Dental Burs: Design, Function, and Performance

Dental burs are essential tools in operative dentistry, used for cutting, shaping, and finishing tooth structure and restorative materials. This guide will cover the key features of dental burs, including blade design, rake angle, clearance angle, run-out, and performance characteristics.

1. Blade Design and Flutes

A. Blade Configuration

• Blades and Flutes: Blades on a bur are uniformly spaced, with depressed areas between them known as flutes. The design of the blades and flutes affects the cutting efficiency and smoothness of the bur's action.
• Number of Blades:
  • The number of blades on a bur is always even.
  • Excavating Burs: Typically have 6-10 blades, designed for efficient material removal.
  • Finishing Burs: Have 12-40 blades, providing a smoother finish.

B. Cutting Efficiency

• Smoother Cutting Action: A greater number of blades results in a smoother cutting action at low speeds.
• Reduced Efficiency: As the number of blades increases, the space between subsequent blades decreases, leading to less surface area being cut and reduced efficiency.

2. Vibration Characteristics

A. Vibration and Patient Comfort

• Vibration Frequency: Vibrations over 1,300 cycles per second are generally imperceptible to patients.
• Effect of Blade Number: Fewer blades on a bur tend to produce greater vibrations, which can affect patient comfort.
• RPM and Vibration: Higher RPMs produce less amplitude and greater frequency of vibration, contributing to a smoother experience for the patient.

3. Rake Angle

A. Definition

• Rake Angle: The angle that the face of the blade makes with a radial line from the center of the bur to the blade.

B. Cutting Efficiency

• Positive Rake Angle: Burs with a positive rake angle are generally desired for cutting efficiency.
• Rake Angle Hierarchy: The cutting efficiency is ranked as follows:
  • Positive rake > Radial rake > Negative rake
• Clogging: Burs with a positive rake angle may experience clogging due to debris accumulation.

4. Clearance Angle

A. Definition

• Clearance Angle: This angle provides clearance between the working edge and the cutting edge of the bur, allowing for effective cutting without binding.

5. Run-Out

A. Definition

• Run-Out: Refers to the eccentricity or maximum displacement of the bur head from its axis of rotation.
• Acceptable Value: The average value of clinically acceptable run-out is about 0.023 mm. Excessive run-out can lead to uneven cutting and discomfort for the patient.

6. Load Characteristics

A. Load Applied by Dentist

• Low Speed: The minimum and maximum load applied through the bur is typically between 100 – 1500 grams.
• High Speed: For high-speed burs, the load is generally between 60 – 120 grams.

7. Diamond Stones

A. Abrasive Efficiency

• Diamond Stones: These are the hardest and most efficient abrasive stones available for removing tooth enamel. They are particularly effective for cutting and finishing hard dental materials.

Supporting Cusps in Dental Occlusion

Supporting cusps, also known as stamp cusps, centric holding cusps, or holding cusps, play a crucial role in dental occlusion and function. They are essential for effective chewing and maintaining the vertical dimension of the face. This guide will outline the characteristics, functions, and clinical significance of supporting cusps.

Supporting Cusps: These are the cusps of the maxillary and mandibular teeth that make contact during maximum intercuspation (MI) and are primarily responsible for supporting the vertical dimension of the face and facilitating effective chewing.

Location

• Maxillary Supporting Cusps: Located on the lingual occlusal line of the maxillary teeth.
• Mandibular Supporting Cusps: Located on the facial occlusal line of the mandibular teeth.

Functions of Supporting Cusps

A. Chewing Efficiency

• Mortar and Pestle Action: Supporting cusps contact the opposing teeth in their corresponding faciolingual center on a marginal ridge or a fossa, allowing them to cut, crush, and grind fibrous food effectively.
• Food Reduction: The natural tooth form, with its multiple ridges and grooves, aids in the reduction of the food bolus during chewing.

B. Stability and Alignment

• Preventing Drifting: Supporting cusps help prevent the drifting and passive eruption of teeth, maintaining proper occlusal relationships.

Characteristics of Supporting Cusps

Supporting cusps can be identified by the following five characteristic features:

1. Contact in Maximum Intercuspation (MI): They make contact with the opposing tooth during MI, providing stability in occlusion.

2. Support for Vertical Dimension: They contribute to maintaining the vertical dimension of the face, which is essential for proper facial aesthetics and function.

3. Proximity to Faciolingual Center: Supporting cusps are located nearer to the faciolingual center of the tooth compared to nonsupporting cusps, enhancing their functional role.

4. Potential for Contact on Outer Incline: The outer incline of supporting cusps has the potential for contact with opposing teeth, facilitating effective occlusion.

5. Broader, Rounded Cusp Ridges: Supporting cusps have broader and more rounded cusp ridges than nonsupporting cusps, making them better suited for crushing food.

Clinical Significance

A. Occlusal Relationships

• Maxillary vs. Mandibular Arch: The maxillary arch is larger than the mandibular arch, resulting in the supporting cusps of the maxilla being more robust and better suited for crushing food than those of the mandible.

B. Lingual Tilt of Posterior Teeth

• Height of Supporting Cusps: The lingual tilt of the posterior teeth increases the relative height of the supporting cusps compared to nonsupporting cusps, which can obscure central fossa contacts.

C. Restoration Considerations

• Restoration Fabrication: During the fabrication of restorations, it is crucial to ensure that supporting cusps do not contact opposing teeth in a manner that results in lateral deflection. Instead, restorations should provide contacts on plateaus or smoothly concave fossae to direct masticatory forces parallel to the long axes of the teeth.

Bases in Restorative Dentistry

Bases are an essential component in restorative dentistry, serving as a thicker layer of material placed beneath restorations to provide additional protection and support to the dental pulp and surrounding structures. Below is an overview of the characteristics, objectives, and types of bases used in dental practice.

1. Characteristics of Bases

A. Thickness

• Typical Thickness: Bases are generally thicker than liners, typically ranging from 1 to 2 mm. Some bases may be around 0.5 to 0.75 mm thick.

B. Functions

• Thermal Protection: Bases provide thermal insulation to protect the pulp from temperature changes that can occur during and after the placement of restorations.
• Mechanical Support: They offer supplemental mechanical support for the restoration by distributing stress on the underlying dentin surface. This is particularly important during procedures such as amalgam condensation, where forces can be applied to the restoration.

2. Objectives of Using Bases

The choice of base material and its application depend on the Remaining Dentin Thickness (RDT), which is a critical factor in determining the need for a base:

• RDT > 2 mm: No base is required, as there is sufficient dentin to protect the pulp.
• RDT 0.5 - 2 mm: A base is indicated, and the choice of material depends on the restorative material being used.
• RDT < 0.5 mm: Calcium hydroxide (Ca(OH)₂) or Mineral Trioxide Aggregate (MTA) should be used to promote the formation of reparative dentin, as the remaining dentin is insufficient to provide adequate protection.

3. Types of Bases

A. Common Base Materials

• Zinc Phosphate (ZnPO₄): Known for its good mechanical properties and thermal insulation.
• Glass Ionomer Cement (GIC): Provides thermal protection and releases fluoride, which can help in preventing caries.
• Zinc Polycarboxylate: Offers good adhesion to tooth structure and provides thermal insulation.

B. Properties

• Mechanical Protection: Bases distribute stress effectively, reducing the risk of fracture in the restoration and protecting the underlying dentin.
• Thermal Insulation: Bases are poor conductors of heat and cold, helping to maintain a stable temperature at the pulp level.

Dental Amalgam and Direct Gold Restorations

In restorative dentistry, understanding the properties of materials and the techniques used for their application is essential for achieving optimal outcomes.  .

1. Mechanical Properties of Amalgam

Compressive and Tensile Strength

• Compressive Strength: Amalgam exhibits high compressive strength, which is essential for withstanding the forces of mastication. The minimum compressive strength of amalgam should be at least 310 MPa.
• Tensile Strength: Amalgam has relatively low tensile strength, typically ranging between 48-70 MPa. This characteristic makes it more susceptible to fracture under tensile forces, which is why proper cavity design and placement techniques are critical.

Implications for Use

• Cavity Design: The design of the cavity preparation should minimize the risk of tensile forces acting on the restoration. This can be achieved through appropriate wall angles and retention features.
• Restoration Longevity: Understanding the mechanical properties of amalgam helps clinicians predict the longevity and performance of the restoration under functional loads.

2. Direct Gold Restorations

Requirements for Direct Gold Restorations

• Ideal Surgical Field: A clean and dry field is essential for the successful placement of direct gold restorations. This ensures that the gold adheres properly and that contamination is minimized.
• Conservative Cavity Preparation: The cavity preparation must be methodical and conservative, preserving as much healthy tooth structure as possible while providing adequate retention for the gold.
• Systematic Condensation: The condensation of gold must be performed carefully to build a solid block of gold within the tooth. This involves using appropriate instruments and techniques to ensure that the gold is well-adapted to the cavity walls.

Condensation Technique

• Building a Solid Block: The goal of the condensation procedure is to create a dense, solid mass of gold that will withstand occlusal forces and provide a durable restoration.

3. Gingival Displacement Techniques

Materials for Displacement

To effectively displace the gingival tissue during restorative procedures, various materials can be used, including:

1. Heavy Weight Rubber Dam: Provides excellent isolation and displacement of gingival tissue.
2. Plain Cotton Thread: A simple and effective method for gingival displacement.
3. Epinephrine-Saturated String:
   • 1:1000 Epinephrine: Used for 10 minutes; not recommended for cardiac patients due to potential systemic effects.
4. Aluminum Chloride Solutions:
   • 5% Aluminum Chloride Solution: Used for gingival displacement.
   • 20% Tannic Acid: Another option for controlling bleeding and displacing tissue.
   • 4% Levo Epinephrine with 9% Potassium Aluminum: Used for 10 minutes.
5. Zinc Chloride or Ferric Sulfate:
   • 8% Zinc Chloride: Used for 3 minutes.
   • Ferric Sub Sulfate: Also used for 3 minutes.

Clinical Considerations

• Selection of Material: The choice of material for gingival displacement should be based on the clinical situation, patient health, and the specific requirements of the procedure.

4. Condensation Technique for Gold

Force Application

• Angle of Condensation: The force of condensation should be applied at a 45-degree angle to the cavity walls and floor during malleting. This orientation allows for maximum adaptation of the gold against the walls, floors, line angles, and point angles of the cavity.
• Direction of Force: The forces must be directed at 90 degrees to any previously condensed gold. This technique ensures that the gold is compacted effectively and that there are no voids or gaps in the restoration.

Importance of Technique

• Adaptation and Density: Proper condensation technique is critical for achieving optimal adaptation and density of the gold restoration, which contributes to its longevity and performance.

Liners

Liners are relatively thin layers of material applied to the cavity preparation to protect the dentin from potential irritants and to provide a barrier against oral fluids and residual reactants from the restoration.

Types of Liners

1. Solution Liners

• Composition: Based on non-aqueous solutions of acetone, alcohol, or ether.
• Example: Varnish (e.g., Copal Wash).
  • Composition:
    • 10% copal resin
    • 90% solvent
• Setting Reaction: Physical evaporation of the solvent, leaving a thin film of copal resin.
• Coverage: A single layer of varnish covers approximately 55% of the surface area. Applying 2-3 layers can increase coverage to 60-80%.

2. Suspension Liners

• Composition: Based on aqueous solvents (water-based).
• Example: Calcium hydroxide (Ca(OH)₂) liner.
• Indications: Used to protect dentinal tubules and provide a barrier against irritants.
• Disadvantage: High solubility in oral fluids, which can limit effectiveness over time.

3. Importance of Liners

A. Smear Layer

• The smear layer, which forms during cavity preparation, can decrease dentin permeability by approximately 86%, providing an additional protective barrier for the pulp.

B. Pulp Medication

• Liners can serve an important function in pulp medication, which helps prevent pulpal inflammation and promotes healing. This is particularly crucial in cases where the cavity preparation is close to the pulp.

Window of Infectivity

The concept of the "window of infectivity" was introduced by Caufield in 1993 to describe critical periods in early childhood when the oral cavity is particularly susceptible to colonization by Streptococcus mutans, a key bacterium associated with dental caries. Understanding these windows is essential for implementing preventive measures against caries in children.

• Window of Infectivity: This term refers to specific time periods during which the acquisition of Streptococcus mutans occurs, leading to an increased risk of dental caries. These windows are characterized by the eruption of teeth, which creates opportunities for bacterial colonization.

First Window of Infectivity

A. Timing

• Age Range: The first window of infectivity is observed between 19 to 23 months of age, coinciding with the eruption of primary teeth.

B. Mechanism

• Eruption of Primary Teeth: As primary teeth erupt, they provide a "virgin habitat" for S. mutans to colonize the oral cavity. This is significant because:
  • Reduced Competition: The newly erupted teeth have not yet been colonized by other indigenous bacteria, allowing S. mutans to establish itself without competition.
  • Increased Risk of Caries: The presence of S. mutans in the oral cavity during this period can lead to an increased risk of developing dental caries, especially if dietary habits include frequent sugar consumption.

Second Window of Infectivity

A. Timing

• Age Range: The second window of infectivity occurs between 6 to 12 years of age, coinciding with the eruption of permanent teeth.

B. Mechanism

• Eruption of Permanent Dentition: As permanent teeth emerge, they again provide opportunities for S. mutans to colonize the oral cavity. This window is characterized by:
  • Increased Susceptibility: The transition from primary to permanent dentition can lead to changes in oral flora and an increased risk of caries if preventive measures are not taken.
  • Behavioral Factors: During this age range, children may have increased exposure to sugary foods and beverages, further enhancing the risk of S. mutans colonization and subsequent caries development.

4. Clinical Implications

A. Preventive Strategies

• Oral Hygiene Education: Parents and caregivers should be educated about the importance of maintaining good oral hygiene practices from an early age, especially during the windows of infectivity.
• Dietary Counseling: Limiting sugary snacks and beverages during these critical periods can help reduce the risk of S. mutans colonization and caries development.
• Regular Dental Visits: Early and regular dental check-ups can help monitor the oral health of children and provide timely interventions if necessary.

B. Targeted Interventions

• Fluoride Treatments: Application of fluoride varnishes or gels during these windows can help strengthen enamel and reduce the risk of caries.
• Sealants: Dental sealants can be applied to newly erupted permanent molars to provide a protective barrier against caries.