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. Thus, good communication with the patient is essential to help prevent unwanted movement. Dual-arch impression techniques can be challenging to stabilize due to possible jaw movement before the material sets. Accordingly, it is important to keep the material stabilized within the mouth throughout its setting time.
Additionally, most materials did not completely cease to flow until 2 to 2.5 minutes (PVS) or 2.5 to 3 minutes (polyether). 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. 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. One source of these types of errors is movement of the impression material in the mouth before the set is complete. Pulls and drags appear as elongated distortions within the impression. The clinician should be cognizant of the different working/setting times manufacturers offer to ensure a proper match between materials. This error also will occur if a fast-set material is used in combination with a regular-set material. If this is a common occurrence, the clinician may benefit by switching to a material with a longer working time. 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. Light-Bodied Material Pulling Away From Heavy-Bodied Material Using the wrong sized mixing tip would also be problematic. 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. When extruding impression material through a mixing tip, the base and catalysts are pushed through a spiral to mix both components. 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.Īnother issue may be that the impression material did not mix properly. Also, the residue of some build-up materials or bis-acryl provisional material may inhibit the set of impression material. Such contact could be the source of the problem. Often the clinician may swipe a gloved finger across a tooth preparation prior to impression or roll a retraction cord between two gloved fingers. The sulfur coating on latex gloves, for example, may inhibit the set of impression materials. One possible cause of unset impression material is the material contacting a substance that inhibits its setting. Potential Impressioning Mistakes Unset Material Within the Impression
This article reviews some common errors that are present in final impressions and suggests possible causes for and solutions to these mistakes. 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. Various factors such as time, cost, patient comfort, and soft-tissue health may prevent a dentist from retaking an impression. A recent evaluation conducted within a commercial dental laboratory determined that 86% of crown-and-bridge impressions contained at least one detectable error and 55% contained a critical error related to the finish line. Making an accurate impression requires understanding the anatomy surrounding the finish line, the material being used both for impressions and gingival displacement, and the correct use of impression trays. Perhaps one of the most challenging procedures in dentistry is obtaining an ideal impression for a fixed dental prosthesis.
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Diagnosing a Failed Impression: Common Errors and How to Overcome Them
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