CATEGORY AUSTRALASIAN DENTIST 67 LINICAL suitable storage location, the impact of the surrounding equipment (e.g. proximity to a refrigerator or sterilizer producing heat) should be taken into account. Material and tray selection: Recommendations For accurate full arch impressions, the use of a rigid, non-perforated metal or plastic tray is highly recommended. Rigidity is important as it avoids inaccuracies caused by deformation. The tray size should be checked intraorally to ensure that uniform space (2 to 3 mm) is available around the teeth equator line (this takes undercuts into account). If a tray is too small, the risk is high that the impression will have defects and a limited reproduction of details. Impression taken with a metal tray that is too small and an exceeded working time, which is responsible for a poor bond between the materials. Long flow defects are visible in the palatal area. Poor bond between tray and wash material, presumably caused by an exceeded working time. Measures facilitating removal of the impression from the mouth. quality: before starting to fill the tray or elastomeric syringe, a small amount of material should be dispensed onto a mixing pad or paper towel to enable you to visually check the mix quality. This is also necessary for materials from a hand dispenser. u Dispensing for a void-free mixture: the end of the mixing tip should always be immersed in the dispensed material during tray filling. u The right timing: tray filling should be completed as soon as the dentist is done syringing the wash material (1step technique), which is particularly important for quick-setting materials like 3M™ Impregum™ Super Quick Polyether Impression Material. This is usually possible when syringing and filling the tray start at the same time. Another important factor is the proper insertion of the loaded tray into the mouth. It needs to be moved slowly parallel to the longaxisof the teethbeforeseating it straight up (maxilla) or straight down (mandible). Once the tray is in its final position with the handle aligned with the patient’s midline (assuming a full arch tray is being used), any movement needs to be avoided until the setting reaction is completed. Proper seating prevents distortions. It is useful to stabilize the tray in the premolar area with the fingers. Some impression materials from 3M offer specific features that ensure a quick transition from the set to the unset state (such as self-warming of 3M™ Imprint™ 4 VPS Impression Material and snap setting of 3M™ Impregum™ Polyether Impression Material), which means less time for unintended movements. With regard to tray removal, releasing the vacuum with a finger aided by a stream of air is essential. Follow-up measures: Doing it right Any impression removed from the mouth should be rinsed with water, dried with air and disinfected with a mild solution before any other action is taken. Both, VPS and polyether impressions need to be stored in a cool, dry and dark place, and they should be shipped to the dental laboratory wrapped in dry paper towel and placed in an open bag if still moist. The dry paper will draw any moisture from the surface, leading to an optimal condition for transportation. Variations in temperature should be avoided. Conclusion The quality and accuracy of an impression has a huge impact on the quality and fit of the final restoration. Despite the complexity of the procedure, it is possible to ensure consistently accurate results with the effective measures described above. The key to success is standardization of the complete procedure, from tray filling to shipping of the final impression. u More information on how to avoid potential errors in the impressioning procedure is available here: http://go.3m.com/4Psr Precise impressions will lead to perfectly fitting restorations! (Images courtesy of Dr. Gunnar Reich). When selecting the impression material, different factors such as the compatibility between the material and the preferred technique need to be taken into account. Polyether is preferable for the monophase and 1-step techniques, while VPS is particularly well-suited for the 2-step technique and also works well with the 1-step technique. When choosing a VPS impression material, the user should select a product with good flow properties, plus hydrophilicity in the unset stage, which is not offered by every available product. The setting times should be selected based on the number of prepared teeth or implants to be captured. Impressioning procedure: Tips and tricks If you use an automatic mixing device, the impression taking procedure usually begins with automatic mixing and tray filling. Care needs to be taken with regard to: u Correct insertion of the cartridge into the device u Proper attachment of the mixing tip before each use: if the tips are not seated tightly on the outlet openings, inhomogeneous mixing results, a long setting time or a high amount of material waste may be the result. u Verification of a homogeneous mix
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