Introduction

In this article we describe the“Carrot Model” technique, alsoknown as the “Geller Model”.Model work is a step that is oftenneglected by dental technicians, orin some cases a task that is assignedto the least experienced person in the laboratory. Wedisagree with this philosophy and strongly believe thatmodel work is a crucial step in the fabrication of a dentalprosthesis. As a reminder, this is the first phase in any newcase and subsequently, the precision achieved on aporcelain crown/veneer will have no meaning if the cast isnot treated with proper care. Additionally, the cast is ouronly reference of the patient’s mouth, and it is the linkbetween the laboratory and the dental office. Thissignificance is the reason why we developed a reliabletechnique to achieve a highly esthetic and precise model.

The Technique

The “Carrot Model” consists of a slightly conical die(s)that is removable from the alveolar base, while keeping

the soft tissue untouched. This type of cast, or modelpreparation technique, presents several advantages: first itis esthetically pleasant. Second, we can repour one singledie on a large case, without (re-)preparing a complete newcast. In addition, the “Carrot Model” allows us to switchthe dies from investment to stone material very easily.Lastly, and most importantly, it allows us to focus on thetooth tissue relationship and design crowns, veneers, orbridges with a more adequate emergence profile in order toclose dark spaces or push the papillae. Despite theseadvantages, just as you would observe with the Pin Model,we have found that we still need a master cast to controlcontacts with the adjacent teeth and the occlusion inorder to reduce or eliminate clinical adjustments.

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“The CarrotModel”Olivier Tric, MDT, CDT

Fig. 1: Original cast

Fig. 2: Original cast ready to be duplicated Fig. 3: Duplication of the master cast

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This Specific Case

This particular case presents diastemas in the anteriorregion. Teeth 6 through 11 are unprepared teeth whichwill be later restored with some “no prep veneers”.Because the original impression was not available, the firststep is to duplicate the master cast (Fig. 1).Manufacturers offer different type of duplicatingmaterials. A proper material selection is essential when

having diastemas or undercuts. Figures 3-5 illustrate thefirst duplicate made with a material with an insufficientcoefficient of elasticity for the teeth shape/positioning ofthis specific case. The silicone was not resistant enoughand was breaking between the central (Fig. 5).A new duplicate with appropriate physical propertieswas made (vinyl polysiloxane from GC America)(Fig. 6). Identical stretch does not tear the material now(Figs. 7-8).

Figs. 4 and 5: Testing the hardness of the impression material

Fig. 4 Fig. 5

Fig. 6: Reduplicationof the master cast

Figs. 7 and 8:Testing thehardness of thesecond impressionmaterial

Fig. 7 Fig. 8

Fig. 9: Pour of the silicon impression Fig. 10: Duplicated cast

Fig. 11: Cutting each tooth Fig. 12: Each tooth is separated

Fig. 13: Side view of a tooth/die Fig. 14: Visualization of the carrot before trimming

Preparing the Carrots

The next step is to create a new cast (Figs. 9-10). We use atype 4 stone (Fujirock from GC America). Every tooth ordie is separated using a diamond disk (Fig. 11).In order to properly trim the dies into “carrots” we tracewith a pencil the shape of a cone on the slice of eachtooth/preparation (Figs. 13- 14). The bulk of the carrotcan be first shaped with a disk (Fig.15). Controlling theprecision of carrot trimming is a key step to achieving ahigh quality cast. Using the right instrumentation is

important (Figs. 16-19). Each die/ carrot must be of aconic shape, very smooth without undercuts while leavingenough space in between the other dies and silicone(Figs. 21-24).The different level of angulations or shape of each die(from base to CEJ) will determine the tightness of eachcarrot onto its alveoli (Fig. 20). A more parallel shapewill bring more stability but may be technique sensitive(Fig. 20 –right die). A less parallel shape (Fig. 20-left die)is suggested when starting this technique; however, it maydecrease the stability of the dies into their base.

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Fig. 15: Shaping the carrot 16: Proper bur shape 17: Incorrect bur shape

Figs. 18 and 19: Refining the carrot shape

Fig. 20: Comparison of different carrot shapes Fig. 21: Replacing the tooth/die in the silicon impression

We can also create a groove or a flat site to minimizeany rotation or movement.After the master dies are created, we must duplicatethem. It is not recommended to use the master dies tomake the alveolar base. Because of the small diameter atthe base of each die, we prefer to use a differentduplicating material (Figs. 26-30). Double Take (fromIvoclar) has a higher modulus of elasticity than theprevious vinyl polysiloxane and will allow us to stretchthe material at the entrance of each die while pouringstone or investment into it (Fig. 52).

Pouring and “Glazing” the “Sacrificed” Dies.

The replicas of the master dies are made (Figs. 34-35).Because, we will not use those dies for anything else otherthan to create the alveolar base, we also call them the“sacrificed” dies. In order to facilitate their separationfrom the base, we find advantages to pour them with asofter stone such a mounting stone (Whip-Mix).We like to call the following steps “glazing the carrot”.We add a thin, smooth and homogenous coat of paraffinwax around each duplicate die.

Fig. 18 Fig. 19

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Fig. 24: Flat sides are created to increase stability of the diesand allow room for the buccal wall of the base.

Fig. 25: Master dies/carrot

Fig. 26: Master dies with sticky wax onporcelain bottle cap.

Figs. 22 and 23: Verification of space around the tooth/die

Fig. 27: Master dies/carrots ready to beduplicated

Figs. 28 and 29: Duplication of the master dies

Fig. 28

Fig. 22 Fig. 23

Fig. 29

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Fig. 31: Cutting and removing the container Fig. 32: Removal of the stone dies

33: Silicone impression ready to be poured Fig. 34: Pour the second set of die

We must only apply it on the carrot area while avoidingthe cervical margin, tooth or surface preparation. The waxwe use is soft and has a low fusion temperature. Theapplication of the wax to the surface of the dies fills allsmall pores of the stone and creates a thin space with thealveolar base which will help to separate each tooth orpreparation from it (Fig. 36).Each die or tooth must be carefully replaced into theimpression. A little bit of sticky wax can be used at themargin area to hold the carrots during the pour(Figs. 37-38). However, it is very difficult to access thecircumference of every part when the case presents two or

more consecutive carrots. In this case, we prefer to extendeach tooth with a casting sprue which is also covered witha thin coat of wax. Each extension is then stabilized to themold or the impression with some pins and sticky wax(Fig. 39).

Pouring the Alveolar Base

The impression is poured with some type 4 stone(Fujirock from GC). There, the challenge is to captureevery detail of the mold such as the papillaes withoutmoving the carrots from their positioning. The use of a

Fig. 36: Glazing of the stone diesFig. 35: Original and duplicated dies.

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Figs. 37 and 38: Setting and fixing carrot back to position into mold.

Fig. 38Fig. 37

Fig. 39: Carrots in place. Fig. 40: Pouring of the alveolar base with type 4 stone.

Fig. 41: Poured alveolar base. Fig. 42: Alveolar base after 90 min.

vibrator is recommended, however, it should be set at thelowest power (Fig. 40).We fill the mold completely and we check if thecontacts between the sticky wax and the silicone remainsealed (Fig. 41). The stone can now set for about 60-90minutes, before removing the pins (Fig. 42) and usingsome water to separate the cast from the impression(Figs. 43-44).

Removing the Dies from the Base

Removing the carrots from the alveolar base is sometimesintricate and it is more than ever the quality and theaccuracy of the prior steps that will determine itssimplicity. In order to avoid any breakage, we like tosoften the wax in a bain-marie or water bath at 55-60degree Celsius for 2-3 minutes (Fig. 45).

Figs. 43 and 44: Using water to separate the base from the mold.

Fig. 44Fig. 43

Fig. 45: Bain-marie. Fig. 46: Removal of the sprues

Fig. 47: Access to the bottom of the carrots. Fig. 48: Pushing out the carrots.

Fig. 49: Trimming and cleaning the alveolar base.

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The Paraffin wax is now soft and each sprue can bepulled from the bottom of the cast (Fig. 46). We nowaccess the base of each die (Fig. 47). The previous stepcan be repeated if needed, before pushing each carrot withan instrument. The use of a steamer can also berecommended to melt the wax and facilitate the removalof the carrots (Fig. 48). All the dies are not being kept orreused. As said previously, they were only needed to createthe base.

Fig. 51: Material for the refractory dies. Fig. 52: Pouring the refractory dies.

53: Thermo-treatment of the refractory dies inburn-out oven.

54: Thermo-treatment of the refractory dies inporcelain oven.

55: Model withrefractory diesin place.

Fig. 50:Finished alveolar base.

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About the author

Olivier Tric, MDT, CDT, began his studies in France at the College of Leonardo De Vinci and the University of Pharo while concurrentlypursuing an apprenticeship in Dental Technology. After a 5 year apprenticeship, he launched a 15+ year career devoted to a thorough

understanding of the principles of Dental Aesthetics and the mastery of specialized techniques in Dental Laboratory Technology. His specialties include allfacets of porcelain restorations on both natural teeth and osseointegrated implants. He is recognized by his peers as a pioneer in the industry by developing newand unique methodologies that are taught worldwide.

He is widely published on the topic of ceramic layering techniques and aesthetic dentistry in industry journals such as Dental Dialogue, Practical Procedures& Aesthetic Dentistry and Quintessence Dental Technology. He also serves on the Editorial Board for Spectrum dialogue, and TeamWork Magazine. Hisprofessional affiliations include Design Technique International (DTI), Oral Design International Foundation, and others. Mr. Tric is a highly respectedlecturer and educator, teaching hands-on courses to dentists and technicians throughout the United States, Canada and Europe. He is regularly consulted byleading Dental Manufacturers and Laboratories for input on new product development. He currently operates Olivier Tric Dental Laboratory and EducationalCenter in the Chicago suburb of Elmhurst, Illinois.

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Figs. 56 and 57: Final cast withstone dies.

Fig. 57

Fig. 56

The base is totally cleaned from the melted wax withsome steam or boiling water. The base is then trimmedand dried (Figs. 49-50). Different set of dies can be madefrom the silicone. In this particular case a first set ispoured in refractory investment (orbit-vest from GC)(Figs. 51- 52). Two hours of setting time are necessarybefore removing them and proceeding with the usual stepsfor thermo treatment (Figs. 53-54).We can use different materials such as epoxy or gypsum

to pour a final, clean, and untouched set of dies. Anymaterial used must have its expansion controlled with theuse of the appropriate liquids, in order for the dies to havethe right tightness inside the alveolar base. The completedcast with its removable dies ready for presentation ordelivery is shown in Figs. 56-57.

Conclusion

The Geller Model is a simple project, but requiresaccuracy all along the process. No step is less importantthan another and each stage is determinant for a definiteand trustworthy outcome. However, no cast can be moreaccurate than a plain uncut cast. All veneers and crownswe create must be verified on a master cast before beingclinically tried. “Carrot Models” offer us essentialinformation on the soft tissue, and therefore determinethe contour of our porcelain work.