The reliability of diagnosis can be improved by collecting adequate records of a patient, which means that trustworthy data that faithfully reproduces the conditions prior to commencing treatment is readily to hand. Study models (sometimes with registrations including face bow for mounting on an articular), with or without a diagnostic set-up, photographs, and the necessary radiographs, provide a treasure-house of information important in the diagnosis, as well as during assessment of the treatment’s progress. Records taken during and after treatment provide the means of being able to mark the intermediate and finished stages, as well as being able to compare and analyse the effect of treatment and of growth, if any.
However, when collecting extra data, a clear distinction should be made between techniques that can be damaging (e.g., radiography) and those which, even if uncomfortable (e.g., impression taking), produce no lasting effect.
If a patient attends a university clinic for treatment, the records required are often more comprehensive, due to the associated teaching and research carried out and to the availability of facilities not normally found in the practice.
A distinction should be made between impressions taken for prosthetic treatment and those needed for study and record purposes in orthodontics. In prosthetic treatment, for example, the impression of the sulcus for a full denture should provide a peripheral seal at rest and in function; on orthodontic models, the tissue in the sulcus should be pushed away as far as possible during the impression taking so that as much of the underlying structure is reproduced.
For a reliable orthodontic diagnosis to be made and a decision taken on the correct therapy, the anatomy of the alveolar process and its junction with the basal mandible and maxilla is of real significance. To take such an extended impression, impression trays should be used that have much higher flanges than those used in prosthetics. Secondly, the peripheries of the trays should be extended, using soft wax or a similar material, to not only obtain good adaptation but to also push up the tissues of the mucobuccal fold very low, or very high. In doing this, one should relieve structures (such as the frenula) that cannot be pushed away. Figure 4-1A shows which areas of the skull can be included in impressions; that this is indeed possible can be seen from Figure 4-1B where the casts are reproduced in correct relation with the matching skull radiograph. Naturally the bases of the models have not been superimposed and only the anatomical part may be seen.
Fig. 4-1 In taking impressions for orthodontic study models as much as possible of the mandible and maxilla should be reproduced.
A The area to include in the impression is indicated.
B Dental casts are superimposed on a lateral skull radiograph of the patient, indicating their spatial orientation and extent.
(Van der Linden, F.P.G.M. and Boersma, H.: Taking impressions for study casts. Quintess. Internat. 1: 53–60, 1970).
The extra high metal impression trays shown in Figure 4-2A do not appear to be high enough (Fig. 4-2B–D), and this also applies to most trays—including the foam disposable type. In order to get an impression of all the required features, the tray may be built up with wax (Fig. 4-3).
In fitting the tray, the teeth should be situated at the center of the appropriate part of the tray. However, if the dental arch is narrow, a compromise often has to be made because the tray is too wide transversely and too short sagittally. To achieve a good reproduction of the buccal segments, the tray must be fitted fairly close, though not too close, to the alveolar process buccally. This also applies to the lingual side of the lower jaw. A general requirement may be set for there being a play of 2 to 4 mm both sagittally and transversely within the tray.
Building up the peripheries may be done with Keur & Sneltjes yellow wax No. 2873, which is specially formulated for this purpose. The wax has such a consistency that good adaptation to the structures is obtained. It is recommended that strips of wax 25 mm in width be cut lengthwise from the sheets, which are about 170 mm long. The strips may then be kept ready for use, when they are warmed, folded along their length and adapted along the rim of the trays (Fig. 4-3). The tray has to be dry for the wax to adhere well. On the lingual side of the lower tray, a particularly deep extension (going well distally) is used to obtain an impression of as much as possible of the lingual aspect of the body of the mandible. The upper tray is provided with a postdam a little dorsal from the Ah-line, mainly to prevent the impression material from flowing too easily back over the soft palate. The postdam is extended laterally behind the tuberosities where it joins the wax that has been built up into the buccal sulcus. The wax build-up is checked in the mouth where, by applying firm pressure, it will be molded to conform with the tissues. The postdam can be seen in Figure 4-4A whilst Figure 4-4B shows the labial extension of the upper tray. The wax periphery on the tray also provides additional retention of the impression material to the tray.
Instead of using Keur & Sneltjes yellow wax, Columbus Dental large white utility wax strips No. 94191 may be employed that are easier to clean off than the yellow wax and, if using metal trays, may be more economical to use because time is saved during the wax removal period.
The technique of impression taking used here, differs from that generally used, and is set out below:
Before choosing trays, the mouth should be cleaned up. Shortly before taking the impression the patient should remove saliva by rinsing the mouth thoroughly with water. The lower impression is best taken first because this is usually easier for the patient, and also helps prepare the patient for the more uncomfortable upper impression.
The filled lower tray is held above the arch so that the occlusal surfaces of the teeth are in the middle of the impression material. The tray is then pressed downward with small tilting movements and pushed slightly dorsal. The heel of the tray should firstly be seated to its full depth and the front pressed down. The lower lip and front part of the cheek should be eased out first on one side, then on the other, to allow any air to escape. The patient should then be asked to put his tongue over the tray so that the material on the lingual side may be gathered up. By first seating the tray posteriorly, the impression material is able to flow forward and the tissues may be extended where it is most desirable. The initial centering of the tray over the arch requires careful handling to avoid the front of the tray descending too far labially, which would result in the lingual flange of the tray pressing painfully into the alveolar process. If surplus material does accumulate in the posterior region, it is wise to remove this at once since it will hinder removal of the tray if left to harden and, by contact with the maxillary teeth, will prevent easy lifting of the impression away from the mandibular teeth. Every effort must be made not to damage the impression. The tray is held motionless whilst the impression material is setting. To do this, the tray is held steady by the index and middle fingers, and the thumb placed under the patient’s chin. To remove the impression, the mandible should be held firmly, the patient asked to wriggle his tongue to break part of the peripheral seal and the impression removed with one short strong movement. The patient may then rinse. The impression is rinsed with cold water and inspected. If acceptable, it is placed tray upwards on a damp paper towel, which is then folded over the impression to provide 100% humidity and to retain the accuracy of the impression as much as possible. Figure 4-5 shows an impression that has been taken as described above.
Fig. 4-2 Even extra-high-flanged impression trays are inadequate to cover the desired structures.
A Impression trays with extra-high flanges.
B The impression tray is still too short.
C Lingually in the mandible there is insufficient extension.
D The upper tray likewise needs to be built up individually.
(From: see Fig. 4-1)
Fig. 4-3 The impression trays are built up with wax in order to cover the structures of which an impression is required.
A The peripheries of the trays are built up with wax.
B The tray now covers all the required structures.
C The lingual peripheries of the tray are much extended.
D The extended upper tray.
(From: see Fig. 4-1)
Fig. 4-4 The extended upper tray is tried in the mouth and adapted. It is often desirable to build up the anterior part higher than the rest in order to reach as high as possible under the nose. In the mandible good extension of the anterior region also is desired.
A The wax postdam closes off the palate dorsally.
B The extended tray periphery elevates the mucobuccal fold.
(From: see Fig. 4-1)
Approximately the same procedure is followed for the maxilla, but here the impression tray should be filled in such a way that there is more material in the front, rather than the posterior, part. It is wise not to overfill the tray because of an increased risk of air bubbles and of the excess material flowing back past the postdam into the pharynx. In any event, it is wise to have a mouth mirror handy because this useful instrument will—with one movement—wipe away any excess material that does escape onto the soft palate. Before inserting the filled tray, a small portion of impression material is placed in the palate behind the maxillary. This is done with a wiping movement made with the index finger. High narrow palates are especially vulnerable to trapped air bubbles. Initially, the impression should be seated on the incisors and care taken that enough alginate is present labially to fill the sulcus (Fig. 4-6A). While filling the anterior part of the vestibule, the loose upper lip is held away and the tray is tilted up at the distal and pressed a little dorsally. With this movement the impression material, which is surplus in the anterior part, is squeezed dorsally to cover the palate without trapping air bubbles, but at the same time not bypassing the postdam which closed off when the tray was tilted upwards distally. Simultaneously, the lateral parts of the vestibule fill but, as with the lips, the cheeks should be held away to allow trapped air to escape. The tilting movement with the upper impression ceases when the wax postdam closes off the palate. The patient is then asked to bite lightly on the tray to allow extra space to develop between the tuberosities and the inner surface of the ramus of the mandible for the impression material to flow into. The tray is then rotated further upwards in front, whereupon the surplus alginate flows up under the lip over the anterior peripheries of the tray and provides a good high impression of the labial region (Fig. 4-6B). By lifting the lip, it is possible to see if there is sufficient material in the sulcus. If necessary, air bubbles can be massaged away by molding the alginate with the lip. If by now there is any further excess alginate behind the postdam, this may be removed with the mouth mirror and, providing it is not advanced in setting (possibly if one works quickly), this surplus may then be placed under the upper lip if required. Figures 4-6C and D show an upper impression taken according to this technique.
Fig. 4-5 Alginate impression of the mandible for provision of an orthodontic study model.
A The deep impression reproduces the greater part of the lingual side of the mandible.
B The impression of the sulcus covers as wide an area as possible.
(From: see Fig. 4-1)
In order to achieve a good impression, it is most important that the patient’s facial musculature be quite relaxed. It is therefore recommended that the whole impression taking procedure be rehearsed with an empty tray beforehand. This way the patient will know what is involved and required of him, and be able to cooperate easily during the actual take.
Fig. 4-6 Some steps in taking an upper impression, and the result.
A There must be sufficient alginate labial to the incisors.
B The impression must go as high as possible buccally and labially.
C The completed upper impression; it encompasses the tuberosities.
D Buccally and labially there is a maximum of the anatomy reproduced.
(From: see Fig. 4-1)
Impression taking—especially in the maxilla—can sometimes provoke gagging and to help overcome this possibility, any excess material should be removed from the region of the soft palate. If gagging does occur, however, the patient should be distracted by asking him to look and follow closely the finger of one’s free hand, to hold the legs stiffly and raise them a little, or breathe heavily through the nose. It is essential that, should gagging occur, the operator remains calm and confident in dealing with the situation. However, it is not worth mentioning the possibility of gagging to the patient beforehand, especially as quick-setting alginate—used at the correct temperature—is not usually />