Materials Used in Orthopaedic Implants

Metal Fabrication

Up to now, we have talked about the general characteristics of certain biomaterials. But these characteristics can be changed depending on the methods used to shape the material into the desired implant (fabrication), and the methods used to treat the implant after it is fabricated.

When fabricating an implant, the material may be machined, molded, or compressed. Any of these processes can change the characteristics of the material. Some fabrication methods, for example, can make some materials stronger, while others can weaken them. So it is not only important to choose the right material, but also to choose the fabrication method that will either maintain or, in some cases, improve the desirable characteristics of the material.

Metal Fabrication Process

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. 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 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 pliable 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).

After fabrication, some implants can be treated to improve the physical characteristics of the material. The most common method is heat treating or annealing, where the metal is heated and then slowly cooled. This makes the metal softer and less brittle.

Polyethylene Fabrication

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 (machining) 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 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.