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Conservative Dentistry

Resistance Form in Dental Restorations

Resistance Form

A. Design Features

  1. Flat Pulpal and Gingival Floors:

    • Flat surfaces provide stability and help distribute occlusal forces evenly across the restoration, reducing the risk of displacement.

  2. Box-Shaped Cavity:

    • A box-shaped preparation enhances resistance by providing a larger surface area for bonding and mechanical retention.

  3. Inclusion of Weakened Tooth Structure:

    • Including weakened areas in the preparation helps to prevent fracture under masticatory forces by redistributing stress.

  4. Rounded Internal Line Angles:

    • Rounding internal line angles reduces stress concentration points, which can lead to failure of the restoration.

  5. Adequate Thickness of Restorative Material:

    • Sufficient thickness is necessary to ensure that the restoration can withstand occlusal forces without fracturing. The required thickness varies depending on the type of restorative material used.

  6. Cusp Reduction for Capping:

    • When indicated, reducing cusps helps to provide adequate support for the restoration and prevents fracture.

B. Deepening of Pulpal Floor

  • Increased Bulk: Deepening the pulpal floor increases the bulk of the restoration, enhancing its resistance to occlusal forces.

2. Features of Resistance Form

A. Box-Shaped Preparation

  • A box-shaped cavity preparation is essential for providing resistance against displacement and fracture.

B. Flat Pulpal and Gingival Floors

  • These features help the tooth resist occlusal masticatory forces without displacement.

C. Adequate Thickness of Restorative Material

  • The thickness of the restorative material should be sufficient to prevent fracture of both the remaining tooth structure and the restoration. For example:
    • High Copper Amalgam: Minimum thickness of 1.5 mm.
    • Cast Metal: Minimum thickness of 1.0 mm.
    • Porcelain: Minimum thickness of 2.0 mm.
    • Composite and Glass Ionomer: Typically require thicknesses greater than 2.5 mm due to their wear potential.

D. Restriction of External Wall Extensions

  • Limiting the extensions of external walls helps maintain strong marginal ridge areas with adequate dentin support.

E. Rounding of Internal Line Angles

  • This feature reduces stress concentration points, enhancing the overall resistance form.

F. Consideration for Cusp Capping

  • Depending on the amount of remaining tooth structure, cusp capping may be necessary to provide adequate support for the restoration.

3. Factors Affecting Resistance Form

A. Amount of Occlusal Stresses

  • The greater the occlusal forces, the more robust the resistance form must be to prevent failure.

B. Type of Restoration Used

  • Different materials have varying requirements for thickness and design to ensure adequate resistance.

C. Amount of Remaining Tooth Structure

  • The more remaining tooth structure, the better the support for the restoration, which can enhance resistance form.

Various dyes have been tried to detect carious enamel, each having some Advantages and Disadvantages:

‘Procion’ dyes stain enamel lesions but the staining becomes irreversible because the dye reacts with nitrogen and hydroxyl groups of enamel and acts as a fixative.

‘Calcein’ dye makes a complex with calcium and remains bound to the lesion.

‘Fluorescent dye’ like Zyglo ZL-22 has been used in vitro which is not suitable in vivo. The dye is made visible by ultraviolet illumination.

‘Brilliant blue’ has also been used to enhance the diagnostic quality of fiberoptic transillumination.

Implications for Dental Practice

A. Health and Safety Considerations

  • Mercury Exposure: Understanding the amounts of mercury released during these procedures is crucial for assessing potential health risks to dental professionals and patients.
  • Regulatory Guidelines: Dental practices should adhere to guidelines and regulations regarding mercury handling and exposure limits to ensure a safe working environment.

B. Best Practices

  • Use of Wet Polishing: Whenever possible, wet polishing should be preferred over dry polishing to minimize mercury release.
  • Proper Ventilation: Ensuring adequate ventilation in the dental operatory can help reduce the concentration of mercury vapor in the air.
  • Personal Protective Equipment (PPE): Dental professionals should use appropriate PPE, such as masks and gloves, to minimize exposure during amalgam handling.

C. Patient Safety

  • Informed Consent: Patients should be informed about the materials used in their restorations, including the presence of mercury in amalgam, and the associated risks.
  • Monitoring: Regular monitoring of dental practices for mercury exposure levels can help maintain a safe environment for both staff and patients.

 

 

1. Noise Levels of Turbine Handpieces

Turbine Handpieces

  • Ball Bearings: Turbine handpieces equipped with ball bearings can operate efficiently at air pressures of around 30 pounds.
  • Noise Levels: At high frequencies, these handpieces may produce noise levels ranging from 70 to 94 dB.
  • Hearing Damage Risk: Exposure to noise levels exceeding 75 dB, particularly in the frequency range of 1000 to 8000 cycles per second (cps), can pose a risk of hearing damage for dental professionals.

Implications for Practice

  • Hearing Protection: Dental professionals should consider using hearing protection, especially during prolonged use of high-speed handpieces, to mitigate the risk of noise-induced hearing loss.
  • Workplace Safety: Implementing noise-reduction strategies in the dental operatory can enhance the comfort and safety of both staff and patients.

2. Post-Carve Burnishing

Technique

  • Post-Carve Burnishing: This technique involves lightly rubbing the carved surface of an amalgam restoration with a burnisher of suitable size and shape.
  • Purpose: The goal is to improve the smoothness of the restoration and produce a satin finish rather than a shiny appearance.

Benefits

  • Enhanced Aesthetics: A satin finish can improve the aesthetic integration of the restoration with the surrounding tooth structure.
  • Surface Integrity: Burnishing can help to compact the surface of the amalgam, potentially enhancing its resistance to wear and marginal integrity.

3. Preparing Mandibular First Premolars for MOD Amalgam Restorations

Considerations for Tooth Preparation

  • Conservation of Tooth Structure: When preparing a mesio-occluso-distal (MOD) amalgam restoration for a mandibular first premolar, it is important to conserve the support of the small lingual cusp.
    • Occlusal Step Preparation: The occlusal step should be prepared more facially than lingually, which helps to maintain the integrity of the lingual cusp.
  • Bur Positioning: The bur should be tilted slightly lingually to establish the correct direction for the pulpal wall.

Cusp Reduction

  • Lingual Cusp Consideration: If the lingual margin of the occlusal step extends more than two-thirds the distance from the central fissure to the cuspal eminence, the lingual cusp may need to be reduced to ensure proper occlusal function and stability of the restoration.

4. Universal Matrix System

Overview

  • Tofflemire Matrix System: Designed by B.R. Tofflemire, the Universal matrix system is a commonly used tool in restorative dentistry.
  • Indications: This system is ideally indicated when three surfaces (mesial, occlusal, distal) of a posterior tooth have been prepared for restoration.

Benefits

  • Retention and Contour: The matrix system helps in achieving proper contour and retention of the restorative material, ensuring a well-adapted restoration.
  • Ease of Use: The design allows for easy placement and adjustment, facilitating efficient restorative procedures.

5. Angle Former Excavator

Functionality

  • Angle Former: A special type of excavator used primarily for sharpening line angles and creating retentive features in dentin, particularly in preparations for gold restorations.
  • Beveling Enamel Margins: The angle former can also be used to place a bevel on enamel margins, enhancing the retention of restorative materials.

Clinical Applications

  • Preparation for Gold Restorations: The angle former is particularly useful in preparations where precise line angles and retention are critical for the success of gold restorations.
  • Versatility: Its ability to create retentive features makes it a valuable tool in various restorative procedures.

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.

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

the convenience or point angles and where the line of force will not permit use of other methods. Powdered golds are also

known to be better condensed with hand pressure. Small condenser points of 0.5 mm in diameter are generally

recommended as they do not require very high forces for their manipulation.

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

over the surface, while the assistant provides rhythmic blows from the mallet. Long handled condensers and leather faced

mallets (50 gms in weight) are used for this purpose. The technique allows greater control and the condensers can be

changed rapidly when required. However, with the introduction of mechanical malleting, use of this method has decreased

considerably.

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

spring is released. (Disadvantage is that the blow descends very rapidly even before full pressure has been exerted on the

condenser point.

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

mallet in the handle itself which remains dormant until wished by the operator to function. The intensity or amplitude

generated can vary from 0.2 ounces to 15 pounds and the frequency can range from 360-3600 cycles/minute.

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

Dr. George M. Hollenback. Pneumatic mallets consist of vibrating nit condensers and detachable tips run by

compressed air. The air is carried through a thin rubber tubing attached to the hand piece. Controlling the air

pressure by a rheostat nit allows adjusting the frequency and amplitude of condensation strokes. The construction

of the handpiece is such that the blow does not fall until pressure is placed on the condenser point. This continues

until released. Pneumatic mallets are available with both straight and angled for handpieces.

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.

Fillers in Conservative Dentistry

Fillers play a crucial role in the formulation of composite resins used in conservative dentistry. They are inorganic materials added to the organic matrix to enhance the physical and mechanical properties of the composite. The size and type of fillers significantly influence the performance of the composite material.

1. Types of Fillers Based on Particle Size

Fillers can be categorized based on their particle size, which affects their properties and applications:

  • Macrofillers: 10 - 100 µm
  • Midi Fillers: 1 - 10 µm
  • Minifillers: 0.1 - 1 µm
  • Microfillers: 0.01 - 0.1 µm
  • Nanofillers: 0.001 - 0.01 µm

2. Composition of Fillers

The dispersed phase of composite resins is primarily made up of inorganic filler materials. Commonly used fillers include:

  • Silicon Dioxide
  • Boron Silicates
  • Lithium Aluminum Silicates

A. Silanization

  • Filler particles are often silanized to enhance bonding between the hydrophilic filler and the hydrophobic resin matrix. This process improves the overall performance and durability of the composite.

3. Effects of Filler Addition

The incorporation of fillers into composite resins leads to several beneficial effects:

  • Reduces Thermal Expansion Coefficient: Enhances dimensional stability.
  • Reduces Polymerization Shrinkage: Minimizes the risk of gaps between the restoration and tooth structure.
  • Increases Abrasion Resistance: Improves the wear resistance of the restoration.
  • Decreases Water Sorption: Reduces the likelihood of degradation over time.
  • Increases Tensile and Compressive Strengths: Enhances the mechanical properties, making the restoration more durable.
  • Increases Fracture Toughness: Improves the ability of the material to resist crack propagation.
  • Increases Flexural Modulus: Enhances the stiffness of the composite.
  • Provides Radiopacity: Allows for better visualization on radiographs.
  • Improves Handling Properties: Enhances the workability of the composite during application.
  • Increases Translucency: Improves the aesthetic appearance of the restoration.

4. Alternative Fillers

In some composite formulations, quartz is partially replaced with heavy metal particles such as:

  • Zinc
  • Aluminum
  • Barium
  • Strontium
  • Zirconium

A. Calcium Metaphosphate

  • Recently, calcium metaphosphate has been explored as a filler due to its favorable properties.

B. Wear Considerations

  • These alternative fillers are generally less hard than traditional glass fillers, resulting in less wear on opposing teeth.

5. Nanoparticles in Composites

Recent advancements have introduced nanoparticles into composite formulations:

  • Nanoparticles: Typically around 25 nm in size.
  • Nanoaggregates: Approximately 75 nm, made from materials like zirconium/silica or nano-silica particles.

A. Benefits of Nanofillers

  • The smaller size of these filler particles results in improved surface finish and polishability of the restoration, enhancing both aesthetics and performance.
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