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ain is yet to be added and the palatal spur on the pontic will act as a handle until the bridge is finished, when it will be removed.

Figure 95
A cast-mesh bridge. It is difficult to achieve good adaptation of the mesh over the entire retainer surface and neither of these retainers has retentive features right up to the periphery. The added thickness of the retainer can also be see.
∙Casting the metal retainer and then etching the fit surface produces micro-mechanical retention by one of three methods: electrolytic etching in acid in the laboratory, electrolytic etching using chairside equipment. Or chemical etching with a hydrofluoric acid gel either in the laboratory or at the chairside. Although these three systems produce different etch patterns, they are all very retentive. The size of the retentive features is approximately one-tenth that of the medium-mechanical retentive systems and the retentive features are undercut from the surface. The smaller size of these etch pits and the absence of unnecessary non-retentive features (as in the medium-mechanical retentive systems) allow thinner metal retainers and a thinner cement film thickness.

Figure 96
a A Maryland bridge, the designe is unsatisfactory in that the extension distally on to the third molar tooth to increase retention would give rise tn an impossible cleaning problem between the second and third molar. This Is a technique bridge, not made for e patient. It is shown to illustrate a common design error.

∙ Chemically retentive resins are now available. Several have been marketed and some (for example. Panavia-Ex) have performed well in some laboratory and short-term clinical trials. They adhere chemically to recently sandblasted metal surfaces and are retained on the tooth by conventional acid-etching of the enamel.
Comparison of indirect minimal- preparation retention systems
A number of laboratory studies and clinical trials have shown that the micro-mechanical and chemical adhesive systems are the most retentive. However, the chemical adhesive systems have only been available for a short time and longer-term clinical trials are necessary before it is safe to rely too heavily on this system.
The disadvantage of the mici0-mechanical retention system is that the metal framework should not be tried in the mouth after the surface has been etched. This is because the very delicate etch pattern may well be damaged or clogged by deposits from saliva. This means that the ideal is to try in the unetched framework and then either return it to the laboratory for etching or etch it at the chairside. This takes time and therefore adds to the cost.
The macro-mechanical retentive design (Rochette) overcomes this problem but is less retentive in most cases and because it is cemented with a conventional composite and the composite comes through the perforations to the mouth, it is prone to degradation over a period of years. However, the main advantage of the Rochette bridge is that it can be removed from the mouth fairly easily. The composite is drilled out from the holes and the bridge can usually be removed without too much force. For this reason the Rochette bridge is still used when the abutment teeth have a poor prognosis and when further modifications are likely to be necessary for example, when one lower incisor is being replaced for periodontal reasons