Keys to Predictable Digital Impressions

Conventional impressions are the benchmark for any new replication technique due to the demonstrated accuracy of PVS and polyether impression materials.¹ However, it is well known that throughout the impression process there are potential risks to final accuracy, including setting of the impression material, removal of the impression, and fabrication of the stone model.^2-4 Despite the documented accuracy of PVS and polyether impressions, studies have repeatedly shown that most impressions sent to dental laboratories have some sort of deficiency. One study evaluated 193 FPD impressions made by 41 dentists immediately after they arrived at 11 dental laboratories and reported that 50.7% of all the impressions had voids or tears in the finish line area, 40.4% had air bubbles at the margin line, and 26.9% had both.⁵ Thus, although impression material has documented accuracy, making an accurate impression involves more than just the precision of the material.

The increased integration of CAD/CAM technology is enabling intraoral scanners (IOSs) and computer software to be used for the replicating of the intraoral condition instead of conventional impression materials. Studies have documented the accuracy of optical impressions as being comparable, if not better, than that of conventional impressions for single-tooth restorations and FPDs.¹ For clinicians transitioning from conventional to digital impressions a common concern is how to achieve accurate results.

To make an accurate conventional impression it is generally accepted that there must be good isolation of the area to be impressed and soft-tissue retraction to allow unimpeded access to the tooth preparation. Isolation includes keeping moisture, whether saliva or blood, from preventing intimate registration of the tooth preparation surface. Impression materials are hydrophobic, which leads to an inability of the material to flow and capture fine detail if there is a lack of hemostasis or poor moisture control. Retraction involves mechanical displacement of the adjacent soft tissue so as to not interfere with the adaptation of the impression material to the tooth preparation surface. PVS and polyether impression materials lack sufficient stiffness to physically deflect the soft tissue from the tooth preparation, so a space must be created to allow the impression material to intimately flow across the tooth surface.

Digital scans require a similar attention to retraction and isolation. Retraction can be thought of as a technique to improve access to the dentition and to the tooth preparation. The IOS needs unimpeded access to the area to be scanned; this usually involves preventing the cheeks and tongue from interfering. For some patients, the back side of the head (opposite the lens) of the IOS can be sufficiently used to retract the cheek and tongue away from the dentition while scanning. However, various techniques may be used in cases where additional retraction of the cheek and/or tongue is necessary. One option is to use lip and cheek retractors to hold the lips and cheeks away from the dentition while scanning (Figure 1). This affords a single operator improved access to scan the dentition rather than needing a second assistant to retract the cheeks or tongue while the scanning is being done. Another technique is to place a dry angle between the mandibular molars and tongue (Figure 2); when the IOS is inserted to scan, the head of the IOS will push against the dry angle, effectively preventing the tongue from interfering with the scanning process.

A dental isolation device, such as Isolite^® 2 (Zyris, zyris.com), or similar retraction device may be useful, as it controls the cheek and tongue with silicone retractors while providing high-volume suction. Although such a device may retrack well, in the author's experience it can potentially restrict movement of the IOS as the camera head approaches the second molars in patients with smaller arch widths.

An IOS records data predictably when the lens surface is held parallel to the surface being recorded. For example, holding the IOS lens parallel to the occlusal surface as would be done with the handpiece, positions the scanning lens parallel to the occlusal surface allowing unfettered data recording. However, if the IOS is not rotated to the facial and lingual it is less likely to completely record vertical surfaces of the tooth preparation. Rotating the IOS allows it to be positioned parallel to the recorded surface to record the data. A simple analogy would be viewing a billboard on the side of the road. As one approaches the billboard it is easy to see and read. However, as one comes closer to passing it, the billboard is viewed at a more severe angle and becomes more difficult to read. This analogy demonstrates why the IOS should be rotated during digital scanning.

A common question regarding digital scanning is, "How far subgingivally can data be recorded?" A simple axiom for digital scans is that the IOS cannot scan what is not visible. The IOS records surface topography and does not see "through" liquids to scan surface detail. Considering that an IOS will record any visible surface data, a better question may be, "Is the subgingival area visible to the clinician?" (Figure 3). IOSs will record data that is visible within the depth of focus, which may be up to 20 mm from the surface of the lens depending on the IOS. The limitation in data scanning is not a function of the IOS, but rather is dependent on the ability to provide sufficient soft-tissue retraction for visual access to the tooth preparation margin. An analogy is when looking down a narrow space, it is not as easy to distinguish detail the further one looks. Similar to the billboard analogy, deep restricted areas will be more difficult to scan because the angle of scanning cannot be widely rotated to allow for more data to be recorded.

Unfortunately, accurately recording subgingival margins without effective moisture control is no more possible with an IOS than a conventional impression. Moisture, including saliva and blood, is not distinguished from the surface of hard-tooth or soft-tissue surface contours and will preclude accurate recording of the tooth surface underlying the saliva or blood. The surface does not need to be desiccated, but it must be essentially free of pooled liquids and debris.

One difference in soft-tissue retraction between conventional impressions and digital scans involves the depth of retraction. Conventional impressions must record at least 1 mm beyond the margin to ensure the margin has been recorded accurately in the impression. Without seeing impression material sufficiently beyond the preparation margin, it is problematic to determine whether the entire margin has been impressed. Tearing of thin sections of impression material can also complicate evaluating the outcome of the impression. Digital impressions do not require a similar extension beyond the margin as long as there is differentiation with the adjacent soft tissue to easily identify the margin location (Figure 4). In essence, conventional impressions require more focus on the depth of retraction, and digital scans require more focus on lateral retraction of the soft tissue.

Similar soft-tissue retraction techniques can be used for both conventional impressions and digital scanning. Commonly used two-cord techniques work very well. Some clinicians prefer removing both cords prior to making impressions, creating maximum space for impression material. Other clinicians prefer removing only the larger second cord and leaving the smaller first cord in place to avoid disturbing the newly created hemostasis (Figure 5 and Figure 6). This technique works well for digital scanning as lateral retraction is more important than vertical retraction past the margin. Dark-colored cords may be preferred for digital scans, because they provide a good color contrast to the soft tissue and tooth preparation for easy identification of equigingival or subgingival margins.

Clinicians are aware of the limitations and problems encountered with margins that are located more than 1 mm subgingivally. Dental lasers, electrosurgery, and soft-tissue cutting diamonds are all effective means of creating access to subgingival margins. The challenge of controlling hemostasis after cutting the tissue may require the use of chemotherapeutic agents, laser, or electrosurgery. The clinician's skill and comfort with these techniques may be the most important factor in successfully recording an accurate replica of the margin regardless of digital or conventional impression. One advantage of digital scanning is that it may be done with an open tissue flap without fear of leaving a foreign body from the use of impression material.

Retraction/hemostatic paste systems consist of 15% aluminum chloride in an astringent paste that can be carefully injected into the sulcus adjacent to the preparation margin. The paste expands while setting providing a degree of soft-tissue retraction as well as hemostasis. Paste retraction systems may be more popular for digital scans as they tend to be effective hemostatic agents but without the same tissue displacement capability as retraction cords.

One significant advantage of digital scans compared to conventional impressions is the ability to immediately evaluate the outcome at high magnification on the computer screen to determine if the margins have been accurately recorded rather than waiting for a solid model from a conventional impression to determine margin accuracy. Correction of defects can be done efficiently by deleting the area of discrepancy of the scanned data and rescanning the area.

While studies have shown that digital systems are as accurate as, or more accurate than, conventional methods for recording intraoral hard and soft tissues, the basic principles of control of the operating field, good soft-tissue retraction, and isolation from moisture contamination are still vital for recording an accurate digital scan.

Dennis J. Fasbinder, DDS
Clinical Professor of Dentistry, University of Michigan School of Dentistry, Ann Arbor, Michigan