Dental Impressions: FCSP Module

• Dental impressions are crucial in dentistry; they are taught by Mrs. Priti Songra in the FCSP module.

GDC Learning Outcomes

• Focuses on scientific principles underpinning dental materials and biomaterials usage, limitations, and selection.
• Covers special investigations and diagnostic procedures like radiography.
• Includes implementing effective decontamination and infection control following current guidelines.
• Stresses maintaining accurate patient records in accordance with legal requirements and best practices.

Intended Learning Outcomes

• Learning outcomes consist of describing ideal impression material features.
• Learning outcomes consist of understanding the uses of impressions, including the stages for dental patients.
• Learning outcomes consist of outlining available impression material types.
• Learning outcomes consist of recognizing alginate's clinical properties

Glossary of Terms

• Exothermic: Heat produced upon mixing, accelerating setting.
• Hydrophilic: Attracts, absorbs, or saturates with water.
• Hydrophobic: Repels water.
• Tear strength: Material's resistance to tearing.
• Working time: Time from mixing until elastic properties develop, usually at room temperature.
• Setting time: Time from mixing until fully set, typically measured in the mouth.
• Dimensional stability: Ability to maintain 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: Ability to return to original shape after stretching.
• Mucostatic: Material that doesn't displace mucosa; suited for impressions of tissues without load, especially flabby ridges.
• Mucocompressive: Material causing mucosa displacement, to record oral tissue in a functional state.
• Thermoplastic: Materials softening to a liquid in high heat, hardening again when cooled.
• Wash impression: Thin layer of low viscosity material for fine details, often a second stage for crown and bridge work.

Introduction

• Dental impressions record the shape of teeth.
• Dental impressions create a negative imprint.
• Dental materials can be made of a variety of materials
• Dental impressions recreate teeth contours, soft tissue, spaces, and voids
• Dental impressions lead to a positive reproduction

Uses of Impressions

• Impressions are used in orthodontics, prosthodontics.
• Impressions are used in oral maxillofacial surgery, prevention, and aesthetics.
• Prosthodontics uses removable and fixed dental impressions
• There are preliminary impressions.
• Impressions can be final or second stage impressions

Qualities of a Good Impression Material

• Should be biocompatible.
• Should be dimensionally stable.
• Should be chemically compatible and flexible.
• Should be easy to mix and insert, also be a good value
• Should resist distortion when casting.
• The material should be non-toxic, non-irritant, clean to use, acceptable taste and smell, long shelf life, and relatively cheap.

Types of Impression Materials: Non-Elastic

• Non Elastic materials includes zinc oxide eugenol, impression compound, impression plaster, and impression wax.

Non-Elastic Impressions: Zinc Oxide Eugenol

• Two pastes: a white base (ZnO) and a brownish-transparent activator (eugenol).
• Typically a secondary impression material for complete dentures.
• Possesses good surface detail and dimensional stability.
• Has a good shelf life, but not for very deep undercuts.
• It sets quickly in thin sections and can cause eugenol allergy.

Non-Elastic Impressions: Impression Compound

• Mix of thermoplastic resin/waxes with fillers.
• Solid at mouth temperature, softens between 55-60°C.
• Mucocompressive with good shelf life.
• It requires suitable setting time and can be modified by reheating.
• Has poor surface detail with a high coefficient of thermal expansion.
• It distorts when removed, especially in undercut areas, and has poor dimensional stability.

Non-Elastic Impressions: Plaster

• Composed of (CaSO4)2.H2O with accelerators, retarders, and coloring agents.
• Exhibits good surface detail and excellent dimensional stability.
• Quick setting but exothermic, causing setting expansion.
• It fractures with undercuts, and its taste and roughness may cause vomiting.

Non-Elastic Impressions: Impression Wax

• Thermoplastic, flows at mouth temperature; soft at room temperature.
• Mainly used for inlay wax, either directly in the mouth or more widely in the lab.
• Used for bite registration but readily distorted.

Types of Impression Materials: Elastic

• This includes hydrocolloids as well as synthetic elastomers
• Hydrocolloids can be reversible like, Agar-agar; they can also be irreversible, like Alginate
• Synthetic Elastomers include Polysulphide, Polyether, Addition Silicone, and Condensation Silicone

Hydrocolloids

• Are composed of two or more substances where one doesn't dissolve but is suspended in another.
• It is a dispersion in water, with two phases: a liquid (sol) and a semisolid (gel).

Hydrocolloids: Agar

• Derived from seaweed.
• Reversible sol/gel via heating/cooling.
• Hydrophilic with poor dimensional stability.

Hydrocolloids: Alginate

• Derived from seaweed.
• Irreversible: once set, cannot revert.
• Hydrophilic with poor dimensional stability.
• Casting quickly is key for accuracy.

Alginate Composition

• Contains potassium alginate
• Contains calcium sulphate
• Includes trisodium phosphate: it slows down reaction time for mixing.
• Includes fillers like diatomaceous earth (broken glass fragments) and zinc oxide.
• Calcium sulphate reacts with the potassium alginate to form the gel

Properties of Alginate: Advantages

• Elastic and non-toxic.
• Non-irritant with good surface detail.
• Has ease of mix and use.
• Is a cheap and setting can be controlled somewhat
• Has a shelf life of approximately 1 year

Properties of Alginate: Disadvantages

• Alginate has poor dimensional stability and incompatibility with some dental stones.
• Its setting time/operator handling can be difficult, messy, and storage issues.
• It can tear easily
• Storage can be an issue with moisture and temp

Impression Materials: Trays

• Stock trays are ready-made in different sizes.
• Stock trays can be metal or plastic, perforated or solid.
• Custom trays are more accurate, comfortable, and uses less material.
• Custom trays have more even material thickness for accuracy

Impression Trays: Types

• Includes full arch, sectional, and quadrant.
• Should carry impression material into the oral cavity and hold material close to the teeth.
• Functions to avoid breaking during removal; preventing warping of the completed impression

Adhesives

• Are essential for impression material to stick onto the tray.
• There are different adhesives for different materials.
• Alginate adhesives contain 10–12% toluene dissolved in 45-50% isopropanol.
• Maximum adhesion results from using adhesive with perforated trays.

Tray Selection

• Trays should feel comfortable to the patient, extending slightly beyond the facial surfaces of the teeth.
• Trays should extend approximately 2 to 3 mm beyond the third molar, retromolar, or tuberosity area of the arch.
• Must be sufficiently deep to allow 2 to 3 mm of material between the tray and incisal or occlusal edges of the teeth, extending into the sulcus.

Adhesive Placement

• Requires applying two thin coats, drying between each coat.
• Do not place impression on pooled adhesive.
• Adhesive applied to the tray's internal surface and over the margins for binding the outer edge of the impression material.

Stages and Considerations for Alginate Impressions

• Signed treatment plan and patient consent.
• Awareness of MH & allergic reactions, using full PPE
• Assess the patient's ability to open their mouth widely, gag reflex & retching.
• Proper positioning of the patient in the dental chair and clinician's posture.

Alginate Placement

• The patient should be seated upright.
• For the lower impression, the clinician should be in front of the patient.
• For the upper impression, the clinician stands behind the patient.
• Check the tray size, place adhesive, and have alginate and water ready.
• Mix thoroughly, ensuring no air bubbles.
• Load the tray and place it into the patient's mouth sideways first.
• Reassure the patient, remove and check if adequate.
• Disinfect and wrap in moist tissue/gauze
• Ensure paperwork is filled

More Recent Developments in Impressions

• Digital impressions via scanners (e.g., iTero, Cerec) and CAD/CAM
• Functions similarly to intra-oral photography, scanning the dental arch to produce a model.
• Literature is not conclusive on this method.

Assessment

• May include formative and summative assessments
• Practical tasks may be implemented.