When the Tru-Step prosthetic foot hit the market around 1986, it was a great improvement over traditional prostheses. The standard SACH foot was a solid-ankle-cushioned-heel device made of wood or plastic, and incapable of flexing side-to-side. But the Tru-Step used three "bones" and various elastomers to allow ankle roll. Finally, amputees could walk along a hillside without tipping over.

It worked great, but children couldn't wear the large, adult-sized feet. So College Park Industries (Frasier, MI, www.college-park.com) decided to create a kids' version, called the Tru-Per.

The challenge was that they couldn't fit all the parts in a smaller size. And the Tru-Step already sold near the top of market prices, so they had to control costs. So they took the Tru-Step and reduced the number of moving parts, or "bones," from three to two; just an ankle and foreheel (combination of forefoot and heel).

The next challenge was mobility. Prosthetic feet are passive-there's no muscular control, just reaction, explains Chris Johnson, director of engineering for College Park.

So the Tru-Per must do some of the work itself, using a pivoting, composite-and-elastomer construction to provide variable resistance through the gait. That lets the foot perform motions like plantar (toe down) and dorsi (toe up) flexion, inversion and eversion (walking across that hillside), and pronation and supination (walking on the instep or outside). In contrast, most SACH feet use rigid carbon plates-the toe is merely a big leaf spring.

The second challenge was that it had to be tough. Really tough. "If you come away with anything about artificial feet, it's that they get the snot beat out of them," Johnson says. "Humans put seven times their body weight on them, through a huge number of cycles." Living limbs have a secret advantage: they can grow new cells, healing as you sleep. That's not an option for prosthetic feet.

But the feet had to be realistic as well as functional. So College Park partnered with prosthetists, clinical specialists who fit patients with prosthetic limbs. They gathered advice on incorporating a toe split into the design (so kids could wear sandals), and whether to show veins.

Finally, College Park had to create a cosmetic covering, or synthetic skin. They turned to ProFICIENT Engineering Services Inc. (Lawton, MI, www.proficient-engineering.com). But Johnson was still designing the foot. How do you wrap a package that's changing shape?

"We needed concurrent design," says Lars Chrisman, president of ProFICIENT. "We had to have parametric control over the surface model, and manage key driving parameters and dimensions that drive the shape, so design changes made on one end or the other could relate to each other."

Yet Chrisman was doing high-end surfacing, while Johnson was doing 3D solid modeling. Fortunately, they were both using Pro/ENGINEER from PTC (Needham, MA, www.ptc.com), so collaborating was as simple as trading CDs in the mail.

"We wanted an integrated solution to model an anatomically accurate and aesthetically pleasing cosmesis on the computer," Chrisman says. The variables included heel height, foot width, and toe spacing (in the split-toe design). "It is like a tent frame defining the structure and the skin is stretched over it, so it floats as the design evolves."

He used a polyester foam for the cosmetic covering. Yet he had to control the wall thickness very closely, to ensure an integrated fit to the structure inside, keep weight down, and help provide spring and cushion.

But his job wasn't done yet-College Park needed a range of foot sizes from 16 to 21 cm, and a left and right version of each size.

"We can do rapid tooling with Pro/E, directly from part geometry," Chrisman says. "So we can make variations of parts very quickly, as opposed to clay modeling." In the traditional approach, artists make hand-sculpted models, and they can't always make the left match the right. "But in our parts database, you can just do mirror images automatically, so they're exactly the same." Thanks to having such control over the design, College Park can now offer cosmesis varieties.

The great majority of amputations are caused by vascular problems, with the remainder caused by traumatic accidents or birth defects, Johnson says. In marketing terms, this means that prosthetic feet are designed for adults, since very few kids have lived long enough to suffer from the first two causes.

In practice, many kids try to hide their prosthetic limb, so the company offers a foam calf which fits over the aluminum post ("pilon"), and fits into the foot with a sealing boot ("bellows"). The bellows also protect the foot's moving parts from dirt and mud, which cause noise and friction. But some kids like to wear the pilon alone, so the company offers bright, anodized colors for the ankle, pilon, and socket (where the leg meets the Tru-Per). "We even make them with cartoon characters," Chrisman says.

"Knowing that we intended to propagate an entire product family out of one model, we made it in Pro/E," he says. "So when we were ready to generate multiple sizes, it was very quick."

The first beta for the Tru-Per was in summer 2000, and the product launched in summer 2001. One of those first users was Brigit, an eight-year-old bilateral amputee who was accustomed to SACH feet. She could walk only awkwardly, but when she tried on the new Tru-Pers, "she just took off running, and her dad was in tears," Chrisman says. "In terms of feeling good about what you do, when we did that first beta, and she put it on and ran, it was remarkable."

The product was so successful that College Park plans to funnel several improvements from the new Tru-Per back into the original Tru-Step.

One is the strength of moving straight from CAD models to RP to make tooling for manufacturing. This saves time and boosts accuracy compared to creating clay models.

Another lesson was the advantage of simplicity: "My design philosophy comes from a quote by Benjamin Franklin," Johnson says. "He wrote a friend a note, and said at the end, 'I would have written you a shorter letter but I didn't have the time.' Well, simplicity is the result of hard work. If you pick up a design that's too complex, either the engineer didn't know what he was doing, or it's not going to stand up to the demands and the punishment."