DM CH 16 Case Study

How would each of the following situations be best handled? -You have fast-set plaster in your office and wish to mix enough material to pour two arches. How would you increase working time?

Decreasing temperature mixing water will give you more working time Increasing water to powder ratio can give you more time

For each of the following situations, which gypsum material would be the best choice? -The dentist has taken an impression for a space maintainer

Dental stone (Type III)

For each of the following situations, which gypsum material would be the best choice? -The hygienist has taken an impression for a custom bleaching tray

Dental stone (Type III)

How would each of the following situations be best handled? -When you are working on a cast, the teeth chip and crumble easily. What factors may have caused this?

Gypsum has very low tensile strength and rocking back and forth while separating can cause teeth to chip and break

For each of the following situations, which gypsum material would be the best choice? -The expanded functions assistant has taken an impression for a full gold crown

High-strength/low expansion stone (Type IV) die stone

How would each of the following situations be best handled? -Several air voids are present on the surfaces of the teeth. What factors may have caused this?

If the mixture wasn't poured slowly in small increments under vibration then it didn't allow the flow from tooth to tooth, causing air to get trapped as it moved, thus causing voids

For each of the following situations, which gypsum material would be the best choice? -The assistant has taken an impression for an orthodontic case study

Model plaster (Type II)- diagnostic cast/study model

A gypsum model is articulated with a wax bite registration and is left in the dental laboratory over a hot weekend. The assistant, on coming in on Monday, discovers that the model is no longer in the correct occlusion -What could have been done to avoid this problem?

Store waxes at or slightly below room temperature to avoid distraction

After the preceding problem is corrected, the impression is poured. The hygienist, on separating the boxed model, finds that there is a thin layer of wax on the base portion What property of the wax most likely caused this and what property of the gypsum product contributed to this problem?

The exothermic property of the gypsum increases the temperature causing the wax to melt forming a thin layer at the base (the temperature of the wax should be room temperature). Failure of this will result in melting and will cause wax to stick to the base

A gypsum model is articulated with a wax bite registration and is left in the dental laboratory over a hot weekend. The assistant, on coming in on Monday, discovers that the model is no longer in the correct occlusion -What property of the dental wax most likely caused the problem?

Thermal expansion Waxes expand when heated and contract when cooled; the thermal expansion and contraction of waxes are greater than those of any other dental material. This property is especially important for pattern waxes. If a wax is heated too far above the melting range or is heated unevenly, expansion above acceptance standards will result

A final impression for an edentulous case is corrected with corrective impression wax; the impression is then sent to the dental laboratory What precautions must be considered when sending the impression?

This wax has low melting point that flows at mouth temperature. When sending it off, to keep it from distorting, it must be stored in conditions that will not allow the corrective wax to melt. It should always be kept at a cooler temperature during the shipping process

A gypsum model is being poured, using boxing wax. The wax is formed around the impression but will not hold in place. What can the assistant do to the wax to help it adhere to itself and the tray?

Waxes, such as boxing and utility wax, are slightly tacky at room temperature to help them adhere to themselves, however, they must remain dry if the assistant is wanting to stick better. Use heat of hands and warm water but not heating via flame

How would each of the following situations be best handled? -You need to make a custom tray for an appointment in progress and have just taken the alginate impression. How would you accelerate the setting time of the gypsum product you select?

You can alter setting time by: Altering water-to-powder ratio Performing spatulation Changing temperature Using accelerators or retarders