1. 1. DR ACHYUT SINHA 1ST YEAR PG 1
2. 2. INTRODUCTION 2
3. 3. PARTS OF FIXED PARTIAL DENTURE RETAINER ABUTMENT PONTIC CONNECTOR
4. 4. Teeth do not posses the regenerative ability found in most other tissues. Therefore, once enamel or dentin is lost as a result of caries, trauma, or wear, restorative materials must be used to reestablish 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 prosthodontic treatment. 4
5. 5. The principles of tooth preparation may be divided into three broad categories: 1. Biologic considerations, which affect the health of the oral tissues 2. Mechanical considerations, which affect the integrity and durability of the restoration 3. Esthetic considerations, which affect the appearance of the patient (ROSENSTIEL) 5
6. 6. BIOLOGIC CONSERVATION OF TOOTH STRUCTURE AVOIDANCE OF OVERCONTOURING SUPRAGINGIVAL MARGINS HARMONIOUS OCCLUSION PROTECTION AGAINST FRACTURE MECHANICAL RETENTION FORM RESISTANCE FORM DEFORMATION ESTHETIC MINIMUM DISPLAY OF METAL MAXIMUM THICKNESS OF PORCELAIN PORCELAIN OCCLUSAL SURFACE SUBGINGIVAL MARGINS 6
7. 7. PRESERVATION OF TOOTH STRUCTURE RETENTION & RESISTANCE FORM STRUCTURAL DURABILITY MARGINAL INTEGRITY PRESERVATION OF THE PERIODONTIUM (SHILLINGBURG) 7
8. 8. BIOLOGIC CONSIDERATIONS 8
9. 9. PREVENTION OF DAMAGE DURING TOOTH PREPARATION -ADJACENT TEETH -SOFT TISSUES -PULP 9
10. 10. 1. ADJACENT TEETH :- Iatrogenic damage to adjacent tooth Plaque retention in damaged area Increased caries 10
11. 11. PREVENTION:- Use of metal matrix band around adjacent tooth structure. Use of proximal enamel of tooth being prepared for protection of adjacent structure. 11
12. 12. Damage to adjacent teeth is prevented by making a thin "lip" of enamel as the bur passes through a proximal contact. 12
13. 13. 2. SOFT TISSUE DAMAGE PREVENTION By careful retraction with Aspirator tip Mouth mirror Flanged saliva ejector 13
14. 14. Mouth mirror protecting the soft tissues during tooth preparation. 14
15. 15. 3. PULP :- Heat produced during tooth reduction/chemical irritation/ Microorganisms Cause reversible/irreversible pulp hyperemia Which may lead to pulp damage 15
16. 16. PREVENTION 1. Evaluation of radiograph 2. Proper cooling system must be used with water spray 16
17. 17. Causes of Injury 1. Temperature , 2. Excessive Pressure , 3. Rotational speed , 4. Chemical action of restorative material, 5. Bacterial Action 17
18. 18. TEMPERATURE: Heat is generated by friction between rotary instrument and surface being prepared. Other factors which may lead to increase in temperature -Excessive pressure -Higher rotational speed -Type,Shape,Condition of cutting instrument 18
19. 19. PREVENTION: Use of water spray. Use of slow speed hand piece or hand instrument. Care should be taken while preparing grooves or pin holes, as coolant cannot reach the cutting edge of bur With a high-speed handpiece, a feather-light touch allows efficient removal of tooth material with minimal heat generation. 19
20. 20. CHEMICAL ACTION: Chemical action of certain DENTAL MATERIALS (bases, restorative resins, solvent) may cause pulpal damage particularly when applied to freshly cut dentin. PREVENTION -Use of Cavity varnish or Dentin bonding agent will form effective barrier. 20
21. 21. BACTERIAL ACTION: Microleakage may cause bacteria to enter the dentin and may cause pulp damage. PREVENTION -Use of zinc phosphate cement, as it provide anti- Bacterial effect to vital dentin which seems to resist infection. NOTE ;all carious dentin should be removed before placing a restoration that will serve as a foundation for a fixed prosthesis. 21
22. 22. CONSERVATION OF TOOTH STRUCTURE USE OF PARTIAL COVERAGE CROWN RATHER THAN COMPLETE COVERAGE. PREPARATION OF TEETH WITH MINIMUM PRACTICAL CONVERGENCE ANGLE (taper) B/W AXIAL WALLS 22
23. 23. PREPARATION OF THE OCCLUSAL SURFACE SO REDUCTION FOLLOWS THE ANATOMIC PLANES TO GIVE UNIFORM THICKNESS IN THE RESTORATION. PREPARATION OF AXIAL SURFACES SO TOOTH STRUCTURE IS REMOVED EVENLY; IF NECESSARY,TEETH SHOULD BE ORTHODONTICALLY REPOSITIONED 23
24. 24. 24
25. 25. SELECTION OF CONSERVATIVE MARGIN. AVOIDANCE OF UNNECESSARY APICAL EXTENSION OF THE PREPARATION. 25
26. 26. CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH AXIAL REDUCTION INSUFFICIENT AXIAL REDUCTION OVERCONTOURED RESTORATION PERIODONTAL DISEASE/DENTAL CARIES. OCCLUSAL REDUCTION Inadequate occlusal reduction may result in occlusal dysfunction. MARGIN LOCATION Poor margin placement such as in the area of occlusal contact, may lead to chipped enamel or cusp fracture. 26
27. 27. SUFFICIENT TOOTH STRUCTURE MUST BE REMOVED TO ALLOW THE DEVELOPMENT OF CORRECTLY FORMED AXIAL CONTOURS IN INTERPROXIMAL AND FURCATION AREAS OF POSTERIOR TEETH. 27
28. 28. MARGIN PLACEMENT MARGIN OF PREPARATION SHOULD BE SUPRAGINGIVAL ADVANTAGES- -CAN BE EASILY FINISHED -EASIER TO PREPARE ACCURATELY WITHOUT TRAUMA TO THE SOFT TISSUE -MORE EASILY KEPT CLEAN -IMPRESSIONS ARE MORE EASILY MADE -RESTORATIONS CAN BE EASILY EVALUATED AT RECALL APPOINTMENTS 28
29. 29. If finish line is given SUB-GINGIVALLY it will result in :- Gingival inflammation Loss of alveolar crest height Periodontitis Periodontal pocket 29
30. 30. SUBGINGIVAL MARGINS (INDICATIONS) DENTAL CARIES , CERVICAL EROSION ,RESTORATIONS EXTEND SUBGINGIVALLY OR WHEN CROWN LENGHTHENING PROCEDURE IS NOT INDICATED PROXIMAL CONTACT AREA EXTEND TO THE GINGIVAL CREST. ADDITIONAL RETENTION IS NEEDED. THE MARGIN OF METAL-CERAMIC CROWN IS TO BE HIDDEN BEHIND THE LABIOGINGIVAL CREST. 30
31. 31. Root sensitivity cannot controlled by conservative procedure. Modification of axial contour is indicated. 31
32. 32. MARGINAL INTEGRITY: The restoration can survive in the biological environment of the oral cavity The configuration of the preparation finish line dictates the shape & bulk of restorative material in the margin of the restoration 32
33. 33. Marginal Integrity.. Incompetent Adaptation Gingival impingement Periodontal deterioration OVER EXTENSION 33
34. 34. OVER EXTENSION 34
35. 35. Marginal Integrity.. Incompetent Adaptation UNDER EXTENSION Sensitivity Accumulation of Debris Plaque formation Marginal leakage 35
36. 36. UNDER EXTENSION 36
37. 37. UNDER EXTENSION 37
38. 38. Marginal Integrity.. Incompetent Adaptation THICK Accumulation of debris Plaque formation Periodontal deterioration Susceptibility to root 38
39. 39. Incompetent Adaptation THICK 39
40. 40. Incompetent Adaptation Marginal Integrity.. OPEN Sensitivity Accumulation of debris Marginal leakage Periodontal deterioration 40
41. 41. Incompetent Adaptation 41
42. 42. MARGINAL ADAPTATION: 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. The more accurately the restoration is adapted to the tooth, the lesser the chance of recurrent caries or periodontal disease . 42
43. 43. ACCEPTABLE MARGINAL ADAPTATION FOR CASTING - 10m FOR PORCELAIN - 50m (BELSER 1985) A WELL DESIGNED PREPARATION HAS A SMOOTH AND EVEN MARGIN 43
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45. 45. MARGIN GEOMETRY (GUIDELINES) EASE OF PREPARATION WITHOUT OVEREXTENSION OR UNSUPPORTED ENAMEL EASE OF IDENTIFICATION IN THE IMPRESSION AND ON THE DIE. A DISTINCT BOUNDARY TO WHICH THE WAX PATTERN CAN BE FINISHED. SUFFICIENT BULK OF MATERIAL CONSERVATION OF TOOTH STRUCTURE. 45
46. 46. FINISH LINE The peripheral extension of a tooth preparation The planned junction of different material The terminal portion of a prepared tooth Line of demarcation 46
47. 47. Finish line.. Types of finish lines Featheredge Chisel edge Chamfer Shoulder Sloped shoulder Shoulder with bevel 47
48. 48. Finish line.. FEATHEREDGE Advantage Conservative of tooth structure Disadvantage * Does not provide sufficient bulk Indication * Not recommended 48
49. 49. CHISEL EDGE Advantage * Conservative of tooth structure Disadvantage * Location of margin difficult to control Indication * On tilted teeth Finish line.. 49
50. 50. CHAMFER Advantage * Distinct margin, adequate bulk Disadvantage *care needed to avoid unsupported lip of enamel Indication * Cast metal restoration, lingual margin of metal-ceramic crowns Finish line.. 50
51. 51. CHAMFER FINISH LINE Full Veneer Crown Preparation 51
52. 52. BIOMECHANICS OF CROWN & BRIDGE HEAVY CHAMFER LINE All Ceramic Crown 52
53. 53. Finish line.. SHOULDER Advantage * Bulk of restorative material Disadvantage * Less conservative of tooth structure Indication * Facial margin of metal ceramic crowns, complete ceramic crowns 53
54. 54. SHOULDER FINISH LINE All ceramic crown (PJC) 54
55. 55. Finish line.. SLOPED SHOULDER Advantage Bulk of restorative material, Advantage of bevel Disadvantage * Less conservative of tooth structure Indication * Facial margin of metal ceramic crowns 55
56. 56. SHOULDER WITH BEVEL Advantage Bulk of restorative material, removes unsupported enamel , allows finishing of the metal Disadvantage * Less conservative of tooth structure, Extends preparation apically Indication * Facial margin of posterior metal ceramic crowns with supragingival margins Finish line.. 56
57. 57. OCCLUSAL CONSIDERATION TOOTH PREPARATION SHOULD ALLOW SUFFICIENT SPACE FOR DEVELOPING FUNCTIONAL OCCLUSAL SCHEME IN THE FINISHED RESTORATION. CONSIDERABLE REDUCTION IS NEEDED TO COMPENSATE FOR THE SUPRAERUPTED ABUTMENT TEETH 57
58. 58. A, Nonreplacement of missing teeth has led to supraocclusion and a protrusive interference (arrow). B, Teeth reduced with the help of trial tooth preparations and diagnostic waxing. C, Restorations with anterior guidance. 58
59. 59. PREVENTING TOOTH FRACTURE: RESTORED TOOTH FRACTURE CAN BE LESSENED IF TOOTH PREPARATION IS DESIGNED TO MINIMIZE POTENTIALLY DESTRUCTIVE STRESSES. PROVIDING A CUSPAL COVERAGE RESTORATION RATHER THAN AN INLAY LESSENS THE CHANCE OF FRACTURE A COMPLETE CROWN IS OFTEN A BETTER SOLUTION,BECAUSE IT OFFERS THE GREATEST PROTECTION AGAINST TOOTH FRACTURE, TENDING TO HOLD THE CUSP OF THE TOOTH TOGETHER. 59
60. 60. A, An intracoronal cast restoration (inlay) can act as a wedge during cementation or function.If the cusps are weakened, fracture will occur. B, A cuspal-coverage onlay provides better protection but often lacks retention. C, A complete crown provides the best protection against fracture. It also has the best retention. 60
61. 61. MECHANICAL CONSIDERATIONS 61
62. 62. MECHANICAL CONSIDERATIONS PROVIDE RETENTION FORM PROVIDE RESISTANCE FORM PREVENTING DEFORMATION OF THE RESTORATION 62
63. 63. RETENTION FORM RETENTION THE QUALITY OF A PREPARATION THAT PREVENTS THE RESTORATION FROM BEING DISLODGED BY FORCES PARALLEL TO THE PATH OF WITHDRAWL. 63
64. 64. FACTORS FOR ADEQUATE RETENTION MAGNITUDE OF DISLODGING FORCES GEOMETRY OF THE TOOTH PREPARATION ROUGHNESS OF THE FITTING SURFACE OF THE RESTORATION MATERIALS BEING CEMENTED FILM THICKNESS OF THE LUTING AGENT 64
65. 65. MAGNITUDE OF DISLODGING FORCES Forces that tend to remove a cemented restoration along its path of withdrawal. DEPENDS ON THE STICKINESS OF THE FOOD SURFACE AREA AND TEXTURE OF THE RESTORATION BEING PULLED 65
66. 66. GEOMETRY OF TOOTH PREPARATION Most fixed prostheses depend on the geometric form of the preparation rather than on adhesion for retention because most of the traditional cements (e.g., zinc phosphate) are nonadhesive (i.e., they act by increasing the frictional resistance between tooth and restoration). CEMENT IS EFFECTIVE ONLY IF THE RESTORATION HAS A SINGLE PATH OF WITHDRAWL. 66
67. 67. THE RELATION SHIP B/W TWO BODIES , ONE RESTRAINING MOVEMENT OF THE OTHER CLOSED LOWER PAIR OF KINEMATIC ELEMENTS. IN FPD SLIDING PAIR- i.e FORMED BY TWO CYLINDRICAL SURFACES CONSTRAINED TO SLIDE ALONG ONE ANOTHER A tooth preparation will be cylindrical if the axial surfaces are prepared by a cylindrical bur held at a constant angle. 67
68. 68. However, if one wall of the complete crown preparation is over-tapered, it will no longer be cylindrical, and the cemented restoration will not be constrained by the preparation because the restoration then has multiple paths of withdrawal. 68
69. 69. TAPER The axial walls of the preparation must taper slightly to permit the restoration to seat.. That is.. Two opposing external walls must gradually converge.. 69
70. 70. The term Angle of Convergence can be used to describe the relationship of the two opposing walls of a preparation, The more nearly parallel the opposing walls of the preparation, the greater should be the retention. 70
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72. 72. SURFACE AREA The greater the surface area for the preparation, the greatest its retention. The preparation on larger teeth are more retentive than preparation on small teeth. Surface area can be increased by adding boxes and grooves 72
73. 73. A full crown preparation is more retentive on a molar than on a premolar 73
74. 74. STRESS CONCENTRATION STRESSES ARE NOT UNIFORM THROUHOUT THE CEMENT BUT CONCENTRATED AROUND THE JUNCTION OF THE AXIAL AND OCCLUSAL SURFACES.NICHOLLAS (1974) ROUNDING OF INTERNAL LINE ANGLES REDUCE STRESS CONCENTRATION 74
75. 75. TYPE OF PREPARATION Different types of preparation have different retentive values that correspond fairly closely to the surface area of the axial walls, as long as other factors (e.g., taper) are kept constant. PREPARATION IN WHICH ADDITION OF A GROOVE LIMITS THE PATH OF WITHDRAWL ,RETENTION IS INCREASED 75
76. 76. RETENTION OF DIFFERENT PREPARATION DESIGN. 76
77. 77. INTERNAL FEATURES Retention from an excessively tapered preparation can be increased by adding grooves and pinholes,because this will limits path of with drawl. 77
78. 78. ROUGHNESS OF SURFACE BEING CEMENTED: RETENTION WILL BE INCREASED IF RESTORATION IS ROUGHENED OR GROOVED.SMITH(1970). AIR ABRADING THE FITTING SURFACE WITH 50m OF ALUMINA INCREASE RETENTION BY 64 %. 78
79. 79. MATERIALS BEING CEMENTED: THE MORE REACTIVE THE ALLOY IS THE MORE ADHESION THERE WILL BE WITH LUTING AGENTS. BASE METAL ALLOYS ARE BETTER RETAINED THAN LESS REACTIVE HIGH GOLD CONTENT METALS.SAITO C et al (1976) 79
80. 80. TYPE OF LUTING AGENTS: ADHESIVE RESIN CEMENTS ARE MOST RETENTIVE FOLLOWED BY GLASS IONOMER POLYCARBOXYLATE CEMENT ZINC OXIDE EUGENOL 80
81. 81. 81
82. 82. RESISTANCE FORM: THE QUALITY OF PREPARATION THAT PREVENT THE ROTATION OF THE RESTORATION THAT ARE PLACED IN COMPRESSION RESISTANCE DEPENDS ON -MAGNITUDE AND DIRECTION OF THE DISLODGING FORCES - GEOMETRY OF THE TOOTH PREPARATION - PHYSICAL PROPERTIES OF THE LUTING AGENT. 82
83. 83. MAGNITUDE AND DIRECTION OF DISLODGING FORCE: In one laboratory study,-" a complete crown cemented on a nickel-chromium test die was found to be capable of withstanding over 13,500 N (1400 kg) -a far greater force than would occur in the mouth before becoming displaced. 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 favorably directed. 83
84. 84. GEOMETRY OF TOOTH PREPARATION: The tooth preparation must be shaped so that particular areas of the axial wall will prevent rotation of the crown. Lateral forces tend to displace the restoration by causing rotation around the gingival margin. Rotation is prevented by any areas of the tooth preparation that are placed in compression, called resistance areas . Multiple resistance areas cumulatively make up the resistance form of a tooth preparation. 84
85. 85. 85
86. 86. increased preparation taper and rounding of axial angles tend to reduce resistance. Short tooth preparations with large diameters were found to have very little resistance form. In general, molar teeth require more parallel preparation than premolar or anterior teeth to achieve adequate resistance form. 86
87. 87. A partial-coverage restoration may have less resistance than a complete crown because it has no buccal resistance areas. 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. 87
88. 88. Resistance form of partial and complete crowns. A, The buccoaxial wall (RA) of a complete crown should provide good resistance to rotation around a lingual axis. B, In a partial crown, resistance must be furnished by mesial and distal grooves. C, In a short or excessively tapered complete crown, resistance form is minimal because most of the buccal wall is missing. A mesiodistal groove should be placed to increase resistance form. D, Poor resistance form is less a problem in a short partial crown, provided the grooves have sufficient definition. However, lack of retention form may indicate the need for complete coverage. 88
89. 89. The resistance of short preparation can be improved by adding grooves 89
90. 90. 90
91. 91. DEFORMATION: MAY BE DUE TO IN APPROPRIATE ALLOY SELECTION IN ADEQUATE TOOTH REDUCTION POOR FRAMEWORK DESIGN 91
92. 92. ALLOY SELECTION: TYPE III AND TYPE IV GOLD ALLOYS ARE APPROPRIATE FOR CROWNS AND FPD. IN LONG SPAN FPDS HIGH NOBLE METAL ALLOYS OR NICKEL CHROMIUM ALLOYS. 92
93. 93. ADEQUATE TOOTH REDUCTION 93
94. 94. A restoration must contain a bulk of material that is adequate to withstand the forces of occlusion.. This bulk must confined to the space created by the tooth preparation. This helps in Harmonious occlusion, Axial contours normal, Preventing periodontal problems around the restoration. 94
95. 95. OCCLUSAL CLEARANCE: Occlusal clearance is an important feature for providing adequate bulk of metal and strength. TYPE OF FUNCTIONAL CUSP NONFUNCTIONAL RESTORATION REDUCTION CUSP REDUCTION GOLD ALLOYS 1.5mm 1mm METALCERAMIC 1.5-2mm 1-1.5mm ALL CERAMIC 2mm 2mm 95
96. 96. The basic inclined pattern of the occlusal surface should be duplicated to produce adequate clearance without over shortening the preparation whose length is already minimal to provide adequate retention Inadequate clearance makes a restoration weaker 96
97. 97. FUNCTIONAL CUSP BEVEL: It provides space for an adequate bulk of metal in an area of heavy occlusal contact If bevel is not given than deflective occlusal contact & unnecessary destruction of the tooth structure will likely to occur. 97
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102. 102. 102
103. 103. 103
104. 104. AXIAL REDUCTION: It secures space for an adequate thickness of the restorative material If reduction is not proper then it will lead to distortion. Other feature that provide space for metal that will improve rigidity and durability of the restoration: Offset , The Occlusal Shoulder, The Isthmus, The Proximal Groove, and The Box. 104
105. 105. MARGIN DESIGN: Distortion of the restoration margin is prevented by designing the preparation outline to avoid occlusal contact in this area. Also, tooth reduction should provide sufficient room for bulk of metal at the margin to prevent distortion. 105
106. 106. ESTHETIC CONSIDERATIONS 106
107. 107. ESTHETIC CONSIDERATIONS The restorative dentist should develop skill in determining the esthetic expectations of the patient Patients prefer their dental restorations to look as natural as possible At the initial examination it is important to make a full assessment of the appearance of each patient noting which areas of which teeth show during smiling talking and laughing 107
108. 108. MINIMUM METAL DISPLAY When mandibular partial cast crowns are made metal display is unavoidable because the occlusal surface of mandibular teeth can be seen during speech Anterior partial coverage restoration can be fabricated to show no metal but their preparation require considerable care 108
109. 109. MAXIMUM THICKNESS OF PORCELAIN: 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 To correct all these deficiencies certain principles are recommended during tooth preparation 109
110. 110. If there is to be sufficient bulk of porcelain for appearance and metal for strength adequate reduction of the facial surface is essential. A minimum reduction of 1.5 mm typically is required for optimal appearance An incisal reduction of 2 mm is recommended for good esthetics The proximal surfaces of anterior teeth will look most natural if they are restored as the incisal edges without metal backing 110
111. 111. Adequate porcelain thickness is essential for preventing direct light reflection from the highly pigmented opaque porcelain. The most critical areas are the gingival and incisal thirds; in practice, opaque modifying stains are often used in these areas. 111
112. 112. PARTIAL COVERAGE RESTORATIONS Esthetic partial coverage restorations depend on accurate placement of the potentially visible facial and proximal margins Many patients will not readily accept a visible display of metal Place margin buccal to the proximal contact area where metal will be hidden by the distal line angle of the neighboring tooth 112
113. 113. The facial margin should be extended just beyond the occlusofacial line angle A short bevel is needed to prevent enamel chipping 113
114. 114. PORCELAIN OCCLUSAL SURFACES The mandibular occlusal surfaces are visible during speech. Hence metal display is unavoidable. to avoid unaesthetic appearance, a metal-ceramic restoration with porcelain coverage on occlusal surface can be made. 114
115. 115. ACCEPTABLE RESULTS 115
116. 116. CONCLUSSION THE PRINCIPLES OF TOOTH PREPARATION CAN BE CATEGORIZED INTO BIOLOGIC , MECHANICAL , AND ESTHETIC CONSIDERATIONS.EACH TOOTH PREPARATION MUST BE MEASURED BY DEFINED CRITERIA ,WHICH CAN BE USED TO IDENTIFY AND CORRECT PROBLEMS.THUS SUCCESSFUL PREPARATION CAN BE OBTAINED MOST EASILY BY SYSTEMATICALLY FOLLOWING THE STEPS. 116
117. 117. REFERENCES Contemporary fixed prosthodontics Rosenstiel, Land and Fujimoto 3rd edition pages 209- 257 Fundamentals of fixed prosthodontics Shillingburg et al 3rd edition pages 119 138 Fundamentals of tooth preparation for cast metal and porcelain restorations Shillingburg et al. 1991 ed pages 13-45 Ayad ME et al: Influence of tooth surface roughness and type of cement on retention of complete cast crowns, J Prosthet Dent 77:116,1997. 117
118. 118. el-Mowafy OM et al: Retention of metal ceramic crowns cemented with resin cements: effects of preparation taper and height, J Prosthet Dent 76:524, 1996. Wilson AH, Chan DC: The relationship between preparation convergence and retention of extracoronal retainers, J Prosthod 3:74, 1994. 118