Designing of rpd

Designing of rpd

1. DESIGNING OF REMOVABLE PARTIAL DENTURES Dr. Paavana II MDS

2. CONTENTS INTRODUCTION TERMINOLOGIES BASIC PRINCIPLES OF RPD CONSTRUCTION BIOMECHANICS AND DESIGN SOLUTIONS POSSIBLE MOVEMENTS OF PARTIAL DENTURE FACTORS INFLUENCING MAGNITUDE OF STRESS TRANSMITTED TO THE ABUTMENT TEETH

3. DIFFERENCES IN PROSTHESIS SUPPORT AND THE INFLUENCE ON DESIGN CONTROLLING STRESS BY DESIGN CONSIDERATIONS PHILOSOPHY OF DESIGN STRESS EQUALISATION PHYSIOLOGIC BASING BROAD STRESS DISTRIBUTION DESIGN PROCEDURE ESSENTIALS OF DESIGN CLASS I AND II CLASS III CLASS IV REVIEW OF LITERATURE CONCLUSION

4. INTRODUCTION The primary objective of partial denture design is the preservation of the remaining teeth, their supporting structures, the residual alveolar ridges and the oral mucosa in a healthy condition , while at the same time replacing the missing teeth for improving aesthetics, mastication and speech. Emphasis must thus be placed first on the biological aspects of Partial Denture restorations, rather than upon the purely technical aspects.

5. Great controversy continues to exist as to what constitutes correct design and adequate support for the removable partial dentures. The method for using and equalizing support from the edentulous ridge and remaining teeth remains the main issue. The different methods used have given rise to various design philosophies.

6. TERMINOLOGIES [GPT-8]

7. DESIGN : To plan and /or delineate by drawing the outline of a proposed prosthesis.

8. SURVEYOR : A paralleling instrument used in construction of dental prosthesis to locate and delineate the contours and relative positions of the abutment teeth and associated structures. SURVEYING : An analysis and comparison of the prominence of intraoral contours associated with the fabrication of dental prosthesis .

9. UNDERCUT: The portion of the surface of an object that is below the height of contour in relationship to the path of placement. GUIDING PLANE: vertically parallel surfaces on abutment teeth and/or dental implant abutments oriented so as to contribute to the direction of the path of placement and removal of a removable prosthesis

10. SURVEY LINE : Line produced on a cast by a surveyor marking the greatest prominence of contour in relation to the planned path of placement of a restoration.

11. The PATH OF INSERTION is the direction in which a restoration moves from the point of initial contact of the rigid part with the supporting teeth to the terminal resting position , with the rest seated and the denture base in the contact with the tissues. The PATH OF REMOVAL is the direction in which a restoration moves from its terminal rest position to the last contact of its rigid part with the supporting teeth.

12. MAJOR CONNECTOR: The part of a partial removable dental prosthesis that joins the components on one side of the arch to those on the opposite side MINOR CONNECTOR: the connecting link between the major connector or the base of a partial removable dental prosthesis and other units of the prosthesis, such as the clasp assembly, indirect retainers, occlusal rests, or cingulum rests

13. OCCLUSAL REST: a rigid extension of a removable partial dental prosthesis that contacts the occlusal surface of a tooth or restoration, the occlusal surface of which may have been prepared to receive it DIRECT RETAINER: That component of partial removable dental prosthesis used to retain and prevent dislodgement, consisting of a clasp assembly or precision attachment

14. INDIRECT RETAINER: That component of a partial removable dental prosthesis that assists the direct retainer 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 moves away from the tissues in pure rotation around the fulcrum line

15. BASIC PRINCIPLES OF RPD CONSTRUCTION First expounded by A H Schmidt in 1956 The dentist must have a thorough knowledge of both the mechanical and biological factors involved in RPD design Treatment plan must be based on complete examination and diagnosis of the individual patient

16. 3. The dentist must correlate the pertinent factors and determine a proper plan of treatment – he alone can modify the conditions in the mouth to enhance the success of the treatment 4. The RPD should restore the form and function without injury to the remaining oral structure 5. “A removable partial denture is a form of treatment and NOT a cure”

17. BIOMECHANICS AND DESIGN SOLUTIONS

18. Removable partial dentures by design are intended to be placed into and removed from the mouth. As they are not fixed to the tissues, they are 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

19. If the potentially destructive forces can be minimized, then the physiological tolerances of the supporting structures are not exceeded and pathological change does not occur. It is important for clinicians providing RPD service 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

20. An understanding of simple machines applied to the design of removable partial dentures helps to accomplish the objective of preservation of oral structures Machines can be classified as Simple Complex Simple machines include – lever, wedge, screw, wheel and axle, inclined plane and pulley

21. LEVER 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.

22. FIRST CLASS LEVER: fulcrum is in center, resistance at one end, and effort / force is at the opposite end. This is the most efficient and easily controlled lever In dental terms, E- force of occlusion / gravity, F- tooth surface such as an occlusal rest and R – resistance provided by a direct retainer/guide plane surface

23. SECOND CLASS LEVER: fulcrum is at one end, effort at opposite end and resistance in center. This type is seen as indirect retention in RPDs.

24. THIRD CLASS LEVERS: fulcrum is at one end, resistance at opposite, and effort in center. This class is not commonly encountered in RPDs.

25. INCLINED PLANE Forces against the inclined plane may result in deflection of that which is applying the force or may result in movement to the inclined plane, neither of these results are desirable. Inclined planes are not a factor when the partial denture is tooth supported.

26. POSSIBLE MOVEMENTS OF PARTIAL DENTURE Differences in displaceability of the periodontal ligament of the supporting abutment teeth and soft tissue covering the residual ridge permit this rotation. The rotation of the prosthesis is in a combination of directions rather than unidirectional. There are three possible movements of the distal extension partial dentures

27. The rotational movement of an extension base type removable partial denture, when a force is placed on the denture base .

28. SAGITTAL PLANE : Rotation around the fulcrum line passing through the most posterior abutments when the denture base moves vertically toward or away from the supporting residual ridge Rotational movement around this fulcrum line or axis is of the greatest magnitude of that around the three fulcrums but not necessarily the most damaging

29. 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

30. FRONTAL PLANE: rotation around a longitudinal axis formed by the crest of the ridge it extends through the occlusal rest on the terminal abutment and the crest of the residual ridge on one side of the arch. In a class I situation there will be 2 of these fulcrums, one on each side of the arch. This fulcrum controls rotational movements of the denture- rocking, side- to- side movements over the crest of the ridge

31. This type of 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.

32. HORIZONTAL PLANE Rotation around a vertical axis located near the center of the arch. The fulcrum is located in the vicinity of the midline just lingual to to the anterior teeth. This fulcrum line is vertical, and it controls the rotational movement of the denture in the horizontal plane or the flat circular movements of the denture

33. This type of 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. Stabilizing components on one side of the arch act to stabilize the partial denture against horizontal forces applied from the opposite side.

34. Horizontal forces always will exist to some degree because of lateral stresses occurring during mastication and bruxism. These forces are accentuated by the failure to consider the orientation of the occlusal plane, the influence of malpositioned teeth and effect of abnormal jaw relationships. The amount of horizontal shift occurring in the partial denture will therefore depend on the magnitude of lateral forces applied and effectiveness of stabilizing components.

35. Since 3 movements are possible in a distal extension partial denture, occlusal rest should not have steep vertical walls or locking dove tails, which could possibly cause horizontal and torquing forces to be applied intracoronally to the abutment teeth

36. FACTORS INFLUENCING MAGNITUDE OF STRESSES TRANSMITTED TO ABUTMENT TEETH

37. LENGTH OF SPAN: Longer the edentulous span, the longer will be the denture base and greater will be the force transmitted to the abutment teeth. The fulcrum is located at or near the occlusal rest on the terminal abutment tooth.

38. When treatment is being planned, every effort should be made to retain a posterior abutment tooth to avoid a class I or class II situations. A base that begins next to the cuspid will have a greater degree of movement than will the one that begins distal to the second bicuspid.

39. QUALITY OF SUPPORT OF RIDGE: The form of the residual ridge can play a large part in dissipating forces created by function of the partial denture. Large, well formed ridges are capable of absorbing greater amounts of stress than are small, thin, or knife-edged ridges.

40. A healthy mucoperiosteum approximately 1mm thick is capable of bearing a great functional load than is a thin atropic mucosa. Soft, flabby, displaceable tissue contributes little to the vertical support of the denture and nothing to the lateral stability of the denture base. This type of tissue allows excessive movement of the denture, with resultant transmission of stress to the adjacent abutment tooth.

41. QUALITIES OF CLASPS The more flexible the retentive arm of the clasp, the less stress is transmitted to the abutment tooth. This is the reason the combination or wrought wire retentive clasp was suggested for the terminal abutments for class I or II partial dentures A flexible clasp arm contributes less resistance to the more destructive horizontal stresses. Therefore, as flexibility of the clasp increases, both the lateral and vertical stresses transmitted to the residual ridge increase.

42. If the periodontal support of the abutment tooth is good, a less flexible clasp such as a vertical projection clasp would be indicated because the tooth would more likely be able to withstand a greater amount of stress. If, on the other hand, the periodontal support has been weakened, a more flexible clasp such as the combination clasp with a wrought wire retentive arm should be used so that the residual ridge would share more of the resistance to horizontal forces acting on the partial denture

43. CLASP DESIGN: A Clasp that is designed so that it is passive when it is completely seated on the abutment tooth will exert less stress on the tooth than one that is not passive. Only when the frame work is completely seated, will the retentive clasp arms be passive. A clasp should be designed so that during insertion or removal of the prosthesis, the reciprocal arm contacts the tooth before the retentive tip passes over the greatest bulge of the abutment. This will stabilize / neutralize the stress to which the abutment tooth is subjected as the retentive terminal passes over the greatest bulge of the tooth

44. LENGTH OF CLASP: The more flexible a clasp, the less stress it will exert on the abutment tooth. Flexibility can be increased by increasing the length of the clasp. Doubling the length of the clasp will increase the flexibility five times. Clasp length may be increased by using a curved rather than a straight course on an abutment tooth

45. MATERIAL USED IN CLASP CONSTRUCTION : A clasp constructed of chrome alloy will normally exert greater stress on abutment tooth, than a gold clasp, all other factors being equal, because of greater rigidity of the chrome alloy. To compensate for this property, clasp arms of chrome alloys are constructed with a smaller diameter than a gold clasp would be to accomplish the same purpose.

46. ABUTMENT TOOTH SURFACE: The surface of a gold crown / restoration offers more frictional resistance to clasp arm movement than does the enamel surface of a tooth. Therefore, greater stress is exerted on a tooth restored with gold than on a tooth with intact enamel .

47. OCCLUSAL HARMONY: A disharmonious occlusion generates horizontal forces that, when magnified by the factors of leverage, can transmit destructive forces to both the abutment teeth and the residual ridges. The area of the denture base against which the occlusal load is applied significantly influences the amount of stress transmitted to the abutment teeth and ridge. If occlusal load is applied to the base adjacent to the abutment tooth, there will be less movement of the denture base and less stress transmission than if the load is applied at the distal end of the denture base

48. Ideally, the occlusal load should be applied in the center of the denture bearing area, both antero-posteriorly and bucco-lingually. In most mouths, the second premolar and first molar represent the best areas for application of the masticating load. Artificial teeth should be arranged so that the bulk of the masticatory force is applied in that area

49. DIFFERENCES IN PROSTHESIS SUPPORT AND THE INFLUENCE ON DESIGN

50. The methods adopted to control the movements of the partial denture depends on whether the prosthesis is tooth-supported or tooth-tissue supported. For a tooth supported prosthesis , the movement potential is less because teeth provide resistance to functional loading. Teeth do not vary widely in ability to provide this support; consequently, designs for prostheses is less variable

51. For tooth-tissue –supported prosthesis, the residual ridge presents a quite variable potential for support. The underlying alveolar bone demonstrates a highly variable form following extraction, and it continues to change with time The overlying connective tissue also undergoes changes along with the alveolar bone changes, that place the soft tissue at risk for pressure-induced inflammatory changes. This variable tissue support potential adds complexity to design considerations while dealing with tooth-tissue-supported prosthesis

52. This is because unlike the efficient support provided by the teeth, which results in limited prosthesis movement, the reaction of the ridge tissue to functional forces can be highly variable, leading to variable amounts of prosthesis movements Factors relating to the opposing arch tooth position, the existence and nature of prosthesis support in the opposing arch, and the potential for establishing a harmonious occlusion can greatly influence the partial denture design

53. Opposing tooth positions that apply forces outside the primary support of the prosthesis can introduce leverage forces that act to dislodge the prosthesis Such an effect is variable based on the nature of the opposing occlusion – natural teeth, complete denture or removable partial dentures. In general, RPDs opposing natural teeth will require greater support and stabilization over time because of greater functional load demands.

54. DIFFERENTIATION BETWEEN TOOTH – AND TOOTH-TISSUE SUPPORTED PROSTHESIS DIFFERENCES IN SUPPORT DIFFERENCES IN IMPRESSION REGISTRATION DIFFERENCES IN CLASP DESIGN

55. DIFFERENCES IN SUPPORT Tooth-tissue supported partial denture derives primary support from the tissue underlying the base and secondary support from the abutment teeth. Length and contour of the ridge influence amount of available support & stability The movement of the base under function also influences the occlusal efficiency of the partial denture and also the degree to which the abutment teeth are subjected to torque and tipping stresses

56. Tooth supported partial denture derives all support from the abutment teeth

57. IMPRESSION REGISTRATION REQUIREMENTS: The anatomic form and relationship of the remaining teeth in the dental arch & surrounding soft tissue must be recorded accurately so that the denture will not exert pressure on those structures beyond their physiologic limits. A type of impression material that can be removed from undercut areas without permanent distortion must be employed E.g., alginate, mercaptan rubber base, silicone impression materials and poly ethers best suited

58. 2. The supporting form of the soft tissue underlying the distal extension base should be recorded so that the firm areas are used as primary stress bearing areas and the readily displaceable tissues are not overloaded. An impression material capable of displacing tissue sufficiently to register the supporting form of the ridge will fulfill the second requirement e.g., mouth temperature waxes, rubber base – supporting form. ZOE paste can be used when only the extension base is involved in the impression

59. No single impression material can satisfactorily fulfill both the requirements.

60. DIFFERENCES IN CLASP DESIGN TOOTH-SUPPORTED PARTIAL DENTURE : It is totally supported by abutment teeth, so it is retained and stabilized by a clasp at each end of the each edentulous space As this type of prosthesis does not move under function, the only requirement of such clasps is that they flex sufficiently during placement and removal of the denture to pass over the height of contour of the teeth, in approaching/escaping from an undercut area.

61. In its terminal position, the retentive clasp should be passive & should not flex except when engaging the undercut area of the tooth to resist a vertical dislodging force

62. Cast retentive arms may be used for this purpose. These may be either of the circumferential type , arising from the body of the clasp and approaching the undercut from an occlusal direction, or of the bar type , arising from the base of the denture and approaching the undercut area from the gingival direction

63. TOOTH-TISSUE-SUPPORTED PARTIAL DENTURE: Due to the anticipated functional movement of the distal extension base, the direct retainer adjacent to the distal extension base must perform still another function in addition to that of resisting vertical displacement. Because of the lack of tooth support distally, the denture base will move tissueward under function proportionate to the displaceability of supporting soft tissue, the accuracy of the denture base, & the total occlusal load applied

64. Because of this tissueward movement, those elements of a clasp that lie in an undercut area mesial to the fulcrum for a distal extension, must be able to flex sufficiently to dissipate stresses that otherwise would be transmitted directly to the abutment tooth as leverage

65. Only the retentive arm of the circumferential clasp, should be made of wrought metal. Reciprocation and stabilization against lateral and torquing movement must be obtained through the use of rigid cast elements, which make up the remainder of the clasp – COMBINATION CLASP .

66. Advantages of combination clasp: Greater flexibility Adjustability Minimum tooth contact Better esthetics The amount of stress transmitted to the supporting edentulous ridge and abutment teeth will depend upon Direction and magnitude of force Length of the denture base lever arms Quality of resistance Design characteristics of the partial denture

67. CONTROLLING STRESS BY DESIGN CONSIDERATIONS

68. The statement “ no removable partial denture can be designed or constructed that will not be destructive in the mouth” can be thoroughly justified if all rotational forces and other stresses are considered At present, there is no way that all forces can be totally countered or negated. However, long term clinical observation has proved that a design philosophy that strives to control these factors within the physiologic tolerance of the teeth and supporting structures can be successful

69. 1. DIRECT RETENTION The retentive clasp arm is the element of the partial denture that is responsible for transmitting most of the destructive forces to the abutment teeth. A RPD should always be designed to keep clasp retention to a minimum yet provide adequate retention to prevent dislodgement of the denture by unseating forces

70. There are several components of the denture that can be used to contribute to the retention of the prosthesis so that the amount of retention provided by the clasps can be reduced. Exploiting this retentive potential in widely separated areas of the mouth can result in stress on the abutment teeth being effectively reduced; the support and stability of the prosthesis may be enhanced as well.

71. FORCES OF ADHESION AND COHESION: To secure the maximum possible retention through the forces of adhesion and cohesion, the denture base should cover the maximum area of available support and must be accurately adapted to the underlying mucosa. Adhesion is the attraction of saliva to the denture and the tissues, and cohesion is the internal attraction of the molecules of saliva to each other

72. Although it is not possible to develop a complete peripheral seal around the borders of a partial denture because of the presence of teeth, atmospheric pressure may still contribute a slight amount of retention This may be noted especially on a maxillary complete palatal major connector when an accurate metal casting is used and the margins of the connector are beaded. A partial vacuum can occur beneath the major connector

73. FRICTIONAL CONTROL The partial denture should be designed so that guide planes are created on as many teeth as possible. Guide planes are areas on teeth created so that they are parallel to the path the denture takes as it is inserted and withdrawn from the mouth The planes may be created on enamel surfaces of teeth / restorations placed on the teeth

74. Frictional contact of the prosthesis against these parallel surfaces can significantly contribute to the retention of the denture

75. NEUROMUSCULAR CONTROL: The innate ability of the patient to control the action of the tongue, lips and cheeks can be a major factor in the retention of the denture Design and contour of the denture base can greatly affect the ability of the patient to control / retain the prosthesis Overextensions will contribute to loss of retention and the abutment teeth bearing the direct retainers will be overly stressed because of the denture being constantly dislodged

76. A properly contoured borders of a denture base can aid in retention by permitting patient to use neuromuscular skills to avoid dislodging base.

77. QUADRILATERAL CONFIGURATION: It is indicated most often for class III arches particularly when there is a modification space on the opposite side of the arch. A retentive clasp should be positioned on each abutment tooth adjacent to the edentulous spaces. This results in the denture being confined within the outline of the four clasps,, and leverage on the denture is effectively neutralised.

78. For a class III arch where no modification space exists, the goal should be to place one clasp as far posterior on the dentulous side as possible and one as far anterior as space and esthetics permit. This retains the quadrilateral concept and is most effective way to control stress

79. TRIPOD CONFIGURATION Tripod clasping is primarily used in class II arches. If there is a modification space on the dentulous side, the teeth anterior and posterior to the space are clasped to bring about the tripod configuration.

80. If a modification space is not present, one clasp on the dentulous side of the arch should be positioned as far posterior as possible, and the other as far anterior as factors such as interocclusal space, retentive undercut, and esthetic considerations will permit. By separating the the two abutments on the tooth supported side as far as possible, the largest possible area of the denture will be enclosed in the triangle formed by the retentive clasps

81. This design is not as effective as the quadrilateral configuration, but is most effective in neutralizing leverage in the class II situation

82. BILATERAL CONFIGURATION: Most of the removable partial dentures fall into the bilateral distal extension group, or class I. Ideally, the single retentive clasp on each side of the arch should be located near the center of the dental arch or denture bearing area. For practical purposes , however, the terminal abutment tooth on each side of the arch must be clasped regardless of where it is positioned

83. In bilateral configuration, the clasps exert little or no neutralizing effect on the leverage induced stresses generated by the denture base. These stresses should be controlled by other means(indirect retainers

84. CLASP DESIGN CIRCUMFERENTIAL CAST CLASP: The conventional circumferential cast clasp originating from the distal occlusal rest on the terminal abutment tooth and engaging a mesiobuccal undercut should not be used on a distal extension RPD.

85. The terminal of this clasp reacts to movement of the denture base toward the tissue by placing a distal tipping, or torquing force on the abutment tooth. This particular force is the most destructive force a retentive clasp can exert. A reverse circlet clasp, a cast circumferential clasp that approaches a distobuccal undercut from the mesial surface of a terminal abutment tooth, is acceptable.

86. An occlusal load applied to the denture base, moves the retentive terminal into a greater vertical undercut but engages the mesiodistal height of contour

87. VERTICAL PROJECTION / BAR CLASP It is used on the terminal abutment tooth on a distal extension partial denture when the retentive undercut is located on the distobuccal surface.

88. As the denture base is loaded towards the tissue, the retentive tip of the T clasp rotates gingivally to release the stress being transmitted to the abutment teeth One school of thought on the philosophy of RPD design has advocated omitting the distoocclusal rest from the terminal abutment in favor of a mesial rest when a bar clasp is used. The belief is that a distal rest would cause the fulcrum line around which the denture tends to rotate to be distal to the retentive clasp terminal

89. The advantage claimed for moving the occlusal rest more anteriorly is that the lever arm is increased, which causes the force directed toward the residual ridge to be more vertical and thus better tolerated by the ridge Omitting a rest adjacent to the edentulous space permits packing of food between minor connector of partial denture and tooth

90. COMBINATION CLASP: When a mesiobuccal undercut exists on an abutment tooth adjacent to a distal extension edentulous ridge, the combination clasp can be employed to reduce the stress transmitted to the abutment Wrought alloy wire, by virtue of its cross sectional shape and internal structure, is more flexible than a cast clasp. It can flex in any spatial plane, whereas a cast clasp flexes in the horizontal plane only. The wrought wire retentive arm has a stress breaking action that absorbs torsional stress in both vertical and horizontal plane

91. SPLINTING OF ABUTMENT TEETH Adjacent teeth may be splinted by means of crowns to control stress transmitted to a weak abutment tooth. Splinting two or more teeth actually increases the periodontal ligament attachment area and distributes the stress over a larger area of support Splinting is also indicated when the proposed abutment tooth has either a tapered / short roots such that there is not an acceptable amount of periodontal ligament area present

92. Splinting is also indicated if the terminal abutment tooth on the distal extension side of the arch stands alone – an edentulous space exists exists both anterior and posterior to it. Usually seen often in second premolars. Such a premolar is potentially a weak abutment because of the rotational forces it must withstand. Splinting of this tooth to the tooth anterior to it, usually the canine, should be accomplished with a fixed partial denture Principal advantage - cross arch stabilization

93. INDIRECT RETENTION : An indirect retainer is a part of the removable partial denture that helps the direct retainer prevent displacement of the distal extension denture by resisting the rotational movement of the denture around the fulcrum line established by the occlusal rests. 98

94. In class I arch the indirect retention must always be used but is not critical in case of class II. The indirect retainer or retainers must be positioned as far anterior to the fulcrum line as possible. If a modification space exists on the tooth supported side, abutment teeth on both sides of the space should be selected. 99

95. For class III arch, indirect retention is not ordinarily required, because there is no distal extension denture base to create a lever arm. The consideration for the class IV arch is the reverse of that for class I and class II arches. The lever arm is anterior to the fulcrum line so the indirect retainer must be located as far posteriorly as possible. 100

96. OCCLUSION A Smoothly functioning occlusion that is in harmony with the movements of both the TMJs and the neuromusculature will minimize the stress transmitted to the abutment teeth and residual ridge. The contacts of the remaining natural teeth must be same when the removable partial denture is in the mouth as when the prosthesis is not in place

97. DENTURE BASE should cover as extensive an area of supporting tissue as possible – stress is distributed over a large area The distal extension denture base must always extend on to the retromolar pad area of the mandible and cover the entire tuberosity in the maxilla

98. Avoid overextensions as it interferences with the functional movements of the surrounding tissues and transmit significant stresses to the remaining teeth The more accurate the adaptation of the denture base to the residual ridge, the better will be the retention, in part because of the forces of adhesion and cohesion

99. The type of impressions used to record the mucoperiosteum of the ridge will influence the amount of stress the residual ridge can effectively absorb

100. MAJOR CONNECTOR In the mandibular arch, the lingual plate major connector that is properly supported by rests can aid in the distribution of functional stresses to the remaining teeth. It is particularly effective in supporting periodontally weakened anterior teeth

101. In the maxillary arch, the use of broad palatal major connector that contacts several of the remaining natural teeth can distribute stress over a large area. The major connector must be rigid and must receive vertical support through rests from several teeth

102. MINOR CONNECTORS The most intimate tooth- to- partial denture contact takes place between the minor connector joining the clasp assembly to the major connector and the guiding planes on the abutment tooth surface. This close contact serves two purpose

103. 1. It offers horizontal stability to the partial denture against lateral forces on the prosthesis. 2. Provides a distinct path of insertion and removal thereby helping in prosthesis retention.

104. RESTS Properly prepared rests seats help control stress by directing forces transmitted to abutment teeth down along the long axis of those teeth. The pdl is capable of withstanding vertical forces of far greater magnitude than horizontal/ torsional forces. During function , these forces average 196 Newtons (44lb) and during parafunction , 295 Newtons (66lb).

105. The floor of the rest seat preparation must form an angle of 90 with the perpendicular line dropped down the long axis of the tooth – permits the rest to grasp the tooth securely and prevents its migration

106. The number of abutment teeth influences the amount of force each tooth must absorb. The more teeth that bear rest seats, the less will be the stress placed on each individual tooth

107. PHILOSOPHY OF DESIGN The variations in the concept of design are multitudinous. However, there are three basic underlying approaches to distributing the forces acting on the partial denture between the soft tissue and teeth. Stress equalisation Physiologic basing Broad stress distribution

108. STRESS EQUALISATION Also referred to as stress directing approach This concept emphasises that the resiliency of the tooth secured by the periodontal ligament in an apical direction is considerably lesser than that of the greater resiliency and displaceability of the mucosa covering the edentulous ridge

109. This school of thought believes that the rigid connection between the denture bases and the direct retainer on the abutment teeth is damaging and that some type of stress director or stress equaliser is essential to protect the vulnerable abutment teeth Eg., a hinge device interposed between the minor connector of the abutment tooth and the denture base. The hinge is designed to permit vertical movement of the denture base as occlusal forces are applied to the artificial teeth. The amount of vertical movement permitted is usually the estimated thickness of the mucosa covering the ridge

110. ADVANTAGES The stress director design usually calls for minimal direct retention, because the denture base operates more independently than in a conventional denture Internal attachments for retention of the stress-broken prosthesis are widely used. Advocates of this theory, stress the importance of stress equaliser in case of class I and class II partial dentures because of the positive lock on the abutment tooth caused by the internal attachment. Thus, the stress director eliminates the tipping strain on the tooth, thereby preventing bone resorption around the tooth

111. ACTION OF STRESS EQUALISER Resiliency of the resiliency of stress equaliser + periodontal ligament = resiliency of mucosa Thereby, the forces are equally distributed between the teeth and soft tissue

112. DISADVANTAGES Fragile Construction – complex + costly Need for constant maintenance Difficult/impossible to repair Lacks the ability to prevent damaging lateral stresses from occuring on the edentulous ridge, resulting in the rapid resorption of bone and settling of the denture

113. If sufficient thickness of metal in the hinge region is used to prevent lateral movement, the prosthesis becomes heavy, bulky and annoying to the patient. Of the three schools of thought of partial denture design, the stress equalising school has the least advocates .

114. PHYSIOLOGIC BASING This school denies the use of stress directors to equalise the disparity of vertical movement between the tooth and mucosa. They believe that equalisation can best be accomplished by some form of physiologic basing/lining of the denture base. Physiologic basing is produced either by displacing/depressing the ridge mucosa during the impression making procedure or by relining the denture base after its construction

115. The reason for displacing the mucosa during the impression procedure is to record the soft tissue in its functioning, and not anatomic form. The rationale – if the tissue is recorded in its functioning form when occlusal forces take place on the denture, the denture base, formed over the displaced tissue, will adapt more readily to the depressed tissue and will be able to withstand the forces that are generated

116. The artificial teeth of a RPD constructed from a tissue displacing impression will be positioned above the plane of occlusion when the denture is in the mouth and not functioning To permit vertical movement of the partial denture from the rest position to the functioning position, the direct retainers/retentive clasps must be designed with minimum retention and the no. of direct retainers must be limited

117. The occlusal rests and direct retainers will also be slightly unseated at rest and will be completely seated only when the mucosa beneath the denture base is displaced to its functional form.

118. ADVANTAGES Intermittent pressure against the mucosa caused the movement of the denture base as occlusal loads are applied and removed has a stimulating effect on the underlying bone and soft tissue– this stimulation reduces tissue changes as well as the necessity of relining/rebasing to compensate for tissue change as is required for most distal extension partial dentures.

119. Simplicity of design and construction because of minimal retention requirements results in light weight prosthesis needing minimum maintenance and repair

120. Minimal direct retention: the looseness of the clasps on the abutment tooth reduces the functional forces transmitted to the tooth – preserving the abutment teeth

121. DISADVANTAGES Denture is not stabilised against lateral forces because of the minimum number and flexibility of the direct retainers. The residual ridge receives a greater proportional amount of the forces that are transmitted by the denture As the artificial teeth are always slightly above the occlusal plane when the denture is not in function, there will always be slightly premature contacts between the opposing teeth and the denture teeth when the mouth is closed

122. It is difficult to produce effective indirect retention because of the vertical movement of the denture and the minimal retention of the direct retainer. By the time the indirect retainer engages a rest seat to prevent the denture base from being dislodged, a direct retainer will have lost contact with the abutment teeth

123. BROAD STRESS DISTRIBUTION Advocates of this school of partial denture design believe that excessive trauma to the remaining teeth and residual ridge can be prevented by distributing the forces of occlusion over as many teeth as possible and as much soft tissue as possible. This is accomplished by the use of additional rests, indirect retainers, clasps and broad coverage denture bases.

124. Maximum coverage of teeth and soft tissues – distribution of forces over as wide an area as possible

125. ADVANTAGES The forces of occlusion are reduced on any tooth or area of the ridge because all the teeth and entire available ridge collectively bear the load. Multiple tooth contacts by direct retainers, additional rests, and minor connectors cause distribution of lateral forces over as many teeth as possible

126. Multiple clasps also aid in lateral stability – for the prosthesis as well as the periodontally compromised teeth. This constitutes a form of removable splinting which can be useful in instances where fixed splinting is not indicated The prosthesis is easier to fabricate and less expensive

127. No flexible parts – so there is less danger of distorting the denture Less subject to breakage Indirect retainers and other rigid components prevent rotational movements of the denture and provide excellent horizontal stabilization. Due to increased stability and decreased movement, it does not require frequent relining

128. DISADVANTAGES Greater amount of tooth and soft tissue coverage results in increased bulk – less patient comfort, less patient acceptance Constant monitoring for dental caries Meticulous oral hygiene

129. DESIGN PROCEDURE

130. During design, simplicity is of prime importance, but not at the expense mechanical and biologic standards that are necessary for maintenance of patient’s health A knowledge of the components of the partial denture and functions of the individual parts is absolutely necessary to make meaningful decisions for any given situation.

131. COLOR CODING A color coding system for various parts of the removable partial denture should be included on the diagnostic casts to help prevent confusion on the part of the dental technician or anyone trying to understand the design being proposed A well designed diagnostic cast also serves as a blueprint for the dentist during the mouth preparation appointment

132. At present there is no universally accepted color coding system. Any system agreed to and understood by the dental lab and the dentist is acceptable COLORS USED: Red crayons Blue crayons Brown crayons Black lead pencil – 2H or 3H

133. The brown crayon pencil – to outline metallic portion of the partial denture Blue – acrylic resin portion Red – to indicate areas on the teeth that will be prepared, relieved or contoured. Solid red – rest seats Red – tooth surfaces that are to be recontoured

134. Black pencil and carbon marker in the surveyor are used to denote survey lines, soft tissue undercuts, & other information to be included, such as the type of tooth replacement or the use of wrought wire for retentive clasps

135. STEPS IN DESIGN PROCEDURE

136. 1. EXAMINATION OF THE OCCLUDED DIAGNOSTIC CASTS

137. Indicate the proposed rest areas by a short vertical line on the cast below the tooth with the black pencil – in event of any change in rest seat location, corrections will not have to be made on the teeth

138. Indicate by outlining in red any cuspal relief that will be needed to provide adequate clearance for rest spaces.

139. Examine the lingual aspect of the occluded casts for adequate space for cingulum rests, indirect retainers, and so on. Using the black pencil from the rear surface of the casts, draw a line on the lingual surfaces of the maxillary anterior teeth using the incisal edges of the mandibular teeth as a guide. This line shows the incisal limit of proposed metal extension [ rests/lingual plating] onto those teeth

140. 2. Indicate with a pencil, using the following symbols, the type of tooth replacement desired T – tube tooth F – facing M - metal pontic RAP – reinforced acyrlic pontic NO SYMBOL – denture teeth on denture base Place these symbols on the soft tissue portion of the cast, adjacent to the edentulous area. One symbol should be used for each tooth replacement

141. Facings- when strength is greatest requirement, limited esthetics Tube teeth – little resorption of ridge, very esthetic RAP - little resorption of ridge, slightly stronger than tube teeth Acrylic resin teeth – resorbed ridges

142. 3. Place the cast on the cast holder at a horizontal tilt.

143. RETENTIVE UNDERCUTS Examine the teeth to be clasped for favorable retentive undercuts .

144. ESTHETICS Examine the anterior edentulous areas for esthetic considerations .

145. GUIDING PLANES Examine the proximal and lingual tooth surfaces for guiding planes.

146. SOFT TISSUE UNDERCUTS Beware of soft tissue undercuts that may interfere with the placement of the partial denture.

147. 4. Tripoding the cast To tripod the cast, the tip of the carbon marker must contact the cast at 3 widely separated points while the cast remains at a fixed tilt and the marker remains at a constant height. The marker scribes a 4-to -5 mm horizontal line at the 3 selected points

148. 5. Place the carbon marker in the vertical arm of the surveyor and scribe the survey line on the teeth that will be contacted by the partial denture

149. Survey lines may be transferrred to the teeth and other structures on the cast by releasing the vertical arm of the surveyor and rotating the cast while the side of the carbon marker remains in contact with the tooth Everything gingival to the survey line will be undercut to the path of insertion

150. The survey line is also transferred to soft tissue areas that will be contacted by the partial denture No rigid component of the prosthesis can lie below the survey line

151. 6 . Replace the carbon marker with the appropriate undercut gauge For most clasps of chrome cobalt alloy, a 0.010 inch undercut is adequate. for wrought wire retentive clasps, 0.02 inch is usually indicated Place the gauge on the desired retentive undercut area, so that the head and shank of the gauge touch the tooth simultaneously With a red pencil, mark the spot that the head touches the tooth. This mark represents the gingival edge of the clasp tip in the desired retentive undercut

152. The 0.01 – inch undercut gauge is used to position the lower border of the tip of the retentive clasp The shank contacts the molar at the survey line as the lid of the gauge contacts the tooth. This point should be marked with the red pencil

153. 7 . With a red pencil draw the extent of the rest areas to be prepared in the mouth The full extent of the rest seat should now be colored solid red. It has to be drawn in actual size so that the effect of the rest seat on the surrounding and opposing structures can be accurately forecast

154. 8.Using a red pencil, outline tooth surfaces that will require recontouring to produce the desired results Place evenly spaced diagonal lines to ensure that these areas are highly visible. Areas of soft tissue relief should be outlined in red and accompanied by the word relief.

155. 9 . Using a blue pencil, outline the exact position of each acrylic resin denture base

156. 10 . With a brown pencil outline the frame work design to harmonise and join the major connectors, rest areas, indirect retainers, minor connectors, denture bases and replacement teeth. Use a carbon marker to outline soft tissue undercuts that will influence the design

157. Maximum support from the hard palate must be the goal The anterior extent of the major connector is scalloped to simulate the necks of the tube teeth

158. Lingual bar mandibular major connector – superior margin should not be closer than 3 mm to the gingival margin of the teeth

159. The minor connector is added to the design – open latticework It should cover the tuberosity

160. 11. With a brown pencil, draw the clasp arms to the actual shape, size and location desired. If wrought wire clasps are to be used, place the symbol WW on the soft tissue below the tooth

161. The size, position, and contour of the clasps should be drawn accurately so that possible interferences with opposing arch can be detected Circumferential clasp - only terminal third of clasp arm below survey line Modified T clasp – approach arm is positioned superior to soft tissue undercut

162. Retentive clasp should be smoothly tapered and should be curved as it crosses the tooth surface – it should never run straight across the tooth Retentive clasps should be kept as low on the crown of the tooth as the survey line permits, preferably in the gingival third

163. The final components to be added are the reciprocal clasp arms. The arms should not be tapered as flexibility should be avoided It should always be positioned above the survey line, at the junction of gingival and middle thirds of the crown If survey line is too high to permit this, the enamel surface must be recontoured to lower the survey line

164. 11. The design should now be complete. Re-examine for accuracy and clarity

165. ESSENTIALS OF DESIGN

166. CLASS I AND II DIRECT RETENTION: Retention should NOT be considered the prime objective of design Main objective: restoration of function and appearance and maintenance of comfort, with great emphasis on preservation of health and integrity of all the oral structures that remain

167. Close adaptation and proper contour of an adequately extended denture base and accurate fit of the framework against multiple, properly prepared guide planes should be used to help the retentive clasp arms retain the prosthesis

168. CLASPS Clasps that will accomplish the design objectives should be employed Should have good stabilizing qualities, remain passive until activated by functional stress, and accommodate a minor amount of movement of the base without transmitting torque to the abutment Should be strategically positioned in the arch to achieve greatest possible control of stress

169. A class I prosthesis usually requires only two retentive clasp arms : one on each terminal tooth - If distobuccal undercut is present, vertical projection retentive clasp preferred If mesiobuccal undercut is present a wrought wire clasp is indicated Reciprocal/ bracing arm must be rigid. This component can be replaced by lingual plating

170. A class II prosthesis should usually have three retentive clasps arms Distal extension side – same as class I prosthesis Tooth supported side should have 2 retentive clasps arms – one as far as posterior and one as far anterior as tooth contours and esthetics permit If modification space exists, its convenient to clasp a tooth anterior and a tooth posterior to the edentulous space The type of clasp & position of the retentive undercut can be selected for convenience Rigidity is required for all bracing arms. Lingual plating may be substituted

171. RESTS Teeth selected for rest preparation should provide maximum possible support for the prosthesis Rest seats should be prepared so that stress will be directed along the long axis of the teeth Rests should be placed next to the edentulous space with few exceptions

172. INDIRECT RETENTION Indirect retention should be employed to neutralize the unseating forces - It should be located as far anterior to the fulcrum line as possible - Two indirect retainers should generally be used in a class I design, whereas one placed on the side opposite the distal extension base may be adequate in a class II

173. The indirect retainers should be positioned in teeth prepared with positive rest seats that will direct forces along the long axis of the tooth. Lingual plating can be used to extend the effectiveness of indirect retention to several teeth. It must always be supported by positive rest seats

174. MAJOR CONNECTOR The simplest connector that will accomplish the objectives should be selected - it must be rigid in nature - must not impinge on gingival tissue

175. Support from hard palate should be used in the design of maxillary major connector when it would be beneficial Extension on to the lingual surfaces of the teeth may be employed to increase rigidity, distribute lateral stresses, improve indirect retention, or eliminate potential food impaction areas. Lingual plating should always be supported by adequate rest seats

176. MINOR CONNECTORS It must be rigid Should be positioned to enhance comfort, cleanliness, and the placement of artificial teeth

177. OCCLUSION Centric occlusion and centric relation should coincide A harmonious occlusion should be established with no interceptive contacts and with all eccentric movements dictated by or in harmony with, the remaining natural teeth Artificial teeth should be selected and positioned to minimize stresses produced by the prosthesis

178. Smaller and/or fewer teeth, and teeth that are narrower buccolingually may be selected For mechanical advantage, teeth should be positioned over the crest of the mandibular ridge whenever possible Teeth should be modified if necessary to produce sharp cutting edges and ample escapeways

179. DENTURE BASE The base should be designed with broad coverage so that the occlusal stresses can be distributed over as wide an area of support as possible The extension of the borders must not interfere with functional movements of the surrounding tissues A selective pressure impression should record the residual ridge in its functional form The polished surfaces should be contoured to enable the patient to exercise maximum neuromuscular control

180. MANDIBULAR CLASS I SYMMETRICAL

181. ASYMMETRICAL

182. WITH MODIFICATION SPACES

183. MAXILLARY CLASS I WITHOUT MODIFICATION SPACE

184. WITH MODIFICATION

185. MANDIBULAR CLASS II WITH NO MODIFICATION

186. WITH POSTERIOR MODIFICATION

187. WITH ANTERIOR MODIFICATION

188. WITH ANTERIOR AND POSTERIOR MODIFICATION

189. MAXILLARY CLASS II WITH NO MODIFICATION

190. WITH POSTERIOR MODIFICATION

191. WITH ANTERIOR MODIFICATION

192. WITH ANTERIOR AND POSTERIOR MODIFICATION

193. CLASS III DIRECT RETENTION Retention can be achieved with much less potential harmful effect on the abutment teeth than with class I or II arch The position of the retentive undercut on abutment teeth is not critical

194. CLASPS The quadrilateral positioning of the direct retainer is ideal The type of clasp selected is not critical Tooth and tissue contours and esthetics should be considered, and the simplest clasp possible should be selected If restorations are required to correct the tooth contours, the wax patterns must be shaped with the surveyor Bracing arms must be rigid

195. RESTS Rest seats should be prepared next to the edentulous space when possible Rests should be used to support the major connector and lingual plating

196. INDIRECT RETENTION Indirect retention is usually not required

197. MAJOR AND MINOR CONNECTORS They must be rigid and meet the same requirements as for a class I or II design OCCLUSION The requirements for occlusion are the same as for a class I or II design

198. DENTURE BASE A functional type of impression is not needed The extent of coverage of the residual ridge areas should be determined by appearance, comfort, and the avoidance of food impaction areas

199. MANDIBULAR WITH NO MODIFICATION

200. WITH POSTERIOR MODIFICATION

201. WITH ANTERIOR MODIFICATION

202. WITH ANTERIOR AND POSTERIOR MODIFICATION

203. MAXILLARY ANTERIOR EDENTULOUS SEGMENTS

204. POSTERIOR EDENTULOUS SEGMENTS

205. BOTH ANTERIOR AND POSTERIOR EDENTULOUS SEGEMENTS

206. CLASS IV The movements of this type of removable partial denture and the resulting stresses transmitted to the abutment teeth are unlike the pattern seen in any other type of prosthesis The esthetic arrangement of the anterior replacement teeth may necessitate their placement anterior to the crest of the alveolar ridge, resulting in potential tilting leverage

207. Every effort should be made in order to minimize these stresses. Some of the possibilities – As much of the labial alveolar process should be preserved as possible A central incisor or other tooth should be retained to serve as an intermediate abutment or as an overdenture abutment A critical evaluation of each remaining tooth in the arch should be made with the intent of retaining as many teeth as possible

208. Strategic clasp position should be used. The quadrilateral configuration with the anterior clasps placed as anterior and the posterior clasps placed as far posterior as possible, would be ideal The major connector should be rigid, and broad palatal coverage should be used in the maxillary arch

209. Indirect retention should be as far posterior to the fulcrum line as possible An ideal quadrilateral configuration of clasping may preclude the need for an additional indirect retainer A functional type of impression may be indicated if the edentulous area is extensive

210. MANDIBULAR SYMMETRICAL

211. ASYMMETRICAL

212. MAXILLARY SYMMETRICAL

213. ASYMMETRICAL

214. REVIEW OF LITERATURE

215. Frechette AR in 1951 studied partial denture planning with special reference to stress distribution and concluded that Wide distribution of vertical stress is obtained by use of rigid connector with broad saddles and properly applied rests The rest seat should be spoon shaped and at right angles to the long axis of the tooth Clasp arm should embrace more than 180 0 of the tooth circumference and remain passive unless actively retaining the denture

216. Jordan LG in 1952 studied designing removable partial dentures with external attachment [clasps] and stated that vertical, lateral and anteroposterior occlusal forces can be well distributed by proper occlusal rest seats and occlusal rest, use of rigid retainer connector, proper clasp arm design, use of rigid base connectors

217. Osborne J, Lammie GA in 1954 presented the treatment of free end saddle and advocated use of Narrow occlusal table for reduction of vertical load Use of a stress breaker by distributing load between teeth and alveolus Distributing load widely over more than one abutment tooth on each side

218. Henderson D in 1967 studied force distribution with removable partial dentures and concluded that Use of rigid major connector linking abutment teeth on each side of the arch was an effective means of decreasing the force occuring to abutment teeth nearest the site of application of non-vertical force Abutment of a removable partial denture positioned farthest from the site of application of force to the dentures participate the least in resisting the force

219. Cecconi BT in 1974 studied effect of rest design on transmission of forces to abutment teeth and concluded that Precision rests and deep rests affect abutment tooth movement in a similar manner Rests with gingival seats at maximum depth in abutment teeth can significantly decrease abutment tooth movement Bilateral loading of a RPD causes significantly less abutment tooth movement than does unilateral loading

220. Krol AJ in 1973 studied clasp design for extension base removable partial denture and advocated RPI clasp because it minimises tooth coverage and reduces stress on the abutment tooth

221. Green LK, Hondrum SO in 2003 studied effect of design modification on the torsional and compressive rigidity of U shaped palatal major connectors and concluded that doubling the thickness of the anterior strap of a U shaped maxillary major connector improved the rigidity of the framework to torsional loads

222. CONCLUSION

223. Authorities in the field of removable partial denture design differ in their approach in developing the design of each individual prosthesis. Any of the above mentioned philosophies can be successful if applied to the correct partially edentulous situation and that all will fail if used under incorrect circumstances

224. There is, however complete agreement that the correct design incorporates proper use and application of mechanical and biologic principles.

225. REFERENCES Alan B Carr, Glen P, David T Brown. Mc Cracken’s Removable Partial Prosthodontics. 11 th edition Kenneth Stewart , Kenneth Rudd. Clinical Removable Partial Prosthodontics. Second edition Russell Stratton, Frank Wiebelt. An Atlas Of Removable Partial Denture Design

226. Frechette AR . Partial denture planning with special reference to stress distribution. J Prosthet. Dent. 1:710-24;1951 Jordan LG . Designing removable partial dentures with external attachments. JPD.2:716-22;1952 Osborne J, Lammie GA. The bilateral free end saddle lower denture. JPD.4:640-53;1953

227. Henderson D, Steward TE . Design and force distribution with removable partial dentures. JPD 17:350-64;1967 Cecconi BT . Effect of rest design on transmission of forces to abutment teeth. JPD.32:141-51;1974 Krol AJ. RPI [ rest, proximal plate, I-bar] clasp retainer and its modification. Dent Clin North Am. 17[4]:631-49;1973

228. Green LK, Hondrum SO. The effect of design modifications on the torsional and compressive rigidity of U shaped palatal major connector. JPD.131-5;2003

229. THANK YOU