Principles of tooth preparation

Introduction
Teeth do not possess the regenerative ability found in most other tissues. Therefore, once the enamel or dentine is lost as a result of caries, trauma, or wear, restorative materials must be used to re-establish the form and function. Teeth require preparation to receive restorations and these preparations must be based on fundamental principles from which basic criteria can be developed to help predict the success of the prosthodontic treatment.
Definition
Objectives of tooth preparation
Principles of tooth preparation
Biological considerations
Mechanical considerations
Esthetic considerations
Conclusion
Each tooth preparation must be measured by clearly defined criteria that can be used to identify and correct problems. It is important to understand the pertinent theories underlying each step is crucial. Successful preparation can be obtained most easily by systematically following the steps which will ensure optimal quality of final restoration, which will serve the patient for a long time.

1. PRINCIPLES OF TOOTH PREPARATION Dr Kumari Kalpana PG - 1 1

2. CONTENTS Introduction Definition Objectives of tooth preparation Principles of tooth preparation Biological considerations Mechanical considerations 2

3. Esthetic considerations Conclusion References 3

4. INTRODUCTION 1 Teeth do not posses the regenerative ability found in most other tissues. Therefore, once the enamel or dentine is lost as a result of caries, trauma or wear, restorative materials must be used to re-establish the form and function. 1. Rosensteil SF, Land FM, Fujimoto J. Contemporary fixed prosthodontics.5 th ed. St.Louis:Mosby;2016. 4

5. 5 Teeth require preparation to receive restorations and these preparations must be based on fundamental principles from which basic criteria can be developed to help predict the success of the prosthodontic treatment.

6. DEFINITIONS 2,3 TOOTH PREPARATION - The process of removal of diseased and/or healthy enamel and dentin and cementum to shape a tooth to receive a restoration. 2 (GPT -9) 6 2. Glossary of prosthodontic terms. J Prosthet Dent 2017;117(5):10-92.

7. DEFINITIONS 3 The mechanical treatment of dental disease or injury to hard tissue that restores a tooth to the original form. 3 (Tylman) 7 3. Malone FP, Koth LD in ed. Tylman’s theory and practice of fixed prosthodontics.8 th ed. All india publisher and distributor;1997.

8. OBJECTIVES OF TOOTH PREPARATION 3 Reduction of the tooth in miniature to provide retainer support. Preservation of healthy tooth structure to secure resistance form. Provision for acceptable finish lines. 8 3. Malone FP, Koth LD. Tylman’s theory and practice of fixed prosthodontics.8 th ed. All india publisher and distributor;1997.

9. Performing pragmatic axial tooth reduction to encourage favorable tissue responses from artificial crown contours e.g., fluting of molars. 9

10. PRINCIPLES OF TOOTH PREPARATION 1,4 10 4. Shillingburg HT. Fundamentals of tooth preparation. 1991. Quintessence. China.

11. According to shillingburg 4 11 Preservation of tooth structure Retention and resistance Structural durability Marginal integrity Preservation of the periodontium

12. According to Rosensteil 1 12 Biologic Affect the health of the oral tissues Mechanical Affect the integrity and durability of the restoration. Esthetic Affect the appearance of the patient

13. ESTHETIC Minimum display of metal Maximum thickness of porcelain Porcelain occlusal surfaces Subgingival margins MECHANICAL Retention form Resistance form Deformation BIOLOGIC Conservation of tooth structure Avoidance of over contouring Supragingival margins Harmonious occlusion Protection against tooth fracture 13

14. BIOLOGIC CONSIDERATIONS 14

15. PREVENTION OF DAMAGE DURING TOOTH PREPARATION 1 Adjacent teeth Iatrogenic damage to the adjacent tooth is a common error while doing tooth preparation. The technique of tooth preparation must avoid and prevent damage to the adjacent tooth surfaces. 15

16. Prevention : Use of metal matrix band around the adjacent tooth for protection. 16

17. A thin tapered diamond bur can be used that passes through the inter-proximal contact area to leave a slight lip or fin of enamel without resulting in excessive tooth reduction or necessitating undesirable angulation of the rotary instrument. 17

18. SOFT TISSUE By careful retraction with aspirator tip mouth mirror flanged saliva ejector. 18

19. Moisture Control 5 rubber dam suction devices absorbents 19 5. Roberson TM, Heymann HO, Swift EJ. Sturdevant’s art and science of operative dentistry. 5 th Edition. St. Louis, Missouri:Mosby;2006.

20. Retraction and Access PROVIDES maximal exposure of the operating site usually involves maintaining an open mouth depressing or retracting the gingival tissue, tongue, lips, and cheek rubber dam, high-volume evacuator, absorbents, retraction cord and mouth prop are used for retraction and access. 20

21. Local Anesthesia – Local anesthetics play a role in eliminating the discomfort of dental treatment and controlling moisture. Reduces salivation 21

22. Pulp 1,6,7 Prevention of pulp injury is of great importance during crown preparation. Extreme temperatures, chemical irritation, or microorganisms can cause irreversible pulpitis. 22 6. Ottl P, Lauer HC. Temperature response in the pulpal chamber during ultrahigh-speed tooth preparation with diamond burs of different grit. J Prosthet Dent 1998;80:12-9. 7.Lauer HC, Kraft E, Rothlauf W, Zwingers T. Effects of the temperature of cooling water during high-speed and ultrahigh-speed tooth preparation. J Prosthet Dent 1990;63:407-14.

23. Temperature Considerable heat is generated by friction between a rotary instrument and the surface being prepared . Excessive pressure, higher rotational speeds, and the type and condition of the cutting instrument may result in increased generation of heat . Prevention- With high speed hand piece, a feather light, intermittent touch along with continuous water spray. 1 23

24. Sato and Schuchard reported that excessive heat adduction can result in structural changes of the hard dental tissues and damage the dental pulp . Zach and Cohen recorded in animal studies that 15% of dental pulps became necrotic when the temperature in the pulpal chamber increased by 5.6° C and 60% became necrotic if the temperature increases by 11.1° C . 24

25. Pohto and Scheinin documented increased capillary permeability , that is a first sign of heat related pulp damage, when the temperature increased between 5° and 7° C. Coarser the grit of diamond bur , the more pronounced the temperature elevation within a pulpal chamber during tooth preparation. 25

26. The relative temperature within the pulp increases by a maximum of 2.5° C for fine , 2.6° C for coarse , and 3.2° C for ultra coarse diamond burs . Preparation with cooling water, temperatures of 29.8°C to 33.7° C with the turbine or the high-speed angle did not produce an increase in temperature in the pulp chamber after three grinding steps 7 26

27. Air-water spray when accurately directed : Reduces the heat generated Prevents clogging Increases the cutting efficiency of the bur. While preparing grooves or pin holes, as the coolant cannot reach the cutting edge of the bur, prevention can be done by using low rotational speed. 27

28. Chemical action The chemical action by materials like restorative resins, solvents, bases & luting can cause pulpal damage particularly when applied to freshly cut dentin. Prevention - Cavity varnish or dentin bonding agents form an effective barrier but their effect on retention of the restoration is controversial. 28

29. Bacterial Action Pulpal damage under restorations- left behind bacteria or gained access to the dentin due to microleakage . Prevention - All Carious dentin must be removed before placement of the restoration. 29

30. CONSERVATION OF TOOTH STRUCTURE 1 One of the basic tenets of restorative dentistry is to conserve as much tooth structure as possible consistent with the mechanical and esthetic principles of tooth preparation. This will reduce the harmful pulpal effects of the various procedures and materials used. 30

31. Tooth structure is conserved by using the following guidelines: Use of partial-coverage rather than complete coverage restorations. Preparation of teeth with the minimum practical convergence angle (taper) between axial walls. 31

32. 3. Preparation of the occlusal surface so reduction follows the anatomic planes to give uniform thickness in the restoration. 32

33. 4. Preparation of the axial surfaces so that a maximal thickness of residual tooth structure surrounding pulpal tissues is retained, if feasible , teeth may be orthodontically repositioned which allows less axial convergence than necessary when tooth alignment is less than optimal to accommodate preparations for fixed dental prosthetic retainer 33

34. 5. Selection of a conservative margin compatible with the other principles of tooth preparation. Avoidance of unnecessary apical extension of the preparation. 34

35. Considerations affecting future dental health Improper tooth preparation may have adverse effects on long-term dental health. 1. AXIAL REDUCTION 1,4 : Axial reduction also plays an important role in securing space for an adequate thickness of restorative material. Gingival inflammation is associated with excessive axial contours. 35

36. If restorations are made with normal contours over preparations with inadequate axial reduction, they will have thin walls that will be subject to distortion. The crown should duplicate the contours & profile of the original tooth unless it is malformed or malpositioned. 1 36

37. MARGIN PLACEMENT 1 Three types of margins- 1. Supragingival 2. Subgingival 3. Equigingival 37

38. 38 The margins should be placed supra-gingival whenever possible as sub-gingival margins are a major etiologic factor in periodontal disease SUPRAGINGIVAL Advantages : 1 . Easily finished without soft tissue trauma 2. More easily kept plaque free 3. Impressions are more easily made, with less potential soft tissue damage 4. Restorations can be easily evaluated at recall appointments

39. Contraindications 8 39 Supragingival placement is not indicated when ( 1) the clinical crown is short, (2) old restorations with subgingival margins are present ( 3) crowns with facings are to be placed in areas requiring an esthetic appearance. (4) caries, abrasion, or erosion has degraded the gingival third of the clinical crown. 8. Richer WA, Ueno H. Relationship of crown margin placement to gingival inflammation. J Prosthet dent 1973:30(2);156-161

40. SUBGINGIVAL Indications: 1 40 Dental caries , cervical erosion and restorations extend subgingivally / Crown lengthening not indicated The proximal contact area extends to the gingival crest Additional retention / resistance is needed Margin of metal ceramic crown is to be hidden behind labio-gingival crest Root sensitivity cannot be controlled by more conservative procedures Modification of axial contour is indicated to provide an undercut to provide retention for the clasp of partial removable dental prosthesis

41. Gottelib stated “The practice of extending the margin of any metal restoration underneath the gingiva must prove detrimental. It is well known that restorations placed underneath the gingiva are commonly ill-fitting and, in that way, serve as an additional source of irritation.” 7 41

42. MARGIN ADAPTATION 1 The junction between a cemented restoration and the tooth is always a potential site for recurrent caries because of dissolution of the luting agent and inherent roughness. 42

43. A well-designed preparation has a smooth and even margin. Rough, irregular, or "stepped“ junctions greatly increase the length of the margin and substantially reduce the adaptation of the restoration. Smooth accurately placed preparation margins are particularly important when restoration are fabricated with CAD/CAM process. 43

44. MARGIN INTEGRITY 4 The restoration can survive in the biological environment of the oral cavity only if the margins are closely adapted to the cavo -surface finish line of the preparation. The configuration of the preparation finish line dictates the shape and bulk of restorative material in the margin of the restoration. 44

45. It also can affect both marginal adaptation and the degree of seating of the restoration. Historically, the bevel was used as a device for compensating for the solidification shrinking of alloys used in fabricating cast restorations 45

46. To bevel… Bevels have been advocated as means of diminishing marginal discrepancy.. D – vertical discrepancy in fit d - closest distance between margin & surface of preparation d = D sin µ d = D cos ø 46

47. 47

48. … or not to bevel D = d/sin µ or D = d/cos ø The film thickness of cement imposes a limit on reduction of perpendicular distance from margin to tooth,d The more nearly the bevel parallels the path of insertion , the greater the distance by which the restoration fails to seat 48

49. 49

50. MARGIN GEOMETRY 1,4 A. Feather edge B. Chamfer C. Beveled D. Chisel edge 50 Chisel edge Beveled

51. E. Shoulder F. Sloped shoulder G. Shoulder with a bevel 51

52. Feather edge: 1 Advantages: conservative of tooth structure. Disadvantages: does not provide sufficient bulk. Indications: not recommended. 52

53. Chisel Edge 1 A variation of the feather edge. Occasionally on tilted teeth Location of margin difficult to control Unfortunately, this margin is frequently associated with an excessively tapered preparation or one in which the axial reduction is not correctly aligned with the long axis of the tooth. 53

54. Chamfer 1,4 It is distinct and easily identified, provides room for adequate bulk of material, and can be placed with precision, although care is needed to avoid leaving a ledge of unsupported enamel. 1 This finish line has been shown experimentally to exhibit the least stress, so that the cement underlying it will have less likelihood of failure. 54

55. The chamfer should never be prepared wider than half the tip of the diamond; otherwise, an unsupported lip of enamel could result. 1 Indication- cast metal restoration, lingual margin of the metal-ceramic restoration. 4 55

56. Heavy Chamfer / deep chamfer 4 A heavy chamfer is used to provide a 90-degree cavo -surface angle with a large-radius rounded internal angle The heavy chamfer provides better support for a ceramic crown than does a conventional chamfer Stress concentration is less in the tooth structure. 56

57. Bevel 1,4 Wide, shallow bevels that are nearly parallel with the outer surface of the tooth should be avoided. They are likely to lead to over contouring. 4 57

58. Advantages : removes unsupported enamel, allows finishing of metal. Disadvantages : extends preparation into sulcus if used on apical margin. Indications : facial margin of maxillary partial coverage restorations and inlay/ onlay margins. 1 58

59. Shoulder 4 The shoulder has long been the finish line of choice for the all-ceramic crown. The wide ledge provides resistance to occlusal forces and minimizes stresses that might lead to fracture of the porcelain. It produces the space for healthy restoration contours and maximum esthetics. However, it does require the destruction of more tooth structure than any other finish line. 59

60. Radial shoulder modified shoulder finish line Initial instrumentation of ledge (coarse ,flat end tapered diamond) A small radius rounded internal angle (fine, flat end tapered diamond) 60

61. Sloped Shoulder 1 Advantages: bulk of material, Disadvantages : less conservative of tooth structure. Indications: facial margins of metal ceramic crowns. 61

62. Shoulder with a Bevel 4 This design can also be used for the facial finish line of metal-ceramic restorations Proximal box & occlusal shoulder of inlays and onlays Mandibular three- quarter crowns It can be used in those situations where a shoulder is already present, either because of destruction by caries or the presence of previous restorations. It is also a good finish line for preparations with extremely short walls, since it facilitates axial walls that are nearly parallel. 62

63. MECHANICAL CONSIDERATION 63

64. Mechanical considerations can be divided into 1 : 1. Providing retention form, 2. Resistance form 3. Preventing deformation of the restoration. 1,11 64

65. RETENTION FORM 1,2,4,11 Retention is that quality inherent in the dental prosthesis acting to resist the forces of dislodgment along the path of placement. 2 (GPT-9) Retention form is the feature of a tooth preparation that resists dislodgment of a crown in a vertical direction or along the path of placement. 2 (GPT-9) 65 11.Narula S, Punia V, Khandelwal M, Sharma V, Pamecha S. Retention in Conventional Fixed Partial Dentures: A Review. Journal of Clinical and Diagnostic Research. 2011;5(5):1128-1133.

66. 66

67. 67

68. MAGNITUDE OF DISLODGING FORCES 1,11 Forces that tend to remove a cemented restoration along path of withdrawal are small as compared to those that tend to tilt it. The magnitude of the dislodging forces depend on the stickiness of the food and the surface area and texture of the restoration being pulled. 68

69. GEOMETRY OF THE TOOTH PREPARATION 1,11 Fixed prosthesis depend on the geometric form of the preparation rather than on adhesion for retention. Cement is effective only if the restoration has a single path of placement. 69

70. For effective retention, a tooth preparation must constrain the movement of a restoration. For this to occur, it must be cylindrical. 70

71. If the preparation has multiple path of withdrawal, then the particles of the cement will tend lift away from rather than slide along the preparation and the only retention will be a result of the limited adhesion of the cement 71

72. TAPER 1,4,11 It is defined as the convergence of two opposing external walls of tooth preparation as viewed in a given plane. Theoretically, maximum retention is obtained if a tooth preparation has parallel walls. 72

73. A slight convergence, or taper, is necessary in the completed preparation. As long as this taper is small, the movement of the cemented restoration will be effectively restrained by the preparation and will have what is known as a limited path of withdrawal . As the taper increases, however, so does the free movement of the restoration, and retention will be reduced . 73

74. The axial walls of the preparation must taper slightly to permit the restoration to seat; i.e., two opposing external walls must gradually converge. Too small taper may lead to unwanted undercuts and too large will no longer be retentive. 10 To minimize stress in the cement interface between the preparation and restoration, a taper of 2.5 to 6.5 degrees has been suggested as optimum. 4 74

75. 75

76. Reports by various studies Nordlander et al 12 - 4 to 10 degrees Gilboe and teteruck 13 - 2 to 5 degrees El-Ebrashi and Craig 14 – 2.5-6.5 degrees Ghafoor et al 15 - 11.13±8.3 degrees El-Mubarak 16 – 39.98 degrees 76

77. A negative taper or undercut must be eliminated or it will prevent the seating of the restoration Preparation taper can be evaluated by viewing it with one eye from a distance of approximately 30 cm or 12 inches 77 VIEWING PREPARATION TAPER

78. In this way it is possible to see all the axial walls with an ideal taper of 6 An undercut as great as 8 can be overlooked if both the eyes are used 78 BINOCULAR VISION SHOULD NEVER BE EMPLOYED

79. SURFACE AREA 1,4,13 If the restoration has a limited path of placement, its retention is dependent on the length of this path or more precisely on the surface area in sliding contact. Therefore, crowns with tall axial walls are more retentive than those with short axial walls and molar crown of same taper are more retentive than premolar crown of the same taper. 1 79

80. Features such as grooves & boxes that are placed in the preparation increase the surface area thereby increasing retention. These are substituted for the missing axial wall. These features are also useful for augmenting retention on severely damaged teeth. 80

81. These secondary factors should be located with the line of withdrawal as far as possible from its reciprocal retentive feature, and at a point which permits the maximum length. The appropriate factor to be incorporated is the one which resists the displacing forces while conserving the greatest amount of structure. 13 81

82. Groove- A groove is the secondary factor which best achieves resistance form while conserving maximum tooth structure. It is, thus, the most commonly incorporated secondary factor. Box - A box may be regarded as a wide groove with an increased surface area. 82

83. 83

84. Pinhole - The pinhole may have tapered or parallel walls. Both types contribute more toward retention than resistance. The amount of retention is directly related to the area of the pin contacting the tooth and to the intimacy of the contact. Retention is a function of the length and diameter of the pin. 13 84

85. Secondary factors should be placed within 2 to 5 degrees of parallelism with a primary or another secondary factor for maximum effectiveness. 13 85

86. FREEDOM OF DISPLACEMENT 1,4 Retention is improved by geometrically limiting the number of paths along which a restoration can be removed from the tooth preparation. Maximum retention is achieved when there is only one path. 86 More the number of paths , less will be the retention

87. STRESS CONCENTRATION 1,11 When a retentive failure occurs, cement often adheres to both the tooth preparation and the fitting surface of the restoration. In these cases, cohesive failure occurs through the cement layer because the strength of the cement is less than the induced stresses . Changes in the geometry of the preparation (e.g. rounding of the internal line angles) reduces stress concentrations and hence increases the retention of the restoration. 87

88. TYPE OF PREPARATION Retention of a complete crown is about double that of partial-coverage restorations. Adding grooves or boxes to a preparation will increase retention because they will limit the paths of withdrawal. 88

89. ROUGHNESS OF THE FITTING SURFACES OF THE RESTORATIONS When the internal surface of a restoration is very smooth, retentive failure occurs not through the cement but at the cement-restoration interface. Under these circumstances, retention will be increased if the restoration is roughened or grooved . The casting is most effectively prepared by air-abrading the fitting surface with 50 um of alumina Retention increases by 64% 89

90. MATERIALS BEING CEMENTED Retention is affected by both the casting alloy and the core or build up material. The more reactive the alloy is, the more adhesion there will be with certain luting agents. Therefore, base metal alloys i.e , Ni, Co & Cr are better retained than less reactive high-gold content metals 90

91. ROLE OF LUTING CEMENTS IN RETENTION Type of luting agent chosen affects the retention of cemented restoration Five kinds of luting agents are most commonly used: 1. Zinc Phosphate 2. Zinc Polycarboxylate 3. Glass ionomer 4. Zinc oxide eugenol 5. Resin bonded cement. 91

92. FILM THICKNESS OF LUTING AGENT 1 92 CATEGORY HIGHER RESISTANCE LOWER RESISTANCE TAPER MINIMUM EXCESSIVE HEIGHT LONG SHORT TYPE OF PREPARATION COMPLETE COVERAGE PARTIAL COVERAGE/ ONLAY LUTING AGENT GLASS IONOMER (24 µm) ZINC PHOSPHATE( 20µm) Zinc Polycarboxylate ( 21µm) A uniform thickness of cement between restoration and tooth provides more retention than a non-uniform thickness. A film thickness of 25 μm or less has been preferred for successful restoration. 11

93. ADDITIONAL METHODS OF GAINING RETENTION 93

94. FACTORS AFFECTING RETENTION IN FIXED PARTIAL DENTURES 94

95. 95

96. 1. Length of Span In addition to the increased load placed on the periodontal ligament by long span bridge, the longer spans are less rigid and so less retentive. 2. Curvature of Arch Arch curvature has its effect on stresses occurring in a fixed bridge. When pontic lie outside the inter abutment axis line , it acts as lever arm producing torquing movement. 96

97. 3. Type of Bridge There are two types of bridges made according to the prevalent condition and position of abutments in the arch. a. Rigid connector b. Non-rigid connector 4. Occlusion Interference with undesirable occlusal contacts produce deviation during closure of maximum intercuspation, hinder smooth passage to and from the intercuspation position and lead to deflective occlusal force on the bridges which may lead to damaging effects on abutment and also on the retention of the casting. 97

98. 5. Periodontal Condition The abutment tooth must be able to provide good support for the bridge. This support is related to both the amount of root and the amount of bone present. 6. Tooth or Teeth Being Replaced A bridge replacing a maxillary canine is subjected to more stresses than the mandibular since forces are transmitted outward (labially) on the maxillary arch against the inside of the curve (its weakest point). 98

99. 7. Type of Retainer Used There are two types of retainers which are generally used • Intra coronal • Extra coronal 8. Materials Employed in the Construction of Retainers The material used in the construction of the fixed partial dentures calls for certain requirements which help to increase the longevity of the restoration. 99

100. Cobalt chromium or nickel chromium alloys generally used for making fixed bridges fulfill majority of these ideal requirements . On the other hand acrylic is generally weak , is not rigid and cannot provide strong connectors . It also has lower compressive and tensile strength compared to other alloys and is thus easily subjected to fracture. Hence acrylic is used for interim or temporary restorations in the mouth. 100

101. 9. Arch Position of the Abutment Teeth and Retention When the abutment teeth are more or less parallel to each other, complete or partial crown retainers can be made. 10. Spring Cantilever Bridges and Retention This bridge provides a method of supporting a pontic at some distance from the retainers. This type of bridge is both tooth and tissue supported. A gold bar which fits in contact with the palatal mucosa connects the pontic to the retainers. 101

102. DIFFERENT TOOTH PREPARATION AND WAYS OF ACHEIVING RETENTION 102

103. Complete Cast Crown Preparation Ways of gaining retention while tooth preparation: guiding grooves on the axial walls a taper of 3-6° width of chamfer should be approximately 0.5 mm which will allow adequate bulk of metal at the margin 103

104. The Metal Ceramic Crown Preparation Factors affecting retention that should be taken into consideration while preparation Reduction of the incisal edge on an anterior tooth should allow 2 mm of adequate material thickness to permit translucency in the completed restoration. 104

105. Labial reduction of 1.5 mm 90° butt joint taper of 6 degree all the line angles and point angles should be rounded .This will help in reducing the stress concentration and thus will enhance retention. 105

106. THE PARTIAL VENEER CROWN PREPARATION Posterior Teeth-Three Quarter Crown During axial reduction, place grooves for axial alignment in the centre of the lingual surface and in the mesio-lingual and distolingual transitional line angles. These grooves should be made parallel to the long axis of tooth. 106

107. During proximal reduction the proximal grooves are placed parallel to the path of withdrawal. The groove should not be deeper than 1mm and is best done with a tapered carbide bur. If additional bulk is needed to ensure rigidity of the restoration, it can be provided with an occlusal offset. This V-shaped groove extends from the proximal grooves along the buccal cusp. 107

108. RETENTIVE FEATURES FOR ALL CERAMIC RESTORATION An all ceramic restoration remains the most aesthetic restoration for duplicating individual anterior teeth. Adequate tooth reduction is created to achieve space for the porcelain bulk required for the strength of the restoration. 108

109. RETENTION IN ENDODONTICALLY TREATED TEETH It has been demonstrated experimentally that endodontically treated teeth are weaker and more brittle than vital teeth. So for this reason attempts have been made to strengthen the teeth by removing part of the root canal filling and replacing it with a metal post. Retention and support is derived from within the root canal. 109

110. Canal Retention It is recommended that the root canal should be enlarged only to amount necessary to enable the post to fit snugly for strength and retention 110

111. RESISTANCE FORM It is the feature of a tooth preparation that enhance the stability of a restoration and resist dislodgment along an axis other than the path of placement. 2 (GPT-9) 111

112. Mastication and para functional activity may subject a prosthesis to substantial horizontal or oblique forces. Adequate resistance depends on the following: 1. Magnitude and direction of the dislodging forces 2. Geometry of the tooth preparation 3. Physical properties of the luting agent 112

113. MAGNITUDE AND DIRECTION OF THE DISLODGING FORCES 1 In a normal occlusion, biting force is distributed over all the teeth; most of it is axially directed. If a fixed prosthesis is carefully made with a properly designed occlusion, the load should be well distributed and favourably directed. 113

114. However, if a patient has a biting habit such as pipe smoking or bruxing, it may be difficult to prevent fairly large oblique forces from being applied to a restoration. Consequently the completed tooth preparation and restoration must be able to withstand considerable oblique forces as well as the normal axial ones 114

115. Length of the preparation 4 Occluso -gingival length is an important factor in both retention and resistance. Because the axial wall occlusal to the finish line interferes with displacement, the length and inclination of that wall become factors in resistance to tipping forces. 115

116. The walls of shorter preparations should have as little taper as possible to increase the resistance. 116

117. It may be possible to successfully restore a tooth with short walls if the tooth has a small diameter. The preparation on the smaller tooth will have a short rotational radius for the arc of displacement. The longer rotational radius on the larger preparation allows for a more gradual arc of displacement. 117

118. Resistance to displacement for a short-walled preparation on a large tooth can be improved by placing grooves in the axial walls. This reduces the rotational radius, and that portion of the walls of the grooves near the occlusal surface of the preparation will interfere with displacement. 118

119. GEOMETRY OF TOOTH PREPARATION 1 The tooth preparation must be shaped so that particular areas of the axial wall will prevent rotation of the crown. A partial-coverage restoration may have less resistance than a complete crown because it has no buccal resistance areas. 119

120. Resistance must be provided by boxes or grooves and will be greatest if they have walls that are perpendicular to the direction of the applied force The resistance form of an excessively tapered preparation can be improved by adding grooves or pinholes, because these interfere with rotational movement and in so doing subject additional areas of the luting agent to compression. 120

121. PHYSICAL PROPERTIES OF THE LUTING AGENT 1 Resistance to deformation is affected by physical properties of the luting agent, such as compressive strength and modulus of elasticity . Zinc phosphate cements have a higher modulus of elasticity than do polycarboxylate cements, which exhibit relatively large plastic deformation. This may account for the observation that the retentive ability of polycarboxylate cement is more dependent on the taper of the preparation than is the retention with zinc phosphate cement. 121

122. Method to analyse resistance form 122 17. Parker MH. Resistance form in tooth preparation. DCNA 2004, 48:387-396.

123. For tilted molars A- center of rotation AB- distal wall CD- mesial wall 123 NR R CENTRE OF ROTATION

124. Adding a groove (HIJKLM) to the preparation affects the resistance form. 124 17. Parker MH. Resistance form in tooth preparation. DCNA 2004, 48:387-396

125. DEFORMATION 1 A restoration must have sufficient strength to prevent permanent deformation during function. Otherwise, it will fail (typically at the restoration-cement, or the metal-porcelain, interface). This may be a result of inappropriate alloy selection, inadequate tooth preparation, or poor metal-ceramic framework design . 125

126. ESTHETIC CONSIDERATION 1 126

127. Patients prefer their dental restorations to look as natural as possible. However, care must be taken that esthetic considerations are not pursued at the expense of a patient's long-term oral health or functional efficiency. 127

128. METAL CERAMIC RESTORATION 1 The poor appearance of some metal-ceramic restorations is often due to insufficient porcelain thickness. On the other hand, adequate porcelain thickness is sometimes obtained at the expense of proper axial contour (such overcontoured restorations almost invariably lead to periodontal disease). In addition, the labial margin of a metal-ceramic crown is not always accurately placed 128

129. Shade problems are frequently encountered in maxillary incisor crowns at the incisal and cervical thirds of the restoration, where direct light reflection from the opaque layer can make the restoration appear very noticeable. The labial surfaces of anterior teeth should be prepared for metal-ceramic restorations in two distinct planes. If they are prepared in a single plane, insufficient reduction in either the cervical or the incisal area of the preparation will result. 129

130. Incisal Reduction An incisal reduction of 2 mm is recommended for good esthetics. 130

131. Proximal Reduction The proximal surfaces of anterior teeth will look most natural if they are restored as the incisal edges, without metal backing. This will allow some light to pass through the restoration in a manner similar to what occurs on a natural tooth 131

132. Labial Margin Placement Supragingival margin placement has many biologic advantages. Nevertheless, subgingival margins may be indicated for esthetic reasons, particularly when the patient has a high lip line and the use of a metal collar labial margin is contemplated. 132

133. Patients with a high lip line, which exposes considerable gingival tissue, present the greatest problem if complete crowns are needed. Where the root surface is not discoloured, appearance can be restored with a metal-ceramic restoration having a supra-gingival porcelain labial margin. 133

134. If the patient has a low lip line, a metal supra-gingival collar may be placed because the metal is not seen during normal function. Metal margins generally have a more accurate fit than porcelain margins. Metal collars can be hidden below the gingival crest, although there will be some discoloration if the gingival tissue is thin. 134

135. The margin should follow the contour of the free gingiva, being further apical in the middle of the tooth and further incisal inter-proximally. A common error is to prepare the tooth so the margin lies almost in one plane, with exposure of the collar labially and irreversible loss of bone and papilla proximally. 135

136. ALL CERAMIC RESTORATIONS Not conservative of tooth structure. 90 degree heavy chamfers margin has to be prepared all around the natural teeth to ensure material thickness. This limits the use of these restorations on facio-lingually thin teeth. 136

137. PARTIAL-COVERAGE RESTORATIONS 1 Esthetic partial-coverage restorations depend on accurate placement of the potentially visible facial and proximal margins. 137

138. Proximal margins Place the margin just buccal to the proximal contact area, where metal will be hidden by the distal line angle of the neighbouring tooth. Tooth preparation angulation is critical and should normally follow the long axes of posterior teeth and the incisal two thirds of the facial surface of anterior teeth. 138

139. If a buccal or lingual tilt is given to the tooth preparation, metal may be visible. The distal margin of posterior partial-coverage restorations is less visible than the mesial margin. Often in this area it is advantageous to extend the preparation farther beyond the contact point for easier preparation and finishing of the restoration and better access for oral hygiene. 139

140. Facial Margin The facial margin of a maxillary partial-coverage restoration should be extended just beyond the occluso-facial line angle. A short bevel is needed to prevent enamel chipping. A chamfer can be placed where appearance is less important because this will provide greater bulk of metal for strength. 140

141. PRESERVATION OF THE PERIODONTIUM 4 Whenever possible, the finish line should be placed in an area where the margins of the restoration can be finished by the dentist and kept clean by the patient. In addition, finish lines must be placed so that they can be duplicated by the impression, without tearing or deforming the impression when it is removed past them. 141

142. 142

143. Preservation of periodontium 4 Finish line has a direct bearing on ease of fabrication & ultimate success of restoration Margins should be smooth as possible & fully exposed to cleansing - in enamal - supragingival margins 143

144. 144

145. 145

146. CONCLUSION Each tooth preparation must be measured by clearly defined criteria which can be used to identify and correct problems 146

147. Successful restoration Accurate diagnosis Thoughtful treatment planning Preparation design 147

148. 148 REFRENCES

149. 1 . Rosensteil SF, Land FM, Fujimoto J. Contemporary fixed prosthodontics.5 th ed. St.Louis:Mosby;2016. 2. Glossary of prosthodontic terms. J Prosthet Dent 2017;117(5); 10-92. 3. Malone FP, Koth LD in ed. Tylman’s theory and practice of fixed prosthodontics.8 th ed. All india publisher and distributor;1997. 4. Shillingburg HT. fundamentals of tooth preparation. 1991. Quintessence. China. 5. Roberson TM, Heymann HO, Swift EJ. Sturdevant’s art and science of operative dentistry. 5 th Edition. St. Louis, Missouri:Mosby;2006 6. O ttl P, Lauer HC. Temperature response in the pulpal chamber during ultrahigh-speed tooth preparation with diamond burs of different grit. J Prosthet Dent 1998;80:12-9. 149

150. 7 . Lauer HC, Kraft E, Rothlauf W, Zwingers T. Effects of the temperature of cooling water during high-speed and ultrahigh-speed tooth preparation. J Prosthet Dent 1990;63:407-14. 8. Richer WA, Ueno H. Relationship of crown margin placement to gingival inflammation. J Prosthet dent 1973:30(2);156-161 9. Padbury A, Eber R, Wang H. Interactions between the gingiva and the margin of restorations. J Clin Periodontol 2003; 30: 379–385 10. Sharma A, Rahul G, Gupta B, Hafeez M. Biological width: No violation zone. European Journal of General Dentistry 2012;1(3):37-41. 11. Narula S, Punia V, Khandelwal M, Sharma V, Pamecha S. Retention in Conventional Fixed Partial Dentures: A Review. Journal of Clinical and Diagnostic Research. 2011;5(5):1128-1133. 150