Final Bridge Fabrication

Having satisfied all of the above concerns with a transitional prosthesis, one is now in a strong position to proceed with final bridge fabrication. Clearly the fit of the transitional prosthesis is an indication of the accuracy of the master cast though not a guarantee, but it should be a sound basis for using the same working cast for final bridge construction.

The metal framework will likely be constructed around prefabricated gold alloy cylinders that relate precisely to the geometry of the abutment and abutment replica. Cylinders are screwed to the replica by means of a bridge screw and the wax up (Fig 5-11), which incorporates each cylinder into the eventual framework, is cast in precious or semi-precious bonding alloys. Frameworks will vary according to the type of prosthesis to be fabricated. For the type of screw retained restorations being presented in this chapter, there are essentially two alternatives. The strictly ad rnodum Brânemark screw retrievable bridge will present a framework that is splinted as a single unit (Fig 5-12): whereby each bridge screw will contribute to the overall fixation of the bridge, with porcelain being fired directly to the framework. In this scenario it is possible to remove the bridge screws through access holes in the occlusal surfaces of the bridge (Fig 5-13).

The alternative concept is termed double construction, and is based on the principle of cementing a superstructure to individual screw retained copings or to a splinted substructure. The individual copings (Fig 5-12: maxilla) and the splinted substructure (Fig 5-14) are both fabricated around the same precision fit bridge cylinders, so that they locate accurately to both the abutment replica and the abutment and are secured with the bridge screws. In order to relate the position of individual copings from the master cast to the mouth, it is essential to use an acrylic template which will maintain their position during the transfer (Fig 5-1 5). On securing the copings, it is then possible to slip off the transfer template (Fig 5-16). The superstructure isthen fabricated in the laboratory to accurately fit over the substructure with a small cementation space. The value of such a double construction is that it is possible to achieve a more aesthetically pleasing result with the absence of occlusal access holes (Fig 5-1 7a and b). Also it has the ability to correct inclination problems by means of waxing the subcopings back into the arch line, thus avoiding the use of angulated abutments.

Fig 5-11 Framework wax up incorporates precision fit, prefabricated, gold alloy bridge cylinders, which are encased in a plastic waxing sheath. These semi-burnout cylinders have a melting point of 1450°C and can be cast with most precious and semi precious bonding alloys.

Fig 5-12 Mandible: The cast framework splints all three implants. Porcelain will be fired directly to this metal framework, which will be a screw retrievable prosthesis

Maxilla: Individual sub-copings have been cast and are secured separately to their respective abutments. A porcelain fused to metal bridge will subsequently be fabricated to cement over the top as part of a double construction.

Fig 5-13 Access to bridge screws is facilitated through the occlusal surfaces of the screw retrievable bridge. Access holes are eventually occluded with light cured composite

Fig 5-14 When opting for the double construction, it is possible to splint the sub copings for additional support of shorter implants.

Fig 5-15 In order to transfer multiple copings from the master cast to the mouth, a transfer template is fabricated In resin, splinting copings In their correct positions.

Fig 5-16 On tightly securing all copings to their respective abutments, It is possible to slip the transfer template off. Bridge copings remain In the correct orientation for subsequent cementation of the superstructure.

Fig 5-17a

Fig 5-17a (top) and Fig 5-17b (below) demonstrate that double construction offers additional flexibility and superior aesthetics for poorly orientated implants by improving their position in the arch and by obviating the need for bridge screw access holes.

Fig 5-17b

Fig 5-18 The excessive buccal or tabiaf inclination of implants will often render a prosthesis unaesthetic, or functionally impaired as demonstrated by this bulky transitional lower fixed bridge, which Is in cross bite.

Fig 5-19 The use of sub copings and a double construction have corrected the cross bite and bulk of the bridge shown in figure 518. This has been achieved without having to resort to angled abutments, which can unnecessarily complicate prosthetic reconstruction.

Fig 5-20 In order to facilitate the removal of a cemented superstructure, small niches are incorporated into the palatal surfaces of this three unit bridge. A small flat plastic or screw driver can be inserted to lever the superstructure off of the underlying sub copings.

Figures 5-18 and 5-19 compare the results of a direct screw retrievable bridge and a double construction in the same posterior mandible, resulting in the complete realignment of an unfavourable cross bite. Superstructures should be cemented only with a temporary cement since the accurate laboratory fit will guarantee a retentive prosthesis. Indeed it is probably necessary to include some kind of mechanism to aid removal of the superstructure should one require access to those components hidden below. One technique is the incorporation of lateral (palatal/lingual) hybrid screws, which are small grub screws incorporated into the superstructure, that can be screwed in, thus separating the structures and breaking the cementation bond. Alternatively and more simply the incorporation of small niches between the two structures (Fig 5-20) enable the clinician to place a flat instrument and lever the superstructure away from the screw retained substructure.

Passive Fit

Regardless of the type of bridge to be fabricated it is essential that all screw retained frameworks have a passive fit. This is the term used to describe a splinted framework which, when placed on two or more implants, locates accurately and does not create unfavourable stresses to the underlying, supporting implant/abutment complexes. Clearly the use of individual copings in the double construction eradicate the possible problems of passive fit. It is necessary at this stage to reflect upon the importance of the technician in the overall team approach to implant work. It is essential for a technician to be suitably trained in implant dentistry and to have a thorough understanding of the systems with which he has to work. A knowledge of the components that are utilised in the clinic as well as the laboratory will ensure a close team environment which in many respects should be dictated by the technician's needs to help fabricate a prosthesis that is not only functionally correct and aesthetically pleasing, but is seen to fit passively on the abutments.

The difficulty with the concept of passive fit is in clinical perception, since it may be difficult to detect small discrepancies between the framework and underlying abutments. However the use of magnifying loupes may help to detect unwanted gaps which appear at sequential framework/abutment joints as each individual cylinder is screwed down, securing it firmly to the abutment below. Some manufacturers insist on the use of a torque driver, to control the degree of tightening of bridge screws. The presence of a framework which rocks and which will not locate all abutments at once, also indicates non passive fit. It is advisable to work through the tightening of bridge screws in an alternating sequence rather than simply tightening neighbouring implants one following the other. Should the patient fee! any pressure whatsoever, as the bridge screws are tightened, this is a cardinal indication of a non passive fit of the framework. Such frameworks require sectioning, indexing and soldering, in order to achieve the desired fit.

It would seem sensible that assessment of passivefit is carried out at the metalwork stage, as a metalwork try-in, so that any adjustments can be facilitated prior to porcelain firing. However it should be understood that the framework could distort during porcelain firing and it is therefore important to recognise that a passive framework at metal try-in may not be passive on insertion, though fortunately this is rarely the case.

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