Metal frame construction
Most of the times, dental crowns, dental bridges and some removable partial dentures need a supporting frame that is designed to provide strength for the entire restoration and to support the aesthetic materials (porcelain, composite, acrylics).
base metal alloy frame
for a porcelain reconstruction
The frame can be made of various metal alloys or zirconia. There are some exceptions (all porcelain restorations) when the restoration is designed without a supporting frame.
Zirconia frames are constructed exclusively by computerized systems. All details in the next chapter.
The fabrication of the metal core is a process that requires patience and time. Various types of metal alloys can be used: noble metal alloys (gold, palladium) or base metal alloys (chromium-nickel, chromium-cobalt or titanium).
The procedure is performed on the dental cast; several steps are involved:
The first step is to build up the metal frame using a wax-up technique. Basically, the entire metal core is sculpted in wax at the precise shape and size considering all aspects demanded by the particular clinical situation.
After the molten metal alloy is cast, the whole wax pattern is converted to a replicate in dental alloy.
There are various types of wax up techniques and the buildup directly depends on the type of restoration that is manufactured. If, for example, a large dental bridge is constructed, first the metal frames for all retainer crowns are modeled. After that, the pontics are sculpted between the retainer crowns.
Obviously, when dental crowns are constructed, the process is somewhat simplified (because there are no pontics).
a. Wax-up for dental crowns metal frame
Porcelain fused to metal crowns
A smooth and thin (0.3-0.5 mm in thickness) "cap" is modeled; the cap is closely adapted to the prepared tooth. The cap is distanced from the adjacent and opposing teeth (1.5 to 2 mm) so that all porcelain layers can be correctly built up on top of the frame.
porcelain crown metal frame wax up
Metal-acrylic or metal-composite crowns
The frame has a greater thickness and it is not so closely adapted from the underneath tooth.
The contacts with neighboring teeth are carefully shaped and all details from the occlusal side (cusps and grooves) are built up in wax. Because the aesthetic materials are normally placed only on the visible side of the tooth, the rest will remain bare metal. The bite is carefully checked.
The aesthetic material (acrylic or composite) will be positioned in a specific window carved inside the wax. The window may contain small retention beads that will hold the aesthetic material in place.
Full metal crowns
In this situation, the entire crown is shaped up in wax. Because there are no aesthetic materials, all aspects are built up in this stage (cusp, grooves, occlusal contacts, contacts with adjacent teeth etc).
wax-up for a full metal crown
b. Wax-up for pontics metal frame
When a dental bridge is fabricated, the wax is placed between the already shaped retainer crowns following the line of the dental arch. The metal frames of all artificial teeth are separately sculpted in wax.
Porcelain fused to metal
When aesthetic materials are placed all around the metal frame, the frame is distanced from opposite teeth to create enough space for porcelain build-up.
porcelain fused to metal dental bridge;
metal frame wax-up front view
porcelain fused to metal dental bridge;
metal frame wax-up back view
Metal composite, metal acrylic and full metal bridges
The frame is waxed up in contact with both opposing teeth and the underneath edentulous ridge (there are various types of contacts that can be designed depending on the clinical situation).
The occlusal side is shaped similar to natural teeth, with cusps and grooves, and the restoration should not be modeled "too high" or "too low".
Most often, the aesthetic material is placed in the form of partial veneers that cover only the visible aspects of the restoration. In this part of the metal frame, specific windows are carved and some retention beads or ditches can be placed inside.
c. Attaching the casting rods (spruing)
After wax-up is completed, the wax pattern is detached from the plaster cast in order to attach the casting rods.
Casting rods (or sprues) are cylindrical pieces of wax that are attached to the wax pattern. After wax burnout, they will turn into passages through which the molten metal alloy will be introduced into the mould pattern.
Depending on the extent of the reconstruction, one or more casting rods can be attached.
Investing is the operation of surrounding the wax pattern with a material that can accurately duplicate its shape and anatomic features.
The wax pattern and the sprue are "invested" in a material (each metal alloy has a specific investing material), and the invested pattern is heated until all remnants of the wax are burned away. After burnout, molten metal will be cast into the void created by the wax pattern and sprue.
The specific investing material for the particular metal alloy is selected.
The material is prepared (normally by mixing powder with water together) until reaching a "creamy" consistency.
The wax pattern whit the sprues attached is placed in a conformer and fully covered with the investing material.
After the material sets, the invested pattern is heated at high temperatures. The investing material is very resistant to high temperatures so it will not change its form in any way.
The wax will be burned away from the pattern until all remnants are eliminated.
After burnout, a void space that is an accurate "negative" replica of the wax pattern is obtained. The molten metal will be cast into this mould through the casting channels (or sprues).
3. Casting the metal alloy
Casting is the process by which a wax pattern of a restoration is converted to a replicate in dental alloy.
If all steps have been done well, the final cast restoration would be same as the original wax pattern and would require minimal modification during the next stages.
The designed metal alloy is heated to a slightly higher temperature than its melting point.
When the alloy becomes fluid, the molten metal is cast into the void created by the wax pattern and sprue through the casting channels. There are specific devices that can help the process by forcing the molten metal inside the mould.
It is very important for the metal alloy to occupy the entire space inside the mould. Otherwise, the accuracy of the reproduction will suffer.
Once the metal alloy solidifies, the investment is broken away and the rough casting is removed. The metal frame appears attached to the metal sprues.
4. Polishing and finishing the metal frame
Even if all the steps were perfectly completed, minor touches are still required. The rough casting is pickled to remove oxides.
The casting rods are cut off and the metal frame is polished using specific burs. The insertion on the abutment teeth is double-checked and the occlusion is examined. If necessary, minor adjustments can be made.
metal frame polishing
When everything is ready, the metal structure is sent to the dental office for fitting.
Last review and update: November 2020