There are two schools of thought regarding triple tray impressions:
1. they are the greatest technique ever;
2. they are absolute garbage, resorted to by lazy dentists.
As it happens, neither extreme position is true. Triple tray impressions are not inherently better than full arch or quadrant separate upper and lower impressions, but they do have several desirable features, when properly carried out.
These features include locked-in occlusal registration, greater comfort for the patient, reduced impression material, reduced chair time, better marginal impressions, better fit and fewer adjustments. In addition, a crown can be done on both upper and lower posterior quadrants on the same side simultaneously These benefits can consistently be achieved if careful and precise procedures are followed. By a careful and precise dentist who would use those same qualities in a full-arch impression, which is the standard of comparison.
Carelessness or blurred vision (literally and figuratively) will result in poor results no matter which technique is used.
Several articles have appeared in Oral Health over the last year, which deal with triple tray impressions and techniques and other writers and lecturers promote their own triple tray techniques. I feel that each of them has been somewhat deficient, leading to misunderstanding by the reader and consequent frustration when results are not as impressive or consistent as suggested or desired.
Still, there are some situations where triple-tray impressions are more complicated or not indicated, such as major anterior aesthetic procedures or full-arch reconstruction. In such situations full-arch custom trays are indicated.
Here are my criteria for a successful triple tray impression:
1. Cases selection should be limited to one to three units per quadrant.
2. The patient must be able to be guided into a centric closed position and stay there during the full setting time.
3. The impression cannot be reliably done in a single stage — there must be a two-stage impression in which the first essentially creates a custom tray and records the occlusal relationship, while the second, subsequent impression captures detail of working and opposing arches.
4. There must be adequate venting of the first impression to reduce hydraulic pressure and eliminate consequent distortion of the carrier tray.
5. Triple tray impressions are not substitutes for proper soft tissue preparation. Compressive techniques which attempt to eliminate retraction cord or other tissue displacement methods will usually result in crowns which are too tight, require significant occlusal adjustment and may have margins which are short of their intended tooth margins.
6. Appropriate materials should be used (see below).
That said, it is not difficult to select a method which captures all of the advantages and results in a very high percentage of finished prostheses which meet all criteria for success.
MATERIALS
The tray must be rigid, have a thin web (not plastic mesh) and not interfere with the patient’s closing comfortably. One good example is the Clinician’s Choice Quad-Tray.
The first impression must be carried out with an accurate material designed for the purpose. PVS materials designed for bite registration or fabrication of provisional crowns are not suitable. They are not accurate enough, they set too quickly and may be too brittle. Two syringeable materials that are suitable are Parkell BluMousse, and Clinician’s Choice Affinity InFlex. Most puttys are too soft for this technique and will distort. One putty which does work well is Ivoclar’s SilTech Super (Please note: this is a two-part, hard-setting putty system. The similarly-named Sil-Tech is a condensation reaction paste-plus-gel catalyst system which is not suitable.)
The wash material may be any good PVS material. It should be light enough to flow into all detailed areas, but it should not drip out of the tray. It should not be so viscous that it distorts the carrier impression. A strong colour contrast with the tray material is advisable. A very quick setting material should not be used – for the sake of an extra minute, the working time needed to seat the tray without premature setting is a strong predictor of success.
The pressure relief venting of the preliminary impression is carried out by a layer of aluminum foil or Glad freezer wrap (not plastic sheets) lightly applied to the teeth when the preliminary impression is taken before the teeth are prepared. The foil can be lightly smeared with PolyGrip denture adhesive to help it cling to the teeth, but it should not be tightly adapted. Good flow of the excess material without tray distortion is necessary.
TECHNIQUE SEQUENCE
1. Tray is tried in prior to tooth preparation, usually while anaesthetic is taking effect, and modified if necessary. The tray is dried and adhesive is applied.
2. Spacer foil or film is applied to upper and lower teeth on the side to be treated.
3. First impression is taken with the rigid material.
4. Impression is removed and then tried in again immediately to ensure that there is no soft tissue impingement which might keep the patient from closing completely. Any such portions can be carved away with a pocket knife.
5. Tooth or teeth are prepared and tissue control is established (techniques to be described in a subsequent article).
6. Spacer foil is removed from the first impression, and a small amount of medium body (monophase) impression material fills the tooth impressions only, upper and lower.
7. Light body material is syringed around the teeth
8. The tray is seated, the patient is guided into full occlusion while the dentist observes the contra-lateral side to confirm full closure. The dentist remains with the patient to ensure that there is no movement.
9. The assistant sets a timer for the full setting time (usually 4 minutes) and the impression is not removed until the timer sounds!
10. The impression is removed and examined carefully under magnification, the teeth are temporized (techniques to follow in a subsequent article) and the patient is dismissed.
If the impression is not satisfactory for any reason, this is the time to redo it. It is important that the same impression not be used to re-impress, because distortion will invariably result. Instead go through all of the above procedures again. It should not take more than six or seven minutes, but it will eliminate a wasted subsequent appointment.
The laboratory technician must be instructed to cast and mount both sides of the impression before separating the dies from the impression. This is essential to make sure that the correct yaw-axis orientation of the models is not changed. A hand-articulated or separate registration patty will reduce chances of success.
If the patient has failed to close all the way but the impressions of the working and opposing sides are satisfactory, the impression can still be used if a separate PVS occlusal registration is taken. In this case the technician must then be instructed to ignore the previous paragraph and mount to the register.
Using these techniques we have provided many hundreds of restorations with a very high degree of accuracy and simplicity. In its place, properly applied, the triple tray impression technique is reliable and predictable in the production of high quality restorations.
Dr. Wyman practices in Toronto. For information on courses in which he teaches this and other techniques visit www.roelwyman.ca.
Oral Health welcomes this original article.
