**DENT4214 2026** **THE UNIVERSITY OF WESTERN AUSTRALIA** | **Oral Health Centre of Western Australia** # Lecture 1-Principles of<br>Tooth Preparations **By Dr Cheryl Fu** #### Lecture 1: Principles of Tooth Preparations Presented by Dr Cheryl Fu # Learning Outcomes: ## The University of Western Australia | Oral Health Centre of Western Australia ### Learning Outcomes: - Understand basic terminology for crown preparations - Understand indications of crowns - Understand the importance of the mechanical, biological, and aesthetic principles of tooth preparation ### Reading for this lecture: **Fundamentals of Fixed Prosthodontics: Chapter 9 Principles of Tooth Preparations** **Contemporary of Fixed Prosthodontics: Chapter 7 Principles of Tooth Preparations** #### Educational Objectives - Understand basic terminology for crown preparations - Understand indications of crowns - Understand the importance of the mechanical, biological, and aesthetic principles of tooth preparation #### Required Reading - **Fundamentals of Fixed Prosthodontics**: Chapter 9, Principles of Tooth Preparations - **Contemporary Fixed Prosthodontics**: Chapter 7, Principles of Tooth Preparations **Why Does a Tooth Need a Crown?** As part of DMD1 you have learnt about: * **Composite restorations** * **GIC restorations** * **Amalgam restorations** So why do we need crown? Considerations for a crown: * Destruction of tooth structure * Aesthetics * Plaque Control/Moisture Control * Retention * Finance* * Other prosthodontic treatment (survey crowns for RPDs) **Fig 6-1 A comparison of resistance to removal forces for four types of crowns (P = .05).¹,² MOD, mesio-occlusodistal.**  Building upon foundational knowledge of direct restorations (Composite, GIC, and Amalgam), crown restorations are indicated based on several clinical factors. #### Factors for Crown Consideration - Destruction of tooth structure - Aesthetics - Plaque Control / Moisture Control - Retention - Finance - Other prosthodontic requirements (e.g., survey crowns for Removable Partial Dentures) #### Mechanical Resistance Clinical data comparing resistance to removal forces indicates varying performance across different crown types, including MOD (mesio-occlusodistal) configurations. Bar chart titled 'Fig 6-1 A comparison of resistance to removal forces for four types of crowns (P = .05)'. The y-axis represents 'Removal force (KgF)' ranging from 0 to 120. The x-axis lists four restoration types: 'MOD onlay' (value 36), '3/4 crown' (value 48), '7/8 crown' (value 52), and 'All-metal crown' (value 110). Error bars are visible above the first three categories. **Why Does a Tooth Need a Crown?** **Considerations for a crown:** * **Destruction of tooth structure** * **Aesthetics** * **Longevity** * **Retention** Figure 3a. Figure 3b. Figure 3. Patients abusing methamphetamine often present with rampant caries. (Photos courtesy of Dr. Jinus Emrani) THREE WISE STARS THE UNIVERSITY OF WESTERN AUSTRALIA Fig 6-1 A comparison of resistance to removal forces for four types of crowns ($P = .05$).$^{1,2}$ MOD, mesio-occlusodistal. Fundamental of Fixed Prosthodontics *** ^ $Terry E. Donovan (2006) *Longevity of the Tooth/Restoration Complex: A Review* $    #### Primary Indications - Destruction of tooth structure - Aesthetics - Longevity - Retention #### Clinical Examples Patients presenting with rampant caries, such as those associated with methamphetamine abuse, often require extensive crown restorations to address significant tooth destruction. #### Research Context Studies on the longevity of the tooth/restoration complex emphasize that the choice of material and technique significantly impacts the long-term success of the restoration. Clinical photo (Figure 3a) showing a patient's mouth with severe rampant caries and existing metallic crowns on posterior teeth. The caption notes this is from patients abusing methamphetamine. Clinical photo (Figure 3b) showing another patient's mouth with extensive dental decay and compromised teeth requiring restoration. Bar chart (Fig 6-1) comparing the removal force (KgF) for four types of crowns: MOD onlay (36), ¾ crown (48), 7/8 crown (52), and All-metal crown (110). It demonstrates that full coverage all-metal crowns offer significantly higher retention than partial coverage options. Why Does a Tooth Need a Crown? **Considerations for a crown:** ? Destruction of tooth structure :: . Aesthetics 4. Plaque Control/Moisture Control ? Retention Incidence and influential factors in pulp necrosis and periapical pathosis following indirect restorations: a systematic review and meta-analysis Kholod Khalil Al-Manei, Shuruq Alzaidi, Ghadah Almalki, Khaled Al-Manei, and Nabeel Almotairy **Results** A total of 5,814 studies were identified, of which 37 were included in the meta-analysis. The overall incidences of **pulp necrosis** and **periapical pathosis following indirect restorations were determined to be 5.02% and 3.63%, respectively.** All studies were assessed as having a moderate-low risk of bias. The incidence of pulp necrosis following indirect restorations increased when the pulp status was objectively assessed (thermal/electrical testing). The presence of pre-operative caries or restorations, treatment of anterior teeth, temporization for more than two weeks, and cementation with eugenol-free temporary cement, all increased this incidence. Final impression with polyether and permanent cementation with glass ionomer cement both increased the incidence of pulp necrosis. Longer follow-up periods (>10 years) and treatment provided by undergraduate students or general practitioners were also factors that increased this incidence. On the other hand, the incidence of periapical pathosis increased when teeth were restored with fixed partial dentures, the bone level was < 35%, and the follow-up was > 10 years. The certainty of the evidence overall was assessed as low. **Conclusions** Although the incidences of pulp necrosis and periapical pathosis following indirect restorations remain low, many factors affect these incidences that should thus be considered when planning indirect restorations on vital teeth. #### Clinical Factors - Destruction of tooth structure - Aesthetics - Plaque Control / Moisture Control - Retention #### Risks of Indirect Restorations A systematic review and meta-analysis regarding pulp necrosis and periapical pathosis following indirect restorations revealed the following: - **Incidence Rates:** The overall incidence of pulp necrosis is approximately 5.02%, and periapical pathosis is 3.63%. - **Risk Factors for Pulp Necrosis:** - Presence of pre-operative caries or existing restorations. - Treatment of anterior teeth. - Temporization exceeding two weeks. - Use of eugenol-free temporary cement. - Final impressions taken with polyether. - Permanent cementation with glass ionomer cement. - Longer follow-up periods (>10 years). - Treatment provided by undergraduate students or general practitioners. - **Risk Factors for Periapical Pathosis:** - Teeth restored with fixed partial dentures. - Bone level < 35%. - Follow-up periods exceeding 10 years. While the overall incidence remains low, these factors must be integrated into the treatment planning process for vital teeth. Why Does a Tooth Need a Crown? Destruction of tooth structure Aesthetics Longevity Retention PFM crowns Photo by tetracycline staining by Dr. Alessandro Martini Song MY, Park JM, Park EJ. Full mouth rehabilitation of the patient with severely worn dentition: a case report.    #### Indications for Treatment - Destruction of tooth structure - Aesthetics (e.g., addressing tetracycline staining) - Longevity - Retention #### Complex Cases Full mouth rehabilitation may be necessary for patients presenting with severely worn dentition, often utilizing Porcelain-Fused-to-Metal (PFM) crowns to restore function and appearance. Clinical photo showing severe tetracycline staining on the anterior dentition. The teeth exhibit dark brown to grey discoloration and opacities. Caption reads: 'Photo by tetracycline staining by Dr. Alessandro Martini'. Clinical photo of an upper anterior view showing severely worn, discolored teeth with multiple existing metal crowns (silver color). Caption at bottom references Song MY et al. case report on full mouth rehabilitation. Clinical photo of a smile after rehabilitation, showing restored anterior teeth with PFM (Porcelain Fused to Metal) crowns. A red arrow points to one of the new crowns. Caption identifies these as 'PFM crowns'. **Why Does a Tooth Need a Crown?** Considerations for a crown: * Destruction of tooth structure * Aesthetics * **Longevity** * Retention Some studies report a lower annual failure rate in indirect restorations (ceramic restorations 1.9%) compared to direct (composite 2.2%, amalgam 3%) restorations. The predominant mode of failure of ceramic restorations is bulk fracture. The predominant mode of failure for composite restorations were reported to be marginal breakdown (secondary caries or deterioration) or fracture. Pooled Survival Rate Studies Success Rate Indirect (n = 24) Direct (n = 23) months 0 50 100 150 9 10 Figure 2. Comparison of the published survival curves of direct restorations (amalgam and composite) vs indirect adhesive inlays with the 2-parameter fit. Manhart J, Chen HY, Hamm G, Hickel R. Review of the clinical survival of direct and indirect restorations in posterior teeth of the permanent dentition.  #### Comparative Annual Failure Rates Research indicates that indirect restorations often demonstrate lower annual failure rates compared to direct restorations: - **Ceramic (Indirect):** 1.9% - **Composite (Direct):** 2.2% - **Amalgam (Direct):** 3.0% #### Primary Modes of Failure - **Ceramic Restorations:** Predominantly fail due to bulk fracture. - **Composite Restorations:** Predominantly fail due to marginal breakdown (secondary caries or deterioration) or fracture. #### Survival Analysis Comparative survival curves show that indirect adhesive inlays generally maintain higher success rates over time (measured in months) when compared to direct amalgam and composite restorations. Line chart titled 'Pooled Survival Rate Studies' comparing success rates over time. The Y-axis represents 'Success Rate' from 50% to 100%, and the X-axis represents 'months' from 0 to 150. Two data series are plotted: an orange line for 'Indirect (n = 24)' restorations showing a slow decline in survival rate, and a blue line for 'Direct (n = 23)' restorations showing a steeper decline. Vertical black lines mark the 9-year and 10-year intervals on the timeline. **In summary:** • If correctly used, indirect restorations may be superior to direct restorations in terms of mechanical properties and longevity • **HOWEVER:** it is more technically demanding for the clinician, and financially expensive for the patient. ($400 vs $1500+). • **Always keep in mind the balance between the pros vs cons** • **Is this the best option for the patient?** • **Have you carefully considered all aspects of the clinical scenario/patient’s details?** • **If the patient was a family member, would you still recommend the same treatment?** #### Clinical Summary - **Mechanical Properties:** When utilized correctly, indirect restorations may be superior to direct restorations regarding mechanical properties and longevity. - **Practical Constraints:** Indirect procedures are more technically demanding for the clinician and significantly more expensive for the patient (e.g., approximately $1500+ compared to $400 for direct restorations). #### Ethical and Clinical Decision Making Clinicians must balance the pros and cons of each approach by considering: - Is this the best option for the specific patient? - Have all aspects of the clinical scenario and patient details been considered? - Would the same treatment be recommended if the patient were a family member? In summary: - If correctly used, indirect restorations may be superior to direct restorations in terms of mechanical properties and longevity - HOWEVER: it is more technically demanding for the clinician, and financially expensive for the patient. ($400 vs $1500+). - **Always keep in mind the balance between the pros vs cons** - **Is this the best option for the patient?** - **Have you carefully considered all aspects of the clinical scenario/patient’s details?** - **If the patient was a family member, would you still recommend the same treatment?** <!-- Image found at bottom right corner, but it is too blurry to specify --> #### Summary of Findings - Indirect restorations offer potential superiority in mechanical properties and longevity if executed correctly. - The transition from direct to indirect restorations involves increased technical difficulty and higher financial costs for the patient ($400 vs. $1500+). #### Final Considerations - Maintain a careful balance between the advantages and disadvantages of the chosen restoration type. - Ensure the treatment is the optimal choice for the individual patient. - Perform a thorough review of all clinical and patient-specific factors. - Apply the "family member" standard to treatment recommendations. | | | | | | :--- | :--- | :--- | :--- | | **CLINICAL PROCEDURES** | | | | | | Oral Health Centre of Western Australia | | **The University of Western Australia** | | | | Clinical examination $\rightarrow$ Primary Impression $\rightarrow$ Tooth preparation + Temporisation | | Definitive Impression | | | | $\rightarrow$ Intraoral scanning | | | | | | | | $\downarrow$ | | | | CAD All-ceramic | | Review $\leftarrow$ Cementation $\leftarrow$ **Try-in** $\rightarrow$ Milling | | Wax up (gold crown, PFM) | | | $\downarrow$ | $\rightarrow$ Casting and veneering | | **Slide by Dr Matsubara** | | | |        #### Clinical Sequence 1. **Initial Phase** - Clinical examination - Primary Impression - Tooth preparation and Temporisation 2. **Impression and Data Acquisition** - Definitive Impression - Intraoral scanning 3. **Laboratory Fabrication Paths** - **CAD All-ceramic**: Milling - **Wax up (Gold crown, PFM)**: Casting and veneering 4. **Final Phase** - Try-in - Cementation - Review Clinical photo of a dental articulator (bite registration device) with a white bite rim and metal components, illustrating the 'Primary Impression' step. Clinical photo showing a dental drill on a tooth, illustrating the 'Tooth preparation + Temporisation' step. Visual representation of CAD All-ceramic design software interface alongside a physical stone cast of prepared teeth. This corresponds to the digital workflow branch of the procedure. Photo of a milled ceramic crown on a stem during the 'Milling' or 'Try-in' phase. Close-up clinical photo of a gold crown being seated onto a tooth model, illustrating the 'Casting and veneering' step. Photo of blue wax patterns on a stone cast representing a 'Wax up (gold crown, PFM)' for traditional fabrication. Clinical photo of a patient's mouth showing temporary cementation or try-in of crowns, illustrating the 'Cementation' and 'Review' steps. | Slide by Dr Matsubara | |---| | **Preparation** | | **Impression** | **Fabrication of models** | **Scanning** | | **Conventional** | **CAD-CAM** | | **Casting** | **Scanning** | | **Manual finishing** | **Milling** | | **Veneering/Polishing** | **Sintering** | | **Insert** | **Veneering/Polishing** | | | **Insert** | | **THE UNIVERSITY OF WESTERN AUSTRALIA** | #### Workflow Comparison - **Preparation** - **Impression** - **Fabrication of models** - **Scanning** #### Conventional Process - Casting - Manual finishing - Veneering/Polishing - Insert #### CAD-CAM Process - Scanning - Milling - Sintering - Veneering/Polishing - Insert Flowchart diagram illustrating two dental restoration workflows: 'Conventional' and 'CAD-CAM'. The process begins at 'Preparation', followed by 'Impression' for the Conventional path leading to 'Casting', 'Manual finishing', 'Veneering/Polishing', and 'Insert'. For the CAD-CAM path, it branches to 'Scanning', then 'Milling', 'Sintering', 'Veneering/Polishing', and 'Insert'. Red arrows indicate workflow progression. <table><tr><td><b>Preparation</b></td></tr><tr><td>Impression</td></tr><tr><td>Fabrication of models</td></tr><tr><td rowspan="4"><b>Conventional</b></td></tr><tr><td>Casting</td></tr><tr><td>Manual finishing</td></tr><tr><td>Veneering/Polishing</td></tr><tr><td>Insert</td></tr><tr><td rowspan="2">Scanning</td></tr><tr><td><b>CAD-CAM</b></td></tr><tr><td>Scanning</td></tr><tr><td>Milling</td></tr><tr><td>Sintering</td></tr><tr><td>Veneering/Polishing</td></tr><tr><td>Insert</td></tr><tr><td>Slide by Dr Matsubara</td></tr></table>  #### Clinical and Laboratory Steps - **Preparation** - **Impression** - **Fabrication of models** #### Conventional Pathway - Casting - Manual finishing - Veneering/Polishing - Insert #### CAD-CAM Pathway - Scanning - Milling - Sintering - Veneering/Polishing - Insert Flowchart diagram illustrating the dental fabrication process comparing 'Conventional' and 'CAD-CAM' methods. The chart begins with a central 'Preparation' box branching into 'Impression' (leading to Conventional) and 'Scanning' (leading to CAD-CAM). The Conventional path details steps including 'Casting', 'Manual finishing', 'Veneering/Polishing', and 'Insert'. The CAD-CAM path outlines steps such as 'Scanning', 'Milling', 'Sintering', 'Veneering/Polishing', and 'Insert'. Red arrows indicate the workflow sequence between stages. ```mermaid graph TD Preparation --> Impression Preparation --> Scanning Impression --> Fabrication_of_models Impression --> Scanning subgraph Conventional Fabrication_of_models --> Casting Casting --> Manual_finishing Manual_finishing --> Veneering_Polishing_Conv[Veneering/Polishing] Veneering_Polishing_Conv --> Insert_Conv[Insert] end subgraph CAD_CAM Fabrication_of_models --> Scanning_Lab[Scanning] Scanning --> Milling Scanning_Lab --> Milling Milling --> Sintering Sintering --> Veneering_Polishing_CAD[Veneering/Polishing] Veneering_Polishing_CAD --> Insert_CAD[Insert] end ``` Slide by Dr Matsubara #### Integrated Workflow Diagram - **Preparation** leads to either **Impression** or **Scanning**. - **Impression** leads to **Fabrication of models** or **Scanning**. #### Conventional Branch - Fabrication of models - Casting - Manual finishing - Veneering/Polishing - Insert #### CAD-CAM Branch - Fabrication of models → Scanning (Lab) - Scanning (Intraoral) → Milling - Milling - Sintering - Veneering/Polishing - Insert Flowchart diagram comparing two dental prosthetic fabrication workflows: Conventional and CAD-CAM. The process begins with 'Preparation' at the top, which branches into 'Impression' (leading to 'Fabrication of models') and 'Scanning'. From 'Fabrication of models', the path splits into two distinct vertical columns. The left column is labeled 'Conventional' and includes steps: 'Casting', 'Manual finishing', 'Veneering/Polishing', and 'Insert'. The right column is labeled 'CAD-CAM' and includes steps: 'Scanning', 'Milling', 'Sintering', 'Veneering/Polishing', and 'Insert'. Arrows indicate the sequence of operations in each workflow. ```mermaid graph TD Preparation --> Impression Preparation --> Scanning Impression --> Fabrication_of_models Impression --> Scanning subgraph Conventional Fabrication_of_models --> Casting Casting --> Manual_finishing Manual_finishing --> Veneering_Polishing_Conv[Veneering/Polishing] Veneering_Polishing_Conv --> Insert_Conv[Insert] end subgraph CAD_CAM[CAD-CAM] Scanning --> Milling Milling --> Sintering Sintering --> Veneering_Polishing_CAD[Veneering/Polishing] Veneering_Polishing_CAD --> Insert_CAD[Insert] end ``` Slide by Dr Matsubara  #### Workflow Logic - **Preparation** - Impression → Fabrication of models - Scanning #### Conventional Path - Fabrication of models - Casting - Manual finishing - Veneering/Polishing - Insert #### CAD-CAM Path - Scanning - Milling - Sintering - Veneering/Polishing - Insert Flowchart diagram comparing Conventional and CAD-CAM dental restoration fabrication workflows. The diagram starts with 'Preparation' at the top, branching into two main paths: a 'Conventional' path on the left involving Impression, Fabrication of models, Casting, Manual finishing, Veneering/Polishing, and Insert; and a 'CAD-CAM' path on the right involving Scanning, Milling, Sintering, Veneering/Polishing, and Insert. Arrows indicate process flow between steps. # Workflow **Preparation** ↓ **Impression** ↓ **Fabrication of models** This process branches into two paths: ### **Conventional** 1. **Casting** 2. **Manual finishing** 3. **Veneering/Polishing** 4. **Insert** ### **CAD-CAM** 1. **Scanning** 2. **Milling** 3. **Sintering** 4. **Veneering/Polishing** 5. **Insert** *** **THE UNIVERSITY OF WESTERN AUSTRALIA** *Slide by Dr Matsubara*  #### Workflow Stages 1. **Preparation** 2. **Impression** 3. **Fabrication of models** This process branches into two paths: #### Conventional 1. Casting 2. Manual finishing 3. Veneering/Polishing 4. Insert #### CAD-CAM 1. Scanning 2. Milling 3. Sintering 4. Veneering/Polishing 5. Insert A workflow flowchart comparing two dental fabrication methods: Conventional and CAD-CAM. The diagram starts with 'Preparation' which leads to 'Impression' and 'Fabrication of models'. From there, it branches into two paths: the left path labeled 'Conventional' includes steps for 'Casting', 'Manual finishing', 'Veneering/Polishing', and 'Insert'; the right path labeled 'CAD-CAM' includes steps for 'Scanning', 'Milling', 'Sintering', 'Veneering/Polishing', and 'Insert'. Red arrows indicate the process flow. # Crown Preparation Terminology FIGURE 8-25 The completed preparation is characterized by a smooth, even chamfer margin; a 6-degree taper; and gradual transitions between all prepared surfaces. Contemporary Fixed Prosthodontics  The completed preparation is characterized by the following features: - A smooth, even chamfer margin - A 6-degree taper - Gradual transitions between all prepared surfaces Labelled diagram of a tooth crown preparation showing the 'Occlusal' surface at the top and 'Axial Margins' on the side. The figure includes an attached caption (FIGURE 8-25) describing the completed preparation as having a smooth, even chamfer margin; a 6-degree taper; and gradual transitions between all prepared surfaces. # Crowns **Materials:** * All ceramic - Zirconia or Glassy Ceramics (eg Lithium Sillicate or Lithium Disillicate) * Metal - Varying alloys * Porcelain Fused to Metal (PFM) Each material may require different preparations.  Each restorative material may require different preparation techniques. Common materials include: #### All-Ceramic - Zirconia - Glassy Ceramics (e.g., Lithium Silicate or Lithium Disilicate) #### Metal - Varying alloys #### Porcelain Fused to Metal (PFM) - Combination of metal substructure with porcelain overlay University logo for The University of Western Australia located in the top right corner. # Crown Preparation Terminology **Margins:** Supra-gingival of subgingival Types of margins - Feather edge - Bevel - Chamfer - Shoulder - Shoulder with bevel **Supragingival Margins** - Easily prepared - Easier to take impressions - Easily maintained by patient - Easily evaluated at recalls - Preserve periodontium **Subgingival Margins** **Indications** - Subgingival dental caries, erosion, fracture line or restoration - Additional retention is needed - Aesthetic margins - Coverage of root surface - Modification of axial contour #### Clinical Advantages of Supragingival Margins - Easily prepared - Easier to take impressions - Easily maintained by patient - Easily evaluated at recalls - Preserve periodontium #### Indications for Subgingival Margins - Subgingival dental caries, erosion, fracture line, or restoration - Additional retention is needed - Aesthetic margins - Coverage of root surface - Modification of axial contour **Margins** Margins: • Supra-gingival of subgingival • **Types of margins** **• Feather edge** • Bevel • Chamfer • Shoulder • Shoulder with bevel <img>Dental burs (860, 861, 862, 863) shown on model and diagram< Advantages: • Conservation of tooth structure Disadvantages: • Fail to provide adequate bulk at margins **CONTRA-INDICATED IN MOST CASES!**   #### Characteristics of Feather Edge Margins - **Advantages:** Conservation of tooth structure - **Disadvantages:** Fail to provide adequate bulk at margins **CONTRA-INDICATED IN MOST CASES!** Clinical photograph showing a dental bur (labeled with numbers 860, 861, 862, 863) cutting into a model tooth. The image demonstrates the preparation of a feather edge margin. Schematic diagram illustrating a 'feather edge' margin on a tooth. It shows a curved line representing the prepared enamel surface tapering to a sharp edge against the vertical axis of the tooth. # Margins **The University of Western Australia** Margins: * Supra-gingival of subgingival * Types of margins * Feather edge * **Bevel** * Chamfer * Shoulder * Shoulder with bevel Commonly for cast restorations **Advantages:** * Allow the margin of the crown to be burnished against tooth structure * Protect the unprepared tooth structure from chipping by removing unsupported enamel * May reduce marginal discrepancy when complete crown fails to seat completely. (But doesn't work for oversized crown) **Disadvantages:** * Can lead to subgingival extension Slide by Dr Nedelcu #### Clinical Applications and Benefits Bevel margins are commonly used for cast restorations. - **Advantages:** - Allow the margin of the crown to be burnished against tooth structure - Protect the unprepared tooth structure from chipping by removing unsupported enamel - May reduce marginal discrepancy when a complete crown fails to seat completely (Note: This does not work for oversized crowns) - **Disadvantages:** - Can lead to subgingival extension **Margins** **Margins:** • Supra-gingival of subgingival • **Types of margins** • Feather edge • Bevel • Chamfer • **Shoulder** • Shoulder with bevel Advantages: • Easy to prepare and finish well • Easy to judge on impressions Disadvantages: • More aggressive preparation Can be used for **all materials**. Internal angle is **ROUNDED** NOT **SHARP** 90 degrees. **tooth** Slide by Dr Nedelcu  #### Clinical Considerations for Shoulder Margins - **Advantages:** - Easy to prepare and finish well - Easy to judge on impressions - **Disadvantages:** - More aggressive preparation #### Material Compatibility and Geometry - Can be used for **all materials**. - Internal angle is **ROUNDED**, NOT a **SHARP** 90 degrees. Labeled diagram illustrating a 'Shoulder' margin preparation on a tooth. The visual shows a cross-section of a tooth with a U-shaped preparation outline and a pink shaded area representing the prepared structure. A blue arrow points to the bottom corner of the preparation to highlight that the internal angle is ROUNDED NOT SHARP 90 degrees. **Margins** Margins: • Supra-gingival of subgingival • Types of margins • Feather edge • Bevel • **Chamfer** • Shoulder • Shoulder with bevel Advantages: • Conserves tooth structure whilst providing distinct finish lines • Easy to read in impressions **Commonly used for metal, or ceramic crowns** Use a tapered diamond bur with round tip! Slide by Dr Nedelcu   #### Clinical Advantages and Usage - **Advantages:** - Conserves tooth structure whilst providing distinct finish lines - Easy to read in impressions - **Commonly used for metal or ceramic crowns** #### Instrumentation - Use a tapered diamond bur with a round tip. Labeled diagram of a 'Shoulder' margin type showing the cross-section of a tooth preparation with a flat, 90-degree finish line at the gingival level. Labeled diagram of a 'Chamfer' margin type showing the cross-section of a tooth preparation with a rounded, concave finish line at the gingival level. **Margins** Margins: Supra-gingival of subgingival Types of margins Feather edge Bevel Chamfer Shoulder **Shoulder with bevel** Sometimes used for PFM crowns with a metal labial margin (hidden in the sulcus). But a shoulder/chamber is preferred for biologic and aesthetic considerations **Uncommonly used.** 52° Bevel Properly seated castings should have minimal marginal gap widths FIGURE 7-25 Effect on marginal fit of beveling the gingival margin. A. If the internal cross section of a crown is the same as or less than that of the prepared tooth, a 45-degree bevel decreases the marginal discrepancy by 70%. B. If the internal diameter is slightly larger than the prepared tooth, beveling increases the marginal discrepancy. In practice, crowns are made slightly larger than the prepared tooth to allow for the luting agent.  #### Clinical Application - Sometimes used for PFM crowns with a metal labial margin (hidden in the sulcus). - A shoulder or chamfer is generally preferred for biologic and aesthetic considerations. - **Uncommonly used.** #### Technical Specifications and Fit - 52° Bevel - Properly seated castings should have minimal marginal gap widths. #### Effect on Marginal Fit - If the internal cross section of a crown is the same as or less than that of the prepared tooth, a 45-degree bevel decreases the marginal discrepancy by 70%. - If the internal diameter is slightly larger than the prepared tooth, beveling increases the marginal discrepancy. - In practice, crowns are made slightly larger than the prepared tooth to allow for the luting agent. Figure 7-25 illustrating the effect on marginal fit of beveling the gingival margin. The figure contains two comparative panels, A and B, showing cross-sections of a prepared tooth (left), a crown restoration (center), and the resulting marginal gap (right). Panel A depicts a scenario where the internal cross section of the crown is the same as or less than that of the prepared tooth, demonstrating how a 45-degree bevel decreases the marginal discrepancy by 70%. Panel B shows the opposite case where the internal diameter is slightly larger than the prepared tooth, and beveling increases the marginal discrepancy. The figure includes labels for 'Shoulder' and '45° Bevel', and a text box stating 'Properly seated castings should have minimal marginal gap widths.' The caption explains that in practice, crowns are made slightly larger to allow for the luting agent. - Numerica incorporation **PRINCIPLES OF TOOTH PREPARATION** **ABUTMENT TOOTH** **MECHANICAL** **BIOLOGICAL** Principles of tooth preparation Partial oral complete preparation Retention form Resistance form Deformation Conservation of tooth structure Avoidance of overcontouring Supragingival margins Organious occlusion Protection agalnst toath fracture 1 **AESTHETIC** Minimum display of metal Maximum thickness of porcelain Porcelain occlusion surfaces Subgingival margins 2 **Slide by Dr Matsubara**  #### Biological Considerations - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture #### Mechanical Considerations - Retention form - Resistance form - Deformation #### Aesthetic Considerations - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusion surfaces - Subgingival margins Venn diagram illustrating the principles of tooth preparations. The diagram consists of three overlapping circles: a yellow circle labeled 'BIOLOGICAL' (containing points like conservation of tooth structure and protection against fracture), a green circle labeled 'MECHANICAL' (containing retention form and resistance form), and a blue circle labeled 'AESTHETIC' (containing minimum display of metal and maximum thickness of porcelain). A section titled 'ABUTMENT TOOTH' lists general principles on the left. ### Principles of Tooth Preparations **THE UNIVERSITY OF WESTERN AUSTRALIA** **ABUTMENT TOOTH** • Principles of tooth preparation • Partial or complete preparation **BIOLOGICAL** • Conservation of tooth structure • Avoidance of overcontouring • Supragingival margins • Harmonious occlusion • Protection against tooth fracture **MECHANICAL** • Retention form • Resistance form • Deformation **AESTHETIC** • Minimum display of metal • Maximum thickness of porcelain • Porcelain occlusal surfaces • Subgingival margins **Occlusal onlays as a modern treatment concept for the reconstruction of severely worn occlusal surfaces** Article in Quintessence international (Berlin, Germany: 1985) - January 2018 DOI: 10.3290/j.qi.a40482 **CITATIONS** 32 **READS** 12,502 **2 authors:** **Daniel Edelhoff** Ludwig-Maximilians-University of Munich 262 PUBLICATIONS 10,264 CITATIONS SEE PROFILE **M. Oliver Ahlers** University Medical Center Hamburg - Eppendorf 60 PUBLICATIONS 677 CITATIONS SEE PROFILE <br> <br> **Fig 1** Approximately 70% of the volume of the clinical crown of a posterior tooth is removed for a full-crown preparation (internal contour), whereas only about 30% is lost for an occlusal onlay, unless more tissue has already been lost (see Fig 3b).  #### Preparation Categories - Principles of tooth preparation - Partial or complete preparation #### Biological Principles - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture #### Mechanical Principles - Retention form - Resistance form - Deformation #### Aesthetic Principles - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins #### Clinical Research Reference **Occlusal onlays as a modern treatment concept for the reconstruction of severely worn occlusal surfaces** - **Source:** Quintessence international (Berlin, Germany: 1985) - January 2018 - **DOI:** 10.3290/j.qi.a40482 - **Authors:** Daniel Edelhoff (Ludwig-Maximilians-University of Munich) and M. Oliver Ahlers (University Medical Center Hamburg - Eppendorf) **Fig 1:** Approximately 70% of the volume of the clinical crown of a posterior tooth is removed for a full-crown preparation (internal contour), whereas only about 30% is lost for an occlusal onlay, unless more tissue has already been lost. Clinical figure (Fig 1) showing a 3D rendered posterior tooth. The image demonstrates the concept of an occlusal onlay versus a full-crown preparation. A faint overlay illustrates the internal contour of a full crown, while the caption explains that this removes approximately 70% of the clinical crown volume compared to only about 30% for an occlusal onlay. **Principles of Tooth Preparations** <div> <table> <caption>Solve for</caption> <tr><th>volume</th></tr> </table> </div> $V \approx 785.4$ <table> <tr> <td>r</td><td>Radius</td><td>5</td> </tr> <tr> <td>h</td><td>Height</td><td>10</td> </tr> </table> <div> <table> <caption>Solution</caption> <tr><td> $V = \pi r^2 h = \pi \cdot 5^2 \cdot 10 \approx 785.39816$ </td></tr> </table> </div> <div> <table> <caption>Solve for</caption> <tr><th>volume</th></tr> </table> </div> $V \approx 402.12$ <table> <tr> <td>r</td><td>Radius</td><td>4</td> </tr> <tr> <td>h</td><td>Height</td><td>8</td> </tr> </table> <div> <table> <caption>Solution</caption> <tr><td> $V = \pi r^2 h = \pi \cdot 4^2 \cdot 8 \approx 402.12386$ </td></tr> </table> </div> <math>V = \pi r^2 h</math> Obvious limitations for this model such as tooth is not a cylinder shape, and tooth has a pulp chamber resulting in this simplified model giving us an underestimate. **This represents almost 50% decrease in volume!!** **Even worse if you include a pulp chamber space!**   #### Volumetric Analysis of Tooth Reduction **Initial State (Cylindrical Model):** - Radius (r): 5 - Height (h): 10 - Formula: $V = \pi r^2 h$ - Volume ($V$): $\approx 785.4$ **Prepared State (Reduced Model):** - Radius (r): 4 - Height (h): 8 - Volume ($V$): $\approx 402.12$ **Clinical Implications:** - This represents almost a 50% decrease in volume. - The loss is even more significant if the pulp chamber space is included. - **Limitations:** This model is a simplification as a tooth is not a perfect cylinder and contains a pulp chamber; therefore, this model provides an underestimate of actual tissue loss. Labeled diagram of a cylinder representing a simplified tooth model. Labels include 'r' for radius and 'h' for height. Labeled diagram of a smaller cylinder representing a simplified tooth model with reduced dimensions. Labels include 'r' for radius and 'h' for height. # Principles of Tooth Preparations ## ABUTMENT TOOTH * Principles of tooth preparation * Partial or complete preparation ## BIOLOGICAL * Conservation of tooth structure * Avoidance of overcontouring * Supragingival margins * Harmonious occlusion * Protection against tooth fracture ## MECHANICAL * Retention form * Resistance form * Deformation ## AESTHETIC * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins Occlusal onlays as a modern treatment concept for the reconstruction of severely worn occlusal surfaces **Fig 1** Approximately 70% of the volume of the clinical crown of a posterior tooth is removed for a full-crown preparation (internal contour), whereas only about 30% is lost for an occlusal onlay, unless more tissue has already been lost (see Fig 3b). Article in Quintessence international (Berlin, Germany: 1985) - January 2018 DOI: 10.3290/j.qi.a40482 CITATIONS 32 READS 12,502 **2 authors:** **Daniel Edelhoff** Ludwig-Maximilians-University of Munich 262 PUBLICATIONS 10,264 CITATIONS **SEE PROFILE** **M. Oliver Ahlers** University Medical Center Hamburg - Eppendorf 60 PUBLICATIONS 677 CITATIONS **SEE PROFILE**  #### Preparation Overview - Principles of tooth preparation - Partial or complete preparation #### Biological Factors - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture #### Mechanical Factors - Retention form - Resistance form - Deformation #### Aesthetic Factors - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins #### Research Context **Occlusal onlays as a modern treatment concept for the reconstruction of severely worn occlusal surfaces** **Fig 1:** Approximately 70% of the volume of the clinical crown of a posterior tooth is removed for a full-crown preparation (internal contour), whereas only about 30% is lost for an occlusal onlay, unless more tissue has already been lost. - **Publication:** Quintessence international (January 2018) - **DOI:** 10.3290/j.qi.a40482 - **Authors:** Daniel Edelhoff and M. Oliver Ahlers Fig 1: A clinical photograph or digital rendering of a posterior tooth showing the difference in preparation volume between a full-crown and an occlusal onlay. The caption states that approximately 70% of the clinical crown volume is removed for a full-crown preparation, whereas only about 30% is lost for an occlusal onlay. # Principles of Tooth Preparations ## ABUTMENT TOOTH * Principles of tooth preparation * Partial or complete preparation ## CORVID DIAGRAM ## CATEGORIES ### BIOLOGICAL * Conservation of tooth structure * *Avoidance of overcontouring* * Supragingival margins * Harmonious occlusion * Protection against tooth fracture ### MECHANICAL * Retention form * Resistance form * Deformation ### AESTHETIC * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins ## DETAILED CROSS-SECTION ILLUSTRATION   #### Preparation Types - Principles of tooth preparation - Partial or complete preparation #### Biological and Mechanical Categories - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture - Retention form - Resistance form - Deformation #### Aesthetic Categories - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins #### Visual Documentation - CORVID DIAGRAM - DETAILED CROSS-SECTION ILLUSTRATION Labeled diagram illustrating tooth preparation principles. The top panel shows a side view of a tooth with a crown being placed, with an inset zoomed-in view showing the internal structure. Below are two cross-sectional views comparing different margin preparations. Detailed cross-section illustration comparing three scenarios labeled A, B, and C. Each scenario shows a tooth with a crown and gingiva, demonstrating different crown contours (undercontoured vs overcontoured) and their effects on food flow, injury, or plaque retention. Labels include 'Flow of food', 'Gingiva', 'Injury', 'Undercontoured crown', 'Overcontoured t crown', 'Plaque retention', and 'Patient can't clean this!' with a red arrow pointing to the problematic area in scenario C. **Principles of Tooth Preparations** **ABUTMENT TOOTH** * Principles of tooth preparation * Partial or complete preparation **BIOLOGICAL** * Conservation of tooth structure * Avoidance of overcontouring * **Supragingival margins** * Harmonious occlusion * Protection against tooth fracture **MECHANICAL** * Retention form * Resistance form * Deformation **AESTHETIC** * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins Slide by Dr Matsubara  #### Abutment Tooth Considerations - Principles of tooth preparation - Partial or complete preparation #### Biological Principles - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture #### Mechanical Principles - Retention form - Resistance form - Deformation #### Aesthetic Principles - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins Venn diagram illustrating the three main principles of tooth preparations: BIOLOGICAL (yellow circle), MECHANICAL (green circle), and AESTHETIC (blue circle). The diagram visually demonstrates how these principles overlap to guide clinical decision-making for an ABUTMENT TOOTH. Each circle contains specific bullet points detailing requirements such as 'Conservation of tooth structure' in Biological, 'Retention form' in Mechanical, and 'Minimum display of metal' in Aesthetic. This visual grouping effectively summarizes the complex relationships between biological, mechanical, and aesthetic factors. # Principles of Tooth Preparations Sub-gingival preparation- aesthetic, BUT..... Slide by Dr Matsubara    Sub-gingival preparation is aesthetic, but carries specific biological risks. Clinical photo of anterior teeth showing a sub-gingival preparation margin located at or below the gum line, illustrating the aesthetic benefit mentioned in the text. Clinical photo (labeled 'A') of lower anterior teeth with visible gingival inflammation and recession, demonstrating the potential complications associated with sub-gingival preparations. Clinical photo of posterior teeth showing tooth preparations where the margins are positioned sub-gingivally relative to the gingival tissue. # Principles of Tooth Preparations **biologic width!** KNOW THIS WORD! Slide by Dr Matsubara  Biologic width! (Essential concept to know). Clinical photo of anterior teeth with a red arrow pointing to the gingival margin between two central incisors, illustrating a potential violation of biologic width. The image demonstrates soft tissue impingement or inflammation at the margin, serving as a visual teaching aid for the concept of biologic width in tooth preparations. # Principles of Tooth Preparations - Connective tissue 1.07 mm - Junctional epithelium 0.97 mm - Gingival sulcus 0.5-1 mm - Biologic Width 2 - 2.25 mm ## Margin location - If the margins has to be subgingival it should only be placed intracrevicularly - Maximum depth around 0.7mm Slide by Dr Matsubara  #### Components of Biologic Width - Connective tissue: 1.07 mm - Junctional epithelium: 0.97 mm - Gingival sulcus: 0.5-1 mm - Total Biologic Width: 2 - 2.25 mm #### Margin Placement Guidelines - If the margins must be subgingival, they should only be placed intracrevicularly. - Maximum depth should be around 0.7mm. Anatomical cross-section diagram illustrating the periodontal tissues and biologic width. The image shows a tooth root (yellow), gingival tissue (pink/red), and bone structure with labeled measurements for Connective tissue (1.07 mm), Junctional epithelium (0.97 mm), Gingival sulcus (0.5-1 mm), and Biologic Width (2 - 2.25 mm). Blue brackets indicate the respective tissue layers. ```html <table> <tr> <td>Principles of Tooth Preparations</td> </tr> <tr> <td>Take Home Message: If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself</td> </tr> <tr> <td>Normal Crest 3 mm (4 approx)</td> <td>High Crest < 3 mm (4 approx)</td> <td>Low Crest > 3 mm (4 approx)</td> </tr> <tr> <td><u><b>Want to know more about biologic width? Read:</b></u><br/>Mulla SA, Patil A, Mali S, Jain A, Sharma D, Jaiswal HC, Saoji HA, Jakhar A, Talekar S, Singh S. Exploring the Biological Width in Dentistry: A Comprehensive Narrative Review. Cureus. 2023 Jul 18;15(7).</td> </tr> <tr> <td>Email me Dr Matsubara</td> </tr> </table> ```  ##### Clinical Rule If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself. ##### Crest Classifications - Normal Crest: 3 mm (approx. 4 mm) - High Crest: < 3 mm (approx. 4 mm) - Low Crest: > 3 mm (approx. 4 mm) ##### Recommended Reading Mulla SA, Patil A, Mali S, Jain A, Sharma D, Jaiswal HC, Saoji HA, Jakhar A, Talekar S, Singh S. Exploring the Biological Width in Dentistry: A Comprehensive Narrative Review. Cureus. 2023 Jul 18;15(7). A comparative diagram illustrating four scenarios of tooth preparations relative to the crestal bone. The images show cross-sections of a tooth with surrounding gum tissue and bone. From left to right: (1) A preparation margin placed correctly within normal biological width; (2) 'Normal Crest' scenario showing approximately 3mm clearance; (3) 'High Crest' scenario where the margin is less than 3mm from the bone (<3 mm); and (4) 'Low Crest' scenario where the margin is more than 3mm away (>3 mm). Each image visually demonstrates the distance between the tooth preparation margin and the alveolar bone crest. # Principles of Tooth Preparations **Take Home Message:** If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself <br> Normal Crest 3 mm (4 approx) High Crest < 3 mm (4 approx) Low Crest > 3 mm (4 approx) <br> Slide by Dr Matsubara Want to know more about biologic width? Read: Mulla SA, Patil A, Mali S, Jain A, Sharma D, Jaiswal HC, Saoji HA, Jakhar A, Talekar S, Singh S. Exploring the Biological Width in Dentistry: A Comprehensive Narrative Review. Cureus. 2023 Jul 18;15(7).     ##### Clinical Rule If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself. ##### Crest Classifications - Normal Crest: 3 mm (approx. 4 mm) - High Crest: < 3 mm (approx. 4 mm) - Low Crest: > 3 mm (approx. 4 mm) ##### Recommended Reading Mulla SA, Patil A, Mali S, Jain A, Sharma D, Jaiswal HC, Saoji HA, Jakhar A, Talekar S, Singh S. Exploring the Biological Width in Dentistry: A Comprehensive Narrative Review. Cureus. 2023 Jul 18;15(7). Labelled diagram showing a tooth preparation margin placed at a distance from the crestal bone. The image depicts a cross-section of gum tissue (pink) and bone (yellow). This panel is associated with the 'Take Home Message' regarding inflammation and bone loss when margins are too close to the bone. Diagram illustrating a 'Normal Crest' scenario where the biological width is approximately 3 mm (4 approx), showing the relationship between the tooth preparation margin and the underlying bone structure. Diagram illustrating a 'High Crest' scenario where the biological width is less than 3 mm (4 approx). A grey instrument is shown interacting with the tissue, demonstrating the violation of biological width principles. Diagram illustrating a 'Low Crest' scenario where the biological width is greater than 3 mm (4 approx). Similar to the previous panels, it shows the anatomical relationship between the margin, soft tissue, and bone. **Principles of Tooth Preparations** The margins of the restoration can be positioned 0.5 mm below the gingival tissue crest if the probing depth of the sulcus is 1.5 mm or less. If the sulcus depth is more than 1.5mm, then the margins of the restoration should be inserted in the sulcus at a depth that is half its probing depth. Lastly, a gingivectomy may be enacted to extend the tooth and design a 1.5 mm sulcus if the probing depth of the sulcus is greater than 2 mm. **Take Home Message:** If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself. #### Margin Positioning by Sulcus Depth - **Sulcus depth ≤ 1.5 mm:** Margins can be positioned 0.5 mm below the gingival tissue crest. - **Sulcus depth > 1.5 mm:** Margins should be inserted in the sulcus at a depth that is half its probing depth. - **Sulcus depth > 2 mm:** A gingivectomy may be enacted to extend the tooth and design a 1.5 mm sulcus. #### Clinical Rule If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself. # Principles of Tooth Preparations ## We risk violation of **biologic width**! > <=> KNOW THIS WORD! --- ### Take Home Message: If you place a margin within 2mm of the crestal bone you will get inflammation Followed by bone loss as the biological width tries to re-establish itself --- ``  We risk violation of biologic width! #### Clinical Rule If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself. Clinical photo showing a close-up of upper front teeth with the gingival margin. A red arrow points to the gingival tissue between two central incisors, highlighting inflammation or recession likely due to violation of biologic width as described in the text. # **Principles of Tooth Preparations** ABUTMENT TOOTH * Principles of tooth preparation * Partial or complete preparation ## **BIOLOGICAL** * Conservation of tooth structure * Avoidance of overcontouring * Supragingival margins * **Harmonious occlusion** * Protection against tooth fracture ## **MECHANICAL** * Retention form * Resistance form * Deformation ## **AESTHETIC** * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins *Slide by Dr Matsubara*  #### Biological Principles - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - **Harmonious occlusion** - Protection against tooth fracture #### Mechanical Principles - Retention form - Resistance form - Deformation #### Aesthetic Principles - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins Venn diagram illustrating the principles of tooth preparations for an abutment tooth. It consists of three overlapping circles labeled 'BIOLOGICAL' (yellow), 'MECHANICAL' (green), and 'AESTHETIC' (blue). The Biological circle lists: Conservation of tooth structure, Avoidance of overcontouring, Supragingival margins, Harmonious occlusion (highlighted in red), and Protection against tooth fracture. The Mechanical circle lists: Retention form, Resistance form, and Deformation. The Aesthetic circle lists: Minimum display of metal, Maximum thickness of porcelain, Porcelain occlusal surfaces, and Subgingival margins. # Principles of Tooth Preparations ## ABUTMENT TOOTH - Principles of tooth preparation - Partial or complete preparation ## BIOLOGICAL - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - **Protection against tooth fracture** ## MECHANICAL - Retention form - Resistance form - Deformation ## AESTHETIC - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins Slide by Dr Matsubara  #### Biological Principles - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - **Protection against tooth fracture** #### Mechanical Principles - Retention form - Resistance form - Deformation #### Aesthetic Principles - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins Venn diagram illustrating the three principles of tooth preparations for an abutment tooth. The diagram consists of three overlapping circles: a yellow circle labeled 'BIOLOGICAL' containing bullet points on conservation of structure and protection against fracture; a green circle labeled 'MECHANICAL' covering retention and resistance forms; and a blue circle labeled 'AESTHETIC' listing requirements for porcelain display and occlusal surfaces. The text is presented in black with key terms highlighted in red. **Principles of Tooth Preparations** Cuspal Protection: Coverage of one or more cusps by a indirect or direct restorative material. **Fracture/Uncontrolled tooth structure loss** **Redistribution of stresses** Slide by Dr Nedelcu    #### Cuspal Protection Cuspal Protection is the coverage of one or more cusps by an indirect or direct restorative material. #### Objectives - Prevention of fracture or uncontrolled tooth structure loss - Redistribution of stresses Clinical photo of a molar tooth with significant fracture or uncontrolled tooth structure loss. The image shows the occlusal surface of a posterior tooth where a large portion of the cusp is missing or fractured. A dental explorer (metal instrument) is inserted into the cavity to demonstrate the depth and extent of the defect. Diagram illustrating 'Fracture/Uncontrolled tooth structure loss'. It depicts a cross-section of a tooth where a yellow block represents a restoration or prepared area on the occlusal surface. A red arrow points downward onto the restoration, indicating biting force, while another red arrow points laterally from the side, suggesting potential fracture propagation or structural weakness due to lack of cuspal support. Diagram illustrating 'Redistribution of stresses'. This diagram shows a similar tooth cross-section but with a reinforced preparation design. Yellow arrows point inward toward the center of the restored area, demonstrating how the indirect restorative material covers one or more cusps and redistributes occlusal forces (indicated by the top red arrow) away from vulnerable tooth structures. The University of Western Australia ### During preparation: - **Removal of tooth structure less than 2mm thick** - **Removal of any low quality/undermined enamel on the cusps** #### Below 2mm thick Also avoid occlusal contacts on the margin of the preparations Slide by Dr Nedelcu   #### Preparation Requirements - Removal of tooth structure less than 2mm thick - Removal of any low quality or undermined enamel on the cusps #### Clinical Considerations - Address structures below 2mm thick - Avoid occlusal contacts on the margin of the preparations Diagram illustrating tooth preparation guidelines. A cross-section of a molar shows a prepared cavity with an arrow pointing to the remaining tooth structure labeled '2mm', indicating the minimum thickness required for structural integrity. Diagram showing occlusal contacts on a prepared tooth. A black circle highlights an area where occlusal contact occurs on the margin of the preparation, which should be avoided according to the accompanying text. Picture of three overlapping circles labeled **BIOLOGICAL** (Yellow), **MECHANICAL** (Green), and **AESTHETIC** (Blue). **BIOLOGICAL** contains: Conservation of tooth structure, Avoidance of overcontouring, Supragingival margins, Harmonious occlusion, Protection against tooth fracture. **MECHANICAL** contains: Retention form, Resistance form, Deformation. **AESTHETIC** contains: Minimum display of metal, Maximum thickness of porcelain, Porcelain occlusal surfaces, Subgingival margins. <table> <thead> <tr> <th> <table> <thead> <tr> <th colspan="2"> PRINCIPLES OF TOOTH PREPARATIONS </th> </tr> </thead> <tbody> <tr> <th colspan="2"> THE UNIVERSITY OF WESTERN AUSTRALIA </th> </tr> </tbody> </table> </th> </tr> </thead> <tbody> <tr> <td colspan="2"> <table> <thead> <tr> <th colspan="2"> ABUTMENT TOOTH </th> </tr> </thead> <tbody> <tr> <td> <strong> Principles of tooth preparation </strong> </td> </tr> <tr> <td> <strong> Partial or complete preparation </strong> </td> </tr> </tbody> </table> <table> <thead> <tr> <th> <table> <thead> <tr> <th> BIOLOGICAL </th> </tr> </thead> <tbody> <tr> <td> <ul> <li> Conservation of tooth structure </li> <li> Avoidance of overcontouring </li> <li> Supragingival margins </li> <li> Harmonious occlusion </li> <li> Protection against tooth fracture </li> </ul> </td> </tr> </tbody> </table> </th> <th> <table> <thead> <tr> <th> MECHANICAL </th> </tr> </thead> <tbody> <tr> <td> <ul> <li> Retention form </li> <li> Resistance form </li> <li> Deformation </li> </ul> </td> </tr> </tbody> </table> </th> </tr> <tr> <th> <table> <thead> <tr> <th> AESTHETIC </th> </tr> </thead> <tbody> <tr> <td> <ul> <li> Minimum display of metal </li> <li> Maximum thickness of porcelain </li> <li> Porcelain occlusal surfaces </li> <li> Subgingival margins </li> </ul> </td> </tr> </tbody> </table> </th> </tr> </tbody> </table> <table> <thead> <tr> <th> Slide by Dr Matsubara </th> </tr> </thead> <tbody> <tr> <td> </td> </tr> </tbody> </table> </td> </tr> </tbody> </table>  Tooth preparation involves balancing three primary categories of principles: biological, mechanical, and aesthetic. #### Biological Principles - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture #### Mechanical Principles - Retention form - Resistance form - Deformation #### Aesthetic Principles - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins Labelled Venn diagram illustrating the three principles of tooth preparations for an abutment tooth. The diagram consists of three overlapping circles: a yellow circle labeled 'BIOLOGICAL' containing bullet points on conservation of tooth structure and protection against fracture; a green circle labeled 'MECHANICAL' containing retention form and resistance form; and a blue circle labeled 'AESTHETIC' containing details on porcelain thickness and margins. Principles of Tooth Preparations **Retention:** prevents removal of restoration/crown from coming off the prepared tooth when a force is applied parallel to the path of insertion. **Resistance:** prevents dislodging of restoration/crown when apical or oblique forces applied, and stops movement of restoration during chewing. **Good Retention But Bad Resistance** <div align="center"></div> **Bad Retention and Bad Resistance** <div align="center"></div>   #### Definitions - **Retention:** Prevents removal of restoration/crown from coming off the prepared tooth when a force is applied parallel to the path of insertion. - **Resistance:** Prevents dislodging of restoration/crown when apical or oblique forces are applied, and stops movement of restoration during chewing. #### Clinical Scenarios - Good Retention But Bad Resistance - Bad Retention and Bad Resistance Labelled diagram illustrating 'Good Retention But Bad Resistance'. It shows a cylindrical crown fitting over a prepared tooth. A vertical blue double-headed arrow indicates movement along the path of insertion (parallel force), demonstrating retention. The caption below confirms this scenario. Labelled diagram illustrating 'Bad Retention and Bad Resistance'. It depicts a conical tooth with two crowns positioned obliquely above it. Blue arrows point outwards from the crowns to the sides, representing oblique forces applied during chewing that would cause dislodgement. The caption explains this demonstrates poor resistance against lateral forces. **Principles of Tooth Preparations** **Retention**: prevents removal of restoration/crown from coming off the prepared tooth when a force is applied parallel to the path of insertion. **Resistance**: prevents dislodging of restoration/crown when apical or oblique forces applied, and stops movement of restoration during chewing. **Bad Resistance Form** TOOTH CROWN Excessive rounding of preparation can decrease resistance form **Good Resistance Form** TOOTH CROWN   #### Definitions - **Retention:** Prevents removal of restoration/crown from coming off the prepared tooth when a force is applied parallel to the path of insertion. - **Resistance:** Prevents dislodging of restoration/crown when apical or oblique forces are applied, and stops movement of restoration during chewing. #### Resistance Form Analysis - **Bad Resistance Form:** Excessive rounding of preparation can decrease resistance form. - **Good Resistance Form:** Proper preparation geometry maintains stability between the tooth and crown. Labeled diagram illustrating 'Bad Resistance Form'. Shows a cylindrical preparation (labeled TOOTH) inside a larger cylinder (CROWN). A blue curved arrow indicates an oblique force attempting to dislodge the crown. Text below notes that 'Excessive rounding of preparation can decrease resistance form'. Labeled diagram illustrating 'Good Resistance Form'. Shows a square preparation (labeled TOOTH) inside a larger cylinder (CROWN). A red prohibition symbol overlays a blue curved arrow indicating oblique force, demonstrating that this shape prevents movement. **Principles of Tooth Preparations** Fundamental of Fixed Prosthodontics Factors influencing Retention and Resistance: **Taper** and Length of axial walls **Evaluation of the degree of taper and convergence angle of full ceramo-metal crown preparations by different specialists centers at Assir Region, Saudi Arabia** Mohammed M. Al-Moalem, M. Shariff, *2 authors, S. Tikare* • Published 1 September 2015 • Medicine • Saudi Journal of Medicine and Medical Sciences   #### Factors Influencing Retention and Resistance - Taper - Length of axial walls #### Research Context Evaluation of the degree of taper and convergence angle of full ceramo-metal crown preparations by different specialists centers at Assir Region, Saudi Arabia (Al-Moalem et al., 2015). Diagram illustrating tooth preparation geometry. Shows a central tooth structure with superimposed lines defining the 'Convergence angle' and 'Taper'. Arrows point to these geometric features on the axial walls of the prepared tooth. Line chart (Fig 9-4) plotting 'Retention (g/mm²)' against 'Taper (degrees)'. The graph shows an inverse relationship where retention decreases as taper increases from approximately 0 to 60 degrees. Caption below states: 'As taper increases, retention decreases.' **convergence angle** |kon-vr'jens ăng'gel|: the angle, measured in degrees as viewed in a given plane, formed by the axial walls when a tooth or machined surface on a metal or ceramic material is prepared for a fixed dental prosthesis; *Editorial note for usage*: the term *total occlusal convergence* applies to the angle formed between two opposing axial walls; the term *taper* applies to the angle formed between an axial wall and the path of placement onto the tooth or machined surfaces of a metal or ceramic material; syn., *angle of convergence*, *TOTAL OCCLUSAL CONVERGENCE*; comp., *TAPER* **taper**: in dentistry, the angle, measured in degrees as viewed in a given plane, formed between an external wall and the path of placement of a tooth preparation or machined surfaces on a metal or ceramic material when prepared for fixed dental prosthesis; comp., CONVERGENCE ANGLE, TOTAL OCCLUSAL CONVERGENCE ANGLE #### Glossary of Terms - **Convergence Angle:** The angle, measured in degrees as viewed in a given plane, formed by the axial walls when a tooth or machined surface on a metal or ceramic material is prepared for a fixed dental prosthesis. - *Total Occlusal Convergence:* Applies to the angle formed between two opposing axial walls. - **Taper:** In dentistry, the angle, measured in degrees as viewed in a given plane, formed between an external wall and the path of placement of a tooth preparation or machined surfaces on a metal or ceramic material. Preparations JPD THE JOURNAL OF PROSTHETIC DENTISTRY THE GLOSSARY OF PROSTHODONTIC TERMS Ninth Edition comp **JP** THE UNIVERSITY OF WESTERN AUSTRALIA convergence angle |kon-vür'jens äng'gel|: the angle, measured in degrees as viewed in a given plane, formed by the axial walls when a tooth or machined surface on a metal or ceramic material is prepared for a fixed dental prosthesis; Editorial note for usage: the term *total occlusal convergence* applies to the angle formed between two opposing axial walls; the term *taper* applies to the angle formed between an axial wall and the path of placement onto the tooth or machined surfaces of a metal or ceramic material; *syn*, *angle of convergence*, TOTAL OCCLUSAL CONVERGENCE; *comp*, TAPER #### The Glossary of Prosthodontic Terms (Ninth Edition) - **Convergence Angle:** The angle, measured in degrees as viewed in a given plane, formed by the axial walls when a tooth or machined surface on a metal or ceramic material is prepared for a fixed dental prosthesis. - **Usage Notes:** - The term *total occlusal convergence* applies to the angle formed between two opposing axial walls. - The term *taper* applies to the angle formed between an axial wall and the path of placement onto the tooth or machined surfaces. # Google is **_not_** your best friend! ### Google Search Result (Left) **Search term:** ideal taper preparation **About 5,160,000 results (0.4...)** Recommended tape... to Shillinberg 6 deg... Minimum taper that... absence of undercu... total convergence o... proposed as being achievable clinically while still affording adequate retention. **Dentosphere : World of Dentistry** https://www.dentaldevotee.com > 2020/06 > minimum-a... --- ### THE JOURNAL OF PROSTHETIC DENTISTRY (Right) **THE GLOSSARY OF PROSTHODONTIC TERMS** **Ninth Edition** **convergence angle** \kən-vûr'jens ăng'gel\: the angle, measured in degrees, between two opposing axial walls when viewed in a given plane, formed by the axial walls when a preparation is made for a metal or ceramic material is part of a dental prosthesis; *Editorial note for usage:* the term *total occlusal convergence* applies to the angle formed between two opposing axial walls; the term *taper* applies to the angle formed between an axial wall and the path of placement onto the tooth or machined surfaces of a metal or ceramic material; *syn*, angle of convergence, TOTAL OCCLUSAL CONVERGENCE; *comp*, TAPER  #### Comparison of Information Sources **General Search Results (e.g., Google)** - Often cites Shillingburg's 6-degree ideal taper. - Discusses minimum taper achievable clinically while affording adequate retention. **Professional Standards (The Glossary of Prosthodontic Terms)** - **Convergence Angle:** The angle, measured in degrees, between two opposing axial walls when viewed in a given plane. - **Taper:** The angle formed between an axial wall and the path of placement onto the tooth. A composite figure illustrating a dental concept. The central element is the Google logo with the text 'is not your best friend!' superimposed below it. This figure serves as a visual metaphor to critique or highlight the unreliability of general internet search results (represented by the blurred background) compared to authoritative academic sources (represented by the journal text on the right). Principles of Tooth Preparations **Factors influencing Retention and Resistance:** **Taper** and Length of axial walls **Fundamental of Fixed Prosthodontics** <img:A graph showing that as taper (degrees) increases, retention (g/mm²) decreases (Fig 9-4).</img> Evaluation of the degree of taper and convergence angle of full ceramo-metal crown preparations by different specialists centers at Assir Region, Saudi Arabia (Arrows pointing left, "Taper")   #### Impact of Taper on Retention - Retention (measured in g/mm²) decreases as the taper (measured in degrees) increases. - Clinical evaluations of ceramo-metal crown preparations focus on the relationship between taper and the total convergence angle. Labeled diagram illustrating tooth preparation principles. Shows a central vertical axis with two diverging lines forming an angle labeled 'Convergence angle'. An arrow points to the taper of the preparation walls labeled 'Taper'. The base of the diagram shows a schematic representation of a prepared tooth structure. Line graph titled 'Fundamental of Fixed Prosthodontics' showing the relationship between Taper (degrees) on the x-axis and Retention (g/mm²) on the y-axis. The curve demonstrates that as taper increases from approximately 5 degrees to 60 degrees, retention decreases significantly from over 100 g/mm² to near zero. Caption below reads: 'Fig 9-4 As taper increases, retention decreases.'. Principles of Tooth Preparations Fig 9-13 To examine a preparation for undercuts, one eye should be closed. Fig 9-14 If both eyes are open when the preparation is viewed, undercuts may remain undetected. Fig 9-15 Preparations in the mouth are viewed through a mouth mirror using one eye. Fundamental of Fixed Prosthodontics  #### Clinical Examination for Undercuts - To examine a preparation for undercuts, one eye should be closed. - If both eyes are open when the preparation is viewed, undercuts may remain undetected due to binocular vision. - Preparations in the mouth are viewed through a mouth mirror using one eye to ensure a proper path of insertion. Three-panel diagram (Fig 9-13 to Fig 9-15) illustrating principles of tooth preparation examination. Panel 1 (Fig 9-13): Shows a clinician examining a dental model with one eye closed to detect undercuts. Panel 2 (Fig 9-14): Shows the same scenario but with both eyes open, indicating that undercuts may remain undetected due to binocular vision masking depth cues. Panel 3 (Fig 9-15): Depicts a side view of a clinician using a mouth mirror and instrument to examine an intraoral preparation through one eye. All panels include detailed line drawings of facial features, instruments, and dental anatomy. Principles of Tooth Preparations Factors influencing Retention and Resistance: Taper and **Length** of axial walls Fig 9-9 The preparation with longer walls (a) interferes with the tipping displacement of the restoration better than the short preparation (b). <a href="#Fig9-9"></a> Fig 9-10 A 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). <a href="#Fig9-10"></a> Fundamental of Fixed Prosthodontics   #### Impact of Axial Wall Length - **Wall Height:** A preparation with longer walls interferes with the tipping displacement of the restoration better than a short preparation. - **Tooth Diameter:** A preparation on a tooth with a smaller diameter resists pivoting movements better than a preparation of equal length on a tooth of larger diameter. Diagram labeled Fig 9-9 illustrating 'The preparation with longer walls (a) interferes with the tipping displacement of the restoration better than the short preparation (b).' It shows two tooth cross-sections: (a) with a long axial wall and (b) with a short axial wall. A red box highlights the axial wall area in both diagrams, and arrows indicate tipping movement vectors. Diagram labeled Fig 9-10 illustrating 'A 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).' It shows two tooth cross-sections: (a) with a narrower diameter and (b) with a wider diameter. Arrows indicate pivoting forces acting on each preparation. **Principles of Tooth Preparations** Factors influencing Retention and Resistance: Taper and **Length** of axial walls What can we do to increase retention/resistance? • Grooves and boxes BUT USE WITH CARE! Fundamental of Fixed Prosthodontics   #### Enhancing Retention and Resistance - **Supplemental Features:** Grooves and boxes can be used to increase retention and resistance. - **Precaution:** These features should be used with care to maintain structural integrity. Labelled diagram Fig 9-6 showing two tooth preparation cross-sections. Panel (a) shows a preparation with vertical grooves limiting withdrawal paths to improve retention; panel (b) shows a smooth preparation with unlimited freedom of displacement resulting in less retention. Arrows indicate direction of potential movement. Labelled diagram Fig 9-7 illustrating groove wall angles relative to axial walls and forces. Panel (a) depicts oblique groove walls that fail to provide necessary resistance against rotational force; panel (b) shows perpendicular groove walls correctly resisting rotating forces. Black arrows indicate the direction of applied force. # Principles of Tooth Preparations Other factors that influence retention and resistance: a. Magnitude of the dislodging force **b. Geometry of tooth preparation (MOST IMPORTANT, YOU DIRECT IMPACT THIS)** c. Roughness of fitting surface of restoration d. Material being cemented e. Properties of luting agent #### Additional Influencing Factors 1. Magnitude of the dislodging force. 2. **Geometry of tooth preparation:** This is the most important factor as the clinician has a direct impact on it. 3. Roughness of the fitting surface of the restoration. 4. Material being cemented. 5. Properties of the luting agent. # Principles of Tooth Preparations **ABUTMENT TOOTH** * Principles of tooth preparation * Partial or complete preparation **BIOLOGICAL** * Conservation of tooth structure * Avoidance of overcontouring * Supragingival margins * Harmonious occlusion * Protection against tooth fracture **MECHANICAL** * Retention form * Resistance form * **Deformation** **AESTHETIC** * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins Slide by Dr Matsubara  #### Abutment Tooth Considerations - Principles of tooth preparation - Partial or complete preparation #### Summary of Preparation Principles - **Biological:** Conservation of tooth structure, avoidance of overcontouring, supragingival margins, harmonious occlusion, protection against tooth fracture. - **Mechanical:** Retention form, resistance form, deformation. - **Aesthetic:** Minimum display of metal, maximum thickness of porcelain, porcelain occlusal surfaces, subgingival margins. A Venn diagram illustrating the principles of tooth preparations for an abutment tooth. It consists of three overlapping circles labeled 'BIOLOGICAL', 'MECHANICAL', and 'AESTHETIC'. The 'BIOLOGICAL' circle (yellow) lists items such as conservation of tooth structure and protection against fracture. The 'MECHANICAL' circle (green) includes retention form, resistance form, and deformation (highlighted in red). The 'AESTHETIC' circle (blue) mentions minimum display of metal and subgingival margins. # Principles of Tooth Preparations ## ABUTMENT TOOTH * Principles of tooth preparation * Partial or complete preparation ### BIOLOGICAL * Conservation of tooth structure * Avoidance of overcontouring * Supragingival margins * Harmonious occlusion * Protection against tooth fracture ### MECHANICAL * Retention form * Resistance form * Deformation ### AESTHETIC * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins --- Slide by Dr Matsubara  #### Abutment Tooth Preparation Overview Tooth preparation for abutments involves balancing biological, mechanical, and aesthetic requirements. Preparations may be categorized as partial or complete. #### Biological Principles - Conservation of tooth structure - Avoidance of overcontouring - Supragingival margins - Harmonious occlusion - Protection against tooth fracture #### Mechanical Principles - Retention form - Resistance form - Prevention of deformation #### Aesthetic Considerations - Minimum display of metal - Maximum thickness of porcelain - Porcelain occlusal surfaces - Subgingival margins Venn diagram illustrating the 'Principles of Tooth Preparations' for an abutment tooth. It consists of three overlapping circles labeled 'BIOLOGICAL', 'MECHANICAL', and 'AESTHETIC'. The Biological circle (yellow) lists: Conservation of tooth structure, Avoidance of overcontouring, Supragingival margins, Harmonious occlusion, Protection against tooth fracture. The Mechanical circle (green) lists: Retention form, Resistance form, Deformation. The Aesthetic circle (blue) lists: Minimum display of metal, Maximum thickness of porcelain, Porcelain occlusal surfaces, Subgingival margins. # Principles of Tooth Preparations **ABUTMENT TOOTH** * Principles of tooth preparation * Partial or complete preparation **BIOLOGICAL** * Conservation of tooth structure * Avoidance of overcontouring * Supragingival margins * Harmonious occlusion * Protection against tooth fracture **MECHANICAL** * Retention form * Resistance form * Deformation **AESTHETIC** * Minimum display of metal * Maximum thickness of porcelain * Porcelain occlusal surfaces * Subgingival margins Slide by Dr Matsubara  Venn diagram illustrating the 'Principles of Tooth Preparations' for an 'ABUTMENT TOOTH'. The diagram consists of three overlapping circles labeled 'BIOLOGICAL', 'MECHANICAL', and 'AESTHETIC'. Each circle contains specific bullet points detailing principles such as conservation of tooth structure, retention form, and minimum display of metal. The layout visually represents the intersection of these three concepts. # **Principles of Tooth Preparations** **Any questions? Please email me, or come up to me in CSSL.** [^1]: Original PDF page 1: [[DMD2 L1 Principles of tooth prep.pdf#page=1|DMD2 L1 Principles of tooth prep, p.1]] [^2]: Original PDF page 2: [[DMD2 L1 Principles of tooth prep.pdf#page=2|DMD2 L1 Principles of tooth prep, p.2]] [^3]: Original PDF page 3: [[DMD2 L1 Principles of tooth prep.pdf#page=3|DMD2 L1 Principles of tooth prep, p.3]] [^4]: Original PDF page 4: [[DMD2 L1 Principles of tooth prep.pdf#page=4|DMD2 L1 Principles of tooth prep, p.4]] [^5]: Original PDF page 5: [[DMD2 L1 Principles of tooth prep.pdf#page=5|DMD2 L1 Principles of tooth prep, p.5]] [^6]: Original PDF page 6: [[DMD2 L1 Principles of tooth prep.pdf#page=6|DMD2 L1 Principles of tooth prep, p.6]] [^7]: Original PDF page 7: [[DMD2 L1 Principles of tooth prep.pdf#page=7|DMD2 L1 Principles of tooth prep, p.7]] [^8]: Original PDF page 8: [[DMD2 L1 Principles of tooth prep.pdf#page=8|DMD2 L1 Principles of tooth prep, p.8]] [^9]: Original PDF page 9: [[DMD2 L1 Principles of 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