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NYC hospital bones up on wire EDM.

A generation ago, people with severely deteriorated or damaged bones and/or joints caused by disease or traumatic injury faced loss of function or even amputation. Today, they can often be restored to full functionality, thanks to places like the Biomechanics Department of The Hospital for Special Surgery, New York City, and its state-of-the-art machining capabilities.

The department is a custom manufacturer of the bone and joint prostheses that are machined from exotic metals like titanium alloys. It was created about 15 years ago to make instruments for implantation and to modify prostheses purchased from outside sources.

Originally, the department's machining equipment consisted of a mill, a lathe, a bandsaw, and a grinding wheel. A profile of the part would be cut from flat stock using the bandsaw. Next, the part would be fixtured on the milling machine for machining of reference surfaces. Centers would also be machined so that the part could be turned on the lathe.

Processing the blank through the various machining operations was labor-intensive and time-consuming. Because the bandsawing process was not accurate or rigid enough to closely approximate finished size, substantial additional machining was required.

One technology in particular, however, has made it easier and less expensive to produce the implants. Electrical discharge machining (EDM), when coupled with the use of CNC machine tools and CAD/CAM systems, is playing a major role in reducing production time and costs for the Biomechanics Department.

The department is using two Sodick A280L wire EDM machines to machine hip prostheses. Initially, the part profile is cut in the flat bar (X-Y plane) on the first machine. On the second machine, the cutout is turned 90 degrees, secured in a fixture, and cut lengthwise in the X-Y plane.

"It would take several days to cut that shape on a manual machine tool," says Matt Naimoli, machining supervisor of the Biomechanics Department. "We can do it in several hours on our Sodick EDMs. And we can not only rough out a more intricate shape with the EDM, but we get much closer to the finish size to minimize the amount of metal removed in secondary operations on a CNC lathe or machining center," he adds.

Parts the department is most commonly called on to produce are hip and knee joints. The hip prostheses and other long, contoured parts are machined from flat bars of Ti-6A1-4V titanium alloy, which is used because it is a relatively inert metal in the human body. The material also is corrosion-resistant, and its stiffness is compatible with that of bone.

The shaft end of the hip prosthesis is anchored in the upper leg bone. A tapered post is machined (turned) on the upper end of the hip prosthesis. The post seats the ball half of the ball-and-socket joint.

Finish requirements of the hip stem and hip ball are different. The ball, which rotates against a plastic hemisphere machined from a solid block of polyethylene and seated in the patient's pelvis, receives a mirror finish to minimize rotating friction. The stem receives a plasma spray coating that increases surface porosity and encourages bone ingrowth. Stems are finish-machined to final dimensions and surface contours before being sent out for surface treatment.

For every hip implant, an almost identical cutting instrument must be machined from stainless steel for use in the surgical procedure. "The cutting tool is used only once, but is critical to inserting the hip. The surgeon uses the tool like a rasp or file to hollow out the inside of the bone to the same shape as the implant being inserted," Mr Naimoli explains.

Typical time to make a prosthesis, from design to completion, is three to four weeks. Machining the stainless steel cutting tool and the titanium hip blank on the EDM saves as much as a week's worth of machining time, says Mr Naimoli. In an emergency, implants can be produced in as little as three days.

Dr Timothy Wright, director of the Department, says EDM has substantially reduced production costs by cutting back machining time and allowing one operator to tend two or three machines. The biggest impact of EDM, however, has been on the accuracy of parts produced, Dr Wright says. "Today, we're creating a porous or roughened surface on the implant so it will become biologically fixed to the bone. That requires matching anatomies as closely as possible, and that takes sophisticated control of geometry."

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Title Annotation:Manufacturing Solutions; Hospital for special surgery; electrical discharge machining
Publication:Tooling & Production
Date:Mar 1, 1993
Words:735
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