Understanding Direct Retainers in Removable Partial Dentures

1. Direct Retainers

2. DEFINITION That component of a removable partial denture used to retain and prevent dislodgement consisting of a clasp assembly or precision attachment.

3. Types of direct retainers Intra coronal retainers Extra coronal retainers

4. Intra coronal retainers (internal attachment) Developed by Dr Herman E. S. Chayes Consists of two units Receptacle (female part) Insert (male part)

5. Extra coronal retainers (clasps) The extra coronal direct retainers operates on the principle of the resistance of metal to deformation. There are two basic category of clasps 1.circumferential or Akers clasps. 2.vertical projection or bar or roach clasps

6. Difference between circumferential clasp and bar clasp

7. Components of the clasp assembly. • Rest • Body • Shoulder • Reciprocal arm • Retentive clasp arm • Retentive terminal • Minor connector • Approach arm

8. Rest : Part of the clasp that lies on the occlusal lingual or incisal edge or surfaces of a tooth . Function Resist the tissue ward movement of the clasp by ensuring that the retentive terminal of the clasp remains fixed in the desired or the planned dept of undercut.

9. Body: Part of the clasp that connects the rest and the shoulder of the clasp to the minor connector . Shoulder : Part of the clasp that connects the body to the clasp terminal . Reciprocal arm: A rigid clasp arm placed above the height of contour on the side of the tooth opposing the retentive clasp arm

10. 1.Rest 2.Body 3.reciprocal arm

11. Function : • Resists the tipping force generated by the retentive terminal as it passes over the height of contour during insertion and removal. • Helps to stabilize the RPD against lateral forces. • Contributes to the vertical support of the prosthesis. • Position: • It must be designed to contact the tooth before the retentive clasp does ,and to remain in contact while the retentive terminal passes the height of contour.

12. Retentive clasp arm: Part of the clasp comprising the shoulder which is not flexible and is located above the height of contour ,and the retentive terminal. Retentive terminal : The distal third of the retentive terminal clasp arm .It is the only component of the RPD to lie on the tooth surface cervical to the height of contour. It is this position of the flexible terminal in the undercut that provides the direct retention.

13. Minor connector : Part of the clasp that joins the body of the clasp to the remainder of the frame work. Approach arm: It is the component of the bar or the vertical projection clasp. The approach arm is a minor connector that projects from the framework, runs along the mucosa ,and turns to cross the gingival margin of the abutment tooth. the body and the retentive terminal are attached to it.

14. . 4.Retentive clasp arm 5.Retentive terminal 6.Minor connector

15. Requirements of a clasp design. All the clasps are so designed that they satisfy the following six basis requirements . 1.retention 2.support 3.stability 4. reciprocation 5.encirclement 6.passivity

16. Cast circumferential clasps • Advantages • Easier to design. • Excellent support ,bracing and retentive qualities .so it is the logic clasp of choice of tooth supported partial dentures. • Easier to repair and few problems of food retention. • Different varities are available which can be used in practically all cases .

17. Disadvantages • More tooth surface is covered . • It changes the morphology of the abutment tooth. • If positioned too high on the tooth surface they increase the width of the food table , which in turn causes greater occlusal forces to be exerted on the tooth. • Not possible to truly adjust the clasp with the pliers . • It can be only adjusted in a plane perpendicular to its flat inner surface .

18. Rule of use • The retentive clasp arm should originate above the height of contour ,and its terminal third should be positioned below the height of contour. • The retentive terminal should always point towards the occlusal surface ,never towards the gingival. • The retentive terminal should always terminate at the mesial or distal line angle and never at the centre of the abutment tooth. • The clasp arm should be kept as low on the tooth as possible with out violating its relation with the height of contour.

19. Types of cast circumferential clasps • Simple circlet clasp • Reverse , or reverse approach circlet clasp • Multiple circlet clasp • Embrasure clasp or modified crib clasp • Ring clasp • C, fish hook or hair pin clasp • Onlay clasp • Combination clasp

20. Simple circlet clasp • Most versatile and widely used. • Clasp of choice on tooth supported removable partial dentures . • Approaches the undercut on the abutment tooth from the edentulous area and engages the undercut remote to the edentulous area.

21. Advantages • Good support, stability ,reciprocation , encirclement and passivity. • Easy to construct . • Simple to repair . • Disadvantages • Similar to disadvantages of circumferential clasp in general. • Not esthetic to be used on the anterior .

22. Simple circlet clasp

23. Reverse or reverse approach circlet clasp Used when the retentive undercut is located on the surface of the abutment tooth adjustment to the edentulous space .

24. Disadvantages • Requires lot of occlusal clearance . • Esthetically poor. • Not a clasp of choice for the premolars. • If occlusal rests are not properly placed on the abutment tooth wedging of food may occur.

25. Multiple circlet clasp It is two opposing simple circlet clasp jointed at the terminal end of the two reciprocal arm.

26. Use To share retention responsibility among several abutment teeth on one side of the arch when a principal abutment teeth has lost some of its periodontal support Advantages Similar to simple circlet clasp

27. Embrasure clasp or modified crib clasp These are two simple circlet clasps joined at the body.

28. Use • Used on the side of the arch where there is no edentulous space. • Disadvantages • Breakage • It is not the clasp of choice

29. Ring clasp Indicated on tipped molars.

30. Disadvantages • Increase the load on the tooth. • Difficult to adjust and repair . • Contra indication • On a mandibular molar where the buccinator attachment is too close to the tooth . • When the bracing arm has to cross the soft tissue.

31. C, fish hook or hairpin clasp • Indication • Used when retentive clasp must engage an undercut adjacent to the occlusal rest or edentulous space and a soft tissue undercut precludes the use of the bar clasp. • Also indicated when the reverse circlet clasp cannot be used due to lack of occlusal space .

32. Disadvantages • Tends to accumulate food debris . • Not indicated on young individuals who are prone to caries. • Esthetically unacceptable especially on premolars

33. Onlay clasp It is an extended occlusal rest with buccal and lingual clasp arm.

34. Indication • When the occlusal surface of the abutment teeth is below the occlusal plane , usually as a result of the teeth being tipped or rotated. • It is used to restore the normal occlusal plane . • Contraindicated • Caries prone mouth

35. Combination clasp Consists of a wrought wire retentive clasp and a clasp reciprocal clasp arm. Indication Used on an abutment tooth adjacent to a distal extension space when the usable undercut on the tooth is on the mesiobuccal surface .

36. Advantages • Esthetically acceptable • The round wrought wire clasp can move in all spatial planes , which allows it to dissipate torquing force excerted on the abutment tooth and can be adjusted in all planes . • Can be used in caries prone mouth becauseit has minimum tooth contact .

37. Disadvantage • Requires an extra step in lab procedure . • Prone to breakage or damage than cast clasp. • Does not provide bracing or stabilization because it is flexible .

38. Bar or vertical projection clasp. • Advantages • The push type of retention is more effective than the pull type of retention provided by the circumferential clasp. • It is easy to inset and difficult to remove . • The flexibility of the clasp can be maintained by varying the length and the taper of the clasp. • .

39. Disadvantages • Greater tendency to collect food debris . • Does not have much bracing and stability due to increase flexibility of the retentive arm. • Additional stabilizing unit should be used

40. Type of bar clasps • T –clasp • Modified T-clasp • Y-clasp • I- clasp and I- bar

41. T clasp

42. Modified t clasp

43. Y clasp

44. I bar

45. Thank you………