Digital Impression System Considerations
A patient-friendly way to expedite clinical workflow
Impressions are a critical component in the fabrication of fixed dental prostheses. However, the traditional impression-taking technique with polyvinyl siloxane (PVS) or polyether materials remains a cumbersome procedure for both the dentist and the patient. During the impression-making procedure, many errors can occur that can produce a less than ideal prosthesis. Some factors that limit the accuracy and ease of traditional impression-making include errors in tray selection; inadequate adhesive application; poor hemorrhage control; inadequate soft tissue retraction; limitations in the impression material’s flow and hydrophilicity; short working time; patient movement while the impression is setting; tearing and deformation of the impression during removal; dimensional stability of the set impression; required disinfecting protocol; and inadequate wetting and voids when pouring the cast.1 Digital impressions eliminate many of the steps required with traditional impressions such as selecting the tray, disinfecting the impression, shipping the impression, pouring the stone model, manual die trimming, and reducing the steps required for articulation.
Clinical skill, experience, and the particular clinical situation can also substantially affect the quality of a final impression. One report stated that more than 89% of impressions sent to the laboratory for prosthesis fabrication were inadequate, with one or more observable errors.2 A large clinical study evaluating 300 impressions stated that bleeding was most often associated with inadequate impressions.3 Some material and technique limitations found in traditional impression-making techniques were addressed when digital impressions were introduced in the 1980s. Christensen described advantages of the digital impression technique as improved patient acceptance, reduced impression material distortion, 3D pre-visualization of the preparation, and potential savings in cost and time.4
Types of Intraoral Scanners
Digital impressions obtained by intraoral scanning devices were originally a part of CAD/CAM systems. These in-office CAD/CAM systems produce an oral scan (digital impression) of prepared teeth. Computer-assisted design of the final prosthesis is completed, and the file is sent to an in-office milling machine, where the final prosthesis is milled from a ceramic or composite block. More recently, intraoral scanners were introduced as stand-alone devices that capture a digital impression and send the file to a dental laboratory for prosthesis fabrication. With these stand-alone intraoral scanners, the design of the crown cannot be performed with the device; however, some systems allow the clinician to indicate the location of the margins prior to sending the scan to the lab.
Each scanning system offers benefits which determine convenience for individual practitioners, such as size of wand, use of powder, method of scanning (individual images or video capture), wand positioning (hovering above tooth or resting on tooth), ability to capture color, ability to capture full-mouth scans, method of obtaining intra-occlusal record, ease of software, portability of device, and time/scans required for scanning. After scanning, the final prostheses can either be fabricated directly (in-office or lab milled) from digital information or indirectly (cast or pressed) from a resin model. Each company determines if the digital impression can be sent directly to the lab in an STL (stereolithography or “send-to-lab”) file or sent to the company first and if a fee is applied. The monthly service or data plans included with some systems must also be factored when calculating a cost analysis. All of these intraoral scanners are also capable of full-arch scans, and some systems are also capable of fabricating orthodontic retainers and integrating with cone-beam computed tomography images for implant planning.
Advantages and Disadvantages of Digital Impressions
Digital impressions offer advantages to both the clinician and the patient. For clinicians, digital impressions may reduce laboratory remakes because the computer screen image of the prepared teeth is magnified, improving the view of the preparation quality. If defects or inadequate preparation reduction are noted in the preparation, that area may be rescanned rather than making another impression, as with conventional impression techniques. Additionally, digital impression devices measure the occlusal clearance between the prepared and opposing teeth and allow the dentist to make changes if required before sending the impression to the laboratory. A major advantage of digital impression systems is their ability to stop the imaging process at any time and continue, which allows the dentist to remove blood and saliva and then continue scanning; this is helpful in large cases with multiple preparations. The accuracy and durability of the model produced by some digital systems are also significant advantages. Made of resin, these models are significantly more abrasion-resistant and precise than gypsum models. Peripheral advantages of digital impressions include elimination of impression disinfection and the need for shipping and cast storage. Another advantage of digital systems is that the laboratory prescription accompanies the digital impression file and is completed before making the digital impression. In fact, the digital impression cannot be started the laboratory prescription is completed.
These devices are also extremely patient friendly. They eliminate the uncomfortable, messy impression-taking process, which causes some patients to gag. A recent study reported that patients found digital impressions more convenient than conventional impressions and would prefer a digital impression in future visits.6 Digital impressions may shorten prosthesis delivery time, because Internet transfer of the file and fewer laboratory procedures (pouring stone models, mounting casts, etc.), reduce the total prosthesis fabrication time. In addition, there is the “wow factor” produced when the digital impression appears on the screen and the patient asks, “Are those my teeth?”
Digital impression systems do have some disadvantages. Like conventional impressions, the gingival margin must be captured to produce an accurate impression. Digital impressions require that the entire margin is exposed along with 0.5 mm of tooth structure apical to the margin to ensure a favorable emergence profile.5 Digital impression taking is also associated with a significant initial cost investment, an operating learning curve, the need for equipment repair and updates, and a cost for processing the digital impression. Also, a larger wand can be a problem for patients with restricted opening. The use of powder with some systems is a disadvantage that discourages some dentists from using a particular system.
Accuracy and Efficiency of Digital vs. Conventional Impressions
As stated earlier, digital impressions are just as accurate and can be more cost- and time-efficient than traditional impressions. Several studies have compared the clinical success and accuracy of digital impressions to conventional impressions. Henkel7 compared crowns generated from conventional and digital impressions (iTero, Align Technology, www.aligntech.com). In 68% of the cases, the digital-impression–generated crowns were selected as the crown of choice for insertion based on the clinical acceptability criteria; 85% of all crowns produced with the iTero system were clinically acceptable compared with 74% of conventionally produced crowns. Crowns made from the iTero impressions required less adjustment time. Kugel and colleagues8 compared copings made by the Lava™ C.O.S. (3M ESPE, www.3mespe.com) digital impressions and PVS conventional impressions. The marginal accuracy of the Lava™ zirconia crowns (3M ESPE) produced from the Lava C.O.S. showed no statistically significant difference compared to crowns made from conventional impressions. Syrek and colleagues9 compared crowns fabricated from PVS impressions and the Lava C.O.S. They determined that crowns fabricated from digital impressions had better marginal fit and proximal contact and required equal occlusal adjustment. Ender and Mehl10 compared the accuracy and precision of digital and convention impressions. Direct scans of a reference die were superimposed over scans of a plaster model of the die made from a polyether impression and digital impressions of die taken with Lava C.O.S and CEREC AC. They reported that digital impressions were equally or more accurate and precise than plaster models.
Several recent studies compared crowns fabricated from current generation CAD/CAM systems with those from a conventional impression material and reported similar marginal fit.11-13 These studies demonstrate that impressions, models, and crowns fabricated from digital intraoral scanners were equally or more accurate than those produced from traditional impression materials. One area of limitation identified by laboratory studies is that not all systems produced comparable accuracy and precision of full-arch scans.14
The time required to make digital impressions compared to conventional impressions is controversial. Although the 3- to 5-minute scanning process requires roughly the same amount of time as traditional impression materials setting time (fast set is 2.5 minutes) there is no need to select a tray, apply tray adhesive, clean impression trays, assemble impression guns and tips, or disinfect impressions. There are still time-consuming steps associated with some digital impression systems, including wheeling the device to the operatory, plugging it in and loading the software. Additionally, the unit and wand require disinfection. Another time-saving feature of a digital impression compared to a conventional impression is that a portion of a deficient area may be re-scanned with the digital system rather than retaking the entire impression. Lee and Gallucci15 reported that the total treatment time for iTero digital impressions of single implant restorations was approximately half the treatment time of a traditional impression. A study by Ahrberg and colleagues16 reported saving approximately 5 minutes with digital impressioning of a quadrant and saving 1 minute 30 seconds with a full-arch tray. A controlled double-blind clinical study by Givan and colleagues17 reported no difference in 50 crown margins made from digital and conventional impressions. This study reported significantly higher impression time with iTero-produced digital impressions (8 minutes, 40 seconds) compared to fast-set traditional impressions (4 minutes, 23 seconds); however, conventional impression set-up and clean-up time were not included.
Future Developments
In the evolution of digital impression systems, some potential improvements remain. One obstacle that remains for all impression systems is management of soft tissue and oral fluids surrounding the crown margins. Development of high-resolution ultrasound technology for digital intraoral scanning would allow the scanner to “see-through” soft tissue and oral fluids.
Another area for improvement is integration of digital impressions with patient management software. When intraoral scans can be accessed in patient management software as easily as digital radiographs, the clinician may appreciate additional benefits of digital models for treatment planning and patient education. Additional efficiency could be achieved by making an intraoral scanner that could be plugged directly into the USB port of an operatory computer similar to a digital radiograph sensor.
Conclusion
Digital impressions offer advantages over conventional impressions by expediting the laboratory process, avoiding an uncomfortable conventional impression material, and allowing a magnified view of the preparation. Current studies suggest that digital impressions produce crowns as or more accurate than conventional impressions.6-13 More data must to be collected, however, to substantiate that digital impressions are more time efficient than conventional impressions.15-17
Disclosures
John O. Burgess, DDS, MS; Nathaniel C. Lawson, DMD, PhD; and Augusto Robles, DDS, MS, have received research/grant support from 3M ESPE.
About the Authors
John O. Burgess, DDS, MS
University of Alabamaat Birminghamc
Birmingham, Alabama
Nathaniel C. Lawson DMD, PhD
University of Alabamaat Birmingham
Birmingham, Alabama
Augusto Robles, DDS, MS
University of Alabamaat Birmingham
Birmingham, Alabama
References
1. Christensen GJ. The state of fixed prosthodontic impressions: room for improvement. J Am Dent Assoc. 2005;136(3):343-346.
2. Samet N, Shohat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. J Prosthet Dent. 2005; 94(2):112-117.
3. Cakir D, Anabtawi M, O’Neal S, et al. Clinical comparison of two impression materials: effectiveness for inexperienced operators. J Dent Res. 2010;89(Spec Iss A):130104. Abstract 2951.
4. Christensen GJ. Impressions are changing: deciding on conventional, digital or digital plus in-office milling. J Am Dent Assoc. 2009;140:1301-1304.
5. Lowe RA. CAD/CAM Dentistry and Chairside Digital Impression Making. PennWell; 2009.
6. Joda T, Brägger U. Patient-centered outcomes comparing digital and conventional implant impression procedures: a randomized crossover trial. Clin Oral Implants Res. 2015;26(4):1-5. doi: 10.1111/clr.12600.
7. Henkel GL. A comparison of fixed prostheses generated from conventional vs digitally scanned dental impressions. Compend Contin Educ Dent. 2007:28(8):422-431.
8. Kugel G, Chaimattayompol N, Perry R, et al. Comparison of digital vs. conventional impression systems for marginal accuracy. J Dent Res. 2008;87(spec iss a):1119.
9. Syrek A, Reich G, Ranftl D, et al. Clinical evaluation of all-ceramic crowns fabricated from intraoral digital impressions based on the principle of active wavefront sampling. J Dent. 2010;38(7):553-559.
10. Ender A, Mehl A. Full arch scans: conventional versus digital impressions—an in-vitro study. Int J Comput Dent. 2011;14(1):11-21.
11. Abdel-Azim T, Rogers K, Elathamna E, et al. Comparison of the marginal fit of lithium disilicate crowns fabricated with CAD/CAM technology by using conventional impressions and two intraoral digital scanners [published online ahead of print June 19 2015]. J Prosthet Dent. 2015. doi: 10.1016/j.prosdent.2015.04.001. Accessed July 30, 2015.
12. Alfaro DP, Ruse ND, Carvalho RM, Wyatt CC. Assessment of the Internal Fit of Lithium Disilicate Crowns Using Micro-CT. J Prosthodont. 2015;24(5):381-386.
13. Boeddinghaus M, Breloer ES, Rehmann P, Wöstmann B. Accuracy of single-tooth restorations based on intraoral digital and conventional impressions in patients [published online ahead of print February 20 2015]. Clin Oral Investig. 2015. http://link.springer.com/article/10.1007/s00784-015-1430-7. Accessed July 30, 2015.
14. Patzelt SB, Emmanouilidi A, Stampf S, et al. Accuracy of full-arch scans using intraoral scanners. Clin Oral Investig. 2014;18(6):1687-1694.
15. Lee SJ, Gallucci GO. Digital vs. conventional implant impressions: efficiency outcomes. Clin Oral Implants Res. 2013;24(1):111-115.
16. Ahrberg D, Lauer HC, Ahrberg M, Weigl P. Evaluation of fit and efficiency of CAD/CAM fabricated all-ceramic restorations based on direct and indirect digitalization: a double-blinded, randomized clinical trial [published online ahead of print June 14 2015]. Clin Oral Investig. 2015. http://link.springer.com/article/10.1007%2Fs00784-015-1504-6#page-1. Accessed July 30, 2015.
17. Givan DA, Burgess JO, O’Neal SJ, Aponte AA. Prospective evaluation of ceramic crowns by digital and conventional impressions. J Dent Res. 2011;90(spec iss a):380.
Digital Impression Showcase
Figure 1 | 3M True Definition Scanner
3M ESPE’s 3M True Definition Scanner delivers powerful 3D video–based scanning and provides precision, accuracy, and repeatability for taking digital impressions. The technology is contained in a small, ergonomic, lightweight wand.
800-634-2249
3mespe.com
Figure 2 | CS 3500
With the CS 3500 intraoral scanner, practitioners can easily acquire precise true color, 2D and 3D digital impressions. The CS 3500 works as part of the integrated CS Solutions CAD/CAM restoration portfolio or as a standalone solution in a practice’s restorative workflow.
800-944-6365
carestreamdental.com
Figure 3 | Planmeca PlanScan
Planmeca PlanScan captures the oral environment with a high level of precision. The two-piece cradle features a weighted base or it can be wall-mounted or separated to fit into a standard handpiece caddy.
800-537-6070
planmecacadcam.com
Figure 4 | iTero Element
Align Technology Inc.’s iTero Element features industry-leading 3D imaging designed to capture 20 color scans per second. Scanned images appear instantly on the 19” high-definition multi-touch display.
800-577-8767
aligntech.com
Figure 5 | Dental Wings DWIO
Dental Wings dwio brings ease-of-use to a new level with its innovative design. The small, light handpiece allows the dentist to take digital impressions in a natural, fluid manner while focusing on the patient’s mouth.
888-856-6997
dentalwings.com
Figure 6 | TRIOS
Shaped much like an operatory impression gun, TRIOS digital scanner balances firmly in the hand, providing steady control of the device. A unique autoclavable scanner tip allows the user to rotate the scan source from the mandible to the maxilla, capturing up to a quadrant in 25 seconds.
908-867-0144
3shape.com
Figure 7 | CEREC AC with Omnicam
Sirona’s CEREC Omnicam’s revolutionary design features an ergonomic handpiece and special optics. It has a rounded camera tube that allows allows easy rotation of camera and offers easy-to-learn, intuitive, full-color scanning without powder or an opaquing agent.
800-659-5977
sirona.com
