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.