Analogy from Civil Engeneering:
Concrete is a composite material composed of cement paste and little stones. It can a carry great compression load, but is very weak in tension. When bent, the structure can crack and fail. This is because bending is a combination of two stresses: compression and tension. If a material is to withstand bending, it needs to be strong both in compression and in tension. To ensure this, metal bars are used to reinforce the concrete in those parts that are subjected to great tension.
Back to Dentistry:
Composite filling materials have the exact same weakness in tension as does concrete. This is because they have a very similar composition: resin and milled glass. Composite dental structures, such as posterior bridges, are very often subjected to great pressure from occlusion and as a result bend easily and crack. To prevent this from happening, the bridge needs to be reinforced with Dentapreg® glass fiber strips. To ensure the best results, the strip should be shaped in a way to reinforce the bridge exactly where it needs to be: in the bottom part where it is subjected to great tension. Dentapreg® strips should therefore be bent as close to the gingiva as possible.