Comparing Digital and Conventional Impressions
Assessing the accuracy, efficiency, and value of today’s systems
Impressions are a critical component in the fabrication of fixed dental prostheses. Traditional impression taking with polyvinyl siloxane or polyether materials remains a cumbersome procedure for both dentist and patient. During the impression-making procedure, many errors can occur that can produce a less than ideal prosthesis.
Some factors that limit the accuracy 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; tearing and deformation of the impression during removal; dimensional stability of the set impression; required disinfection;; and inadequate wetting and voids when pouring the cast.1 Digital impressions eliminate many of the steps required with traditional impressions such as tray selection, disinfecting and shipping the impression, pouring the stone model, manual die trimming, and some steps required for articulation.
Clinical skill, experience, and the particular clinical situation can also substantially affect the quality of a final impression. One study reported that more than 89% of impressions sent to the laboratory for prosthesis fabrication were inadequate, with one or more observable errors.2 Another clinical study reported 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 This review will summarize digital impression systems, generalize their advantages and disadvantages, and compare their accuracy and efficiency to traditional impression materials.
Types of Intraoral Scanners
Digital impressions obtained by intraoral scanning devices were originally a part of CAD/CAM systems. These in-office systems (eg, CEREC®, Sirona Dental Systems, www.cereconline.com; E4D® Dentist, E4D Technologies, www.e4d.com) produce a 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. Intraoral scanners that are commercially available include the iTero™ Digital Impression System (Align Technology, www.aligntech.com), True Definition Scanner (3M ESPE, www.3mespe.com), and IOS FastScan™ (IOS Technologies, www.ios3d.com) In addition, both Sirona (CEREC AC Bluecam and CEREC Omnicam) and E4D (NEVO™) have developed units and software for sending digital impressions from in-office scanners to dental laboratories.
Intraoral scanning systems use fundamentally different imaging technology to capture their 3D images. CEREC bases its imaging on triangulation, a technique in which a light source is reflected off an object and depth-dependent shifts in the light are detected by a sensor. Light triangulation limits accuracy when scanning curved surfaces—especially those that do not reflect light evenly, such as teeth with amalgam restorations. To correct this problem, surfaces to be imaged are coated with an opaque, reflective coating. An uneven or overly thick coating may compromise the accuracy of these systems. Their original stand alone intraoral scanner, CEREC AC (Sirona Dental Systems) with Bluecam technology, projects visible blue light with a shorter wavelength than previous models, improving scanner accuracy and reducing the amount of powder required for light reflection (Figure 1). Sirona recently released the Omnicam, which uses a powder-free, full-color video capturing system. The scanner collects white light video, which can be used to create a photorealistic image of the tooth. The files from these systems are then sent to an external laboratory using Sirona Connect software.
The original system from E4D, the E4D Dentist, uses oscillating laser light to capture a digital image. Lasers differ from traditional light sources in that they are produced from a single wavelength of light, making them more accurate but unable to capture color information. The new NEVO system (which has replaced the E4D Dentist) is a powder-free sensor that operates by use of a blue laser (Figure 2). The scanner can be plugged into a laptop and digital impressions can be sent to an external lab through the E4D Sky network.
3M ESPE developed Active Wavefront Sampling (AWS) for its original intraoral scanner, the Lava C.O.S. AWS uses blue light detected by sensors at different angles and compares image focus on each sensor to determine 3D perspective. Lava C.O.S. has been replaced by the True Definition Scanner, which also operates by using AWS to capture 20 frames of video imagery per second (Figure 3). The images are converted to 3D data sets and displayed in real time as the 3D model on the computer screen. This system requires a light dusting of powder. A major improvement of the True Definition Scanner is the smaller intraoral wand, which contains a series of lenses fixed with an imaging sensor.
The iTero system uses a parallel confocal system, in which light is filtered by passing through a small pinhole (Figure 4). Only the light reflected from the object at the proper focal distance will pass back through the pinhole. The iTero system also uses a telecentric system to maintain the same field of view of the area being scanned regardless of distance from the object that is being imaged. As a result, there is no need to hover the scanner above the object being scanned, and the scanning probe can be placed directly on the surface of the teeth being scanned. Additionally, no reflective coating powder is applied to the oral cavity.
The IOS FastScan is one of the newest intraoral scanning systems (Figure 5). This device operates by projecting a red laser sheet, which moves along with a camera within the wand. This technology allows a large scanning area to be captured with only three positions (buccal, lingual, and occlusal). The FastScan uses a powder and the wand can be placed directly on the tooth surface.
Each scanning system offers benefits that determine convenience for individual practitioners, including the size of the wand, use of powder, method of scanning (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 using 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 must be sent to the company first, and if a fee is applied. Some of this information is summarized in Table 1.
Value 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.
Another 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. The accuracy and durability of the model produced by some digital systems is another significant advantage. Made of resin, they are significantly more abrasion-resistant and precise than gypsum models. Another advantage of digital systems is that the laboratory prescription accompanies the digital impression file and is completed before making the digital impression.
These devices are also extremely patient friendly. They eliminate the uncomfortable, messy impression-taking process, which causes some patients to gag. They also shorten prosthesis delivery time, because Internet transfer of the file and reduced laboratory procedures shorten the total prosthesis fabrication time. Finally, there is the “wow factor” that occurs when the patient sees the digital impression on the screen for the first time.
Digital impression systems do have some disadvantages. 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, and a cost for processing the digital impression. Also, the wand size can be a problem for patients with restricted opening.
Comparing Accuracy and Efficiency
Several studies have compared the clinical success and accuracy of digital impressions to conventional impressions. Henkel6 compared crowns generated from conventional and digital impressions (iTero). 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; and 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 et al7 compared copings made by the Lava C.O.S. digital impressions and polyvinyl siloxane (PVS) conventional impressions. The marginal accuracy of the Lava zirconia crowns produced from the Lava C.O.S. showed no statistically significant difference compared with crowns made from conventional impressions. Syrek et al8 compared crowns fabricated from PVS impressions and the Lava C.O.S. Crowns fabricated from digital impressions had better marginal fit and proximal contact and required equal occlusal adjustment. Ender and Mehl9 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. 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.
The idea that digital impressioning may be faster is controversial. Although the 3-to-5-minute scanning process requires roughly the same amount of time as traditional impression materials setting time, there is no need to select a tray, apply tray adhesive, clean impression trays, assemble impression guns and tips, or disinfect impressions. Another timesaving feature of a digital impression is that a portion of a deficient area may be rescanned with the digital system rather than retaking the entire impression. Lee and Gallucci10 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 controlled double-blind clinical study by Givan et al11 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 with fast-set traditional impressions (4 minutes, 23 seconds); however, conventional impression set-up and clean-up time were not included.
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.6-14 More data need to be collected, however, to substantiate that digital impressions are more time efficient than conventional impressions.10,11
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 [poster 245]. Presented at: AADR Annual Meeting; March 4, 2010; Washington, DC.
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. http://intl.invisaligngallery.com/wp-content/uploads/2012/01/Cad-Cam-Dentistry-and-Chairside-Digital-Impression-Making-by-Dr-Bob-Lowe-062609.pdf. 2009. Accessed June 19, 2012.
6. Henkel GL. A comparison of fixed prostheses generated from conventional vs digitally scanned dental impressions. Compend Contin Educ Dent. 2007:28(8):422-431.
7. 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.
8. 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.
9. Ender A, Mehl A. Full arch scans: conventional versus digital impressions—an in-vitro study. Int J Comput Dent. 2011;14(1):11-21.
10. Lee SJ, Gallucci GO. Digital vs. conventional implant impressions: efficiency outcomes [published online ahead of print February 22 2012]. Clin Oral Implants Res. 2013;224(1):111-115. doi: 10.1111/j.1600-0501.2012.02430.x.
11. 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.
12. Doherty E, Kugel G, Dunne P, Nelson M. Margin characteristics of PFM crown copings fabricated on stereolithography models. J Dent Res. 2010;88:1058.
13. Ogledzki M, Wenzel K, Doherty E, Kugel G. Accuracy of 3M-Brontes stereolithography models compared to plaster models. J Dent Res. 2010;88:1060.
14. Balakrishnama S, Wenzel K. Bergeron J, et al. Dimensional repeatability from the LAVA COS 3D intra-oral scanning system. J Dent Res. 2009; Abstract 2951.
About the Author
John O. Burgess, DDS, MS
University of Alabama at Birmingham
Nathaniel C. Lawson, DMD, PHD
University of Alabama at Birmingham
Augusto Robles, DDS, MS
University of Alabama at Birmingham