DIRECT RETAINERS IN REMOVABLE PARTIAL DENTURE

Direct retainer: That component of a partial removable dental
prosthesis used to retain and prevent dislodgment, consisting of a
clasp assembly or precision attachment (GPT9).

•Direct retention: Retention obtained in a partial removable dental
prosthesis by the use of clasps or attachments that resist removal from
the abutment teeth.

1. DIRECT RETAINERS DR SYLVANA AM PROFESSOR DEPARTMENT OF PROSTHODONTICS SREE ANJANEYA INSTITUTE OF DENTAL SCIENCES

2. INTRODUCTION De vans Law : Aim of prosthodontics is preservation of what remains rather than meticulous replacement of what is missing” – MM Devan . MM Devan’s dictum has been the guiding principle of dentistry.

3. COMPONENTS n. A, Major connector. B, Rests. C, Direct retainer. D, Minor connector. E, Guide plane .F , Indirect retainer.,

4. DEFINITION Direct retainer: That component of a partial removable dental prosthesis used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment (GPT9). Direct retention: Retention obtained in a partial removable dental prosthesis by the use of clasps or attachments that resist removal from the abutment teeth.

5. Clasp is the direct retainer

6. PARTS OF A CLASP ASSEMBLY

7. A clasp assembly should consist of four component parts. One or more minor connectors :from which the clasp components originate. Principal rest should be designed to direct stress along the long axis of the tooth. Retentive arm :should engage a tooth undercut. Non retentive arm :should be present on the opposite side of the tooth for stabilization and reciprocation against horizontal movement of the prosthesis (rigidity of this clasp arm is essential to its purpose).

8. ROLE OF A CLASP Forces acting to displace the prosthesis from the tissue can consist of 1. gravity acting against a maxillary prosthesis, 2. action of adherent foods acting to displace the prosthesis or functional forces to unseat the prosthesis. A direct retainer is any unit of a removable dental prosthesis to resist displacement of the prosthesis away from basal seat tissue.

9. Basic Principles of Clasp Design PRINCIPLE OF ENCIRCLEMENT : It means that more than 180 degrees in the greatest circumference of the tooth, passing from diverging axial surfaces to converging axial surfaces, must be engaged by the clasp assembly.

10. The occlusal rest must be designed to prevent movement of the clasp arms toward the cervical. Each retentive terminal should be opposed by a reciprocal component. Clasp retainers on abutment teeth ,must act as stress-breakers - flexible clasp arm The amount of retention should always be the minimum necessary to resist reasonable dislodging forces

11. Types Two basic types of direct retainers are available: the I ntra coronal retainer and the Extra coronal retainer.

12. Clasp arms : two types T wo basic categories : One is the Circumferential clasp arm, which approaches the retentive undercut from an occlusal direction. Bar clasp arm, which approaches the retentive undercut from a cervical direction. A clasp assembly may comprise various retentive arms (i.e., a cast circumferential, a bar clasp arm, or a wrought-wire retainer)

13. TYPES OF CLASPS 1. Circumferential clasp Definition: A retainer that encircles a tooth by more than 180°, including opposite angles, and which generally contacts the tooth throughout the extent of the clasp, with at least one terminal located in an undercut.

14. 1. Circumferential clasp

15. I. Cast circumferential clasp • It is also called Akers’ clasp. • All components of the clasp assembly are made of cast alloy.

16. Types of cast circumferential clasps Simple circlet clasp • Most versatile and widely used . • Clasp of choice for tooth-supported (class III) removable partial dentures . • Contraindicated for distal extension partial dentures as it can only engage mesio buccal undercut

17. 2. Reverse circlet clasp Indicated for distal extension situations when bar clasp is contraindicated . In distal extensions it will move further into undercut when a force is applied on the denture base just like bar clasp and when dislodging forces (sticky food) are applied, it engages the undercut and denture is retained.

18. 3. Multiple circlet clasp • This is a combination of two opposing simple circlet clasps joined at terminal end of reciprocal arms. • When principal abutment has lost some of its periodontal support, this clasp is used to share retention responsibilities among other teeth. • It is used in periodontally compromised abutment teeth in distal extension situations.

19. 4. Embrasure clasp or modified crib clasp • Two simple circlet clasps joined at the body. • Used on the side of the arch where there is no edentulous space. • Indications : Kennedy’s class II and class III without any modifications . • It crosses both marginal ridges and engages undercut on opposing line angles . The occlusal rest must be prepared on both the adjacent teeth and buccal inclination of cusp is reduced to get the required strength and rigidity for the clasp.

20. 5. Ring clasp • Indicated on tipped molars . • It starts on the opposite side of the undercut adjacent to edentulous space and engages the undercut by encircling the entire tooth almost from its origin. Bracing arm will have to cross a soft tissue undercut. • Because of great length of the clasp, it is usually supported by an auxiliary bracing arm from the minor connector of denture base to the centre of reciprocal arm : for proper reciprocation and stabilization.

21. 6. Back action clasp • It is a modification of ring clasp. • Minor connector is attached to the clasp arm on the lingual surface unlike other clasps where the minor connectors are attached to the occlusal rest. • Indicated in Kennedy’s class I and class II where only a mesio buccal undercut is present.

22. 7. Fish hook or hairpin clasp • This ‘C’ clasp is essentially a simple circlet clasp where the retentive arm , after crossing the facial surface, loops back in a hairpin turn to engage the proximal undercut below its point of origin. The upper retentive arm is rigid, while the lower part is flexible and tapered because it engages the undercut. • The abutment should have sufficient crown height to accommodate the two arms. • It is indicated in distal extension base abutments

23. 8. Onlay clasp • It is an extension of occlusal rest with buccal and lingual clasp arms. • Indicated : abutments is below occlusal plane and the onlay will restore the same. • As it covers a large amount of tooth, it may lead to enamel breakdown . Hence, used only in caries free mouth alloy of choice is gold alloy (margins can be burnished). • Metal occlusal surfaces can be lined with tooth coloured acrylic resin to reduce the wear of opposing teeth.

24. II. Combination clasp

25. Combination clasp A circumferential retainer for a removable dental prosthesis that has a cast reciprocal arm and a wrought wire retentive clasp (GPT9). The wrought wire can flex in all three planes and has greater flexibility than a cast arm. It is indicated on an abutment adjacent to a distal extension base where a mesio buccal undercut is present . The use of flexible wrought wire clasp will help dissipate this functional stress better . More aesthetically acceptable as it can be placed in gingival third of facial surface.

26. 2.Bar clasps

27. 2. Bar clasp Definition : A clasp retainer whose body extends from a major connector or denture base, passing adjacent to the soft tissues and approaching the tooth from a gingivo-occlusal direction ( GPT9). Arises from denture base minor connector and approaches the undercut from a gingival direction resulting in ‘push type’ retention. Indications -1 . Small undercut (0.01 inch) existing in cervical third of abutment 2.Most tooth-supported partial dentures including modification spaces where aesthetics is a concern.

28. Types of Bar clasps 1.T-clasp 2. Modified T- clasp 3. Y- clasp 4. I- Clasp

29. 1.T-clasp Approach arm extends till the height of contour at which point the retentive terminal leaves approach arm and engages an undercut. The other terminal is positioned above the height of the contour

30. Modified T-clasp • Essentially a T-clasp without the nonretentive (usually mesial) finger of the T-terminal. • It has better aesthetics and is used on canines and premolars. • It does not possess 180° encirclement.

31. Y-clasp It is used when the height of contour on the buccal surface of abutment is high near the mesial and distal line angles, but low at the centre

32. I-clasp • Used on disto buccal surface of maxillary canines for aesthetics. • Compromise on encirclement and horizontal stabilization. • This is also used in the RPI concept.

33. CLASSFICATION

34. Surveying: The procedure of locating & delineating the contour and position of the abutment teeth and associated structures before designing a partial denture.

35. PROTHEROS CONCEPT Proposed ‘cone theory’ of clinical crown in 1916 that provides conceptual basis of mechanical retention. He described that crown shape of posterior teeth to be like two cones sharing a common base . The line formed at the junction of this represents the greatest diameter of the tooth: Height of contour . The part of the clasp that ends on the cervical cone would resist movement in an occlusal direction as it would be forced to deform to come out of the undercut .

36. Types of clasps : Infrabulge & Suprabulge Suprabulge : It is the portion of an abutment that converges towards the occlusal or incisal surface and is considered as suprbulge aspect of the abutment : Circumferential clasps 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 : Vertical projection or bar type

37. Infrabulge clasps retentive arms are pushed over the height of contour Suprabulge clasps –retentive arms are pulled over the height of contour Push type is better for retention over pull type.

38. Requirements of Clasp Assembly Retention support Stability Reciprocation encirclement Passivity

39. RETENTION Clasp retention is based on resistance to deformation of the metal . For a clasp to be retentive, it must be placed in an undercut area of the tooth, where it is forced to deform upon application of a vertical dislodging force Clasp flexibility is the product of clasp length (measured from its point of origin to its terminal end), clasp relative diameter (regardless of its cross-sectional form), clasp cross-sectional form or shape (whether it is round, half-round, or some other form), and the material used in making the clasp.

40. Retention is provided primarily by the flexible portion of the clasp assembly. Retentive terminals are ideally located in measured undercuts in the gingival third of abutment crowns . When force acts to dislodge the restoration in an occlusal direction, the retentive arm is forced to deform as it passes from the undercut location over the height of contour.

41. SUPPORT Support is the quality of 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

42. PASSIVITY 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 forces to the abutment.

43. 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. This component is known as a reciprocal element. The reciprocal element may be cast clasp ,lingual plating or a combination of mesial and distal minor connectors.

44. Types of clasp assemblies: RPI, RPA and Bar Clasp Mesial rest concept clasps have been proposed to accomplish movement accommodation by changing the fulcrum location. This concept includes the RPI and RPA [rest, proximal plate, Akers] clasp

45. Mesio occlusal rest The abutment tooth contains the mesiocclusal rest with the minor connector placed into the mesiolingual embrasure but not contacting the adjacent tooth.

46. 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.

47. I - bar The approach arm of the I bar extends from the framework so as to remain at least 3mm from the gingival margin and then crosses gingival margin at right angles Approx 2mm of the bar contacts the tooth surface, usually at gingival 1/3 rd of the tooth

48. Contraindications of RPI Insufficient depth of vestibule to permit the approach arm of the I bar to be located at least 3mm from gingival margin. A tooth which has a severe lingual tilt and no labial or buccal undercut Tissue undercut so severe that 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 buccaly A tooth which has only a distobuccal retention undercut and doenot require a restoration A mouth with a high floor in which lingual plate is used on the lingual plate is used on the lingual surface of abutment tooth.

49. RPA Clasps ( rest-proximal plate- Akers clasp) RPA : Eliason 1983 It consists of 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.

50. ADVANTAGES The circumferential claps is easier for removal Easy to fabricate This concept can be used in several clinical conditions

51. Esthetic solutions in the smile zone

52. MGR clasp design MGR clasp design: Retention is provided by 19 gauges round I-bar and retentive dimple located at distobuccally on the tooth. Reciprocation is provided by mesial groove or rest and distal proximal plate

53. L-clasp: The design consists of the clasp arm extending from lingual minor connector with an inde p endent reciprocal rest. The L-clasp has greater ri g idity than the C-clasp.

54. C- clasp: It consists of a modified back-action clasp with rest incorporated in clasp

55. Equipoise RPD system Proposed by J. J. Goodman Rests are placed away from edentulous span The equipoise clasp is a lingual backaction clasp that is fully reciprocated and extremely esthetic with no facial clasp displays. When this clasp is used on a premolar adjacent to a distal extension area, a prominent occlusal rest seat should be placed on the side of the abutment tooth opposite the edentulous area.

56. METAL-FREE CLASPS Optiflex invisible clasp partials: With the OptiFlex acetyl resin clasps, metal-free, lightweight partial dentures that provide natural esthetics and a comfortable fit can be designed. 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

57. Proflex clear wire clasps: Clear wire is an excellent new way to fabricate clear, strong, flexible clasps .

58. Alternative framework materials PEEK: Poly Ether Ether Ketone Nylon Acetal resins

59. Thank You