Impressions in crown and bridge

Question 1

Impression definition

Answer 1

An impression is an imprint or negative likeness. (Shillingburg)

a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry; (GPT-9)

Question 2

An impression for a cast restoration should meet the following requirements:

Answer 2

• It should be an exact duplication of the prepared tooth
• Other teeth and tissue adjacent to the prepared tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration
• It must be free of bubbles, especially in the area of the finish line and occlusal surfaces of the other teeth in the arch.

Question 3

Impression materials for c&b

Answer 3

• Reversible hydrocolloid
• Polysulfide
• Condensation silicone
• Polyvinyl siloxane (PVS)
• Polyether

Question 4

Custom Resin Trays

Answer 4

Custom resin trays have been used in elastomeric impression techniques because these materials are more accurate in uniform, thin layers of 2 to 3 mm

The custom tray must be rigid, and it should have stops on the occlusal surfaces of the teeth to orient the tray properly when it is seated in the mouth.

The impression material must adhere firmly to the tray. This is achieved with a rubber adhesive packaged with the impression material

The bonding strength of adhesives used with PVS materials can be improved by nearly 50% by adding perforations to the tray and by approximately 140% by roughening the inner surface of an acrylic resin tray with 80-grit silicon carbide paper before adding the adhesive

PVS putty does not adhere to its adhesive making good mechanical retention in the tray, such as perforations, mandatory when putty is used.

The tray should be prepared at least 6 hours in advance of making the impression. As the monomer undergoes polymerization, it shrinks. The dough could shrink as much as 8% before curing is completed.

Significant linear changes occur during the first 40 minutes of the fabrication of a tray, with some changes continuing to occur for up to 6 hours

If the tray is made immediately before the impression is made, however, polymerization shrinkage and stress relaxation will be occurring while the impression is in the tray, resulting in distortion of that impression.

Question 5

Dual-Arch Impressions

Answer 5

These include the use of only one tray, which captures an impression of the prepared tooth, the adjacent and opposing teeth, and the relationship between them in maximal intercuspation without the need for an interocclusal record

This technique is best used for a patient with an intact, mutually protected Angle Class I occlusion. The restoration should be for a single tooth with intact adjacent and opposing teeth. There should be no arc-of-closure interference in closure to maximal intercuspation.55

Question 6

Types of putty

Answer 6

Light-body, which is used with a syringe and placed directly on hard and soft tissues;

Heavy-body, which is placed in the tray to support the light-body material; and

Putty, which is useful for materials that exhibit significant polymerization shrinkage.

Question 7

MIXING SYSTEMS

Answer 7

Types of mixing systems are available to mix the catalyst and the base:

1. Using paper pad ( Glass slab) and spatula.
   Light , medium and heavy consitencies
2. Hand kneading
   Heavy or putty consistency
3. Static automixing
   Catridge form. ( Light,medium, heavy)
4. Dynamic mechanical mixing
   All consitencies

Question 8

Three common methods for making impressions for fixed restorations are:

Answer 8

• A simultaneous, dual viscosity technique
• A single viscosity or monophase technique
• A putty-wash technique
  1. Single stage/Twin-mix technique
  2. Two-stage with spacer technique
  3. Two-stage without spacer technique

Question 9

Disinfectants

Answer 9

• Chlorine solutions. These tend to be harmful to skin, eyes etc., and they bleach clothing, have an unpleasant odour and are highly corrosive to metals.
• Aldehyde solutions. These give off a suffocating odour and are irritating to the skin and eyes. Commercial products tend to be made from glutaraldehyde-based solutions rather than formaldehyde-based solutions.Glutaraldehyde 2% solutions are the preferred disinfectants.
• Iodine solutions (Iodophors 1%).
• Phenols

Finish Line Exposure
* Mechanical
* Chemicomechanical
* Surgical