Conventional machining methods cannot handle certain difficult-to-machine materials used for medical applications. Therefore, firms entering the medical market typically try several. In some applications, only wire EDM can produce the required shapes, forms, and dimensions and provide the necessary surface finish.

For example, Allendale, N.J.-based Zimmer TMT Inc. (zimmer.com) makes a proprietary material it uses in the manufacturing of orthopedic surgical parts. Called Trabecular Metal (named after the natural geometry of human Trabecular bone), the biomaterial is made with tantalum metal and vapor-deposition techniques that create a metallic strut configuration. The material does not require a solid metal substrate and can be fabricated into complex implant shapes.

Trabecular Metal's porous cellular structure mirrors the physical and mechanical properties of bone. It has a high strength-to-weight ratio and its compressive strength and elastic modulus are similar to bone. The 80% porous material facilitates bone formation to allow two to three times more bone ingrowth than conventional prosthetic load-bearing materials. It also has a higher biocompatibility compared to titanium and cobalt chrome, standard metals used for orthopedic implants.

“But trying to cut this material with conventional machines produced a smeared surface layer,“ says manufacturing supervisor Georgios A. Gerou. “I come from a tool-and-die making background, so I tried almost every machine tool imaginable. For parts cut on conventional machines, bone ingrowth may not be as good with a smeared cut surface. To be as beneficial as engineers specify, the implant designs require an open porous surface against bone.”

Eventually the company selected wire electrical-discharge machining (EDM) and now uses wire EDMs from Mitsubishi (www.mitsubishiworld.com) to cut the different anatomical shapes for hip, knee, shoulder, and spine implants. “Wire EDMs cut the complex implant geometries we require and maintain the material's open-cellular structure,“ says Gerou.

To date, the company manufactures thousands of spine, knee, shoulder, and hip implants monthly with three FA-S wires, an EA 12 sinker, and an EDM small-hole drill. “The machines cut incredibly fast without compromising the high accuracy we need. In fact, we've doubled output on one product,“ explains Gerou.