Comprehensive Study Notes on Impression Materials: Alginate and Elastomers

Overview and Indications of Irreversible Hydrocolloid (Alginate)

• Alginate, also referred to as alginate hydrocolloid or irreversible hydrocolloid, is the most widely used impression material in dentistry.
• Key characteristics of alginate include:
  • Inexpensive cost
  • Ease of manipulation
  • No requirement for special equipment
  • Reasonable accuracy for numerous dental procedures
• Clinical applications and indications for alginate impressions:
  • Diagnostic casts
  • Partial denture frameworks
  • Repairs of broken partial or complete dentures
  • Fabrication of provisional restorations
  • Fluoride and bleaching trays
  • Sports protectors
  • Preliminary impressions for edentulous arches
  • Removable orthodontic appliances
• Contraindications and limitations of alginate:
  • Alginate is not accurate enough to produce final impressions for inlay, onlay, crown, and bridge preparations.
  • It fails to capture the fine detail of the preparation required for a precise restoration fit.
  • It is thick and lacks sufficient flow into embrasures or occlusal surfaces.
• Reversible Hydrocolloid (Agar) background:
  • Use of reversible hydrocolloid has declined over time due to the introduction of elastic (rubber) impression materials.

Chemical Composition and Setting Reaction of Alginate

• Alginate is manufactured from derivatives of seaweed, with sodium or potassium alginate acting as the primary active ingredient.
• Proportions of ingredients vary by manufacturer and depend on whether the material is fast-, regular-, or slow-set.
• Components of alginate impression material and their functions (Table 5.1):
  • Sodium or potassium alginate ($15\%$ to $20\%$): Provides colloidal particles that form the structural basis of the gel.
  • Calcium sulfate dihydrate ($14\%$ to $20\%$): Reacts with sodium alginate to create an insoluble gel.
  • Potassium sulfate ($10\%$): Ensures the proper setting of gypsum materials poured into the impression.
  • Trisodium phosphate ($2\%$): Acts as a retarder to delay the setting reaction and control setting time.
  • Diatomaceous earth ($55\%$ to $60\%$): Functions as a filler to increase stiffness and strength while preventing the surface from becoming sticky.
  • Other additives present in very small quantities:
    • Organic glycols or glycerin: Added to dustless powders to prevent silica particles from becoming airborne during dispensing.
    • Flavoring agents: Added to improve the taste of the material.
    • Coloring agents: Added to provide pleasant visual colors.
    • Disinfectants: Added to impart antibacterial action.
• Health Hazard Caution: Dispensing and mixing alginate requires wearing a mask. Alginate dust contains silica particles from diatomaceous earth fillers (silicon dioxide), which present a serious respiratory hazard if inhaled. Dustless alginate formulations minimize but do not entirely eliminate this risk.
• Chemical Setting Mechanism:
  • When alginate powder is mixed with water, calcium sulfate dihydrate reacts with sodium alginate to form calcium alginate.
  • Calcium alginate is insoluble and precipitates the conversion of the mixed sol (liquid) into a solid gel.
  • Because gelation occurs via a chemical reaction, it cannot be reversed back to the sol state, making it an irreversible hydrocolloid.
• Color-changing indicators: Certain manufacturers incorporate chemical indicators that change color as the chemical reaction progresses, signaling the exact time to insert the impression into the mouth and changing color again when gelation is complete and ready for removal.

Working Time, Setting Time, and Temperature Manipulation

• Working Time parameters (begins from initial mixing of materials until seating in the mouth):
  • Regular-set alginate: $2\text{ to }3\,\text{minutes}$.
  • Fast-set alginate: $1.25\text{ to }2\,\text{minutes}$ (American Dental Association / ADA Specification No. 18 establishes the absolute minimum working time at $1.25\,\text{minutes}$).
  • The longer the time spent mixing the alginate, the faster it must be loaded into the impression tray and seated in the patient's mouth.
• Setting Time parameters:
  • Regular-set alginate: Sets in $2\text{ to }5\,\text{minutes}$.
  • Fast-set alginate: Sets in $1\text{ to }2\,\text{minutes}$.
• Temperature Controls:
  • Setting time can be lengthened by using cold water or shortened by using warm water.
  • Environmental air temperature and oral cavity temperature also alter working and setting times.
  • Altering the powder-to-water ratio to adjust set time is strongly discouraged because it adversely affects the physical and mechanical properties of the impression material.
• Clinical Tip for Sensitive Teeth:
  • Mixing alginate with cold water can induce pain in patients with tooth sensitivity.
  • In these patients, utilize regular-set alginate mixed with warm water. This shortens working and setting times while maintaining patient comfort.

Physical and Mechanical Properties of Alginate

• Permanent Deformation:
  • Alginate compresses as it is pulled past undercuts in the mouth. Higher compression increases the likelihood of permanent deformation.
  • A minimum thickness of $2\text{ to }4\,\text{mm}$ of alginate is required between the impression tray and the teeth or tissue undercuts; alginate that is too thin deforms excessively and tears easily.
  • Removal technique: The impression must be removed from the mouth with a rapid, quick "snap" to prevent permanent deformation of critical surfaces.
  • Strain Recovery: Allowing $8\text{ to }10\,\text{minutes}$ to elapse between removal from the mouth and pouring the model permits elastic recovery (rebound) from compression. Unrecovered deformation causes permanent distortion in the poured gypsum cast.
  • Standard disinfection procedures require at least $10\,\text{minutes}$, during which time most elastic rebound occurs naturally.
• Dimensional Stability:
  • Alginate is highly susceptible to moisture loss, which causes shrinking.
  • Immediate post-removal handling protocol:
    1. Rinse and disinfect the impression (Procedure 5.5).
    2. Wrap in a damp (not dripping wet) paper towel.
    3. Seal inside a zippered plastic bag.
    4. Alternative method: Place a few drops of water inside the zippered plastic bag alongside the impression.
  • Warning: Alginate will imbibe water and swell if wrapped in a towel that is excessively wet.
  • Enclosing the impression creates a $100\%$ relative humidity environment to minimize water loss.
  • Syneresis: A process inherent to gels where standing causes contraction and forces liquid out onto the gel surface, forming a wet film. Syneresis alters material properties even in $100\%$ humidity environments.
  • Pouring recommendations: Impressions should ideally be poured immediately after disinfection. If storage is required for a few hours, it must remain in $100\%$ humidity. Delayed pouring increases the likelihood of distortion.
• Tear Strength:
  • Tear strength is more clinically critical than compressive strength because commercial alginates far exceed minimum requirements for compressive strength.
  • Factors that weaken alginate tear strength:
    • Excessive water in the mix ratio.
    • Thin sections of alginate material.
    • Slow removal rate from the oral cavity.
  • Methods to increase tear strength:
    • Leaving the impression in the mouth for $1\,\text{minute}$ beyond initial gelation significantly increases tear strength and undercut rebound.
    • Addition of silicone polymers by select manufacturers.

Tray Selection, Preparation, and Customization

• Tray Types and Compatibility:
  • Stock trays (metal or plastic) work well by accommodating adequate material thickness ($2\text{ to }4\,\text{mm}$).
  • Alginate must adhere tightly to the tray; detachment causes severe distortion.
  • Perforated and solid trays are both acceptable.
  • Disposable plastic trays must be completely rigid; flexible plastic trays distort under the heavy weight of wet gypsum or when pulled past anatomical undercuts.
  • Custom acrylic trays can be fabricated on edentulous casts (Fig. 5.3).
• Anatomical Criteria for Full-Arch Tray Selection:
  • Must cover all teeth completely.
  • Must extend into facial and lingual vestibules without impinging on soft tissues.
  • Must extend posteriorly to include the retromolar area for mandibular trays and the hamular notch area for maxillary trays.
  • Must provide at least $2\,\text{mm}$ of space for alginate beyond incisal and occlusal surfaces.
  • Must provide approximately $5\,\text{mm}$ of alginate clearance between tray walls and oral tissues.
• Tray Modification with Utility Wax (Rope Wax):
  • Utility wax can be added to tray borders to extend coverage in individuals with large jaws (e.g., third molar region).
  • Utility wax can be added to the mid-palatal region of the tray to support heavy alginate in patients with a deep palatal vault (Procedure 5.1).
• Patient Preparation:
  • Patients should rinse with water to clear loose debris and thick saliva prior to impression placement.
  • An antimicrobial mouth rinse may be used to lower the concentration of oral pathogens.

Dispensing and Mixing Procedures

• Dispensing Measures:
  • Always utilize manufacturer-specific scoops for powder and cylinders for water. Proportions and measure sizes differ between brands and must never be interchanged.
  • If a clinic utilizes multiple alginate brands, color-code the measuring scoops and cylinders to avoid cross-mixing.
• Fluffing Powder:
  • Powder settles and compacts tightly during shipping and storage, causing ingredients to separate.
  • Compacted powder yields an excessively dense scoop, producing a mix that is too thick and sets prematurely.
  • Containers (cans or plastic drums) must be rotated end-over-end several times to decompress (fluff) the powder before dispensing.
• Prepackaged Alginate:
  • Supplied in premeasured, watertight foil packets containing a quantity suitable for a medium arch (equivalent to $2\,\text{scoops}$ with most brands, Fig. 5.4).
  • Prepackaged units offer superior mix consistency and minimize cross-contamination, though at a higher unit cost.
• Proportions and Scoop Guidelines:
  • Water ratio: Exactly $1\,\text{unit}$ of water per $1\,\text{scoop}$ of powder.
  • Upper adult arch:
    • Large tray: $3\,\text{scoops}$
    • Medium tray: $2\text{ to }3\,\text{scoops}$
    • Small tray: $2\,\text{scoops}$
  • Lower adult arch:
    • Large tray: $2\text{ to }3\,\text{scoops}$
    • Medium tray: $2\,\text{scoops}$
    • Small tray: $2\,\text{scoops}$
• Mixing Protocol:
  1. Measure water and place it into a clean, flexible mixing bowl first.
  2. Add the measured alginate powder to the water.
  3. Stir powder gently into water until fully wetted.
  4. Aggressively press and wipe the wetted mix against the inner sides of the mixing bowl using a wide-bladed spatula (or use an automated mechanical mixing machine, Fig. 5.5).
  5. Forcing the material against bowl walls incorporates all powder and expels entrapped air bubbles.
• Mixing Durations and Consistency:
  • Regular-set alginate: Complete spatulation within $45\,\text{seconds}$.
  • Fast-set alginate: Complete spatulation within $30\,\text{seconds}$.
  • Finished mix appearance: Must possess a smooth, creamy consistency. A grainy appearance indicates incomplete mixing.

Loading, Seating, and Impression Handling

• Tray Loading Procedure:
  • Gather mixed alginate onto the spatula and force it into the deepest aspects of the tray to expel air pockets.
  • Load the tray in large increments as rapidly as possible to prevent air entrapment.
  • Fill the tray until material is flush and even with the top edges of the tray borders.
  • Moisten a gloved finger with water to smooth the alginate surface and create a shallow trough along the tooth ridge line, aiding alignment and reducing air pockets.
• Pre-Seating Steps and Seating Technique:
  • Wipe a small quantity of residual alginate from the bowl onto the occlusal surfaces, embrasures, deep vestibules, or palatal vault with a gloved index finger before inserting the tray.
  • Retract the patient's lip outward.
  • Seat the tray from posterior to anterior. Seating the posterior region first forces excess material forward into the anterior vestibule rather than backward into the throat.
• Stabilization:
  • Hold the impression tray steady without movement throughout the entire setting reaction.
  • Never permit the patient to hold the impression tray.
• Removal and Post-Impression Care:
  • Verify complete set by evaluating the residual alginate left in the mixing bowl.
  • Allow impression to remain in the mouth for $1\,\text{additional minute}$ after gelation to maximize tear strength.
  • Break the tissue seal and remove rapidly with a sudden "snap".
  • Rinse, disinfect, wrap, and seal impression as required.
  • Clean all residual alginate from the patient's teeth, oral cavity, and facial skin.

Comprehensive Troubleshooting for Alginate Impressions

• Problem: Premature set
  • Causes: Too much powder in mix; prolonged mixing or loading time; water or room temperature too warm.
  • Solutions: Fluff powder in container prior to dispensing; use exact powder/water measures; use a timer to track working time; use cool water to retard the set.
• Problem: Slow set
  • Causes: Water temperature too cold; excessive water in mix.
  • Solutions: Use warmer water; adhere to correct water and powder measures.
• Problem: Grainy mix lacking surface detail
  • Causes: Incomplete mixing of powder and water.
  • Solutions: Ensure all powder is wetted, then spatulate aggressively to a creamy consistency.
• Problem: Incomplete coverage of teeth or soft tissues
  • Causes: Selected tray too small or short for arch; incomplete seating of tray.
  • Solutions: Select a larger tray size or extend tray borders with utility rope wax; check complete tray seating visually using a mouth mirror.
• Problem: Voids on occlusal surfaces
  • Causes: Air trapped under tray during seating.
  • Solutions: Pre-wipe fresh alginate directly onto occlusal surfaces prior to seating the tray.
• Problem: Large voids at the vestibule or mid-palate region
  • Causes: Trapped air; insufficient volume of alginate loaded into tray.
  • Solutions: Place alginate directly into the vestibule or palatal vault before seating tray; load adequate alginate volume into tray.
• Problem: Small voids distributed throughout impression
  • Causes: Air incorporated during manual spatulation.
  • Solutions: Firmly wipe alginate against bowl walls with wide-blade spatula during spatulation to force out entrapped air.
• Problem: Distortion or double imprint
  • Causes: Improper seating direction; lip caught under tray; impression removed too soon; tray movement during gelation.
  • Solutions: Seat posterior border first, allowing flow forward; pull lip outward out of the way; evaluate residual material in bowl and wait $1\,\text{additional minute}$ post-set; hold tray completely motionless without allowing patient assistance.
• Problem: Torn alginate material
  • Causes: Slow removal from oral cavity; excessively thin mix.
  • Solutions: Remove impression rapidly with a quick snap; utilize correct powder-to-water proportions.
• Problem: Excess alginate overflowing back of tray
  • Causes: Tray seated anterior first then posterior (forcing mass backward); overfilling tray.
  • Solutions: Seat posterior border first to direct flow anteriorly; load tray level with sides; create shallow trough for ridge; scrape off excess alginate from posterior tray wall prior to insertion.

Overview and Chemistry of Elastomeric Impression Materials

• General Characteristics:
  • Elastomers are highly accurate, elastic impression materials possessing rubber-like qualities (frequently called rubber impression materials).
  • Extensively utilized in restorative dentistry for fabricating:
    • Metal castings
    • Ceramic restorations
    • Bridges
    • Implant restorations
    • Partial denture frameworks
    • Complete dentures
• Four Chemical Types of Elastomers:
  1. Polysulfides
  2. Condensation silicones
  3. Addition silicones (polyvinyl siloxane / PVS)
  4. Polyethers
• Most widely used elastomers: Polyvinyl siloxane (PVS) and Polyether.
• Hybrid Formulation: Vinyl polyether combines the favorable characteristics of polyethers and polyvinyl siloxane.
• Matrix and Stability Properties:
  • Elastomers feature a flexible matrix composition that is non-water-based.
  • Unlike hydrocolloids, elastomers are not sensitive to water loss or imbibition (water uptake).
• Shelf Life and Storage Requirements:
  • Standard shelf life ranges from $12\text{ to }18\,\text{months}$.
  • Refrigeration extends product shelf life.
  • Refrigerated materials must return to room temperature prior to clinical use, unless an extended working time is explicitly desired.
• Tray Adhesives:
  • Rubber impression materials do not adhere naturally to solid metal stock trays or custom acrylic trays.
  • Tray adhesive must be applied to prevent material separation from the tray, which leads to severe distortion.
  • Each elastomeric chemical class requires its own specific tray adhesive; adhesives must never be interchanged between different elastomer types.
  • Tray adhesive must be allowed to dry completely before loading the tray with impression material.
• Elastic Recovery and Wettability:
  • Elastomers feature high elastic recovery (rebound).
  • Wettability varies among formulations: polyethers are hydrophilic (easily wetted by water or saliva), whereas addition silicones are hydrophobic unless modified with surfactants.