CHAPTER 8

Impression techniques

1.8.1 Introduction

In this chapter:

■ Biological width

■ Methods for temporary tissue retraction

■ Conventional impressions

■ Optical impressions

To ensure well-adapted restoration margins, an accurate impression of the abutment teeth or implants and the surrounding tissues is required. Even though the entire procedure is executed meticulously, a restoration will theoretically never fit perfectly to the preparation margin. Microscopically, a gap of a few microns between the restoration and the tooth will always be present. Studies have shown that when the gap between the tooth and restoration exceeds 150 µm, permanent damage of the periodontium is more likely to occur1^–3. Gingival health is an important factor that can influence both conventional and optical impressions. Taking an impression of abutment teeth suffering from gingivitis demands more aggressive retraction measures, to keep the area clean and dry, which increases the risk of gingival recession4^,5. Well-contoured provisional restorations of good quality facilitate oral hygiene measures and are fundamental in maintaining the gingival health of a tooth undergoing restorative procedure. This in turn, facilitates taking the impression6^–9.

1.8.2 Biological width

Biological width is a term frequently used to describe the dimensions of soft tissue around teeth and implants (Fig 1-8-1). The biological width concept goes back to the early studies of Gottlieb and co-workers from 1921, which documented that the soft tissue attached to teeth consisted of two parts: fibrous tissue and epithelial tissue10^–13. Histomorphometric measurements performed to evaluate the soft-tissue dimensions reported an average sulcus depth of 0.7 mm (range 0.4–1.1 mm), an average dimension of the junctional epithelium attachment of 1 mm (range 0.4–1.6 mm), and an average dimension of the connective tissue attachment of 1.1 mm (range 0.7–1.5 mm). There is evidence that when the preparation margin extends into the junctional epithelium or the connective tissue, that is, violates the biological width, the risk of attachment loss is significantly increased14^–18. To maintain healthy gingival tissues the crown margins should be placed in the gingival sulcus. Hence, on average the preparation margin should not be deeper than 0.7 mm from the gingival margin. One way to mark the sulcus depth before preparing a tooth is to carefully insert a fine moist retraction cord (#000, #00, or #0) into the sulcus and then stay with the preparation margin above the inserted cord.

1.8.3 Methods for temporary tissue retraction

Gingival retraction methods used today can be divided into mechanical methods, and chemical methods or combinations of those methods19^,20. It is important to carefully place the retraction cord (Fig 1-8-2) into the gingival sulcus to prevent unnecessary damage to the junctional epithelium and supracrestal connective tissue fibers21. Studies have shown that the placement of a dry cotton cord into the gingival sulcus can cause damage to the gingival epithelium. In the purely mechanical method for gingival retraction a moist cotton cord without any additional agent is placed in the gingival sulcus22. However, in the mechanical-chemical method the cotton cord is impregnated with different medicaments, eg, epinephrine, aluminum chloride, zinc chloride, aluminum sulfate, tannin, or ferric sulfate23. Today, the chemical-mechanical method is most frequently used for temporary gingival retraction24.

Basically, there are two methods used for temporary gingival retraction with cotton cords: single cord and double-cord technique:

■ “Single cord technique”: One cotton cord is placed into the gingival sulcus to make the preparation margin accessible for the impression material or the optical scan (Fig 1-8-3). It is important that the cord length is adequate and fits the circumference of the tooth. The cord is either removed immediately prior to taking the impression, or it is left in the gingival sulcus below the preparation margin while the impression is made. Upon removal, it is important that the cord is slightly wet to prevent unnecessary bleeding and damage to the junctional epithelium5.

■ “Double-cord technique” is also a well-known method for gingival retraction25^,26. The first cord is fine (#000, #00, or #0) (Fig 1-8-4) and is placed into the sulcus usually without any medication. It should remain there throughout the period of preparation and impression taking. The purpose of this fine cord is to gain vertical retraction for access to the preparation margin. The second cord, which is usually coarser (#0, #1, or #2), is placed on top of the first one and then removed shortly before the impression is made (Fig 1-8-5). The purpose of the second cord is to gain lateral gingival deflection and create space for the impression material25.

Guidelines on the amount of time the cords should stay in the gingival sulcus in order to gain sufficient tissue retraction have ranged from 3 to 15 minutes27^–29. To minimize the potential for soft-tissue damage, it is important to remove cotton cords from the gingival sulcus as soon as possible after the impression is taken. Leaving a cord in the sulcus over a long period of time can cause inflammation, infection, abscess formation and major gingival recession28^,29. For purely chemical temporary tissue retraction, gingival retraction paste with hemostatic effect that contains aluminum chloride and aluminum-silicate-hydrate has been introduced19^,30^–33. The paste is applied with a syringe into the gingival sulcus and left there for 1–2 mins to give tissue retraction (Fig 1-8-6). Before impression taking the paste is rinsed away with water and the area is air-dried. A recent clinical study31 that compared the effect of a retraction paste (Expasyl) and the double thread technique for temporary tissue retraction concluded that minor or moderate gingival recession (<1 mm) were more likely to occur when conventional cords are used compared with the retraction paste (Fig 1-8-7). However, when using the double-cord technique, the dental technicians found die preparation significantly less challenging compared with impressions made using the retraction paste for tissue retraction31.

1.8.4 Conventional impressions

The aim of a conventional as well as optical impression is to get an exact impression of the abutment tooth with all its details: the preparation margins, the relation between abutment teeth and/or implants, the soft tissue, and the alveolar ridge, the neighboring teeth, and the remaining teeth of the dental arch. Before impression taking it is important to double-check whether adequate temporary tissue retraction has been achieved. A clearly visible retraction cord around the abutment tooth usually represents an adequate tissue retraction (Fig 1-8-8). For tooth-supported restorations it is recommended to utilize impression material consisting of a light body with high viscosity with excellent flow properties for the syringe and medium or heavy body with lower viscosity for the impression tray (Fig 1-8-9). For implant impressions, however, it makes more sense to use stiffer impression materials like medium or heavy body that give more exact relation between implants or between implants and neighboring teeth (Fig 1-8-10).

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