When a single tooth requires a crown, the prosthetic crown will in most instances rest upon whatever tooth structure was originally supporting the crown of the natural tooth. However, when restoring an edentulous area with a bridge, the bridge is almost always restoring more teeth than there are root structures to support. For instance, in the photo at right, the 5-unit bridge will only be supported on three abutment teeth. In order to determine whether or not the abutment teeth will be able to support a bridge without a virtually guaranteed failure due to lack of proper support from the remaining root structures, the dentist should employ Ante's rule, which states that the roots of the remaining abutment teeth must possess a combined total surface area in three dimensions more than that of the missing root structures that are to be restored with the bridge. When the situation yields a poor prognosis for proper support, double abutments may be required to properly conform to Ante's rule.
When a posterior tooth that is intended to become an abutment tooth already possesses an intracoronal restoration, it might be in order to make that bridge abutment into an inlay or an onlay, instead of a crown. However, this may concentrate the torque of the masticatory forces onto a less enveloping restoration, thus making the bridge more prone to failure.
In some situations, a cantilever bridge may be constructed to restore an edentulous area that only has adequate teeth for abutments either mesially or distally. This must also conform to Ante's rule but, because there are only abutments on one side, a modification to the rule must be applied, and these bridges possess double abutments in the majority of cases, and the occlusal surface area of the pontic is generally decreased by making the pontic smaller than the original tooth.
As with preparations for single-unit crowns, the preparations for multiple-unit bridges must also possess proper taper to facilitate the insertion of the prosthesis onto the teeth. However, there is an added dimension when it comes to bridges, because the bridge must be able to fit onto the abutment teeth simultaneously. Thus, the taper of the abutment teeth must match in order to properly seat the bridge; this is known as requiring parallelism among the abutments. When this is not possible, due to severe tipping of one of more of the abutments, for example, an attachment may be useful, so that one of the abutments may be cemented first, and the other abutment, attached to the pontic, can then be inserted, with an arm on the pontic slipping into a groove on the cemented crown to achieve a span across the edentulous area.
As with single-unit crowns, bridges may be fabricated using the lost-wax technique if the restoration is to be either a multiple-unit FGC or PFM. As mentioned in the paragraph above, there are special considerations when preparing for a multiple-unit restoration in that the relationship between the two or more abutments must be maintained in the restoration; that is, there must be proper parallelism in order for the bridge to be able to seat properly on the margins. Sometimes, the bridge does not seat, but the dentist is unsure whether or not it is only because the spacial relationship of the two or more abutments is incorrect, or whether the abutments do not actually fit the preparations. The only way to determine this would be to section the bridge and try in each abutment by itself. If they all fit individually, it must have simply been that the spacial relationship was incorrect, and the abutment that was sectioned from the pontic must now be reattached to the pontic according to the newly confirmed spacial relationship. This is accomplished with a solder index.
The proximal surfaces of the sectioned units (that is, the adjacent surfaces of the metal at the cut) are roughened and the relationship is preserved with a material that will hold onto both sides, such as GC pattern resin. With the two bridge abutments individually seated on their prepared abutment teeth, the resin is applied to the location of the sectioning to reestablish a proper spacial relationship between the two pieces. This can then be sent to the lab where the two pieces will be soldered and returned for another try-in or final cementation.