Lecture 4: Alginate and Stone

What are the characteristics of ideal impression material?

Easy to mix and handle, suitable working time, suitable setting time, compatible with stone, non-toxic, and allergenic.

What is measured by dimensional stability in impression materials?

The ability to retain its absolute dimensional size over time.

What is tear resistance in impression materials?

The ability to resist tearing in thin sections.

What is the importance of accuracy in impression materials?

It evaluates the ability to replicate the intraoral surface details.

What are the two classifications of impression materials?

Elastic and Nonelastic.

What is Alginate?

An irreversible hydrocolloid developed during World War II as a substitute for agar impression material.

What is the main component of Alginate?

Alginic acid derived from brown seaweed.

What is a property of underwater impression materials?

They are hydrophilic, allowing for effective use on moist tissue surfaces.

How is the sol-gel reaction in Alginate described?

A reaction of soluble alginate with calcium sulfate forming insoluble calcium alginate gel.

What is the recommended mixing time for Alginate?

45 seconds to 1 minute.

What is the significance of the 3 mm thickness in Alginate impressions?

To provide sufficient bulk for the impression to be strong.

How can the setting time of Alginate be affected?

By altering the temperature of the water used for mixing.

What do impression disinfectants do?

Utilize a disinfectant-soaked paper towel or plastic bag for 10 minutes.

Name one type of elastic impression material.

Polyethers.

Name one nonelastic impression material.

Zinc oxide Eugenol.

What happens during the mixing of alginates with water?

They form a sol which transforms into a gel.

What is the purpose of custom trays in alginate impressions?

To construct a more accurate second impression after using initial Alginate.

What happens to Alginate when exposed to air bubbles?

It can compromise the quality of the impression.

What is VPS in relation to impression disinfection?

Vinyl Polysiloxane, which is not adversely affected by disinfectants.

What is the main chemical in gypsum used for dental applications?

Calcium sulfate dihydrate (CaSO4 · 2H2O).

How is gypsum commonly used in dentistry?

For the preparation of study models and dental prostheses.

What is the role of silica in gypsum products?

To regulate mixing properties when added as fillers.

What physical property is determined by the W/P ratio in gypsum?

It affects the physical and chemical properties of the final product.

What is defined as the time from the addition of powder to water until mixing is complete?

Mixing Time (MT).

What is the time allowed to use a workable mix in dental materials?

Working Time (WT).

What is the time from mixing until the material hardens called?

Setting Time (ST).

What is an accelerator in relation to setting time of plaster?

A substance that decreases the setting time.

What causes setting expansion in dental impressions?

The formation of crystals during the crystallization process.

How can setting expansion be controlled?

Through the addition of certain chemicals by manufacturers.

What is Impression Plaster (Type I) used for?

It is rarely used anymore for dental impressions.

What is the strength of model plaster (Type II)?

Compressive strength is as low as 9 MPa.

What are dental stones (Type III) preferred for?

For casts used to process dentures.

What characteristic does Type V dental stone exhibit?

Higher compressive strength and increased setting expansion.

What is meant by 'maximum intercuspation' in dentistry?

All teeth contact in a stable way.

How is centric occlusion defined?

First contact when a leaf gauge is in position on the anterior teeth.

How should Alginate not behave when mixed?

It should not readily drip off the spatula.

What does a smooth, creamy mixture of Alginate indicate?

It is ready for use in making impressions.

What can affect the properties of alginate gel?

Changes in the W/P ratio or mixing time.

Why are alginates used for preliminary impressions?

To help in constructing custom trays or making study models.

What can prolonged exposure of polysulfides and polyethers lead to?

Potential expansion of the impression material.

What is the primary method for controlling the final properties of gypsum products?

Through chemical modifiers added during mixing.

What does tear strength depend on when removing an alginate impression?

It increases with a snap-like removal.

What makes the gel formed from alginate useful in impressions?

It accurately records all aspects of teeth and tissue.

What electrolyte can be used to reduce setting expansion?

Potassium sulfate.

What is the common setting reaction associated with gypsum?

A dissolution and precipitation process leading to set solid formation.

What gauge time informs mixing practices in dentistry?

Mechanical mixing typically completes in 20 to 30 seconds.

What type of impression material has high strength for detailed work?

Addition silicone.

What happens when soluble alginates mix with water?

They create a sol that turns into gel.

What is the maximum thickness of Alginate impressions recommended?

At least 3 mm thick.

What is one advantage of digitally recorded impressions?

They eliminate certain physical materials, offering quicker processes.

What factors affect the setting time of plaster?

The addition of chemical modifiers or ambient temperature.

What is a common limitation of alginate materials?

They can be weak and susceptible to tearing.

How can digital impression systems aid dental work?

By scanning and electronically transmitting files for processing.

What is the role of dental stones in creating prostheses?

They serve as a basis for casting and fabricating dental devices.

How does setting expansion affect impression accuracy?

It must be controlled to prevent errors in fitting restorations.

What chemical changes occur during gypsum setting?

Dissolution of hemihydrate and precipitation of dihydrate crystals.

What should be avoided to prevent compromising impression accuracy?

Air bubbles during the impression process.

What is a common feature of alginate from brown seaweed?

It is a natural, hydrophilic polymer.

How long is the mixing time typically required for plaster?

About 20 to 30 seconds for mechanical mixing.

What is a key criticism of non-aqueous elastomers?

They may have complex handling requirements.

In what scenario would you choose condensation silicone?

When certain detail preservation is necessary.

What factors regulate the hydration of dental plaster?

The shape and compactness of the crystals in the mixture.

What is one downside of using model plaster?

It has relatively low strength and is not durable.

What material is often used for impressions requiring high detail?

Addition silicone and polysulfides.

What is the common error when handling alginate impressions?

Failing to mix thoroughly may lead to inaccuracies.

What is one property of reversible hydrocolloids?

They can change back and forth between sol and gel states.

What happens after the addition of water to gypsum powder?

A transformation to a workable mix occurs.

How should the alginate impression material behave during the impression process?

It should adhere well to the tray and not flow excessively.

What is one advantage of using VPS in dental impressions?

Good dimensional stability and accuracy.

What is the result of proper alginate handling and mixing?

Accurate and representative impressions for treatment planning.

What are the desired traits of impression materials in clinical use?

Economical, easy to disinfect, and accurate.