First impressions count

Infection control

Prior to casting an impression, it must be disinfected without losing dimensional stability and care must be taken to follow manufacturer's instructions with respect to timings – less than the recommended time will lead to inadequate disinfection, whereas an excessive time period may cause distortion of the impression material.

Once removed from the mouth, impressions should be rinsed thoroughly under water until visibly clean then inspected to ensure all details have been recorded. It is important to check that the material has not pulled away from the tray, leading to inaccuracies (with perforated trays ensure that the material has flowed through the perforations). Disinfect using an appropriate method for the advised time period, eg immersion in a proprietary disinfectant such as Perform®-ID (Schülke & Mayr UK Ltd, Sheffield, UK) for ten minutes. Following removal from the disinfectant, re-rinse and store appropriately (Table 2). When completing the laboratory prescription it must be confirmed that disinfection has been carried out.

Primary impressions (Figure 3)

The purpose of the primary impression is to enable study model manufacture and construction of custom-fit special trays for both the partially dentate or edentate patient.

With edentulous cases, these impressions enable the areas involved with denture support to be outlined as well as the correct extensions for the special trays – the latter helping facilitate a peripheral seal in the completed denture (Table 1). If the primary impression for an edentulous case fails to meet these criteria it will prove impossible to record a satisfactory secondary impression – hence the first impression not only counts but is crucial for success.

With partial denture cases, casts can be surveyed and provisional designs drawn up, having assessed features such as:

Metal or plastic disposable stock trays can be used, though the former are losing popularity owing to difficulties in sterilization. The flanges of plastic trays can be trimmed easily if overextended or silicone putty or wax is used to modify them if underextended. It is important to ensure that the selected trays are tried in the mouth and assessed carefully to ensure that they engage left and right sulci simultaneously, but are not so wide as to distort the buccal mucosa. Their length should also be assessed to ensure that they cover the landmarks required (Table 3).

Alginate is used commonly for primary impressions. Prior to using alginate, in partially edentulous cases, silicone putty or impression composition can be used to customize the stock tray by adding it to large saddle areas or in the palatal region (Table 2). Silicones or impression composition alone can also be used for edentate mouths. An advantage of silicone putty is that it also serves to extend the stock tray should it be under-extended. If surface detail is inadequate having used putty, a thin wash of light-bodied silicone or alginate (using an appropriate adhesive) may be applied to the fit surface of the set putty and the tray re-inserted.

Care is required when taking an impression as regards operator and patient positioning and the method used to seat the tray. For the upper impression, with the patient sitting upright, the operator stands behind the dental chair looking down over the patient's head as the tray is rotated into position to ensure that the tray handle is positioned centrally. The anterior part of the tray is seated first so the amount of material flowing from its posterior edge can be monitored visually. With the lower impression, the operator stands in front of the patient and rotates the tray into place, ensuring its alignment centrally. Care must be taken to avoid trapping the tongue with the lingual flanges by asking the patient to protrude the tongue such that its tip rests on the lower lip as the tray is seated. It is important to remove flexible impression materials from the mouth using a snap movement to avoid deforming the material.3

Special trays

In a survey of general dental practitioners, 75% reported using laboratory-constructed special trays to make their complete denture secondary impressions, with much variation in their design.1 Decisions need to be made concerning the design of the special tray and these decisions communicated effectively to the laboratory. Such decisions include whether or not to perforate the tray, the amount, if any, of spacing needed, the handle design and whether features such as windows are required (see the anterior fibrous change section). Such decisions will depend in part upon the chosen impression material and the nature of the denture-bearing tissues (Table 3). Appropriate spacing would be 3 mm for alginate and 2 mm for medium or low viscosity elastomers4 and close-fitting for zinc oxide eugenol. With edentate arches the advantage of close-fitting, non-perforated trays is that it is readily apparent, when the tray is tried in, whether modifications are required. A correctly extended maxillary tray (possibly with the addition of appropriate border tracing material such as greenstick composition) should exhibit similar retentive properties to the final denture base as a peripheral seal can be obtained, whereas if a perforated tray is used, such a seal will not be possible. Stub handles, rather than L-shaped ones, are preferred in edentate arches, as they do not cause distortion of the lip or errors in recording the labial sulcus (Figure 4).

Secondary impressions

The purpose of the secondary impression is to record the denture-bearing area and relevant abutment teeth. In addition, the tissues in contact with the denture base during function and the sulci, with the lips and cheeks supported, must be recorded (RKF Clark – personal communication, 2010). In their explanation of complete denture retention, Darvell and Clark discuss the importance of close adaptation of the denture base to the underlying tissues and the relevance of impression technique in achieving this.5

Necessary modifications are made to the special tray, reducing over-extensions with an acrylic bur and correcting under-extensions using additions of greenstick composition/silicone putty. Greenstick composition can be used to border-mould the tray by tracing it incrementally around the periphery, tempering in warm water then massaging the surrounding musculature whilst holding the tray in place. This moulding can also be achieved by asking the patient to contract his/her oral musculature by pursing the lips and offering a broad smile. These techniques will ensure that the periphery of the tray has adequate thickness, thus helping to achieve the desired peripheral border seal in edentate cases.

The posterior extension of the edentate maxillary tray needs careful assessment to check that it extends to cover the necessary landmarks (Tables 1 and 3). Greenstick composition can be added to the fit surface in this distal post-dam region in order to displace the underlying tissues and help facilitate a posterior border seal in the completed denture (Figure 5).

In edentate cases, the disto-lingual extension of the mandibular tray is crucial. The lingual flange should rest against the floor of the mouth when the mylohyoid muscle is contracted to obtain a seal and must extend into, and contact, the medial aspect of the ridge in the retromylohyoid fossae (Figure 5). See Table 3 for tips on how to achieve this.

Having completed necessary modifications to the special tray, as outlined above, the impression can then be made using the material of choice (Tables 2 and 3). For partial dentures, alginate or a medium-bodied, addition-cured silicone are the materials of choice. Prior to using these materials, any necessary tooth alterations, eg guide plane or rest seat preparations, must be completed.

As with the primary impression, it is important to ensure that the quantity of material used is assessed carefully. If a close-fitting tray is used in an edentate mouth, only a thin wash of impression material is required. Once the tray is seated, the tissues should be massaged to facilitate border moulding of the impression. For the lower impression, the patient should raise his/her tongue and rest the tip on the lower lip such that mylohyoid contracts and the floor of the mouth is recorded in the raised position. Figure 6 shows a mandibular secondary impression recorded with a medium-bodied silicone.

Study casts

Study casts can be used to/for:

In cases where an accurate impression is not possible in a stock tray, a custom-made special tray may be indicated. If there is an adequate number of teeth present and intercuspal position (ICP) is stable, it should be possible to articulate the models without the need for an occlusal registration. However, if ICP is not stable, or the occlusion needs to be assessed in retruded contact position (RCP), an occlusal registration is essential. This can be registered in RCP or ICP using pink wax, beauty wax, silicone materials or zinc oxide eugenol (ZOE). In cases where detailed analysis of the occlusion is necessary, registrations are also needed in lateral excursions and protrusion.6,7

Management of the gagging patient

The patient that gags during impression-taking can present a challenge. There may be underlying systemic causes resulting from stomach problems or excessive alcohol and tobacco use, but psychological factors often also play a role (RKF Clark, 2010 – personal communication). In many cases, operator error is the cause as a result of overloading the tray with impression material. The following practical tips may help overcome this problem:

Anterior fibrous change

If lower anterior teeth occlude against an edentulous maxillary ridge, then fibrous change to the maxillary anterior ridge can occur over time. Use of a conventional impression technique is likely to result in displacement of this ridge, which in turn may lead to an unretentive denture. One method for overcoming this problem involves constructing a close-fitting special tray with a window cut out over the area of fibrous change. The impression is taken in two stages. Firstly, a wash of material is applied to the fit surface and the tray seated. When set, the tray is removed and any material that has flowed into the window area is cut away using a scalpel. The tray is then re-seated and held in place while a low viscosity material, such as ZOE or light-bodied silicone, is painted over the exposed window area. Once set, the tray is removed and the resulting impression should have recorded the area of fibrous change in an un-displaced state.

Lynch and Allen describe a technique which involves constructing a special tray with additional spacing and perforations in the area of displaceable tissue.8 A heavy-bodied addition cured polyvinylsiloxane is used to record the ‘normal’ denture-bearing tissues and is applied to the tray periphery. Once set, any excess material is removed from the area of displaceable tissue and light-bodied polyvinylsiloxane is applied over the whole fit surface of the tray and the tray re-seated.

Replica/copy technique

There have been numerous methods utilized to replicate dentures.9 Most involve taking an impression of the fit and polished surfaces of the denture to be copied and then a reline impression taken within the copy.

Prior to making the copy of the original denture, any under-extended borders should be corrected as outlined above.

A large stock tray is filled with the material of choice (alginate or silicone putty) and the denture embedded in it, polished surface down, to a level just short of the top of the flanges. A laboratory silicone putty material is the authors' material of choice as it is less costly than silicones for intra-oral use and is more dimensionally stable than alginate (Figure 7).

Once set, apply a thin smear of petroleum jelly to the top of the impression. A second large impression tray is filled with impression material and seated over the fitting surface of the embedded denture, thus sandwiching it.

Once set, the two impression trays can be separated and the denture retrieved; any attached greenstick composition is removed and the denture returned to the patient.

At the second clinical visit, an impression is made of the patient's denture-bearing tissues using ZOE or a light-bodied silicone wash impression within the constructed replica and a jaw registration taken. It has been suggested that this impression should be made before the jaw registration, as the impression material may lead to movement of the denture, in relation to the underlying ridge, thus potentially affecting occlusal vertical dimension (OVD) and occlusal plane orientation.9 McCord describes a modification to this replica technique for patients with a displaceable anterior maxillary ridge.10 The base of the replica denture is constructed of a solid material, such as cold or light-cured polymethylmethacrylate (PMMA) and, having completed the wash impression within the maxillary denture using a medium-bodied material, a window is cut out over the area of displaceable tissue and a light-bodied impression material syringed into the window to record the anterior ridge in an undisplaced state.

Reline/rebase impression (complete dentures

One must always question the desirability of relining or rebasing a denture as there is a risk that, unless care is taken, both clinically and in the laboratory, the orientation of the occlusal plane and the OVD may be altered in an undesired fashion.

Firstly remove any undercuts on the fit surface of the denture using a bur to enable the laboratory to remove the denture from the cast.

To reduce the risk of altering the OVD or occlusal plane orientation, a thin wash of flowable impression material should be used (eg ZOE or a light-bodied addition-cured silicone) and care taken when seating the denture to ensure the occlusal plane is unaltered.

Both a closed and an open-mouthed technique have been described, the former involving the patient lightly closing together against the opposing denture in the position of maximum intercuspation and the latter treating the denture like a special tray, with the impression recorded with the patient's mouth open.11 It is important to appreciate that the process may result in movement of one denture relative to the other with a consequential effect on the occlusion. Whichever technique is used, it is important, following return of the denture from the laboratory, to re-assess the occlusion carefully and to undertake any necessary adjustments.

‘Neutral zone’ technique

The neutral zone is defined as ‘a zone in which the forces of the cheeks and lips are said to be in equilibrium with those of the tongue’.2 Impression techniques exist to determine its location so that the teeth on a mandibular complete denture can be set within it, thus assisting its stability. This can be helpful in patients that have powerful oral musculature or poor neuromuscular control.12

Firstly, undertake conventional impression and jaw registration stages. Construct a mandibular baseplate with vertical stops attached to the upper surface that contact the maxillary wax try-in denture at the pre-determined OVD.13 A wire running between these vertical stops allows a visco-elastic material (eg temporary soft lining material) to be retained.

The surrounding musculature moulds the impression material resulting in an outline indicating where the ‘neutral zone’ lies. Further details of this technique and the associated laboratory work are outwith the scope of this paper.

Altered cast technique

In mandibular Kennedy Class I and II cases, with distal free-end saddles, a cobalt-chromium denture will be tooth and mucosal borne. As the support offered by the teeth differs from that offered by the mucosa, there is a theoretical possibility that, if an impression is made of the distal free-end saddle tissues in an undisplaced state, then during function the denture may sink down into the tissues in this region. This difference in support may be overcome by using the altered cast technique, which involves altering the edentulous section of the master cast.14

Having tried in the metal framework successfully, close-fitting, self-cure acrylic saddles are attached to the distal of the framework such that their extension is the same as that of a lower special tray, extending into the retromylohyoid fossa region, over the retromolar pads and covering the buccal shelves.

A thin wash of ZOE or light-bodied silicone is applied to the fit surface of the tray and the metal framework placed into the patient's mouth, ensuring the framework is fully seated, with rest seats engaged, and finger pressure applied to the metal work (but not to the saddle area). This technique will then result in an impression of the distal tissues in the displaced state and the laboratory can undertake the cast alterations.

The altered cast technique is used today less commonly, in part because if silicone putty is used during the primary impression stage it tends to produce the desired displacement of the distal tissues, and in part because of the need for an additional clinical visit which provides a questionable clinical advantage.

Open tray method

Closed tray method

There is no doubt that errors can be introduced during impression-taking, model pouring, die trimming and restoration construction extra-orally. In most cases, these are minor and traditional methods provide accurately fitting restorations. Since the introduction of CAD/CAM to dentistry in 1980, various chairside and laboratory procedures have become widely available and popular.20 These techniques offer advantages over traditional methods by reducing errors and, in the case of chairside systems, providing patients with highly aesthetic restorations in a single visit.