Making Better Impressions
Even the most experienced practitioner, using the best materials, can encounter difficulties when making an impression. The information assembled in this guide is intented to help identify common impression problems and offer solutions. It is hoped this will help you avoid costly and time consuming remakes or adjustments to crown and bridge restorations.
Indications of a Good Impression
Making a highly accurate impression is the first and most important step in creating superior crown and bridge restorations for your patients.
Uniform, homogeneous mix
Tray adhesive applied thoroughly
Rigid, sturdy impression tray
No voids or pulls on margin detail
Detailed margins with no tears or rough surfaces
No burn-through of heavy body material
Strong bond between heavy body and light body material
Strong bond between impression materail and tray
No tooth contact with tray
Surface Inhibition or Slow Set
Visual Appearance - Impression not completely set. Tacky to the touch.
Result - Inadequate surface detail on stone reproductions, poor fitting crowns.
For Vinyl Polysiloxane Materials.
Causes
Sulfur in latex gloves or rubber dam inhibits the setting reaction of VPS, but does not affect polyether impression material.
Touching prepared teeth or surrounding tissue with latex glove
Rolling Retraction cord with latex glove fingers
Exposure to residues from custom temporary materials
Exposure to air inhibited methacrylates (i.e. composites, adhesives)
Solutions
Wear nitrile gloves or gloves proven not to inhibit set of VPS impression materials immediately prior to making final impression.
If contamination suspected, scrub affected area with diluted hydrogen peroxide.
Wear nitrile gloves or gloves proven not to inhibit set of VPS impression materials immediately prior to making final impression.
Do not use impressions already used to fabricate the temporary restoration. Fabricate the temporary crown or bridge after final impression has been made.
Remove air inhibited later on the exposed surface with an alcohol wipe before making final impression.
For Polyether Materials
Causes
Exposure to epinephrine or iron III sulphate.
Retraction solutions not removed sufficiently.
Wetting agent too aggressive
Inadequate mix
Solutions
Select retraction cords or hemostatic agents not containing these chemicals when using polyether impressioning materials.
Carefully remove the retraction solution with water spray.
Clean polyether impressions with water and dry. Wetting agents are not necessary.
Ensure mixing instructions are followed and materials have a streak-free appearance. Bleed 50ml cartridge before applying mix tip to ensure even dispensing.
Lack of Impression Detail
Visual Appearance - Muted detail reproduction and inadequate margins.
Result - Crowns may be too tight or small and extensive occlusal adjustments may be required.
Causes
Blood/saliva contamination around prep.
Inadequate retraction of sulcus around prep.
Exceeding the working time of the impression material.
Solutions
Rinse and dry prep area just prior to making impression.
Good retraction technique with proper moisture control and proper tissue retraction.
Follow manufacturer’s working time specifications
Delamination
Visual Appearance - Heavy body and light body materials not blending, or adhering together.
Results - Indirect restoration will not seat properly.
Causes
Exceeding the working time of the material.
Contamination of pre-set heavy body material in two-step technique.
Solutions
Follow manufacturer’s working time specifications.
Ensure VPS impression material does not come into contact with latex gloves. Ensure impression material does not come into contact with methacrylate residue from acrylate temporary materials.
Voids on Margin
Visual Appearance - Voids on margin or prepared teeth
Result - The fit and function of the final restoration may be compromised.
Causes
Air incorporated in the intra-oral syringe or while filling impression tray.
Improper syringe technique.
Blood saliva contamination around prep.
Poor retraction around prep.
Retraction cord not left in place adequate amount of time so that no blood or saliva is present.
Solutions
Front load syringe by inserting mix tip directly into syringe and keep mix tip immersed in heavy body material while filling the tray.
Keep syringe tip immersed in wash material to avoid entrapping air.
Good retraction technique. Rinse and dry prep area.
Good retraction technique. Consider two-cord retraction to displace tissue and control fluids.
Good retraction technique, leave cord in sulcus until no blood or saliva is present before syringing the light body impression material. Consider two-cord retraction.
Tearing at the Margin
Visual Appearance - Tearing visible on the margin of the preparation
Result - Short crown margins and/or open margins
Causes
Poor retraction technique
Surface inhibition
Slow setting material
Early removal from mouth
Solutions
Improve retraction and where appropriate use two-cord technique
For VPS, avoid contamination from sulphur or methacrylate. For polyether, avoid epinephrine or iron III sulphate because setting time could be compromised.
Follow manufacturer’s setting time recommendation. Check expiration date of material.
Follow manufacturer’s setting time recommendation.
Inadequate Capture of Margins
Visual Appearance - Incomplete Margin.
Result - Short crown margins and/or open margins
Causes
Inadequate coverage of marginal area with light body impression material.
Fluids such as blood or saliva present within the sulcus.
Keep syringe tip immersed while syringing
Tearing of margin.
Exceeding the working time of the material.
Solutions
Good retraction technique.
Maintain a clean and dry field.
Gently stir while syringing.
Good retraction technique. Check manufacturer’s instructions for working and setting times.
Follow the manufacturer’s working time specification.
Inadequate Mix
Visual Appearance - Non-homogeneous mix.
Result - Slow setting impression material.
Causes
Improper ration of catalyst to base.
Mix tip not attached correctly.
Air in the 50ml cartridge.
Solutions
Bleed cartridge prior to attaching mix tip.
Use manufacturer’s recommended mix tip.
Bleed cartridge to ensure equal catalyst/base expulsion.
Facial-Lingual Pulls
Visual Appearance - V-shaped void, trough-like.
Result - Failure to capture complete and accurate dentition.
Causes
Improper tray seating.
Improper syringe technique.
Too little material.
Solutions
Seat tray slowly.
Use proper syringe technique.
Use more material.
Improper Tray Seating
Visual Appearance - Burn-through of light body impression material. Impression tray exposed.
Result - Crowns are too tight, too small, or rock when seated. Tight fitting crowns.
Causes
Prepared teeth contacting the sides or the bottom of impression tray.
Tooth contact with the pre-set tray material when using the two-set technique.
Tray seated too quickly or forcefully.
Tray movement or rocking during the impression.
Weaker plastic trays can allow deflection of the tray which may rebound upon removal.
Solutions
Avoid contact of teeth with any surface of the tray. Test various sizes to ensure proper size.
Relieve the heavy body impression material to ensure a 2-3mm space.
Slowly position tray into patient’s mouth.
Use passive pressure to immobilize the tray for the recommended set time.
Use a stiffer, more rigid stock tray.
Poor Bond of Impression Material to the Tray
Visual Appearance - Impression pulling away from the sides/bottom of tray
Result - Crown(s) may not seat fully, or require excessive occlusal adjustment.
Causes
No tray adhesive used.
Incompatible tray adhesive used.
Inadequate drying time for tray adhesive.
Solutions
Use appropriate tray adhesive.
Follow manufacturer’s instructions for application and drying time.
Stone Model Discrepancies
Visual Appearance - Voids on margin, powdery cusp tips or incisal edges on prepared tooth.
Result - Incomplete seating of indirect restorations
Causes
Small Voids due to hydrogen gas evolution from the polymerization of VPS materials.
Tooth contact with impression tray or gauze of double bite tray causes water to leach out of the tray dehydrating the stone.
Large voids present due to poor model pouring technique.
Solutions
Follow manufacturer’s instructions for pouring.
Avoid tooth contact with any surface of the impression tray.
Follow manufacturer’s instructions for pouring. Use a surfactant for a VPS impression.