The Art of Taking a Good Impression

Successful restorations depend upon successful impressions. If an impression does not accurately represent the teeth, the stone model will have distortions which will in turn result in poorly fitting restorations. Such restorations may be too small, too tight, rock when seated, or require excessive occlusal adjustment. Based on "3M Dental Impression Tips" published by 3M Dental Products, the following are some suggestions for avoiding problems encountered when taking impressions. We hope this information will make precise and reliable results easier to achieve.

Problem: Tight crowns

Solution: Tight crowns may result from seating partially set impression material, removing the impression from the mouth too early, double wash impression (when you impress, remove and re-impress using the same material twice), or from a poor bond of impression material to the tray. Make sure to follow the indicated working time for materials, as well as the full set time for the impression. To ensure a strong bond of impression materials to the tray, always use the adhesive recommended for the material you've chosen. Clean the tray with alcohol before applying adhesive and follow drying time instructions.

Problem: Short Crowns

Solution: Tooth contact with any surface of the impression tray or pre-set material can result in restorations that are too short or rock when seated. Short crowns also result from using trays with weak or low walls. To avoid tooth contact with impression tray or putty, provide 2 to 3 mm of space around the teeth by relieving the tray material, and use plenty of wash both on the teeth and in the tray. Use custom or inflexible stock trays.

Problem: Slow-setting wash

Solution: Slow-setting wash can be caused by contamination. Avoid rolling retraction cords in gloved fingers or touching prepared teeth and surrounding tissue when wearing latex gloves. Latex gloves and rubber dams leave traces of sulfur which inhibit the set of the impression materials. Contaminated wash or putty may also result in poor adhesion of tray material to wash. To avoid contamination, wear vinyl gloves or gloves guaranteed to be non-contaminating. Apply diluted hydrogen peroxide to any area you think might be contaminated just before making the final impression. Other contaminates to watch out for are residues from temporary materials, composites, adhesives, and core buildup materials. Contamination due to the presence of these materials may be neutralized by applying an alcohol wipe before taking the final impression. Residues from temporary materials may be avoided by taking the final impression before fabricating the temporary.

Problem: Slow-setting putty

Solution: Slow-setting putty may be caused by contamination, inadequate mixing, mixing unequal amounts of catalyst and base, combining materials from different batch numbers, or from using outdated materials. Taking an impression with inadequately set putty may result in lack of detail and poor adhesion of wash to tray material. To ensure putty sets properly, wear vinyl gloves when mixing, and wash hands well with a noncontaminating soap when degloving. Mix materials until the color is consistent, and seat putty within one minute of mixing.

Problem: Rough occlusal or incisal surfaces and tearing at margins

Solution: Rough surfaces and tearing at margins may be caused by the presence of moisture, removing the impression from the mouth too early, failing to observe the recommended work time for wash, or slow-setting wash due to contamination. Poor adhesion of tray material to wash may also result. To avoid these problems, control bleeding and keep the area as free from water and saliva as possible. Closely observe work time for wash and full set time for impression. Follow recommendations given above for avoiding contamination.

Problem: Voids on marginal and occlusal surfaces

Solution: While voids on the margin compromise the fit and function of the final restoration, if they occur on the occlusal surfaces, they make articulation of stone models difficult. Voids are caused by the incorporation of air in the material when loading the syringe. They can also be caused by the syringe technique used, as well as the presence of moisture on tooth surfaces or in the sulcus. To reduce the number of voids, control bleeding and keep the area as free from water and saliva as possible. The two cord retraction technique and haemostatic agents can help accomplish this.

Remember to front load the syringe. The tip should remain submerged in the material and a stirring motion used while you syringe.

Most effective method: Using the dual cord technique is by far the best way to produce a quality impression. This technique will allow maximum space around the gingival of the substructure which will provide deep and clear margins.