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difficult to clean. It is therefore not commonly used.

Figure 86
A conventional fixed-fixed bridge should have all the occluding surfaces of the abutment teeth protected by the retainers. otherwise an occlusal force directed at the unprotected area will depress the abutment tooth in its socket while the bridge and the other abutment tooth hold the retainer. This will break down the cement lute, causing leakage. The retainer is held in place by the bridge and so secondary caries develops rapidly (see Figure 146. page 232)

Figure 87
a A niveable cibbectirm seorated.

b Acryllc burn-out patterns for patterns for moveable connectors. The blue Is very tapered, the red more parallel-sided.
Cantilever bridge
A cantilever bridge provides support for the pontic at one end only. The pontic may be attached to a single retainer or to two or more retainers splinted together, but has no connection at
the other end of the pontic. The abutment tooth or teeth for a cantilever bridge may be either mesial or distal to the span but for small bridges are usually distal.
Spring cantilever bridge
Spring cantilever bridges are restricted to the replacement of upper incisor teeth. Only one
pontic can be supported by a spring cantilever bridge. This is attached to the end of a long metal arm running high into the palate and then sweeping down to a rigid connector on the palatal side of a single retainer or a pair of splinted retainers. The arm is made long and fairly thin so that it is springy, but not so thin that it will deform permanently with normal occlusal surfaces (i. e. , exceed the elastic limit). Forces applied to the pontic are absorbed by the springiness of the arm and by displacement of the soft tissues of the palate so that excessive leverage forces do not disturb the abutment teeth. The abutments are usually the two premolar teeth splinted together, or a single premolar or molar tooth.
Combination designs
The four basic designs can be combined in a variety of ways. In particular, the fixed-fixed and cantilever designs are often combined. In larger bridges additional cantilever pontics may be suspended from the end of a large fixed-fixed section (see Figure 88). Similarly, it is possible to combine fixed-fixed and fixed-moveable designs.
It is less common for spring cantilever bridges to be combined with other types of bridge, but occasionally a spring cantilever arm is attached to a posterior fixed-fixed bridge.
It is possible to combine a bridge with a removable buccal flange that replaces lost alveolar tissue.
Hybrid designs
There are three different hybrid designs:
∙ Fixed-fixed with one conventional and one minimal- preparation retainer
∙ Fixed-moveable with a minimal- preparation retainer carrying the moveable connector
∙Fixed-movable with the conventional retainer carrying the moveable connector.
The first two designs should not be used. In either case if the minimal- preparation retainer becomes debonded then it will not be possible to recement it without removing the conventional retainer who may well involve destroying the bridge.
The third design is acceptable and may well be the one of choice given circumstances in which one of the abutment teeth (usually the mesial one) already has a restoration which coul