Principles of tooth preparation

PROSTHODONTICS

1. Principles Of Tooth Preparation By Dr.Sayli Patil 2nd year PG Dept. Of Prosthodontics

2. Contents Introduction Defination Principles of Tooth Preparation Biologic considerations Conservation of Tooth structure Mechanical Considerations Esthetic considerations Planning and Evaluating Tooth Preparations Recent advances Conclusion References

3. Introduction 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. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

4. Defination Tooth Preparation: the process of removal of diseased and/or healthy enamel and dentin and cementum to shape a tooth to receive a restoration. GPT 9. According to Tylman : as the mechanical treatment of dental disease or injury to hard tissue that restores a tooth to the original form Glossary of Prosthodontic Terms, Ninth Edition, GPT‐9. The Academy of Prosthodontics Foundation. J. Prosthet. Dent.. 2017;117(5S):e1-05.

5. Principles of tooth preparation may be divided into: Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

6. Optimum restoration should satisfy biologic, mechanical, and esthetic requirements

7. Design and Preparation of Tooth are governed by 5 principles: Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

8. BIOLOGIC CONSIDERATIONS The adjacent teeth, soft tissues, and the pulp of the tooth being prepared are easily damaged in tooth preparation. If poor preparation leads to inadequate marginal fit or deficient crown contour, plaque control around fixed restorations will become more difficult. This will impede the long-term maintenance of dental health Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

9. PREVENTION OF DAMAGE DURING TOOTH PREPARATION: Adjacent Teeth: A metal matrix band around the adjacent tooth for protection may be helpful; however, the thin band can still be perforated and the underlying enamel damaged. The preferred method is to use the proximal enamel of the tooth being prepared for protection of the adjacent structures. Teeth are 1.5 to 2 mm wider at the contact area than at the cementoenamel junction (CEJ), and a thin, tapered diamond can be passed through the interproximal contact area to leave a slight lip or fin of enamel without causing excessive tooth reduction or undesirable angulation of the rotary instrument

10. Soft Tissues : Damage to the soft tissues of the tongue and cheeks can be prevented by careful retraction with an aspirator tip, a mouth mirror or a flanged saliva ejector. Great care is needed to protect the tongue when the lingual surfaces of mandibular molars are being prepared. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

11. Pulp : Extreme temperatures, chemical irritation, or microorganisms can cause an irreversible pulpitis , particularly when they occur on freshly sectioned dentinal tubules. Pulp size can be evaluated on a radiograph, and it decreases with age. Up to about age 50, it decreases more so occlusocervically than faciolingually .

12. Causes of Injury Temperature : The spray must be accurately directed at the area of contact between tooth and rotary instrument. It also washes away debris, which is important because rotary instrument clogging reduces cutting efficiency. Irrigation also prevents desiccation of the dentin. To prevent heat buildup,retention features should be prepared at low rotational speed or with a high-speed handpiece with a feather-light touch Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

13. Chemical Action The chemical action of certain dental materials (bases, restorative resins, solvents, and luting agents) can cause pulpal damage, particularly when applied to freshly cut dentin. Dentin bonding agents form an effective barrier, but their effect on the retention of cemented restorations is controversial. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

14. Bacterial Action Pulpal damage under restorations has been attributed to bacteria that either were left behind or gained access to the dentin because of microleakage . Because vital dentin seems to resist infection,the routine use of antimicrobial agents may not be advantageous. Of importance is that all carious dentin must be removed before placement of a restoration that will serve as a foundation for a fixed prosthesis. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 3rd ed St. Louis: Elsevier. 2001:108-35.

15. Conservation of Tooth Structure Tooth structure is conserved through adherence to 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

16. Preparation of teeth with the minimum practical convergence angle (taper) between axial walls. Preparation of the occlusal surface so that reduction follows the anatomic planes and produces uniform thickness in the restoration

17. Preparation of the occlusal surface so that reduction follows the anatomic planes and produces uniform thickness in the restoration. A shoulder margin (2) is indicated when esthetic restorations are planned, but it is much less conservative than a chamfer margin (1). An anatomically prepared occlusal surface results in adequate clearance without excessive tooth reduction. A flat occlusal preparation will result in either insufficient clearance (1) or an excessive amount of reduction (2). Selection of a margin geometry that is conservative and yet compatible with the other principles of tooth preparation

18. Avoidance of unnecessary apical extension of the preparation which would result in loss of additional tooth structure Apical extension of the preparation can necessitate additional tooth reduction because coronal diameter becomes smaller. B, Preparations for periodontally involved teeth may necessitate considerable reduction if the margins are to be placed subgingivally for esthetic reasons. C, Supragingival margins are preferred where applicable

19. MECHANICAL CONSIDERATIONS Mechanical considerations can be divided into three categories: Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

20. Retention & Resistance form the feature of a tooth preparation that resists dislodgment of a crown in a vertical direction or along the path of placement- GPT 9 Certain forces (e.g., when the jaws are moved apart after biting on very sticky food) act on a cemented restoration in the same direction as the path of placement. The quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of placement is known as its retention form. Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

21. Resistance form: the features of a tooth preparation that enhance the stability of a restoration and resists dislodgment along an axis other than the path of placement - Gpt 9 Resistance prevents dislodgment of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces - Shillingburg

22. There are 4 factors which are in operators control during tooth preparation which influence retention. Degree of Taper Total Surface area of cement film Area of cement under shear Roughness of the tooth surface.

23. The ability of a cement bond to withstand force depends largely on the direction of he force in relation o the cemented surfaces. From this we would expect that more parallel the opposing walls of a preparation the greater will be the retention. Jorgensen, found that retention decreases as taper increases. Extracoronal restoration (a) uses opposing external surfaces for retention (b). Intracoronal restoration (a) uses opposing internal surfaces for retention (b).

24. Taper the axial walls of the preparation must taper slightly to permit the restoration to seat; ie , in the occlusal direction, two opposing external walls must gradually converge or two opposing internal surfaces of tooth structure must diverge. A tapered diamond or bur will impart an inclination of 2 to 3 degrees to any surface it cuts if the shank of the instrument is held parallel to the intended path of insertion of the preparation. Two opposing surfaces, each with a 3-degree inclination, would give the preparation a 6-degree taper. External (top row) and internal (bottom row) opposing surfaces

25. By calculating the average height-to-base ratio of preparations of incisors, canines, premolars , and molars, minimally acceptable guidelines for preparation tapers by tooth group were determined. The values expressed as total occlusal convergence were found to be 58 for incisors, 66 for canines, 20 for premolars, and 16 for molars . These values are based on resistance form recommended for minimally acceptable average tapers Resistance form is easily obtained on anterior teeth , although it is more difficult to obtain with posterior teeth because they are shorter and wider, making the height-to-base ratio less . Shillingburg recommended valuefor average ideal tapers (total convergence angle) increase from anterior (10 ) to posterior (19 to 22) Parker MH. Resistance form in tooth preparation. Dental Clinics. 2004 Apr 1;48(2):387-96.

26. Methods to analyze resistance form Lewis and Owen showed that for preparations with straight walls, the dividing point between the resistive and nonresistive sections of a preparation wall is the point of intersection with the perpendicular line from the center of rotation on the opposing margin. Using an ingenious application of geometry, Zuckerman used a boundary circle but centered his circle at the center of the base of the preparation with a radius one half of the base radius used by Weed .

27. Three techniques for evaluating the resistance form of side AC. The Lewis method of evaluating resistance form of side AC is applied by extending a perpendicular line to side AC from the center of rotation E at the opposite margin. The intersection of the Lewis line and side AC is point B, so all points occlusal to B (section BC) are resistive and all points gingival to B (section AB) are not resistive. The same results are obtained with the Zuckerman circle, where all points outside of the circle (section BC) are resistive and all points inside (section AB) are nonresistive . Parker MH. Resistance form in tooth preparation. Dental Clinics. 2004 Apr 1;48(2):387-96.

28. Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

29. Ward was one of the first to recommend tapering as such, prescribing 5% to 20% per inch (3 to 12 degrees, respectively). Jorgensen and Kaufman et al have demonstrated experimentally that retention decreases as taper is increased. Recommendations for optimum axial wall taper of tooth preparations for cast restorations have ranged from 3 to 5 degrees,6 to 6 degrees, 7 to 10 to 14 degrees. 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. There is only a slight increase in stress as taper is increased from 0 to 15 degrees9; however, at 20 degrees, stress concentration was found to increase sharply.

30. Consciously attempting to create a taper can easily result in an overtapered and nonretentive preparation. It can be as low as 10 degrees on preparations on anterior teeth and as high as 22 degrees on molars.

31. 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. A full veneer preparation with long, parallel axial walls and grooves would produce such retention By limiting the paths of withdrawal, retention is improved. (b) A preparation with unlimited freedom of displacement is much less retentive

32. The walls of a groove that meet the axial wall at an oblique angle do not provide the necessary resistance. (b) The walls of a groove must be perpendicular to rotating forces to resist displacement. The facial and lingual walls of a box will not resist rotational displacement if they form oblique angles with the pulpal wall. (b) They must meet the pulpal wall at angles near 90 degrees

33. Length Occlusogingival length is an important factor in both retention and resistance. The preparation with longer walls (a) interferes with the tipping displacement of the restoration better than the short preparation (b). preparation on a tooth with a smaller diameter (a) resists pivoting movements better than a preparation of equal length on a tooth of larger diameter (b). The shorter the wall, the more important its inclination. The walls of shorter preparations should have as little taper as possible to increase the resistance. However, even this will not help if the walls are too short.

34. Substitution of internal features Kent et al reported a marked difference between the degree of taper of full crown preparations (18.4 to 22.2 degrees) and that of boxes and grooves in the axial surfaces of those preparations (7.3 degrees). The taper of these internal features is nearly the same as the taper of the instruments used to cut them (4 to 6 degrees). These features offer an excellent means of enhancing the overall retention and resistance of an otherwise overinclined axial wall. Internal preparation features Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

35. Path of insertion The path of insertion is an imaginary line along which the restoration will be placed onto or removed from the preparation. The correct technique must be used to survey a preparation visually because this is the primary means of ensuring that the preparation is neither undercut nor overtapered . If the center of the occlusal surface of a preparation is viewed with one eye from a distance of approximately 30 cm (12 inches), it is possible to sight down the axial walls of a preparation with a minimum taper. However, it is also possible to sight down the axial walls of a preparation with a reverse ( ie , undercut) taper of 8 degrees when both eyes are open. Therefore, it is important that preparations be viewed with one eye closed

36. For a preparation to be surveyed in the mouth, where direct vision is rarely possible, a mouth mirror is used. It is held at an angle approximately ½ inch above the preparation, and the image is viewed with one eye. The path of insertion must be considered in two dimensions: faciolingually and mesiodistally

37. The path of insertion of a preparation must parallel the adjacent proximal contacts (a) or it will be prevented from seating (b). (a) The path of insertion of a preparation for a metalceramic crown should parallel the long axis of the tooth. (b) If the path is directed facially, the prominent facioincisal angle may create esthetic problems of overcontouring or opaque show-through. (c) However, if the path is directed lingually , the facial surface will intersect the lingual surface, creating a shorter preparation. It also may encroach on the pulp.

38. The correction of inadequate retention and resistance form should always be attempted first through the application of compensatory measures (principles) to the primary factor (axial walls) and then, if necessary, through the addition of secondary factors . Gilboe DB, Teteruck WR. Fundamentals of extracoronal tooth preparation. Part I. Retention and resistance form. Journal of Prosthetic Dentistry. 1974 Dec 1;32(6):651-6.

39. Structural Durability A restoration must contain a bulk of material that is adequate to withstand the forces of occlusion. This bulk must be confined to the space created by the tooth preparation. Inadequate occlusal reduction does not provide the needed space for a cast restoration of adequate thickness Occlusal reduction should reproduce basic inclined planes rather than being cut as one flat plane

40. Occlusal reduction: One of the most important features for providing adequate bulk of metal and strength to the restoration is occlusal clearance. For gold alloys, there should be 1.5 mm of clearance. Metal-ceramic crowns will require 2.0 mm of clearance to receive ceramic coverage. There should be 2.0 mm of clearance on preparations for all ceramic crowns. Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

41. Functional cusp bevel: A wide bevel on the palatal inclines of the maxillary palatal cusps and the facial inclines of mandibular facial cusps provides space for an adequate bulk of metal in an area of heavy occlusal contact. Lack of a functional cusp bevel can cause a thin area or perforation in the casting. Lack of a functional cusp bevel may result in overcontouring and poor occlusion. Overinclination of the facial surface will destroy excessive tooth structure and lessen retention

42. Axial reduction: If restorations are made with normal contours over preparations with inadequate axial reduction, they will have thin walls that will be subject to distortion. Laboratory technicians often attempt to compensate for this by overcontouring the axial surfaces. While this intended solution to the problem strengthens the restoration, it can have a disastrous effect on the periodontium . Inadequate axial reduction can cause thin walls and a weak restoration (a) or a bulbous, overcontoured restoration (b).

43. There are other features that serve to provide space for metal that will improve the rigidity and durability of the restoration: the offset, the occlusal shoulder, the isthmus, the proximal groove, and the box. The isthmus connects the boxes, and the offset ties the grooves together to enhance the reinforcing “truss effect.” (a) The three-quarter crown is reinforced by the bulk of gold that fills the offset and grooves. (b) The occlusal shoulder strengthens the lingual margin, and the isthmus and boxes reinforce the main body of an MOD onlay Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

44. Marginal Integrity The configuration of the preparation finish line dictates the shape and bulk of restorative material in the margin of the restoration. It also can affect both marginal adaptation and the degree of seating of the restoration. Bevels have been advocated as a means of diminishing marginal discrepancy Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

45. The following criteria for margin selection seem reasonable: 1. The selected margin must provide a predictable level of marginal integrity. 2. To minimize plaque accumulation, the selected margin must present smooth materials to the gingival sulcus . 3. In some situations, the margin also must provide acceptable esthetics. Donovan TE, Chee WW. Cervical margin design with contemporary esthetic restorations. Dental Clinics of North America. 2004 Apr;48(2):vi-417.

46. Margin Placement: Whenever possible, the margin of the preparation should be supragingival . Advantages of supragingival margins: 1. Can be easily finished without associated soft tissue trauma. 2. More easily kept plaque free. 3. Impressions are more easily made, with less potential for soft tissue damage. 4. Restorations can be easily evaluated at the time of placement and at recall appointments. Subgingival margins indications: 1. Dental caries, cervical erosion, or restorations extend subgingivally , and a crown-lengthening procedure is contraindicated. 2. Proximal contact area extends apically to the level of the gingival crest. 3. The margin of an esthetic restoration is to be hidden behind the labiogingival crest. 4. Axial contour modification is indicated Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

47. Margin Adaptation Poor preparation design, leading to increased margin length. C, A rough, irregular margin makes the fabrication of an accurately fitted restoration almost impossible.

48. Margin Geometry : Guidelines for margin design should be considered: Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

49. Finish line configurations The preferred gingival finish line for veneer metal restorations is the chamfer,it has been shown experimentally to exhibit the least stress, so the cement underlying it will have less likelihood of failure. Deep chamfer (heavy chamfer) is used to provide a 90-degree cavosurface angle with a large-radius rounded internal angle. It provides better support for a ceramic crown than does a conventional chamfer, but it is not as good as a shoulder. Chamfer finish line demonstrated on a full veneer crown preparation (a) Deep chamfer on a preparation for an all-ceramic crown. (b)90 degree finish line formation.

50. Lip formation an undesirable fragile lip of enamel at the cavosurface.This friable, unsupported enamel is very easily fractured during or after cementation of the restoration. Classic shoulder on a preparation for an all-ceramic crown wide ledge provides resistance to occlusal forces and minimizes stresses that might lead to fracture of the porcelain a) A radial shoulder on an allceramic crown preparation combines maximum support of the ceramic with a stress reducing, rounded gingivoaxial angle. (b) Minimal radius of curvature reduces stress

51. 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. for preparations with extremely short walls finish lines that permit an acute margin of metal is the knife Edge. It may have to be used on the lingual surface of mandibular posterior teeth, on teeth with very convex axial surfaces, and on the surface toward which a tooth may have tilted Shoulder with a bevel on the occlusal shoulder of an MOD onlay Knife edge on the lingual of a mandibular three-quarter crown

52. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

53. Becker CM, Kaldahl WB. Current theories of crown contour, margin placement, and pontic design. The Journal of prosthetic dentistry. 1981 Mar 1;45(3):268-77. Crown contours which promote favorable tissue response follow these guidelines: buccal and lingual contours are flat embrasure spaces should be open contacts should be high ( incisal one third) and buccal to the central fossa (except between first and second molars) furcations should be ‘‘fluted’’ or ‘‘barreled out.’’ Margins should be supragingival where possible. The pontic design of choice is the modified ridge lap for posterior spaces and the ridge-lap facing for anterior spaces.

54. Preservation of the Periodontium

55. Instrumentation The excavation of caries should be accomplished with sharp spoon excavators and round burs (no. 4 or no. 6).Small diamond points, used with an air-water spray in a high-speed handpiece , will remove precisely amounts of tooth structure Cross-cut or dentate burs are employed for removal of old restorations, but the horizontal ridges they leave on tooth structure make them unacceptable for planing tooth surfaces. Acceptable finish lines on vertical flares can be obtained through the use of abrasive paper disks. Nondentate tapered burs (169L, 170L, and 171L) are used for grooves, boxes, isthmuses, and offsets where they are needed. Gross reduction is most efficiently accomplished with coarse diamonds. Using fine diamonds of the same size and shape, it is possible to maintain the finish line configuration developed by the coarse diamond instrument. Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013.

56. Rotary instruments for tooth preparations Shape S i l houette Use Round-end tapered diamond Depth orientation grooves , Occlusal reduction,Functional cusp bevel Round-end tapered finishing diamond Occlusal finishing, Functional cusp bevel finishing Flat-end tapered diamond Axial reduction( MCR,all ceramic crown) Shoulder( MCR,all ceramic crown) Flat-end tapered finishing diamond Axial wall finishing (MCR, allceramic crown)Radial shoulder(MCR, allceramic crown Tapered torpedo diamond Axial reduction Chamfer finish line Tapered torpedo finishing diamond Axial wall Finishing Chamfer finishing Short needle diamond Initial proximal reduction (posterior teeth)

57. Shape S i l houette Use Long needle diamond Initial proximal reduction (anterior teeth) Small football shaped diamond Lingual Reduction (anterior teeth) Small footballshaped Finishing diamond Lingual reduction finishing (anterior teeth) Flame diamond Proximal Flare Gingival bevel Flame finishing diamond Flare and bevel finishing Tapered fissure bur Seating groove ,Proximal groove (posterior teeth) Offset,Occlusal shoulder,Isthmus,Proximal box, Smoothing and finishing,Occlusal and Incisal bevels Tapered groove alignment fissure burs Initial groove alignment Angles of Proximal boxes Smoothing and finishing Occlusal and incisal bevels

58. ESTHETIC CONSIDERATIONS Options for esthetic restorations include partial veneer crowns, which maintain an intact labial or buccal surface in original tooth structure; metal-ceramic restorations, which consist of a metal cast substructure that in visible areas has an esthetic porcelain veneer; and all-ceramic restorations Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

59. All-Ceramic Restorations Not all ceramic crown preparations are conservative of tooth structure, in as much as a wide 90-degree heavy chamfer margin must be prepared around the entire tooth to ensure increased material thickness and material strength. For the same reason, additional reduction on the lingual surface is needed for these restorations A minimal material thickness of approximately 1 to 1.2 mm is necessary to ensure optimal esthetics. This limits the use of these restorations on faciolingually thin teeth and on teeth with large pulps, as in young individuals. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

60. Metal-Ceramic Restorations Facial Tooth Reduction: A minimum reduction of 1.5 mm is typically required for optimal appearance. Adequate thickness of porcelain is needed to create a sense of color depth and translucency. 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, the reduction in either the cervical or the incisal area of the preparation is insufficient Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

61. Recommended tooth preparation for maxillary (A) and mandibular (B) metal-ceramic restoration. In each case, the facial reduction has two distinct planes. 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.

62. Incisal Reduction: An incisal reduction of 2 mm is recommended for good esthetics. Excessive incisal reduction must be avoided because it reduces the resistance and retention form of the preparation. Proximal Reduction: The proximal surfaces of anterior teeth look most natural if they are restored at the incisal edges, without metal backing Optimal esthetics require proximal light transmission through the esthetic veneer. Occluding lingual surfaces are in metal, which extends into the proximal aspect.

63. Labial Margin Placement: subgingival margins may be indicated for esthetic reasons, particularly when the patient has a high lip line and when the use of a metal collar labial margin is contemplated. If the patient has a low lip line, a metal supragingival collar may be placed because the metal is not seen during normal function. The periodontium must be healthy before the tooth is prepared. If periodontal surgery is needed, the sulcular space should not be eliminated completely; rather, a postsurgical depth of about 2 mm should be the objective. Margins should not be placed so far apically that they encroach on the attachment; extension to within 1.5 mm of the alveolar crest leads to bone resorption

64. Partial-Coverage Restorations Whenever possible, an esthetically acceptable result without the use of complete crowns is preferred because tooth structure is conserved and because no restorative material can approach the appearance of intact tooth enamel. Esthetic partial-coverage restorations depend on accurate placement of the potentially visible facial and proximal margins. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28

65. Proximal Margin Correct placement of the mesial margin of a partial-coverage restoration is essential for good esthetics. To allow proper access for finishing, the restoration must extend just beyond the contact area, but the metal must remain hidden from the casual observer. B, The tooth should be prepared in its long axis; otherwise, metal is displayed.

66. Facial Margin The facial margin of a partial cast crown should be shaped so that light is not reflected directly to the observer. B, A three-unit fixed dental prosthesis. The mesial abutment is a canine, shaped to look like a lateral incisor. The distal abutment is a partial cast crown, which proved to be esthetically acceptable because the metal had been correctly contoured.

67. PLANNING AND EVALUATING TOOTH PREPARATIONS Diagnostic Tooth Preparations: Diagnostic tooth preparations are performed on articulated casts before the actual clinical preparation Selecting the appropriate path of placement for a fixed dental prosthesis, particularly for abutment teeth that are tilted, are rotated, or have an atypical coronal contour Deciding on the amount of tooth reduction necessary to accomplish a planned change in the occlusion. Determining the best location for the facial and proximal margins of a partial-coverage restoration so that the metal is not visible.

68. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: an art form based on scientific principles. The Journal of prosthetic dentistry. 2001 Apr 1;85(4):363-76. Teeth should be prepared so that they exhibit the following characteristics: 10 to 20 degrees of total occlusal convergence, a minimal occlusocervical dimension of 4 mm for molars and 3 mm for other teeth, and an occlusocervical -to- faciolingual dimension ratio of 0.4 or greater. Facioproximal and linguoproximal line angles should be preserved whenever possible. When the above features are missing, the teeth should be modified with auxiliary resistance features such as axial grooves or boxes, preferably on proximal surfaces. Finish line selection should be based on the type of crown/retainer, esthetic requirements, ease of formation, and personal experience Expectations of enhanced marginal fit with certain finish lines could not be validated by recent research. Esthetic requirements and tooth conditions determine finish line locations relative to the gingiva , with a supragingival location being more acceptable. Line angles should be rounded, and a reasonable degree of surface smoothness is desired.

69. Recent advances the study evaluated the operator preferences and tooth preparation performance of electric handpieces compared with those of traditional air-turbine handpieces . Conclusion: Despite heavier weight, poorer grip feel, and less flexibility, the electric handpiece emitted lower noise, had better feel of the tooth preparation in general, and was preferred for its greater smoothness in the finishing step of tooth preparation than the air-turbine handpiece . The surface roughness of the prepared teeth and the crown fit between the teeth and ceramic crown were not affected by the choice of air-turbine or electric handpiece Pei DD, Meng YC, Fayed AS, You YF, Wu ZX, Lu Y. Comparison of crown fit and operator preferences between tooth preparation with electric and air-turbine handpieces . The Journal of Prosthetic Dentistry. 2020 Feb 7.

70. Green et al 2018,The purpose of this study was to measure the influence of tooth substrate type (all enamel, all-dentin, or half-dentin-half-enamel) and veneer thickness on the loads needed to cause initial and catastrophic porcelain veneer failure. Conclusion: 1. Porcelain veneers bonded to enamel were substantially stronger and more damage-tolerant than those bonded to dentin or mixed substrates. 2. Veneers on half-enamel-half-dentin substrates were no more resistant to catastrophic failure than those placed completely on dentin. 3. Increased porcelain thickness substantially raised the loads to catastrophic failure on all-enamel substrates. 4. Increased porcelain thickness only moderately raised the loads to catastrophic failure on all-dentin or half-enamel-half-dentin substrates. Ge C, Green CC, Sederstrom DA, McLaren EA, Chalfant JA, White SN. Effect of tooth substrate and porcelain thickness on porcelain veneer failure loads in vitro. The Journal of prosthetic dentistry. 2018 Jul 1;120(1):85-91.

71. The purpose of this in vitro study was to compare the marginal and internal fit accuracy of lithium disilicate glass-ceramic inlays fabricated with conventional, milled, and 3-dimensional (3D) printed wax patterns. Conclusion: Lithium disilicate glass-ceramic inlays produced from digital impressions and subtractive milling of wax patterns resulted in better marginal and internal fit accuracy than either conventional impression/fabrication or additive 3D manufacturing. Three-dimensional printed wax patterns yielded fit values similar to those of the conventionally waxed inlays Homsy FR, Özcan M, Khoury M, Majzoub ZA. Marginal and internal fit of pressed lithium disilicate inlays fabricated with milling, 3D printing, and conventional technologies. The Journal of prosthetic dentistry. 2018 May 1;119(5):783-90.

72. The purpose of this clinical study was to examine the 10-year survival of pressed lithium disilicate glass-ceramic restorations and the relationship between clinical parameters on outcomes. Conclusion: The probability of survival for lithium disilicate monolithic complete-coverage restorations (n=1410) was 96.5% over 10.4 years and 100% over 7.9 years for the bilayered group. Pressed lithium disilicate restorations in this study survived successfully over the 10.4-year period studied with an overallfailure rate below 0.2% per year and were primarily confined to molar teeth. Malament KA, Natto ZS, Thompson V, Rekow D, Eckert S, Weber HP. Ten-year survival of pressed, acid-etched e. max lithium disilicate monolithic and bilayered complete-coverage restorations: Performance and outcomes as a function of tooth position and age. The Journal of prosthetic dentistry. 2019 May 1;121(5):782-90.

73. Conclusion The principles of tooth preparation can be categorized into biologic,mechanical and esthetic considerations. Often those principles conflict, and the clinician must decide how the restoration should be designed. Each tooth preparation must be measured by clearly defined criteria, which can be used to identify and correct problems. It is important to understand the pertinent theories underlying each step. Successful preparations can be obtained by systematically following these steps. It is important to critically evaluate each step before proceeding to the next step to ensure an optimal quality final restoration, which will serve the patient for a longtime.

74. References Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Pub Co (4th edn ), Chicago. 2013. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics 5 th ed St. Louis: Elsevier. 2015 Jul 28. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: an art form based on scientific principles. The Journal of prosthetic dentistry. 2001 Apr 1;85(4):363-76. Ge C, Green CC, Sederstrom DA, McLaren EA, Chalfant JA, White SN. Effect of tooth substrate and porcelain thickness on porcelain veneer failure loads in vitro. The Journal of prosthetic dentistry. 2018 Jul 1;120(1):85-91. Homsy FR, Özcan M, Khoury M, Majzoub ZA. Marginal and internal fit of pressed lithium disilicate inlays fabricated with milling, 3D printing, and conventional technologies. The Journal of prosthetic dentistry. 2018 May 1;119(5):783-90 . Gilboe DB, Teteruck WR. Fundamentals of extracoronal tooth preparation. Part I. Retention and resistance form. Journal of Prosthetic Dentistry. 1974 Dec 1;32(6):651-6.

75. Pei DD, Meng YC, Fayed AS, You YF, Wu ZX, Lu Y. Comparison of crown fit and operator preferences between tooth preparation with electric and air-turbine handpieces . The Journal of Prosthetic Dentistry. 2020 Feb 7. Becker CM, Kaldahl WB. Current theories of crown contour, margin placement, and pontic design. The Journal of prosthetic dentistry. 1981 Mar 1;45(3):268-77. Donovan TE, Chee WW. Cervical margin design with contemporary esthetic restorations. Dental Clinics of North America. 2004 Apr;48(2):vi-417. Malament KA, Natto ZS, Thompson V, Rekow D, Eckert S, Weber HP. Ten-year survival of pressed, acid-etched e. max lithium disilicate monolithic and bilayered complete-coverage restorations: Performance and outcomes as a function of tooth position and age. The Journal of prosthetic dentistry. 2019 May 1;121(5):782-90. Glossary of Prosthodontic Terms, Ninth Edition, GPT‐9. The Academy of Prosthodontics Foundation. J. Prosthet. Dent.. 2017;117(5S):e1-05 . Parker MH. Resistance form in tooth preparation. Dental Clinics. 2004 Apr 1;48(2):387-96.