Introduction
The complete metal crown covers all surfaces of the clinical crown and is made entirely of metal: classically a gold alloy, today also base-metal alloys (nickel-chromium, cobalt-chromium) or titanium.
Fabrication: the lost-wax technique
1. The coping
Two methods are taught for making the base of the wax pattern:
- "Adapta" system: two polypropylene foils, a 0.6 mm basic foil with a 0.1 mm spacer foil on top, are heated until transparent and pressed over the die in a silicone container. The spacer is trimmed 2 mm and the basic foil 1 mm short of the finish line; cervical wax fills the margin.
- Dipping technique: the die, painted with spacer varnish, is dipped into molten wax for 2–3 seconds to form a wax cap.
Overheating the foil gives thin walls; underheating gives thick, distorted copings. Keep 0.5 mm of unprepared tooth visible below the finish line on the die.
2. Wax-up
Axial contours are built level with the cap's occlusal surface, then the occlusal anatomy is added cone by cone (topic 33).
3. Spruing and investing
A sprue is attached to the thickest part of the pattern and the pattern is invested in a casting ring straight away, before the wax relaxes and distorts.
4. Burnout and casting
The ring is heated so the wax melts and burns out, leaving a mould. The alloy is melted quickly (flame or induction) with oxidation kept low, and forced into the mould under pressure: centrifugal, vacuum, air or vapour pressure machines. Enough pressure gives sharp detail.
5. Recovery and finishing
The casting is broken out of the investment, sandblasted, fitted, adjusted and polished (topic 34).