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04.principles of tooth preparation


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principles of tooth preparation

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04.principles of tooth preparation

  1. 1. Principles of Tooth Preparation 1
  2. 2. PURPOSE OF TOOTH PREPARATION • Restoration is usually required to repair a diseased, injured or defective tooth structure. • The restoration helps in maintaining proper form, function and esthetics. 2
  8. 8. TYPES OF RESTORATION • Tooth restoration may be classified as intracoronal, when it is placed within a preparation made in the crown of a tooth • extracoronal, when it is placed outside the tooth as in the case of a crown. • Intracoronal restoration is placed directly into the tooth preparation while extracoronal restoration uses an indirect technique. 8
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  10. 10. Factors to be considered before restoration of a tooth • Tooth factors Primary or permanent Occlusal stresses Quality of tooth (hypoplasia) Location of tooth Type of tooth Type of tooth preparation. 10
  11. 11. • General patient factors Patient’s exposure to floride Age Xerostomia Socioeconomic status Diet Caries status General health Presence of any parafunctional habit. 11
  12. 12. Factors related to clinician and the restoration to be used Type of restoration Physical properties of the restoration Whether moisture control can be achieved or not Technical experties. 12
  13. 13. TERMINOLOGY OF TOOTH PREPARATION • Simple Tooth Preparation A tooth preparation involving only one tooth surface is termed simple preparation for example occlusal preparation. 13
  14. 14. • Compound Tooth Preparation A tooth preparation involving two surfaces is termed compound tooth preparation for example mesioocclusal or disto-occlusal preparation. 14
  15. 15. • Complex Tooth Preparation A tooth preparation involving more than two surfaces is called as complex tooth preparation for example MOD preparation. 15
  16. 16. Tooth Preparation Walls • Internal Wall It is a wall in the preparation, which is not extended to the external tooth surface 16
  17. 17. • External Wall An external wall is a wall in the prepared tooth that extends to the external tooth surface. External wall takes the name of the tooth surface towards which it is situated. 17
  18. 18. • Pulpal Wall A pulpal wall is an internal wall that is towards the pulp and covering the pulp. • It may be both vertical and perpendicular to the long axis of tooth. 18
  19. 19. • Axial Wall It is an internal wall which is parallel to the long axis of the tooth 19
  20. 20. • Floor Floor is a prepared wall which is usually flat and perpendicular to the occlusal forces directed occlusogingivally,for example, pulpal and gingival walls 20
  21. 21. Cavosurface Angle Margin/Tooth Preparation Margin • Cavosurface angle is formed by the junction of a prepared tooth surface wall and external surface of the tooth. • The acute junction is referred to as preparation margin or cavosurface margin. • The cavosurface angle may differ with the location of tooth and enamel rod direction of the prepared walls and also differ according to the type of restorative material to be used. 21
  22. 22. • Line Angle It is a junction of two surfaces of different orientations along the line and its name is derived from the involved surfaces. • Point Angle It is a junction of three plane surfaces or three line angles of different orientation and its name is derived from its involved surfaces or line angles. 22
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  24. 24. Steps in tooth preparation Stage I: Initial tooth preparation steps • Outline form and initial depth • Primary resistance form • Primary retention form • Convenience form. Stage II: Final tooth preparation steps • Removal of any remaining enamel pit or fisure, infected dentin and/or old restorative material, if indicated • Pulp protectin, if indicated • Secondary resistance and retention form • Procedures for finishing the external walls of the tooth preparation • Final procedures: Cleaning, inspectig and sealing • Under special conditions these sequences are changed. 24
  25. 25. • Outline form defines the external boundaries of the preparations. 25
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  28. 28. Following principles are kept in mind while preparing an outline form: • • Removal of all weakened and friable tooth structure • • Removal of all undermined enamel • • Incorporate all faults in preparation • • Place all margins of preparation in a position to afford good finishing of the restoration. 28
  29. 29. Features for establishing a proper outline form are • Preserving cuspal strength • Preserving strength of marginal ridge • Minimizing the buccolingual extensions • If distance between two faults is less than 0.5 mm, connect them • Limiting the depth of preparation 0.2 to 0.8 mm into dentin • Using enameloplasty wherever indicated 29
  30. 30. Primary Resistance Form • Definition: Primary resistance form is that shape and placement of preparation walls to best enables both the tooth and restoration to withstand, without fracture the stresses of masticatory forces delivered principally along the long axis of the tooth. 30
  31. 31. Factors affecting resistance form • Amount of occlusal stresses • Type of restoration used • Amount of remaining tooth structure. 31
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  36. 36. Primary Retention Form • Primary retention form is that form, shape and configuration of the tooth preparation that resists the displacement or removal of restoration from the preparation under lifting and tipping masticatory forces. • Usually, resistance and retention forms are obtained by providing same features, hence they are sometimes described together. The retention form is affected by the type of the restorative material used 36
  37. 37. Factors affecting retention form • Amount of the masticatory stresses falling on the restoration • Thickness of the restoration • Total surface area of the restoration exposed to the masticatory forces • The amount of remaining tooth structure 37
  38. 38. Retention form for different restorations • Amalgam: • Retention is increased in amalgam restoration by the following: • – Providing occlusal convergence (about 2–5%) of the dentinal walls towards the tooth surface • – Giving slight undercut in dentin near the pulpal wall • – Conserving the marginal ridges • – Providing occlusal dovetail 38
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  42. 42. Cast metals: • Retention is increased in cast restorations by the following: • – Close parallelism of the opposing walls with slight occlusal divergence of two to five degrees • – Making occlusal dovetail to prevent tilting of restoration in class II preparations • – Use of secondary retention in the form of coves, skirts and dentin slot • – Give reverse bevel in class I compound, class II, and MOD preparations to prevent tipping movements 42
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  44. 44. Composites: • In composites, retention is increased by: • – Micromechanical bonding between the etched and primed prepared tooth structure and the composite resin • – Providing enamel bevels. 44
  45. 45. Direct filling gold: • Elastic compression of dentin and starting point in dentin provide retention in direct gold fillings by proper condensation. 45
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  47. 47. Convenience Form • The convenience form is that form which facilitates and provides adequate visibility, accessibility and ease of operation during preparation and restoration of the tooth. 47
  48. 48. Features of convenience form • Suffcient extension of distal, mesial, facial or lingual walls to gain adequate access to the deeper portion of the preparation. • The cavosurface margin of the preparation should be related to the selected restorative material for the purpose of convenience to marginal adaptation. • In class II preparations access is made through occlusal surface for convenience form. 48
  49. 49. Features of convenience form • Proximal clearance is provided from the adjoining tooth during class II tooth preparation. • To make Class II tunnel preparation, for convenience, the proximal caries in posterior teeth is approached through a tunnel initiating from the occlusal surface and ending on carious lesion on the proximal surface without cutting the marginal ridge. • In tooth preparation for cast gold restorations occlusal divergence is one of the feature of convenience form. 49
  50. 50. Final Stages of Tooth Preparation • After initial stages of the preparation, the prepared tooth should be carefully examined • The remaining carious portion should be removed only after the initial tooth preparation has been completed. It provides two advantages: • 1. It allows optimal visibility and convenience form for removal of remaining carious lesion. • 2. Completion of the initial preparation permits immediate placement of a base and the restoration. 50
  51. 51. • Removal of Any Remaining Enamel Pit or Fissure, Infected Dentin and/or Old Restorative Material, if Indicated • After the establishment of external and internal outline form, if any of the remaining carious tooth structure or defective restorative material is left in tooth, it is to be removed in this stage. • Infected dentin must be removed even if it leads to exposure of pulp which is treated accordingly 51
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  53. 53. Pulp Protection • When remaining dentin thickness is less, pulpal injury can occur because of heat production, high speed burs with less effective coolants, irritating restorative materials, galvanic currents due to restoration of dissimilar metals, excessive masticatory forces transmitted through restorative materials to the dentin and ingress of microorganisms and their noxious products through microleakage. 53
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  55. 55. Bases • cements used on pulpal and axial walls in thickness of about 0.5 to 2 mm beneath the permanent restorations. • They provide thermal, galvanic, chemical and mechanical protection to the pulp. • Commonly used restorative materials as base are • zinc phosphate cements, • Glass ionomers, • polycarboxylate cements, • zinc oxide eugenol, • calcium hydroxide cement. 55
  56. 56. Secondary Resistance and Retention Forms • This step is needed in complex and compound tooth preparations where added preparation features are used to improve the resistance and retention form of the prepared tooth. • These are as follows: 56
  57. 57. 57 Grooves and coves:
  58. 58. 58 Slots or internal boxes:
  59. 59. 59 Locks
  60. 60. Pins: • Different types of pins of various shapes and sizes are used to provide additional retention. • They can be used in all types of restorations like amalgam, composite and cast restorations. 60
  61. 61. Skirts: • Skirts are prepared for providing additional retention in cast restorations. • They increase the total surface area of the preparation. • Skirts can be prepared on one or all four sides of the preparation depending upon the required retention 61
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  63. 63. Amalgam pins • Amalgam pins are vertical posts of amalgam anchored in dentin. • Dentin chamber is prepared by using inverted cone bur on gingival floor 0.5 mm in dentin with 1 to 2 mm depth and 0.5 to 1 mm width. • Amalgam pins increase the retention and resistance of complete restoration. 63
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  65. 65. Beveled enamel margins: • Beveling of the preparation, margins increases the surface area and thus the retention in composite restorations. 65
  66. 66. Enamel wall etching: • Etching results in microscopic roughness, which increases the surface area and thus helps in enhancing the micromechanical retention. 66
  67. 67. • Dentin conditioning (etching and priming): Etching and priming of the dentin surface done in some restorative materials increases the retention. • Adhesive luting cements: Adhesive luting cements increase the retention of indirect restorations 67
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