Clinicians Reaping Benefits of New Concepts In Impressioning
While conventional elastomeric impression materials are still being used to generate excellent impressions, the future of impression-taking is undoubtedly with digital intraoral scanners. Advances in scanning systems are making this technology easier to use and more practical for dentists.
Impressions are a vital component in the fabrication of fixed dental prostheses. Current-generation elastomeric impression materials and digital scanners are capable of quickly and accurately capturing prepared teeth. Perhaps the greatest challenge clinicians face while capturing an impression is achieving complete reproduction of the prepared finish line, particularly with sub- or equigingival margins. In these cases, the impression material must resist displacement from the gingival tissue, sulcular fluid, and/or blood. Several impression materials have been developed for either increased compatibility with fluids, simplified retraction of the gingival tissue, or injection of the material directly into the sulcus. Although excellent impressions may be made with conventional elastomeric materials, the future of impression-taking is with digital intraoral scanners. Improvements in these scanners have made this technology easier to use and more practical for dentists.
Hydrophilic Impression Materials
Polyvinyl siloxane (PVS) impression material, the most commonly used final impression material, is hydrophobic. Crevicular fluid and blood present in the gingival sulcus prevent hydrophobic materials from entering the sulcus and preclude accurate marginal detail reproduction in subgingival margins. Therefore, surfactants have been added to PVS materials to increase their hydrophilicity. A new class of PVS impression materials has been released that contains more efficient surfactants. Materials in this category include Imprint™ 4 (3M ESPE) and Panasil® (Kettenbach). Polyether elastomeric impression material is inherently hydrophilic due to its chemical composition. Hybrid materials, such as Identium® (Kettenbach) and EXA’lence™ (GC America), are PVS with grafted polyether functional chains that claim to combine advantages of both types of impression materials—the hydrophilicity of polyethers and the tear strength of PVS.
A recent study reported that the new PVS materials (Imprint 4; Panasil) and hybrid material (Identium) were more hydrophilic than traditional PVS materials when exposed to water. When exposed to saliva, however, the hybrid material and a polyether material (Impregum™, 3M ESPE) were shown to be the most hydrophilic.1 This study suggests that polyether and some hybrid materials are preferred for capturing margins exposed to saliva. Regardless of the type of impression material used, it is always indicated to thoroughly dry the tooth preparation immediately prior to taking the impression.
When attempting to capture subgingival margins, it is also important to use an impression material capable of flowing into the sulcus. Previous research has shown that a polyether material (Impregum) has higher and more prolonged flow than PVS materials.2 Using a tray/wash technique with PVS materials will improve flow, as the hydrostatic pressure from the more viscous tray material will push the less viscous wash material into the sulcus. Another clinical tip for capturing subgingival margins is to blow air onto an initial circumferential layer of wash impression material, directing it into the sulcus. This air-thinned wash layer is followed by a second syringed layer of wash material to completely cover the preparation and ultimately the material-filled tray. The use of an extra-low viscosity syringe material will help impression material flow into a deep sulcus.
Retraction Pastes and Caps
The traditional method of retracting soft tissue prior to taking an impression involves packing cord into the sulcus. This procedure, however, can be difficult for the dentist and uncomfortable for the patient and has been shown to cause 0.2 mm of gingival recession.3 An alternative concept for displacement of gingival tissue involves injection of a retraction paste into the sulcus in order to mechanically separate the tooth from the surrounding gingival tissue. After a waiting period, the paste is rinsed away to reveal an expanded sulcus.
Some retraction pastes, such as Expasyl® Gingival Retraction Paste (Kerr Corp.), Traxodent® Hemodent® Paste Retraction System (Premier Dental), and Retraction Capsule (3M ESPE), are composed of clay. These products also contain aluminum chlorate or aluminum sulfate to provide simultaneous retraction and hemostasis. Other retraction materials are composed of a type of PVS material that undergoes expansion during setting; these include GingiTrac™ Gingival Retraction Material (Centrix) and Magic FoamCord® retraction paste (Coltène/Whaledent). These materials release hydrogen while setting, which helps to expand sulcus after placement. There are no hemostatic agents in the PVS retraction systems.4
The clinical challenge associated with retraction pastes is ensuring the paste is completely inserted into the sulcus. Manufacturers have designed extra-fine and flexible tips for these pastes to aid in their placement. It is important to visualize the presence of paste in the sulcus and slight blanching of the tissue to confirm adequate placement of the material. These systems may be used in combination with retraction caps, which are hollowed cotton cylinders that are placed over the prepared tooth following application of a retraction paste. The patient bites on the cap to apply pressure on the paste, pushing it into the sulcus.
Clinical studies have shown that retraction pastes are less likely to separate junctional epithelium in the sulcus5 or cause gingival recession than traditional cord.6 Another clinical study showed that the use of a clay-based retraction paste resulted in less sulcular penetration of impression material (0.1 ± 0.41 mm) than using a double-cord technique (0.34 ± 0.3 mm).7 A recent study reported that all paste retraction systems, although less traumatic than retraction cord, produced 10 times less displacement pressure than the cord. Also, Expasyl and Expasyl New Generation produced significantly greater pressure than Retraction Capsule and Magic FoamCord.8 These studies suggest that retraction paste is less harmful to the gingival tissue but it may not provide as much retraction as cord. Therefore, if the margin is too far subgingival, use of retraction cord may be more effective.
Tissue Management Impression System
A newer retractionless tissue management impression system (Aquasil Ultra Cordless Tissue Managing Impression System, DENTSPLY Caulk) directly injects PVS impression material into the gingival sulcus. The system employs a pneumatic dispenser, which pushes an individual cartridge of impression through an extra-fine applicator tip. The system utilizes the thixotropic nature of impression materials, meaning that shear forces applied to the material will decrease its viscosity. As the material is forced through the extra-fine applicator tip, the walls of the tip apply shear forces to the impression material, allowing for better flow and direct placement into the gingival sulcus.
Future of Impressions
As digital impression systems continue to improve, their role in prosthetic dentistry will increase. Several recent advancements have made these systems more feasible in clinical practice. Some digital impressioning systems (CEREC® Omnicam, Sirona Dental; PlanScan®, Planmeca; iTero®, Align Technology; TRIOS® Color, 3Shape) have been simplified by eliminating or reducing the titanium dioxide powder used to coat prepared teeth. Many systems allow the clinician to continuously capture the impression with a video-based scanner instead of point and click. Some systems produce color images. The scanners themselves have also evolved; some can be plugged directly into a laptop, while others (3M™ True Definition, 3M ESPE) have been scaled down to the size of a dental handpiece. The software on some of the devices has been modified to allow the clinician to save the scans in an “open” format file. These open STL files can then be sent directly to any in-office milling machine or a dental laboratory milling device that accepts an STL file. In addition, open systems allow seamless integration of the digital impression with cone-beam computed tomography (CBCT) data for implant and orthodontic treatment planning.
Capturing the crown finish line with a digital impression system requires visualization of the margin and, in many cases, still involves retraction of the gingival tissue. One advantage of digital impression systems is that if an area of the margin is not captured clearly, that area of the scan can be deleted and rescanned. The dream of restorative dentistry would be an x-ray CT scan that could accurately capture hard-tissue impressions for fixed prosthesis fabrication and subtract interfering soft tissue. As CT devices become more commonplace in dental offices, this technology may be closer to realization than thought.
About the authors
Nathaniel C. Lawson, DMD, PhD
Assistant Professor, Department of Clinical and Community Sciences, Division of Biomaterials, University of Alabama at Birmingham School of Dentistry, Birmingham, Alabama
John O. Burgess, DDS, MS
Assistant Dean for Clinical Research, Department of Clinical and Community Sciences, Division of Biomaterials, University of Alabama at Birmingham School of Dentistry, Birmingham, Alabama
References
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