Principles of Tooth Preparation PDF

DR. Shaimaa Ahmed Principles of Tooth Preparation DR. Shaimaa Ahmed Professor of Fixed Prosthodontics Faculty of Dentistry, Mansoura University  DR. Shaimaa Ahmed III- Principles of to...

DR. Shaimaa Ahmed Principles of Tooth Preparation DR. Shaimaa Ahmed Professor of Fixed Prosthodontics Faculty of Dentistry, Mansoura University  DR. Shaimaa Ahmed III- Principles of tooth preparation Teeth are not regenerative tissue, once enamel or dentine is lost as a result of caries, trauma or wear, restorative material must be used to restore form and function. They require preparation to receive restoration The principles of tooth preparation can be divided into three broad categories: 1. Biologic consideration: which affect the health of the oral tissues. 2. Mechanical considerations: Affect the integrity and durability of the restoration ( retention and the resistance form and resistance to deformation). 3. Esthetic considerations: which affect the appearance of the patient. The optimum restoration should satisfy biologic, mechanical, and esthetic requirements.  DR. Shaimaa Ahmed 1) Biological considerations: a) Prevention of damage during tooth preparation 1- Adjacent teeth 2- Soft tissue 3- Pulp b) Conservation of tooth structures c) Consideration affecting future dental health 1- Axial reduction 2- Margin placement 3- Margin adaptation 4- Margin geometry 5- Occlusal considerations 6- Preventing tooth fracture 1-Prevention of damage during tooth preparation to: a) The adjacent tooth. b) Soft tissue. c) The pulp. a) The adjacent tooth. Iatrogenic damage to the proximal contact area of the adjacent tooth makes it more susceptible to dental caries. Therefore, a metal matrix band put around the adjacent tooth for protection may be helpful but the preferred method is to use a fine, tapered diamond stone to pass through the interproximal contact area leaving a slight lip of enamel without causing excessive tooth reduction. Damage to adjacent teeth is prevented by making a thin "lip" of enamel as the stone passes through a proximal contact.  DR. Shaimaa Ahmed Metal matrix band Damage of proximal surface b) Soft tissues: Tongue and cheeks can be preserved by careful retraction with an aspirator tip and mouth mirror. Using gingival retraction cord to reflect the gingiva in case of subgingival finishing line. Retraction with mouth mirror Flanged saliva ejector 4  DR. Shaimaa Ahmed Direct trauma, more likely with subgingival margins Periodontal disease Careless cutting Aggressive use of retraction cord c) The pulp: Tooth preparations must take into consideration the morphology and size of the dental pulp chamber. The most common change in a molar tooth will be the flattening of the pulp chamber due to dentin production in the pulp roof and floor Causes of pulp injury a- Temperature b- Chemical action c- Bacterial action 5  DR. Shaimaa Ahmed Causes of pulpal damage: 1. Extreme temperature: This is generated by friction between a rotary instrument and the surface being prepared, and this depends on: a. Excessive pressure b. Higher rotational speed c. Type and condition of cutting instrument. This heat can be minimized by the use of water coolant at high speed with light touches. Water coolant is useful because it cools the tooth and the bur, and removes the tooth debris from the cutting blades because clogging reduces the cutting efficiency and generates more frictional heat. 2. Chemical irritation. Certain dental materials (bases, restorative resins, solvents, and luting agents) can cause pulpal damage when applied to freshly cut dentin. cavity varnish or dentin- bonding agents will form an effective barrier in most instances but their effect on the retention of a cemented restoration is controversial. Chemical agent may be used for cleaning tooth preparation they have been shown to be pulpal irritants. 3. Bacteria irritation. Bacteria that either left behind or gained access to the dentin because of microleakage lead to pulpal damage. Because the vital pulp resist infection, the common use of antimicrobial agents after tooth preparation has not been documented in clinical trial. All carious dentine must be removed before placement of restoration that serve as foundation for FPD. An indirect pulp cap is not recommended on teeth that will subsequently receive cast restoration Dental crown with open margin 6  DR. Shaimaa Ahmed b) Conservation of tooth structure: One of the basic aims of restorative dentistry is to conserve as much tooth structure as possible because excessive cutting leads to: a) Thermal hypersensitivity and pulpal inflammation may lead to necrosis. b) The tooth might be over-tapered or shortened, thus affecting the prepared tooth's mechanical properties (retention and resistance). Examples: 1- Partial coverage rather than complete coverage 2- Minimal practical convergence angle 3- Occlusal reduction follows anatomical plane giving uniform restoration thickness 4- preparation of the axial wall as uniform as possible even using orthodontic repositioning 5- Selection of margin conservative and compatible with other principles of tooth preparation. 6- Avoidance of unnecessary apical extension of the preparation. 7- proper axial reduction→ proper axial contour → proper oral hygiene measures 1 2 3 4 5 6, 7 7  DR. Shaimaa Ahmed 3- Consideration affecting future dental health 1- Axial reduction 2- Margin placement 3- Margin adaptation 4- Margin geometry 5- Occlusal considerations 6- Preventing tooth fracture 1- Axial reduction - Gingival inflammation is commonly associated with crowns and FPD abutments having excessive axial contours…………. Because it is more difficult for the patient to maintain plaque control around the gingival margin. - A crown should duplicate the contours and profile of the original tooth (unless the restoration is needed to correct a malformed or malpositioned tooth). 2- Margin Placement Whenever possible, the margin of the preparation should be supragingival. Subgingival margins of cemented restorations have been identified as a major factor in periodontal disease, particularly where they encroach on the epithelial attachment. 8  DR. Shaimaa Ahmed Finishing line: is the final margin that separates between the prepared and the unprepared tooth structure. Requirement of the finishing line: 1. Clear, smooth, and well-defined. 2. Continuous from one surface to the other. 3. Lie on sound tooth structure. Otherwise, it will interfere with the seating of the crown if it is poorly done. Position of finishing line (margin placement) Finishing line can be placed either: 1. Supragingival: Placing the margin above the gingival tissue for the following reasons (Advantages): - a- Can be easily prepared and finished. b- The impression can be easily made. c- The patient can clean the area easily. d- less destructive. e- Most of the time such position is situated on hard enamel. f- Restorations can be easily evaluated at the time of placement and at recall appointments. 2. Subgingival: Placing the crown margin below the gingival tissue but not more than 2mm from the free gingival margin. Subgingival finishing line is indicated for the following reasons: - a- The margin of an esthetic restoration is to be hidden behind the labiogingival crest. b- When we need extra retention (in case of short clinical crown). c- Dental caries, cervical erosion, or restorations extend subgingivally. d- Proximal contact area extends apically to the level of the gingival crest. e- Root sensitivity cannot be controlled by more conservative procedures, such as the application of dentin bonding agents. f- Modification of the axial contour is indicated. 9  DR. Shaimaa Ahmed 3. Placing the margin within gingival level. Marginal placement depends on patient lip lines (Smile Line) 10  DR. Shaimaa Ahmed 3- Margin 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. A well-designed preparation has a smooth and even margin. Rough, irregular, or "stepped" junctions greatly increase the length of the margin and substantially reduce the adaptation of the restoration. 4- Margin Geometry The cross-sectional configuration of the margin (Finish line): Line of demarcation between prepared & unprepared tooth structure. The following guidelines for margin design should be considered: 1. Ease of preparation without overextension or unsupported enamel 2. Ease of identification in the impression and on the die 3. A distinct (clear) boundary to which the wax pattern can be finished 4. Sufficient bulk of material (to enable the wax pattern to be handled without distortion and to give the restoration strength and esthetics when porcelain is used) 5. Conservation of tooth structure (provided the other criteria are met) 11  DR. Shaimaa Ahmed Margin designs: A, Featheredge. B, Chisel. C, Chamfer. D, Bevel. E, Shoulder. F, Sloped shoulder. G, Beveled shoulder. Types of finishing lines according to its design or configuration The following designs for finishing line could be used depending on the type of the crown restoration: 1. Knife edge (also named "feather end") 2. Chisel 3. Bevel 4. Chamfer, and Heavy chamfer 5. Shoulder, Sloped shoulder and Shoulder with bevel 6. Radial shoulder, 12  DR. Shaimaa Ahmed 1. Knife edge or feather end finishing line (Shoulderless) A pointed-end tapered fissure bur (long needle diamond fissure bur) provides this type of margin design. It is the most conservative type of finishing line since the least amount of tooth structure is removed, but the margin is weak since this margin design does not provide enough bulk or thickness for the material. Advantages of knife edge finishing line 1. It is the most conservative type of finishing line. 2. It is easy to prepare. 3. It is a burnishable type of finishing line. i.e. it provides a burnishable margin. “Burnishing is the further adaptation of the margin of metal restoration to the tooth structure.” Disadvantages of knife edge finishing line 1. Difficult to be identified by the laboratory technician. 2. It provides a thin margin that is difficult to accurately wax and cast. 3. The margin of the restoration is susceptible to distortion since this type of margin design does not provide enough thickness. Indications of knife edge finishing line It is mainly used for: Full Metal Crown (All the surfaces). 13  DR. Shaimaa Ahmed 2. Chisel edges Advantages conservation of tooth structure, Disadvantages Location of margin difficult to control 3. Bevel edges Advantages Remove unsupported enamel allow finishing of metal Disadvantages Extend preparation into sulcus if used on apical margin Indication Margin of maxillary partial coverage restorations and inlay and onlay margin 4. Chamfer finishing line It is a well-defined finishing line somewhat like knife edge finishing line except that the cut is made deeper. A tapered stone with round end is used to obtain this preparation margin. Advantages: It provides adequate space at the cervical region so can make the contour of the crown restoration within the contour of natural tooth without overcontouring of the final restoration it is mainly used for: l - Full Metal Crown (All the surfaces). 2- Lingual and proximal surfaces of partial veneer metal-ceramic crown, 3/4 crown and post crown. 14  DR. Shaimaa Ahmed Heavy chamfer finishing line This type of finishing line provides nearly a 90° cavosurface line angle with a large radius internal angle, so it provides better support for the ceramic crown. It can be used with porcelain fused to metal (PFM) crown and All Ceramic crowns. Tapered stone with round end Chamfer Finish line 5. Shoulder finishing line (Butt shoulder) Shoulder finishing line is the least conservative type of finishing line due to the excessive amount of tooth structure removed to obtain this type of finishing line. In the "butt" type of shoulder finishing line, the axial walls meet the finishing line at a right angle. A flat-end tapered diamond stone is used to obtain this finishing line. 15  DR. Shaimaa Ahmed This type of finishing line is used when bulk is required for strength or esthetic, that's why it is almost used with jacket crown since jacket crown is made of either porcelain or acrylic resin, which are brittle materials and require enough thickness to withstand the occlusal forces without fracture. On the other hand, the increased thickness provides better shade of the material and so better esthetics. Advantages provide adequate bulk of material Disadvantages Less conservative to tooth structure Indication Metal-ceramic restoration (Full veneer), Facial margin of metal ceramic restoration (Partial veneer) Complete ceramic crown Preparation Using tapered diamond stone with a flat tip It should form 90-degree angle with unprepared tooth surface Tapered stone with flat end Shoulder Finish line 16  DR. Shaimaa Ahmed Sloped shoulder margin Advantages provide adequate bulk of material In addition to advantage of bevel Disadvantages Less conservative to tooth structure Indication Facial margin of metal ceramic restoration Complete ceramic crown Shoulder with bevel Shoulder with bevel is another modification of the shoulder finishing line by adding a bevel to the shoulder. The bevel is at 45° angle. Objectives of adding a bevel to the shoulder finishing line 1. The bevel provides a burnishable margin for the metal that may extend subgingivally (The thinner it is, the more adaptable to the tooth surface). 2. To provide enough space for shape and contour. 3. To reduce marginal discrepancies. 4. To remove unsupported enamel. 17  DR. Shaimaa Ahmed Advantages provide adequate bulk of material In addition to advantage of bevel Disadvantages Less conservative to tooth structure Extend preparation epically Indications Facial margin of posterior metal ceramic restoration in which facial metal collar is used It is indicated when we use a combination of metal with facing material (acrylic or porcelain) as in full veneer crown, where it is used for the labial surface. Shoulder with bevel 6. Radial shoulder finishing line Radial shoulder is a modification of the shoulder finishing line. It is a shoulder finishing line with rounded internal line angles. This will reduce the shoulder slightly and minimize stress concentration on the tooth structure from one hand and on the restoration itself from the other hand. 18  DR. Shaimaa Ahmed 5- Occlusal Considerations ❖ Tooth preparation should allow sufficient space for a functional occlusal scheme in the finished restoration. ❖ Disrupted occlusion (tilting, supra-eruption) need occlusal analysis then considerable reduction to compensate for the supra-erupted tooth. ❖ In case of severe occusal disruption, violating the principle of conservation of tooth structure is preferable to the potential harm from a traumatic occlusal scheme. 6- Preventing Tooth Fracture. Tooth preparation should be designed to prevent fracture of restored teeth. Providing a cuspal coverage restoration (onlay) rather than an inlay lessens the chance of such fracture. However, although not conservative of tooth structure, a complete crown is often a better solution 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, but it can be associated with periodontal disease and poor esthetics. 19  DR. Shaimaa Ahmed 2) Mechanical considerations The design of tooth preparations must adhere to certain mechanical principles; otherwise, the restoration may become dislodged or may distort or fracture during service. Mechanical considerations can be divided into three categories: 1. Providing retention form. 2. Providing resistance form. 3. Preventing deformation of the restoration. 1. Providing retention form: Retention: is the ability of the preparation to resist the removal of the crown restoration along its path of withdrawal. 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. Path of insertion: An imaginary line along which the restoration can be inserted and removed without causing lateral forces on the abutment. The crown restoration should have a single path of insertion to be retentive. Which is mostly parallel to the long axis of the tooth but sometimes not parallel for example: in 3/4 crown for anterior teeth the path of insertion should be parallel to the incisal 2/3 of the tooth crown (not to the long axis). 20  DR. Shaimaa Ahmed Factors affecting retention: 1. Magnitude of dislodging force 2. Geometry of tooth preparation a) Taper b) Surface area c) Stress concentration d) Type of preparation. 3. Roughness of the fitting surface of the restoration 4. Material being cemented 5. Type of luting cement. 6. Film thickness of the luting cement. 1.Magnitude of dislodging force Forces that tend to remove a cemented restoration along its path of withdrawal are small compared to those that tend to seat or tilt it. Fixed dental prosthesis can be subjected to such forces by: - Pulling with floss under the connectors - The greatest removal forces arise when exceptionally sticky food (e.g. caramel) is eaten. The magnitude of dislodging forces depends on: - Stickiness of the food. - The surface area. - Texture of the restoration being pulled. Pulling with floss under the connectors 21  DR. Shaimaa Ahmed 2.Geometry of tooth preparation Most fixed prosthesis depends on the geometric form of the Film thickness of the luting cement. Preparation rather than on adhesion for retention because most of the traditional cement (e.g. zinc phosphate) are non-adhesive (i.e. they act by increasing the frictional resistance between tooth and restoration) ……………. The grains of cement prevent two surfaces from being lifted from another. Cement is effective only if the restoration has a single path of withdrawal (i.e. the tooth is shaped to restrain the free movement of the restoration). a) Taper: An undercut is defined as a divergence between opposing axial walls segments in a cervical- occlusal direction. i.e. if the cervical diameter of a tooth preparation at the margin is narrow than at the occlusoaxial junction (reverse taper) it will be impossible to seat a complete cast crown of similar geometry 22  DR. Shaimaa Ahmed Path of insertion It is (imaginary line) the direction through which the restoration could be precisely seated on to or removed from the prepared teeth. Path of insertion could be classified into: A) For single restoration (crown): Line of insertion; It is the single direction through which the restoration could be precisely seated on the prepared tooth. B) For bridge: Common line of insertion: It is the single direction through which all the retainers of a fixed-fixed bridge could be precisely seated on the corresponding abutment teeth. Factors affecting the path of insertion: 1) Reduction of axial surfaces. 2) Rounding of axial line angles. 3) Proximal grooves. 4) Ledges and/ or indentation. 5) Pinholes. 23  DR. Shaimaa Ahmed B) Surface area Provided the restoration has a limited path of withdrawal, its retention depends on the length of this path. So, crown with long axial walls are more retentive than those with short axial walls. Molar crowns are more retentive than premolar crowns of similar taper. C) Stress concentration Cohesive failure……… When a retentive failure occurs, cement adheres to both the tooth preparation and the fitting surface of the restoration. In these cases, cohesive failure occurs through the cement layer because the strength of the cement is less than the induced stresses. d) Type of preparation Retention of a complete crown is about double that of partial-coverage restorations. Addition groove which limits the path of withdrawal, increase retention. 3. Roughness of the fitting surface of the restoration When the internal surface of a restoration is very smooth…..retentive failure occurs not through the cement but at the cement-restoration interface. Retention will be increased if the restoration is roughened or grooved. The casting is most effectively prepared by air-abrading the fitting surface with 50 µm of alumina. 4.Material being cemented ❖ Retention is affected by both o Type of casting alloy o The core or build up material One laboratory study examining adhesion between cements and core materials found that the cement adheres better to amalgam than to composite or cast gold. However, when crowns were tested for retention…...higher values were found with the composite resin than with amalgam cores. The difference may be due to the dimensional changes of the core materials. 24  DR. Shaimaa Ahmed 5.Type of the luting agent The type of luting agent chosen affects the retention of a cemented restoration. But the decision about luting agent to use is also based on other factors. In general, the data suggest that adhesive resin cements are the most retentive. 6.Film thickness of the luting cement There is conflicting evidence about the effect of increased thickness of the cement film on retention of a restoration. but ………… increase the film thickness Will endanger the cement strength Ideal dimension for luting agent space is 20-40 µm for each wall 2. Providing resistance form. Definition: It is the features of the preparation that enhance the stability of a restoration and resist dislodgement along an axis other than the path of withdrawal. Tipping path: It is the path along which the restoration could be displaced under the displacing occlusal force Rotation is prevented by any areas of the tooth preparation that are placed in compression, called resistance areas Adequate resistance depends on: - a. Magnitude and direction of the dislodging forces. b. Geometry of the tooth preparation. c. Physical properties of the luting agent. 25  DR. Shaimaa Ahmed a. Magnitude and direction of the dislodging forces: In normal occlusion biting force is distributed over all the teeth and most of it is axially directed. If the restoration is carefully made with a properly designed occlusion……. the load should be well distributed and favorably directed. However, if a patient has a biting habit as pipe smoking or bruxing…...it may be difficult to prevent large oblique forces from being applied to a restoration. So, the competed tooth preparation and restoration must be able to withstand considerable oblique forces as well as the normal axial ones. b. Geometry of the tooth preparation: As in retention, preparation geometry plays a key role in attaining desirable resistance form. The tooth preparation must be shaped so that particular areas of the axial walls prevent rotation of the crown. 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. Molar teeth require more parallel preparation than premolar and anterior teeth to achieve adequate resistance form. A partial-coverage restoration may have less resistance than complete crown because it has no buccal resistance area. Resistance must be provided by boxes or grooved and will be greatest if they have walls that are perpendicular to the direction of the applied force. So, the U-shaped grooves or flared boxes provide more resistance than V-shaped ones. The resistance form of an excessively tapered preparation can be improved by adding grooves or pinholes because these interfere with the rotational movement and subject additional areas of the luting agent to compression. 26  DR. Shaimaa Ahmed c. Physical properties of the luting agent: Physical properties as compression strength and modulus of elasticity affect the resistance form Higher resistance is gained by using Adhesive resin > Glass ionomer > Zinc phosphate Polycarboxylate > Zinc oxide–eugenol 3-Resistance to deformation and structural durability: A restoration must have sufficient strength to prevent permanent deformation during function, otherwise it will fail (at the restoration-cement or metal porcelain interface). This may be due to:- In appropriate alloy selection Inadequate tooth preparation Poor metal-ceramic framework design Structural durability The restoration must be rigid enough not to flex, perforate (metal) or even fracture (plastic) during occlusal forces. Sufficient tooth structure should be removed to create enough thickness to the ideal occlusion. -Preparation features related to Structural durability: Factors affecting deformation and structural durability: - 1- Alloy Selection 2- Adequate tooth reduction A) Occlusal reduction B) Functional cusp bevel C) Axial reduction 3- The Path of insertion 4- Margin and framework design 27  DR. Shaimaa Ahmed 1- Alloy Selection Metal ceramic alloy with high noble alloy or nickel-chromium alloys → ↑↑ hardness 2- Adequate tooth reduction A) Occlusal reduction Should be as uniform as possible, following the cuspal planes of the teeth → sufficient occlusal clearance + preservation of as much tooth structure as possible. ▪ Occlusal reduction must reflect the geometric inclined planes of occlusal surface with beveling of the functional cusps (palatal cusps of the upper posterior teeth and the buccal of lower posterior). ▪ Avoid creating steep planes with sharp angles that lead to stress concentration. ▪ Flat occlusal reduction leads to thin metal and perforation in the future. ▪ Lowering the entire occlusal surface as an attempt to provide enough space will destruct the tooth structure. Occlusal clearance: is the space between occlusal surface of prepared tooth and that of the opposing tooth. It should evaluate in centric and eccentric relation. 28  DR. Shaimaa Ahmed B) Functional cusp bevel ◼ Allow sufficient space for optimal restoration thickness in this functional area ◼ If not performed → thin casting → perforation ◼ In mandibular molars on buccal cusps, In maxillary molars on palatal cusp Beveling of the functional cusp c) Axial reduction: it must be sufficient to provide enough space to the restoration to duplicate the normal contour of the tooth without flexure during occlusal forces. Removal of the undercut area, Alignment with the path of insertion 3- The Path of insertion ◼ It is the direction through which the restoration could be precisely seated on the prepared tooth or teeth ◼ Failure in establishing a path of insertion for a single restoration may result in either: a. Distortion of the wax pattern, or b. Failure in seating the restoration. 29  DR. Shaimaa Ahmed Factors Affecting the Path of Insertion 1. Reduction of the opposing axial surfaces: The reduced opposing axial surfaces may be: a. Parallel to each other to give a line of insertion, or with b. Slight occlusal convergence to give a range of insertion. 2. Rounding the axial line angles 3. Alignment of the axial grooves 4. Alignment of ledges and indentation 5. Pinholes 4- Margin and framework design · Occlusally, the dentist should design the preparation outline form so that occlusal contact is avoided in the junction between metal and ceramic in case of metal-ceramic partial veneered restorations. Keeping preparation margins approximately 1 to 1.5 mm away from occlusal contact locations The configuration of the finishing line determines the shape and bulk of the restoration margins. The requirements of the restoration margins: ▪ It must be well adapted (as fit as possible) to the finishing line of tooth preparation. ▪ It must have enough strength. ▪ Should be placed in the area that is easily polished by dentist and cleaned by the patient. To minimize plaque accumulation, the selected margin must present smooth materials to the gingival sulcus. In some situations, the margin also must provide acceptable esthetics. 30  DR. Shaimaa Ahmed 3) Esthetic consideration: Teeth are important not only from the functional point but also because they contribute substantially towards the psychological well-being of the person further, tooth loss in the aesthetic zone in younger patients is a catastrophic event, which is mainly attributed to the genetic, caries, and traumatic injuries. Replacement of this by a faulty prosthesis, leads to compromised esthetics, inflammation and hyperplasia in the tissues and mechanical failures of the prosthesis. Appearance Zone This is the anterior oral area where aesthetics is of prime concern & which is visible on smiling, from maxillary premolar to premolar (usually 1st molars also). Depends on the person’s self-image, mouth size, teeth size, smile width, lip size and tightness. 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 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. 31  DR. Shaimaa Ahmed Esthetic restorations include: 1- All ceramic restorations 2- Metal-ceramic restorations 3- Partial veneer crowns Certain principles are recommended during tooth preparation that will ensure sufficient room for porcelain and accurate placement of the margins. These principles include: 1- Facial Tooth Reduction. 2- Incisal Reduction. 3- Proximal Reduction. 4- Labial Margin Placement. 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 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 32  DR. Shaimaa Ahmed Metal collar Partial-Coverage Restorations Whenever possible, an esthetically acceptable result without the use of complete crown 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. The current focus is on conservative tooth preparation that is non-invasive and minimally involves dentin. Principles of tooth preparation are presently being modified to accommodate modern approaches i.e., acid etching with minimum reduction. Dentistry is changing from macro tooth preparation to esthetic bonding (Laminate veneers). GOOD LUCK 33