Fabrication
The most common fabrication methods used for metal implants are machining, casting, hot forging, and cold forging. Sometimes a specific fabrication method is necessary in order to achieve a complex implant shape. Other times the fabrication can be selected based on the effect it will have on the characteristics of the material.
Most orthopaedic materials are available in standard bar forms. Some of the processing to create the bar forms will actually strengthen some materials. These forms can be machined into the final shape. Most of this machining is done with computer controls that allow high precision and repeatability of the final shape.
In casting, the metal is completely melted, then poured into a mold to create the shape of the implant. This is a good method to use when the implant has a complex shape. But cast metal implants are typically not as strong as wrought or forged implants. However, if the implant is large enough, the strength of the material may not be as important because its thickness will make the implant stronger, just as a thick piece of cardboard is harder to bend than a thin piece.
Forging is a fabrication method where the material is not melted, but simply pressed into shape between two molds. Some metals must first be heated to make them more formable so they can be shaped with less force (hot forging), while other metals are naturally more pliable and can be shaped at room temperature (cold forging).
Raw polyethylene begins as a powder. Turning the powder into a solid piece of polyethylene is called consolidation. The way the powder is consolidated and then shaped into the final implant can affect its characteristics.
One way polyethylene powder is consolidated is through a process called ram extrusion. The powder is heated in a chamber and then forced out through a round tube. The combination of pressure and heat consolidates the powder into a solid round bar, which is then cut and shaped by machines to create the implant.
Another way to fabricate polyethylene implants is called compression molding, where the raw powder is heated and then pressed into large sheets that can be cut and shaped by machines (machining) to create the implant.
A variation of this process is known as net-shape compression molding. Instead of compressing the powder into sheets and then using machining to create the final implant shape, the powder is heated and molded directly into the final shape. This method provides smooth surfaces and consistent shapes.
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