Diagnosing a Failed Impression: Common Errors and How to Overcome Them

Various factors such as time, cost, patient comfort, and soft-tissue health may prevent a dentist from retaking an impression. However, if the dentist is unable to identify the cause of the error in the original impression, the mistake may very well be duplicated in the retake.

This article reviews some common errors that are present in final impressions and suggests possible causes for and solutions to these mistakes.

One possible cause of unset impression material is the material contacting a substance that inhibits its setting. The sulfur coating on latex gloves, for example, may inhibit the set of impression materials. Often the clinician may swipe a gloved finger across a tooth preparation prior to impression or roll a retraction cord between two gloved fingers. Such contact could be the source of the problem. Also, the residue of some build-up materials or bis-acryl provisional material may inhibit the set of impression material. To ensure the preparation is entirely clean prior to impression, some clinicians prefer to clean the preparation with pumice/alcohol and use nitrile gloves instead of latex.

Another issue may be that the impression material did not mix properly. When extruding impression material through a mixing tip, the base and catalysts are pushed through a spiral to mix both components. The initial mix of the material should be discarded because it is possible that one side of the mixing tip was filled before the other side at the start of mixing. Using the wrong sized mixing tip would also be problematic.

Poor adaptation between a light-bodied impression material and heavy-bodied material is a sign that too much time has elapsed between injecting the light-bodied material and inserting the tray with heavy-bodied material. If this is a common occurrence, the clinician may benefit by switching to a material with a longer working time. This error also will occur if a fast-set material is used in combination with a regular-set material. The clinician should be cognizant of the different working/setting times manufacturers offer to ensure a proper match between materials.

Pulls and drags appear as elongated distortions within the impression. One source of these types of errors is movement of the impression material in the mouth before the set is complete. A study of the flow properties of polyvinyl siloxane (PVS) and polyether impression materials demonstrated that all PVS materials showed decreasing flow properties within the first minute of extrusion.² Therefore, an impression that has been seated within the mouth has a limited period of time in which it may be adjusted without pulls and drags being created. Additionally, most materials did not completely cease to flow until 2 to 2.5 minutes (PVS) or 2.5 to 3 minutes (polyether). Accordingly, it is important to keep the material stabilized within the mouth throughout its setting time. Dual-arch impression techniques can be challenging to stabilize due to possible jaw movement before the material sets. Thus, good communication with the patient is essential to help prevent unwanted movement.

To prevent air voids, the light-bodied material should be injected around the sulcus continuously without removing the mixing tip from the body of the material. Accurately positioning and controlling a 50-mL cartridge in a dispensing gun can be difficult. Smaller single-use dispensing systems reduce the lever arm distance between the handgrip and dispenser tip and lessen the force required to extrude material, thus allowing increased control.

Voids also can be caused by the presence of saliva or blood. Saliva should be rinsed off the tooth before impressing, and the tooth should then be air-dried thoroughly immediately before application of the light-bodied material. If a two-cord technique is used, a final application of air can be applied to the sulcus after the first cord is removed. If excessive bleeding is present, use of retraction pastes containing ferric sulfate and compression caps, which apply pressure to the bleeding tissue, may serve as an effective alternative to liquid or gel hemostatics. If bleeding cannot be controlled, having the patient return for a final impression after using a chlorhexidine mouthrinse for a week may be beneficial.

A common cause of insufficient margin reproduction is inadequate tissue retraction. A recent study from the National Practice-Based Research Network reported that most dentists use either a single-cord (35%) or double-cord (35%) retraction technique, while a smaller percentage of dentists (16%) prefer an injectable retraction paste.³ Another possible cause of this problem is insufficient flow of the impression material. A lower viscosity material may offer better flow into a tight sulcus. Use of polyether material may also be beneficial when dealing with deeper subgingival margins, as these materials have higher reported flow properties than PVS materials.²

Finally, it is possible that a thin portion of the margin may tear during removal. Both PVS and polyether impression materials have shown to increase in tear strength throughout their setting time. These materials develop their optimal strength within the final minute of setting, with some materials showing increased tear strength after their recommended set time.⁴ Therefore, all impressions should be kept in the mouth for their entire set time, and any impression with thin portions of the margin may benefit from additional time before being removed. It is important that the impression be kept in the patient's mouth for the manufacturer's recommended setting time, which should be monitored with the use of a timer or clock. Judging the set time of the impression simply by feel should be avoided, as this could result in an overestimation of the set of PVS and polyether impression materials.

After removing an impression, dentists often focus only on the capture of the prepared tooth. However, if there is insufficient data in the remainder of the impression, the laboratory will not be able to accurately mount the case and design the crown. Try-in of the impression tray is an important step that must be maintained in the protocol. This try-in allows the dentist to ensure that no tray borders are in contact with any tooth or tissue, and it prepares the patient for the upcoming procedure. Custom-made trays can be effective for this, especially when working with impressions that involve multiple teeth.

Another common error is the use of a dual-arch tray outside of its recommended purpose. A preparation captured in a dual-arch tray should have a tooth mesial, distal, and opposite the prepared tooth in order to prevent distortion while impressing and allow proper articulation when mounting. Additionally, the dual-arch impression should capture opposing canines in order to reproduce canine guidance.

Yet another common error among clinicians is underfilling the impression tray. Properly filling the tray not only ensures that a sufficient number of teeth will be recorded for cast articulation, but it also will allow sufficient capture of the tissue apical to the preparation to enable the technician to design the emergence profile of the crown.

Many of the errors that are possible with conventional impressions can be eliminated with digital impressions. Examples include incomplete material setting, tray and wash material incompatibility, drags, and pulls. Some impression errors, however, can be reproduced by digital impressioning, such as incomplete capture of data or failure to visualize the entire margin.

Some of the unique advantages of digital impressions include the ability to view a positive impression of the preparations, use color to differentiate hard and soft tissue, magnify the scan, and check the reduction of a preparation. Additionally, many scanners allow the clinician to modify problem areas of the preparation and then rescan those modified portions of the preparation rather than starting over again.

In summary, many errors in impression-taking can be avoided through adequate preparation and careful attention to detail. Often, by spending a few extra seconds checking their work prior to taking the impression, clinicians may save many agonizing minutes and wasted material after the impression is already seated.

Taiseer A. Sulaiman, DDS, PhD

Director, Division of Operative Dentistry and Biomaterials, University of North Carolina School of Dentistry, Chapel Hill, North Carolina

Nathaniel C. Lawson, DMD, PhD

Director, Division of Biomaterials, University of Alabama at Birmingham School of Dentistry, Birmingham, Alabama