Primary Retention Form in Dental Restorations

Primary retention form refers to the geometric shape or design of a prepared cavity that helps resist the displacement or removal of a restoration due to tipping or lifting forces. Understanding the primary retention form is crucial for ensuring the longevity and stability of various types of dental restorations. Below is an overview of primary retention forms for different types of restorations.

1. Amalgam Restorations

A. Class I & II Restorations

• Primary Retention Form:
  • Occlusally Converging External Walls: The walls of the cavity preparation converge towards the occlusal surface, which helps resist displacement.
  • Occlusal Dovetail: In Class II restorations, an occlusal dovetail is often included to enhance retention by providing additional resistance to displacement.

B. Class III & V Restorations

• Primary Retention Form:
  • Diverging External Walls: The external walls diverge outward, which can reduce retention.
  • Retention Grooves or Coves: These features are added to enhance retention by providing mechanical interlocking and resistance to displacement.

2. Composite Restorations

A. Primary Retention Form

• Mechanical Bond:
  • Acid Etching: The enamel and dentin surfaces are etched to create a roughened surface that enhances mechanical retention.
  • Dentin Bonding Agents: These agents infiltrate the demineralized dentin and create a hybrid layer, providing a strong bond between the composite material and the tooth structure.

3. Cast Metal Inlays

A. Primary Retention Form

• Parallel Longitudinal Walls: The cavity preparation features parallel walls that help resist displacement.
• Small Angle of Divergence: A divergence of 2-5 degrees may be used to facilitate the seating of the inlay while still providing adequate retention.

4. Additional Considerations

A. Occlusal Dovetail and Secondary Retention Grooves

• Function: These features aid in preventing the proximal displacement of restorations by occlusal forces, enhancing the overall retention of the restoration.

B. Converging Axial Walls

• Function: Converging axial walls help prevent occlusal displacement of the restoration, ensuring that the restoration remains securely in place during function.

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.

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.

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.

Gingival Seat in Class II Restorations

The gingival seat is a critical component of Class II restorations, particularly in ensuring proper adaptation and retention of the restorative material. This guide outlines the key considerations for the gingival seat in Class II restorations, including its extension, clearance, beveling, and wall placement.

1. Extension of the Gingival Seat

A. Apical Extension

• Apical to Proximal Contact or Caries: The gingival seat should extend apically to the proximal contact point or the extent of caries, whichever is greater. This ensures that all carious tissue is removed and that the restoration has adequate retention.

2. Clearance from Adjacent Tooth

A. Clearance Requirement

• Adjacent Tooth Clearance: The gingival seat should clear the adjacent tooth by approximately 0.5 mm. This clearance is essential to prevent damage to the adjacent tooth and to allow for proper adaptation of the restorative material.

3. Beveling of the Gingival Margin

A. Bevel Angles

• Amalgam Restorations: For amalgam restorations, the gingival margin is typically beveled at an angle of 15-20 degrees. This bevel helps to improve the adaptation of the amalgam and reduce the risk of marginal failure.

• Cast Restorations: For cast restorations, the gingival margin is beveled at a steeper angle of 30-40 degrees. This angle enhances the strength of the margin and provides better retention for the cast material.

B. Contraindications for Beveling

• Root Surface Location: If the gingival seat is located on the root surface, beveling is contraindicated. This is to maintain the integrity of the root surface and avoid compromising the periodontal attachment.

4. Wall Placement

A. Facial and Lingual Walls

• Extension of Walls: The facial and lingual walls of the proximal box should be extended such that they clear the adjacent tooth by 0.2-0.3 mm. This clearance helps to ensure that the restoration does not impinge on the adjacent tooth and allows for proper contouring of the restoration.

B. Embrasure Placement

• Placement in Embrasures: The facial and lingual walls should be positioned in their respective embrasures. This placement helps to optimize the aesthetics and function of the restoration while providing adequate support.

Gallium Alloys as Amalgam Substitutes

• Gallium Alloys: Gallium alloys, such as those made with silver-tin (Ag-Sn) particles in gallium-indium (Ga-In), represent a potential substitute for traditional dental amalgam.
• Melting Point: Gallium has a melting point of 28°C, allowing it to remain in a liquid state at room temperature when combined with small amounts of other elements like indium.

Advantages

• Mercury-Free: The substitution of Ga-In for mercury in amalgam addresses concerns related to mercury exposure, making it a safer alternative for both patients and dental professionals.