Impression Materials Overview
Types of Impression Materials
Brands/Nomenclature:
Zhermack (e.g., Hydrogum 5)
Kerr (e.g., Elite HD)
3M ESPE (e.g., Impregum)
SS White: Impression Paste
Densell: algiprint 4109
Lysanda: Kerr impression compound
Desirable Characteristics of Impression Materials
Fluidity:
Must be fluid enough to flow easily and capture fine details of the oral tissues.
Viscosity:
Should be viscous enough to stay in the impression tray without prematurely flowing out, ensuring good tissue contact.
Setting Time:
Should harden into a rubbery solid within a clinically acceptable time frame (typically less than 7 minutes) after being placed in the mouth.
Dimensional Stability:
Must maintain its original shape and size without deforming or tearing after hardening and during removal from the mouth.
Reusability:
Maintain its precise dimensions over time until the cast (model) can be poured from it.
Biocompatibility:
Should not cause any harmful biological reactions or irritation to the patient's oral tissues.
Cost-Effectiveness:
Should offer a reasonable balance between its cost and the quality/benefits it provides.
Note: No single material possesses all of these characteristics (Phillips – Dental Materials, 12th ed, 2013).
Accuracy in Impression Materials
Definition: The material's capacity to precisely reproduce the intricate anatomical details of the oral tissues.
Influencing Properties:
Viscosity: Directly impacts how well the material flows into small crevices.
Wettability: The ability of a liquid impression material to flow over and intimately contact the surface of oral tissues, influenced by surface energy and contact angle. A high wettability promotes better detail reproduction.
Equation for contact angle θ:
High wettability: The liquid spreads entirely, indicating good contact. This is represented by a small contact angle, ideally \theta = 0^{ ext{o}} .
Low wettability: The liquid tends to bead up, which can trap air and prevent capturing fine details. This is represented by a large contact angle, where \theta > 90^{ ext{o}} .
Tear Resistance
Definition: The ability to resist tearing when the impression is removed from the mouth without damage (Phillips – Dental Materials, 12th ed, 2013).
Classification of Impression Materials
Based on Setting Mechanism:
Chemical Reaction (Irreversible): These materials set via an irreversible chemical change and cannot be returned to their original state (e.g., most elastomers like polysulfide, addition silicone).
Physically-Induced Reaction (Reversible): These materials undergo a physical change (e.g., cooling) to set and can be reversed back to their initial state (e.g., hydrocolloids like agar).
Mechanical Characteristics:
Elastic: Materials that can deform under stress and return to their original shape upon removal (e.g., Impression Compound, Alginate, Polysulfide, Polyether, Addition Silicone, Condensation Silicone).
Inelastic: Materials that are rigid after setting and will fracture or distort if removed from undercuts (e.g., Plaster, Zinc Oxide-Eugenol Paste).
Hydrocolloids
Definition: Colloids (materials consisting of particles dispersed in another substance) that are dissolved in water or contain a significant water component, typically setting into a gel.
Types:
Agar (Reversible): Sets via a physically induced change (cooling).
Alginate (Irreversible): Sets via an irreversible chemical reaction.
Properties of Agar
Derived from: Seaweed, specifically a type of red algae, known for its colloidal properties.
Liquefaction Temperature: Between 71^{ ext{o}}C and 100^{ ext{o}}C . This high temperature is required to transform the material from a gel to a fluid sol before use (Phillips – Dental Materials, 12th ed, 2013).
Handling and Applications of Agar Impression Material
Indications: Primarily used for duplicating casts or models due to its excellent accuracy for this purpose; less common for direct clinical impressions due to complex handling requirements. Also used in some removable partial denture impressions.
Handling: Requires a specialized three-compartment conditioning unit (for liquefaction, storage, and tempering) to prepare the material to the correct consistency and temperature for clinical use. A syringe is also needed for accurate tray loading and capturing fine details.
Advantages and Disadvantages of Agar
Advantages: Offers good accuracy and is reusable (though reusability introduces biological risks and potential for contamination).
Disadvantages: Its complex handling system, the need for specialized equipment, potential for patient burns due to high temperatures, and the requirement for storage at controlled temperatures make it less convenient for routine use.
Overview of Alginate
History: Developed during WWII when agar, primarily imported from Japan, became scarce; it is derived from alginic acid found in brown algae.
Characteristics: Easy handling, offers good patient comfort, is low cost, and requires no specialized equipment beyond basic mixing tools, making it very popular.
Applications:
Diagnostic casts (study models).
Orthodontic study models for treatment planning.
Primary impressions for custom trays or mouthguards.
Impressions for removable partial dentures.
Composition and Setting Reaction of Alginate
Setting Reaction: Forms a gel (gelation) when alginate powder mixes with water. The main reaction involves:
Components: Alginate powder (containing soluble sodium alginate, calcium sulfate, and a retarder) + water.
Reaction: Soluble sodium alginate reacts with dissolved calcium sulfate to form insoluble calcium alginate, which creates the gel network that gives the impression its form.
Retarder Incorporation Required: Sodium phosphate (Na3PO4) is added to slow down the setting reaction, providing adequate working time for the clinician to mix and load the tray (Phillips – Dental Materials, 12th ed, 2013).
Mixing Alginate
Powder/Liquid Ratio: Typically around 16 g of powder to 38 ml of water; strictly follow manufacturers’ recommendations as variations can affect material properties.
Procedure: Always add water first to the mixing bowl, then add the powder. This helps wet all the powder particles evenly and prevents dry clumps from forming, ensuring a homogeneous mix.
Dimensional Stability of Alginate
Deformation Risks: Alginate impressions are prone to dimensional changes due to two phenomena:
Syneresis: Loss of water through evaporation, leading to shrinkage of the impression.
Imbibition: Uptake of water if immersed in an aqueous solution, leading to swelling.
Both processes can cause non-linear distortion, making the impression unusable for accurate casting.
Stability Influenced by Air and Water Exposure: Stability decreases rapidly at room temperature when exposed to air (due to syneresis). Immersed in water, stability is also negatively affected (due to imbibition) (Phillips – Dental Materials, 12th ed, 2013). Therefore, impressions should be poured with gypsum and cast promptly, ideally within 10-15 minutes, to minimize distortion.
Working and Setting Time of Alginate
Regular Set: Typically 3 to 4.5 minutes.
Fast Set: Typically 1.5 to 3 minutes.
Tip: Water temperature significantly affects reaction timing; colder water lengthens working and setting times, while warmer water shortens them (approximately a 1-minute change for every 10^{ ext{o}}C difference).
Polysulfide Impression Material
Composition: Typically supplied as two pastes:
Base Paste: Contains a polymer of polysulfide, titanium dioxide (as a filler for strength and opacity), dibutyl phthalate (as a plasticizer for flexibility), and sulfur (as an accelerator).
Catalyst Paste: Contains lead dioxide (the primary reactor), oleic or stearic acid, and other fillers.
Performance: During setting, this material releases volatile by-products (like water), which contribute to its poor dimensional stability. Therefore, impressions must be poured promptly, ideally within 1 hour.
Setting Time Influenced by Temperature and Humidity: Higher temperatures and humidity accelerate the setting process.
Advantages: Offers good working and setting times, decent tear resistance, and can reproduce details accurately.
Disadvantages: Possesses an unpleasant and strong sulfurous smell, can stain clothing, and has relatively low dimensional stability due to the release of by-products.
Condensation Silicone
Composition: Typically a two-paste or paste-liquid system:
Base Paste: Composed mainly of a polymer of polydimethyl siloxane.
Filler: Usually finely divided silica (e.g., quartz, diatomaceous earth) is incorporated for strength and to control viscosity.
Catalyst: Contains stannous octoate (an activator) and tetraethyl orthosilicate (a cross-linking agent).
Performance Characteristics: Releases an alcohol by-product (e.g., ethyl alcohol) during setting, which evaporates and leads to dimensional instability. Therefore, impressions should be poured within 1 hour.
Advantages: Offers good working and setting times, decent tear resistance, and is generally odorless compared to polysulfides.
Disadvantages: Higher cost than alginate, and its dimensional stability can be compromised due to by-product release.
Addition Silicone
Composition: Typically a two-paste or putty-wash system:
Base: Contains polyvinyl siloxane and polymethyl hydrosiloxane.
Catalyst: Contains a platinum salt (as a catalyst) along with various fillers.
Advantages: Known for excellent tear resistance, very high accuracy, good working time, and notably, no volatile by-products are released during setting, contributing to superior dimensional stability. These properties make it very popular for precision work.
Disadvantages: Can be costly. A potential issue is the temporary release of hydrogen gas as a minor by-product shortly after mixing; if the cast is poured too soon (e.g., within 30-60 minutes), this gas can create bubbles in the gypsum model. This can be mitigated by waiting or using hydrogen scavengers in the material design.
Polyether Impression Material
Composition: Typically a two-paste system:
Base Paste: Contains a polymer of polyether, a plasticizer to adjust consistency, and fillers for body and strength.
Catalyst Paste: Contains an aromatic alkyl sulfonate initiator.
Unique Characteristics: Specifically developed for dental applications, known for its inherent hydrophilicity immediately after setting (meaning it's less affected by moisture during pouring). They also exhibit relatively fast setting times.
Pros and Cons:
Advantages: Offers high precision, fast setting times, and excellent rigidity once set, making it highly accurate for detail reproduction. Its hydrophilicity aids in pouring accurate casts even in slightly moist conditions.
Disadvantages: Generally higher cost than other materials. Its inherent stiffness after setting can make removal challenging in areas with severe undercuts. Also, it is sensitive to water absorption (imbibition) if stored in high humidity or immersed in water, which can lead to swelling and dimensional inaccuracy.
Summary of Material Properties and Comparisons
Tear Resistance and Rigidity Comparisons:
Elastic materials, which include agar, alginate, polysulfide, polyether, condensation silicone, and addition silicone elastomers, are designed to deform elastically and return to their original shape upon removal from the mouth.
Inelastic materials, such as Plaster and Zinc Oxide-Eugenol Paste, do not deform elastically and are used for specific applications where rigidity is paramount (e.g., edentulous ridges).
Specific measurements of working times and setting times for elastomers (approximate clinical ranges):
Polysulfide: Working time: 4-7 min, Setting time: 7-10 min.
Polyether: Working time: 3 min, Setting time: 6 min.
Condensation silicone: Working time: 2.5-4 min, Setting time: 6-8 min.
Addition silicone: Working time: 2-4 min, Setting time: 4-6.5 min (Phillips – Dental Materials, 12th ed, 2013). These times can vary based on brand and specific formulation.
Materials and Ingredients in Dental Applications
Zinc-Oxide Eugenol Paste:
Composition: Usually supplied as two pastes:
Base Paste: Primarily zinc oxide.
Catalyst Paste: Contains eugenol (oil of cloves, which is an aromatic oil), fillers (like kaolin or resins for strength), accelerators (e.g., zinc acetate), and sometimes olive oil to adjust consistency.
Setting Reaction: Zinc oxide reacts with eugenol in the presence of water (from the oral environment or catalyst paste) to form zinc eugenolate, a chelate, and water as a by-product: ZnO + H2O + 2C{10}H{12}O2 \rightarrow Zn(C{10}H{11}O2)2 + 2H_2O (Phillips – Dental Materials, 12th ed, 2013).
Indications: Primarily used for final impressions for complete dentures (especially for challenging anatomical features like flabby ridges), and also for bite registration.
Properties: Produces highly accurate and rigid impressions after setting, accurately capturing mucosal details. However, it is not recommended for impressions of highly retentive areas (e.g., presence of teeth or severe undercuts) due to its inelasticity, which would make removal difficult and potentially damage tissues or the impression itself if forced.
Bibliography: Phillips' Science of Dental Materials, 12th ed, 2013.