Fabricating an Accurate Implant Master Cast Notes

Pouring Making of Dental Cast Introduction

A dental cast serves as a replica of the denture foundation in the absence of the patient. A dental cast is made by pouring suitable gypsum products into the impression. The present chapter informs about available methods for making dental casts.

Methods Of Making Cast

• Inversion method
• Beading-boxing method
• Plaster pumice method.

Here, the most widely used two methods, i.e. inversion method for pouring primary impressions and the beading-boxing method for pouring final impressions are discussed in detail.

Read and Learn  More: Preclinical Prosthodontics Notes

Making Primary Cast by Inversion Method:

This method is mainly used for making diagnostic and primary casts.

This method involves three phases namely:

• Pouring the impression
• Retrieval of the cast
• Finishing off the cast. This method is also known as the “two-stage pouring method”.

The armamentarium used for this procedure is:

• Primary impression
• Large rubber bowl
• Dental plaster
• Plaster spatula
• Porcelain tile
• Plaster knife
• Warm water (55–60°C temperature)
• Sandpaper.

Pouring of Impression:

Step 1:

The impression made of the edentulous cast is rinsed with water and disinfectant. A measured quantity of water is taken in a rubber bowl. A weighed quantity of dental plaster (according to the recommended water powder ratio by the manufacturer) is sifted in water. This is spatulated to form a creamy mix.

Step 2:

Put some quantity of this mix in one corner of the impression and allow it to flow to the other corner by vibrating/tapping the impression. This will avoid the incorporation of air bubbles while pouring. Cover the impression completely.

Step 3:

Make some serrations as shown in create undercuts or place some plaster blebs. This helps in the mechanical union between the first pour and the second pour. The base of the cast is made after the initial set of the first pour.

Step 4:

The rest of the creamy mix is placed on the porcelain tile and roughly given the shape of the base of the cast and the impression is inverted over it.

Step 5:

Allow the initial set to take place. Plaster is shaped according to the desired form of cast base using a plaster knife. Excess material is removed beyond the borders of the impression. Allow the material to set completely. In case of mandibular impression, remove plaster from tongue space till the stock tray borders. Impression borders should not be covered more than 2–3 mm. Covering the tray border in excess results in fracture of land area while retrieval of the cast.

Retrieval of Cast:

Step 1:

After the plaster is set completely, the poured impression with base is placed in a large rubber bowl with hot water having a temperature of 55–60°C.

Step 2:

The compound is allowed to soften in hot water and the cast is retrieved slowly.

Step 3:

The cast should be checked for anatomical details. Any air bubbles or nodules should be eliminated or corrected to obtain a clean cast.

Finishing off the Cast:

Step 1:

Sulcus depth is marked using a red pencil. The posterior end of the maxillary cast is marked from the depth of the hamular notch on one side to the hamular notch on the other side. The posterior end of the mandibular cast is marked beyond the retromolar pad on both sides.

Step 2:

The cast is trimmed using a plaster knife in such a way that the land area of the cast is 3 mm high and 3 mm wide. It slopes downward and outward by 45° or should be straight. The base of the cast is 15 mm thick. The base of the cast is smoothened using sandpaper. Do not alter the anatomical area of the cast with sandpaper or by any means.

Step 3:

Label the cast with your name and registration or roll number on its side. (On the patient’s cast OPD number and patient name are written).

Pouring Cast by Beading-Boxing Method:

The final impression is made by recording fine details with precision. The final cast is made with the beading-boxing method. This method gives a more precise cast that needs no or minimum corrections after retrieval. In this method, one can control or predetermine the thickness of the base by adjusting the height of the boxing wax.

Instruments and Material:

• Maxillary and mandibular impressions
• Rubber bowl
• Plaster spatula
• Beading wax
• Boxing wax
• Dental stone
• Sandpaper.

Beading wax is available in square or round shapes and in various colors. Boxing wax is available in various colors and lengths.

Procedure:

Step 1:

Wash the final impression and disinfect it. Place markings 2–3 mm away from the impression borders as shown.

Step 2:

Beading wax is placed 3 mm below the sulcus extension (borders) of the impression. It is sealed with a hot wax spatula to bead the impression. The placement of beading wax 2–3 mm below the impression borders forms the height of the land area on the cast. The thickness of the beading wax ensures the thickness of the land area on the cast. It should be 3 mm.

Step 3: Boxing wax is sealed to the beading wax such that the impression is boxed in. The height of boxing wax provides the thickness of the base of the cast.

Note: In the case of mandibular impression, a tongue-shaped modeling wax is placed in tongue space after beading to create tongue space in the cast.

Step 4:

Pour some water into the boxed-in impression to ensure that it is sealed perfectly. This ensures perfect sealing of beading and boxing wax and there is no seepage from where plaster can flow out.

according to the manufacturer’s recommended water-powder ratio. The mix should have a creamy consistency. For pouring maxillary impression, a small quantity of a creamy mix of dental stone/plaster is placed in the center of the palate and it is allowed to flow toward the periphery by putting the boxed impression on the vibrator or by tapping the impression. Plaster/stone is added in small increments till the boxed impression is completely covered to the height of the boxing wax.

For pouring mandibular impressions, dental stone/plaster is placed on the retromolar area and an impression is either tapped or placed on the vibrator to make the plaster flow to the other end. Plaster/stone is added in small increments till the boxed impression is completely covered up to a height of boxing wax.

Step 5: Once dental stone/plaster sets completely, beading and boxing wax are removed. Put it in hot water to remove the impression compound. The cast is finished and smoothened as required. Sandpaper is used to smooth the base of the cast. Casts are poured by beading–boxing methods.