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CAST PARTIALS

Checklist for AccurateAlginate Impressions!Gary Wakelam, RDT, CDT

Taking an alginate impression is one of the most common

procedures done in the dental office. In fact, if you follow

directions, it should be as easy as baking a cake. But like

baking, if the dental impression recipe is changed without

thinking about how it is going to affect the result, it can lead to

failure. For example, simply adding a little extra water to algi-

nate can adversely change the water/powder ratio, thinning

the mix and increasing the setting time.

Unfortunately, because taking an alginate impression

appears to be so easy, it has become one of the most abused

techniques in dentistry. Not taking the time to do this proce-

dure correctly is one of the most expensive mistakes you can

make in your dental practice.

Checklist for Alginate Impressions

The following is a general set of steps and tips regarding tak-

ing an alginate impression. Various alginate brands may vary

from this checklist based on their own individual properties.

Please refer to the individual manufacturers instructions for

specific technique and auxiliary material recommendations.

1. Select the proper tray. Ensures dimensional elasticimpression materials accuracy and that all essential areas

are included in impression. Solid non-perforated metal

rim-lock tray recommended (will force material around

teeth). Rim-lock design holds impression material in place

without adhesive.

If tray too large: Alginate will not flow into edentulous

regions and palate unless supported, reach full extent of

the tissue folds or may slump in palate.

If tray too small or without adequate clearance: impression

may be distorted due to impingement of soft tissues (will

distort or tear upon removal).

2. Modify the selected tray. Rim periphery wax dental

compound or heavy-bodied polyvinyl siloxane impressionmaterial around entire upper and lower trays. Seat trays

in mouth and muscle trim.

If the patient has high vault, missing teeth or limited

space in molar areas, soften modeling compound in water

bath, place in tray and make impression of distal exten-

sion, edentulous areas, palate and posterior palatal seal

area. Note where tooth imprints occur. Cut these out with

sharp knife so compound has definite stops against the

tuberosity (assures teeth will not touch tray and distort

impression). Trim compound to allow for 1/8 to 1/4 inch

Alginate tools.

Solid rim lock trays are best for alginate

impressions.

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clearance between compound and mucosa except in

posterior palatal seal area. Ensure compound not loose in

tray. If loose, heat wax spatula and carefully force some of

compound into rim-lock borders of tray. Soften compound

surface with a flame. Press bulk cotton into surface,

leaving short cotton fibres embedded. Place tray in cold

water to harden compound.

3. Clean teeth and prepare mouth as necessary.

For regular-bodied alginates, have the patient rinse with

mouthwash mixture. This will cut mucin and lower sur-

face tension helping to eliminate air bubbles. Lightly dry

teeth with compressed air just prior to insertion of

impression material and tray.

For heavy-bodied alginates, saliva makes an excellent sep-

arating medium. Do not over dry teeth prior to insertion

or heavy-bodied material will stick to teeth.

4. Place gauze in mouth to remove excess saliva, if

necessary.

5. Mix pre-weighed alginate and precisely measureddistilled water for 15 seconds if a vacuum mechani-cal mixer is employed or 60 seconds with handspatulation (timed with a clock).

Mechanical spatulation under vacuum produces the best

results: a smooth, air-free mix. Distilled water is recom-

mended as calcium and other minerals often found in tap

water will contaminate mix. Setting time should be con-

trolled by varying water temperature, not consistency of

mix. Warmer water allows mix to set faster than cold

water.

6. Dip tray in water to wet the cotton fibres and blowexcess water away with air.

7. Load tray, being careful to force the alginate intorim-locks and cotton fibres. Smooth surface withfinger dipped in cold water.

8. Remove any gauze from mouth.

9. Gently place some alginate on teeth and in palate.

10. Seat tray immediately with slight rocking motion.

11. When tray is seated, release pressure and hold traylightly in place to prevent unseating.

Imperative to release pressure as soon as tray seated.

Alginate materials have a directional set from tooth

surface to the tray. Pressure will cause impression to set

under strain. On removing impression from mouth, these

strains will be released, causing distortion and an

Other Considerations

(a) Varying Setting TimeThere are occasions when you will need to vary

the setting time of the alginate. For example, if

your patient gags you may want the alginate to

set faster. This can be controlled by varying the

temperature of the water. Warmer water allows

the mix to set faster than cold water. Never con-

trol the setting time by varying the consistency of

the mix. Changing this ratio will also effect the

impressions permanent deformation, flexibility

and strength.

(b) Short-Term Impression Storage

Alginate impressions lose their accuracy ratherquickly. If the impression is stored in air, water

evaporates and the impression shrinks. If it is

stored in water it will absorb the water and

expand. Therefore it is always best to pour up an

alginate impression immediately. But, when you

cannot, keep it in a humidor. Storing it in 100%,

relative humidity will give you about an hour

before serious dimensional changes occur.

Conclusion

In todays economic environment, any steps you

can take to make your office run more efficiently

are of the utmost importance. If you follow theinstructions laid out here, taking an accurate algi-

nate impression can be as easy as baking a cake!

inaccurate cast. Moving tray during gelation will incorpo-

rate similar strains.

12. Place small amount of alginate mix on bracket tableas test sample.

13. When test sample has lost its tackiness, set timer for two minutes.

14. At the end of two minutes, remove impression with firm, quick snap. Do not rock or twist impressionbefore or during removal.

15. Inspect impression carefully for defects as soon asit is removed and before it is washed.