DENT4216 2024
---
**Lecture 1-Principles of Tooth Preparations**
By Dr Cheryl Fu
# **Lecture 1-Principles of Tooth Preparations**
**By Dr Cheryl Fu**
---
**Expected Reading:**
THE READING WILL
BE EXAMINABLE
CONTENT!
![]()
OneSearch interface showing search results for "fundamentals of fixed prosthodontics" with arrows pointing to two book entries, alongside a cover image of "Fundamentals of Fixed Prosthodontics, Fourth Edition".
# **Expected Reading**
THE READING WILL BE EXAMINABLE CONTENT!
**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*
# **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*
> [!tip] Study Tip
> It is recommended to watch the lecture first to gain a general understanding, then use the textbook chapters to review and gain more in-depth knowledge.
# Why Does a Tooth Need a Crown?
As part of DMD1 and DMD2 (Semester 1) you have learnt about:
- Composite 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)
![]()
Bar chart comparing resistance to removal forces for four types of crowns: MOD only (36 KgF), ½ crown (48 KgF), ¾ crown (52 KgF), and All-metal crown (110 KgF). Caption: Fig 6-1 A comparison of resistance to removal forces for four types of crowns (P = .05).¹,² MOD, mesio-occlusodistal.
# **Why Does a Tooth Need a Crown?**
As part of DMD1 and DMD2 (Semester 1) you have learnt about:
- Composite restorations
- Amalgam restorations
So why do we need crown?
## **Considerations for a crown**
- **Destruction of tooth structure**
This is indicated when there is extensive loss of tooth structure due to:
- ==**Large carious lesions.**==
- ==**Fracture of cusps**, often in teeth with existing large restorations (e.g., MOD amalgams). A crown protects the remaining weakened tooth structure from further fracture under occlusal loads.==
- ==**Endodontically treated teeth**, especially posterior teeth, which are brittle and have lost structure from the access cavity. Full occlusal coverage is crucial to prevent catastrophic fracture.==
# Why Does a Tooth Need a Crown?
**Considerations for a crown:**
- **Destruction of tooth structure**
- **Aesthetics**
- **Plaque Control/Moisture Control**
- **Retention**
**Figure 6-1.** A comparison of resistance to removal forces for four types of crowns (P = .05).¹² MOD, mesio-occlusodistal.
**Fundamental of Fixed Prosthodontics**
Terry E. Donovan (2006) Longevity of the Tooth/Restoration Complex: A Review
![]()
Figure 3a and Figure 3b showing patients with rampant caries, photos courtesy of Dr. Jinus Emrani
- According to Terry E. Donovan (2006) in *Fundamental of Fixed Prosthodontics*, a study comparing resistance to removal forces for four types of crowns found a significant difference (P = .05).
- **Aesthetics**
# Why Does a Tooth Need a Crown?
**Considerations for a crown:**
- Destruction of tooth structure
- Aesthetics
- Plaque Control/Moisture Control
- Retention
**Photo by tetracycline staining by Dr. Alessandro Martini**
**PFM crowns**
**Song MY, Park JM, Park EJ. Full mouth rehabilitation of the patient with severely worn dentition: a case report.**
- Crowns, such as PFM crowns, can be used for full mouth rehabilitation in cases of severely worn dentition or to address issues like tetracycline staining.
- ==To mask severe discoloration like **tetracycline staining**, a thin ceramic veneer or crown can provide a superior aesthetic result compared to bulky composite additions.==
- ==To restore teeth in cases of **severe tooth wear**, crowns offer better mechanical properties and retention than direct composites for rebuilding incisal edges worn down by bruxism.==
- **Plaque Control/Moisture Control**
# Why Does a Tooth Need a Crown?
**Considerations for a crown:**
- Destruction of tooth structure
- Aesthetics
- **Plaque Control/Moisture Control**
- 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.
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.
- 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.
- *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.*
> [!info] Clinical Considerations
> - In areas with difficult moisture control (e.g., deep subgingival margins), an indirect restoration made from a well-recorded impression may offer a better marginal seal and adaptation than a direct composite.
> - **Prerequisite:** The patient must demonstrate good oral hygiene. If a patient has poor plaque control, placing an expensive crown is risky, as secondary caries can form at the margins. It may be better to place a large direct restoration and monitor the patient's hygiene commitment.
- **Retention**
- **Finance***
> [!info]
> The patient's ability to afford treatment is a significant factor, but it does not change the clinical need for the crown.
- **Other prosthodontic treatment (survey crowns for RPDs)**
**CLINICAL PROCEDURES**
![]()
Flowchart of clinical dental procedures including Clinical examination, Primary Impression, Tooth preparation + Temporisation, Definitive Impression, Intraoral scanning, CAD All-ceramic, Milling, Try-in, Cementation, Review, Casting and veneering, Wax up (gold crown, PFM)
# **CLINICAL PROCEDURES**
```mermaid
flowchart TD
A[Preparation] --> B[Impression]
A --> C[Scanning]
B --> D[Casting]
D --> E[Manual finishing]
E --> F[Veneering/Polishing]
F --> G[Insert]
C --> H[Scanning]
H --> I[Milling]
I --> J[Sintering]
J --> K[Veneering/Polishing]
K --> L[Insert]
style A fill:#fff,stroke:#000,stroke-width:2px
style B fill:#f9f9f9,stroke:#999
style C fill:#f9f9f9,stroke:#999
style D fill:#f9f9f9,stroke:#999
style E fill:#f9f9f9,stroke:#999
style F fill:#f9f9f9,stroke:#999
style G fill:#f9f9f9,stroke:#999
style H fill:#f9f9f9,stroke:#999
style I fill:#f9f9f9,stroke:#999
style J fill:#f9f9f9,stroke:#999
style K fill:#f9f9f9,stroke:#999
style L fill:#f9f9f9,stroke:#999
classDef conventional fill:#fff,stroke:#999,stroke-width:1px
classDef cadcam fill:#fff,stroke:#999,stroke-width:1px
class B,D,E,F,G conventional
class C,H,I,J,K,L cadcam
```
**Slide by Dr Matsubara**
```mermaid
flowchart TD
A[Preparation] --> B[Impression]
A --> C[Scanning]
B --> D[Casting]
D --> E[Manual finishing]
E --> F[Veneering/Polishing]
F --> G[Insert]
C --> H[Scanning]
H --> I[Milling]
I --> J[Sintering]
J --> K[Veneering/Polishing]
K --> L[Insert]
style A fill:#fff,stroke:#000,stroke-width:2px
style B fill:#f9f9f9,stroke:#999
style C fill:#f9f9f9,stroke:#999
style D fill:#f9f9f9,stroke:#999
style E fill:#f9f9f9,stroke:#999
style F fill:#f9f9f9,stroke:#999
style G fill:#f9f9f9,stroke:#999
style H fill:#f9f9f9,stroke:#999
style I fill:#f9f9f9,stroke:#999
style J fill:#f9f9f9,stroke:#999
style K fill:#f9f9f9,stroke:#999
style L fill:#f9f9f9,stroke:#999
classDef conventional fill:#fff,stroke:#999,stroke-width:1px
classDef cadcam fill:#fff,stroke:#999,stroke-width:1px
class B,D,E,F,G conventional
class C,H,I,J,K,L cadcam
```
*Slide by Dr Matsubara*
![]()
Flowchart showing dental restoration processes: Preparation → Impression → Scanning (split into Conventional and CAD-CAM paths). Conventional: Casting → Manual finishing → Veneering/Polishing → Insert. CAD-CAM: Scanning → Milling → Sintering → Veneering/Polishing → Insert. Slide by Dr Matsubara.
```mermaid
graph TD
A[Preparation] --> B[Impression]
B --> C[Fabrication of models]
B --> D[Scanning]
D --> E[CAD-CAM]
E --> F[Scanning]
F --> G[Milling]
G --> H[Sintering]
H --> I[Veneering/Polishing]
I --> J[Insert]
C --> K[Casting]
K --> L[Manual finishing]
L --> M[Veneering/Polishing]
M --> N[Insert]
style E fill:#f9f,stroke:#333,stroke-width:2px
style K fill:#ccc,stroke:#333,stroke-width:2px
style F fill:#ccc,stroke:#333,stroke-width:2px
style G fill:#ccc,stroke:#333,stroke-width:2px
style H fill:#ccc,stroke:#333,stroke-width:2px
style I fill:#ccc,stroke:#333,stroke-width:2px
style J fill:#ccc,stroke:#333,stroke-width:2px
style K fill:#ccc,stroke:#333,stroke-width:2px
style L fill:#ccc,stroke:#333,stroke-width:2px
style M fill:#ccc,stroke:#333,stroke-width:2px
style N fill:#ccc,stroke:#333,stroke-width:2px
style A fill:#fff,stroke:#333,stroke-width:2px
style B fill:#fff,stroke:#333,stroke-width:2px
style C fill:#fff,stroke:#333,stroke-width:2px
style D fill:#fff,stroke:#333,stroke-width:2px
```
Slide by Dr Matsubara
```mermaid
graph TD
A[Preparation] --> B[Impression]
B --> C[Fabrication of models]
B --> D[Scanning]
D --> E[CAD-CAM]
E --> F[Scanning]
F --> G[Milling]
G --> H[Sintering]
H --> I[Veneering/Polishing]
I --> J[Insert]
C --> K[Casting]
K --> L[Manual finishing]
L --> M[Veneering/Polishing]
M --> N[Insert]
style E fill:#f9f,stroke:#333,stroke-width:2px
style K fill:#ccc,stroke:#333,stroke-width:2px
style F fill:#ccc,stroke:#333,stroke-width:2px
style G fill:#ccc,stroke:#333,stroke-width:2px
style H fill:#ccc,stroke:#333,stroke-width:2px
style I fill:#ccc,stroke:#333,stroke-width:2px
style J fill:#ccc,stroke:#333,stroke-width:2px
style K fill:#ccc,stroke:#333,stroke-width:2px
style L fill:#ccc,stroke:#333,stroke-width:2px
style M fill:#ccc,stroke:#333,stroke-width:2px
style N fill:#ccc,stroke:#333,stroke-width:2px
style A fill:#fff,stroke:#333,stroke-width:2px
style B fill:#fff,stroke:#333,stroke-width:2px
style C fill:#fff,stroke:#333,stroke-width:2px
style D fill:#fff,stroke:#333,stroke-width:2px
```
*Slide by Dr Matsubara*
```mermaid
graph TD
A[Preparation] --> B[Impression]
B --> C[Fabrication of models]
C --> D[Casting]
D --> E[Manual finishing]
E --> F[Veneering/Polishing]
F --> G[Insert]
A --> H[Scanning]
H --> I[Scanning]
I --> J[Milling]
J --> K[Sintering]
K --> L[Veneering/Polishing]
L --> M[Insert]
style A fill:#ffffff,stroke:#000000
style B fill:#ffffff,stroke:#000000
style C fill:#ffffff,stroke:#000000
style D fill:#ffffff,stroke:#000000
style E fill:#ffffff,stroke:#000000
style F fill:#ffffff,stroke:#000000
style G fill:#ffffff,stroke:#000000
style H fill:#ffffff,stroke:#000000
style I fill:#f0f0f0,stroke:#000000
style J fill:#f0f0f0,stroke:#000000
style K fill:#f0f0f0,stroke:#000000
style L fill:#f0f0f0,stroke:#000000
style M fill:#f0f0f0,stroke:#000000
classDef conventional fill:#ffffff,stroke:#000000;
classDef cadcam fill:#f0f0f0,stroke:#000000;
class D,E,F,G conventional;
class I,J,K,L,M cadcam;
subgraph Conventional
D
E
F
G
end
subgraph CAD-CAM
I
J
K
L
M
end
```
Slide by Dr Matsubara
```mermaid
graph TD
A[Preparation] --> B[Impression]
B --> C[Fabrication of models]
C --> D[Casting]
D --> E[Manual finishing]
E --> F[Veneering/Polishing]
F --> G[Insert]
A --> H[Scanning]
H --> I[Scanning]
I --> J[Milling]
J --> K[Sintering]
K --> L[Veneering/Polishing]
L --> M[Insert]
style A fill:#ffffff,stroke:#000000
style B fill:#ffffff,stroke:#000000
style C fill:#ffffff,stroke:#000000
style D fill:#ffffff,stroke:#000000
style E fill:#ffffff,stroke:#000000
style F fill:#ffffff,stroke:#000000
style G fill:#ffffff,stroke:#000000
style H fill:#ffffff,stroke:#000000
style I fill:#f0f0f0,stroke:#000000
style J fill:#f0f0f0,stroke:#000000
style K fill:#f0f0f0,stroke:#000000
style L fill:#f0f0f0,stroke:#000000
style M fill:#f0f0f0,stroke:#000000
classDef conventional fill:#ffffff,stroke:#000000;
classDef cadcam fill:#f0f0f0,stroke:#000000;
class D,E,F,G conventional;
class I,J,K,L,M cadcam;
subgraph Conventional
D
E
F
G
end
subgraph CAD-CAM
I
J
K
L
M
end
```
*Slide by Dr Matsubara*
![]()
Flowchart comparing Conventional and CAD-CAM dental restoration workflows, starting from Preparation and ending with Insert.
**Workflow**
Slide by Dr Matsubara
![]()
Flowchart showing two parallel dental restoration workflows: Conventional and CAD-CAM, both starting from Preparation and ending at Insert.
## **Workflow**
Understanding the overall workflow helps contextualize the principles of tooth preparation.
1. **Examination & Treatment Planning:**
- Includes clinical exams, pulp testing, radiographs, and diagnostic impressions to create mounted study models on an articulator.
- This phase determines the prognosis and confirms the need for a crown.
2. **Tooth Preparation & Temporization:**
- The tooth is reduced according to the principles for the chosen material.
- A temporary crown is placed to protect the prepared tooth, prevent sensitivity, maintain occlusal and proximal contacts, and provide aesthetics.
3. **Impression / Digital Scan:**
- **Conventional:** A definitive impression is taken using a material like PVS. The lab then pours a model.
- **Digital:** An intraoral scanner (e.g., TRIOS) is used to create a digital model directly, or the conventional impression/model is scanned by the lab.
4. **Laboratory Fabrication:**
- **Conventional Workflow:** A lab technician waxes up the crown by hand, casts it (for metal/PFM), applies porcelain, and finishes it.
- **Digital (CAD/CAM) Workflow:** The crown is designed on a computer (Computer-Aided Design) and then milled from a block of ceramic or other material (Computer-Aided Manufacturing). It is then sintered, finished, and glazed.
5. **Try-in and Evaluation:**
- The temporary crown is removed, and the definitive crown is tried in.
- Key aspects are evaluated: interproximal contacts, occlusal contacts, marginal fit, shade, and shape.
6. **Cementation:**
- If the try-in is successful, the crown is permanently cemented. Excess cement must be meticulously removed.
7. **Review:** The patient should be reviewed to ensure proper function and health of the surrounding tissues.
*Slide by Dr Matsubara*
**Crown Preparation Terminology**
**Occlusal**
**Axial**
**Margins**
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*
# **Crown Preparation Terminology**
- **Occlusal**
- **Axial**
- **Margins**
- **Occlusal Reduction:** ==The amount of tooth structure removed from the occlusal surface to create space for the restorative material.==
- **Axial Reduction:** ==The reduction of the vertical surfaces (buccal, lingual, mesial, distal walls) of the tooth.==
- **Margins:** ==The finish line of the preparation where the crown will meet the tooth.==
> 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*
**Crowns**
**Materials:**
- **All ceramic**
- Zirconia or Glassy Ceramics (eg Lithium Silicate or Lithium Disilicate)
- **Metal**
- Varying alloys
- **Porcelain Fused to Metal (PFM)**
Each material may require different preparations.
# **Types of Crowns**
## **Materials**
- **All ceramic**
- Zirconia or Glassy Ceramics (eg Lithium Silicate or Lithium Disilicate)
- ==**Zirconia:** Very strong, opaque.==
- ==**Glassy Ceramics:** More aesthetic, includes lithium disilicate and feldspathic porcelain.==
- **Metal**
- Varying alloys
- **Porcelain Fused to Metal (PFM)**
Each material may require different preparations.
# Crown Preparation Terminology
## Margins:
- Supra-gingival of subgingival
- Types of margins
- Feather edge
- Bevel
- Chamfer
- Shoulder
- Shoulder with bevel
## 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
## Supragingival Margins
- Easily prepared
- Easier to take impressions
- Easily maintained by patient
- Easily evaluated at recalls
- Preserve periodontium
# **Margins**
- Supra-gingival of subgingival
- Types of margins
- Feather edge
- Bevel
- Chamfer
- Shoulder
- Shoulder with bevel
## **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
- ==To improve the **emergence profile** and avoid a bulky contour.==
## **Supragingival Margins**
- Easily prepared
- Easier to take impressions
- Easily maintained by patient
- Easily evaluated at recalls
- Preserve periodontium
# Crown Preparation Terminology
## Margins:
- Supra-gingival of subgingival
- Types of margins
- Feather edge
- Bevel
- Chamfer
- Shoulder
- Shoulder with bevel
## 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
## Supragingival Margins
- Easily prepared
- Easier to take impressions
- Easily maintained by patient
- Easily evaluated at recalls
- Preserve periodontium
# Margins
**Margins:**
- Supra-gingival of subgingival
- Types of margins
- **Feather edge**
- Bevel
- Chamfer
- Shoulder
- Shoulder with bevel
**Advantages:**
- Conservation of tooth structure
**Disadvantages:**
- Fail to provide adequate bulk at margins
**CONTRA-INDICATED IN MOST CASES!**
Slide by Dr Nedelcu
## **Types of Margins**
### **Feather edge**
- **Advantages:**
- Conservation of tooth structure
- **Disadvantages:**
- Fail to provide adequate bulk at margins
- ==The finish line is indistinct and difficult for the lab to read and fabricate to.==
- **CONTRA-INDICATED IN MOST CASES!**
> [!note] Indication
> Generally contraindicated for crowns; may be used for some feldspathic veneers.
*Slide by Dr Nedelcu*
**Margins**
**Margins:**
- Supra-gingival of subgingival
- Types of margins
- Feather edge
- **Bevel**
- Chamfer
- Shoulder
- Shoulder with bevel
**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
**Commonly for cast restorations**
Slide by Dr Nedelcu
### **Bevel**
- **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
- **Commonly for cast restorations**
*Slide by Dr Nedelcu*
# 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.
Slide by Dr Nedelcu
### **Shoulder**
- **Advantages:**
- Easy to prepare and finish well
- Easy to judge on impressions
- **Disadvantages:**
- More aggressive preparation
- **Indication:** ==Primarily for **all-ceramic crowns**.==
- **Can be used for all materials.**
- Internal angle is ROUNDED NOT SHARP 90 degrees.
> [!info]
> It is prepared with a flat-end bur, resulting in a rounded internal line angle.
*Slide by Dr Nedelcu*
**Margins**
**Margins:**
- Supra-gingival or 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
### **Chamfer**
- **Advantages:**
- Conserves tooth structure whilst providing distinct finish lines
- Easy to read in impressions
> [!info]
> This is the most common margin type.
- **Commonly used for metal, or ceramic crowns**
- Use a tapered diamond bur with round tip!
*Slide by Dr Nedelcu*
# 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.**
Contemporary Fixed Prosthodontics
### **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
> [!note] Historical Use
> It was historically used for PFM crowns where a metal collar at the margin was hidden subgingivally for a better seal. It is not preferred today due to poor aesthetics and potential violation of biologic width.
- **Uncommonly used.**
*Contemporary Fixed Prosthodontics*
# 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
# **Principles of Tooth Preparations**
The principles of tooth preparation for an abutment tooth (partial or complete preparation) can be categorized as follows:
> [!abstract] The Three Pillars
> A successful preparation balances three key principles:
> - **BIOLOGICAL:** Preserving the health of the tooth and surrounding tissues.
> - **MECHANICAL:** Ensuring the restoration is stable, retentive, and durable.
> - **AESTHETIC:** Achieving a natural and pleasing appearance.
### **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*
# 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
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: 10.1096/j.qi.20180002
CITATIONS
32
2 authors:
Daniel Edelhoff
Ludwig-Maximilians-University of Munich
262 PUBLICATIONS 10,264 CITATIONS
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M. Oliver Ahlers
University Medical Center Hamburg - Eppendorf
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![]()
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).
## **Biological Principles**
### **Conservation of tooth structure**
# Principles of Tooth Preparations
**V ≈ 785.4**
*r* Radius 5
*h* Height 10
**Solution**
*V = π r² h = π · 5² · 10 ≈ 785.39816*
**V ≈ 402.12**
*r* Radius 4
*h* Height 8
**Solution**
*V = π r² h = π · 4² · 8 ≈ 402.12386*
**V = πr²h**
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!**
- **Volume Calculation Example (V = πr²h):**
- A simplified cylindrical model shows a significant volume decrease with small changes in dimensions.
- **Cylinder 1:** Radius = 5, Height = 10 → **Volume ≈ 785.4**
- **Cylinder 2:** Radius = 4, Height = 8 → **Volume ≈ 402.12**
- This represents almost a 50% decrease in volume. The actual loss is even worse considering the pulp chamber.
# 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
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).
- According to a study by Edelhoff and Ahlers (2018), approximately 70% of the volume of the clinical crown of a posterior tooth is removed for a full-crown preparation, whereas only about 30% is lost for an occlusal onlay.
> [!warning] Clinical Best Practices
> - Excessive removal brings the preparation closer to the pulp, increasing the risk of thermal, chemical, or bacterial damage leading to pulpal complications.
> - **Always use sufficient water spray** with a high-speed handpiece to prevent thermal damage.
> - Consider more conservative options when possible, such as partial coverage restorations (onlays, 3/4 crowns) or materials requiring less reduction (e.g., metal crowns).
# 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
**Patient can’t clean this!**
# Principles of Tooth Preparations
## ABUTMENT TOOTH
### BIOLOGICAL
- Principles of tooth preparation
- Partial or complete preparation
- 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
**Patient can’t clean this!**
### **Avoidance of overcontouring**
- **Patient can’t clean this!**
> [!info] Emergence Profile
> The **emergence profile** (the contour of the tooth or crown as it emerges from the gingiva) must be correct.
> - **Over-contoured crowns**, often a result of under-preparation, create a ledge that traps plaque and leads to gingival inflammation.
> - **Under-contoured crowns** can lead to food impaction and direct trauma to the gingiva.
# Principles of Tooth Preparations
## ABUTMENT TOOTH
### 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*
### **Supragingival margins**
![]()
Series of dental images showing sub-gingival tooth preparations highlighting aesthetic outcomes and potential complications.
# Principles of Tooth Preparations
We risk violation of **biologic width**! ← KNOW THIS WORD!
Slide by Dr Matsubara
![]()
Clinical image highlighting the gingival margin and interdental papilla, with a red arrow and circle indicating the area of concern related to biologic width.
We risk violation of **biologic width**! ← KNOW THIS WORD!
> [!danger] Biologic Width Violation
> Placing a restoration margin within the ~2 mm space of the biologic width causes chronic inflammation, attachment loss, pocket formation, and bone resorption as the body tries to re-establish the space.
> [!question] Assessment (Bone Sounding)
> Under local anesthetic, a periodontal probe is pushed through the tissue to the bone crest. Biologic width = (Probe-to-bone measurement) - (Sulcus depth). A value less than 2 mm indicates a potential problem.
*Slide by Dr Matsubara*
# 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*
#### **Biologic Width**
- **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*
**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
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).
Slide by Dr Matsubara
![]()
Diagram showing three cross-sections of tooth and gum tissue labeled "Normal Crest 3 mm (4 approx)", "High Crest < 3 mm (4 approx)", and "Low Crest > 3 mm (4 approx)" with a dental instrument approaching the tissue in each.
# 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
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).
<table>
<thead>
<tr>
<th>Condition</th>
<th>Measurement</th>
</tr>
</thead>
<tbody>
<tr>
<td>Normal Crest</td>
<td>3 mm (4 approx)</td>
</tr>
<tr>
<td>High Crest</td>
<td>&lt; 3 mm (4 approx)</td>
</tr>
<tr>
<td>Low Crest</td>
<td>&gt; 3 mm (4 approx)</td>
</tr>
</tbody>
</table>
![]()
Diagram showing four cross-sections of tooth and gum structures labeled Normal Crest, High Crest, and Low 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
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).
| Normal Crest | High Crest | Low Crest |
|--------------|------------|---------|
| 3 mm (4 approx) | < 3 mm (4 approx) | > 3 mm (4 approx) |
# 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
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).
![]()
Diagram showing three cross-sections of tooth and gum tissue labeled "Normal Crest 3 mm (4 approx)", "High Crest < 3 mm (4 approx)", and "Low Crest > 3 mm (4 approx)"
# 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
# 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
Slide by Dr Matsubara
- **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.
> [!warning] Marginal Integrity
> Margins must be smooth and continuous. Rough or open margins lead to plaque accumulation, cement washout, secondary caries, and restoration failure.
- 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.
- **Crestal Bone Height:**
<center>
| Condition | Measurement |
| :--- | :--- |
| Normal Crest | 3 mm (4 approx) |
| High Crest | < 3 mm (4 approx) |
| Low Crest | > 3 mm (4 approx) |
</center>
> [!info] Patient Categories based on Crestal Bone Height
> - **Normal Crest (85% of patients):** Biologic width is stable. A margin 0.5 mm subgingivally is usually well-tolerated if the sulcus is shallow (≤1.5 mm).
> - **Low Crest (13%):** The bone crest is further from the CEJ. These patients are prone to gingival recession if the attachment is damaged during procedures.
> - **High Crest (2%):** The bone crest is very close to the CEJ. These patients are at high risk for biologic width violation and chronic inflammation if an intracrevicular margin is placed.
- **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).*
# 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
# 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 porcelin
- Porcelain occlusal surfaces
- Subgingival margins
Slide by Dr Matsubara
### **Harmonious occlusion**
> [!note]
> The final restoration must not create occlusal interferences. Occlusal disharmony can cause pain, temperature sensitivity, tooth mobility, and TMD symptoms.
*Slide by Dr Matsubara*
# 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
### **Protection against tooth fracture**
*Slide by Dr Matsubara*
# Principles of Tooth Preparations
**Cuspal Protection**: Coverage of one or more cusps by a indirect or direct restorative material.
Slide by Dr Nedelcu
![]()
Fracture/Uncontrollable tooth structure loss and Redistribution of stresses diagram
- **Cuspal Protection**: Coverage of one or more cusps by a indirect or direct restorative material.
> [!info]
> This binds the tooth together and redirects occlusal forces along the long axis of the tooth, which it is designed to withstand. This prevents the wedging forces that can fracture weakened cusps.
*Slide by Dr Nedelcu*
During preparation:
- Removal of tooth structure less than 2mm thick
- Removal of any low quality/undermined enamel on the cusps
Also avoid occlusal contacts on the margin of the preparations
Slide by Dr Nedelcu
- **During preparation:**
- Removal of tooth structure less than 2mm thick
- Removal of any low quality/undermined enamel on the cusps
- Also avoid occlusal contacts on the margin of the preparations
*Slide by Dr Nedelcu*
# Principles of Tooth Preparations
## ABUTMENT TOOTH
### BIOLOGICAL
- Principles of tooth preparation
- Partial or complete preparation
- 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*
## **Mechanical Principles**
*Slide by Dr Matsubara*
**Principles of Tooth Preparations**
**Retention**: prevents removal of restoration/crown 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.
![]()
Diagram illustrating "Good Retention But Bad Resistance" and "Bad Retention and Bad Resistance" with labeled tooth and crown structures, arrows indicating forces, and a speaker icon.
**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.
![]()
Diagram comparing Bad Resistance Form (excessive rounding of preparation) and Good Resistance Form (angular preparation) for a tooth and crown, illustrating how excessive rounding can decrease resistance form.
- **Retention**
> [!note]
> Full crowns offer significantly higher retention compared to other restoration types.
: 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.
# Principles of Tooth Preparations
## Factors influencing Retention and Resistance:
**Taper and Length of axial walls**
## Fundamental of Fixed Prosthodontics
**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-Moaleem, M. Shariff, +2 authors, S. Tikare • Published 1 September 2015 • Medicine • Saudi Journal of Medicine and Medical Sciences
![]()
Diagram illustrating Taper and Convergence angle on a tooth preparation.
![]()
Graph titled "Fig 9-4: As taper increases, retention decreases. (Modified from Jorgensen* with permission.)" showing a downward curve plotting Retention (g/mm²) against Taper (degrees).
### **Factors influencing Retention and Resistance**
#### **Taper and Length of axial walls**
> [!important]
> These are the most critical factors that the clinician controls.
- *Fundamental of Fixed Prosthodontics*
- *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-Moaleem, M. Shariff, +2 authors, S. Tikare • Published 1 September 2015 • Medicine • Saudi Journal of Medicine and Medical Sciences*
# Principles of Tooth Preparations
**convergence angle** \kon-vûr’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
**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
**Principles of Tooth Preparations**
*Slide by Dr Nedelcu*
---
**convergence angle** \kon-vûr’jens ăng’gəl\:
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
---
![]()
Google search result for "ideal taper preparation" showing recommended taper is 3-12 degrees, with Schillenberg recommending 6 degrees and minimum taper of -12 degrees for undercut avoidance.
#### **Terminology: Convergence Angle vs. Taper**
- **convergence angle** \kon-vûr’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
- **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
> [!info] Key Terminology
> It is crucial to distinguish between these terms.
> - **Taper:** The angle of a *single* axial wall relative to the long axis of the tooth preparation.
> - **Total Occlusal Convergence (TOC) / Convergence Angle:** The angle formed between *two opposing* axial walls. **(TOC = 2 x Taper)**.
> - The ideal TOC is between **6-12 degrees**.
*Slide by Dr Nedelcu*
**Google**
is **not** your best friend!
Slide by Dr Nedelcu
**Google** is **not** your best friend!
*Slide by Dr Nedelcu*
# Principles of Tooth Preparations
**Factors influencing Retention and Resistance:**
**Taper and Length of axial walls**
**Fundamental of Fixed Prosthodontics**
**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-Moaleem, M. Shariff, +2 authors, S. Tikare • Published 1 September 2015 • Medicine • Saudi Journal of Medicine and Medical Sciences
![]()
Diagram illustrating Convergence angle and Taper on a tooth preparation.
![]()
Graph labeled "Fig 9-4 As taper increases, retention decreases. (Modified from Jorgensen* with permission.)" showing a downward curve with "Retention (g/mm²)" on the y-axis and "Taper (degrees)" on the x-axis.
**Principles of Tooth Preparations**
THE UNIVERSITY OF WESTERN AUSTRALIA
Oral Health Centre of Western Australia
---
**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*
#### **Taper and Undercuts**
> [!note]
> Parallel walls (0° convergence) would provide maximum retention. However, this is clinically impossible to achieve without creating **undercuts**. An undercut is any irregularity or convergence of walls that prevents the seating of the restoration. A slight divergence (taper) is necessary to ensure a path of insertion.
- **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*
# 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).
*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).
*Fundamental of Fixed Prosthodontics*
![]()
Figure 9-9 and Figure 9-10 illustrating tooth preparation with longer vs. shorter walls and effect on resistance to tipping and pivoting movements.
#### **Length and Diameter**
- **Fig 9-9** The preparation with longer walls (a) interferes with the tipping displacement of the restoration better than the short preparation (b).
- **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).
*Fundamental of Fixed Prosthodontics*
# 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*
#### **Grooves and Boxes**
- What can we do to increase retention/resistance?
- Grooves and boxes
- **BUT USE WITH CARE!**
> [!warning] Caution
> These are difficult to prepare without creating undercuts and are more commonly used for onlays/overlays. They are not generally recommended for crowns unless absolutely necessary.
*Fundamental of Fixed Prosthodontics*
> [!tip] Clinical Rule
> The shorter the axial walls, the more parallel they must be (i.e., a smaller convergence angle is required) to achieve adequate retention and resistance.
# 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
# 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
### **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
> [!info]
> Roughening the internal surface of a metal or zirconia crown (e.g., by sandblasting) increases surface area and mechanical retention.
d. Material being cemented
e. Properties of luting agent
> [!info]
> Resin cements, which bond to both tooth and restoration, provide significantly more retention than traditional luting cements that rely on friction.
# 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
### **Deformation**
> [!note]
> The restoration itself must have sufficient bulk and thickness to withstand masticatory forces without flexing or fracturing. This is why specific **reduction depths** are required for different materials. For example, functional cusps (palatal on maxillary, buccal on mandibular) require an additional **functional cusp bevel** to provide extra space for material in this high-stress area.
*Slide by Dr Matsubara*
# Principles of Tooth Preparations
## ABUTMENT TOOTH
### 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
# 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
# **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
# Principles of Tooth Preparations
Any questions? Please email me,
or come up to me in CSSL.
# **Principles of Tooth Preparations**
Any questions? Please email me,
or come up to me in CSSL.
[[F5 Principles of Tooth Prep.pdf#page=1|F5 Principles of Tooth Prep, p.1]]
[[F5 Principles of Tooth Prep.pdf#page=2|F5 Principles of Tooth Prep, p.2]]
[[F5 Principles of Tooth Prep.pdf#page=3|F5 Principles of Tooth Prep, p.3]]
[[F5 Principles of Tooth Prep.pdf#page=4|F5 Principles of Tooth Prep, p.4]]
[[F5 Principles of Tooth Prep.pdf#page=5|F5 Principles of Tooth Prep, p.5]]
[[F5 Principles of Tooth Prep.pdf#page=6|F5 Principles of Tooth Prep, p.6]]
[[F5 Principles of Tooth Prep.pdf#page=7|F5 Principles of Tooth Prep, p.7]]
[[F5 Principles of Tooth Prep.pdf#page=8|F5 Principles of Tooth Prep, p.8]]
[[F5 Principles of Tooth Prep.pdf#page=9|F5 Principles of Tooth Prep, p.9]]
[[F5 Principles of Tooth Prep.pdf#page=10|F5 Principles of Tooth Prep, p.10]]
[[F5 Principles of Tooth Prep.pdf#page=11|F5 Principles of Tooth Prep, p.11]]
[[F5 Principles of Tooth Prep.pdf#page=12|F5 Principles of Tooth Prep, p.12]]
[[F5 Principles of Tooth Prep.pdf#page=13|F5 Principles of Tooth Prep, p.13]]
[[F5 Principles of Tooth Prep.pdf#page=14|F5 Principles of Tooth Prep, p.14]]
[[F5 Principles of Tooth Prep.pdf#page=15|F5 Principles of Tooth Prep, p.15]]
[[F5 Principles of Tooth Prep.pdf#page=16|F5 Principles of Tooth Prep, p.16]]
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