Alloy Substrate

The mechanical properties of a metal- ceramic restoration depend largely on the design of the structure that supports the veneer

The average human biting force is about 100lbs The average dog bite is 500lbs!

Unsupported porcelain will result in fracture

Good substructure design!

Sharp angles or pits on the veneering surface of a metal-ceramic restoration should be avoided as they can cause internal stress in the porcelain Always make sure surfaces are convex and round and the porcelain is supported.

The intended metal-ceramic junction should be a 90 degree angle, as smooth as possible and should also be 1.5 mm from all centric occlusal contacts. The junction must be distinct to allow a clear alloy/ceramic border.

An example of porcelain debonding from substrate.

The metal framework is made from a wax pattern and then cast using the lost wax technique Optimum design for a framework/substrate can be achieved by waxing up to full anatomic contour then cutting back to allow for porcelain veneering.

Casting faults Incomplete casting wax pattern too thin Cool mould or melt Inadequate metal

Fins Premature heating Pattern too near edge of investment Too rapid heating Dropped mould

Nodules Excessive surfactant Excessive burnout temperature Air trapped during investment procedure

Nodules can cause marginal discrepancies!

Secondary caries caused by microleakage!

Incomplete casting Incomplete wax elimination