Dental Impressions: FCSP Module

• Dental impressions are covered in the FCSP module

GDC Learning Outcomes

• Scientific principles of dental materials and biomaterials, as well as limitations, are described
• Special investigations and diagnostic procedures (radiography) are undertaken
• Effective decontamination and infection control procedures are performed according to guidelines
• Maintaining accurate patient records according to legal requirements and best practice is important

Intended Learning Outcomes

• Ideal impression material features are described
• Uses and stages of impression taking are understood
• Different types of impression material are outlined
• Clinical properties of alginate are recognized and identified

Glossary of Terms

• Exothermic: Heat is produced when material is mixed, speeding the setting

• Hydrophilic: Likes water, absorbs or saturated by water

• Hydrophobic: Repels water

• Tear strength: Amount of resistance a material has to tearing

• Working time: Time from mixing until elastic properties develop, measured at room temperature

• Setting time: Time from mixing until fully set, measured at mouth temperature

• Dimensional stability: The degree to which a material maintains original dimensions under temperature and humidity changes, resisting distortion

• Polymerization: Chemical reaction joining small molecules (monomers) into polymers

• Viscosity: Resistance to flow or 'thickness'

• Elasticity: Material's ability to return to normal shape after stretching

• Mucostatic: Material does not displace mucosa, ideal for impressions of tissues without load (flabby ridges)

• Mucocompressive: Material displaces mucosa to record oral tissue in functional state

• Thermoplastic: Materials soften into liquid at high heat, then harden when cooled

• Wash impression: Special technique using thin layer of low viscosity material for fine details, often used in the second stage of crown and bridge work with gingival tissues retracted

Introduction to Impressions

• Impressions record the shape of teeth

• Negative imprints are created

• Teeth contours are recorded

• Variety of materials can be used

• Soft tissue reflections are captured

• Spaces and voids are accounted for

• Negative imprints lead to positive reproductions

Uses of Impressions

• Impressions are useful in orthodontics
• Impressions are useful in prosthodontics (removable/fixed)
• Impressions are useful in oral and maxillofacial
• Impressions are useful in prevention
• Impressions are useful in aesthetics

Types of Impressions

• Preliminary impressions are taken first
• Final or second stage impressions are taken last

Qualities of a Good Impression Material

• Mixing and insertion require proper proportioning, biocompatibility, and suitable working time
• Setting and removal needs short set time, low setting and thermal shrinkage, flexibility, and high tear resistance
• Storage and casting needs dimensional stability and chemical compatibility

Additional Requirements

• Nontoxic, non-irritant, clean to use, acceptable taste and smell
• Long shelf life and relatively cheap

Impression Materials: Non-Elastic

• Zinc Oxide Eugenol: Typically used as a secondary impression for complete dentures
• Impression Compound: Mix of thermoplastic resin/waxes with fillers; solid at mouth temperature, softens between 55-60°C
• Impression Plaster: (CaSO4)2.H2O with additives; good surface detail and dimensional stability
• Impression Wax: Thermoplastic, flows at mouth temperature; mainly for inlay wax

Zinc Oxide Eugenol

• Two pastes (base - white = ZnO, activator - brownish transparent = eugenol.)
• Features good surface detail, dimensional stability and shelf life
• Not for very deep undercuts
• Sets quickly in thin sections
• Might cause an eugenol allergy

Impression Compound

• Suitable working time that can be modified by reheating
• Good shelf life and mucocompressive properties
• Poor surface detail, high coefficient of thermal expansion and dimensional stability
• Distrots when removed in undercut areas

Plaster

• Good surface detail and excellent dimensional stability
• Quick setting
• Exothermic
• Setting expansion
• Fractures with undercuts
• Taste and roughness may cause vomiting

Wax

• Mainly used for inlay wax, and bite registration too
• Readily distorted

Impression Materials: Elastic

• Synthetic Elastomers: Polysulphide, Polyether, Addition Silicone, Condensation Silicone.
• Hydrocolloids: Agar-agar (reversible), Alginate (irreversible)

Hydrocolloids Defined

• Colloid: Two or more substances with one suspended within another
• Hydrocolloid: Dispersion in water, featuring a sol (liquid) and a gel (semisolid) phase

Hydrocolloid Types

• Agar is derived from seaweed, reversible with heating/cooling, and has poor dimensional stability.
• Alginate is derived from seaweed, irreversible once set, and has poor dimensional stability.
• Accuracy requires quick casting

Alginate Composition

• Potassium alginate: Derived from seaweed, it acts as a thickening agent (e.g., in ice cream).
• Calcium sulfate: Reacts with potassium alginate to form the gel.
• Trisodium phosphate: Slows down reaction time for mixing.
• Fillers: Diatomaceous earth (broken glass fragments) and zinc oxide.

Alginate Reactions

• K alginate + CaSO4 • 2H2O = Ca alginate gel + K2SO4

Alginate Advantages

• Elastic and nontoxic
• Non-irritant
• Produces good surface detail
• Easy to mix and use with a cheap price
• Good shelf life (approx. 1 year)
• Setting time can be controlled

Alginate Disadvantages

• Poor dimensional stability
• Incompatible with some dental stones
• Messy
• Storage moisture and temperature
• Has setting time/operator handling issues
• Tears easily

Impression Trays

• Stock trays are ready-made in different sizes made of metal or plastic
• Trays are perforated or solid

Custom Trays

• Accurate
• More comfortable
• Less material used
• Material is more even in thickness for accuracy

Impression Tray Types

• Three types: full arch (whole arch), sectional (anteriors), quadrant (covers one half of arch)

Impression Tray Properties

• Tray carries impression material into the oral cavity
• Holds material close to the teeth
• Avoids breaking during removal
• Prevents warping of the completed impression

Adhesives

• Essential for impression material to stick onto tray
• Various adhesives for different materials
• Alginate adhesive: 10–12% toluene in 45–50% isopropanol
• Maximum adhesion achieved with adhesive and perforated trays

Tray Selection

• Should be comfortable for the patient
• Should extends slightly beyond facial surfaces of teeth
• Should extend 2-3 mm beyond the third molar/retromolar/tuberosity area
• Deep enough for 2-3 mm material between the tray and incisal/occlusal edges, extending into the sulcus

Adhesive Placement

• Apply two thin coats
• Dry between coats
• Do not place adhesive on pooled adhesive
• Apply to internal surface and over margins for binding

Stages and Considerations

• Obtain signed treatment plan and consent
• Consider medical history and allergic reactions
• Use fully personal protective equipment

Stages and Considerations Continued

• Ensure patient has the ability to open mouth widely with no gag reflex or retching
• Ensure patient is positioned correctly and clinician has correct posture

Alginate Placement Procedure

• Seat patient upright
• Lower impression: clinician in front of patient, behind the patient for the upper impression
• Check tray size
• Place adhesive and have alginate and water ready
• Mix with no air bubbles and load tray
• Place tray in mouth sideways, reassure and remove

Alginate Procedure Continued

• Check for adequacy (repeat if needed), disinfect, and wrap in moist tissue/gauze
• Ensure paperwork is filled

Recent Developments in Impressions

• Digital impressions using scanners
• Such as iTero or Cerec (CEramic REConstruction) using CAD/CAM.
• This method is used for intra-oral photography, scanning arches and surrounding tissues
• Produces a model

Assessment

• Formative Assessment
• Summative Assessment
• Practical task

This lesson covers dental impressions within the FCSP module. It describes ideal impression material features, uses, stages, and different types of impression materials. Clinical properties of alginate are recognized and identified.