Mercury Exposure and Safety

Concentrations of Mercury in Air

• Typical Levels: Mercury concentrations in air can vary significantly:
  • Pure air: 0.002 µg/m³
  • Urban air: 0.05 µg/m³
  • Air near industrial parks: 3 µg/m³
  • Air in mercury mines: 300 µg/m³
• Threshold Limit Value (TLV): The generally accepted TLV for exposure to mercury vapor for a 40-hour work week is 50 µg/m³. Understanding these levels is crucial for ensuring safety in dental practices where amalgam is used.

Continuous Retention Groove Preparation

Purpose and Technique

• Retention Groove: A continuous retention groove is prepared in the internal portion of the external walls of a cavity preparation to enhance the retention of restorative materials, particularly when maximum retention is anticipated.
• Bur Selection: A No. ¼ round bur is used for this procedure.
• Location and Depth:
  • The groove is located 0.25 mm (half the diameter of the No. ¼ round bur) from the root surface.
  • It is prepared to a depth of 0.25 mm, ensuring that it does not compromise the integrity of the tooth structure.
• Direction: The groove should be directed as the bisector of the angle formed by the junction of the axial wall and the external wall. This orientation maximizes the surface area for bonding and retention.

Clinical Implications

• Enhanced Retention: The continuous groove provides additional mechanical retention, which is particularly beneficial in cases where the cavity preparation is large or when the restorative material has a tendency to dislodge.
• Consideration of Tooth Structure: Care must be taken to avoid excessive removal of tooth structure, which could compromise the tooth's strength.

Resin Modified Glass Ionomer Cements (RMGIs)

Resin Modified Glass Ionomer Cements (RMGIs) represent a significant advancement in dental materials, combining the beneficial properties of both glass ionomer cements and composite resins. This overview will discuss the composition, advantages, and disadvantages of RMGIs, highlighting their role in modern dentistry.

1. Composition of Resin Modified Glass Ionomer Cements

A. Introduction

• First Introduced: RMGIs were first introduced as Vitrebond (3M), utilizing a powder-liquid system designed to enhance the properties of traditional glass ionomer cements.

B. Components

• Powder: The powder component consists of fluorosilicate glass, which provides the material with its glass ionomer properties. It also contains a photoinitiator or chemical initiator to facilitate setting.
• Liquid: The liquid component contains:
  • 15 to 25% Resin Component: Typically in the form of Hydroxyethyl Methacrylate (HEMA), which enhances the material's bonding and aesthetic properties.
  • Polyacrylic Acid Copolymer: This component contributes to the chemical adhesion properties of the cement.
  • Photoinitiator and Water: These components are essential for the setting reaction and workability of the material.

2. Advantages of Resin Modified Glass Ionomer Cements

RMGIs offer a range of benefits that make them suitable for various dental applications:

1. Extended Working Time: RMGIs provide a longer working time compared to traditional glass ionomers, allowing for more flexibility during placement.

2. Control on Setting: The setting reaction can be controlled through light curing, which allows for adjustments before the material hardens.

3. Good Adaptation: RMGIs exhibit excellent adaptation to tooth structure, which helps minimize gaps and improve the seal.

4. Chemical Adhesion to Enamel and Dentin: RMGIs bond chemically to both enamel and dentin, enhancing retention and reducing the risk of microleakage.

5. Fluoride Release: Like traditional glass ionomers, RMGIs release fluoride, which can help in the prevention of secondary caries.

6. Improved Aesthetics: The resin component allows for better color matching and aesthetics compared to conventional glass ionomers.

7. Low Interfacial Shrinkage Stress: RMGIs exhibit lower shrinkage stress upon setting compared to composite resins, reducing the risk of debonding or gap formation.

8. Superior Strength Characteristics: RMGIs generally have improved mechanical properties, making them suitable for a wider range of clinical applications.

3. Disadvantages of Resin Modified Glass Ionomer Cements

Despite their advantages, RMGIs also have some limitations:

1. Shrinkage on Setting: RMGIs can experience some degree of shrinkage during the setting process, which may affect the marginal integrity of the restoration.

2. Limited Depth of Cure: The depth of cure can be limited, especially when using more opaque lining cements. This can affect the effectiveness of the material in deeper cavities.

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.

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.

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.