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