Biomechanical Considerations

• These include the role of the edentulous span These include the role of the edentulous span dimensions, the pontic characteristics and the dimensions, the pontic characteristics and the connectors or joints of the bridge components connectors or joints of the bridge components on the success of the constructed bridge.on the success of the constructed bridge.

• These include the role of the edentulous span These include the role of the edentulous span dimensions, the pontic characteristics and the dimensions, the pontic characteristics and the connectors or joints of the bridge components connectors or joints of the bridge components on the success of the constructed bridge.on the success of the constructed bridge.

• The span, represent the edentulous space that will be occupied by the pontic.

• The dimensions of the span (mesio-distal length and occluso-gingival height) were found to influence the number of the selected abutments, the type of retainers and the material from which the bridge will be constructed.

• All bridges either short or long flex or bend to All bridges either short or long flex or bend to some extent due to forces applied through the some extent due to forces applied through the pontics to the abutment teethpontics to the abutment teeth

• Bending or flexing of the bridge under these Bending or flexing of the bridge under these forces was found to:forces was found to:

a.a.Vary directly with the cube of the span length Vary directly with the cube of the span length ((pp).).

b.b.Vary inversely with the cube of the occluso-Vary inversely with the cube of the occluso-gingival thickness (gingival thickness (TT))

• All bridges either short or long flex or bend to All bridges either short or long flex or bend to some extent due to forces applied through the some extent due to forces applied through the pontics to the abutment teethpontics to the abutment teeth

• Bending or flexing of the bridge under these Bending or flexing of the bridge under these forces was found to:forces was found to:

a.a.Vary directly with the cube of the span length Vary directly with the cube of the span length ((pp).).

b.b.Vary inversely with the cube of the occluso-Vary inversely with the cube of the occluso-gingival thickness (gingival thickness (TT))

1 -span length (p)

2- occluso-gingival thickness (T)

To minimize the flexion or deflection of the long or thin spans:-

• Select pontic designs with a greater occluso gingival dimension.

• The prosthesis may also be fabricated of an alloy with higher yield strength as nickel chromium.

• Secondary abutments carefully selected may be used.

Torque forceTorque forceLonger spans also have the potential for producing more

torquing or displaying forces on the bridge especially on the secondary abutment.

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 preparation should be modified to produce greater resistance and structural durability for ex. multiple grooves, including some of the buccal and lingual surfaces.

• The most effective location of the groove to resist torque is in a plane perpendicular to the axis of torque. The load on the long span bridge with straight pontics produces torque around bucco-lingual axes. Resistance can be best enhanced by the addition of buccal and lingual grooves.

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)

Arch curvatureArch curvature

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

Special problems

Pier abutments

Tilted molar abutments

Special problems

Special problems

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

Special problems

Cantilever fixed partial denture