Most impression materials can be used with both a perforated and an unperforated spoon. A dense spoon is preferred, because this allows maximum pressure on the impression material during placement. In addition, less impression material is needed for a closed spoon because there is no overflow through perforations. As already discussed, a good fit of the impression spoon has a major influence on the quality of the impression.
The following applies for the distal limitation of a suitable spoon: at the upper jaw, a good impression spoon must distally enclose the entire tuberculum retromolare, in short the tuber.
In the lower jaw, the entire trigonum retromolare, called the trigonum, must appear in the print.
The width of the dental arch must be well within the contour of the spoon. On average there should be about 3 to 4 mm space for the print material. If the spoon is too tight on the buccal side, it will cause tension during curing. In addition, there is a risk of tearing the thin spot while taking out the print.
With the help of a special blunt caliper the size of the buccal width at the level of the first molars can be taken in the mouth. The outside of the compass legs must then lie just against the inside of the chosen upper spoon.
A correct linguistic width is important for the lower jaw. To this end, the legs of the blunt compass lingual at the level of the second molars are pressed against the alveolar process. The compass is taken out and a suitable lower spoon is selected with the inside of the legs. If the spoon is upside down, with the edges up, the lingual edges of the chosen spoon should just fall inside the compass legs at the location of the second molars.
In the case of abnormal shapes of the dental arch, a confection spoon can be selected from a confection mold set or an initial impression can be taken to make an individual spoon.
Common dental arch shapes are:
- V-shape: elongated, narrow arch in the front.
- Omega-shape: named after the shape of the Greek letter omega: O. A normal to broad front is followed by some inward (endosteme) premolars.
- U-shape: a dental arch with a broad front because the cuspidates are outside the arch (ectosteme cuspidates, also called Dracula teeth).
The length of the spoon edge must be well matched to the purpose of the print.
For making orthodontic models, it is necessary that a lot of information about the alveolar process is included in the print. This requires that the print tray has very long edges. For this, confection spoons often have to be built up with strips of yellow beeswax. 2.5 cm wide strips are heated and folded in half around the edge of the spoon. The spoon must be thoroughly dry, otherwise the wax will not stick sufficiently. This building up of the edges must usually be done on the entire buccal side and also on the lingual side. Also, a kind of "dam" is applied distal to the upper scoop to prevent too much impression material from going to the throat. The previously described disposable borderlock spoons also have such a distal seal on the palatum.
An individual scoop for a complete prosthesis, on the other hand, will have to have short edges, because the muscles must be moved during printing to thereby determine the extreme boundary of the future prosthetic edge: mucodynamic print, also called functional print.
The shape of the spoon edge can be adjusted with heated gray stents or with Isofunctional. A thin layer of this build-up material is applied warm to the spoon edge. The spoon is then placed in the mouth and the edge is shaped into the soft, warm material by muscle movements.