Making A New Impression
Digitizing the oral environment or the physical impression offers the dentist and laboratory a promising opportunity.
By Chris Brown, BSEE
Beeswax may have been the first material ever used for taking dental impressions back in the 1600s. By the early to mid-1800s, dental professionals had moved on to gutta-percha or plaster of Paris for impression-taking. And today’s dentists have a wide range of impression materials at their disposal. Over the years, every new generation of impression materials has improved in efficiency and accuracy. But it is not just the materials that have changed—the impression-taking process has also transformed dramatically.
For nearly 200 years, the restorative prosthetic workflow began with an impression, followed by the poured model, and finally, the fabrication of the restoration on that model before insertion in the patient’s mouth. Little more than 25 years ago, the dental industry changed when the University of Zurich introduced the original CEREC machine in 1980. It was the first to use digital impression technology to scan a patient’s mouth, virtually design a restoration, and then mill it for immediate seating in the patient’s mouth.
Today, dentists have in-office chairside CAD/CAM systems, such as CEREC AC (Sirona, www.sirona.com) and E4D Dentist (D4D Technologies, www.e4dsky.com). Both have the ability to scan the patient’s mouth or a traditional impression, then design and mill a restoration while the patient waits in the chair. Over the last quarter-century, the list of available materials and indications has expanded, and the scanning, software design, and milling technology has seen vast improvements.
The Lava™ Chairside Oral Scanner C.O.S. (3M ESPE, www.3MESPE.com) and iTero™ (Cadent Inc., www.cadentinc.com ) have been on the market for several years, taking intraoral digital impressions for laboratories to fabricate traditional or CAD/CAM-based restorations. Recently introduced chairside digital impression-taking systems, such as the Progress IODIS (Clōn 3D, www.clon3d.com), MIA3d (Densys3d, www.densys3d.com), and IOS FastScan™(IOS Technologies, ios3d.com), not only offer intraoral scans but are also capable of scanning traditional impressions as well.
For at least 8 years, laboratories have been using desktop scanners to scan poured models created from traditional impressions and then virtually design restorations or restoration substructures. Now, they have the ability to scan physical impressions with systems such as the D700 (3Shape Inc., www.3shape.com), DW-5S (Dental Wings, www.dental-wings.com), NobelProcera (Nobel Biocare, www.nobelbiocardental-wings.come.com), OpenScan 100 (Laserdenta, www.laserdenta.com), and the Identica (Medit, www.medit-group.com).
The Technology
The leading model and intraoral scanners today do an excellent job of scanning tooth and model surfaces and contours, regardless of the particular technology they employ. Light in one form or another is projected onto a tooth or model surface. One or more cameras record the reflected light, and sophisticated software interprets the data, producing a digital model on which restorations can be designed.
As such, the manufacturers have to balance a number of scan design factors: the distance between the light source and object being scanned, the distance and angle between the light source and the camera, the optical depth of field, and other facets critical to data capture. All of these can affect the scanning window and accuracy.
One of the biggest challenges to scanning an impression is the technology’s ability to “see” inside the voids that the teeth create in the impression material. Many older model scanners are unable to see inside these voids because the angle between the light source and the camera is too great. So in order to scan impressions, manufacturers decreased the angle between the light source and the camera, giving them a better view inside those voids (Figure 1). Due to their size, intraoral scanners already have highly condensed optics and seem to adapt quite well to scanning impressions.
The Process
Intraoral digital impression scanning wands require the user to manually hold and manipulate the wand in the mouth during the scanning process. Model scanners typically have fixtures to hold and rotate the model or impression to make the scanning process more automated.
Some scanning systems require that powder be applied to the surfaces of the teeth or the model prior to being scanned. If powder application is required for models or teeth, it will most likely be required for impression scanning. Some systems may not require powder application according to manufacturer recommendations, but may still benefit from a light dusting under certain circumstances.
Those accustomed to scanning models, dies, or natural teeth should be prepared for a slightly longer scan time when scanning physical impressions. The number of images required to gather sufficient data is typically far greater for scanning impressions than for intraoral or model scanning.
Figure 2 shows both sides of a common dual-arch impression scan, which was placed in a Medit Identica scanner. The scanning software guides the user through selecting the operative teeth and areas of the impression to scan. After scanning the operative arch, the user manually flips the impression over in the impression-holding fixture, and the scanning process is repeated for the antagonist arch.
Figure 3 shows the scan data obtained from each side of the impression. The next step is to align both scanned arches together. Some CAD software programs will do this automatically, while others may require identification of common data points on both arch scans so the data sets can be “snapped” together. Once the arches are aligned, then extra data can be trimmed away, leaving only the information needed. Figure 4 shows the aligned arches and an occlusal view of the operative arch.
After Data Capture
The workflow for intraoral scans has been proven. The doctor scans the patient, virtually designs the restoration, and can then produce a restoration in the office using a chairside CAM system. The 3M and Cadent chairside digital impression systems forward the case onto a laboratory with a model for a laboratory-fabricated restoration.
What about scanning impressions or chairside intraoral scanners that do not have a model solution yet? Quite honestly, that is what the industry is waiting to find out. Currently, there are two logical paths to follow. One is to convert the scanned impression data into a physical model so that a traditional restoration or CAD/CAM-based restoration can be fabricated. For instance, Sirona just announced a desktop model milling solution for its MCXL inLab unit. The other is to import the scanned impression into a CAD/CAM system and create a model-less restoration.
While 3M and Cadent currently offer milled or 3-D printed models from intraoral scans, the cost and complexity of the processes used to create the models are all economically challenging for most laboratories and milling centers. The race is on to find an efficient, economical model milling or 3-D printing solution.
3Shape and Dental Wings currently have virtual “Model Manufacturing” modules integrated into their software; others are emerging or in development. These modules take the data obtained from the impression scans and allow an operator to virtually build a model. The bite plane is set, the operative teeth are identified, a means to create a removable die is provided, and additional material is added to the scan data to interface with an articulator.
Once the virtual model has been created, it still needs to be manufactured. This requires a CAM program to take the model data and a CAM milling or 3-D printing system to produce the physical model. Currently, the process is somewhat cumbersome and production yield is relatively low compared to the capital investment. This is likely to change with expected advances in software and hardware.
Dentists who use CEREC AC or E4D Dentist digital intraoral scanners with their respective in-office milling systems are actually performing model-less dentistry every day. Are laboratories and other doctors ready to jump on the bandwagon?
Without a model, layering porcelain on a substructure really is not an option. Milling a full-contour wax pattern for pressing or casting in the laboratory would most likely require extra fitting chairside for the doctor. For laboratories, full-contour, milled monolithic zirconia, lithium disilicate, or other millable ceramics are currently the only restorative material choices for model-less dentistry. While these materials can be used in many situations, they may not always be a doctor or laboratory’s ideal choice. Some are just not suitable for use in highly esthetic cases or in clinical situations, such as heavy bruxism or large-span bridges.
A Promising Future
The CAD/CAM revolution in dentistry is still in full swing. New materials, budding corporate partnerships, increased connectivity between systems, and promising opportunities come onto the market nearly every month. Digitizing the oral environment or the physical impression is now one of those new opportunities, both chairside and in the laboratory.
Nearly every restorative material company has new monolithic materials on their radar, which could mean the industry will see stronger, more esthetic materials that are easier to mill for model-less restorations in the near future. CAD and CAM software is evolving to work with scanned impressions, and model printing and milling systems are being developed and adapted to fill a potentially lucrative market.
Laboratories should not make plans to scrap their model rooms any time soon but instead should keep a close eye on impression scanning technology.
Chris Brown, BSEE, is the business manager of Apex Dental Milling in Ann Arbor, Michigan.
