Cutting Edge Mechanics

Edge Angles and Their Importance

• Edge Angle: The angle formed at the cutting edge of a bur blade. Increasing the edge angle reinforces the cutting edge, which helps to reduce the likelihood of blade fracture during use.
• Reinforcement: A larger edge angle provides more material at the cutting edge, enhancing its strength and durability.

Carbide vs. Steel Burs

• Carbide Burs:
  • Hardness and Wear Resistance: Carbide burs are known for their higher hardness and wear resistance compared to steel burs. This makes them suitable for cutting through hard dental tissues.
  • Brittleness: However, carbide burs are more brittle than steel burs, which means they are more prone to fracture if not designed properly.
  • Edge Angles: To minimize the risk of fractures, carbide burs require greater edge angles. This design consideration is crucial for maintaining the integrity of the bur during clinical procedures.

Interdependence of Angles

• Three Angles: The cutting edge of a bur is defined by three angles: the edge angle, the clearance angle, and the rake angle. These angles cannot be varied independently of each other.
  • Clearance Angle: An increase in the clearance angle (the angle between the cutting edge and the surface being cut) results in a decrease in the edge angle. This relationship is important for optimizing cutting efficiency and minimizing wear on the bur.

Beveled Conventional Preparation

Characteristics

• External Walls: In a beveled conventional preparation, the external walls are perpendicular to the enamel surface.
• Beveled Margin: The enamel margin is beveled, which helps to create a smooth transition between the restoration and the tooth structure.

Benefits

• Improved Aesthetics: The beveling technique enhances the aesthetics of the restoration by minimizing the visibility of the margin.
• Strength and Bonding: Beveling can improve the bonding surface area and reduce the risk of marginal leakage, which is critical for the longevity of the restoration.

Onlay Preparation

Onlay preparations are a type of indirect restoration used to restore teeth that have significant loss of structure but still retain enough healthy tooth structure to support a restoration. Onlays are designed to cover one or more cusps of a tooth and are often used when a full crown is not necessary.

1. Definition of Onlay

A. Onlay

• An onlay is a restoration that is fabricated using an indirect procedure, covering one or more cusps of a tooth. It is designed to restore the tooth's function and aesthetics while preserving as much healthy tooth structure as possible.

2. Indications for Onlay Preparation

• Extensive Caries: When a tooth has significant decay that cannot be effectively treated with a filling but does not require a full crown.
• Fractured Teeth: For teeth that have fractured cusps or significant structural loss.
• Strengthening: To reinforce a tooth that has been weakened by previous restorations or caries.

3. Onlay Preparation Procedure

A. Initial Assessment

• Clinical Examination: Assess the extent of caries or damage to determine if an onlay is appropriate.
• Radiographic Evaluation: Use X-rays to evaluate the tooth structure and surrounding tissues.

B. Tooth Preparation

1. Burs Used:

   • Commonly used burs include No. 169 L for initial cavity preparation and No. 271 for refining the preparation.

2. Cavity Preparation:

   • Occlusal Entry: The initial occlusal entry should be approximately 1.5 mm deep.
   • Divergence of Walls: All cavity walls should diverge occlusally by 2-5 degrees:
     • 2 degrees: For short vertical walls.
     • 5 degrees: For long vertical walls.

3. Proximal Box Preparation:

   • The proximal box margins should clear adjacent teeth by 0.2-0.5 mm, with 0.5 ± 0.2 mm being ideal.

C. Bevels and Flares

1. Facial and Lingual Flares:

   • Primary and secondary flares should be created on the facial and lingual proximal walls to form the walls in two planes.
   • The secondary flare widens the proximal box, allowing for better access and cleaning.

2. Gingival Bevels:

   • Should be 0.5-1 mm wide and blend with the secondary flare, resulting in a marginal metal angle of 30 degrees.

3. Occlusal Bevels:

   • Present on the cavosurface margins of the cavity on the occlusal surface, approximately 1/4th the depth of the respective wall, resulting in a marginal metal angle of 40 degrees.

4. Dimensions for Onlay Preparation

A. Depth of Preparation

• Occlusal Depth: Approximately 1.5 mm to ensure adequate thickness of the restorative material.
• Proximal Box Depth: Should be sufficient to accommodate the onlay while maintaining the integrity of the tooth structure.

B. Marginal Angles

• Facial and Lingual Margins: Should be prepared with a 30-degree angle for burnishability and strength.
• Enamel Margins: Ideally, the enamel margins should be blunted to a 140-degree angle to enhance strength.

C. Cusp Reduction

• Cusp Coverage: Cusp reduction is indicated when more than 1/2 of a cusp is involved, and mandatory when 2/3 or more is involved.
• Uniform Metal Thickness: The reduction must provide for a uniform metal thickness of approximately 1.5 mm over the reduced cusps.
• Facial Cusp Reduction: For maxillary premolars and first molars, the reduction of the facial cusp should be 0.75-1 mm for esthetic reasons.

D. Reverse Bevel

• Definition: A bevel on the margins of the reduced cusp, extending beyond any occlusal contact with opposing teeth, resulting in a marginal metal angle of 30 degrees.

5. Considerations for Onlay Preparation

• Retention and Resistance: The preparation should be designed to maximize retention and resistance form, which may include the use of proximal retentive grooves and collar features.
• Aesthetic Considerations: The preparation should account for the esthetic requirements, especially in anterior teeth or visible areas.
• Material Selection: The choice of material (e.g., gold, porcelain, composite) will influence the preparation design and dimensions.

Triple Antibiotic Paste is an intracanal medicament used primarily in regenerative endodontics and for disinfecting necrotic root canals.

 Composition:

• Ciprofloxacin – targets Gram-negative bacteria

• Metronidazole – effective against anaerobes

• Minocycline – broad-spectrum, but may cause tooth discoloration

Uses:

• Disinfection in immature teeth with necrotic pulps

• Regenerative procedures like revascularization

• Lesion sterilization and tissue repair (LSTR)

Limitations:

• Tooth discoloration (especially from minocycline)

• Potential cytotoxicity to stem cells

• Alternatives include double antibiotic paste or modified TAP (replacing minocycline with clindamycin or cefaclor)

Condensers/pluggers are instruments used to deliver the forces of compaction to the underlying restorative material. There are

several methods for the application of these forces:

1. Hand pressure: use of this method alone is contraindicated except in a few situations like adapting the first piece of gold to

2. Hand malleting: Condensation by hand malleting is a team work in which the operator directs the condenser and moves it

3. Automatic hand malleting: This method utilizes a spring loaded instrument that delivers the desired force once the spiral

4. Electric malleting (McShirley electromallet): This instrument accommodates various shapes of con-denser points and has a

5. Pneumatic malleting (Hollenback condenser): This is the most recent and satisfactory method first developed by