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∙With small bridges the length of span is limited to one or perhaps two pontics because of
the leverage forces on the abutment teeth; if more teeth are to be replaced with a cantilever
bridge a large number of abutments widely spaced round the arch must be used.
∙The construction of the bridge must be rigid to avoid distortion.
Choice of materials
Metal only
Many posterior bridges can be made entirely of cast metal, whether they are fixed-fixed, fixed-moveable or cantilever, If the retainers or pontics do not show when the patient smiles and speaks then an all-metal bridge is the best choice. The material necessitates the least destruction of tooth tissue and, depending on the choice of metal, may be the least costly. The margins are also easier to adapt to the preparations.
Metal-ceramic
When the strength of metal is required together with a tooth-coloured abutment or pontic, metal-ceramic is the best material. This has now replaced an other crown and pontic facing materials, including acrylic except in special circumstances, such as the patient removable bridges.
Proprietary ceramic pontic facings have also been superseded by metal-ceramic pontics.
A ranger of composite crown and bridge facing materials is now available but it is too early to say whether these have any advantage over metal-ceramic materials. It seems unlikely that they do because the retention of the facing material depends upon mechanical undercuts on the surface of the metal instead of the very reliable physicochemical metal-ceramic bond.
Ceramic only
The all porcelain bridge is limited by its relatively poor strength to two-unit cantilever bridges or three-unit fixed-fixed bridges. All porcelain bridges made from conventional feldspathic porcelain can have a very satisfactory appearance (see Figure 85c). However, with improvements in metal-ceramic materials. These all porcelain bridges are now falling into disuse. The newer cast ceramic and reinforced porcelain materials may produce a new generation of all porcelain bridges.
One advantage of the all-porcelain bridge is the effuse box' principle. All porcelain bridges, if properly designed and constructed have sufficient strength to survive normal functional forces but will break if subjected to excessive forces. This potential for fracture may save the roots of the abutment teeth from fracturing if the bridge receives a blow. It is not uncommon for patients who lose a tooth as a result of an accident to have a further accident, either because of their occupation or sport or because with a class Ⅱ Division I incisor relationship, their upper incisors are vulnerable to trauma. A broken bridge is better than broken roots.
Combinations of materials
Many combinations are possible, but three deserve special mention. The first two are common.
∙ A metal-ceramic retainer and pontic with a moveable connector to a gold inlay or other minor retainer.
∙ An all-metal retainer (a full or partial crown) towards the posterior end of the bridge with anterior metal-ceramic units.
Initially soldering standard casting alloys to metal-ceramic alloys after the porcelain had been added was difficult and failures were common. With improved materials and techniques, however, this is no longer the same problem. The solder joint is