44. Fabrication of a gypsum fixator with “a key”

whats in it

• a working cast

• key of occludator

• fabrication of hypsum fixator with a key

a working cast

is a positive and accurate replica of the prepared tooth (or teeth), ridge area, and other parts of the dental arch. It is obtained from the final impression and on which the wax pattern is made. The working cast is strong enough to resist the stresses of fabricating an indirect restoration or prosthesis.

a key of ocludator

used to fix the upper and lower in the same position according to the relationship of the bite of the lower and upper teeth. The key must follow the shape of the masticatory sulcus (it is shaped by the grooves of the premolars and molars). The key must be placed after the last molar.

Fabrication of gypsum fixator with the key:

🦷 Simplified Steps for Fabricating a Gypsum Fixator with a Key:

1. Keep the working cast inside the impression — do not cut out the dies yet.

2. The goal of the gypsum fixator is to:

   • Hold the upper and lower arches in a stable bite

   • Use a "key" shape (like a T or +) to lock the parts together accurately

3. Once the working cast is dry and retentions (like paper clips) are in place:

   • Isolate the area around the pins — so you can remove the dies later.

   • Mix gypsum with the correct water-to-powder ratio.

   • Pour some gypsum over:

     • The pins and retention areas, and

     • The tabletop/work surface near you.

4. Flip the working cast upside down into the gypsum on the table:

   • Make sure the distal (back) teeth are oriented correctly.

   • The pins should be parallel and touch the surface at the same height.

5. While the gypsum is still soft:

   • Shape it neatly.

   • Create the “key” feature (T-shape or + shape) at the level of the back (posterior) biting surfaces.

   • This key ensures the upper and lower parts fit together precisely.

6. Once it hardens:

   • Isolate the key (so new gypsum won’t stick to it).

   • Repeat the previous step:

     • Add more gypsum into the key and onto the antagonistic side (the opposing model).

     • Shake it gently or use a vibrating table to remove air bubbles.

7. Trim the model with a knife or trimming machine to clean it up and make it more compact.

8. When everything is fully dry:

   • Open the impression carefully.

   • Now you have a complete gypsum fixator with a key and your working cast on top.

9. Finally, you can use a flat saw to cut and remove the individual dies from the cast.