Understanding Impression Materials and Techniques

There is no single procedure that a dentist can do, or any single procedure that a lab can do for the dentist, that will improve the final result of indirect restorations more than improving the quality of the impression. The impression is the basic starting point for all laboratory procedures, as well as the basis for quality and cost-effective dentistry for the clinician.

Unfortunately, there are a great many choices regarding the best tray to use (full-arch, quadrant, or dual-arch [triple" trays, stock or custom, plastic or metal trays) and regarding which technique is best (1- or 2-step putty/wash, heavy-body tray/wash, or monophase), and an even greater number of choices regarding which impression material to use to take the impression. At least 43 different brands of polyvinyl siloxane impression materials are offered by at least 29 different companies. Confusion abounds and is further complicated by the omission from marketing literature of some information about PVS materials that is instrumental in using them and understanding how to get the best results from them.
This article will attempt to clarify some of the areas of confusion regarding impression materials and techniques.

TRAY SELECTION

Consider first the type of case that is being done. Laboratories estimate that about 85% of units are single crowns, and most of those are done with a dual-arch quadrant or triple tray. Because of their ease of use, conservation of material, and elimination of the need for an opposing impression and bite registration, dual-arch quadrant impressions have become the overwhelming choice for most dentists. When employed properly they can be used satisfactorily; however, the overuse and abuse observed in commercial laboratories have frustrated both technicians and clinicians.
This is further complicated by the fact that many commercial laboratories employ plastic, disposable articulators to use in the fabrication process, adding to the inaccuracies that can occur. Evaluation of the 6 most popular plastic, disposable articulator systems used with dual-arch impressions found that with all systems, it was virtually impossible to maintain and repeat a centric occlusal position stop. The conclusion was that they should only be used for single units, not when the terminal unit in the arch is being prepared.1
Another study evaluated the effect of the viscosity of the impression materials in plastic and metal dual-arch trays.2 There were statistically significant differences noted in the accuracy of the dies. Rigid materials in metal trays were the most accurate, while monophase materials in plastic trays produced dies that were dramatically shorter.

Figure 1. Dual-arch impression that was correctly taken in maximum intercuspation shows tooth-to-tooth contact anterior and posterior to the prepared tooth.	Figure 2. Soft-tissue contact with the plastic tray leading to distortion of the final impression.
Figure 3. Tooth contact with the plastic tray leading to distortion of the final impression.	Figure 4. Even soft-tissue contact with a plastic tray can cause the tray to distort.
Figure 5. CLINICIAN`S CHOICE metal Quad-Tray.	Figure 6. Photograph illustrating the difficulty in stabilizing a quadrant tray.

The 2 most common problems observed in commercial labs with dual-arch trays are tissue or tooth contact with the tray and failure to record maximum intercuspation. Recent visits to 3 commercial labs in 3 different states in 3 different regions of the country showed that only about 40% of the 100 randomly selected dual-arch quadrant impressions were in maximum intercuspation. Maximum intercuspation can be visually verified by observing tooth-to-tooth contact anterior and posterior to the prepared tooth (Figure 1). Few metal trays were observed, even though the preceding evidence, as well as the anecdotal advice of many prominent clinicians, suggest they are more accurate. Soft-tissue contact was frequently observed in plastic trays, with an occasional tooth contact to the tray (Figures 2 and 3). Any contact between soft or hard tissue and a plastic tray applies an occlusal force to the tray that is likely to cause it to distort. When the patient opens-removing the occlusal force that distorted the tray-the tray returns to its original dimension because of the memory of the plastic, and the final impression is distorted, causing inaccuracies in dies as well as occlusal and contact discrepancies (Figure 4).
A properly designed metal dual-arch tray assists in tray placement and increases the ease of reaching maximum intercuspation, as well as provides the proper rigidity to decrease the chance of 3-D distortion (Figure 5). For more involved cases where a full-arch impression is indicated, research utilizing an optical laser scanner showed that dies from a stock impression tray were as accurate as those produced from a custom tray.3 Custom trays do save money in impression material because they generally use less material. That savings is more than offset, though, by the additional cost of tray material and labor to fabricate a custom tray. Custom trays should be employed when the patient`s anatomy dictates their necessity, most commonly in cases of mandibular tori or a particularly large or small arch form.
Quadrant single-arch trays are the most difficult tray to use without introducing distortion because of the difficulty in stabilizing the tray while the material sets. It is virtually impossible to keep the tray steady-particularly on the mandibular arch-if the patient makes any kind of movement with his or her lips or tongue while swallowing. Because the clinician is unable to obtain cross-arch stabilization during setting of the impression material, movement permits a slight rocking of the tray that causes distortion (Figure 6).

TECHNIQUE CHOICES

Dentists can use a number of different techniques that employ a variety of different materials and viscosities to take impressions. Traditionally, a heavy-body tray material is utilized with a lighter body wash material that is syringed around the tooth just before the tray is seated. Since the 1980s, with the introduction of vinyl putties, several putty/wash techniques have been advocated. Various innovations of these techniques have emerged, further confusing the clinician. Although it is possible to get many different techniques to work, the final choice as to technique should center on which is least problematic and most likely to produce consistent results.
The putty/wash techniques fall into 2 basic categories: (1) a single-step procedure where the putty is loaded into the tray and inserted immediately after syringing a wash material around the prepared tooth or teeth, or (2) a 2-step procedure where the putty is used to take an impression before starting the preparation, allowed to set, and removed from the mouth. After tooth preparation is complete, the tooth has a wash material syringed over it, and the initial tray and putty impression are reinserted over the wash. Variations of this include routing out part of the putty impression around the prepared tooth to produce space for the wash, and relining the entire impression with wash material.
Statistically, laboratories see more problems with 2-step impressions than any other technique. This is a result of the difficulty in repositioning the impression in the exact same position without creating a [step" between the original putty impression and the detailed wash impression. It is much more difficult to reseat the tray than it appears, and the result is more chairside occlusal adjustments in the completed restoration. There appears to be little difference in reducing the creation of [steps" by routing out some of the material around the prepared tooth or washing the entire arch.
The single-step putty/wash technique can also be equally difficult to produce a consistent result with because the viscosity of the putty material is most often considerably lower than heavy-body tray material produced from a mixing machine or by hand- held gun extrusion mixing cartridges. The extremely stiff putty material, when seated in the mouth, forces the much thinner wash material away from the prepared tooth, and the resultant impression is captured in more putty than wash. ADA/ANSI specifications require that a wash material capture details to 20 µm, while tray materials are required to capture only 70 µm of detail.4 For this reason, most if not all of the prepared tooth should be captured in the fine detail of the wash material.
Recently, variations of these putty/wash techniques suggest that they can be successfully utilized to obtain impressions without the use of retraction cords. The concept is that the wash material can hydrostatically be forced into the gingival sulcus when the set putty impression is reseated in the mouth, thereby capturing the impression of the margins without having to place retraction cords or remove gingival tissue with a laser or electrosurgically. Some techniques further suggest that this technique can hydrostatically displace crevicular fluids as well as tissue.
Again, with these techniques the commercial laboratories report inconsistent results with their clients regardless of the brand of material used. The degree of preparation taper plays an integral role in the amount of hydrostatic pressure that is placed at the margin of the preparation during this impression technique. The greater the preparation taper, the greater the dimensional accuracy.5 Too much hydrostatic pressure at the margin causes a [pull-back" or rebound effect when the impression is removed from the mouth after setting. This is why all full-arch impressions should be held passively while setting. Any active force while the impression is setting causes an increase in elastic recoil when the impression is removed from the mouth and creates a reduction in the size of the casting. The greater the preparation taper, the less the hydrostatic pressure at the margin, creating (in theory) enough hydrostatic pressure to displace fluids and tissue, but not enough to create 3-D distortion. Unfortunately, an increase in the degree of preparation taper to fulfill the requirements to make this technique work adds to the clinician`s problems by ultimately increasing retention difficulties because of excessive taper.
Finally, the reason for using retraction cords, laser, or electrosurgical techniques is not limited to taking the impression. Precise viewing of the margin of the preparation is imperative, unless placed completely supragingival, for final preparation of the margin. Most clinicians advocate the use of a dual-cord impression technique, where the first cord is placed to fill about one half of the sulcus as soon as the contact is broken. This process expands the sulcus to allow for better visualization of the preparation margins for final preparation, and greatly benefits the procedure because the first cord is placed prior to any gingival bleeding. It is much easier to do this and prevent gingival hemorrhage than it is to arrest hemorrhage after it begins. A correctly chosen diameter of cord can be placed quickly and atraumatically. After the preparation is completed, the second cord is placed to expand the sulcus further so that the margin is visible for impression taking. The second cord is left in position for 3 to 5 minutes and removed for the impression, while the first cord is left in place to ensure hemostasis until completion of the impression, when it is removed.
If the gingival sulcus is very shallow (1 to 2 mm), particularly on anterior teeth, veneer preparations, and when the gingival margin is being placed just at or just below the height of the sulcus, it is frequently better to use only one cord. In these situations the preparation is completed supragingival, and then the retraction cord is placed, again filling about one half of the depth of the sulcus. The preparation can then be refined to the height of the sulcus or slightly below without trauma to the gingival tissues. The impression can be taken after careful removal of the retraction cord, or the cord can be left in place, as there should be adequate access to the supragingival margin to take the impression.
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.

MATERIAL CHOICES

Figure 7. Two-dimensional representation of polymerization shrinkage that occurs toward a rigid wall and to the center of its mass when no rigid wall is present.

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.
All impression materials-polyvinyls, polyethers, polysulfides, and hydrocolloids-polymerize toward the rigid wall of the tray that has adhesive and/or perforations to retain the material. Unfortunately, the interproximal material has no tray, so it polymerizes toward its center (Figure 7). This explains why an impression of a perfectly round machined steel die produces an elliptical casting, which is observed regardless of the type or brand of material used.6 Polymerization shrinkage accounts for the 3-dimensional differences in dies made from impressions taken with the same material, but with different techniques-trays, dual-arch trays with sides, and sideless dual-arch trays. There is one material that is marketed specifically for use in the dual-arch quadrant metal tray (Inflex [CLINICIAN`S CHOICE]).
Monophase materials are limited in their clinical application. An ideal material must be resilient enough to be withdrawn from the mouth easily, particularly when mobile teeth or prepared teeth are thin. The material must also be flexible enough to be removed from the model without breaking off a thin preparation. This has led to the development of the [soft" formulas of polyethers. However, any material when used in a sideless, dual-arch tray must be firm enough to support the weight of the gypsum when the model is poured, without sagging under the weight of the stone. It appears that one viscosity of material would not be able to ideally fulfill all of these parameters simultaneously.

CONCLUSION

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.
When the clinical procedure calls for a full-arch impression, a stock tray coated with a compatible adhesive can be utilized with a slightly less rigid tray material to aid in removal from the mouth.
In all cases it is imperative to be able to see the preparation for final margination, and then take the impression. The dual-cord technique accomplishes both, and when done properly in the preparation sequence can lead to minimal hemorrhage or gingival trauma.