This guide will provide information about digital impressions and scanning within dentistry, including their roles, comparisons between new and traditional techniques and the benefits they offer to dentists and dental patients. A colleague recently pointed out that it is appropriate to change terminology from gingival retraction to the more correct sulcus expansion. Gingival retraction, which may have been a more viable term in years past when a patient’s tissue health might have been less than ideal, is no longer descriptive of the procedure that is actually performed. When a patient has good gingival health—and there should be no reason to begin an indirect restoration until that has been achieved—the procedure that is accomplished is a temporary expansion of the sulcus to permit access for final preparation of the margin and then taking the impression. Removing the cords after completing the impression then permits the tissue to return to its normal biological position without a change in the occlusal-apical height of the tissue. Changing the terminology helps the dentist, chairside assistant, and patient more accurately visualize and then achieve the ideal result. Two-stage un-spaced impression – putty impression is recorded first and after it has set it is relined with a thin layer of wash material. Traditionally, creating an impression involves using dental putty to form a mould of the mouth; not only is this messy and potentially inaccurate; it may also be very uncomfortable and unpleasant for the patient, especially if they feel anxious or they have a very sensitive gag reflex. When a dentist creates a mould in this way, they insert the putty into the mouth and then shape it, so that it covers the teeth and gums; when the putty sets, a model of the mouth is formed. These models are then sent away to a laboratory where dental technicians will use them as a basis for a new restoration, such as a crown or bridge, or a gum shield for sport, for example.

When the clinical procedure is limited to 1 or 2 units, the clinician may choose a dual-arch tray. A metal tray would be preferred over a plastic tray because of its rigidity and resultant dimensional stability. A tray material that is very inflexible should be used for its rigidity and ability to support gypsum when the model is poured. Panichuttra R et al. Hydrophilic poly(vinyl siloxane) impression materials: dimensional accuracy, wettability, and effect on gypsum hardness. Int J Prosthodont 1991; 4: 240–248. Generally, polyether and VPS impression materials are suitable for making impressions of natural teeth and implants. Due to the higher initial hydrophilicity and constant flow behavior of polyethers, they may show a superior reproduction of the relevant details, especially in more complex cases where extra working time is needed. Based on this assumption, they are often preferred in clinically challenging situations and recommended for implant cases with multiple units. Our recommendation is that if a VPS impression material is used, the clinician should select a product, which is hydrophilic already in the unset state and offers good flow properties; e.g. 3M™ Imprint™ 4 VPS Impression Material. It has a limited working time based on the temperature of the water used and it is available in both standard and fast-setting varieties. The most effective way of recording a putty wash impression is to use the one-stage technique in a rigid metal tray.Do your best to control these by using retraction cord, haemostatic agents, or possibly even surgery (electrosurgery, or crown lengthening surgery) to give you a clear, clean field. The most difficult aspect of a crown preparation is the interproximal margin. Therefore it makes sense to reduce the amount of preparation required in the area.

Sulfur contamination from natural latex gloves inhibits the setting of addition silicone. Some vinyl gloves may have the same effect because of the sulfur-containing stabilizer used in the manufacturing process. The contamination is so pervasive that touching the tooth with latex gloves before seating the impression can inhibit the setting of the critical surface next to the tooth.Silicone impression material consists of a base and catalyst in a putty consistency. Silicone impressions are classified according to their method of polymerisation on setting. They are available in a range of viscosities including light, medium, heavy and very high viscosity (or putty) material. When taking a putty impression it is usually combined with a low viscosity silicone. This is also known as the putty wash technique. Putties were developed initially to reduce the shrinkage of condensation silicones. There are three ways of recording a putty wash impression: With the elimination of the putty viscosity as a recommended choice, the remaining choices are heavy-body, light-body, and monophase materials. To logically choose the best to use, it is important to consider how all materials react during setting and the impact that has on the previous discussion of trays and techniques.

Certain materials require more reduction than others. This is due to physical or optical properties. Margins should be smooth and continuous with no steps. This is difficult to achieve. I find that a fine grit bur (yellow or white) is very helpful in refining the margins. Mixing of cements, bases, and liners is not a procedure that the dental hygienist typically performs on a routine basis. However, the hygienist may be asked by the dentist to mix these materials if the dental assistant is absent. This situation would include using them as a temporary restoration. Step-by-step instructions for measuring and mixing some of the commonly used cements, bases, and liners serve as the basis for this chapter. II.Calcium Hydroxide Base/Liner Material A.Use The setting time for both consistencies of zinc phosphate cement is between 5 and 9 minutes. F.CleanupImpression materials are introduced into the mouth as viscous pastes with precisely adjusted flow properties. The viscosity and flow behavior of the unmixed components are also important in regard to the ease of mixing, air entrapment during mixing, and the tendency for the trapped air to escape before the impression is made.