Utilizing Reversible-Irreversible Hydrocolloid Impressions for Composite Onlays

Dentistry Today


Obtaining an accurate impression is a critical aspect of treatment to provide an indirect restoration. In many cases, the indirect fabrication of inlays and onlays offers certain clinical advantages when compared to direct fabrication, and if an indirect restoration can be fabricated and seated in a single visit, the patient benefits in terms of efficiency and convenience. This article pre-sents a technique for using reversible-irreversible hydrocolloid impression material to fabricate an indirect onlay at a single visit.



Impression materials are used to make accurate reproductions of the hard and soft tissues. Desirable properties of an impression material include the following: ease of use,  readily wets the oral tissues, adequate tear strength, no permanent deformation after strain, dimensional stability, accuracy, compatibility with die materials, and easy disinfection. Agar hydrocolloid (reversible), alginate hydrocolloid (irreversible), and a variety of elastomeric impression materials (polyethers and polyvinyl siloxanes) are the most commonly used impression materials.

Sears introduced reversible hydrocolloid  in 1937. This material has been considered the standard for dental impressions because a casting made from one impression will fit the die from a second impression.1 Agar hydrocolloid materials are derived from reversible agar gels. The material liquefies between 70°C and 100°C and becomes a gel again at 30°C to 50°C. Agar is an organic, hydrophilic colloid that is derived from seaweed and is a sulfuric ester of linear polymer galactose.2 The material is viscoelastic, and its tear and compressive strengths are dependent on how quickly the material is removed from the mouth. Higher compressive and tear strengths occur at higher loading, and tearing decreases with the  increasing rate of impression removal from the mouth. Therefore, these impressions should be removed quickly with a snap action.

Reversible hydrocolloids offer the following advantages: a custom tray is not required;  no separate components are mixed, thus the chance for error is reduced;  they are relatively hydrophilic and inexpensive; they are  pleasant tasting and clean to use; casts are easily removed from the impression; and compatibility with stone allows the creation of bubble-free casts. The disadvantages are the initial start-up cost of the conditioning unit, casts must be poured immediately, and the material has a low tear strength when impressing narrow, deep sulci.3

When using agar (reversible hydrocolloid) impression material, the tray is removed with a snap-out method. The impression is placed in a glass or stainless steel covered dish containing a 2% potassium sulfate solution, and the impression is left in the solution for 5 to 20 minutes. (The potassium sulfate solution will result in a harder surface of the stone die material.) Many agar hydrocolloid materials contain borax as the filler. When the impression is left on the bracket table for a few minutes, water leaches out with some borax. The borax retards the set of the die stone, creating a weak surface. The potassium sulfate counteracts the effect of the borax, resulting in a more accurate die. The potassium sulfate should not be washed out of the impression, but should be carefully blown away with a stream of air.4

Alginate impression materials (irreversible hydrocolloid) change from a sol  phase to a gel phase by means of a chemical reaction. Unlike the agar hydrocolloids, the formation of a gelation stage prevents the material from being reliquified. In an alginate impression material the powder contains calcium sulfate dehydrate, soluble alginate, and sodium phosphate. Adding water causes the calcium from calcium phosphate dehydrate to react with the phosphate ions from sodium phosphate and pyrophosphate to form insoluble calcium phosphate. After depletion of the phosphate ions, the calcium reacts with soluble alginate to form the irreversible calcium alginate gel. Irreversible materials are most often used to make impressions for diagnostic or opposing casts.3 Ideally, preweighed and prepackaged alginate and measured water will provide the most consistent results. Vacuum mixing will provide a more thorough mix with less porosity.

Alginate does not flow well, and areas that are difficult to access require finger placement or the use of a syringe before the tray is placed into position. The mixing time is 1 minute for regular alginate and 45 seconds for fast-setting alginate. These mixing times should be observed as closely as possible, as overmixing and undermixing weaken the final impression. The properly proportioned powder and water are mixed in a bowl using a spatula or in an automatic mixing system. The impressions should be poured as soon as possible and preferably in less than 10 minutes. If the impression cannot be poured in 10 minutes, some materials may be stored in 100% relative humidity. The alginate should be rinsed with cool water to remove any exudates formed by alginate gel  syneresis (spontaneous shrinking of a gel with exudation of a liquid). The exudates may retard setting of the gypsum material used to fabricate the model. Wrap the alginate in a wet paper towel and place it in a sealed plastic bag. Aluminum sulfate solution should be sprayed on the alginate after disinfection to improve the surface quality of the cast. The impression should not remain in contact with the stone more than 60 minutes or after the exothermic reaction is completed.4

Impression Technique

The clinical technique for taking an impression using a reversible hydrocolloid material is as follows: Ivory liquid soap (Proctor and Gamble) is diluted with tap water in a 1:3 ratio. A cotton pellet is placed in the solution, removed, and  rubbed first on the prepared tooth (teeth) and then on all the other teeth in the arch after completion of the tooth preparation. The dental assistant sprays air and water  on the pellet to remove any debris that accumulated on the teeth during preparation and was loosened by the soaked cotton pellet. Then a No. 100 brush is dipped into the detergent solution, over the prepared tooth, and into the sulcus. A light stream of air will remove visible air bubbles. It is important to remove excess but leave some detergent on the teeth to lessen surface tension and reduce bubbles in the impression. The detergent acts as a lubricant to prevent thin edges of the hydrocolloid from adhering to the inside of the sulcus.5

Almost an entire cartridge of material is injected into the sulcus of the prepared tooth. The excess allows the material to retain heat. This keeps the material soft and flowable as the tray material is placed over it.6


An ideal disinfecting solution should disinfect while maintaining surface detail and the dimensional stability of the impression material. Disinfecting reversible or irreversible hydrocolloid impression material can be accomplished with 1% sodium hypochlorite or 2% potentiated glutaraldehyde. No dimensional changes occur, although minor surface detail may be lost.7-9



Reversible and irreversible hydrocolloid can be used together to achieve the following benefits: reduced cost;  the agar does not irritate tissue at the temperature used (the alginate cools the agar to an ideal gelation temperature); agar is more compatible with gypsum model materials than alginate; less equipment is required; and time is reduced.

Reversible-irreversible hydrocolloid combinations are formed by the alginate setting in approximately 3 minutes, and the agar gel will also set in the same time frame because of the cooling effects of the alginate. The combination of the setting alginate and gelling hydrocolloid creates a bond. However, in order to create an effective bond between these materials, both must be in a flowable state. Some combinations bond better than others, and manufacturer-compatible materials should be used. The accuracy of combination agar-alginate impressions has been demonstrated to be comparable to elastomeric impression materials.10

Reversible hydrocolloid is a hydrophilic material and may either displace or absorb the moisture that may be present. This is important, since dehydration/dessication of the prepared teeth may result in adverse effects on the dentin and pulpal tissue. In a study by Stewart, et al the dies in a wet field were as accurate as those in a dry field.11 The reversible hydrocolloid part of the impression results in a highly accurate facsimile of the preparation, and the irreversible hydrocolloid in the impression tray makes the procedure easier because a tempering bath or water-cooled trays are not required for the tray material.12



There are a number of options for creating inlays/onlays in a single office visit. CAD/CAM units such as CEREC III (Sirona) can be used to provide single-visit inlays and onlays that are fabricated chairside. However, one of the main drawbacks of the CEREC unit is its cost, although this device becomes more cost-effective if more cases are treated. Another drawback is the uniform shade of the restoration, as the restoration is carved from a block of material that has only 1 shade. Therefore, for the clinician who places a limited number of this type of restoration, the ease of use, low cost, and accuracy of hydrocolloid suggest its use when the treatment plan calls for a single-visit inlay/onlay.

Directly placed composite restorations are recommended for conservative restorations of posterior teeth.13 The location and size of the restoration and the material used are important factors in determining clinical longevity.14-17 Composite resins placed in molar teeth wear faster than those inserted in premolar teeth; as the buccal-lingual dimension increases, the wear rate also increases. Two types of wear patterns, generalized and localized, occur on the occlusal surface.14

Futher, direct composite resins are technique sensitive.17 Polymerization shrinkage may result in recurrent caries, increased marginal leakage, and post-placement sensitivity.18 Interproximal contour and contacts are also harder to develop properly with this material. Product improvement and innovative placement techniques have improved, but not eliminated, these problems.15 One technique to improve marginal adaptation is the indirect fabrication of a posterior composite restoration and cementing of the inlay/onlay with resin cement.17

Among the advantages of an indirect resin inlay are its ability to blend with tooth structure; increased strength compared to direct resin restorations; polymerization shrinkage that occurs only at the margins; better contacts and contours as compared to direct resin restorations; good marginal seal; a tempering process that improves physical properties; resin filler composition that ensures optimal water absorption and color stability; the ability to adjust and repair the restoration at chairside; and reduced potential for postoperative sensitivity compared to direct resin restorations.17-21

An accurate indirect restoration can be made using stone models, but removal of the composite from the models may result in  model breakage or composite inlay/onlay damage. A flexible model reduces these problems.

The following case reports describe fabrication of indirect composite onlays. The materials used were a combination of reversible and irreversible hydrocolloid for the impression, and Mach-2 die silicone and Blu-Mousse for the flexible model (Parkell).


The patient presented with a large amalgam restoration in tooth No. 30. A fractured lingual cusp was present (Figure 1). To minimize the amount of additional tooth structure that would be removed, an onlay was chosen as the restoration of choice. Due to the patient’s concern about cost, local anesthesia, and the time involved, a 1-visit procedure was selected.

Figure 1. Tooth No. 30 had an extensive amalgam with a fractured lingual cusp. Figure 2. Dry Processor II hydrocolloid heater.

Prior to initiating tooth preparation, Identic Syringable (Dux Dental) was placed into the Dry Processor II (Dux Dental, Figure 2). After turning the unit on, the boil switch was activated. When the ready (green) light is illuminated, the material is ready for use.

The tooth preparation consisted of a 5° to 15° taper toward the occlusal surface, rounded internal line angles, and no bevels on cavosurface margins. The isthmus width should be a minimum of 2 mm. A 2-mm reduction is needed to onlay the cusps. The shade for the final restoration should be selected prior to the teeth becoming dehydrated during the preparation phase. A resin-modified glass ionomer material can be placed to block out any undercuts and thereby avoid excess removal of tooth structure. Retraction cord is placed, or if the margins are subgingival, a laser or electrosurgical unit can be used to create both retraction and a dry field during the luting stage.

Figure 3. Identic Alginate is mixed in the Alginator II. Figure 4. Identic syringe material.
Figure 5. A reversible-irreversible hydrocolloid impression is taken of the arch.

Identic Alginate (Dux Dental) was mixed in the Alginator II (Dux Dental, Figure 3). The areas to be impressed were moistened, then the sulcus and coronal surface of the preparation were filled with Identic Syringable material (Figure 4). Immediately, a rigid tray filled with Identic Alginate was seated. After the alginate was completely set, the tray was removed with a quick snap (Figure 5). The impression was rinsed for 30 seconds using gently flowing water at room temperature. The impression was then sprayed with disinfectant (DisCide Ultra, Palmero Health Care).

The impression was then sealed in a plastic bag for disinfection. The time for disinfection was based on the manufacturer’s recommendations. The impression was then removed from the bag and rinsed again for 30 seconds. A second impression was obtained in the same manner. One impression was poured as usual with die stone (VanRock, Dux Dental) and removed after setting for 1 hour. This model would be used as the master model to minimize chairside adjustments.


Figure 6. A syringe with the Mach-2 Die Silicone is placed at the bottom of the impression and kept submerged to minimize bubbles. Figure 7. Mach-2 Die Silicone is used to fill the coronal aspect of the teeth in the arch.

After ascertaining the accuracy of the second impression, debubbleizing solution was sprayed into the impression.  The debubbleizer was removed with compressed air. A specially formulated, fast-setting vinyl polysiloxane (Mach-2 Die Silicone, Parkell) cartridge was loaded into an impression gun. After ascertaining free flow of the material, a mixing tip with intraoral attachment was affixed. The die silicone was expressed into the impression of the prepared tooth, starting at the deepest portions of the preparation and allowing the material to flow across the impressed tooth (Figure 6). The crown portions of all impressed teeth were filled (Figure 7).

Figure 8. Blu-Mousse is used to fill the remaining portion of the impression and placed into an indexed articulator to be used as a base.The impression is then inverted onto this base before it hardens. Figure 9. Blu-Mousse is used to take a bite registration for mounting of the opposing model.

The cartridge was removed from the impression gun, and a cartridge of Blu-Mousse Superfast (Parkell) was inserted without a mixing tip. Free flow of the material was ascertained, and a model base was created by expressing a layer of Blu-Mousse over the Mach-2 Die Silicone in the impression. The plastic base former was filled with the additional Blu-Mousse. The impression was inverted and set on the base former so that the 2 layers of the material could fuse (Figure 8). This was allowed to set for approximately 2 minutes. While the material is hardening, a bite registration can be otained with Blu-Mousse (Figure 9). The base former was removed from the model.

The impression was separated from the model. Individual dies were created by cutting through the interproximal areas mesial and distal to the prepared teeth with a single-edged razor blade. The cut should be made at least two-thirds, but no more than three-quarters, through the model. It is important not to cut all the way through.

The remaining part of the model was snapped apart. This creates a relatively jagged edge that ensures accurate reassembly of the model in the base former, which is important for establishing proper contour and contacts in the final restoration. A light-cured provisional can be fabricated at this time if the restoration is not to be completed at the same visit.

A light-cured hybrid composite material was used to build the onlay (Vitalescence, Ultradent). Vitalescence was chosen because of its ability to replicate natural tooth structure. The color of a tooth is the mean value of the sum of the chromas distributed over the entire tooth surface. The low-translucency, high chromatic saturation of the dentin body provides the basic hue. The cervical third of the tooth has the highest saturation due to the thin enamel, making the dentin more visible. This area is used to determine hue for the restoration. The organic component of the dentin provides the phenomenon of fluorescence. Enamel provides the opalescence effect, created by short-wave components of the light spectrum striking the surface. In order to replicate natural tooth structure for anterior or posterior restorations, the material utilized should provide low-translucency, high-fluorescent dentin and high-translucency, low-fluorescent enamel.

Figure 10. The die is separated from the indexed model, and the dentin build-up is initiated. Figure 11. Upon completion of the dentin buildup, the enamel layer is placed.
Figure 12. Cusps and fossae are finalized.

Minimizing the polishing step was accomplished by using composites that can be shaped and molded to the desired anatomical shape. A dentin shade was placed to build up each cusp individually (Figure 10). Metal instruments can be used to shape the composite (examples: Axis, Newton Fahl instruments, Hu-Friedy, XTS instruments). Each layer was light-cured for 20 to 40 seconds. Stain can be placed in the grooves using an endodontic instrument. This dentin layer was brought up to a point 0.5 mm shy of the final margin. A translucent enamel shade was then used to build up the remaining portion (Figure 11). The anatomy developed in the dentin was maintained (Figure 12).

Finishing burs, polishing points, and polishing paste were then used to refine the anatomy and achieve a smooth surface. The onlay was removed from the flexible model and fitted to the master stone model. An accurate fit on the stone model will minimize chairside adjustments. Complete seating, contacts, and occlusion were verified. The onlay was removed from the model and placed in the oven (Coltène/Whaledent) at 110ºF for 7 minutes.

The onlay was removed from the oven, and the fit was verified intraorally. Fit Checker (GC America) can be used to determine any binding points. After the fit was verified, the internal aspect of the onlay was sandblasted with 50 µm of aluminum oxide; 32% to 40% phosphoric acid was used to remove contamination that may have occurred during try-in procedures, and then silane was applied to this surface and dried with compressed air.

The tooth was isolated with a rubber dam and packing cord was placed, then the tooth was disinfected with Concepsis (Ultradent). Dentin and enamel were etched with 37% phosphoric acid, then rinsed with water. Excess etchant was removed with air and blotted to leave the surface slightly moist.

Scotchbond Multi-Purpose Plus Primer (3M ESPE) was placed and lightly blown dry to evaporate the solvent, and the adhesive was placed.

A handle such as Vivastick (Figure 13, Ivoclar/Vivadent) or Pic-N-Stic (Pulpdent) can be used to hold the restoration. The restoration was luted with dual-cure luting material.

Figure 13. A Vivastick is used to transfer the onlay to the preparation. Figure 14. The final restoration is luted into position with Appeal Posterior.

The restoration was held in place with a plastic instrument and tacked into place with a 2-mm light probe. Floss was passed through the contact occluso-gingivally to remove excess composite, and a brush or plastic instrument can be used to remove gross excess. Glycerin can be used to reduce the oxygen-inhibited layer.

Light curing from the occlusal, buccal, and lingual aspects was accomplished by exposure for 40 seconds each. The occlusion can be adjusted as necessary with carbide finishing burs. The restoration was polished with finishing points and composite polishing paste. Interproximal strips can be used to smooth the gingival margin.

A surface sealant (Opti-guard, Kerr) may be placed to seal any micro-cracks that develop during trimming (Figure 14). If a color change is desired and the tooth is located in an aesthetic zone, the buccal margin can also be placed at the gingival margin or 0.5 mm subgingivally. A translucent composite can be placed at this margin to achieve a “contact lens” effect. This will be illustrated in the next case.


Figure 15. Tooth No. 12 with an extensive MOD amalgam and fractured buccal cusp. Figure 16. A combined reversible-irreversible quadrant impression is obtained.
Figure 17. Buccal view of luted onlay demonstrates excellent color match. Figure 18. Occusal view with established proximal contacts.

The patient presented with a large MOD amalgam in tooth No. 12 with a fractured buccal cusp (Figure 15). The old restoration was removed. The preparation was flared to eliminate any undercuts and provide a definite path of insertion. Because the buccal aspect was in an aesthetic zone, a short, 45° bevel was placed. By placing a translucent composite at this aspect, the underlying dentin can show through so that the restoration blends with the surrounding tooth structure (“contact lens” effect). An impression was obtained (Figure 16), and the restoration was fabricated in the same manner as described in the first case (Figures 17 and 18).



A technique for fabricating an indirect composite onlay in a single visit using a combination of reversible-irreversible hydrocolloid impression material has been presented. Composite resin can be used in a variety of techniques for the restoration of posterior teeth: direct placement, chairside-indirect, and indirect. The direct technique is most often used for small to medium class I and II cavities. It is the least expensive and most conservative of natural tooth structure.  The disadvantages are more chairside time (especially if several restorations are involved) and the dependence on the clinician’s artistic ability. Indirect chairside posterior composite restorations can be performed in a single appointment, as demonstrated here. The benefits of an indirect restoration include the following: ideal proximal contour, contacts, and occlusion, and less polymerization shrinkage. Restoration of most of the teeth in an arch is best achieved by utilizing the services of a laboratory.


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Dr. Trushkowsky is a fellow in the Academy of General Dentistry, the Academy of Dental Materials, the International College of Dentists, the American College of Dentists, and the Pierre Fauchard Academy. He wrote a chapter on direct composites in Esthetic Dentistry published by CV Mosby and a chapter on complex single-tooth restorations in Dental Clinics of North America. He has published more than 80 articles and abstracts in a variety of journals and magazines. He is on the editorial boards of Contem-porary Esthetics and Restorative Practice and Collaborative Techniques. He can be reached at (718) 948-5808 or composidoc@aol.com.