Principles of bridge construction. Biomechanical Considerations. These include the role of the edentulous span dimensions, the pontic characteristics and the connectors or joints of the bridge components on the success of the constructed bridge.
a.Vary directly with the cube of the span length (p).
b.Vary inversely with the cube of the occluso-gingival thickness (T)
The forces on the retainers of bridges are different in magnitude and direction from those applied to single restorations, the dislodging forces on a bridge retainer tend to act in a mesio-distal direction, compared to the more common buccolingual direction of forces on a single restoration.
The load on the pontic that lies buccal to the interabutment axis line produces torque primarily around that axis. The most effective location for supplemental grooves in this situation is on the mesial and distal surfaces.
Pressure on a cantilevered pontic produces a strong lifting force on the distal retainer. Resistance can be increased by adding buccal and lingual grooves, and by making the most distal axial surface as nearly as parallel with the wall nearest from the pontic as possible.
when the pontic lies outside the inter-abutment axis line, the pontic act as a lever arm, which can produce a torquing movement. This is a common problem in replacing all four maxillary incisors with a bridge.
To compensate this torque, additional retention is required in the opposite direction from the lever arm and at a distance (R) from the inter-abutment axis equal to the length of the pontic lever arm (p)
Secondary retention(R)must extend a distance from the primary inter-abutment axis equal to the distance that the pontic lever arm(P)extends to the opposite side
Tilted molar abutments
Canine-Replacement fixed partial denture
The forces are directed more outward and the pontic lies farther outside the inter-abutment axis
The forces are directed inward and the pontic is closer to the interabutment axis
Cantilever fixed partial denture