4. Direct Retainers and Indirect Retainers.pptx

Prosthodontics

1. DIRECT RETAINERS IN REMOVABLE PARTIAL DENTURES DEPARTMENT OF PROSTHODONTICS AND CROWN & BRIDGE

2. Removable partial dentures (RPD) for partially edentulous individuals is for restoration of oral function, preservation of remaining oral structures, and prevention of oral disease to the greatest extent possible. However, after treatment with the RPD, functional forces transmitted from the artificial teeth to the abutment teeth sometimes exceed the threshold of the physiological movements of the abutment teeth, then it is possible to speculate an increase in tooth mobility. INTRODUCTION

3. 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.

4. That component of a removable partial denture used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment. (GPT 9 TH EDITION) A direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth to resist displacement of the prosthesis away from the basal seat tissue. (Mc Cracken’s 11 th EDITION)

5. CLASSIFICATION OF DIRECT RETAINERS

6. Also called internal attachments or precision attachments. It is an interlocking device, one component of which is integrated into a removable prosthesis to stabilize and/or retain it. This principle was first formulated by Dr.Herman E.S. Chayes in1906.   INTRA-CORONAL RETAIERS Chayes attachment Crismaniattachments . McCollum unit Ancra attachment. T- Geschiebe 123 Dalla Bone ( Dalbo ) Gerber Lekia

7. Advantages:- Elimination of visible rest and support component Better vertical support Horizontal stabilization Disadvantages:- Require prepared abutments and castings They eventually wear with progressive loss of frictional resistance to denture removal. Difficult to repair and replace Complicated clinical and laboratory procedures Difficult to place completely within the circumfernce of tooth Expensive

8. Prothero's Concept Proposed “cone theory” of clinical crown in 1916. Provided 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.

9. The basic parts of clasp assembly include the following

10. 1. 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 by ensuring that the retentive terminals of the clasp remain fixed in the desired or planned depth of undercut .

11. Body of the clasp It is the part of the clasp that connects the rest and shoulder of the clasp to the minor connector must be rigid. above the height of contour.

12. Shoulder It is the part of the clasp that connects the body to the clasp terminals. The shoulder must lie above the height of contour and provide some stabilization against horizontal displacement of the prosthesis.

13. Reciprocal arm A rigid clasp arm placed above the height of contour on the side of the tooth, opposing the retentive clasp arm.

14. 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 . 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.

15. 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.

16. Basic Principles of Clasp Design Retention Stability Encirclement Support Reciprocation Passivity

17. Retention “Retention is the inherent quality of the clasp assembly that resists forces acting to dislodge components away from the tooth structure.” No single component of a clasp assembly is solely responsible for prosthesis retention . Rather, it is effective design and accurate construction that make the removable partial denture retentive. The amount of retention designed into a removable partial denture should always be the minimum necessary to resist reasonable dislodging forces.

18. Factors affecting retention can be divided into:-

19. Prosthesis factors: Length of clasp arm- Longer the clasp arm the more flexible it will be. The clasp arm should taper from the point of origin to provide its flexibility. Circumferential clasps more retentive than bar clasp for a given clasp length.

20. Diameter of the clasp: The greater the diameter of a clasp arm the less flexible it will be. If its taper is absolutely uniform ,the avg diameter will be at a point midway between its origin and its terminal end. will exist .

21. Cross-sectional form: Flexibility may exist in any form, but is limited to only one direction in the case of the half-round form Clasp arm should only flex away from tooth so half round is used. The only universally flexible form is the round form. Round shaped clasp arm used only in distal extension denture bases so that it can flex in all directions during functional movement and minimize stresses.

22. Material used for construction:

23. Support “Support is the quality of a clasp assembly that resists displacement of a prosthesis in an apical direction.” Provided by occlusal rest. 1. A properly prepared rest seat and corresponding rest serve to resist displacement of the prosthesis toward the supporting teeth and soft tissues, thereby ensuring that the clasp assembly maintains its intended relation to the abutment, and 2. Transmit functional forces parallel to the long axes of the abutments.

24. Stability “Stability is the quality of a clasp assembly that resists displacement of a prosthesis in a horizontal direction.” It is provided by :- 1. Reciprocal element. 2. The shoulder(s) of a cast circumferential retentive clasp. 3. Vertically oriented minor connectors. It helps the denture be steady constant firm and resist displacement due to function stresses and also prevent change in position of the denture.

25. 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.” The reciprocal element may be a 1. Retentive arm of clasp 2. Lingual plating, 3. Combination of mesial and distal minor connectors. damage to the tooth. As the retentive arm passes over the height of contour it flexes creating lateral forces

26. Encirclement “Encirclement is the characteristic of a clasp assembly that prevents movement of an abutment away from the associated clasp assembly”.

27. 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.”

28. First proposed by Dr N B Nebbit . Later modified by Dr Polk E Aker. Simple and easy to construct . Large amount of tooth surface that is covered by the clasp assembly leads to enamel decalcification. AKER’S CLASP

29. Design rules 1. The clasp should arise from the main body of the clasp assembly above the height of contour. The retentive arm should extend cervically and circumferentially in a gently arcing manner. 2. All the components of the C clasp should be present above the height of contour except the retentive tip. 3. The retentive terminus should always be directed towards the occlusal surface never towards the gingiva. 4. It should always terminate at the mesial or distal line angle never at midfacial or midlingual surface.

30. 5. The retentive arm should be positioned as far apically on the abutment as is practical.(not impinging the gingiva) 6. Special considerations in case of distal extension case:-

31. Simple Circlet Clasp It is one of the most commonly used clasps as it has retentive and stabilizing ability. The basic design consist of 1. buccal retentive arm and 2. a lingual reciprocal arm originating from a common body. 3. Occlusal rest. With retentive arm projecting away from the edentulous space.

32. Advantages:- 1. Fulfils the design requirements of support, stability, reciprocation, encirclement, and passivity. 2. Its uncomplicated design features make it easy to construct and relatively simple to repair. Disadvantages:- 1. If used in distal extension base due to fulcrum rotation of the clasp it can cause damage to teeth. 2. Can increase the circumference of teeth and lead to food accumulation and decalcification.

33. Reverse circlet clasp It consists of a mesial occlusal rest, a horizontal reciprocal arm, and a retentive arm engaging the distobuccal undercut adjacent to the edentulous area. Used when undercut is located at the facial distoangle adjacent to an edentulous space. Advantages:- Poor aesthetics' if used in premolars and cuspids. We a k clasp if sufficient preparation is not done. Disadvantages:- Decreases the harmful stresses to teeth.

34. Multiple circlet clasp A multiple circlet clasp design involves two simple circlet clasps joined at the terminal aspects of their reciprocal elements. Used in periodontally weekend teeth to splint them

35. Embrasure clasp Also known as the Bonwill clasp. fabrication of unmodified Class II or Class III partial denture situation when there are no edentulous spaces available on the opposite side of the arch to aid in clasping.

36. Design Double occlusal rests, two retentive clasp arms and the two reciprocal clasp arms either bilaterally or diagonally opposed . Prevent interproximal wedging by the prosthesis, which could cause separation of the abutment tooth and result in food impaction and clasp displacement. In addition to providing support, occlusal rests also serve to shunt food away from the contact area. Disadvantages:- Improper clearance can give rise thin section of clasp leading to breakage.

37. Ring clasp Circumferential clasp encircles nearly all of the tooth from its point of origin. It is usually used when a proximal undercut cannot be approached by any other means. In case of tilted molars . A support strut is provided on the non retentive arm. Advantages:- Provide adequate encirclement. excellent retention with adequate flexibility due to increased length of clasp arm Disadvantages:- Decalcification of teeth. Poor structure of clasp. Increased occlusal table.

38. Hairpin clasp or reverse action or fish hook clasp 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.

39. This clasp is indicated when the 1. soft 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. Consideration:- Occlusal arm shouldn’t interfere within the occlusion there should be Space between occlusal and apical arm. Sufficient clinical crown height.

40. Onlay clasp Indicated when the occlusal surface of the abutment lies noticeably apical to the occlusal plane. It establishes the occlusal plane. Occlusal surface should be restored with gold acrylic inserts Indicated in caries free individual.

41. 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. More aesthetic than c clasp. Flexibility of clasp from length and taper. Ex :- y clasp, t clasp, I clasp.

42. Design Rules 1. The approach arm of an infrabulge clasp must not impinge on the soft tissues adjacent to the abutment. 2. The approach arm should cross perpendicular to the free gingival margin. It shouldn’t impinge the underlying gingiva . 3. Shouldn’t be used in area of tissue undercut. 4. Uniform length and adequate taper should be given for sufficient flexibility. 5. The clasp terminus tip should be placed as apical as possible on the abutment teeth.

43. T clasp Name is derived from the shape of the retentive terminal. Used in class 1 and class 2 situation. The retentive terminal consist of horizontal two projection the one on the distal side engages the undercut and the one on the mesial side is above the height of contour. T clasp is contraindicated when the height of contour is at the occlusal one thirds.

44. In modified t clasp the non retentive arm is absent. Y clasp is similar to t clasp with the approach arm ending cervical to the retentive arm. I clasp lack the horizontal retentive arms but only a horizontal retentive tip. Only 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.

45. RPI concept RPI stands for Rest Proximal plate I bar. Introduced by Kratochvil in 1963 it consisted of three different parts connected to the metal framework. Mesial occlusal rest, a distal guide plan, and an I bar retainer. The guide plan contacts the full length of the of the proximal surface of the tooth.

46. The guide plan contacts the full length of the of the proximal surface of the tooth. Physiologic relief was required to prevent impingement of gingival tissues during function. Since the proximal plate covers a greater surface area of the tooth, the functional forces are directed in the horizontal direction, thus the tooth is located more than the edentulous ridge.

47. BASIC PRINCIPLES OF RPI CONCEPT The mesiobuccal rest with the minor connector is placed into the mesiolingual embrasure, but not contacting the adjacent tooth. A distal guiding plane, extending from the marginal ridge to the junction of the middle and gingival thirds of the abutment tooth, is prepared to receive a proximal plate The proximal plate in conjunction with the mesial occlusal rest and minor connector provides the stabilizing and reciprocal aspects of the clasp assembly. The I-bar contributes to the retentive aspect and should be located in the gingival third of the buccal or labial surface of the abutment in 0.01 inch undercut.

48. Design modification I : Here, the proximal plate is designed to extend from the marginal ridge to the junction between the middle and cervical third of the tooth. ( Remember,in an l-bar retainer the proximal plate extends about 2mm across the tooth tissue junction). Proximal Plate Modification

49. Design modification II: Here, the proximal plate is designed to extend along the entire length of th e proximal surface of the abutment with a minimum tissue relief. A relief is provided near the gingival margin at tooth tissue junction. This allows the proximal plate to disengage into proximal under cut during occlusal loading.

50. Design modification III: Here, the proximal plate is designed to contact just about 1 mm 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.

51. RPA clasps 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.

52. References Clinical removable prosthodontics:- STEWART’S 3rd edition Mc cracken removable partial denture prosthodontics – 1 1 th edition. Hansen CA, Everson GW. An esthetic removable partial denture retainer for the maxillary canine. J Prosthet Dent 1986;56:199-203. Gupta S, Mehta A. Esthetic removable partial denture – a step towards achieving realism. JIPS 2002;1:25-8. Krol A.J. “Clasp design for extension base RPD”. J. Prosthet . Dent., 1973; 29 : 408-415.