Written by Robert Margeas, DDS Saturday, 31 October 2009 19:00
Clinicians are faced with many choices when determining which materials to use for taking impressions. The situation is made more intricate in light of the fact that each type of material offers different strengths and features, and the procedure and handling recommendations for each material can vary. However, some materials have now been around long enough to establish a significant track record of success. With materials such as these, innovations in features and delivery options can help make them even more useful in our practices.
POLYETHERS: A BRIEF OVERVIEW
Polyethers, introduced in the 1960s, have long been popular with clinicians for providing outstanding accuracy and dimensional stability. Most importantly, however, polyethers have proven themselves reliable in their ability to produce restorations that fit precisely even in challenging clinical situations. This performance is largely due to the material’s initial hydrophilicity, which allows the material to capture accurate impressions in the presence of moisture. In addition to hydrophilicity, polyether’s structural makeup makes the material thixotropic an important characteristic contributing to the material’s high reliability.
Hand-Dispensed Polyether Introduced
Polyether materials have previously been only available for use with automatic mixing systems, or in tubes via hand-mixing. Recently, a polyether (Impregum Soft Polyether Tray Impression Material), that can be dispensed from hand-dispensed cartridge mixing system, has been introduced. The material can be used with either a heavy body/light body impression technique, or a monophase technique using a medium body viscosity. This new delivery system allows dentists to achieve the easy and convenient dispensing style of a VPS with the clinical benefits of a polyether. This system is ideal for dentists who have not yet made the investment in an automatic mixing unit, or who are limited to using an automix unit only in selected operatories, the ability to easily use polyether in a portable system.
POLYETHER’S UNIQUE PROPERTIES
Hydrophilicity is a standout characteristic of polyether. While VPS impression materials are actually water-repellent by nature (surfactants are added to increase the “hydrophilicity” of these materials), polyether’s chemical makeup allows it to remain moisture tolerant throughout its working and setting time. In practice, this results in an easier path to void-free impressions. This is due to the fact that when an impression material with a surfactant comes into contact with moisture, the surfactant then has to travel to the surface of the material. The “migration” time needed for this chemical process inhibits VPS materials from reaching a point of fully effective hydrophilicity during their material working time, and this delay in the action of the material can lead to inaccurate, voided impressions.
An additional advantage offered by polyether material is that of its flow properties, which help the product flow to critical areas without a high amount of pressure being exerted. This characteristic is especially helpful in cases with undercut areas or with a deep sulcus. The “shark fin” test is most frequently used to measure flow properties, via a process in which the impression material is injected into a small receptacle, a fin-shaped mold is placed over it, and a weight is added that represents the pressure applied to the material during clinical use. The weight slowly sinks into the material for the specified setting time, after which the height of the resulting “fin” is measured. A taller fin represents better flow properties with this method. The results of this test for Impregum Soft impression material show that it outperforms VPS at both the beginning and end of its working time, exhibiting significantly better flow properties. Results show the flow properties for Impregum remain almost constant throughout the entire working time of the material.4 Additional findings on the cartridge-dispensed polyether are consistent with this data. A study comparing the flow of Impregum Soft Polyether Tray Impression Material delivered from the hand dispenser versus commercial VPS impression materials found that the polyether material showed higher flow than the VPS tested.5
Structural Viscosity and Snap-Set Behavior
Complementing its hydrophilicity and flow properties, polyether’s structural makeup also results in thixotropy, meaning it changes viscosity under stress. This characteristic adds to the reliability of a polyether, as its viscosity is reduced at increased shearing speeds; such as those that occur when the material is being injected or applied to the tray. However, when no stress is being exerted, the material quickly returns to a highly viscous state which prevents it from oozing out of the tray or away from the tooth preparation. This property is the result of the triglycerides used in polyethers. Triglygerides form a 3-dimensional (3-D) lattice that harbors its liquid parts, giving the material the required stability. Under stress, the crystals in the material align evenly and its flowability increases, but once the pressure is released the 3-D lattice is restored, the material reverts to its original viscosity.
A 19-year-old female patient presented with congenitally missing teeth Nos. 5, 7, and 10 (Figure 1). The patient had previously undergone orthodontic treatment in hopes of creating space for implants to be placed. Unfortunately, the orthodontist encountered problems with the roots of the teeth and was unable to torque the existing teeth enough to create adequate space for implants (Figures 2 and 3). Following this treatment attempt, the patient was seeking another option for filling in the missing spaces in order to achieve a more aesthetic smile.
Proper technique is vital in taking an accurate impression. In addition, the impression material itself is vital for capturing the preparation and other morphological details in a way that will meet the goals of the dentist and the patient. With an outstanding impression material, dentists will have less retakes and intraoral adjustments. Doctors and their dental laboratory teams will also benefit from fewer remakes while creating restorations that are outstanding both aesthetically and functionally.
- Klettke T, Kuppermann B, Führer C, et al. Hydrophilicity of precision impression materials during working time. Paper presented at: International Association for Dental Research—Continental European Division (IADR-CED); 2004; Istanbul, Turkey.
- McCabe J, Carrick T. Impression accuracy when recording impressions of moist surfaces. Paper presented at: International Association for Dental Research—Continental European Division (IADR-CED); July 1, 2006; Brisbane, Australia.
- Perry R, Kugel G, Appelin E, et al. Detail reproduction of impression materials on a wet surface. Paper presented at: IADR/CADR/AADR; March 23, 2007; New Orleans, La.
- 3M ESPE Impression Materials Update. Studies show clinical advantages in using innovative “soft” polyether vs. vinyl polysiloxane. 2004; Seefeld, Germany.
- Durack JL, Hudson C, Kuppermann B, et al. Flow of impression materials during working time. Espertise Scientific Facts 2008. 3M ESPE; Seefeld Germany. Paper presented at: International Association for Dental Research; September 2008; Toronto, Canada.
Dr. Margeas received his DDS from the University of Iowa College of Dentistry in 1986 and completed an AEGD residency in 1987. He is an adjunct professor in the Department of Operative Dentistry at the University of Iowa. He is board-certified by the American Board of Operative Dentistry and is a fellow of the Academy of General Dentistry. He has authored numerous articles on implant and restorative dentistry and lectures on those subjects. He is the director of The Center for Advanced Dental Education and maintains a private practice in Des Moines, Iowa. He can be reached at (515) 277-6358 or via e-mail at email@example.com.
Disclosure: Dr. Margeas reports no conflicts of interest.
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