Introduction
A bridge takes part in every chewing stroke. To survive, it must be mechanically stable (not bend, crack or come loose) and it must load its abutments within their biological limits. Every bridge design is a balance between the two, along with esthetics and hygiene.
The anatomic-physiological triad
Jaw movement depends on three coordinated factors: the relation of the dental arches, symmetrical function of the TMJs and muscle activity. A bridge is designed to fit into this triad, not to disturb it: the occlusion should be checked, and any dysfunction treated, before work starts.
Mechanical stability
- Span length: a bridge bends with the cube of its span: twice as long means about eight times more bending. Long spans are a classic reason for failure.
- Connector height: stiffness grows with the cube of the height: double the height, eight times stiffer. Width adds stiffness only in proportion.
- Alloy: a higher modulus of elasticity and proof stress means less bending. Weaker alloys need thicker frameworks.
- Framework: one-piece casting gives the best strength; at least 0.3–0.5 mm of alloy under 1.5–2.0 mm of veneer.
- Occlusion: forces kept along the long axes; no interferences; narrower occlusal tables.
Why bridges fail mechanically
- Poor tooth preparation (short, over-tapered walls).
- Wrong choice of material.
- Poor framework design: thin connectors, unsupported veneer.
- Poor occlusion.
- Too long a span.