Composite Materials- Mechanical Properties and Clinical Considerations

Introduction

Composite materials are essential in modern dentistry, particularly for restorative procedures. Their mechanical properties, aesthetic qualities, and bonding capabilities make them a preferred choice for various applications. This lecture will focus on the importance of the bond between the organic resin matrix and inorganic filler, the evolution of composite materials, and key clinical considerations in their application.

1. Bonding in Composite Materials

Importance of Bonding

For a composite to exhibit good mechanical properties, a strong bond must exist between the organic resin matrix and the inorganic filler. This bond is crucial for:

• Strength: Enhancing the overall strength of the composite.
• Durability: Reducing solubility and water absorption, which can compromise the material over time.

Role of Silane Coupling Agents

• Silane Coupling Agents: These agents are used to coat filler particles, facilitating a chemical bond between the filler and the resin matrix. This interaction significantly improves the mechanical properties of the composite.

2. Evolution of Composite Materials

Microfill Composites

• Introduction: In the late 1970s, microfill composites, also known as "polishable" composites, were introduced.
• Characteristics: These materials replaced the rough surface of conventional composites with a smooth, lustrous surface similar to tooth enamel.
• Composition: Microfill composites contain colloidal silica particles instead of larger filler particles, allowing for better polishability and aesthetic outcomes.

Hybrid Composites

• Structure: Hybrid composites contain a combination of larger filler particles and sub-micronsized microfiller particles.
• Surface Texture: This combination provides a smooth "patina-like" surface texture in the finished restoration, enhancing both aesthetics and mechanical properties.

3. Clinical Considerations

Polymerization Shrinkage and Configuration Factor (C-factor)

• C-factor: The configuration factor is the ratio of bonded surfaces to unbonded surfaces in a tooth preparation. A higher C-factor can lead to increased polymerization shrinkage, which may compromise the restoration.
• Clinical Implications: Understanding the C-factor is essential for minimizing shrinkage effects, particularly in Class II restorations.

Incremental Placement of Composite

• Incremental Technique: For Class II restorations, it is crucial to place and cure the composite incrementally. This approach helps reduce the effects of polymerization shrinkage, especially along the gingival floor.
• Initial Increment: The first small increment should be placed along the gingival floor and extend slightly up the facial and lingual walls to ensure proper adaptation and minimize stress.

4. Curing Techniques

Light-Curing Systems

• Common Systems: The most common light-curing systems include quartz/tungsten/halogen lamps. However, alternatives such as plasma arc curing (PAC) and argon laser curing systems are available.
• Advantages of PAC and Laser Systems: These systems provide high-intensity and rapid polymerization compared to traditional halogen systems, which can be beneficial in clinical settings.

Enamel Beveling

• Beveling Technique: The advantage of an enamel bevel in composite tooth preparation is that it exposes the ends of the enamel rods, allowing for more effective etching compared to only exposing the sides.
• Clinical Application: Proper beveling can enhance the bond strength and overall success of the restoration.

5. Managing Microfractures and Marginal Integrity

Causes of Microfractures

Microfractures in marginal enamel can result from:

• Traumatic contouring or finishing techniques.
• Inadequate etching and bonding.
• High-intensity light-curing, leading to excessive polymerization stresses.

Potential Solutions

To address microfractures, clinicians can consider:

• Re-etching, priming, and bonding the affected area.
• Conservatively removing the fault and re-restoring.
• Using atraumatic finishing techniques, such as light intermittent pressure.
• Employing slow-start polymerization techniques to reduce stress.

Atraumatic Restorative Treatment (ART) is a minimally invasive approach to dental cavity management and restoration. Developed as a response to the limitations of traditional drilling and filling methods, ART aims to preserve as much of the natural tooth structure as possible while effectively managing caries. The technique was pioneered in the mid-1980s by Dr. Frencken in Tanzania as a way to address the high prevalence of dental decay in a setting with limited access to traditional dental equipment and materials. The term "ART" was coined by Dr. McLean to reflect the gentle and non-traumatic nature of the treatment.

ART involves the following steps:

1. Cleaning and Preparation: The tooth is cleaned with a hand instrument to remove plaque and debris.
2. Moisture Control: The tooth is kept moist with a gel or paste to prevent desiccation and maintain the integrity of the tooth structure.
3. Carious Tissue Removal: Soft, decayed tissue is removed manually with hand instruments, without the use of rotary instruments or drills.
4. Restoration: The prepared cavity is restored with an adhesive material, typically glass ionomer cement, which chemically bonds to the tooth structure and releases fluoride to prevent further decay.

Indications for ART include:

- Small to medium-sized cavities in posterior teeth (molars and premolars).
- Decay in the initial stages that has not yet reached the dental pulp.
- Patients who may not tolerate or have access to traditional restorative methods, such as those in remote or underprivileged areas.
- Children or individuals with special needs who may benefit from a less invasive and less time-consuming approach.
- As part of a public health program focused on preventive and minimal intervention dentistry.

Contraindications for ART include:

- Large cavities that extend into the pulp chamber or involve extensive tooth decay.
- Presence of active infection, swelling, abscess, or fistula around the tooth.
- Teeth with poor prognosis or severe damage that require more extensive treatment such as root canal therapy or extraction.
- Inaccessible cavities where hand instruments cannot effectively remove decay or place the restorative material.

The ART technique is advantageous in several ways:

- It reduces the need for local anesthesia, as it is often painless.
- It preserves more of the natural tooth structure.
- It is less technique-sensitive and does not require advanced equipment.
- It is relatively quick and can be performed in a single visit.
- It is suitable for use in areas with limited resources and less developed dental infrastructure.
- It reduces the risk of microleakage and secondary caries.

However, ART also has limitations, such as reduced longevity compared to amalgam or composite fillings, especially in large restorations or high-stress areas, and the need for careful moisture control during the procedure to ensure proper bonding of the material. Additionally, ART is not recommended for all cases and should be considered on an individual basis, taking into account the patient's oral health status and the specific requirements of each tooth.

_{Hand Instruments - Design and Balancing}

_{Hand instruments are essential tools in dentistry, and their design significantly impacts their effectiveness and usability. Proper balancing and angulation of these instruments are crucial for achieving optimal control and precision during dental procedures. Below is an overview of the key aspects of hand instrument design, focusing on the shank, angulation, and balancing.}

_{1. Importance of Balancing}

_{A. Definition of Balance}

• _{Balanced Instruments: A hand instrument is considered balanced when the concentration of force can be applied to the blade without causing rotation in the grasp of the operator. This balance is essential for effective cutting and manipulation of tissues.}

_{B. Achieving Balance}

• _{Proper Angulation of Shank: The shank must be angled appropriately so that the cutting edge of the blade lies within the projected diameter of the handle. This design minimizes the tendency for the instrument to rotate during use.}
• _{Off-Axis Blade Edge: For optimal anti-rotational design, the blade edge should be positioned off-axis by 1 to 2 mm. This slight offset helps maintain balance while allowing effective force application.}

_{2. Shank Design}

_{A. Definition}

• _{Shank: The shank connects the handle to the blade of the instrument. It plays a critical role in the instrument's overall design and functionality.}

_{B. Characteristics}

• _{Tapering: The shank typically tapers from the handle down to the blade, which can enhance control and maneuverability.}
• _{Surface Texture: The shank is usually smooth, round, or tapered, depending on the specific instrument design.}
• _{Angulation: The shank may be straight or angled, allowing for various access and visibility during procedures.}

_{C. Classification Based on Angles}

_{Instruments can be classified based on the number of angles in the shank:}

1. _{Straight: No angle in the shank.}
2. _{Monoangle: One angle in the shank.}
3. _{Binangle: Two angles in the shank.}
4. _{Triple-Angle: Three angles in the shank.}

_{3. Angulation and Control}

_{A. Purpose of Angulation}

• _{Access and Stability: The angulation of the instrument is designed to provide better access to the treatment area while maintaining stability during use.}

_{B. Proximity to Long Axis}

• _{Control: The closer the working point (the blade) is to the long axis of the handle, the better the control over the instrument. Ideally, the working point should be within 3 mm of the center of the long axis of the handle for optimal control.}

_{4. Balancing Examples}

_{A. Balanced Instrument}

• _{Example A: When the working end of the instrument lies within 2-3 mm of the long axis of the handle, it provides effective balancing. This configuration allows the operator to apply force efficiently without losing control.}

_{B. Unbalanced Instrument}

• _{Example B: If the working end is positioned away from the long axis of the handle, it results in an unbalanced instrument. This design can lead to difficulty in controlling the instrument and may compromise the effectiveness of the procedure.}

Radiographic Advancements in Caries Detection

Advancements in dental technology have significantly improved the detection and quantification of dental caries. This lecture will cover several key technologies used in caries detection, including Diagnodent, infrared and red fluorescence, DIFOTI, and QLF, as well as the film speeds used in radiographic imaging.

1. Diagnodent

• Technology:

  • Utilizes infrared laser fluorescence for the detection and quantification of dental caries, particularly effective for occlusal and smooth surface caries.
  • Not as effective for detecting proximal caries.

• Specifications:

  • Operates using red light with a wavelength of 655 nm.
  • Features a fiber optic cable with a handheld probe and a diode laser light source.
  • The device transmits light to the handheld probe and fiber optic tip.

• Measurement:

  • Scores dental caries on a scale of 0-99.
  • Fluorescence is attributed to the presence of porphyrin, a compound produced by bacteria in carious lesions.

• Scoring Criteria:

  • Score 1: <15 - No dental caries; up to half of enamel intact.
  • Score 2: 15-19 - Demineralization extends into the inner half of enamel or upper third of dentin.
  • Score 3: >19 - Extending into the inner portion of dentin.

2. Infrared and Red Fluorescence

• Also Known As: Midwest Caries I.D. detection handpiece.
• Technology:
  • Utilizes two wavelengths:
    • 880 nm - Infrared
    • 660 nm - Red
• Application:
  • Designed for use over all tooth surfaces.
  • Particularly useful for detecting hidden occlusal caries.

3. DIFOTI (Digital Imaging Fiber Optic Transillumination)

• Description:
  • An advancement of the Fiber Optic Transillumination (FOTI) technique.
• Application:
  • Primarily used for the detection of proximal caries.
• Drawback:
  • Difficulty in accurately determining the depth of the lesion.

4. QLF (Quantitative Laser Fluorescence)

• Overview:
  • One of the most extensively investigated techniques for early detection of dental caries, introduced in 1978.
• Effectiveness:
  • Good for detecting occlusal and smooth surface caries.
  • Challenging for detecting interproximal caries.

Film Speed in Radiographic Imaging

• Film Types:
  • Film D: Best film for detecting incipient caries.
  • Film E: Most commonly used film in dentistry for caries detection.
  • Film F: Most recommended film speed for general use.
  • Film C: No longer available.

Film Thickness of Dental Cements

The film thickness of dental cements is an important property that can influence the effectiveness of the material in various dental applications, including luting agents, bases, and liners. .

1. Importance of Film Thickness

A. Clinical Implications

• Sealing Ability: The film thickness of a cement can affect its ability to create a proper seal between the restoration and the tooth structure. Thicker films may lead to gaps and reduced retention.
• Adaptation: A thinner film allows for better adaptation to the irregularities of the tooth surface, which is crucial for minimizing microleakage and ensuring the longevity of the restoration.

B. Material Selection

• Choosing the Right Cement: Understanding the film thickness of different cements helps clinicians select the appropriate material for specific applications, such as luting crowns, bridges, or other restorations.

2. Summary of Film Thickness

• Zinc Phosphate: 20 mm – Known for its strength and durability, often used for cementing crowns and bridges.
• Zinc Oxide Eugenol (ZOE), Type I: 25 mm – Commonly used for temporary restorations and as a base under other materials.
• ZOE + Alumina + EBA (Type II): 25 mm – Offers improved properties for specific applications.
• ZOE + Polymer (Type II): 32 mm – Provides enhanced strength and flexibility.
• Silicophosphate: 25 mm – Used for its aesthetic properties and good adhesion.
• Resin Cement: < 25 mm – Offers excellent bonding and low film thickness, making it ideal for aesthetic restorations.
• Polycarboxylate: 21 mm – Known for its biocompatibility and moderate strength.
• ** Glass Ionomer: 24 mm – Valued for its fluoride release and ability to bond chemically to tooth structure, making it suitable for various restorative applications.

Beveling in Restorative Dentistry

Beveling: Beveling refers to the process of angling the edges of a cavity preparation to create a smooth transition between the tooth structure and the restorative material. This technique can enhance the aesthetics and retention of certain materials.

Characteristics of Ceramic Materials

• Brittleness: Ceramic materials, such as porcelain, are inherently brittle and can be prone to fracture under stress.
• Bonding Mechanism: Ceramics rely on adhesive bonding to tooth structure, which can be compromised by beveling.

Contraindications

• Cavosurface Margins: Beveling the cavosurface margins of ceramic restorations is contraindicated because:
  • It can weaken the bond between the ceramic and the tooth structure.
  • It may create unsupported enamel, increasing the risk of chipping or fracture of the ceramic material.

Beveling with Amalgam Restorations

Amalgam Characteristics

• Strength and Durability: Amalgam is a strong and durable material that can withstand significant occlusal forces.
• Retention Mechanism: Amalgam relies on mechanical retention rather than adhesive bonding.

Beveling Guidelines

• General Contraindications: Beveling is generally contraindicated when using amalgam, as it can reduce the mechanical retention of the restoration.
• Exception for Class II Preparations:
  • Gingival Floor Beveling: In Class II preparations where enamel is still present, a slight bevel (approximately 15 to 20 degrees) may be placed on the gingival floor. This is done to:
    • Remove unsupported enamel rods, which can lead to enamel fracture.
    • Enhance the seal between the amalgam and the tooth structure, improving the longevity of the restoration.

Technique for Beveling

• Preparation: When beveling the gingival floor:
  • Use a fine diamond bur or a round bur to create a smooth, angled surface.
  • Ensure that the bevel is limited to the enamel portion of the wall to maintain the integrity of the underlying dentin.

Clinical Implications

A. Material Selection

• Understanding the properties of the restorative material is essential for determining the appropriate preparation technique.
• Clinicians should be aware of the contraindications for beveling based on the material being used to avoid compromising the restoration's success.

B. Restoration Longevity

• Proper preparation techniques, including appropriate beveling when indicated, can significantly impact the longevity and performance of restorations.
• Regular monitoring of restorations is essential to identify any signs of failure or degradation, particularly in areas where beveling has been performed.