Indirect retainer in removable partial denture

Indirect retainers in removable partial denture

1. Indirect retainers ANN SALES Postgraduate Student

2. INTRODUCTION Removable partial denture is an appliance that allows controlled movement in function, under load to avoid impingement of tissues and injury to abutments. Dentists have long advocated the use of indirect retainers for distal-extension removable partial dentures. The function of the indirect retainer is to prevent the distal-extension base from moving away from its seat because of cheek and tongue forces, sticky foods, or gravity on a maxillary prosthesis.

3. DEFINITIONS EXTENSION-BASE REMOVABLE PARTIAL DENTURE (GPT 9) - It is a type of removable partial denture that is supported and retained by natural teeth anterior to the denture base and in which a portion of the functional force vector of the base is carried by the residual ridge. INDIRECT RETAINER (GPT 9) - The component of a removable partial denture that assists the direct retainer(s) in preventing displacement of the distal-extension denture base by functioning through lever action on the opposite side of the fulcrum line when the denture base attempts to move away from the tissues in pure rotation around the fulcrum line.

4. BIOMECHANICS

5. Removable partial dentures by design are intended to be placed into and removed from the mouth. Because of this, they cannot be rigidly connected to the teeth or tissue. This makes them subject to movement in response to functional loads, such as those created by mastication. Consideration of the forces inherent in the oral cavity is critical. This includes the direction, duration, frequency, and magnitude of the force. In the final analysis, it is bone that provides the support for a removable prosthesis ( i . e., the alveolar bone by way of the periodontal ligament and the residual ridge bone through its soft tissue covering). It is important for clinicians to understand the possible movements in response to function and to be able to logically design the component parts of the removable partial denture to help control these movements.

6. The forces occurring with removable prosthesis function can be widely distributed and directed, and their effect minimized by appropriate design of the removable partial denture. An understanding of simple machines applied to the design of removable partial dentures helps to accomplish the objective of preservation of oral structures. The lever system represented by a distal extension removable partial denture could magnify the applied force of occlusion to the terminal abutments, which would be undesirable.

7. Concept of Levers In its simplest form, a lever is a rigid bar supported somewhere along its length. It may rest on the support or may be supported from above. The support point of the lever is called the fulcrum, and the lever can move around the fulcrum. The rotational movement of an extension base type of removable partial denture, when a force is placed on the denture base will be in relation to the three cranial planes because of differences in the support characteristics of the abutment teeth and the soft tissue covering the residual ridge. Even though the actual movement of the denture may be small, a lever force may be imposed on abutment teeth. This is especially detrimental when prosthesis maintenance is neglected. There are three types of levers: first, second, and third order levers. Each lever modifies the intensity of the force acting on the denture to a different degree.

8. First Order Levers In this lever the fulcrum is in the center, resistance is at one end and effort (force) is at the other end. These levers are more efficient and easily controlled. This type of lever can occur in patients with distal extension partial dentures. The direct retainer will be the fulcrum, effort end lies on the point where the denture takes up the occlusal load (area where the artificial teeth are located) the load is the region of the anterior end of the major connector. Using an additional rest (indirect retainer) to shift the fulcrum line prevents lever action in these dentures.

9. Second Order Levers In this lever, the fulcrum is at one end, effort is at the opposite end and resistance or load is the center. This type of lever action occurs in indirect retention of a removable partial denture. When a displacing force tends to lift a denture from one end (effort), the anterior most point of the major connector will act as the axis of rotation (fulcrum), the intermediate zone of the denture, which is lifted by the force will form the resistance (load) of the lever

10. Third Order Levers In this lever, the fulcrum is at one end, resistance is at the opposite end and effort is at the centre .

11. Forces Acting on the Denture The response of the prosthesis to various forces acting on it plays an important role in indirect retention. Movements of the denture can occur in three planes: Movement towards the tissue : this movement is prevented by the presence of rests. Rests are a part of the clasp assembly located on the occlusal surface of the teeth. The rests act as a vertical stop when the occlusal forces push the denture down (towards the tissue). Movement towards the teeth (away from the tissue) : These forces affect the retention of the denture. This movement is prevented by the activation of the direct retainer, which is passive in normal conditions. Movement in buccolingual direction (horizontal) : This movement affects the stability of the denture. The movement is prevented by the action of the stabilizing components of the denture. For example, major connector, denture flange and so on.

12. Rotational Movement Presuming that direct retainers are functioning to minimize vertical displacement, rotational movement will occur about some axis as the distal extension base or bases move either toward, away, or horizontally across the underlying tissue. Rotational movement is possible around various axes in the mouth : Horizontal axis Longitudinal axis Vertical axis

13. Horizontal Axis This axis may be through occlusal rests or any other rigid portion of a direct retainer assembly located occlusally or incisally to the height of contour of the primary abutments. This axis known as the fulcrum line, is the center of rotation as the distal extension base moves toward the supporting tissue when an occlusal load is applied. The axis of rotation may shift toward more anteriorly placed components, occlusal or incisal to the height of contour of the abutment, as the base moves away from the supporting tissue when vertical dislodging forces act on the partial denture. These dislodging forces result from the vertical pull of food between opposing tooth surfaces, the effect of moving border tissue, and the forces of gravity against a maxillary partial denture.

14. Presuming that the direct retainers are functional and that the supportive anterior components remain seated, rotation—rather than total displacement—should occur. Vertical tissue ward movement of the denture base is resisted by the tissue of the residual ridge in proportion to the supporting quality of that tissue, the accuracy of the fit of the denture base, and the total amount of occlusal load applied. Movement of the base in the opposite direction is resisted by the action of the retentive clasp arms on terminal abutments and the action of stabilizing minor connectors in conjunction with seated, vertical support elements of the framework anterior to the terminal abutments acting as indirect retainers. Indirect retainers should be placed as far as possible from the distal extension base, affording the best possible leverage against the lifting of the distal extension base.

15. Longitudinal Axis A second movement is rotation about a longitudinal axis as the distal extension base moves in a rotary direction about the residual ridge. This movement is resisted primarily by the rigidity of the major and minor connectors and their ability to resist torque. If the connectors are not rigid or if a stress-breaker exists between the distal extension base and the major connector, this rotation about a longitudinal axis either applies undue stress to the sides of the supporting ridge or causes horizontal shifting of the denture base.

16. A third movement is rotation about an imaginary vertical axis located near the center of the dental arch. This movement occurs under function because diagonal and horizontal occlusal forces are brought to bear on the partial denture. It is resisted by stabilizing components, such as reciprocal clasp arms and minor connectors that are in contact with vertical tooth surfaces. Such stabilizing components are essential to any partial denture design regardless of the manner of support and the type of direct retention employed. Stabilizing components on one side of the arch act to stabilize the partial denture against horizontal forces applied from the opposite side. It is obvious that rigid connectors must be used to make this effect possible. Vertical Axis

17. FULCRUM LINE FULCRUM LINE (GPT 9) - An imaginary line, connecting occlusal rests, around which a removable partial denture tends to rotate under masticatory forces; the determinants for the fulcrum line are usually the cross-arch occlusal rests on the most distally located abutments. There are two types of fulcrum lines: Retentive Fulcrum Line – It is an imaginary line, connecting the retentive points of clasp arms around which the denture tends to rotate when subjected to forces such as the pull of sticky foods. Once the denture begins to rotate the retentive arm becomes active and provides retention by engaging the undercut.

18. Stabilizing Fulcrum Line – It is an imaginary line, connecting occlusal rests, around which the denture tends to rotate under masticatory forces. Moving the stabilizing fulcrum line away from the point of action of occlusal forces provides indirect retention.

19. Secondary Fulcrum Line A second fulcrum line is produced by indirect retention. The prosthesis rotates about the primary fulcrum line if the denture base is forced tissue ward , and about the secondary fulcrum line if the denture base is lifted off the tissue .  However, if a second fulcrum line is not produced by indirect retention, the prosthesis rotates about the primary fulcrum line whether forces move the denture base toward or away from the tissue.

20. INDICATIONS OF INDIRECT RETAINER RPD’s with tooth support and clasp retention at both the ends – No primary retention and fulcrum lines present RPD’s with tooth support at both the ends and clasp retention at only one end – Primary retention line present but no primary fulcrum line RPD’s with tooth support and clasp retention at only one end – Both primary retention and fulcrum lines present THE LAST TWO TYPES SHOULD BE DESIGNED WITH INDIRECT RETAINERS .

21. CONTRAINDICATIONS OF INDIRECT RETAINERS Unfavorable alignment of the anterior tooth Lingually tipped tooth. Adjustability of the patient. Space: lack of horizontal overlap as well as vertical space between the lower incisors and the cingulum of the upper teeth Speech alteration

22. PRINCIPLE OF INDIRECT RETAINERS

23. Tooth supported partial dentures effectively use teeth to control movement away from the tissue. Tooth-tissue supported partial dentures do not have this capability since one end of the prosthesis is free to move away from the tissue. When the distal extension denture base is dislodged from its basal seat, it tends to rotate around the fulcrum lines. Theoretically, this movement away from the tissue can be resisted by the activation of the direct retainer, the stabilizing components of the clasp assembly, and the rigid components of the partial denture framework that are located on definite rests on the opposite side of the fulcrum line away from the distal extension base (indirect retainers). The indirect retainer components should be placed as far as possible from the distal extension base, which provides the best leverage advantage against dislodgment. An indirect retainer consists of one or more rests and the supporting minor connectors. The proximal plates, adjacent to the edentulous areas, also provide indirect retention. Although it is customary to identify the entire assembly as the indirect retainer, it should be remembered that the rest is actually the indirect retainer united to the major connector by a minor connector.

24. Distal extension bases generally act as a class I lever. This is the most hazardous system and causes damage to both the tooth and the supporting tissues But when an indirect retainer is incorporated the fulcrum is converted into a Class II lever Indirect retainer becomes the fulcrum, the retentive clasps the resistance and the power is the force that tends to move the denture base away from the tissue

25. An indirect retainer should be placed as far from the distal extension base as possible in a prepared rest seat on a tooth capable of supporting its function. Although the most effective location of an indirect retainer is commonly in the vicinity of an incisor tooth, that tooth may not be strong enough to support an indirect retainer and may have steep inclines that cannot be favorably altered to support a rest. In such a situation, the nearest canine tooth or the mesio -occlusal surface of the first premolar may be the best location for the indirect retention, despite the fact that it is not as far removed from the fulcrum line. Whenever possible, two indirect retainers closer to the fulcrum line are then used to compensate for the compromise in distance. INDIRECT RETENTION IS THE APPLICATION OF CLASS 2 LEVERS

26. Mesial Rest vs Distal Rest As the rest is moved anteriorly , it moves the point of rotation anteriorly as compared to the conventionally placed disto -occlusal rest. Mesial rest reduces the need for indirect retention. It also transmits forces to the edentulous ridge in a more nearly vertical direction. Reduces tilting potential of distal abutment. Retainer will have a tendency to disengage under load and engage retentive area under dislodging force which leads to equal load distribution .

27. Advantages of Mesial Rests Minor connector placed in mesial aspect tend to resist distal displacement of RPD. In distal rest, when distal extension move towards the tissue the distal rest pulls over the tooth thus applying stress on the abutment, whereas in mesial rest case , it allows the distal extension base to move towards the tissue and clasp to move more towards the retentive area of the tooth , thus applying less stress to the abutment tooth .

28. Retentive clasp engages the retentive area thus resisting movement. Use of a mesial rest instead of a distal rest on the terminal abutment tooth does not decrease indirect retention.

29. FACTORS INFLUENCING EFFECTIVENESS OF INDIRECT RETAINERS

30. The principal occlusal rests on the primary abutment teeth must be reasonably held in their seats by the retentive arms of the direct retainers. If rests are held in their seats, rotation about an axis should occur, which activates the indirect retainers. If total displacement of the rests occurs, there would be no rotation about the fulcrum, and the indirect retainers would not be activated. Distance from the fulcrum line . The following three areas must be considered: Length of the distal extension base Location of the fulcrum line How far beyond the fulcrum line the indirect retainer is placed Rigidity of the connectors supporting the indirect retainer. All connectors must be rigid if the indirect retainer is to function as intended. Effectiveness of the supporting tooth surface . The indirect retainer must be placed on a definite rest seat on which slippage or tooth movement will not occur. Tooth inclines and weak teeth should never be used to support indirect retainers

31. Mechanical Disadvantage of Denture Design The clasp is always nearer to the indirect retainer (fulcrum) than is the displacing force. The clasp is therefore working at a mechanical disadvantage relative to the displacing force. The RPD design should strive to reduce the mechanical advantage of the displacing force by placing the clasp axis as close as possible to the saddle and by placing the indirect retainers as far as possible from the saddle.

32. The mechanical advantage of a lever is expressed as the quotient of the power arm length divided by the resistance arm length. It can be reduced in the following ways: Resistance arm is made longer by moving the indirect retention mesially to prepared rest seat on cingulum of canine tooth. Resistance arm is lengthened again by moving retentive clasp tip from mesial toward distal of primary abutment tooth.

33. AUXILIARY FUNCTIONS OF INDIRECT RETAINERS

34. It tends to reduce anteroposterior-tilting leverages on the principal abutments. This is particularly important when an isolated tooth is being used as an abutment, a situation that should be avoided whenever possible. Ordinarily, proximal contact with the adjacent tooth prevents such tilting of an abutment as the base lifts away from the tissue. Contact of its minor connector with axial tooth surfaces aids in stabilization against horizontal movement of the denture. Such tooth surfaces, when made parallel to the path of placement, may also act as auxiliary guiding planes. Anterior teeth supporting indirect retainers are stabilized against lingual movement. In addition to effectively activating the direct retainer to prevent movement of a distal extension base away from the tissue, an indirect retainer may serve the following auxiliary functions. NOTE : These auxiliary functions derived from indirect retainers are important to consider, especially given the reported controversy as to the effectiveness of indirect retainers.

35. It may act as an auxiliary rest to support a portion of the major connector facilitating stress distribution. For example, a lingual bar may be supported against settling into the tissue by the indirect retainer acting as an auxiliary rest. One must be able to differentiate between an auxiliary rest placed for support for a major connector, one placed for indirect retention, and one serving a dual purpose. Some auxiliary rests are added solely to provide rest support to a segment of the denture and should not be confused with indirect retention. It may provide the first visual indications for the need to reline an extension base partial denture. Deficiencies in basal seat support are manifested by the dislodgment of indirect retainers from their prepared rest seats when the denture base is depressed and rotation occurs around the fulcrum. The tip of the retentive clasp act as fulcrum thus lifting the distal extension base. Indirect retention prevents the retentive clasp tips from becoming a fulcrum about which the prosthesis could rotate when forces move the denture base away from the tissue. Now when the distal extension base is lifted the auxillary rest will prevent this movement as it will enhance the retention property of the clasp.

36. LOCATION OF INDIRECT RETAINERS

37. Fulcrum lines found in various types of partially edentulous arches, around which denture may rotate when bases are subjected to forces directed toward or away from residual ridge. The most advantageous position of indirect retainer differs based on the type of arch : In Class I arch, fulcrum line passes through the most posterior abutments, provided some rigid component of framework is occlusal to abutment's heights of contour.

38. In Class II arch, fulcrum line is diagonal, passing through abutment on distal extension side and the most posterior abutment on opposite side.

39. If abutment tooth anterior to modification space lies far enough removed from fulcrum line, it may be used effectively for support of indirect retainer.

40. In Class IV arch, fulcrum line passes through two abutments adjacent to single edentulous space

41. In Class III arch with non-supporting anterior teeth, adjacent edentulous area is considered to be tissue-supported end, with diagonal fulcrum line passing through two principal abutments as in Class II arch.

42. In Class III arch with posterior tooth on right side, which has a poor prognosis and will eventually be lost, fulcrum line is considered the same as though posterior tooth were not present. Thus its future loss may not necessitate altering original design of the removable partial denture framework.

43. FORMS OF INDIRECT RETAINERS

44. CLASSIFICATION INDIRECT RETAINER ANTERIOR POSTERIOR PALATE TOOTH TOOTH PALATE CUMMER RESTS CONTINUOUS CLASP PALATAL ARM PALATAL BAR RESTS PALATAL BAR

45. Auxiliary Occlusal Rest Canine Rests Canine Extensions From Occlusal Rests Cingulum Bars (Continuous Bars) and Linguoplates Modification Areas Rugae Support Incisor Rests Lingual Rests Cummer Arms Embrasure Hook Denture Base Direct – Indirect Retention The indirect retainer may be in one of several forms. All are effective in proportion to their support and the distance from the fulcrum line.

46. Auxiliary Occlusal Rest The most commonly used indirect retainer is an auxiliary occlusal rest located on an occlusal surface and as far away from the distal extension base as possible. In a mandibular Class I arch, this location is usually on the mesial marginal ridge of the first premolar on each side of the arch. The ideal position for the indirect retainer perpendicular to the fulcrum line would be in the vicinity of the central incisors, which are too weak and have lingual surfaces that are too perpendicular to support a rest. Bilateral rests on the first premolars are quite effective, even though they are located closer to the axis of rotation. The same principle applies to any maxillary Class I partial denture when indirect retainers are used.

47. Bilateral rests on the mesial marginal ridge of the first premolars are generally used in preference to rests on incisor teeth. Not only are they effective without jeopardizing the weaker single-rooted teeth, but also interference with the tongue is far less when the minor connector can be placed in the embrasure between canine and premolar rather than anterior to the canine teeth. Indirect retainers for Class II partial dentures are usually placed on the marginal ridge of the first premolar tooth on the opposite side of the arch from the distal extension base. Bilateral rests are seldom indicated except when an auxiliary occlusal rest is needed for support of the major connector or when the prognosis of the distal abutment is poor and provision is being considered for later conversion to a Class I partial denture.

48. Canine Rests When the mesial marginal ridge of the first premolar is too close to the fulcrum line or when the teeth are overlapped so that the fulcrum line is not accessible, a rest may be used on the adjacent canine tooth. Such a rest may be made more effective by placing the minor connector in the embrasure anterior to the canine, either curving back onto a prepared lingual rest seat or extending to a mesio -incisal rest. Lingual or incisal rests, may be used.

49. Canine Extensions From Occlusal Rests Occasionally a finger extension from a premolar rest is placed on the prepared lingual slope of the adjacent canine tooth. Such an extension is used to effect indirect retention by increasing the distance of a resisting element from the fulcrum line. This method is particularly applicable when a first premolar must serve as a primary abutment. The distance anterior to the fulcrum line is only the distance between the mesio -occlusal rest and the anterior terminal of the finger extension. In this instance, although the extension rests on a prepared surface, it is used in conjunction with a terminal rest on the mesial marginal ridge of the premolar tooth.

50. Even when they are not used as indirect retainers, canine extensions, continuous bar retainers, and linguoplates should never be used without terminal rests because of the resultant forces effective when they are placed on inclined planes alone.

51. Cingulum Bars (Continuous Bars) and Linguoplates Technically, cingulum bars (continuous bars) and linguoplates are not indirect retainers because they rest on unprepared lingual inclines of anterior teeth. The indirect retainers are actually the terminal rests at either end in the form of auxiliary occlusal rests or canine rests. In Class I and Class II partial dentures, a cingulum bar or linguoplate may extend the effectiveness of the indirect retainer if it is used with a terminal rest at each end. In tooth-supported partial dentures, a cingulum bar or linguoplate is placed for other reasons but always with terminal rests. In Class I and Class II partial dentures especially, a continuous bar retainer or the superior border of the linguoplate should never be placed above the middle third of the teeth so that orthodontic movement is prevented during the rotation of a distal extension denture.

52. This guideline is not as important when the six anterior teeth are in nearly a straight line, but when the arch is narrow and tapering, a cingulum bar or linguoplate on anterior teeth extends well beyond the terminal rests—and orthodontic movement of those teeth is more likely. Although these are intended primarily to stabilize weak anterior teeth, they may have the opposite effect if not used with discretion.

53. Modification Areas Occasionally the occlusal rest on a secondary abutment in a Class II partial denture may serve as an indirect retainer. This use will depend on how far from the fulcrum line the secondary abutment is located. The primary abutments in a Class II, modification 1 partial denture are the abutment adjacent to the distal extension base and the most distal abutment on the tooth-supported side. The anterior abutment on the tooth-supported side is a secondary abutment, serving to support and retain one end of the tooth-supported segment and adding horizontal stabilization to the denture.

54. If the modification space were not present, as in an unmodified Class II arch, auxiliary occlusal rests and stabilizing components in the same position would still be essential to the design of the denture. However, the presence of a modification space conveniently provides an abutment tooth for support, stabilization, and retention. If the occlusal rest on the secondary abutment lies far enough from the fulcrum line, it may serve adequately as an indirect retainer.

55. Its dual function then is tooth support for one end of the modification area and support for an indirect retainer. The most typical example is a distal occlusal rest on a first premolar when a second premolar and first molar are missing and the second molar serves as one of the primary abutments. The longest perpendicular to the fulcrum line falls in the vicinity of the first premolar, making the location of the indirect retainer nearly ideal. On the other hand, if only one tooth, such as a first molar, is missing on the modification side, the occlusal rest on the second premolar abutment is too close to the fulcrum line to be effective. In such a situation, an auxiliary occlusal rest on the mesial marginal ridge of the first premolar is needed, both for indirect retention and for support for an otherwise unsupported major connector. Support for a modification area extending anteriorly to a canine abutment is obtained by any one of the accepted canine rest forms, as previously outlined in Chapter 6. In this situation the canine tooth provides nearly ideal indirect retention and support for the major connector as well.

56. Rugae Support Some clinicians consider coverage of the rugae area of the maxillary arch as a means of indirect retention because the rugae area is firm and usually well situated to provide indirect retention for a Class I removable partial denture. Although it is true that broad coverage over the rugae area can conceivably provide some support, the facts remain that tissue support is less effective than positive tooth support and that rugae coverage is undesirable if it can be avoided. The use of rugae support for indirect retention is usually part of a palatal horseshoe design. Because posterior retention is usually inadequate in this situation, the requirements for indirect retention are probably greater than can be satisfied by this type of tissue support alone.

57. In the mandibular arch, retention from the distal extension base alone is usually inadequate to prevent lifting of the base away from the tissue. In the maxillary arch, where only anterior teeth remain, full palatal coverage is usually necessary. In fact, with any Class I maxillary removable partial denture extending distally from the first premolar teeth, except when a maxillary torus prevents its use, palatal coverage may be used to advantage. Although complete coverage may be in the form of a resin base, the added retention and reduced bulk of a cast metal palate make the latter preferable. However, in the absence of full palatal coverage, an indirect retainer should be used with other designs of major palatal connectors for the Class I removable partial denture.

58. Incisor Rest Incisor rests are placed at the incisal angles of the anterior teeth in enamel. These are the least desirable rests for anterior teeth and are indicated only when the canines are absent. Preparation is begun with a small safe-side diamond disk held parallel to the path of insertion. Depth - 1.5-2 mm. It should be approximately 2-3 mm inside the proximal angle of the tooth. Mesiodistally, the rest should be concave. The preparation is completed using a small flame-shaped diamond point. The notch created by the disk should be rounded. Factors to be considered in determining the site and form of rest placed on incisor: Root form Root length Inclination of the tooth Crown / Root ratio

59. Lingual Rests Lingual rests are primarily given on maxillary canines. If a canine is not present, multiple rests are spread over several incisor teeth instead of a single tooth. Lingual rest seat preparations on enamel should never be attempted on mandibular anterior teeth because of lack of thickness of enamel. However, a lingual rest on cast restoration may be used on any anterior tooth, either maxillary or mandibular.

60. Lingual or cingulam rest is preferred over incisal rest due to: Mechanical advantage. Closer to center of rotation More esthetic Less prone to breakage. It is prepared if the enamel of the tooth is sound, there is good oral hygiene & caries index is low. No.2 or 4 round diamond or cylindrical diamond stone with a rounded tip is used to prepare the rest seat The outline form should be half-moon shaped. Should follow a smooth curve from one marginal ridge to the other, crossing the center of the tooth incisal to the cingulum. Deepest point of the rest seat is over the cingulum. It should follow general contour of the cingulum. The base of the rest seat should be placed about 1mm cervical to the height of the cingulum.

61. Cummer Arms Metal arm extending from saddle or connectors to end on unprepared palatal or lingual surface of an anterior tooth (upper canines) Disadvantes : Liable to cause tooth movement as excessive load is placed on a single tooth Can be used only on canines because of their long roots Liable to get distorted Advantages: Used when insufficient space exists for the continuous clasp

62. Embrasure Hook Placed in the embrasure between the anterior teeth. It covers the occlusal and incisal surfaces of the teeth and continues over to the labial or buccal surfaces. They are attached to continuous clasp or linguoplate . on unprepared tooth and are more esthetic.

63. Denture Base Indirect retention is provided by extending the denture base anteriorly onto the rugae area. Anterior part of the denture is indirectly retaining the posterior part Although the tissue support is les effective than tooth support, denture bases are used as a means of indirect retention This is because of the wide palatal coverage and the palatal tissues being firm in consistency If they become flabby, the efficiency of the bases decrease It provides indirect retention and also some support Cannot be used as effectively in the lower arch

64. Direct – Indirect Retention These are used in certain cases In this example the reciprocal arm of the direct retainer provides the needed indirect retention also

65. CONCLUSION Extension-base partial denture patients are the most common partial denture patients. It is essential for us to understand and utilize indirect retainers in our dentures. Therefore, with the use of proper biomechanics in the design and construction of removable partial denture, the dentist can deliver removable partial denture with high level of predictability and acceptance from the patients.

66. REFERENCES Carr AB, Brown DT. McCracken's Removable Partial Prosthodontics. Elsevier Health Sciences; 2010 Jun 22. Stewart KL, Rudd KD, Kuebker WA. Clinical removable partial prosthodontics. Ishiyaku EuroAmerica ; 1992. Nallaswamy D. Textbook of prosthodontics. JP Medical Ltd; 2017 Sep 30. Nallaswamy D. Textbook of prosthodontics. JP Medical Ltd; 2017 Sep 30. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P. Prosthetics: Indirect retention. British dental journal. 2001 Feb;190(3):128. Richard P. Frank, Jack I. Nicholls . An investigation of the effectiveness of indirect retainers The Journal of Prosthetic Dentistry, Volume 38, Issue 5, November 1977, Pages 494-506. William E. Avant. Indirect retention in partial denture design. J Prosthet Dent 2003 :90;(1):1-5

67. Fulcrum lines can also be classified based on the axes around which the roation occurs : Horizontal Fulcrum Line : passing between two principal abutment teeth It acts along the x-axis of the denture. This controls the rotational motion of the denture towards or away from the supporting soft tissues. Forces across this lever produce the most deleterious effect on the supporting tissues and the abutment teeth. Second Rotational Fulcrum Line (Sagittal) It extends posteriorly from the occlusal rests of the terminal abutment. It passes along the alveolar crest till the posterior extent of the residual ridge on the same side. It acts along the z-axis of the denture. This fulcrum controls rocking or side-to-side movement of the denture that takes place over the crest of the residual ridge. In Class I condition, there are two such fulcrums extending posteriorly from each primary abutment to the respective retromolar pads. These forces also have severe effects on the soft tissues .

68. Third Fulcrum Line (Vertical) It is vertical and located on the midline, lingual to the anterior teeth. It acts long the y-axis of the denture. It controls the movement of the denture around the y-axis. When the forces acting on the denture are vertical, most of the periodontal ligament fibers are activated which provides good resistance to the force. If the forces acting on the denture are not vertical, only a part of the periodontal fibres are activated. This can produce damage to the abutment tooth.