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Compendium

October 2010, Volume 31, Issue 8
Published by AEGIS Communications


Creating Well-Fitting Restorations with a Digital Impression System

Richard Smith, DDS

The introduction of digital impressioning devices is enabling the creation of better-fitting restorations via the improved data-capture capabilities of these devices. This case report explores the creation of a porcelain-fused-to-metal bridge for a patient, outlining the steps involved in capturing a digital impression and the subsequent changes these impressions enable in the laboratory. While still a relatively new technology, statistics show that restorations created with digital impressions result in better fit, faster seating, and fewer remakes.

Impressioning and the resulting fabrication of restorations are procedures that have typically been rife with opportunities for error.1-4 Traditional methods of impressioning require outstanding technique and handling in order to create a well-fitting restoration. Although many dentists reach a point at which they can perform the procedure confidently, they may still encounter an occasional misstep.5

Recently, the introduction of digital impressioning devices has offered the possibility of better-fitting restorations and greater productivity for the dentist.6 These systems offer users the ability to capture a detailed digital image of the oral environment and submit that information electronically to the laboratory. One such device-the Lava Chairside Oral Scanner C.O.S. (3M ESPE, http://www.3mespe.com)-uses a wand and chairside monitor to capture 3D impression data in a video sequence and allows the dentist to view the image in real time. This helps enable the user to ensure that all necessary data are captured and to complete any missing information before sending the image to the laboratory.7The device can be used in the creation of zirconia, pressed, lithium disilicate, and porcelain-fused-to-metal (PFM) restorations, and results in an outstanding marginal fit. Research has shown that the overall fit of crowns created with a Lava C.O.S. impression is significantly better than those made with traditional impression materials.8,9 A comparison of crowns made with a digital scan versus those created with a traditional impression found that the scanned restorations showed a greater number of perfect interproximal contacts, better fit and stability, better margins, and more perfect occlusion.8 In addition, the scanner's dimensional repeatability for single preparations has been shown to perform beyond the requirement to achieve clinical acceptability for marginal adaptation in a crown or bridge restoration.10 The time savings are significant,11 with one study demonstrating a 33% greater efficiency at the crown-seating appointment.8

Because the models for these restorations are created via automated stereolithography (SLA), additional research has been performed on the accuracy of the models. These studies have found no significant difference in accuracy of models produced using the Lava C.O.S. versus a traditional vinyl polysiloxane (VPS) material.12,13 An examination of whether any microlayer steps found on an SLA model would transfer to the margins of copings for PFM crowns found only a slight correlation between the microlayers and marginal characteristics.12 The following case demonstrates the use of this device to enable the creation of a well-fitting PFM bridge.

Case Presentation

A 46-year-old female presented with a missing tooth No. 30 (Figure 1) and a history of her existing resin-bonded bridge debonding twice. Due to deficient buccal lingual width of the edentulous region, the patient would have required ridge augmentation for an implant placement, which she did not want. The authors decided to restore the area with a traditional PFM bridge.

Observations based on an examination showed normal gingival architecture. Probing measurements averaged 1 mm to 3 mm, with a few isolated 4-mm interproximal pockets. The patient did not brux and had no medical contraindications. Bleeding was isolated with probing. A radiographic assessment showed a normal eruption pattern, and all other findings were within normal limits.

The initial preparation and impressioning appointment occurred February 2009, at which time diagnostic casts were created and a waxed tooth was developed in the No. 30 position in order to create a provisional form. The area was anesthetized, and Nos. 29 and 31 were prepared with a single-use diamond bur. Gingival troughing was performed with a diode laser in select areas, primarily interproximal. In nonesthetic regions, the margins were kept supragingival, when possible.

The double-cord technique was used for tissue management. An Ultradent #000 braided cord (http://www.ultradent.com), which was predipped in Hemodent (Premier, http://www.premusa.com) and dried, was placed first, followed by a #00 cord (Figure 2). A lip and cheek retractor was placed, and a Hygoformic (Pulpdent, http://www.pulpdent.com) saliva ejector was used to control saliva and retract the tongue (Figure 3).

The prepared teeth and edentulous area were lightly dusted with specially formulated titanium dioxide powder, and the top retraction cord was removed (Figure 4). The teeth were then powdered lightly again, as well as the remaining teeth in the arch. The Lava C.O.S. was used to scan the prepared teeth for approximately 2 mins, and the scan was reviewed (Figure 5). Then, the remainder of the lower arch was scanned and the image was reviewed once more. The opposing arch was dried, powdered, and scanned, as well as the bite. The full scan of both arches articulated was reviewed (Figure 6), and a digital prescription was submitted electronically to the laboratory. A temporary bridge was then fabricated and placed.

After the dies were marked by a technician at an authorized Lava design center and a virtual model was created, the information was sent to an SLA facility and a durable model was manufactured. The model was then shipped to the author's local laboratory for construction of the restoration (Figure 7). The patient returned for a bisque bake try-in, at which the author verified the occlusion and made minor adjustments. No interproximal adjustments were necessary, and the restoration was returned to the laboratory for final anatomy and glazing. On receipt of the final product, the author was able to seat it quickly and easily (Figure 8).

Discussion

As many clinicians have experienced with the use of traditional impression techniques, the more complicated the case, the greater the chance for error in capturing the impression.14 The time lost to retakes and remakes in a typical dental practice can be significant, resulting directly in lost productivity. Using a digital scanning device and proper tissue management, dentists can be more confident that their impressions will be accurate the first time and the resulting restorations will fit well.

In the author's experience, patients have expressed satisfaction with the scanning procedure, especially those who have experienced traditional impression taking, because this device causes no discomfort or gagging. One study found that out of 122 patients who had both the traditional impression process and the digital scan, three quarters preferred the scan.8

These factors, however important, are not the main benefit of digital impressioning. Its ability to produce exquisitely accurate and predictable restorations while boosting productivity and reducing seating times are the key factors that make this device so ideal. The author has experienced dramatic reductions in seating times. Without question, the author's average seating time has been reduced 50%, providing him with the confidence to reserve less chair time for seating these cases. The author believes that restorations created with this system will seat properly, quickly, and accurately.

References

1. Kaplowitz GJ. Trouble-shooting dual arch impressions. J Am Dent Assoc. 1996;127(2):234-240.

2. Christensen GJ. Will digital impressions eliminate the current problems with conventional impressions? J Am Dent Assoc. 2008;139(6):761-763.

3. Christensen GJ. The state of fixed prosthodontic impressions: room for improvement. J Am Dent Assoc. 2005;136(3):343-346.

4. Christensen GJ. Laboratories want better impressions. J Am Dent Assoc. 2007;138(4):527-529.

5. Christensen GJ. The challenge to conventional impressions. J Am Dent Assoc. 2008;139(3):347-349.

6. Christensen GJ. Impressions are changing: deciding on conventional, digital or digital plus in-office milling. J Am Dent Assoc. 2009;140(10):1301-1304.

7. Feuerstein P, Puri S. CAD/CAM and Digital Impressions. Dental Economics supplement. January 2009.

8. Farah JW, Brown L. Comparison of the fit of crowns based on digital impressions with 3M ESPE Lava Chairside Oral Scanner C.O.S. vs. traditional impressions. The Dental Advisor Research Report. No 22. June 2009.

9. Hirayama H, Chang YC. Fit of zirconia copings generated from a digital impression technique and a conventional impression technique [master's thesis]. Boston, MA: Tufts University of Dental Medicine.

10. Balakrishnama S, Wenzel KW, Bergeron J, et al. Dimensional repeatability from LAVA COS 3D Intra-oral Scanning System. Presented at: International Association for Dental Research General Session; April 4, 2009; Miami, FL.

11. Lowe RA. CAD/CAM Dentistry and Chairside Digital Impression Making. Dental Economics supplement. September 2009.

12. Moore W, Dunne P, Doherty E, et al. Margin characteristics of PFM crown copings fabricated on stereolithography models. Presented at: AADR Annual Meeting; March 5, 2010: Washington DC.

13. Ogledzki M, Wenzel K, Doherty E, et al. Accuracy of 3M-Brontes stereolithography models compared to plaster models. Presented at: AADR Annual Meeting; March 5, 2010; Washington DC.

14. Pyle MA. An impression technique for severely resorbed mandibles in geriatric patients. J Am Dent Assoc. 1999;130(2):255-256.

About the Author

Richard Smith, DDS
Clinical Instructor
Department of Reconstructive Sciences
University of Connecticut School of Dental Medicine
Farmington, Connecticut

Private Practice
Middlebury, Connecticut


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Image Gallery

Figure 1  The original resin-bonded bridge on the patient"s missing tooth No. 30 had debonded twice; the ridge had resorbed significantly.

Figure 1

Figure 2  Two retraction cords were placed around each preparation.

Figure 2

Figure 4  The area was lightly dusted with scanning powder, and the top cord was removed.

Figure 4

Figure 5  The digital scanner displays the image chairside, enabling the dentist to review it for accuracy.

Figure 5

Figure 6  Digital image of scan results for both arches.

Figure 6

Figure 7  The complete articulated SLA model with restoration in place.

Figure 7

Figure 8  Buccal view of the seated restoration.

Figure 8