**PORCELAIN FUSED TO METAL CROWNS** *Principles of Tooth Preparation* *Tooth Preparation - Steps* Dr V Matsubara # **PORCELAIN FUSED TO METAL CROWNS** ## **Principles of Tooth Preparation** ## **Tooth Preparation - Steps** Learning Outcomes: - Understand and discuss the **objectives of tooth preparation** for indirect restorations. - Identify the **indications** for tooth preparation in indirect restorative procedures. - Explain the **principles of tooth preparation**, including **biological**, **mechanical**, and **aesthetic** considerations. - Describe the **general features** of a **Porcelain-Fused-to-Metal (PFM) crown**. - Discuss the **indications and contraindications**, as well as the **advantages and disadvantages** of PFM crowns. - Discuss the **types of metal alloys** used in PFM fabrication and the **basic material science** involved. - Understand the **clinical steps** involved in preparing a tooth for a PFM crown, including the **required amount of tooth reduction**. # **Learning Outcomes** - Understand and discuss the **objectives of tooth preparation** for indirect restorations. - Identify the **indications** for tooth preparation in indirect restorative procedures. - Explain the **principles of tooth preparation**, including **biological**, **mechanical**, and **aesthetic** considerations. - Describe the **general features** of a **Porcelain-Fused-to-Metal (PFM) crown**. - Discuss the **indications and contraindications**, as well as the **advantages and disadvantages** of PFM crowns. - Discuss the **types of metal alloys** used in PFM fabrication and the **basic material science** involved. - Understand the **clinical steps** involved in preparing a tooth for a PFM crown, including the **required amount of tooth reduction**. > [!note] Foundational Principles > This lecture serves as a foundational guide to the principles of tooth preparation for indirect restorations, which will be practiced throughout the semester. Understanding these principles is crucial for clinical decision-making and communication with tutors and patients. These principles are universally applicable to various indirect restorations such as full crowns, onlays, and bridges. PART I **Tooth Preparation for indirect restorations** Clinical process of mechanically altering a tooth to receive a custom-made restoration fabricated outside the mouth - Crown prep. - Bridge prep. - Inlay/onlay prep. - Veneer prep. - Post space prep. # **PART I** ## **Tooth Preparation for indirect restorations** Clinical process of mechanically altering a tooth to receive a custom-made restoration fabricated outside the mouth. > [!info] Contrast with Direct Restorations > Unlike direct fillings (e.g., amalgam), where the cavity must have retentive features like undercuts, preparations for indirect restorations must be free of undercuts to allow the restoration to be seated. Retention is achieved through different mechanical principles. - Crown prep. - Bridge prep. - Inlay/onlay prep. - Veneer prep. - Post space prep. # Objective of Tooth Preparation To optimize the shape and form of a tooth that will receive an indirect restoration: - Create room for the selected restorative material - Remove diseased and/or weakened tooth structure - Provide retention and resistance to the restoration - Allow a proper fit, function, aesthetics and longevity - Facilitate subsequent procedures (provisional restoration, impression, cast pouring, final restoration fabrication) - Preserve healthy tooth structure # **Objective of Tooth Preparation** To optimize the shape and form of a tooth that will receive an indirect restoration: - Create room for the selected restorative material - Remove diseased and/or weakened tooth structure > [!warning] No Remaining Decay > Unlike some direct filling techniques, decay cannot be left near the pulp for crowns due to the cost and complexity of future intervention if symptoms arise. - Provide retention and resistance to the restoration - Allow a proper fit, function, aesthetics and longevity - Facilitate subsequent procedures (provisional restoration, impression, cast pouring, final restoration fabrication) > [!tip] > A well-defined preparation is crucial, as deep or rough margins can make the impression-taking process difficult and unpredictable. - Preserve healthy tooth structure # Indications for Tooth Preparation ## 1) Therapeutic preparations: Restoration of teeth that have lost tooth structure by caries and non-caries lesions: attrition, abrasion, erosion, abfraction; or fractures. ## 2) Preparations of healthy teeth: Preparation of abutments, supports of RPR, occlusal correction, aesthetics. # **Indications for Tooth Preparation** ## **1) Therapeutic preparations:** Restoration of teeth that have lost tooth structure by caries and non-caries lesions: attrition, abrasion, erosion, abfraction; or fractures. - ==**Attrition:** Wear from tooth-to-tooth contact, often seen in patients who grind (bruxism).== - ==**Abrasion:** Mechanical wear from foreign objects (e.g., aggressive toothbrushing).== - ==**Erosion:** Chemical dissolution of tooth structure from acids.== - ==**Abfraction:** Wedge-shaped cervical lesions with sharp internal angles, caused by tooth flexure under occlusal load.== ## **2) Preparations of healthy teeth:** Preparation of abutments, supports of RPR, occlusal correction, aesthetics. > [!example] Aesthetic Indications > An example of preparing a healthy tooth for aesthetics is to mask discoloration (e.g., after root canal treatment) or to alter its shape for a better appearance. # Principles of Tooth Preparation Venn diagram illustrating the intersection of BIOLOGICAL, MECHANICAL, and AESTHETIC principles for optimal tooth preparation, with an arrow pointing to the central overlap labeled "Optimal situation". # **Principles of Tooth Preparation** # Principles of Tooth Preparation **Biological factors** - Affect the health of oral tissues **Mechanical factors** - Determine the integrity and durability of the restoration **Aesthetic factors** - Determine the appearance of the final restoration **Case by case decision: choose the ideal combination** ## **Principles of Tooth Preparation** > [!abstract] > The preparation of a tooth for any indirect restoration is governed by three interdependent principles. An ideal preparation successfully balances the requirements of all three. - **Biological factors** - Affect the health of oral tissues - **Mechanical factors** - Determine the integrity and durability of the restoration - **Aesthetic factors** - Determine the appearance of the final restoration Case by case decision: choose the ideal combination # Principles of Tooth Preparation ## Biological Factors - Prevention of tooth damage during preparation - Preservation of tooth structure - Avoidance of overcontouring the restoration - Supragingival margins - Marginal integrity - Harmonious occlusion - Protection of remaining tooth structures # **Principles of Tooth Preparation** ## **Biological Factors** - Prevention of tooth damage during preparation - Preservation of tooth structure - Avoidance of overcontouring the restoration - Supragingival margins - Marginal integrity - Harmonious occlusion - Protection of remaining tooth structures **Biological Factors** **Prevention of Tooth Damage During Preparation** 1) Proximal teeth - Can lead to caries and gingivitis 2) Soft tissues - Gingival tissues, tongue & cheeks 3) Pulp - Thermal irritation: use copious irrigation - Chemical irritation: bases, resins, etch, solvents & luting agent (4-8% of pulps die in the 10 years following crown preparations) - Bacterial irritation: either left behind or gained access due to microleakage Valderhaug et al; 1997 ### **Prevention of Tooth Damage During Preparation** 1. **Proximal teeth** - Can lead to caries and gingivitis 2. **Soft tissues** - Gingival tissues, tongue & cheeks 3. **Pulp** - **Thermal irritation:** use copious irrigation > [!danger] Avoid Overheating > Overheating can occur from inadequate water irrigation, especially with high-speed handpieces (400,000 RPM) or old/finishing burs. Cutting a tooth dry is unacceptable. - **Chemical irritation:** bases, resins, etch, solvents & luting agent (4-8% of pulps die in the 10 years following crown preparations) > [!info] > This irritation can happen if the prepared dentin is exposed to various chemicals when the tooth is not properly protected with a provisional restoration. - **Bacterial irritation:** either left behind or gained access due to microleakage > [!info] > Bacterial irritation often occurs due to microleakage under a poorly sealed restoration, which can lead to pulp symptoms over time. Valderhaug et al; 1997 # Biological Factors ## Preservation of Tooth Structures - Tooth structures loss due to material selection - Metal coverage alone requires less reduction **Gold crown** **PFM** ### **Preservation of Tooth Structures** - Tooth structures loss due to material selection - Metal coverage alone requires less reduction **Gold crown** **PFM** **Biological Factors** Preservation of Tooth Structures Preparation with minimal taper Occlusal reduction following the anatomical contour Diagram showing tooth preparation with minimal taper and occlusal reduction following anatomical contour. - Preparation with minimal taper - Occlusal reduction following the anatomical contour > [!tip] > This not only preserves more tooth structure but also keeps the preparation further from the pulp. **Biological Factors** Preservation of Tooth Structures - Uniform axial reduction - Conservative margins - Avoid unnecessary apical extension of the preparation Image of a tooth preparation model showing a uniform axial reduction with a marked margin. - Uniform axial reduction - Conservative margins - Avoid unnecessary apical extension of the preparation **Biological Factors** **Avoidance of Over contouring the Restoration** - Favors plaque build ups – gingivitis, periodontitis - Ideal contours of the restoration should follow the natural contour of the tooth and surrounding tissue (emergence profile) Diagram illustrating proper and improper restoration contours and their impact on surrounding tissue. ### **Avoidance of Over contouring the Restoration** > [!warning] Cause of Over-Contouring > If the tooth preparation does not provide enough space, the dental technician will be forced to make the restoration bulky or **over-contoured** to achieve the minimum required material thickness. - Favors plaque build ups – gingivitis, periodontitis - Ideal contours of the restoration should follow the natural contour of the tooth and surrounding tissue (emergence profile) **Biological Factors** **Avoidance of Over contouring the Restoration** - Favors plaque build ups – gingivitis, periodontitis - Ideal contours of the restoration should follow the natural contour of the tooth and surrounding tissue (emergence profile) # Biological Factors ## Supragingival Margins - Easily prepared - Easier to take impressions - Easily maintained by patient - Easily evaluated at recalls - Preserve periodontium Diagram showing sulcular epithelium, junctional epithelium, connective tissue attachment, and alveolar crest with associated measurements. ### **Supragingival Margins** > [!success] Ideal Approach > Placing the restoration margin **above the gumline** is the ideal approach whenever possible. - Easily prepared - Easier to take impressions - Easily maintained by patient - Easily evaluated at recalls - Preserve periodontium **Biological Factors** **Subgingival Margins** **Indications** - Subgingival dental caries, erosion, fracture or restoration - Apical proximal contact area - Additional retention is needed - Aesthetic margins - Coverage of root surface - Modification of axial contour ### **Subgingival Margins** > [!danger] Biological Width Violation > The margin can be placed within the gingival sulcus (up to 0.7 mm) but must **not violate the biological width** (the combined height of the junctional epithelium and connective tissue attachment), as this will cause chronic inflammation. #### **Indications** - Subgingival dental caries, erosion, fracture or restoration - Apical proximal contact area - Additional retention is needed - Aesthetic margins - Coverage of root surface - Modification of axial contour - ==To improve the emergence profile of a crown when closing a diastema (gap).== **Biological Factors** **Marginal Integrity** - Potential site for caries: - Dissolution of cement - Inherent roughness leading to plaque accumulation - Should be even and smooth: - Facilitates subsequent steps till the delivery of the restoration ### **Marginal Integrity** - Potential site for caries: - Dissolution of cement - Inherent roughness leading to plaque accumulation - Should be even and smooth: - Facilitates subsequent steps till the delivery of the restoration # Biological Factors ## Marginal Integrity **Healthy periodontium:** - Mean sulcular depth 0.69 mm - Mean junctional epithelial length 0.97 mm - Mean connective tissue attachment length 1.07mm - JE + CT attachment is known as “biological width” **If you place a margin within the biological width, increase the chance of gingival inflammation Followed by bone loss as the biological width tries to re-establish itself** #### **Healthy periodontium:** - Mean sulcular depth 0.69 mm - Mean junctional epithelial length 0.97 mm - Mean connective tissue attachment length 1.07mm - JE + CT attachment is known as “biological width” If you place a margin within the biological width, increase the chance of gingival inflammation. Followed by bone loss as the biological width tries to re-establish itself. > [!tip] Follow the Gingival Contour > The preparation margin should follow the natural scalloped contour of the gingiva, rising over the interdental papilla to avoid placing it too deep proximally. **Biological Factors** **Marginal Integrity** Follow the gingival and papillary contour to avoid violation of biological width Clinical image showing anterior teeth with healthy gingival and papillary contours. Follow the gingival and papillary contour to avoid violation of biological width. **Biological Factors** **Margin/finishing lines** **Design requirements** - Simplicity of preparation - On intact tooth surface - No unsupported enamel - Ease of identification - Allows sufficient bulk of restoration - Durability - Aesthetics - Conservative **Possible Designs** 1) Feather edge 2) Bevel 3) Chamfer 4) Shoulder 5) Shoulder with bevel ### **Margin/finishing lines** #### **Design requirements** - Simplicity of preparation - On intact tooth surface - No unsupported enamel - Ease of identification - Allows sufficient bulk of restoration - Durability - Aesthetics - Conservative #### **Possible Designs** 1. Feather edge 2. Bevel 3. Chamfer 4. Shoulder 5. Shoulder with bevel **Biological Factors** **Margin/finish lines** 1) Feather edge - **Advantages:** Conservation of tooth structure - **Disadvantages:** Fail to provide adequate bulk at margins Diagram showing dental burs (860, 861, 862, 863) and a cross-section of a tooth with a feather edge margin. #### **1) Feather edge** - **Advantages:** - Conservation of tooth structure - **Disadvantages:** - Fail to provide adequate bulk at margins - ==It is **not recommended** as it is difficult to identify and often leads to over-contoured restorations.== # Biological Factors ## Margin/finish lines ### 2) Bevel - **Advantages:** - Allow the margin of crown to be burnishing against tooth structure - Protect the unprepared tooth structure from chipping by remove unsupported enamel - **Disadvantages:** - Lead to subgingival extension Diagram showing a tooth with a beveled margin. #### **2) Bevel** - **Advantages:** - Allow the margin of crown to be burnishing against tooth structure - Protect the unprepared tooth structure from chipping by remove unsupported enamel - **Disadvantages:** - Lead to subgingival extension - ==It is not suitable for all-ceramic crowns as it results in a thin, fragile ceramic edge.== **Biological Factors** **Margin/finish lines** **3) Shoulder** - Margin for all materials - Easy to prepare and finish well - Easy to judge impressions - Frequently used for metal crowns, metal ceramic crowns and all ceramic crowns **Modified Shoulder** CAD-CAM all ceramic #### **3) Shoulder** - Margin for all materials - Easy to prepare and finish well - Easy to judge impressions - Frequently used for metal crowns, metal ceramic crowns and all ceramic crowns > [!info] Traditional vs. Modern Shoulder > The traditional shoulder has a sharp internal line angle suitable for cast restorations. For modern **CAD/CAM restorations, a rounded internal line angle is required** to allow the milling bur to fabricate it accurately. **Modified Shoulder** CAD-CAM all ceramic # Biological Factors ## Margin/finish lines ### 4) Chamfer - Can be used margin for all metal and all ceramic restorations - Preserves tooth tissue whilst providing distinct finish line - Easy to read in impressions Diagram illustrating Chamfer and Shoulder margin types #### **4) Chamfer** - Can be used margin for all metal and all ceramic restorations - Preserves tooth tissue whilst providing distinct finish line - Easy to read in impressions > [!warning] Gutter Margin > Be careful not to sink the bur too deep, as this can create a 'gutter margin' with unsupported enamel. **Biological Factors** **Margin/finish lines** 5) Shoulder with bevel Two images of a tooth preparation showing a shoulder with bevel finish line from different angles. #### **5) Shoulder with bevel** **Biological Factors** **Harmonious Occlusion** - Confirmative or reorganized approach - Decide upon a satisfactory occlusal scheme prior to tooth preparation - Especially important with overerupted & tilted teeth - Eventual occlusal plane adjustment - Possibility of elective endodontic treatment - Possibility of crown lengthening surgery - Diagnostic wax-ups can be extremely helpful ### **Harmonious Occlusion** - Confirmative or reorganized approach - ==**Conformative Approach:** The restoration is designed to fit into the patient's existing occlusion without changing it. This is the standard approach in the dental school clinic.== - ==**Reorganized Approach:** The entire occlusion is rebuilt, usually in complex cases of severe wear.== - Decide upon a satisfactory occlusal scheme prior to tooth preparation > [!todo] Pre-operative Assessment > Occlusion must be assessed *before* starting the preparation to identify contact points and guidance patterns (e.g., canine guidance, group function). - Especially important with overerupted & tilted teeth - Eventual occlusal plane adjustment - Possibility of elective endodontic treatment - Possibility of crown lengthening surgery - Diagnostic wax-ups can be extremely helpful **Biological Factors** Protection of Remaining Tooth Structures - Ensure all weakened tooth structure is removed prior to core placement Clinical photo showing dental instrument removing decay from a tooth and a diagram illustrating core placement with a red arrow indicating force direction. ### **Protection of Remaining Tooth Structures** - Ensure all weakened tooth structure is removed prior to core placement # Biological Factors ## Protection of Remaining Tooth Structures ### CUSPAL COVERAGE A) Tooth structure less than 2 mm B) Low quality of the enamel covering the cusp Diagram illustrating cuspal coverage with labels A and B, alongside a cross-sectional view showing force distribution on a tooth with a restoration. #### **CUSPAL COVERAGE** A) Tooth structure less than 2 mm B) Low quality of the enamel covering the cusp **Biological Factors** Protection of Remaining Tooth Structures **CUSPAL COVERAGE** Occlusal contacts on the margin of the preparation #### **CUSPAL COVERAGE** Occlusal contacts on the margin of the preparation **Biological Factors** Protection of Remaining Tooth Structures **CUSPAL COVERAGE** Functional cusps: Buccal – IP Lingual – SP Diagram showing tooth cusps with red circles highlighting functional cusps. #### **CUSPAL COVERAGE** Functional cusps: - Buccal – IP - Lingual – SP > [!tip] Supporting vs. Non-Supporting Cusps > **Supporting cusps** (palatal on uppers, buccal on lowers) bear the primary occlusal load and require more reduction than non-supporting cusps. **Biological Factors** Protection of Remaining Tooth Structures **CUSPAL COVERAGE** Endodontically treated tooth ↓ Weakness of cusp #### **CUSPAL COVERAGE** > [!info] > Covering the cusps with the restoration (as in an onlay or full crown) distributes occlusal forces over the entire tooth surface, preventing individual cusps from flexing and fracturing. Endodontically treated tooth ↓ Weakness of cusp # Principles of Tooth Preparation **BIOLOGICAL** - Conservation of tooth structure - Avoidance of overcontouring - Supragingival Margins - Harmonious Occlusion - Protection against tooth fracture **MECHANICAL** - Retention form - Resistance form - Deformation **ESTHETIC** - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins **Optimal situation** SF Rosenstiel, MF Land, R Walter (2022) Contemporary fixed prosthodontics # **Principles of Tooth Preparation** ### **BIOLOGICAL** - Conservation of tooth structure - Avoidance of overcontouring - Supragingival Margins - Harmonious Occlusion - Protection against tooth fracture ### **MECHANICAL** - Retention form - Resistance form - Deformation ### **ESTHETIC** - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins **Optimal situation** SF Rosenstiel, MF Land, R Walter (2022) Contemporary fixed prosthodontics **Mechanical Factors** **Retention form** - Preparation feature which resists displacement occlusally - Avoid relying on cements alone to retain the restoration - Purpose: placing the cement shear instead of tension - Retention form depends on: - Preparation height - Degree of taper - Freedom of displacement - Roughness of the surfaces - Physical properties of luting cement ## **Mechanical Factors** ### **Retention form** - Preparation feature which resists displacement occlusally > [!info] > This means it prevents the restoration from being dislodged along its **path of insertion** (i.e., resists pulling forces). - Avoid relying on cements alone to retain the restoration - Purpose: placing the cement shear instead of tension - Retention form depends on: - Preparation height - Degree of taper - Freedom of displacement - Roughness of the surfaces - Physical properties of luting cement # Mechanical Factors ## Retention form **Geometry of Tooth Preparation** - Enhance the cement function - Cement: - Fills the crown-tooth space and creates friction - Prevents surface from sliding #### **Geometry of Tooth Preparation** - Enhance the cement function - Cement: - Fills the crown-tooth space and creates friction - Prevents surface from sliding - ==**Height:** Taller preparations are more retentive than shorter ones.== - ==**Width:** For a given height, narrower preparations (e.g., incisors) are more retentive than wider ones (e.g., molars).== # Mechanical Factors ## Retention form **Geometry of Tooth Preparation** Parallels: Best retention, but difficult to fit Overtapered: unsatisfactory retention Diagram showing three tooth preparation geometries: two crossed out (parallels and overtapered) and one circled (6°) - **Parallels:** Best retention, but difficult to fit > [!warning] > Parallel walls are clinically impractical, as they make it impossible to fully seat the crown without binding. - **Overtapered:** unsatisfactory retention # Mechanical Factors ## Retention form ### Geometry of Tooth Preparation The axial walls must taper to: - Permit more complete seating of the restoration - Prevent undercuts - Visualize preparation walls and finish line - Compensate for inaccuracies during fabrication Diagram illustrating "Undercuts" and "Overtapered" tooth preparation geometries. The axial walls must taper to: - Permit more complete seating of the restoration - Prevent undercuts - Visualize preparation walls and finish line - Compensate for inaccuracies during fabrication **Mechanical Factors** **Retention form** **Degree of taper** - ideal taper 6° - Clinically 6–20° is acceptable - Tapered burs are designed to cut a 3° taper on each wall and should be hold parallel to the long axis of the tooth #### **Degree of taper** - ideal taper 6° - Clinically 6–20° is acceptable > [!tip] > Short preparations require minimal taper (closer to parallel) to maximize retention. - Tapered burs are designed to cut a 3° taper on each wall and should be hold parallel to the long axis of the tooth # Mechanical Factors ## Retention form ### Degree of taper - One eye vision - All the margins should be clearly visible - In the mouth by mirror - On the cast (distance of 30 cm) - One eye vision - All the margins should be clearly visible - In the mouth by mirror - On the cast (distance of 30 cm) # Mechanical Factors ## Retention form ### Preparation Surface 1) Surface area - The greater the surface area, the greater the retention - Molars are more retentive than premolars with similar degree of taper 2) Surface roughness enhances retention Diagram comparing two preparation surfaces (A and B) with arrows indicating retention force. #### **Preparation Surface** 1. **Surface area** - The greater the surface area, the greater the retention - Molars are more retentive than premolars with similar degree of taper 2. **Surface roughness enhances retention** > [!info] > A rougher prepared surface provides more microscopic retention for the luting cement. # Mechanical Factors ## Resistance form – Stability - Preparation features which resist displacement in all directions other than occlusal (horizontal and oblique forces) - Depends on: - Magnitude and direction of dislodging forces - Geometry of the tooth preparation: - Height - Degree of taper - Cementation material - Resistance to deformation ### **Resistance form – Stability** - Preparation features which resist displacement in all directions other than occlusal (horizontal and oblique forces) > [!important] > Resistance form is considered more critical than retention form because chewing forces are primarily oblique, not vertical. - Depends on: - Magnitude and direction of dislodging forces - Geometry of the tooth preparation: - Height - Degree of taper - Cementation material - Resistance to deformation # Mechanical Factors ## Resistance form - Increasing the proportion of axial surfaces: More surface area interfering with dislodgement: - Reduced taper - Increased preparation length Higher preparations show more stability - Less resistance with partial coverage - Increasing the proportion of axial surfaces: More surface area interfering with dislodgement: - Reduced taper - Increased preparation length - Higher preparations show more stability > [!info] Mechanism of Resistance > A tall preparation with minimal taper will resist rotational forces, whereas a short, over-tapered preparation will offer almost no resistance, allowing the crown to be easily dislodged. - Less resistance with partial coverage # Mechanical Factors ## Resistance form > Tooth with smaller diameter resists more to rotating movements than preparation with equal height, but greater diameter. Short perps - Molars require more parallel surfaces than premolars or anterior teeth Diagram showing two tooth preparations, one with a smaller diameter and one with a greater diameter, illustrating resistance to rotating movements. > Tooth with smaller diameter resists more to rotating movements than preparation with equal height, but greater diameter. Short perps - Molars require more parallel surfaces than premolars or anterior teeth # Mechanical Factors ## Resistance form > Consider additional features > Pin holes > Boxes > grooves All in natural tooth structure > Consider additional features > [!tip] When to Use Additional Features > For short clinical crowns, **retention grooves or boxes** can be added to enhance resistance form. > Pin holes > Boxes > grooves All in natural tooth structure **Mechanical Factors** Resistance to Deformation (Structural Durability) Depends - Depth of preparation (space for restoration) - Type of prep margin - Type of restorative material ### **Resistance to Deformation (Structural Durability)** Depends on: - Depth of preparation (space for restoration) - Type of prep margin - Type of restorative material **Mechanical Factors** Resistance to Deformation (Structural Durability) - Sufficient strength to prevent permanent deformation - Failure at cement-restoration interface or metal-ceramic interface May result from: - Inappropriate material selection - Inadequate preparation - Poor restoration design - Sufficient strength to prevent permanent deformation - Failure at cement-restoration interface or metal-ceramic interface May result from: - Inappropriate material selection - Inadequate preparation - Poor restoration design # Mechanical Factors Resistance to Deformation (Structural Durability) ## Adequate Tooth Reduction - Sufficient reduction to accommodate the restoration material - Controlled by diagnostic procedures - Occlusal position (MIP) and excursive movements (Lateroprotrusion) #### **Adequate Tooth Reduction** - Sufficient reduction to accommodate the restoration material > [!warning] Avoid Flat Reduction > A flat occlusal reduction creates thin spots over the cusps and thick spots in the fossae, leading to stress concentration and a higher risk of fracture. The reduction must follow the occlusal anatomy to create a uniform material thickness. - Controlled by diagnostic procedures - Occlusal position (MIP) and excursive movements (Lateroprotrusion) **Mechanical Factors** Resistance to Deformation (Structural Durability) **Margin Design** - Enough restoration bulk to resist distortion - Avoid occlusal contact on the margin of partial coverage restoration - Avoid feather edge - boxes enhance the rigidity #### **Margin Design** - Enough restoration bulk to resist distortion - Avoid occlusal contact on the margin of partial coverage restoration - Avoid feather edge - boxes enhance the rigidity # Aesthetics Considerations Natural looking restoration depends on: - Facial tooth reduction - Incisal reduction - Proximal reduction - Material selection - Substratum color # **Aesthetics Considerations** Natural looking restoration depends on: - Facial tooth reduction - Incisal reduction - Proximal reduction - Material selection - Substratum color > [!example] Aesthetic Considerations > - **Maximal Ceramic Thickness:** For aesthetic crowns, the preparation must allow for sufficient thickness of porcelain to mask the underlying tooth or metal coping and achieve a natural appearance. > - **Minimal Metal Display:** In PFM crowns, the design aims to hide the metal collar at the margin. # Aesthetics Considerations ## Labial margin placement - Consider intracrevicular placement: especially for high smile line - Inform the patient of the anticipated margin location - Healthy periodontal tissues is mandatory - Sufficient time after periodontal surgery Clinical image showing anterior teeth with visible gingival margins. ## **Labial margin placement** > [!info] > For anterior teeth, the labial margin is often placed slightly **subgingivally** for superior aesthetics, hiding the transition from crown to tooth. In non-aesthetic areas (e.g., posterior teeth), supragingival margins are preferred. - Consider intracrevicular placement: especially for high smile line - Inform the patient of the anticipated margin location - Healthy periodontal tissues is mandatory - Sufficient time after periodontal surgery For each case, select the ideal combination of tooth preparation principles Some principles are non-negotiable For each case, select the ideal combination of tooth preparation principles Some principles are non-negotiable [[MR1 PFM.pdf#page=1|MR1 PFM, p.1]] [[MR1 PFM.pdf#page=2|MR1 PFM, p.2]] [[MR1 PFM.pdf#page=3|MR1 PFM, p.3]] [[MR1 PFM.pdf#page=4|MR1 PFM, p.4]] [[MR1 PFM.pdf#page=5|MR1 PFM, p.5]] [[MR1 PFM.pdf#page=6|MR1 PFM, p.6]] [[MR1 PFM.pdf#page=7|MR1 PFM, p.7]] [[MR1 PFM.pdf#page=8|MR1 PFM, p.8]] [[MR1 PFM.pdf#page=9|MR1 PFM, p.9]] [[MR1 PFM.pdf#page=10|MR1 PFM, p.10]] [[MR1 PFM.pdf#page=11|MR1 PFM, p.11]] [[MR1 PFM.pdf#page=12|MR1 PFM, p.12]] [[MR1 PFM.pdf#page=13|MR1 PFM, p.13]] [[MR1 PFM.pdf#page=14|MR1 PFM, p.14]] [[MR1 PFM.pdf#page=15|MR1 PFM, p.15]] [[MR1 PFM.pdf#page=16|MR1 PFM, p.16]] [[MR1 PFM.pdf#page=17|MR1 PFM, p.17]] [[MR1 PFM.pdf#page=18|MR1 PFM, p.18]] [[MR1 PFM.pdf#page=19|MR1 PFM, p.19]] [[MR1 PFM.pdf#page=20|MR1 PFM, p.20]] [[MR1 PFM.pdf#page=21|MR1 PFM, p.21]] [[MR1 PFM.pdf#page=22|MR1 PFM, p.22]] [[MR1 PFM.pdf#page=23|MR1 PFM, p.23]] [[MR1 PFM.pdf#page=24|MR1 PFM, p.24]] [[MR1 PFM.pdf#page=25|MR1 PFM, p.25]] [[MR1 PFM.pdf#page=26|MR1 PFM, p.26]] [[MR1 PFM.pdf#page=27|MR1 PFM, p.27]] [[MR1 PFM.pdf#page=28|MR1 PFM, p.28]] [[MR1 PFM.pdf#page=29|MR1 PFM, p.29]] [[MR1 PFM.pdf#page=30|MR1 PFM, p.30]] [[MR1 PFM.pdf#page=31|MR1 PFM, p.31]] [[MR1 PFM.pdf#page=32|MR1 PFM, p.32]] [[MR1 PFM.pdf#page=33|MR1 PFM, p.33]] [[MR1 PFM.pdf#page=34|MR1 PFM, p.34]] [[MR1 PFM.pdf#page=35|MR1 PFM, p.35]] [[MR1 PFM.pdf#page=36|MR1 PFM, p.36]] [[MR1 PFM.pdf#page=37|MR1 PFM, p.37]] [[MR1 PFM.pdf#page=38|MR1 PFM, p.38]] [[MR1 PFM.pdf#page=39|MR1 PFM, p.39]] [[MR1 PFM.pdf#page=40|MR1 PFM, p.40]] [[MR1 PFM.pdf#page=41|MR1 PFM, p.41]] [[MR1 PFM.pdf#page=42|MR1 PFM, p.42]] [[MR1 PFM.pdf#page=43|MR1 PFM, p.43]] [[MR1 PFM.pdf#page=44|MR1 PFM, p.44]] [[MR1 PFM.pdf#page=45|MR1 PFM, p.45]] [[MR1 PFM.pdf#page=46|MR1 PFM, p.46]] [[MR1 PFM.pdf#page=47|MR1 PFM, p.47]] [[MR1 PFM.pdf#page=48|MR1 PFM, p.48]] [[MR1 PFM.pdf#page=49|MR1 PFM, p.49]] [[MR1 PFM.pdf#page=50|MR1 PFM, p.50]]