Prosthodontics | Lab Processing of Crowns | INBDE, ADAT

Summary

This final video in the prosthodontics series delves into the lab processing of crowns, detailing the steps from taking a final impression to delivering the crown. It covers the creation of a master cast, die preparation, waxing, investing, burn out, casting, and quenching. The video also addresses common issues like porosity in materials and provides a mnemonic for remembering types of investments. It concludes with a summary slide and a promise of upcoming practice questions for board exam preparation.

Takeaways

• 🦷 The final video in the prosthodontics series discusses lab processing of crowns, detailing the steps from impression to final crown delivery.
• 🗿 The master cast is a replica of the dental arch or quadrant being worked on, and a die is created for the tooth receiving the crown.
• 🔍 'Ditching a die' is the process of cleaning the margin of the prepared tooth to ensure it's clearly visible for accurate crown fitting.
• 🖌 Die spacer is applied to the die to account for the cement layer, ensuring a thin film of cement between the crown and the tooth.
• 🔄 Understanding positive and negative reproductions is crucial for lab processing, starting with an impression (negative) and creating a cast or model (positive).
• 🕯 Waxing is the step where the crown is built on the die using wax, which is easily manipulated but can cause distortion due to internal stress.
• 🗝️ Speuming involves attaching wax channels (sprues) to the wax crown to create a path for the metal during the casting process.
• 🏺 Investing covers the wax and sprue with investment material to create a mold for the metal crown.
• 🔥 Burn out is the process of melting out the wax to leave a void in the investment material for the metal to be cast.
• 🗝️ Casting involves pouring molten metal into the investment material to create the metal framework of the crown.
• 💧 Quenching cools the hot cast metal, making it more malleable for finishing and delivery to the patient.
• 📊 There are dimensional changes and porosity issues to be aware of during lab processing, such as shrinkage and expansion, and different types of porosity related to sprue design.

Q & A

What is the purpose of the final impression in the process of crown preparation?

The final impression is used to create a master cast, which is a replica of the whole dental arch or the specific quadrant being worked on. This cast is essential for accurately reproducing the crown.

What is the term used for the positive reproduction of the prepared tooth?

The term used for the positive reproduction of the prepared tooth is 'die'.

What is the process of 'ditching a die' and why is it important?

Ditching a die involves scraping and cleaning away any stone below the margin of the prepared tooth so that it is clearly visible 360 degrees around the tooth. This is important to ensure that the margin is accurately reproduced in the final crown.

What is the purpose of applying a die spacer?

A die spacer is applied to allow room for the cement layer. It creates a thin film thickness of cement between the intaglio surface of the crown and the prepared tooth, which is necessary for a proper fit.

What is the significance of understanding positive and negative reproductions in lab processing?

Understanding positive and negative reproductions helps in the process of creating an accurate replica of the tooth or dental arch. A negative reproduction is the initial mold or imprint, and a positive reproduction is the final cast or model used for making the crown.

What material is typically used for waxing the crown during the lab processing of crowns?

Wax is used for building the crown on the die. It is chosen because it can be easily manipulated with hot and cold instruments and maintains its form once the waxing is completed.

What is the purpose of 'spuing' in the lab processing of crowns?

Spuing involves making a path with wax for the metal to flow into during the creation of the prosthesis. It is essential for ensuring that the metal can be properly cast into the shape of the crown.

What is the term used for the process of covering the wax and sprue with investment material?

The process of covering the wax and sprue with investment material is called 'investing'.

What are the three types of investments mentioned in the script and their respective uses?

Gypsum bonded investments are used for making gold crowns, phosphate bonded investments are used for the fabrication of PFM (porcelain-fused-to-metal) crowns, and silica bonded investments are used for base metal crowns.

What is the process called where the wax positive is melted out to leave room for the metal?

The process is called 'burn out', where the wax positive is melted out, leaving a void in the investment material for the metal to take its place.

What is the final step in the lab processing of crowns before delivering it to the patient?

The final step is 'quenching', where the hot cast metal is immediately placed in cool water to make it more malleable for finishing, after which the sprue is removed, the crown is polished, and it is delivered to the patient.