Impression Materials

Chapter 8

Copyright © 2017 by Elsevier, Inc. All rights reserved.

Learning Objectives

  • Lesson 8.1: Impression Materials and Alginate Powders
    1. Describe the function of an impression material.
    2. Describe the relationship between a tooth, an impression of the tooth, and the die.
    3. List the requirements for an ideal impression material.
    4. List the components in an alginate powder and describe their function.
    5. Describe the properties of alginate substitute impression materials.

Impression Materials Overview

  • Function: The primary function of impression materials is to accurately record the dimensions of oral tissues and their spatial relationships.
    • A material in a plastic state is applied to the oral tissues, and upon setting, the impression is removed to create a replica of these tissues.
  • Negative and Positive Reproductions:
    • The impression provides a negative reproduction of oral tissues.
    • A positive reproduction is achieved by pouring dental stone or another suitable material into the impression, which, when hardened, is referred to as a model (when large areas are involved) or a die (when single/multiple tooth preparations are recorded).

Ideal Impression Material Properties

  • Requirements for an Ideal Material:
    • Ease of manipulation and reasonable cost: It should be user-friendly and affordable.
    • Adequate flow properties: Should flow easily to capture details.
    • Appropriate setting time and characteristics: Should set within a predictable time frame.
    • Sufficient mechanical strength: Must resist tearing or permanent deformation upon removal from the mouth.
    • Good dimensional accuracy: Should maintain the correct dimensions after setting.
    • Acceptability to the patient: Should be non-irritating and easy to use for the patient.
    • Safe: Must not pose any health risks or complications.
    • No significant degradation with disinfection: Should maintain properties upon sterilization or disinfection procedures.
    • Compatibility with die and cast materials: Should work well with all materials used to create casts and dies.
    • Good shelf life: Should remain usable over time when stored correctly.

Classification of Impression Materials

  • Impression materials can be classified based on:
    1. Setting Mechanism:
    • Chemical reaction-based setting
    • Temperature change-based setting
    1. Flexibility:
    • Flexible materials: Most commonly used due to their ease of removal.
    • Rigid materials: Used where undercuts do not exist and can be safely applied for full crowns or edentulous arches.

Alginate Hydrocolloid Impression Material

  • Overview: Alginate hydrocolloid is among the most commonly used dental impression materials.
  • Advantages:
    • Ease of mixing and manipulation.
    • Requires minimal equipment.
    • Flexible when set.
    • Accuracy if properly handled.
    • Generally low cost.
  • Disadvantages:
    • Low tear strength compared to other materials.
    • Less effective in transferring surface detail than elastomeric impressions.
  • Uses:
    • Preparing study models.
    • Creating gypsum models for athletic mouth protectors.
    • Not recommended for cavity preparation impressions.

Packaging and Composition of Alginate

  • Packaging: Supplied as powder in either bulk or preweighed individual packets. Equipment includes:
    • A plastic scoop for bulk powder.
    • A plastic cylinder for measuring water.
    • A wide-bladed, stiff spatula for mixing.
  • Composition:
    • Sodium or potassium alginate salt: Dissolves in water to form a gel.
    • Calcium sulfate: Reacts with dissolved alginate to form insoluble calcium alginate.
    • Sodium phosphate: Acts as a retarder by preferentially reacting with calcium sulfate.
    • Diatomaceous earth or silicate powder: Controls consistency and flexibility.
    • Potassium sulfate or potassium zinc fluoride: Counteracts inhibition of gypsum model material by alginate.
    • Organic glycol: Minimizes dispensing dust on powder particles.
    • Pigments: Provide color to the powder.
    • Quaternary ammonium compounds or chlorhexidine: For self-disinfection.
    • Aspartame: Acts as a sweetener.

Setting Mechanism of Alginate

  • Simplified Reaction:
    ( ext{Sodium Alginate} + ext{CaSO}4 ullet ext{H}2 ext{O}
    ightarrow ext{Calcium Alginate} ext{(gel)} + ext{Na}^+ + ext{SO}4^{2-} + ext{H}2 ext{O})
  • The amount of sodium phosphate controls the setting time.
  • After sodium phosphate reacts with soluble calcium ions, sodium alginate reacts with remaining calcium ions, leading to the precipitation of calcium alginate, forming a fibrous network that occupies spaces filled with water, yielding a gel termed alginate hydrocolloid, or more commonly known as irreversible hydrocolloid.

Properties of Alginate Hydrocolloid Impression Material

  • Mixing and Setting Times:

    • Normal-set alginate: Should develop a smooth creamy consistency in < 1 minute; sets within 2 to 4.5 minutes after mixing begins; workable up to 2 minutes.
    • Fast-set alginate: Should set within 1 to 2 minutes; workable for at least 1.25 minutes; mixing time of 30 to 45 seconds.
    • Temperature effects: Increased water temperature decreases working and setting times; thinner mixes increase setting time; acceptable setting times range from 2.5 to 5 minutes.

  • Elastic Recovery:

    • ANSI-ADA specifications require at least 95% elastic recovery with no more than 5% permanent deformation; many commercial alginates meet 96% to 98% elastic recovery.

  • Flexibility:

    • ANSI-ADA specification limits compression at the time of model preparation to between 5% to 20%.
    • Water and powder ratio influences flexibility.

  • Strength:

    • The strength depends on the rate of deformation during removal; rapid removal increases tear and compressive strengths.
    • Strength can be increased by using thicker mixes, though it may affect flowability.
    • Delaying removal can also enhance strength, albeit with limitations.

  • Dimensional Change:

    • Accuracy diminishes with extended storage as alginate is a hydrocolloid gel containing substantial water, which can either evaporate or be absorbed by the impression, causing shrinkage or expansion, respectively.
    • Alginate can shrink under 100% relative humidity conditions due to syneresis, but can be stored under such conditions for approximately an hour without significant dimensional changes.

  • Reproduction of Detail:

    • The impression material must effectively record oral tissue detail, ensuring this detail transfers to the resulting model or die accurately.

  • Disinfection:

    • Guidelines recommend rinsing and disinfecting all impressions using disinfectants such as sodium hypochlorite, iodophor, glutaraldehyde, and phenylphenol solutions.
    • Some manufacturers include disinfectants in alginate powder to enhance safety.
    • Studies show immersion for 30 minutes did not affect alginate clinical accuracy.

Learning Objectives - Lesson 8.2: Alginate Impressions, Disinfection, and Impression Systems

  1. List the five objectives for alginate impressions.
  2. List the factors to be considered when selecting a tray for impressions.
  3. Describe tray modifications for alginate impressions.
  4. Detail proper dispensing and mixing of alginate.
  5. Explain how to load alginate into the tray.

Objectives for Taking Alginate Impressions

  • Must record:
    • All teeth in the upper and lower arch.
    • Entire alveolar process.
    • Retromolar area of lower arch.
    • Area of hamular notch in upper arch.
    • A detailed, undistorted, and bubble-free reproduction of oral tissues.

Selection of the Tray

  • Maxillary Tray Requirements:
    • Must cover the tuberosity completely.
    • Should be 4 mm wider than the most apical part of the alveolar process at molar regions.
    • Must cover anterior teeth allowing incisors to contact the flat arch portion about 4 mm from the raised palatal part of the tray.
  • Mandibular Tray Requirements:
    • Must cover all teeth and the retromolar pad.
    • Should be 4 mm wider than both buccal and lingual portions of posterior teeth and labial and lingual portions of anterior teeth.
    • Must allow teeth to be centered correctly while complying with previous requirements.

Modification of the Tray

  • Soft rope wax can be added to the anterior labial flange of the tray to ensure that alginate flows properly into the labial vestibule upon seating.
  • Rope wax may also be added to other tray borders to provide adequate support for alginate extensions.

Dispensing the Alginate

  • Recommended water amount should be measured using a liquid dispensing vial into a rubber bowl at room temperature.
  • Powder should be fluffed before opening; a plastic scoop is used to ensure a full measure, and excess is scraped off with the spatula before sifting into the water in the bowl.

Mixing the Alginate

  • A stiff, wide-bladed spatula is used to mix alginate powder with water, ensuring a smooth, creamy consistency.
  • Regular-set alginate requires about 45 seconds of spatulation, while fast-set requires approximately 30 seconds.

Loading the Tray

  • Use a spatula to transfer mixed alginate from the bowl into the tray, recommended to start from the posterior and push forward.

Making the Impression

  • For Lower Arch Impressions:

    • Push the left corner of the mouth to the side with the tray.
    • Insert the tray with a rotary motion, starting from the left heel, then anterior, and finally the right heel.
    • Instruct the patient to lift their tongue and gently pull the lip down to allow alginate flow into vestibules during seating.
    • The incisal edges and cuspal tips should be about 1-2 mm from the metal surface of the tray at final seating.
    • Hold the tray until alginate sets, approximately 1 minute after losing tackiness.

  • For Upper Arch Impressions:

    • Rotate the filled tray counterclockwise into the mouth while retracting the right cheek.
    • Seats the posterior border first, ensuring excess alginate flows labially or buccally, avoiding the palatal direction.
    • Hold the tray until the alginate sets, breaking the peripheral seal to remove it with a rapid, firm pull.

Steps after Making the Impression

  • Rinse impression with cool water, cut away unsupported alginate, and disinfect impression.
  • Rinse disinfected impression with cold water and remove excess water before pouring gypsum model, or if short-term storage is needed, wrap loosely in a wet paper towel or seal in a plastic bag.
  • Store the impression with the tray supporting the alginate.

Troubleshooting Alginate Impressions

  • If alginate sets too quickly:

    • Incorrect powder-liquid ratio.
    • High ambient temperature.
    • Excessive time spent loading the tray.
    • Recommended solution: cool the mixing water.

  • If voids appear in critical areas:

    • Squeeze alginate initially to wet thoroughly, ensuring it flows correctly into all tray sections.
    • Alginate can be added at one point and allowed to flow to ensure even coverage.
    • Apply alginate to occlusal surfaces before inserting the tray.

  • If excessive material appears in the posterior but not in the vestibule:

    • Ensure the posterior portion of the tray is seated first.
    • Make sure an adequate amount of alginate is loaded in the tray.
    • Pull the lip to help alginate flow into vestibule.

Alginate Substitute Impression Material

  • Overview: Designed to replicate the characteristics of alginate materials, often comprising addition silicone.
    • Compared to alginates, substitutes offer:
    • Improved dimensional stability.
    • Enhanced accuracy.
    • Better elastic recovery.

Elastomeric Impression Materials

  • Predominantly used for impressions, except in study model preparations, featuring:
    • Greater dimensional stability over time.
    • Superior detail recording ability.
    • Compatibility with a broader selection of model and die materials.
    • Types include: polysulfides, condensation silicones, polyethers, and addition silicones.

Addition Silicone Impression Materials

  • Known as vinyl polysiloxanes (VPS), were developed to replace polysulfides and condensation silicones.
  • Composition and Setting Reaction:
    • Supplied in a two-paste system (base and catalyst) mixed in equal volumes that react without producing byproducts.
  • Properties:
    • Reduced dimensional change and higher elastic recovery compared to previous materials.
    • Short working time and limited flexibility.
  • Manipulation:
    • Base and catalyst are extruded using a mixing gun for uniformity and ease of use.

Polyether Impression Materials

  • These exhibit better mechanical properties than polysulfides and less dimensional change than condensation silicones, though they are hydrophilic.
  • Composition and Setting Reaction:
    • Comprised of a base (polyether) and aromatic sulfonic acid ester catalyst resulting in cross-linked rubber.
  • Properties:
    • Shortest working time among rubber impression materials, high viscosity, moderate permanent deformation, and relatively low flexibility.

Digital Impressions

  • Various digital impression systems are available, replacing traditional elastomeric systems.
  • Utilizing CAD-CAM technology to create precise, customized restorations more efficiently and with fewer remakes.

Conclusion

  • Understanding the properties, applications, mixing, and setting times of various impression materials is crucial for successful dental practices. This knowledge helps ensure accurate impressions, contributing to effective treatment outcomes in dentistry.