Impressions in Prosthodontics Notes

Materials used to register or reproduce the form and relationship of the teeth and oral tissues.
The impression gives a Negative reproduction of tissues.
By filling the impression with dental stone or other model material, a Positive cast is made that can be removed after the model material has set.

Primary impression: A negative likeness made for the purpose of diagnosis, treatment planning, and/or the fabrication of a custom impression tray.
Final impression: The impression that represents the completion of the registration of the surface or object.
Working time: The time available for mixing and manipulating a material.
Setting time: The time needed for a material to become fully set.
Undercut: A part in the surface of an object that is below height of contour in relationship to path of placement; this portion of residual ridge prevents insertion of a dental prosthesis.
Edentulism: The state of being edentulous; without natural teeth.

Also called “DENTAL TRAYS”
Definition: A device used to carry, confine, and control the impression material from and to the patient's mouth.

To give rigid support to the impression material.
To facilitate insertion & removal of impression material from and to the patient's mouth.

Metallic trays (Stainless steel, Aluminum):
- Metal Trays provide maximum support for impression materials.
- Rigid, durable, and long-lasting.
- Can be perforated or solid and sterilized to be reused.
Non-metallic trays (Plastic trays, Acrylic resin trays):
- Plastic trays are disposable, eliminate cross-contamination.
- Has thick walls to provide lateral support for impression material during pouring model.

Stock trays (prefabricated):
- Are ready-made and come in specific sizes.
- They are reusable after sterilization.
- Can be sub-classified according to size and shape:
  - Full Tray
  - Quadrant Tray
  - Sectional Tray: used when we want to take impression for 4 or 5 teeth and it can work for a single tooth
  - Upper Arch Tray
  - Lower Arch Tray
Custom trays or Special trays:
- An individualized impression tray made from a cast recovered from a preliminary impression.
- It is used to make a final impression.
- Custom trays are a better fit than stock trays.
- They are used only for the particular patient then discarded.

Edentulous trays.
Dentulous trays
*Dentulous Trays - Square Cross-Section - For patients with teeth!
*Edentulous Trays - Rounded Cross-Section - For patients without teeth!
*When flange height is high this indicates that the tray is for edentulous patients

Perforated trays: The perforation help in mechanical retention for impression materials.
Non-perforated trays
Rim-lock trays: Has metal beading on the edge of the tray wall are ideal for taking alginate impressions.

The type of impression material
- Elastic Impression materials used with perforated trays. Trays has to incorporate mechanical retention like Rim-Lock OR perforations in trays.
- Inelastic materials (ridge impression materials) use Non- Perforated trays, because it can stick on the tray.
Size of the arch
Form of the arch (round, square, taper).
Type of mouth:
- Dentulous Mouth – use Dentulous Trays.
- Edentulous – use Edentulous Trays.
- Partially Edentulous – use Combination Trays.

Once tray style has been identified, tray size that best fits the patient is selected.
The tray should be large enough to avoid impinging on soft tissue or interfering with dentition and should extend to cover last tooth in arch and retro molar pad areas and tuberosity.
There should be a sufficient room (3-5 mm) for the alginate material between the tray wall and dentition, otherwise, there will be a plastic deformation of impression when removing impression material from undercuts.

Shape of dental arches vary in many ways; the operator sometimes finds it difficult to select a tray that fits properly.

Tray flanges are short: using utility wax tray can be extended to desired length.
If the patient's teeth protrude labially or buccally, sides of a metal tray may require bending for both maxillary and mandibular arches.
Patient has arch with long edentulous area, the normal dentulous tray cannot fit. Adding base plate wax (pink) in the edentulous area to make tray more fit.