In the mid-19 century, others materials such as plaster of Paris, gutta-percha and thermoplastic modeling compound became popular for taking impression and after in the early 1900s has been introduced the reversible hydrocolloid alginate and later irreversible hydrocolloid alginate that is yet used nowadays (1).ĭental models can be used to evaluate the occlusion and perform measurement more easily and accurately than in the patient's mouth. Philip Pfaff first described an impression-taking technique by using heated sealing wax to obtain a negative representation of dental arches that was used to pour a cast in plaster of Paris. The information obtained from this dental casts is invaluable to help in the orthodontic diagnosis and plaster models have been the standard for years, since their introduction in the 1700s. Introductionĭental study models are a cornerstone in the armamentarium used by orthodontists to both classify malocclusion and formulate treatment plans. In the most of studies the measurements results underestimated but the differences are clinically insignificant. Results: digital models represents a valid alternative to plaster models in the orthodontic diagnostic process. Materials and methods: rating of studies present in literature from 2003 to 2016, that evaluate the validity and reliability of orthodontic space analysis on digital models. To take an accurate impression of the patient’s oral tissues, a custom-made tray is constructed to support the impression material ( Figure 2.2.1).Objective: the purposes of this review is to evaluate the validity and reliability of orthodontics measurements on digital models Secondary impression casting Introduction 2.2 Construction of a close-fitting custom impression tray Heat-cured acrylic resin is accurate and stable during use, however, it is very expensive in terms of the time taken for manufacture. It is easy to adjust but is brittle, may distort at relatively low temperatures and is less easy to use. Shellac is a form of wax which softens on heating. However, although self-curing acrylic resin is cheap, it has limited working time and easily distorts during curing. They are more expensive than self-curing acrylic. They are supplied in a uniform thickness and are dimensionally stable on curing and in storage. Light-curing composite blanks are the most common and easy to use materials for tray construction. This should transfer to the model on casting. This is drawn on the primary impression by the clinician with an indelible pencil for alginate or a permanent marker pen for silicone. The outline (peripheral extension) required for the custom-made tray.The information you should find on the prescription card and impression is as follows: There are few indications in complete denture prosthetics for the use of such high viscosity materials for working impressions. Spacing is also required for stiff (high viscosity) impression materials because unless there is some spacing between the tray and the tissues it is difficult to seat the tray without using considerable force. A weak material such as alginate requires more spacing than a tough material such as silicone. The amount depends on the amount of undercut to be recorded, the elastic limit and tear strength of the chosen impression material. When undercuts are present, spacing is required.
#A maxillary alginate impression routinely requires free#
In edentulous cases, the majority of denture-bearing areas are free from large undercuts and impressions may be taken in close-fitting trays using impression materials which are rigid (non-elastic) when set, i.e.
Retain its shape throughout the impression procedure and during model production.Allow pressure to be applied by the clinician on selected areas of the denturebearing area.Support the impression material in contact with the oral tissues.The design of the tray will depend on the amount of undercut present and the type of impression material being used. Like stock trays, they are used to support and transport the impression material to the mouth and once set, to the laboratory. Their design and accurate fit allows for better control of impression materials that are fluid and must be guided into place. They are custom-made to fit the denture-bearing area on the primary model. Overview of custom impression traysĬustom impression trays are used to overcome the inaccuracies associated with primary impressions.
Primary impressions are discussed in the previous chapter, here we will begin with the manufacture of a customised tray. (Note: The clinical stages are indicated in bold, and those conducted in the dental laboratory in italics.)
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