02.07.2014
Edentulism is the condition of being toothless to at least some degree; it is the result of tooth loss.
A dimensionally accurate impression is one of the primary determinants for a precise fitting indirect restoration. The design and use of the custom tray offers distinct clinical advantages compared to the stock tray. Adapting the visible light-cured resin material directly over the wax spacer may leave a wax residue remaining in the tray.
It is essential that the impression material be securely attached to the tray, especially during removal of the set material from the oral cavity.
Adhesive drying times of less than 15 minutes reduced the bond strength values of the elastomeric impression materials to the custom tray. The main objective in tray construction is to provide a rigid tray for retention of the impression material.
Fabricating an indirect custom impression tray requires planning, a diagnostic model, and laboratory procedural time.
3 Discuss hydrocolloid impression materials and their uses, mixing techniques, and application. 5 Discuss elastomeric impression materials and their uses, mixing techniques, and application. The dental assistant is responsible for knowing the different types of impression materials used in the office, the setup for the procedure, the mixing of the material, and either assisting in the procedure or the actual taking of the impression. Impression materials are selected because of their unique qualities that allow the dentist to get the most accurate reproduction of the tooth and surrounding tissues. Preliminary impressions are used to create a reproduction of the teeth and surrounding tissues.
Alginate is an irreversible hydrocolloid (“hydro” meaning water and “colloid” meaning a gelatinous substance) and is the material of choice when taking preliminary impressions.
The material is available in two settings: normal set, which has a working time of 2 minutes and a setting time of up to minutes, and fast set, which has a working time of minutes and a setting time of 1 to 2 minutes. It is important for the clinical assistant to be ready to mix the alginate, load the tray, and help keep the patient comfortable while the dentist takes the impression. Model plaster, which is a derivative of plaster of Paris, is used when strength is not essential and dimensional accuracy is not critical. Dental stone, which is a form of gypsum, is stronger than model plaster and is commonly used when a more durable diagnostic cast is required, for example, when used as a working model to make a retainer, a custom tray, or a casting by the laboratory technician.
Although plaster and dental stone have the same chemical formulas, their physical structures are different, and it is necessary to use specific water-to-powder ratios for each.
Each type of cast has an optimal water-to-powder ratio, which has been specified by the manufacturer. It is important to have enough working time to mix the material and transfer it into the impression. Three methods are commonly used for pouring diagnostic casts: the double-pour method, the box-and-pour method, and the inverted-pour method.
When diagnostic casts are to be used for a case presentation or as part of the patient’s permanent record, they should have an esthetic appearance. The types of final impression materials commonly used in dental practice are polysulfide, polyether, condensation silicone, and addition silicone.
Evaluate the stone model for potential undercuts and design the form and dimension of the tray. Heat and adapt layers of base plate wax to the diagnostic model to obtain a 2 to 4 mm thickness following the predetermined guide marks.
The light-cured material is applied over the spacer and carefully adapted to the pre-determined design. After an air barrier coating (Triad air barrier coating [DENTSPLY]) is applied over the tray material, the custom tray is placed in the light-curing unit and undergoes 2 polyme rizations of 5 minutes each.
This residue contamination can interfere with adhesion of elastomeric impression materials to the impression tray. The polymerized custom tray, while still on the diagnostic model, is immersed in boiling water for several minutes.
The tray is evaluated on the diagnostic model and any sharp edges or irregularities are smoothed with a tungsten carbide bur (H251Q [Brasseler USA]).
Evaluation of the custom tray in the patient’s mouth for proper extension, stability and orientation.
A thin layer of adhesive is applied to the internal surface of the tray and should extend several millimeters beyond the borders of the tray.
A precise, predictable, and dimensionally accurate elastomeric impression is obtained using a visible light-cured custom tray (a,b).


Surface preparation of the custom tray can significantly affect the retention of the impression material and can improve adhesion between impression material and tray.
To obtain durable and stable adhesion between elastomeric impression material and tray, the drying time after application of adhesive should be at least 15 minutes.21,22 Also, it is important to remember that each adhesive is specific to the impression material (ie, a polysulfide adhesive can not be used with an addition silicone impression material). The aforementioned consideration factors can provide insight into the optimal fabrication and utilization of the custom tray. Figures 1 to 10c illustrate the laboratory fabrication and clinical utilization of the visible light-cured custom impression tray that can be used to obtain a precise and predictable final impression.
Dimensional accuracy of dental casts: influence of tray material, impression material, and time.
The accuracy of polyvinyl siloxane impressions made with standard and reinforced stock trays. Accuracy of open tray implant impressions: an in vitro comparison of stock versus custom trays.
A clinical study comparing the three-dimensional accuracy of a working die generated from two dual-arch trays and a complete-arch custom tray. The effect of custom tray material type and fabrication technique on tensile bond strength of impression material adhesive systems. Tensile bond strength of polyvinyl siloxane impressions bonded to a custom tray as a function of drying time: Part 1. Terry is a clinical assistant professor in the Department of Restorative Dentistry and Biomaterials, at the University of Texas Health Science Center Dental Branch at Houston. Tric is a master ceramist and specializes in the many facets of porcelain aesthetic restorations on both natural teeth and osseointegrated implants. Blatz graduated and received an additional Doctorate as well as a postgraduate certificate in Prosthodontics from the University of Freiburg, Germany. Burgess graduated from Emory University School of Dentistry and completed graduate training at the University of Texas Health Science Center in Houston.
Impressions in dentistry are used for creating a model for diagnosing conditions, making custom trays, making a temporary crown, or for the laboratory technician to create a permanent indirect casting. The type of tray selected for a procedure will depend on (1) the dentist’s preference and (2) what will provide the most accurate result for the type of impression material being used.
A preliminary impression can be taken by the dentist or by the expanded-functions dental assistant. A plastic scoop is provided for dispensing the powder, and a plastic cylinder is supplied for measuring the water (Figure 22-5).
If this procedure is a legal function in the state in which the dental assistant works, then he or she would proceed with taking the impression. The materials most commonly used to create diagnostic casts are model plaster and dental stone. Model plaster is easy to trim and is excellent for diagnostic casts because of its clean appearance (Figure 22-6).
The water-to-powder ratio has a dramatic effect on the setting time and strength of any gypsum product. Setting time is the length of time it takes for the mixture of stone or plaster to turn into a rigid solid.
This section should be no more than one-half inch thick and should make up one third of the overall cast.
This material is expressed from a syringe around the tooth and into the sulcus of the prepared tooth or teeth. This material is used as a tray material but has the ability to flow more easily, thus requiring improved control from the tray.
This material is most commonly used as a tray material and has the ability to force the light-bodied material into close contact with the prepared tooth and surrounding tissue to ensure a more accurate impression.
The techniques for custom tray fabrication also vary and range from direct intraoral techniques to indirect laboratory procedures on a primary model. A streamlined design can reduce discomfort to the patient during the impression procedure because of the smaller design size and reduced volume of material.
Diagram with a margin liner pencil the boundaries of the wax spacer (red), borders of the custom tray (blue) and the occlusal stops (blue).
After adapting the material to the anticipated boundaries, the excess material is excised using a scalpel blade (No. Even a small release of the impression material can cause a distortion in the impression, so this is critical. The spacer wax is removed and the air barrier is applied to the internal surface of the tray and placed in the light-curing unit for an additional 5 minutes to cure the interior surface. To improve retention of the impression material, perforations are made with a carbide bur (H379 [Brasseler USA]).


Cross-sectional view of the custom impression tray, illustrating a uniform thickness of material throughout the tray (c).
A visible light-cured resin material (Palatray XL [Heraeus Kulzer]) was selected for its rigidity, high dimensional stability, ease of manipulation, and unrestricted working time. He is a member of Oral Design International and currently owns and operates a dental laboratory and an educational center for Oral Design in Elmhurst, Ill.
He is currently a professor of Restorative Dentistry and Chairman of the Department of Preventive and Restorative Sciences at the University of Pennsylvania School of Dental Medicine. He completed a General Practice Residency and a General Dentistry Residency in the Air Force. Because of the many uses of an impression, many types of impression materials are available.
They can be perforated to allow the impression material to form a mechanical lock with the tray, or smooth, which would require an adhesive to be applied to hold the impression material securely in the tray.
When an impression is poured in stone or plaster to make a model, it creates a positive reproduction of the teeth and their surrounding structures.
The water-to-powder ratio for mixing alginate is 1 scoop of powder to 1 “measure line” of water.
When taking an impression, there are specific steps to be followed to gain an accurate impression (Box 22-2). Impression material is packaged several ways for preference of use: tubes of paste, cartridge, or putty system (Figures 22-10 and 22-11). After pressure steaming the internal surface, a wax remover is applied to the internal surface of the tray to remove any residual wax residue. Also, this material provides the ability to be ideally contoured prior to curing, thus eliminating prolonged finishing times.
He serves on the editorial board of Spectrum and has authored numerous scientific articles on ceramic layering techniques and aesthetic dentistry. He is currently the assistant dean for Clinical Research and the director of the Biomaterials Graduate Program at the University of Alabama in Birmingham. When mixing the material for a mandibular impression, generally 2 scoops of powder and 2 measure lines of water are required.
Before taking the impression, the procedure should be explained to the patient to ensure his or her comfort. Other visible light-cured resins include Individo Lux (VOCO), Triad (DENTSPLY International), and Fastray LC (Bosworth Products). Terry is the founder and CEO of design Technique International and the Institute of Esthetic and Restorative Dentistry. He is a consultant in the area of new product development and clinical testing of materials for dental manufacturers and laboratories. Tric has lectured and given hands-on courses to dentists and technicians throughout the United States and Europe. He is a member of multiple professional organizations, including the European Academy of Esthetic Dentistry and OKU Honor Dental Society. Terry is an editorial member of numerous peer-reviewed scientific journals and has published over 230 articles on various topics in aesthetic and restorative dentistry.
Burgess reviews for 4 dental journals, is the author of more than 300 journal articles, textbook chapters and abstracts, and has presented more than 800 continuing education programs internationally.
He has authored the textbooks Natural Aesthetics with Composite Resin and Aesthetic & Restorative Dentistry: Material Selection & Technique. Blatz has published and lectured extensively on various facets of restorative dentistry, implantology, and dental materials. He is an active investigator on clinical trials evaluating posterior composites, adhesives, fluoride releasing materials, impression materials, and Class V restorations.
He has lectured internationally on various subjects in restorative and aesthetic dentistry. He can be reached at (215) 573-3959 or via e-mail at This email address is being protected from spambots. He can be reached at (205) 996-5796 or via e-mail at This email address is being protected from spambots.



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