made in a low fusing solder after the porcelain is added, and the bridge cannot be returned to the furnace for further adjustments to the porcelain after it is made. Figure 91 shows the range of temperatures of the various components in the metal ceramic system.
∙ A framework of standard casting alloy and separately constructed porcelain crowns cemented to them; this type of construction is now uncommon, but they are still seen in a number of patients and need to be maintained, sometimes by the replacement of fractured crowns.
Minimal- preparation bridges
Conventional bridges involve removing tooth tissue, or a previous restoration, and replacing it with a retainer. This may be destructive of tooth tissue and will certainly be time consuming and expensive. The alternative, minimal- preparation bridge involves attaching pontics via a metal plate to the unprepared (or minimally prepared) lingual surfaces or adjacent teeth. The attachment is made by a composite resin material, retained by the acid etch technique to the enamel.
Obviously these bridges can be used only when the abutment teeth have sufficient intact enamel.
Figure 91
Typical temperature ranges for the metal-ceramic process. Thee vary according to the metal, porcelain and solder used, and with the type of furnace, in particular its rate of temperature rise.
Figure 92 shows a simple classification of minimal- preparation bridges; variations of this technique are shown in Figures 94, 95-b.
MINIMAL PREPARATION BRIDGES
DIRECT INDIRECT
Macro Mechanical Medium Mechanical Micro Mechanical Chemically
Retention- Retention- e.g. Vinginia Salt Retention- e.g. Adhesive- e.g.
e.g. Rochette Mesh Crystal Bond Maryland Panavia-ex
(see Fig. 94) (see Fig. 92) (see Fig. 96) (see Fig. 97)
Figure 92 A simple classification of minimal-preparation bridges
∙Direct bridges may be using the crown of the patient’s own tooth. This can often be done as a simple and rapid way of replacing a tooth lost through injury (which cannot be reimplanted) or which has to be extracted urgently. Sometimes metal mesh or wire is added to the lingual surface to increase strength, but this is not always necessary. If the natural crown of the tooth is not available or is not suitable, an acrylic denture tooth can be used in the same way.
∙ Macro-mechanically retentive bridges (Rochette, see Figure 94) have large undercut perlorations through the cast-metal plate, through which the composite flows. These holes are cut in the wax or acrylic pattern with a bur and are then countersunk.
∙ Medium-mechanical retentive systems all involve retentive features cast as part of the metal framework (see Figure 95). They all add significantly to the cement film thickness in some areas, at least, of the retainer and they all produce large, non-undercut lumps of metal on the fit surface which do not contribute to retention but which necessitate a relatively thick retainer. The size of the retentive features is intermediate between macro and micromechanical retentive systems.
Figure 94
A rochette bridge replacing one centrel incisor. The porcel
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