Digital Master Impressions: A Clinical Reality

Digital Master Impressions: A Clinical Reality

The excellence and marginal fit of the definitive laboratory restorations are only as good as the master dies from which they are created. The precision of the master impression cannot be compromised. Marginal detail and tooth structure apical to the restorative margin are both necessary elements of an acceptable final impression.

This article discusses the use of iTero by Cadent, which allows the dentist to accurately "impress" the oral cavity without the use of conventional impression materials. The acquired optical information is then converted by Cadent to a master working model that is sent (within 48 hours of receiving the digital information from the dentist) to the laboratory of the dentist's choice, who immediately begins the restoration fabrication process.The Cadent-produced polyurethane master model is extremely resistant to abrasion, and it preserves the soft tissue architecture around the preparations by creating master dies that "emerge" from the gingival tissues.This permits the ceramist to accurately follow the "gingival silhouette" in the creation of the emergence profile.

Tissue Management

A "digital impression" taken by a scanning device cannot "see" through the gingival tissues or through fluid (hemorrhage, or crevicular fluid) in the sulcus. Because the optical scanner must be able to clearly see the restorative margin and the tooth or root surface just apical to the margin to create an accurate master die, digital impression-making must still include proper tissue management and retraction!

Using iTero, it is convenient to scan the opposing arch while the patient is getting numb. Depending on the number of teeth, or types of restorations planned, a quadrant scan, or full-arch scan can be done. Once the necessary preparation information is entered into the patient's record in iTero, the software indicates which views are required for that particular case. After tooth preparation is completed, the soft tissues surrounding the preparation, if the restorative margin is located equicrevicular or intracrevicular, must be retracted from the margin in such a way that controls fluid contamination in the sulcus prior to completing the scanning process of the prepared arch. There are several ways to accomplish this:

1) Two-Cord Tissue Retraction
First, a #00 cord (UltraPak) is packed around each preparation margin starting from the lingual proximal to the facial aspect, then back through the remaining proximal area to the lingual aspect. The excess at both lingual ends is trimmed, and the ends of the cord are tucked into the lingual gingival sulcus so that the ends butt against one another. If desired, the cords may be soaked in a hemostatic solution then dried with a 2X2 prior to placement.

Next, a #1 cord (UltraPak) is placed on top of the #00 in the same fashion as previously described. The preparation is cleansed with AcQuaSeal Dentin Desensitizer on a cotton pledget. When ready, the #1 cord is teased out of the sulcus from the facial aspect of each preparation using the tyne of an explorer, and the amount of retraction is evaluated.

The optical impression should capture not only the entire restorative margin, but also about 0.5 mm of the tooth/root surface apical to the margin. If the marginal gingiva adjacent to any restorative margin rebounds to contact the tooth/margin, a small piece of a larger diameter cord (#2) is placed into the affected area for an additional minute, and then removed prior to scanning. This added retraction should be sufficient to create a space between the tooth surface and the inner lining of the gingival sulcus.

The goal of retraction is to "create a moat" (space for the optical scanner to clearly "see" the margin and tooth surface) around the castle (tooth preparation).

2) Laser Troughing
The use of both diode and Er, Cr, YSGG wavelengths (EZLase and Waterlase MD) can be natural adjunct to soft tissue management for optical impressions. In many cases, troughing alone will allow visualization of the restorative margin and emergence profile so that an accurate scan can be achieved. Also, lasers are helpful in hemostasis, therefore the ideal scanning environment can be accomplished very easily and quickly using laser technology.

It is recommended to trough a case only when there is enough horizontal thickness of tissue that troughing around the preparation does not result in loss of vertical tissue height. Beware of thin periotypes and low crest bone positions relating to biologic width as these types of patients are prone to gingival recession.

That being said, when gingival troughing is indicated to aid in gingival displacement, use of either the diode laser or the Er,Cr: YSGG laser is much more predictable than using an electrosurgery instrument. When troughing with an electrosurgery instrument, the unpredictable zone of necrosis, particularly in a thin sulcular environment, can lead to excessive recession and gingival sloughing following healing. Since it is reported that the necrosis resulting from a laser wound is only a few cell layers thick, this is much less likely to happen when troughing with a laser.

It is still a good practice to use mechanical tissue retraction when indicated, particularly in the aesthetic zone (facial aspect), and use gingival troughing when localized gingival excess, or chronic inflammation is present, and in areas outside the esthetic zone

3) Retraction Paste
An alternative to retraction cord in troughing procedures to can a bit more hemostasis and tissue deflection would be to use Expasyl after troughing, prior to scanning the preparation. In the case below as illustrated in Figure 4, troughing was performed interproximally, and retraction cord was placed following troughing to create 360 degrees of totally patent sulcus for scanning to take place. This will virtually assure a perfect impression every time. Remember, the goal is not only to capture the margins, but also 0.5mm of root surface apical to the margin so that the laboratory can create the proper emergence angle for the restoration.

Optical Scanning of the Prepared Tooth

The iTero software indicates which views are required for the optical impression. An occlusal view of each preparation is taken and can then be reviewed on the monitor for accuracy prior to taking additional scans. For each preparation, a facial, lingual, mesio-proximal and disto-proximal view is recorded. This takes the operator approximately 15 to 20 seconds per prepared tooth. The adjacent teeth are scanned from the facial and lingual aspect, and finally two interocclusal views are taken to capture the jaw relationship.

iTero software also allows the dentist to view the entire case in three dimensions prior to sending the information to the lab. In fact, the software will indicate occlusal clearance in critical areas of the prepared tooth, so if there is not enough space for the ceramist to create an optimal restoration, the dentist can make an immediate correction, take an additional scan or scans of the corrected preparation, and the software will correct the virtual model. This can eliminate the need for reduction copings and/or additional patient visits since the lack of space required is not usually found out until the technician pours up and mounts a case. iTero provides immediate feedback to the dentist so corrections can be made on the spot!

The entire process for scanning an average case takes about 2 minutes from start to finish. The average fast set impression material takes about 1 to 2 minutes to inject, load and place the tray and an additional 3 to 4 minutes to complete the intraoral set. A provisional restoration is then fabricated and placed prior to dismissal of the patient.

Figure 1. A preoperative view of tooth numbers 3 to 5 prior to preparation for indirect ceramic replacements due to marginal breakdown and recurrent decay.

For the first case demonstrated in this article (Figure 1), tooth numbers 3 through 5 were to be prepared for indirect ceramic restorations. A combination of tissue troughing with a diode laser (EZLase) and retraction cord was used to displace the gingival tissues around the margins of the preparations (Figures 2-4).

Figure 2. A diode laser (EZLase) is used to trough around the preparation in preparation for taking the optical impression.

Figure 3. Retraction cord (Ultrapak) is "laid" into the trough created by the laser to deflect any tissue tags that may remain after the troughing procedure.

Figure 4. An occlusal view of the retraction around the preparations prior to capturing the optical impression.

A conventional elastomeric impression was taken as well as an iTero optical scan (Figure 5).


Figure 5. A dental assistant is shown scanning the opposing arch of the patient with iTero while the local anesthesia in the prepared arch is taking effect.


Also, occlusal relationships for both iTero master models (interocclusal scan) and an opposing model created from an elastomeric impression and interocclusal bite records were taken. The definitive ceramic restorations are shown in both sets of master dies that were created (Figure 6-8).


Figure 6


Figure 7. An occlusal view of the ceramic restorations are shown. (Restorations fabricated by DSG/Americus Laboratory, Jamaica, New York)


Figure 8. An occlusal view of the ceramic restorations on the Itero master model is shown.

Both sets of dies fit very well from the marginal perspective and are basically interchangeable on the two sets of models. This confirms to the operator the accuracy of the scanning technology is excellent as are the models that are generated by Cadent for the laboratory.


Figure 9. After a total etch of the preparations with 37% phosphoric acid for 15 seconds, a small amount of blood in the retracted tissue was noted after rinsing and drying the area. Expasyl is an excellent hemostatic agent and tissue deflector during delivery of ceramic restorations to ensure a "blood free" sulcular environment.

Once the marginal fit, proximal contact, and occlusal contact is verified, and hemostasis is achieved (Figure 9), the restorations are cemented using a total-etch technique with resin cement NX3 (Figure 10).


Figure 10. The resin cement (NX3) is syringed directly into the preparation using an automix tip.

The margins are polished using porcelain polishing points (Porcelain Polishing Points).


Figure 11. After cementation, the margins of the ceramic inlay in tooth number 5 are being polished with a porcelain polishing point. The operative area is isolated during the delivery procedure with Isolite.

The completed restorations are shown in an occlusal view in Figure 12.


Figure 12. An occlusal view of the completed porcelain restorations. Note the precision of the marginal fit.

I've found that the marginal fit and occlusal contacts appears to be better overall on the iTero created dies, and I attribute that to the accuracy of the scan, combined with the precision and durability of the model created by Cadent (Figures 13-19).

Figure 13. This is a lingual view of an Itero master model. Note the accuracy of the occlusal contacts.

Figure 14. A close-up view of the master die as it emerges from the soft tissue from the lingual aspect. Note the amount of occlusal clearance between the preparation and the opposing arch.

Figure 15. A close up proximal view that demonstrates the marginal fit of this full coverage zirconium restoration.

Figure 16. The occlusal contacts are demonstrated using Accufilm II after cementation with a modified resin ionomer cement (FujiCem). This restoration was delivered without any proximal or occlusal adjustment.

Figure 17. This is a view of a full coverage pressed ceramic crown (tooth number 3) and inlay (tooth number 4)on the Itero master dies prior to cementation.(Restorations fabricated by DSG/Americus Laboratory, Jamaica, New York)

Figure 18. An occlusal view of the restorations shown in Figure 17 after cementation.

Figure 19. A facial view of the completed restorations shown in figure 17 in maximum intercuspation (centric occlusion). An excellent clinical result is shown that was built on a master model created by a digital impression.

Of course, ultimately the precision of the restoration itself is depended upon excellence by the laboratory support as well. However keeping in mind that it has been stated that only 20% to 30% of impressions made and received by dental laboratories can be classified as "clinically excellent", we as a profession have a way to go in ensuring the quality of our clinical work is kept to as high a standard as possible.

Optical impressions taken by systems like iTero can help raise the overall quality of the service we provide to our patients. Technology does not come without a price, but remember--our most precious commodity is chair time. If a technology helps us work better, smatter, more efficient, and allows us to produce better results.....it is a small price to pay and the patient ultimately benefits!

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