufficient occlusal clearance. Because a reverse circlet clasp crosses the facial surface from mesial to distal, it is a poor choice from an esthetic viewpoint. As a result, it is not the clasp of choice for canine and premolar abutments  

62. MULTIPLE CIRCLET CLASP A multiple circlet clasp design involves two simple circlet clasps joined at the terminal aspects of their reciprocal elements. Indication: When the principal abutment tooth is periodontally compromised

63. Bonwill clasp Crib clasp Double clasp ( 2 occlusal rest) Back to back clasp Two circlet clasps joined at the body. An embrasure clasp is used when no edentulous space exists at the clasp assembly site. Can be subrabulge or infrabulge type. Can be cast or wrought. Embrasure clasps may be used in Class II, III, or IV RPDs.   EMBRASURE CLASP Contra indication: Short and bulbous crown . Not preferred in teeth with more undercuts .

64. Disadvantages: 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 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.

65. RING CLASP The ring clasp is most often indicated on tipped molars In tipped molars where available retentive undercut will be located on the mesiolingual line angle of a mandibular molar and mesiobuccal line angle of a maxillary molar.   The ring clasp engages this undercut by encircling almost the entire tooth from its point of origin.

66. Because of its additioned length, it must be designed with additioned support usually in the form of a bracing arm(supporting strut). The bracing arm usually projects from the (acrylic resin) denture base minor connector, run across the mucosa and turns upward to engage the buccal arm of the ring clasp near the center of the buccal surface. The entire clasp , except the retention tip must be placed above the height of contour.

67. Disadvantage : -Alteration in the food flow pattern -It cannot retain its physical qualities -Difficult to adjust or repair - increased tooth surface coverage Contraindications: -If the buccinators attachment lies close to the lower molar -If the bracing arm will have to cross a soft tissue undercut.

68. FISH HOOK CLASP/ C- CLASP/ HAIPIN CLASP: a simple circlet clasp in which the retentive arm, after crossing the facial surface of the tooth from its point of origin, loops back to engage a proximal undercut below its point of origin. The upper part of the retentive arm is considered to be a minor connector and should be rigid, the lower part should be tapered and should be flexible.  It should be used in tooth with sufficient occluso gingival height to accommodate the double width of clasp. The upper arm should not interfere with opposing occlusion.

69. Indication : undercut adjacent to the occlusal rest or edentulous space When soft tissue undercut precludes the use of a bar clasp  It also indicated when a reverse circlet cannot be used because of lack of occlusal space. Disadvantage : - As it cover considerable tooth structure it may trap food debris.  Not advisable for caries prone and young patients. Poor esthetics.

70. It is an extension of a metal crown or onlay with buccal and lingual clasp arms. The clasp may originate from any part on the onlay that may not cause occlusal interference.  Indication: -In the occlusal surfaces of submerged abutment teeth(that are below the occlusal plane) -It should be given on caries free mouths. ONLAY CLASP

71. It has a retentive arm arising from one direction and a reciprocal arm arising from another. The buccal arm provides for bracing only. The lingual arm utilizes an undercut adjacent to the edentulous space for retention. Two minor connectors are needed for this design The first minor connector attaches the Occlusal rest and the retentive arm to the major connector. The second minor connector connects the reciprocal arm which is similar to the bar clasp with or with out an auxillary rest. This design is intended to provide dual retention Originally intended for unilateral partial dentures HALF AND HALF CLASP

72. HALF AND HALF CLASP Indication: Lingually inclined premolars where lingual undercut are close to the edentulous space. Contra indication: Buccal inclined premolars – If it is used for distal extension RPD a distal rest should be placed. Mesial rest also can be used in conjunction with the distal rest.

73. BACK ACTION CLASP It is a modification of the ring clasp. It has an unsupported occlusal rest and the minor connector is connected to end of the clasp arm. Disadvantages : lack of support to occlusal rest reduces its function . It has biological and mechanical unsound principles which makes it hard to justify its use.

74. COMBINATION CLASP Uses: It is used on abutment tooth adjacent to a distal extension base where only a mesiogingival undercut exists on the abutment or where a large tissue undercut contraindicates a bar type retainer. The tapered wrought wire retentive arm offers greater flexibility than does the cast clasp arm and therefore better dissipates functional stresses. In 1965, Dr.O.C . Applegate introduced a modified wrought-wire clasp assembly known as the Combination clasp Consists of: an occlusal rest ,a cast metal reciprocal arm , and a wrought wire retentive arm. Cross section of wrought wire retentive arm. : circular, there by permitting flexure in all directions Adapted during framework wax-up procedure

75. Advantages It has a thin line contact, collects less debris and is easy to maintain It can flex in all planes Adjustable Rigid reciprocal arm compensates for any orthodontic force applied by the retentive arm Disadvantages It involves extra steps in fabrication, particularly when high fusing chromium alloys are used. It may be distorted by careless handling on the part of the patient. Since it is bent by hand, it may be less accurately adapted and therefore provide less stabilization above the height of contour.

76. BAR/INFRABULGE/GINGIVALLY APPROACHING/ROACH’S CLASP

77. Infra Bulge Push Clasp Roach Clasp This bar clasp approaches the retentive undercut in a gingival direction resulting in a push type of retention. This push type of retention is more effective than pull type retention characteristic of circumferential clasp- tripping action. This clasp is termed by F.E. Roach in 1930 and hence the name Roach clasp. The bar clasp is classified by shape of the retentive terminal T, modified T, I, Y forms, all of which originate from the denture base frame work and approaches the undercut from gingival direction.

78. Only the retentive arm of a bar clasp varies from that of a cast circumferential clasp. All other components like rest, shoulder, body reciprocal arm, and proximal plate are similar. Retentive arm in a bar clasp comprises of two parts namely the gingival approach arm and a retentive terminal

79. Advantages of bar clasp -It is easy to insert and difficult to remove -It is more esthetic ,as it covers less tooth structure Disadvantages -It tends to collect food debris -reduced bracing and stabilization -Additional stabilizing units are needed

80. Guidelines for use of bar clasp: The approach arm of the bar clasp must not impinge on the soft tissue. It is not desirable to provide an area of relief under the arms but the tissue side of the approach arm should be smoothed and polished . Minor connector that attaches the occlusal rest to the frame work should be strong and rigid to provide some bracing. For flexibility, the approach arm must always be tapered uniformily from its origin to the clasp terminal. The approach arm positioned over a soft tissue undercut will collect food and irritates lips or cheeks. Should not “bridge” a soft tissue undercut. The approach arm should cross the gingival margin at a 90 degree angle . The bar retentive clasp is used only when the retentive undercut is adjacent to the edentulous area from which the approach arm originate. Theclasp terminus should be positioned as far apically on abutment as is practical. This will reduce the leverage induced stresses.s

81. TYPES OF BAR CLASP:

82. The retentive terminal must cross under height of contour to engage the retentive undercut, while other finger stays on the supra bulge of the tooth which improves bracing and stabilization.  contacts the tooth only at the height of contour. Indications Its frequently used in Kennedys class I or Class II distal extention cases where undercut is located on distobuccal surface of abutment tooth It is also used for tooth supported partial denture where the retentive undercut is located adjacent to edentulous space.   T- CLASP

83. Contraindications : -It should not be used on a terminal abutment adjacent to distal extention partial denture iwhen mesiofacial undercut is present   It should not be used when approach arm bridges an undercut. - It should not be used when height of contour is placed closer to occlusal or incisal surface. 

84. Modified T clasp is essentially- T clasp that lacks the , non retentive horizontal projection (usually mesial) Indications often used on canines or premolars for esthetic reasons. Kennedys class I or Class II( when retentive undercut are located adjacent to edentulous area). The main disadvantage is that encirclement, or 180-degree coverage of the abutment tooth may be sacrificial to esthetic. MODIFIED T- CLASP

85. The ‘y’ clasp is basically a T clasp. Its configuration occurs when the height of contour on the facial surface of the abutment tooth is high on the mesial and distal line angles but low on the center of the facial surface The approach arm ends in the cervical third of the abutment. Practically, Y-clasp is equivalent to a T-clasp. Y- CLASP

86. I- CLASP The I-clasp used on the distobuccal surface of maxillary canines far esthetic reasons.  Here the clasp terminal will be touching about 2-3mm in height & 1.5-2mm in width on the abutment tooth. As a result encirclement of horizontal stabilization is compromised. 

87. OTHER DESIGN CONCEPT: RPI, RPA VRHR clasp or Grasso clasps

88. I- bar clasp In 1963, Kratochvil introduced I-bar design philosophy. It consists of a mesial rest, long distal proximal plate and an I- bar retentive element. WHY MESIAL REST ? As the distance from the rotational center to the denture base is increased, the associated radius becomes larger, and the accompanying arc becomes more linear. Consequently, anterior placement of rests helps direct the forces vertically onto the bearing tissues beneath the extension bases.

89. PROXIMAL PLATES: Extend from marginal ridge to tooth-tissue junction and into the attached gingiva for 2 mm. This configuration Permits improved stabilization Improves retentive characteristics by limiting the path of insertion and removal Protects the tooth-tissue junction by reducing food impaction between the tooth and the proximal plate Provides reciprocation during insertion and removal of the prosthesis   I-BAR: The clasp terminus engages an undercut at the height of contour or slightly mesial to it. The contact area bet the clasp and the abutment is 2-3mm in height and 1.5-2mm in width.

90. DRAWBACKS: Bar clasp assembly in which guiding plane (GP) and proximal plate (PP) extend entire length of proximal tooth surface. Physiologic relief is required to prevent impingement of gingival tissues during function. Extending proximal plate to contact greater surface area of guide plane direct functional forces in horizontal direction, thus tooth (teeth) are loaded more than edentulous ridge

91. Krol in 1973 made certain modifications in the design under the “ minimal coverage criteria ” 1) Mesial rest preparation: In molars the mesial rest extends only into triangular fossa. The canine rests are circular, concave depressions prepared on the mesial marginal ridge. It does not cover the entire marginal ridge Arthur Krol JPD 1973;23;408-415 RPI CLASP

92. Design modification I : The proximal plate is designed to extend along the entire length with a minimum tissue relief. Design modification II : The proximal plate is designed to extend from the marginal ridge to the junction between the middle and cervical third of the tooth where as in I-bar retainer the proximal plate extends about 2mm across the tooth tissue junction. Design modification III : The proximal plate is designed to contact just about 1mm of the gingival third of the guiding plane of the abutment tooth. The purpose of reducing the length of the proximal plate is to improve the gingival health. Proximal Plate Modification:

93. I-bar modification: The tip of the I-bar is modified to have a pod shape in order to allow more tooth contact. It is placed more mesially so that it shifts towards the mesial embrasure space under occlusal load and increases reciprocation. The I-bar is designed to compensate for the decreased encirclement, {tooth contact}, provided by the proximal plate and mesial rest.

94. This is a modification of the RPI retainer, again devised by Krol in 1976. The I-bar in the RPI system could not be used for patients with soft tissue undercuts and high frenal attachments. In such cases the I-bar of the RPI retainer is replaced with an Aker’s {circumferential} clasp to form the RPA retainer. RPA CLASP

95. The vertical reciprocal horizontal retentive arm concept was developed by Grasso in 1980 and is characterized by: A distal occlusal rest supported by a minor connector. A lingual vertical reciprocal component originating from the major connector. A horizontal retentive arm attached to either the major connector or the retention latticework for the denture base. Joseph Grasso JPD 1980,43;618-21 VRHR CLASP

96. CIRCUMFERENTIAL CLASP BAR CLASP Approach of undercut From occlusal aspect From gingival aspect Relation to height of contour Arises above HOC Arises below HOC Minor connector RIGID FLEXIBLE ;called the approach arm Removal from undercut Easy to remove Easy to seat but difficult to remove Retention PULL TYPE of retention PUSH TYPE of retention Tooth contact CONTINUOUS; good bracing effect 3 – POINT; less bracing effect Aesthetics Less aesthetics; more metal exposure Better aesthetics Debris accumulation Reduced due to better adaptation to tooth structure Increased due to space between minor connector and abutment surface Repair easy difficult Width of occlusal table Increases; also load No such problem Decalcification Increased; due to more tooth coverage None; due to limited 3 – point contact Tilted abutments and soft tissue undercuts can be used Cannot be used

97. CLASPS UTILIZING PROXIMAL UNDERCUTS (Osborne & Lammie ) 2 Clasps: MESIO-DISTAL CLASP Canines (less buccal undercut / esthetics) Embraces on mesial ,palatal &distal sides Cast in gold Adv : esthetic, good retention Not used in distal – extention cases

98. 2. DE VAN CLASP - Uses near proximal undercut - comprises of : i ) small head on tooth below survey line ii) reciprocated by lingual strut,ends in a lingually placed occlusal rest iii) occlusal rest –near proximal occlusal surface Advantages: 1. esthetic 2. More retention 3. Unfavourable survey lines Disadvantage: 1. Less bracing effect

99. CLASP WITH SPLINTING ACTION (Osborne & Lammie ) 1. EXTENDED ARM CLASP - Arms covers 2 teeth - it remains above survey line of first tooth &crosses into undercut of adjacent tooth - distributes lateral load over 2 teeth - Au: 2 premolars Co-Cr : longer arm

100. 2. COMPOUND CLASP All Classes circumferential clasp arising from same body situated between embrasure of 2 teeth

101. RECENT ADVANCES: ESTHETIC CLASPS

102. 1) Masking the direct retainer with composite and acrylic has been done by 1) Macro mechanical retention Retentive beads and meshwork have been used to retain facing of either acrylic or composite resin. 2) Micromechanical retention Air borne particle abrasion used 3) Silica coating

103. 2) Metal Free Clasps Optiflex invisible clasp partials Flexite plus cast thermoplastic Proflex clear wire clasps Smile-Rite partials Made from acetyl clasps. Examples of these clasps are:

104. 3) The Twin-Flex clasp • This consists of a wire clasp soldered into a channel that is cast in the major connector. • This clasp is flexible, it does not generate as much as torque when the distal extension is depressed. • The ability to adjust this clasp and its conventional path of insertion provides an excellent design option for retention to an adjacent edentulous segment

105. Disadvantages • There is extra thickness of major connector over the wire clasp, • an extra laboratory step incurs extra cost, • Difficulty in repairing the clasp if breakage occurs

106. 4) Mesial groove reciprocation (MGR) clasp Uses the distofacial undercut of the maxillary canines for retention & esthetic advantage Uses a mesiolingual reciprocating groove and rest seat to achieve bracing against distal movement.

107. Successful use of the system requires careful analysis of each component for the function that it provides and thoughtful execution of the system in the abutment preparation. Keep the prosthesis design as simple as possible… Make RPD more comfortable, more efficient and affordable to the patient. CONCLUSION

108. REFERENCES: Clinical removable prosthodontics:- STEWART’S 3 rd edition Mc cracken removable partial denture prosthodontics – 12 th edition. “A color atlas of RPD”, Quintessence Publications Ltd., Russell J Stratton Krol A.J. “Clasp design for extension base RPD”. J. Prosthet . Dent. , 1973; 29 : 408-415. M.M.DeVan JPD 1955;5,208-14 Joseph Grasso JPD 1980,43;618-21 Davenport et al, Clasp Designs , BRITISH DENTAL JOURNAL, VOLUME 190, NO. 2, JANUARY 27 2001