Impression Techniques

Advantage should be taken of prefabricated impression copings which are screwed to the abutments. The direct or squared impression copings are secured to the abutments by means of a central guide pin, which remains exposed during the impression taking, through windowscuĂ® in to the customised impression tray (Fig 5-2).

The tray windows are covered with soft wax to keep the impression

Fig 5-2 Direct or "squared" impression copings are secured 1o abutments by means of central screws or guide pins. Access windows are incorporated into the customised tray to facilitate unscrewing of guide pins, to allow impression withdrawal material, which should ideally be 1mpregum®or a material of similar properties, from oozing out On final set, it is now possible to gain access to the guide pins which are unscrewed. The impression can now be withdrawn with the squared copings remaining in situ (Fig 5-3). Such a technique ensures the accurate position and relation of each coping within the impression. Clearly impression definition is of importance, but retention of copings and their relation to each other is the most critical aspect, with peripheral details perhaps less important than with conventional impressions. Unfortunately, It is not always practical to get copings, impression tray.

guide pins, screwdriver, and hand all in the patient's mouth and it is therefore necessary to sometimes use an indirect technique with tapered copings (Fig 5-4), These are screwed directly to the implants and remain so on withdrawal of the impression. It is then necessary to unscrew each coping, replacing it firmly in to its respective site in the impression. Though it would seem that such a technique can not guarantee the accurate reproduction of position and relation of each coping within the impression, studies have in fact shown it to be as good as the direct technique.42'43

Fig 5-3 Direct impression copings are so-called, because they remain in situ on withdrawal of the impression.

Fig 5-4 Indirect or "tapered" impression copings screw directly on to the abutments. On withdrawal of the impression, these copings need to be unscrewed from the abutments and reseated in the impression

The result of all these studies can be generalised as follows:

1 The greater the number of implants available to support a fixed bridge, the more favourable the load distribution.

2 Implants should ideally be placed to receive axial loading in preference to non axial loading.

3 When three or more implants are placed, it is preferable that they be just offset from a strictly linear arrangement,

4 Cantilevers should be kept to a maximum length of 12mm.

5 Occlusal table width should be reduced, whilst maintaining an axial load.

6 Never place both mesial and distal cantilevers on a bridge supported by only two implants.

7 The use of ceramometal for prosthetic restorations does not appear to be contraindicated as first thought.

To further confound the clinician, the above principles only govern a fixed bridge constructed on osseointe-grated implants. The controversial topic of joining teeth to implants introduces even more complex issues of combining essentially ankylosed units with those that possess an inherent damping or shock absorbing system, the periodontal ligament. It was postulated that a bridge constructed on anchorage units of such a differing reslience would lead to biomechanical breakdown. Indeed one system, the IMZ system (Friatec

AG, Mannheim, Germany) incorporates a shock absorbing intramobile element or IME between fixture and abutment, which acts to dissipate occlusal load during function.37-39 Not all studies have borne out the findings of the IME"10 whilst some studies have shown equally good success rates when using other implant systems in partial reconstruction's joining teeth to implants."" However it may be that for the novice, an eighth rule should apply:

8 It is desirable that implants and teeth should not be combined in an FBR if at all possible.

Some systems, such as the ITI system (Institute Straumann AG, Waldenburg, Switzerland) offer the option to work on abutments, to which a conventional type bridge is directly cemented. For such systems, standard impressions of the abutments are taken as if they were conventional preparations and bridgework is fabricated on stone dies. In an effort to achieve manufacturer precision in the mouth, the use of impression copings directly implicates the use of prefabricated abutment replicas which are screwed to the copings priortocasting(Fig5-5). This allows the technician to fabricate the bridgework on metal dies (abutment replicas) incorporated into the master cast (Fig 5-6), rather than on stone dies. It is now possible to construct bite blocks, temporary bridgework, and final bridgework on prefabricated

Fig 5-5 Stainless steel abutment replicas act as prefabricated metal dies and are secured to the impression copings prior to casting. Note the non linear position of the replicas, which reflects the tripodised position of the implants.

Fig 5-6 The master cast incorporates abutment replicas in exactly the same relation as their respective intra-oral abutments. Metal framework can now be fabricated on precision fit bridge cylinders, which locate accurately on both the replicas and the abutments. Although the replicas appear to be in a straight line, figure 5-5 clearly shows that the replicas are just off a linear arrangement as recommended in the text.

cylinders, which screw accurately to both abutment replicas and implant retained abutments, in the mouth.

0 0

Post a comment