What are the techniques for transferring the implant position to a working cast?

In order to record an accurate impression of the position of an implant, specially designed components called impression copings or transfer aids are used. All implant manufacturers have their own corresponding impression copings, which are machined to fit accurately over an implant. These are either screwed or press-fit (snap-on impression copings) on the implant and are used for open-tray or closed-tray impression techniques, respectively.

Accurate seating of impression coping is important, and the coping has to engage the internal or external antirotational features of the implant. For example, in < ?xml:namespace prefix = "mbp" />Figure 21.1, the Straumann implant has an octagonal antirotational configuration. Its corresponding impression coping has a similar octagonal configuration so that, when inserted accurately, these configurations should slot into each other. If the implant is subgingival and there is doubt about complete seating of the impression coping, a radiograph should be taken to check the seating prior to taking an impression (Figures 21.2 and 21.3, respectively, show incomplete seating of the impression coping and the same impression coping when correctly seated). When the implant is subgingival, snap-on impression copings should not be used.

Fig 21.1 A cross section of an implant showing the internal antirotational feature.

Fig 21.2 Screw-retained impression coping that is incompletely seated.

Fig 21.3 Screw-retained impression coping fully engaging the internal antirotational features and fully seated for accurate impression.

Following confirmation of complete seating of the impression coping, an accurate impression is recorded using either a polyether or addition-cured silicone impression material. It is important to handle the impression tray with care to ensure an accurate impression. It is particularly relevant when using snap-on impression copings because they can be easily dislodged if the tray is forced into position.

Although it is advisable to have a special custom tray made for an implant impression, for a single-tooth and a short-span restoration, a correctly selected stock tray can be adequately used. The author prefers to use a lower impression tray without palatal coverage when recording an impression for a fixed restoration.

As the name implies, an open-tray impression tray has an opening to access the screw head in order to unscrew the impression coping after the material has set (Figure 21.4). After the tray is selected, a wax lid can be added using a warm sheet of modelling wax to prevent outflow of impression material through the window.

Fig 21.4 Custom tray with an opening allowing access to impression coping to facilitate removal after setting of the material.

After the impression material is fully set, the tray is removed. The snap-on impression coping will snap off the implant and remain embedded within the impression material. In contrast, the screwed type will need to be unscrewed to detach the coping from the implant, and the tray is removed from the mouth (Figure 21.5). The impression is checked for its accuracy, and seating of the coping can be verified (Figure 21.6). If impression material is overflowing the fitting surface of the coping, then it is fair to assume that the impression coping was either not fully seated or dislodged during the procedure.

Fig 21.5 The impression coping retained within the set impression material.

Fig 21.6 Inside view of the impression with an embedded impression coping.

The next stage involves attaching an implant analogue (replica implant) to the impression coping within the impression (Figures 21.7 and 21.8). The analogues are also accurately machined to replicate a particular implant type.

Fig 21.7 A typical implant analogue that mimics the shape of a particular implant system and type.

Fig 21.8 Two implant analogues are attached to the two impression copings in this particular case.

In order to easily develop the subgingival emergence of the implant restoration, the peri-implant gingivae is mimicked using pink-coloured silicone material (Figure 21.9). This creates an accurate representation of the clinical situation after the model is poured in stone and allows easier subgingival manipulation than if only stone is used.

Fig 21.9 A working cast with pink-coloured silicone gingival mask and implant abutments.

What is an implant abutment and what are the various available types?

The implant abutment has a very important role to play and constitutes a critical interface between the implant and the prosthesis. An abutment connects the implant to the restoration and also interfaces with the peri-implant soft tissue as it emerges through these tissues. Therefore, abutments should be biocompatible, stable in the oral environment, allow an intimate soft tissue attachment, and withstand occlusal forces.

Abutments can be classified according to their fabrication techniques:

Implant manufacturers offer prefabricated abutments in various shapes, sizes, and angulations (Figure 21.10). These are suitable for certain clinical situations and can be a cost-effective option.

Fig 21.10 Prefabricated (A) screw-retained abutment, (B) straight-cemented abutment, and (C) angled abutment.

Although most of the prefabricated types are noncustomizable, some can be customized, />