Working Models and Day’s

There are several essential requirements for a good working model: No Air Bubbles: There should be no air bubbles along the finish line of the prepared teeth. No Distortion: No part of the model should have any distortion. Moldable: Models should be designed to facilitate the shaping of the wax...
Model Of Teeth
Working Models and Day’s
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There are several essential requirements for a good working model:

  • No Air Bubbles: There should be no air bubbles along the finish line of the prepared teeth.
  • No Distortion: No part of the model should have any distortion.
  • Moldable: Models should be designed to facilitate the shaping of the wax pattern.

Working Model:

  • A working model is a model placed on an articulator that represents a copy of the teeth.
  • This model should represent the entire dental arch and is used in the fabrication of the wax pattern.
  • The wax pattern is used to create interproximal contact, buccal and lingual contour, and occlusion with the opposing teeth.

Day:

  • A Day is a movable duplicate of a single prepared tooth or all the prepared teeth.

There are two main systems for working models and Day’s:

 Working Model with Separate Day’s:

ADVANTAGES:

  • The simplest method of preparing working models and Day’s.
  • Maintains a fixed and immobile relationship between the abutment teeth.

Ensures excellent compatibility with restoration contours, soft tissues, and adjacent teeth.

DISADVANTAGES:

  • Internal adaptation may be compromised when transferring the wax pattern from one model to another.
  • As this model requires casting plaster twice, an elastic impression material should be used.

Model Preparation:

  • When preparing the wax pattern, Type IV or Type V dental stone should be used to prevent surface abrasion (Type IV: high strength, Type V: high strength, high expansion).
  • The impression should be cleaned before pouring the stone.
  • The stone and water should be mixed in the proportions recommended by the manufacturer and mixed in a vacuum mixer.
  • The stone is gently poured into the area of the prepared tooth, making the Day easy to shape.
  • To pour the full arch impression, the stone is gently placed at the most distal edge of one side of the impression.
  • The stone flows to fill the impression of each tooth from the bottom.
  • The model should be allowed to set for at least 1 hour.
  • The model should not be separated from the impression and the Day should not be prepared before 1 hour has elapsed.

 

Day Preparation: Day preparation is an essential step in the working model, involving the preparation of the Day for a tooth on the model. Here are the steps explained in detail:

  • Carefully remove the model from the impression.
  • Remove excess plaster with a plaster motor or suitable tool.
  • There should be no plaster duplicating soft tissue beyond the gingiva (gum line) on the model.
  • A base of sufficient length should be left under the prepared tooth model to easily grasp the Day by hand (typically about 2.5 cm).
  • This base should be either parallel or taper towards the base.
  • Trim just below the finish line using a pear-shaped burr or suitable tool.
  • Smooth the area around the finish line with a scalpel or a thin spatula.
  • The contours of the Day in this region should allow for easy modeling of the axial contours of the restoration. Creating a deep undercut area below the finish line can prevent spatula adaptation, leading to a thick gingival margin in the model.
  • The contours of the Day should resemble those of a normal tooth.
  • Once the Day trimming is complete, mark the finish line with a red pencil or suitable marker.
  • Apply a relief material to the Day to create a cement space. This material should be approximately 2.5 microns thick and should start 0.5 mm above the finish line.

Removable Day’s in Working Models:

ADVANTAGES:

  • Wax patterns and castings can be used without needing to be removed from their Day’s.
  • Prevents impression distortion.
  • Eliminates incompatibility between different types of plaster.

Resolves mismatches caused by Day’s and castings made from different impressions.

DISADVANTAGES:

  • If the removable Day does not fit perfectly into the model, errors can occur in the model.

These methods are important steps used in the preparation of Day’s and the creation of working models.

A removable Day’s model must possess certain characteristics, and these are crucial for the model to function correctly. Here are these features:

  • Day’s should return to their original positions: The Day’s of the model should be able to return to their starting positions. This is important for the accurate placement of the restoration.
  • Day’s should remain stable when the model is inverted: During the inversion of the model, the Day’s should stay securely in place. This prevents slipping or distortion of the model during impression taking.
  • Easy placement of the Day’s model on the articulator: It should be easy to place the model on the articulator (a device that simulates jaw movements), which allows for accurate adjustment of the occlusion (bite) of the restoration.

For this purpose, BRASS PINS can be used. BRASS PINS help to secure the Day’s of the model and ensure the model has the desired characteristics. Different types of pins can be used, including:

  • STRAIGHT PIN (Day Pin Method): A Day pin is placed inside each prepared tooth. The Day pins should be placed parallel to the long axis of the tooth or parallel to each other. Pins can be placed into the impression with wire clasps. Retention elements are placed before the plaster sets. After cutting the pins, they are separated from the model.
  • CURVED PIN: Used in more complex situations and better secures the Day’s.
  • PINDEX SYSTEM: Used to create a removable and highly accurate relocatable Day’s in a master model. The impression is poured with the pins already placed and secured. The machine drills parallel holes from the bottom of the correctly cut plaster model.
  • DI-LOK TRAY: A system where the Day and model can be precisely connected with the help of a tray containing orientation grooves for both Day and model. This method is especially useful when accurate joining of the Day and model is required.

These pin and Day systems assist in preparing working models with Day’s accurately and stably.

 

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