In-Depth Notes on Dental Impressions Technology

Impressions

• Definition: An imprint or negative likeness.
• Creation: Made by placing soft, semi-fluid material in the mouth and allowing it to set.
• Material Properties: Depending on the material, the impression may be hard or elastic; elastic materials are commonly used for cast restorations.
• Importance: Accurate, undistorted impressions are crucial for creating precise restorations that match the prepared tooth.

Requirements for Impressions

• Exact duplication of the prepared tooth, including:
• All preparation features.
• Sufficient uncut tooth surface beyond the preparation for proper location of the finish line.
• Accurate reproduction of adjacent teeth and tissue for proper articulation.
• Must be free of bubbles, particularly around finish lines and occlusal surfaces.

Reversible Hydrocolloid

• Usage: Employed extensively for over 70 years in making cast gold restorations.
• History: Patented in 1925 and introduced in the U.S. in the late 1920s.
• Composition: Approximately 85% water, critical for impression accuracy.
• Distortion: Water loss (syneresis) or absorption (imbibition) can lead to distortion.
• Best Practice: Impression should be poured soon after removal from the mouth to minimize distortion.

Custom Resin Trays

• Purpose: Used for elastomeric impression techniques for improved accuracy.
• Design: Must be rigid with stops to orient correctly in the mouth.
• Adhesion: Impression material must adhere firmly to the tray via a rubber adhesive.

Dual Arch Impressions

• Advantages: Utilizes one tray to capture the prepared tooth, adjacent and opposing teeth, enhancing accuracy.
• Best Use: Suitable for patients with intact, mutually protected Angle class I occlusion and no arch-of-closure interference.
• Tray Options: Available as reusable metal or disposable plastic.

Polysulfide

• Description: An elastomer known for greater dimensional stability than hydrocolloids.
• Limitations: Requires timely pouring (within 1 hour) to maintain accuracy; must not be sent unpoured to a lab.
• Handling: Care needed to avoid moisture on preparations to prevent distortion.
• Radiopacity: Unique property due to lead dioxide, aiding in locating trapped fragments.

Condensation Silicone

• Reaction Type: Polymers cross-link through condensation reactions involving ethyl silicate and organo-tin catalysts.
• Issues: Shrinkage due to alcohol evaporation can lead to poor dimensional stability; should be poured soon after removal.

Polyvinyl Siloxane (PVS)

• Overview: Known for better dimensional stability; referred to as addition silicone.
• Pour Timing: Accurate even when poured 1 week after removal from mouth; generally hydrophobic unless treated with surfactants.

Polyether

• Properties: Hydrophilic elastomer with excellent dimensional stability; accurate even when poured days later.
• Challenges: Stiffness requires blocking out undercuts to avoid locking.

Disinfection of Impressions

• Requirement: Impressions must be made safe before external handling.
• Chemical Disinfectants:

1. Chlorine compounds
2. Synthetic phenolic compounds
3. Glutaraldehydes
4. Iodophors
5. Phenol-alcohol combinations

• ADA Recommendations: Immersion of various impression materials in accepted disinfectants (max 30 minutes).

Digital Impressions

• Cad/Cam Development: First U.S. patent granted in 1987 for CAD/CAM devices in dentistry.
• CEREC System: Introduced in 1987 for fabricating dental restorations using optical scanning.
• E4D System: Advanced laser scanning technology introduced in 2008 for improved efficiency without reflective powder.