Clinical Communication

Reversible agar agar hydrocolloidHomer Vernon Reed*

Reversible agar agar hydrocolloid remaitis an excellent, cost-effective impression mate-rial. A review of the history of the development of the material and a sound techniquefor its use are presented. (Quintessence Int 1990:21:225-229.)

Introduction

In recent years, many new elastomeric materials formaking dental impressions have been introduced.While eaeh demonstrates specific properties that ad-dress problems presented to the dentist, I ftnd that anold standby still ranks as the standard. This material,reversible agar agar hydrocolloid, with little change,has met the test of titne and remains a cost-effective,excellent impression material.

Agar agar hydrocoUoids were first used in industryin 1925. The first "tooth impressions"" (in the mouth)were made in 1937. Until that time, the chemicals usedto strengthen the hydrocolloid material were harmfulto human tissue.

In 1938, Rose, the Dean of the University of Ten-nessee College of Dentistry, assigned to me (then aftrst-year dental student) the task of making complete-arch impressions of the dental students currently en-rolled in the dental college. Rose was a periodontistand was concerned with ocelusion of the teeth as acausative factor of periodontal disturbances.

My assignment was to make the impression, pourthe stone casts, and make interocclusal records. Roseand others mounted the casts in Hanau articulators,studied the occlusion, and adjusted the occlusion totheir satisfaction. Usually the result was monoplaneocclusion, but there were no interferences left whenthey were finished.

The two impression materials available at that timewere plaster of paris and compound. These two ma-

Professor, Department of Proslhodontics, Division or FûedProsthodoiities. University of TenDessee. College of Dentistry.875 Union Avenue, Memphis, Tennessee 3St63,

terials were carried to the mouth in smooth metaltrays. The materials were allowed to "set" or hardento the appropriate time, the tray was removed fromthe mouth, and, after proper cuts, the irnpressionswere removed in sections from the mouth. The abovedescription is an oversimplification of a difficult,tedious exercise.

The plaster .sections were then allowed to dry, re-positioned in the tray, and iuted together with stickywax. The compound sections did not need to dry, Aseparating solution was applied to the plaster impres-sion (none was needed for the compound), and theimpression was poured in cast, or similar, stone.

To circumvent the hard work and to prevent intra-oral lacerations caused by these impression proce-dures. Rose suggested that 1 investigate the possibilityof using agar agar hydrocolloid,

Agar agar chips or fiakes were purchased from thelocal pharmacy in large quantities. It was the samematerial that bacteriologists had been using for yearsin their petri dishes. Remember this should you betempted to reuse the material, especially in anotherpatient's mouth. The material is too inexpensive toreuse.

These fiakes were mixed into a pot of boiling water,and various ingredients were added to the pot in a"heiter skelter'" fashion. We added long eotton fila-ments, flakes of glue, antibacterial chemicals, antifun-gal chemicals, asbestos fibers, and a substance called"Fuller's Earth" to give the material body. This wasboiled until it was the proper consistency, then allowedto cool until it reached a temperature of approximately150 to 160 "F, Water-cooled trays were not available,or at least we did not have access to them. We usedsmooth trays and made undercuts with sticky wax orcompound stops. The post dams and tray extensionswere made in compound. The tray was filled with the

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Clinical Communication

hydrocolloid and placed in a basin of water at ap-proximately 120 to 125 'F and left a few minutes. Itwas then inserted into the student's motitli and heldas still as possible. An assistant (tisitally another sUi-dcnt) tilled a water btilb syringe with cold water andsquirted this cold water into the student's mouth. Aft-er a time, the impression was removed and inspected.and if acceptable at all, was poured in some cast stonematerial.

The above technique was worked out after manytrial and error procedures. The intraoral tissues ofmany sttidents were scorehed, and we almost drowneda few trying to eool the material.

Later, a large metal mixing syringe was purchased.The material was processed as stated previously andwas allowed to cool until it gelled. The material wasthen cut into a cylindrical shape to fit the syringe,placed in the mixing syringe and placed into boilingwater. When the material reached the sol state, aninternal plunger was pushed up and down, and thematerial was mixed. The syringe was placed in 150- toI55-^F water and cooled as it was mixed. Then ihetrays were loaded from the mixing syringe, placed intempering water at about 120''F for a few minutes,and removed. Any remaining surface moisture wasremoved before the trays were inserted into the tnouth.The cooling process was the same until the water-cooled trays came on the market, mainly through theefforts of Thompson and others. The Thompson con-ditioner was a great advancement over the equipmentwe used from 1938 to 1942.

The seaweed, or kelp, plant is the tallest plant, eventaller than the giant redwood tree. Some of the bestseaweed (kelp) plants grow in a reef off the coast oflower California. The height of the seaweed is eon-trolled by the depth of the reefs floor.' Before WorldWar II, Japanese came to this reef and harvested theplants, loaded them on ships and took them to Japan,where the seaweed was cleansed, boiled in water, andallowed to gel. The gel was transported hack to Cal-ifornia, where it was converted into agar agar hydro-colloid for the dental profession.

With the advent of World War 11, Japanese shipswere denied the use of the sea lanes to California, andthe production of agar agar hydrocolloid closed. Thisbrought about the "discovery" of the "alginates," orirreversible hydrocolloids.

According to Johnston et al,' "The accuracy andease of handhng of agar agar hydrocolloid has madepossible actual duplication of cavity or abutmentpreparations and their relationships, with less effort

and lime expended by both the patieril ;iii'l Ihe dentist.. . . success depends upon a good workmy knowledgeand careful control of all variable.̂ ."• namely goodworking equipment and efficient office routines (pay-ing strict attention to all steps). If these things areobserved, authentic reproduction of the areas involvedcan be expected.

"The technique is unexcelled for the indirect con-strtietion of restorations for individual teeth and par-tially edentulous mouths."' The material is unexeelledfor making impressions of the hard struetures of themouth.

When the agar agar hydrocolloid is heated it formsa gel; when it is cooled (to approximately 102 °F itforms a solid. The proeess is reversible, hence, the termreversible agar agar hydrocolloid.

One product on the market consists of 15% agaragar, 80% water, and 5% thermoplastics (inert ma-terials: chemicals, antiseptic, antifungal).

Method and materials

Equipment

There are many good eondilioners on the market tc-day. The temperatures are controlled with either bi-metal or solid-state thermostats. As a general rule, thebimetal controls are less expensive, but are not as ac-eurate and will not last as long as the solid-state con-trols, and repair is more involved.

In my opinion, the conditioners should have re-movable stainless Steel pots. Cleanliness is one im-portant factor in this system, and the removable stain-less pot ean be kept clean with minimal effort, if it isdone routinely. Clean equipment and an aecurate ther-mometer are mandatory. The mercury-glass thermo-meter is the one of choice. It is less expensive andmore aeeurate than the dial type. The eleetric ther-mometers will not be accurate if they are dropped oreven bumped hard, and they are expensive.

Syringes for injecting agar agar material around andinto the gingival crevice may be eonstructed from oneof the dentist's older novacaine anesthetie syringes{without the harpoon). The opening for the disposableneedle is slightly enlarged with a dental bur.

A good syringe on the market is made of plastic, inwhich you place the glass carpule of syringe material.The plastic keeps the material warm and plastic willnot burn or damage the patient's lips as will metal.The carpule syringe system is an advantage over theone in whieh "sticks" of syringe material are loaded

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into a syringe and the syringe and contents are"boiled" in the first bath.

Solid water-cooled trays are manufactured in severalsizes by various companies. Some arc made for par-tially edentulous arches, others for fully dentuiousarches. There are complete-arch trays (mandibularatid maxillary), hall-arch trays, quadrant trays, and aquadrant tray that is supposed to be good for makingthe maxillary and mandibular quadrants at the sametime. However, there is a serious dtsadvantage to thislast technique. The occlusal surfaces of the teeth willbe in occlusion, with only a piece of material (tin foilor filter paper) separating them, and the occlusal sur-faces will not be accurate when the casts are poured.

Complete-arch impressions are recommended, be-cause the casts from the impression can be accuratelymounted in the articulator. However, if the eomplete-arch impression is not used, the half-arch impressionis preferable to the quadrant impression. The half-archimpressions encompass the anterior teeth and the re-sulting casts can be more accurately articulated thancan the cast made from a quadrant impression.

Impression materials

There are many good brands of agar agar hydrocol-loids on the market today They are all basically thesame (80% water, 15% agar agar, and 5% inert ma-tenals). You may purchase either regular- or heavy-bodied consistency In poly tubes or flex skins for com-plete-arch impressions, or quad skins for quadrant orhalf-arch impressions.

The syringe material comes in light- or heavy-bod-ied consistency and is packaged in glass cartridges orsticks.

Gingiva! displacement material

A wide assortment of chemicals are used in conjunc-tion with cord, yam, thread, rings, and other materialsto relax and displace the gingtval tissue and to controlhemorrhaging. Some in use today are epinephrine,alum, aluminum potassium sulfate, aluminumchloride, and hemogent — zinc chloride should not beused.

Materials for laboratory procedures

Any good die stone material of choice may be used-Hardness and color of these materials vary Solutionsare available to harden these materials.

Use of good smooth metal dowel pins and a simplesystem descrihed later are my choice for cast construc-tion.

Preparation of hydrocolloid impre.ssion material

Cleanliness, maintenance, and care of the conditionerand strict adherence to time and temperature are man-datory in the agar agar hydrocolloid impression tech-nique.

The water pots should be clean and free of anyforeign material, eg, wax, calcium deposits, and"scum" from the boihng water. Conditioners stayclean. Failure to keep the pots clean will result in tem-perature variations. Clean, cool, distilled water isplaced in the three pots. The conditioner is turned on.Thirty minutes should be allowed for the water toreach the desired temperatures before proceeding. Theboiling pot temperature should be 150 to 155 E Thestorage pot should be 150 to 155 'F, and the temperingpot should be 110 to 115 "F

The caps on the tubes of hydrocolloid are tightened.The tubes are placed in the conditioner, caps down.If the glass carpules of syringe material are being used,place the rubber plungers down. The water is broughtto a boil, and boihng is continued for 10 minutes.Tubes that have to be rcboiled should now have anextra 2 minutes added to the 10 minutes. Longer boil-ing harms the material. The tubes should be left in theboiling compartment about 15 minutes after the boil-ing stops. Then the tubes of hydrocolloid are trans-ferred to the tempering pot. fhe syringe material isleft m the boiling pot. If the metal or plastic syringeis used, the cap on the needle ts removed, and the stickof syringe material is placed on the syringe plunger,and inserted into the syringe. The plunger is pressedon, the cap on the needle is screwed on, thus forminga vaccum in the syringe (if the 0-ring is in goodshape), and the needle is placed end down into theconditioner.

'footh preparation

When the agar agar hydrocolloid impression tech-nique is to be used, a few cavity preparalton proce-dures must be observed. A more accurate impressioncan be made if all caries is removed and all undercutsare blocked out with cements, composite resins, orother materials. This is a good technique no matterwhich impression material is being used.

If pinholes are to be placed in the preparation, they

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Clinical Communication

should be larger tban, and not placed as deeply intothe tooth as, pinholcs in a nortnal pin-retained prep-aration.

The cervical margins should be distinct. Thereshould be no large undercuts apically to the tnargin.In this ease, the tooth tnust be removed circumferen-tially; if not, varying degrees of distortion or actualtearing of tbe itnpression is inevitable.

Grooves in three-quarters or five-eighths prepara-tions should be larger when the hydrocolloid itnpres-sion material is used than are grooves in a "normar'preparation. Box preparaltons should be used insteadof grooves in the posterior teeth when hydrocolloidmaterial is used to make the impression.

Trays should be selected and tried in the patient'smouth before the itnpression material is placed inthem. Black compound or Van R Tacky Stops (VanR Dent Products, Inc) should be placed to guide andstop the tray in the desired position. There should be3 mm of impression tnaterial around the teeth occlu-sally and laterally. For best results, three stops shouldbe used to give the tripod effect. If possible, the stopsshould rest on the incisai surfaces of the anterior teethand oeelusal surfaces of the right and left posteriorteeth. If there are no teeth available, or if these teethhave been prepared, soft tissue stop areas may be se-lected.

Préimpression steps

The prepared area, abuttnents, and gtngtval tissues areexamined, A spray of diluted, warm mouthwash ofchoice is an excellent means of washing and cleansingthe area. Adequate cotton rolls are placed; cotton rollholders are mandatory in the mandibular areas. Thecotton rolls must be placed and tnaintained apicallyto the gingival crest area. If they are allowed to exertpressure on this area, the gingival displacement pro-cedure will be in vain.

Saliva ejectors are placed in the mouth, and theprepared area is dried cautiously to rid the surface ofmoisture. The gingival displacement material is placedaround the prepared teeth to the exact circumferenceof the preparation. No loose ends should lap over tbegingival tisstie or hang out over the labial or lingualtissue, A small tip end may be left in the interproximalarea to aid in removal.

The prepared area and the oral cavity should bekept free of excessive moisture from the time the dis-placement material is placed until the impression hasbeen removed.

Impression making

The agar agar hydrocolloid that has been stored inthe 150- to 155-"F bath is now removed and placed inthe selected tray Tbe material should be placed in thetray without lapping or trapping air. The palatal areaof the maxillary tray need not be filled unless the pa-tient has a deep vault in the palate or an impressionis being made for a removable partial denture in whicha palatal bar or strap major connector is to be nsed.The filled tray is placed in the tempering bath at 110to 115 'F for 5 to 10 minutes or to the individual den-tist's desire. After a few impressions have been made,tbe dentist has a good idea of the time and tempera-ture best suited to bis or her technique.

The syringe is removed from the bath, and sotnematerial is extruded frotn the needle to eliminate anycontaminated syringe material. The displacement ma-terial is removed, the area is inspected quickly, and thesyringe needle is placed in the free gitigival sulcus. Thetnateriai is injected carefully around the preparedabutment. Care must be taken not to trap any airduring the procedure. The needle should not be liftedout and replaced in the materia!, as this will cause airpockets. Any excess syringe material may be quicklyplaced on the oeelusal surfaces of the unprepared teethto aid in the elimination of voids in the impression ofthese areas.

The prepared and filled tray is removed frotn thetempering bath before or during the insertion of thesyringe material — a gauze square is placed on thesurface to blot any excess water and the hoses areconnected to the tray.

As soon as the syringe material has been placed onthe abuttnents, the gauze is removed and the tray isinserted in the mouth. The gauze square should leavean imprint of the gauze on the hydrocolloid when itis removed. If the imprint is not visible, there is stillexcessive moisture on the surface of the hydroeolloid.The surface must be blotted again before proceeding.The use of mirror and a gentle rocking motion willaid in the placement of the tray. The stops will aid inguiding and seating of the tray.

Tbe patient should be in an uprigbt position, andthe arch of prepared teeth should be parallel with thefloor. The patient should be in a relaxed position, andthe hps should be relaxed.

During and after insertion of the mandibular tray,the patient's tongue should be raised up and back intothe palatal area and then relaxed into its normal po-sition or left to rest on the upper part of the tray. The

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tray should be placed and held in tbe mouth by thedentist until it is removed from the mouth.

The water Is allowed to circulate through the tubesand the tray for the desired period of titne. Tbe watershould be 65 to 72 ^F and should be used for not lessthan 5 minutes. Again, each dentist will determinetime aeeording to his or her technique. The tray isremoved with a .map-out method — do not ease it out.

The impression should be inspected, and, if it meetswith the dentist's approval, it should be placed im-mediately into a glass or stainless steel eovered dishin wbich a 2% potassium sulfate solution has beenplaced.

One teaspoon (5 mL) of potassium sulfate mixedwith 1 pt (0.47 L) water will make a 2% solution. Tbeimpression should be left in tbe solution for 5 to 20minutes.

Tbe potassium sulfate solution gives a harder sur-face to the stone die material. Some, if not ali, agaragar hydrocolloid contains borax as a filler. When theimpression sets on the bracket table just for a fewminutes, a thin film of water forms on the surface.Tbe water film contains borax tbat has been leechedfrom the hydrocolloid material. Borax retards the setof die stone, and the water film contaminates the sur-face of the die, resulting in a weak surfaee that willremain in tbe agar agar hydrocolloid material whenthe dies are removed.

The potassium snifate acts as an accelerator for thestone, counteracting the retarding effect of the borax,thus producing a harder and sharper die. A hardeningsolution may be placed in the water used to mix thedie stone, resulting in a harder die. Rudd has stated."This will also result in a more brittle die." (RuddKD: Personal communication).

The potassium sulfate should not be washed out ofthe impressions before the dies are poured. Theimpression must be blown carefully; the impressionshould not be dehydrated, but an excessive amount ofwater or moisture should not be left in the impression.The working cast should be poured immediately after

tbe removal from tbe potassium sulfate bath. Excesswater may be "blotted" using the end corner of a facialtissue. The occlusiil depths are checked carefully forwater droplets. The stone cast will have holes in areasin wbich the water has been left.

Transfer die me I hod

Only the abutment impressions are poured with agood mixed die stone material. Gentle vibration of theimpression tray will aid in getting the stone into theabutment impression area. As soon as the impressionsof the abutments are filled, the impression is removedfrom the vibrator. Some die stone is picked up with aNo. 7A spatula and the impression is held in one handwhile the hand witb the No. 7A spatula is placed onthe vibrator and the root portion of the die is buiitup. A stiff, well-mixed die stone can be "stacked" astall or high as the operator ehooses. Care must betaken not to let the stone run over into the adjacentareas.

The impression with the "transfer" dies is placedinto a humidor and set for 45 minutes to 1 hour. Theimpression is removed from the humidor, the transferdies are removed earefully. and a solid cast is pouredinto the impression. The impression is again placedinto the humidor and set for at least 1 hour. The solideast can be mounted against the opposing east in thearticuiator and used to wax the abutments and/orfixed partial dentures to the desired morphology andocclusion. The transfer dies are properly trimmed andthe patterns are transferred to them for the final mar-ginal adaption prior to spruing and investing.

ReferencesGeo^r1. Earle SA: Undersea world of a kelp forest.

1980:158:411^26.3. Johnston JF, Phillips RW. Dykema RW: Modern Practice in

Cron-n and Bridge Prosthodontics, ed Î. Philadelptiia, WB Saun-ders Co, 1971, p 206. •

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