Impression materials are used to create indirect models of the oral cavity by capturing shapes.
They offer precise reverse accuracy, reducing the burden on patients and doctors by creating indirect models.
They are classified into elastic and non-elastic impression materials based on their hardened state.
Additionally, they can be categorized into pressure impression and non-pressure impression based on the method of taking impressions.
What are the differences between non-elastic and elastic impressions and their applications?Open
For taking impressions of mucosal tissues, excessive pressure can cause issues with mucosal surface reproducibility, sometimes require the use of impression gypsum.
Due to its non-elastic nature, handling undercut areas becomes complex, leading to the predominant use of elastic impression materials such as alginate and silicone.
What are the raw materials for hydrocolloid impression materials?Open
Agar impression material’s main ingredient･･･Agar derived from red algae like tengusa.
Alginate impression material’s main ingredient･･･Sodium alginate, processed from seaweed like giant kelp.
Agar impression material･･･Utilizes the property of agar solution to gel at lower temperatures and remain liquid at higher temperatures (reversible reaction).
Alginate impression material･･･Utilizes the property of sodium alginate reacting with calcium in gypsum to form a three-dimensional network of calcium alginate, resulting in gelation (irreversible reaction).
How to adjust the gelation time of alginate impression material?Open
Use different types provided by manufacturers･･･fast, normal, and slow, which progressively take longer to set.
Adjust water temperature･･･Lowering the water temperature can delay the setting time significantly.
How long should alginate impression materials be kept in the mouth?Open
2 to 2.5 minutes. The duration can be influenced by the mixing method (shorter with mechanical mixing) and the paste temperature (shorter with higher temperatures)
What are imbibition and syneresis?Open
【Imbibition】: This is the swelling phenomenon where hydrocolloid impression materials expand when immersed in water for an extended period.
【Syneresis】: This is the shrinkage phenomenon where hydrocolloid impression materials contract and expel internal liquid when exposed to air.
Both imbibition and syneresis can lead to dimensional inaccuracies in the impression.
How to store hydrocolloid impressions after taking them?Open
In the air : Minimize exposure time to reduce significant shrinkage.
In water : Minimize immersion time to prevent significant swelling.
At 100% relative humidity : This condition minimizes dimensional changes but avoid long-term storage.
When should gypsum be poured after taking impressions?Open
Impression materials tend to shrink over time, and hydrocolloid impressions can undergo syneresis, which degrades the surface quality of the gypsum. Therefore, it is advisable to pour gypsum as soon as possible.
What is the impact of syneresis from hydrocolloid impressions on gypsum?Open
Syneresis from agar･･･Caused by borax solution, agar solution.
Syneresis from alginate･･･Caused by phosphate buffer solution.
The components causing syneresis not only increase the gypsum mixing ratio but also delay the setting time of gypsum, resulting in surface setting delays and leading to surface roughness due to dehydration ,a phenomenon known as “dry-out.”
Super hard gypsum is particularly prone to surface roughness due to its greater tendency for delay during this process.
What is a fixing solution?Open
Fixing solutions not only wash away the components of syneresis but also have an accelerating effect, making the model surface harden faster and preventing surface roughness.
Dental Gypsum
What are the types of dental gypsum?Open
Based on the type of raw gypsum, dental gypsum can be classified into dental plaster (mainly β-gypsum, also known as ordinary gypsum) and dental stone (mainly α-gypsum).
Within dental stone, there are types with lower expansion and higher strength, referred to as high-strength stone (super hard gypsum).
What are the differences between α-gypsum and β-gypsum?Open
α-gypsum is produced by autoclaving under pressure, where it is dissolved and recrystallized in a hydrothermal environment. β-gypsum is produced by heating in an open kettle, where dihydrate gypsum retains its original shape while losing its water of crystallization.
Although α and β gypsum cannot be distinguished by X-ray analysis and are nearly identical crystallographically, they differ significantly in their physical properties. Both α and β gypsum require the same theoretical amount of water for hydrating 100 grams, however α-gypsum is dense and requires less water for mixing, resulting in higher strength. β-gypsum is porous and requires more water for mixing, leading to lower strength.
What is the mixing amount and mixing ratio?Open
Mixing amount : The quantity of water (g) required to mix with 100g of calcined gypsum.
Mixing ratio : The weight ratio of water to calcined gypsum. This data is crucial as it affects all other performance characteristics.
What is the effect of the mixing ratio on setting time, setting expansion, and strength?Open
Increasing the mixing ratio delays the setting time and reduces both setting expansion and strength.
How to delay the setting time?Open
Increase the mixing ratio, shorten the mixing time, lower the water temperature, or add retarders (e.g., sodium citrate).
How to accelerate the setting time?Open
Decrease the mixing ratio, extend the mixing time, increase the water temperature, or add accelerators (e.g., slurry water, potassium sulfate).
Slurry water is the cloudy liquid from trimming gypsum models, which acts as a nucleation site for gypsum hydration, thus accelerating the setting without adversely affecting the model.
What are the types and causes of bubbles in set gypsum?Open
Trapped air from pouring subsequent layers of gypsum on top of an initial layer.
Trapped air in undercuts and corners if gypsum does not to flow into these areas.
Residual air within gypsum particles if not completely eliminated during manual mixing.
Gases produced at the gypsum surface during exothermic reactions when using silicone impressions.
How to mix gypsum without bubbles?Open
Use a vacuum mixer if available.
For manual mixing, measure water and gypsum according to the standard mixing ratio.
Add gypsum to water in a rubber bowl and let it soak for 30 seconds to replace air with water.
Mix with a spatula for one minute until a uniform paste is achieved.
Use a vibrator to the rubber bowl to release air bubbles before pouring.
How to pour gypsum to minimize surface roughness and bubbles?Open
For agar and alginate impressions, immediately after removing the impression, wash it with water and use a “wick” or similar tool to remove any water in undercuts and corners.
Dry the impression and pour the gypsum as soon as possible before the impression dries. Avoid using an air blower after washing, as it may cause the agar to peel off.
For silicone impressions, after removing the impression, wash it with water, spray it with a surfactant-based surface treatment agent, remove any excess liquid completely, and pour adequately mixed gypsum.
(Adequately mixed gypsum sets quickly after pouring, thereby reducing surface roughness.) Pour the gypsum slowly from a single spot while applying a vibrator.
Carefully observe the leading edge of the flowing gypsum to ensure it properly fills corners and undercuts properly.
Once the required height of the model surface is reached, stop the vibrator and use a spatula to add more gypsum if necessary.
What happens if gypsum gets wet during storage?Open
High temperatures and humidity increase the moisture content of gypsum, initially delaying the setting time. Over time, dihydrate gypsum forms on the particle surfaces, accelerating the setting time.
Although this “weathered” gypsum may function initially, it cannot ensure uniform hardening and will not perform optimally.
How to store impressions after pouring gypsum?Open
For agar and alginate impressions : Store in a humidity chamber after pouring. This prevents shrinkage that can occur in a dry environment, which may deform the model.
For silicone impressions, standard room conditions are usually sufficient.
How to store gypsum models after removing them from impressions?Open
After the manufacturer-specified time, remove the gypsum model from the impression and let it sit in a non-drying area for 2-3 hours before drying at temperatures below 45℃.
Exposing to higher temperatures can gradually dehydrate the model, reducing its strength.
Fully hydrated gypsum takes 2 hours for calcined gypsum and 3 hours for hard gypsum.
Fully dried models achieve twice the strength of wet models.
If dried at temperatures exceeding 45℃, the gypsum will gradually lose its crystalline water, leading to a decrease in strength, so be mindful of the temperature.
