Principles of crown preparation/ orthodontic seminars


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  • A bridge stabilizes the bite of a patient who is missing one or more teeth. Dental bridges are meant to replace missing teeth by "bridging" the gap between two remaining teeth. This involves creation of a pontic (false tooth) with a dental crown on either side. The crowns are fitted over the remaining teeth to hold the false tooth in place.,-India
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Principles of crown preparation/ orthodontic seminars

  1. 1. PRINCIPLES OF TOOTH PREPARATION INDIAN DENTAL ACADEMY Leader in continuing dental education
  3. 3. INTRODUCTION Teeth - no regenerative ability Restorative material must be used if…. Teeth require preparation to receive restoration These preparation must be based on fundamental principles from which basic criteria must be developed to help predict the success of prosthodontic treatment Careful attention to every detail is imperative during tooth preparation
  4. 4.
  5. 5. 3 Broad categories : 1. BIOLOGIC CONSIDERATIONS, which affect the health of oral tissues 2. MECHANICAL CONSIDERATIONS, which affect the integrity and durability of the restoration 3. ESTHETIC CONSIDERATIONS, which affect the appearance of the patient
  6. 6. BIOLOGIC Conservation of tooth structure Avoidance of overcontouring Supragingival margins Harmonious occlusion Protection against tooth fracture MECHANICAL Retention form Resistance form Deformation ESTHETIC Minimum display of metal Maximum thickness of porcelain Porcelain occlusal surfaces Subgingival margins Best combination of compromises
  7. 7. BIOLOGIC CONSIDERATIONS 1. Prevention of damage during tooth preparation -Adjacent teeth -Soft tissue -Pulp 2. Conservation of tooth structure 3. Avoidance of overcontouring -Axial reduction 4. Marginal integrity and preservation of periodontium -Margin location -Margin adaptation -Margin geometry or Finish line configuration 5. Occlusal considerations 6. Preventing tooth fracture
  8. 8. Prevention of damage during tooth preparation Adjacent teeth - iatrogenic damage - damaged proximal contact - To avoid Soft tissues - tongue and cheeks - careful retraction Pulp - excessive temperature - chemical irritation - microorganisms
  9. 9. Conservation of tooth structure  More the remaining dentin – lesser the pulpal damage  Must be considered relative  GUIDELINES: 1. Partial coverage 2. Minimum taper
  10. 10. 3. Planar occlusal reduction 4. Even reduction of axial surfaces
  11. 11. 5. Selection of conservative margin 6. Avoidance of unnecessary apical extension of preparation
  12. 12. Avoidance of overcontouring A crown should duplicate the contours and profile of the original tooth Axial reduction Undercontoured restoration is better than overcontoured restoration Anterior teeth, increasing the proximal contour is better
  13. 13. Marginal integrity and preservation of periodontium Margin location- Biologic width It’s the dimension of space that the healthy gingival tissues occupy above the alveolar bone. It refers to the combined connective tissue-epithelial attachment from the crest of the alveolar bone to the base of the sulcus(2mm;connective tissue-1.07mm and epithelium-0.97mm). Evaluation of the biological width Radiographs Probing Sounding of bone Violation of biological width:-
  14. 14. Nevins and Sukrow in 1984 recommended that for the maintenance of healthy periodontium, no restoration should violate the attachment ,even though its not possible for a clinician to identify the most coronal extent of the junctional epithelium. Wilson and Majnard cautioned against extending restorations so far subgingivally that the attachment complex is damaged.They stated that “Some distance of unprepared tooth structure should remain between the finished line and junctional epithelium and this distance should be ideally 0.5mm.
  15. 15. Location of restorative margins:-  Supragingival  Equigingival  Subgingival
  16. 16. Supragingival margin:-  Least traumatic to the soft tissues  Margin placement…  Easily finished  Impressions are easily made  Most accessible for cleansing  Restorations can be easily evaluated
  17. 17. Subgingival margins:-  Clinical crown is short…  Old restorations with subgingival margin  Esthetic appearance  Caries, abrasion or erosion…  Root sensitivity Disadvantages of subgingival margins  Plaque retention…  Margin placement…  Mechanical irritation…  Violation of biological width  Soft tissue injury…
  18. 18. Criteria for subgingival margin placement:-  Emergence profile  Margins are closed and properly finished  Adequate band of attached gingiva  Margin should not violate the biological width
  19. 19. Crown lengthening procedures:- It’s a procedure similar to the apical repositioning of the flap with concomitant osteoplasty. Indications –  Short clinical crown  Restoration impinge on the biological width  Hopless teeth with extensive subgingival caries, subgingival fracture and root perforation Crown lengthening procedure -  Surgical methods  Orthodontic method
  20. 20. D.A.Felton in 1991 conducted a study on “Effects of in vivo crown margin discrepancies on the periodontal health” in his study he strongly supported the placement of supragingival margins for artificial crowns and FPD’s. William.G.Reeves in his review article concluded that more supragingivally a restorative margin is placed,the less chance that the margin will contribute to gingival inflammation.
  21. 21. MARGINAL GEOMETRY OR FINISH LINE CONFIGURATION shape and bulk of the restorative material in the margin of the restoration Marginal adaptation Degree of seating of restoration Guidelines for evaluation for margin design :  Ease of preparation without overextension or unsupported enamel  Ease of identification in the impression and on the die  Sufficient bulk of the material  Conservation of the tooth structure  A distinct boundary to which wax pattern can be finished
  22. 22. Finish lines  The finish line is the peripherel extension of a tooth preparation  The most important consideration in selecting a cervical margin design is its ability to consistently and predictably provide excellent marginal integrity.  Knife edge  Chisel edge  Chamfer  Shoulder  Sloped shoulder  Shoulder with bevel  Radial shoulder  Heavy chamfer
  23. 23. KNIFE EDGE OR FEATHER EDGE Advantages  Conservation of the tooth structure  Permits an acute margin of the metal Disadvantages:  The axial reduction may fade out  Thin margin may be difficult to accurately wax and cast  More susceptible to distortion  Results in overcontouring Indications:  Not recommended
  24. 24. CHISEL EDGE Variation of feather edge Larger angle between the axial surface and unprepared tooth structure Associated with excessively tapered preparation Historic advantage-impression making with rigid impression compound in coper bands Indications: Lingual surface of mandibular posterior teeth The surfaces towards which tooth has tilted Cementum
  25. 25. Chamfer It’s a finish line design for tooth preparation in which gingival aspect meets the external axial surface at an obtuse angle Advantages  It provides distinct margin  Adequate bulk to the restoration  Easier to control  Exhibits least stresses  Disadvantage -Care needed to avoid unsupported lip of enamel Indications:  Cast metal restorations  Lingual margin of the metal ceramic crowns
  26. 26. Shoulder  Bulk of the restoration  Wide ledge provides resistance to occlusal forces  Provides space for healthy restorative contours  Maximum esthetics  It offers resistance against distortion during processing Disadvantages:  Less conservative of tooth structure  The sharp 900 internal line angle… Indications:  All ceramic restorations  Facial magins of metal-ceramic crowns
  27. 27. SLOPED SHOULDER Cavosurface margin - 120° Reduces the possibility of leaving unsupported enamel and yet leaves sufficient bulk to allow thinning of the metal framework to a knife edge for acceptable aesthetics. Indicated for facial margin of metal ceramic crowns
  28. 28. SHOULDER WITH BEVEL Removes unsupported enamel, allows finishing of metal Disadvantage – extends the preparation into the sulcus if used on apical margin Indications – -facial margins of maxillary partial coverage restorations - Inlay and onlay margin -Shoulder is already present because of destruction by caries or presence of previous restorations
  29. 29. RADIAL SHOULDER: Shoulder with rounded internal line angle Stress concentration is less in the tooth structure HEAVY CHAMFER Internal line angle is large radius rounded Provides better support for a ceramic crown than chamfer, but it is not as good as shoulder Easier to prepare than shoulder
  30. 30. MARGINAL ADAPTABILITY The restoration can survive in the biological environment of the oral cavity only if the margins are closely adapted to the cavosurface finish line Junction between a cemented restoration and tooth…. More accurate the adaptation – lesser the chance of recurrent caries A well designed preparation should have a smooth and even margins Rough, irregular and stepped junctions ……
  31. 31. TO BEVEL OR NOT TO BEVEL4 Controversial Trignometric analysis- more acute the bevel- -lesser the marginal discrepancy -lesser the seating with cement So, bevel improves the marginal adaptation but reduces the seating of restoration Empirical clinical results dictate that acute margin of metal should continue to be used on metal restorations but that angle should be in 30-45°
  32. 32. OCCLUSAL CONSIDERATIONS Occlusal scheme should not be traumatic Tooth preparation should allow sufficient space for developing a functional occlusal scheme in the finished restoration Supraerupted or tilted teeth – reduce the teeth Sometimes even the endodontic treatment is necessary to make enough room Careful judgment is needed and diagnostic tooth preparation and waxing procedures are essential to determining the exact amount of reduction required to develop an optimum occlusion
  33. 33. PREVENTING THE TOOTH FRACTURE The likelihood that a restored tooth will fracture can be lessened if the tooth preparation be designed to minimize the potentially destructive stresses Inlay- greater potential for fracture Onlay – lessens the chance of fracture Complete crown –greatest protection against fracture
  35. 35. RETENTION FORM DEFINITION-The feature of a tooth preparation that resists dislodgement of a crown in a vertical direction or along the path of placement. In practice retention and resistance are closely related and they are not always clearly distinguishable. Only dental caries and porcelain failure outrank lack of retention as a cause of failure of crown and FPD
  36. 36. FACTORS AFFECTING RETENTION Magnitude of dislodging forces Geometry of tooth preparation -Taper -Surface area -stress concetration -Type of preparation and secondary retentive features Roughness of surfaces being cemented Materials being cemented Type of luting agent Film thickness of luting agent
  37. 37. MAGNITUDE OF DISLODGING FORCES small compared to those that tend to seat or tilt it  by pulling the FPD with floss under the connectors,  when exceptionally sticky food is eaten Depends on the stickiness of food and the surface area and texture of the restoration being pulled
  38. 38. GEOMETRY OF TOOTH PREPARATION Essential elements of retention:- opposing vertical surfaces in same preparation. Path of insertion Area under shear stress Freedom of displacement FACTORS Taper Surface area stress concetration Type of preparation and secondary retentive features
  39. 39. Opposing vertical surfaces in same preparation: 1) External surfaces:- eg. Buccal & lingual walls of full veneer crown. An extracoronal restoration is an example of veneer, or sleeve retention. Scan0001.jpg
  40. 40. 2) Internal surfaces:- eg. Buccal & lingual walls of the proximal box of a proximo-occlusal inlay. An intracoronal restoration resists displacement by wedge retention.
  41. 41. PATH OF INSERTION:- It is an imaginary line along which the restoration will be placed onto or removed from the preparation. It is of special importance when preparing teeth to be fixed partial denture abutments, since the paths of all the abutment preparations must parallel each other. Surveying visually, since it is the primary means of insuring that the preparation is neither undercut nor over-tapered.
  42. 42. The path of insertion must be considered in two dimensions:- - Faciolingually - Mesiodistally The faciolingual orientation of the path can affect the esthetics of metal-ceramic or partial veneer crowns.
  43. 43. The mesiodistal inclination of the path must parallel the contact areas of adjacent teeth. So if path is inclined mesially or distally, the restoration will be held up at the proximal contact areas & be “locked out”.
  44. 44. Area under Shear Stress:  Most important feature for retention is that the total surface area of cement which will experience shearing rather than tensile stress.  To achieve this the preparation must have opposing walls nearly parallel to each other.
  45. 45. To obtain the greatest area of cement under shear stress, the direction in which a restoration can be removed must be limited to one path.
  46. 46. FREEDOM OF DISPLACEMENT:- Retention is improved by geometrically limiting the numbers of paths along which a restoration can be removed from the tooth preparation. Maximum retention is achieved when there is only one path.
  47. 47. TAPER The axial walls of the preparation must taper slightly to permit the restoration to seat. i.e. 2 opposing external walls must gradually converge – ANGLE OF CONVERGENCE. 2 opposing internal surfaces of the tooth structure must diverge occlusally – ANGLE OF DIVERGENCE.  The relationship of one wall of a preparation to the long axis of that preparation is the INCLINATION of that wall.
  48. 48. The more nearly parallel the opposing walls of a preparation, the greater should be the retention. Most retentive preparation should be one with parallel walls, but the parallel walls are impossible to create in the mouth without producing preparation undercuts. An UNDERCUT is defined as a divergence between opposing axial walls, or wall segments, in a cervical- occlusal direction.
  49. 49. Tooth preparation taper should be kept minimal because of its adverse effects on retention. ed taper ed retention ed taper ed retention.  Jogensen said as retention decreases taper increases.
  50. 50. Ideal taper should be within the range of 2-6.5°. A taper of 6° have been proposed as being clinically achievable while affording adequate retention.
  51. 51. William A. Kent, Herbert T. Shillingburg, Manville G. Duncamon, Quintessence Int. vol. 19, 1988. - Conducted a study to evaluate the degree of taper of 418 dies of preparations, cut over a time span of 12 years. The dies were categorized by location and type of preparation. The degree of taper of six opposing sets of surfaces were measured and compared. They concluded the mean of tapers of preparations ranged from 8.6 – 26.6°. Internal features such as grooves and boxes have a lower degree of taper compared with external axial walls and preparations in the anterior segment have a lower degree of taper than in posterior segment.
  52. 52.  Jeffrey Nodlander, Dennis Weir, Warren Stoffer and Shigaro Ochi, JPD, vol. 60, 1988. – Conducted a study to measure the convergence angles of a full coverage preparations performed in a clinical environment. Teeth were prepared by the participants to attempt a 4-10° convergence angle. The convergence angles of all preparations were determined by projecting the faciolingual and mesiodistal silhouttes of the dies with an overhead projector. They concluded that, the ideal convergence angle of 4-10° is seldom achieved. The convergence angle for mandibular preparations were greater than maxillary. Auxillary retention should be used in molar region because these preparations were found to have larger convergence angle.
  53. 53. SURFACE AREA Provided the restoration has limited path of withdrawal, Greater the surface area of a preparation, greater is its retention. Length….. Crowns with long axial walls are more retentive… Molar crowns are more retentive than premolar crowns of same taper
  54. 54. STRESS CONCENTRATION If line angle between axial and occlusal surface is sharp, it leads to concentration of stresses around that junction Induced stresses exceeds the strength of the cement Leads to cohesive failure of cement Rounding the internal line angles reduces the stress concentration and thus increases the retention of restoration
  55. 55. TYPE OF RESTORATION AND SECONDARY RETENTIVE FEATURES Full veneer crown has excellent retention when compared to partial veneer crown because reducing the path of insertion to a narrow range.
  56. 56. If axial wall left unveneered, retention is achieved by substituting a grooves, boxes or pinholes for the missing wall. • Secondary retentive features doesnot significantly affect the retention because the surface area is not increased significantly. • But where these features limits the path of withdrawal, retention is
  57. 57. Surface roughness:  Adhesion of dental cements depends primarily on projections of the cement into microscopic irregularities.  Jorgensen found retention of castings cemented with ZnPO4 cement on test dies with a 10° taper to be twice as great on preparations with 40µm scratches than 10µm.  Retention increases when restoration is roughened or grooved.  Failure rarely occurs at the cement tooth interface. So roughening tooth preparation hardly influences retention.
  58. 58. Materials being cemented:  Retention is affected by both the casting alloy and the core material.  More reactive the alloy is more adhesion.  Base metal alloys are better retained than less reactive high gold content metals. Type of luting agent:  Studies show that adhesive resin cements are more retentive than compared to conventional ZnPO4 and GIC cements.
  59. 59. Film thickness of the luting agent. Conflicting evidences Studies showed that increased thickness of the cement film will have some effect on a restoration. This may be important if a slightly oversized casting is made using die spacers.
  60. 60. Stanley G. Vermilyea, Michael J. Kuppler, Eugen F. Hugger, JPD, 50, 1983. – Conducted a study to determine the influence of die relief agent on the retention of cast restorations using 3 cementing mediums. They concluded that forces required to dislodge unspaced castings were higher when ZnPO4 as luting medium. Die relief agents resulted in a 32% reduction in the dislodging forces. Steven M. Carter, Peter R. Wilson, IJP, vol. 9, 1996.- Conducted a study to see the effect on die spacing on pre and post cementation crown removal and crown elevation. Different layers of die spacer was used. They observed that the force required to remove a crowns before cementation decreased with increased layers of die spacers. Following cementation the mean crown elevation decreased and the removal force is increased.
  61. 61. RESISTANCE FORM  The features of a tooth preparation that enhance the stability of restoration and resist dislodgement along an axis other than the path of placement (GPT).  It prevents dislodgement of a restoration by forces directed in an apical, oblique or horizontal direction.  The geometric configuration of tooth structure must place the cement in compression to provide the necessary resistance.  Lateral forces tend to displace the restoration by causing rotation around gingival margin  Rotation is prevented by any areas of the tooth preparation that are placed in compression and are called as RESISTANCE AREA
  62. 62. FACTORS AFFECTING RESISTANCE FORM Magnitude and direction of dislodging forces Leverage Length of the preparation Width of the preparation Taper Type of preparation Rotation about vertical axis Physical properties of luting agent
  63. 63. Magnitude and direction of dislodging forces Normal occlusion - axially directed forces Habits (pipe smoking and bruxing) – large oblique forces to restoration Resistance decreases in following order: - normal occlusion - habits - eccentric interferences - anterior guidance
  64. 64. LEVERAGE AND RESISTANCE:  Leverage occurs when the line of action of a force passes out side the supporting tooth structure.  If the force passes within the margin of a crown no tipping of the restoration when compared to the line of action passing outside the margins of the restoration
  65. 65. Forces are outside the margin in the following cases: - wide occlusal table of restoration - crowns on tipped teeth - retainers for cantilever bridge - force at an oblique angle Fulcrum point – the point on margin that lies closest to the line of action Lever arm – the closest distance between line of action and fulcrum Torque - force × lever arm In equilibrium, this torque is balanced by the sum of all the resisting forces ( tensile, shear, compressive) The farther these resisting forces lie from the fulcrum, greater is their mechanical advantage
  66. 66. If a line drawn from the center of rotation perpendicular to the cement film on the opposite wall of the preparation the point where the line intersects the cement film is known as tangent point.
  67. 67. If the tangent points of all the arcs of rotation around a given axis are connected they form the tangent line. The area above the tangent line is resisting area. To have effective resistance the tangent line should extend atleast halfway down the preparation.
  68. 68. Preparation length and resistance: -Shortening of preparation will reduce the resistance area. -In short crown lifting force is small when compared to long crown. -Small restoration is less likely to fail through tipping than long restoration (on preparation of equal length).
  69. 69. Resistance and tooth width:  A wider preparation has a greater retention but a narrow tooth can have greater resistance to tipping. Because of smaller diameter a tangent line falls low on the wall opposite to axis of rotation. Resulting in a large resisting area. Weak resistance can be enhanced by placing vertical grooves/ boxes/
  70. 70. A grooved lingual wall must be distinct and perpendicular to the axial wall. So U- shaped grooves or flared boxes provides more resistance than V-shaped ones
  71. 71. Taper and Resistance:  More tapered a preparation less is resistance. No taper the resisting area cover half the axial wall. Ideal taper < ½ the axial wall. Over tapered small resisting area near the occlusal surface.
  72. 72. William W. Dodge, Roger M. Weed, Ramon J. Baez and Richard N. Buchanan (Quintessence Int, Vol.3 1985) – conducted a study to compare the effect of varying the convergence angle on retention and resistance in complete veneer crown preparations. 15 stainless steel dies were machined with 10°, 16° and 22° taper. They concluded that resistance is more sensitive to changes in a convergence angle than retention form. There is no significant difference in retention values between preparations with 10° and 16°. 16° of occlusal convergence was considered adequate for retention and resistance.
  73. 73.  Permissible taper of a preparation is directly proportional to height : width ratio.  Taper that permit an effective resisting area for a preparation in which height equals width is double than in a preparation where height is only half width.
  74. 74. Rotation around a vertical axis: A partial veneer crown which has no grooves offer little resistance to rotation. Axial symmetry of a full veneer crown preparation may allow rotation of the restoration.
  75. 75. TYPE OF PREPARATION Partial coverage restoration may have less resistance than a complete crown because it has no buccal resistance area PHYSICAL PROPERTIES OF LUTING AGENT Resistance to deformation is affected by physical properties of the luting agent, such as compressive strength and modulus of elasticity Adhesive resin >GIC > ZnPO4> Polycarboxylate > ZOE
  76. 76. STRUCTURAL DURABILITY A restoration must have sufficient strength to prevent permanent deformation during function DEFINITION- “The ability of a restoration to withstand destruction due to external forces is known as structural durability”.
  77. 77. Factors affecting structural durability 1. Adequate tooth reduction. - occlusal reduction - functional cusp bevel - axial reduction 2. Alloy selection. 3. Metal-ceramic framework design. 4. Margin design
  78. 78. ADEQUATE TOOTH REDUCTION OCCLUSAL REDUCTION:- An important feature for providing adequate bulk of metal & strength to the restoration is occlusal clearance. - Occlusal thickness varies with different restorative materials Gold alloys – 1.5mm (FC) & 1mm(NFC) Metal-ceramic crowns- 1.5-2mm(FC)&1-1.5mm(NFC) All-ceramic crowns – 2mm of clearance on preparation
  79. 79. - Firstly, opposing occlusal equilibration is to be achieved eg. Plunger cusps to be rounded. - Round line and point angles, avoid deep grooves in the center of the occlusal surfaces to prevent stress concentration and to distribute the forces over a larger surface area. - Uniform and planar occlusal reduction.
  80. 80. This ensures: -sufficient occlusal clearance - preservation of tooth structure - gives rigidity to crown because of CORRUGATED EFFECT OF PLANE OCCLUSAL OFFSET can be given on posterior partial veneer crown preparation to provide space for a TRUSS of metal to form reinforcing strap INCISAL OFFSET can be given on anterior partial veneer crown preparation to provide space for metal that helps to strengthen the lingual-incisal margin
  81. 81. FUNCTIONAL CUSP BEVEL:- It is an integral part of the occlusal reduction. A wide bevel on the lingual inclines of the maxillary lingual cusps & the buccal inclines of the mandibular buccal cusps provides space for an adequate bulk of metal in an area of heavy occlusal contact.
  82. 82. If a wide bevel is not placed on the functional cusp, several problems may occur : - If the crown is waxed & cast to normal contour it can cause a thin area or perforation in the casting. - To prevent this the crown may be waxed to optimal thickness resulting in overcontouring & poor occlusion.
  83. 83. - If an attempt is made to obtain space for an adequate bulk in a normally contoured casting without a bevel, it will result in over inclination of the buccal surface which will destroy excessive tooth structure while lessening retention.
  84. 84. AXIAL REDUCTION:- It plays an important role in securing space for an adequate thickness of restorative material. Inadequate axial reduction can cause thin walls & a weak restoration subjected to distortion or a bulbous, overcontoured restoration which will strengthen the restoration but may have a disastrous effect on periodontium.
  85. 85. Other features that provide space for metal and improve the rigidity & durability of the restoration are: The offset, the occlusal shoulder, the isthmus, the proximal groove & the box.
  86. 86. Selection of the alloy: -It is essential that there be sufficient clinical evidence of superiority, before selecting a particular material. -Type I and II gold alloys…
  87. 87. Factors considered when selecting an alloy: 1.Intended use: Traditionally alloys for casting were classified on the basis of their intended use- -Type I: Simple inlays -Type II: Complex inlays -Type III: Crowns and fixed partial dentures -Type IV: Removable partial dentures and pin ledges. Porcelain: metal-ceramic alloys. 2. Physical properties: FDI (1965) classified casting alloys according to their physical properties as: Type I: Soft Type II: Medium Type III: Hard Type IV: Extra-hard
  88. 88. 3. Color: The patients view on the subject should be sought if the metal will be visible in the mouth; otherwise the color is irrelevant. 4. Composition: The percentage composition by weight of the main ingredients must be mentioned. 5. Cost. 6. Clinical performance: A) Biologic properties -Gingival irritation. -Recurrent caries. -Plaque accumulation. -Allergies. B) Mechanical properties: -Wear resistance and strength. -Marginal fit. -Ceramic bond failure. -Connector failure. -Tarnish and
  89. 89. 7. Laboratory performance: Factors like casting accuracy, surface roughness, strength, metal-ceramic bond strength should be considered.
  90. 90. Choice of material: Gold: Indications- 1.In situations of severe occlusal stress. 2.Following endodontic treatment of posterior teeth. 3.Full or partial coverage of posterior teeth where there has been significant loss of coronal dentin. 4.For restoration of adjacent or opposing teeth to avoid problems arising from use of dissimilar metals. Contraindications: -Aesthetics --Cost
  91. 91. Porcelain (Ceramic): Indications: - Large inadequate restorations on the anterior teeth provided there is enough tooth substance. - Severely discolored teeth. - Over an existing post and core substructure. Contraindications: -Teeth with short clinical crown -Edge to edge occlusion -Teeth which do not allow ideal preparation form to support the porcelain.
  92. 92. Metal ceramic: Indications: - Esthetics - Failure of porcelain jacket crowns. -Posterior teeth where esthetics is necessary and partial coverage gold crowns are contraindicated Contraindications: -Young patients at risk of pulp being exposed -large pulp chamber -Traumatic occlusion (heavy occlusal forces) , where wear of opposing occlusal surfaces is expected.
  93. 93. Metal ceramic framework: -A metal occlusal contact requires 1-1.5mm of reduction -A porcelain contact requires 2mm of reduction -Occlusal contacts need to be 1.5-2.0mm from the porcelain- metal junction -The substructure must support an even thickness of the porcelain veneer (1mm minimum and 2mm maximum). -The minimum thickness of the metal is 0.2-0.3mm. -Cut-back: porcelain-metal junction should be 90° or greater. -Metal should preferably be in the area of the centric stop to enhance the durability of the restoration.
  94. 94. MARGIN DESIGN - distortion of restoration margin is prevented by designing the preparation outline to avoid occlusal contact in this area - tooth reduction should provide sufficient room for bulk of metal at the margin to prevent distortion
  95. 95. ESTHETIC CONSIDERATIONS METAL-CERAMIC RESTORATION - Usually poor appearance is due to insufficient porcelain thickness - In addition, the labial margin of metal ceramic crown is not always accurately placed - To correct all these deficiencies, certain principles are recommended during tooth preparation FACIAL REDUCTION - Adequate thickness of porcelain is needed to create a sense of color depth and translucency - Adequate reduction  sufficient bulk of porcelain for appearance and metal for strength - Minimum reduction of 1.5 mm is required - Shade problems incisal and cervical 3rd of restoration (here direct light reflection from the opaque layer can make the restoration appear very noticeable
  96. 96. - Opaque porcelain generally have a different shade from body porcelain, they often need to be modified with special stains in these areas. - with very thin teeth like mandibular incisors…… INCISAL REDUCTION - Incisal edge  no metal backing. So, it can be made with a translucency similar to that of natural tooth structure - reduction  2 mm - Excessive reduction – reduces retention and resistance PROXIMAL REDUCTION - Esthetics depends on exact location of the metal ceramic junction in complete restoration - Proximal surface of anterior tooth will look most natural if they are restored as the incisal edges without metal backing. - In FPD, connectors makes it impossible
  97. 97. LABIAL MARGIN PLACEMENT - Should follow the contour of gingiva - High lip line  sub gingival margin no discoloration of root surface – supragingival porcelain margin - Low lip line  metal supragingival collar - Metal collars can be hidden below the gingival crest
  98. 98. PARTIAL COVERAGE RESTORATION - No restorative material can achieve the appearance of intact tooth enamel - Esthetic depends on accurate placement of potentially visible facial and proximal margins PROXIMAL MARGIN - Mesial margin  buccal to contact area - Distal margin  can be extended beyond the contact point - Tooth preparation angulation long axis of posterior teeth and incisal 2/3rd of facial surface of anterior
  99. 99. FACIAL MARGIN - Maxillary  margin should be extended just beyond the occlusofacial line angle - If buccal margin is correctly shaped following the original cuspal contour, it will not reflect light to an observer and tooth will appear merely a little shorter than normal - Mandibular  metal display is unavoidable because …. - If unacceptable to the patient  metal ceramic restoration with porcelain coverage on occlusal surface
  100. 100. - Anterior partial coverage  facial margin is extended between highest contour of the incisal edge and incisolabial line angle
  102. 102.
  103. 103.
  104. 104.
  105. 105.
  106. 106.
  107. 107.
  108. 108.
  109. 109. ENDODONTICALLY TREATED TEETH Conservation of tooth structure Retention form Resistance form
  110. 110. Conservation of tooth structure PREPARATION OF CANAL Only minimum tooth structure should be removed from the canal Root canal should be enlarged only enough to enable the post to fit accurately yet passively while ensuring strength and retention
  111. 111. PREPARATION OF CORONAL TISSUE Coronal tooth structure should be saved as much as possible because….. Ferrule: - Extension of the axial wall of the crown apical to the missing tooth structure provides what is known as a ferrule. - helps to bind the remaining tooth structure together, preventing root fracture during function
  112. 112. Retention form  Preparation geometry  Post length  Post diameter  Post surface texture  Luting agent
  113. 113. Resistance form  Stress distribution  Rotational resistance
  114. 114. SUMMARY AND CONCLUSION Proper attention should be given during margin placement and the principle of “Do no harm” to the soft tissues should be followed. All preparations require the incorporation of factors to prevent the dislodgement of restoration by functional stresses. If too much emphasis is given on any one of the principles then the success of the procedure may be limited by a lack of consideration of the other factors. An analysis of these principles and factors should enable the dentist to effectively apply them during the design of any preparation.
  115. 115. REFERENCES 1. Rosensteil (2001) Contemporary Fixed Prosthodontics. 3rd edition. 2. Shillingburg (1981) Fundamentals of Fixed Prosthodontics. 2nd edition. 3. Tylman (1989) Theory & Practice of Fixed Prosthodontics. 8th edition. 4. Shillingburg, Richard Jacobi, S.E. Brackett. (1987) Fundamentals of Tooth Preparations for Cast metal and Porcelain restorations. 5. Carranza’s “ Clinical periodontology” 9th edition. 6. Anusavice “Phillips science of dental materials” 10th edition
  116. 116. 7. William A. Richter “Relationship of crown margin placement to gingival inflamation” JPD. 30: 156-161,1973 8. W. Kent, H.T. Shillingburg, M.G. Duncansson: Taper of clinical preparation for cast restoration. Quint. Int. 1988; 19: 339-345. 9. J. Nordlander, D. Weir, W. Stoffer, S. Ochi : The taper of clinical preparations for fixed prosthodontics. JPD, 1988; 60: 148-151. 10.S.M. Carter, Peter Wilson: The effect of die-spacer on crown retention. IJP, 1996, 9: 21-29. 11.S. Vermilyea, M.J. Buffler, E. Maget : The effect of die relief agent on the retention of full coverage castings. 1983; 50: 207-210.
  117. 117.