Invention & Technology, Fall 2009Building a truly useful prosthetic arm remains a major engineering challenge: while new designs show promise, they still can’t beat the utility of a simple hook mechanism that dates back to before World War I.

By Michael Belfiore.

Invention & Technology Magazine, Fall 2009.

The place didn’t look like a cutting-edge research laboratory. The work benches in the little room at Johns Hopkins University’s Applied Physics Laboratory (APL) in Laurel, Maryland, were cluttered with oscilloscopes and other gear for testing electronics, circuit boards, computer workstations, drills, files, miscellaneous bits of wire, and other gear reminiscent of a hobbyist’s well-equipped workshop.

A mannequin at the door, wearing an articulated metal and plastic arm in place of its usual plastic, and a cabinet shelf stocked with realistic rubber hands betrayed the lab’s true purpose: the creation of a prosthetic arm so close in form, appearance, and function as to be virtually indistinguishable from a natural human limb.

Thirty-five-year-old mechanical engineer and former Marine reservist Jon Kuniholm, solidly built and still maintaining a short military-style haircut, had more at stake in the proceedings than his colleagues. In place of his right forearm, he wore a carbon fiber prosthesis that terminated in a split hook. Its design had changed little since 1912, when inventor D. W. Dorrance, who had lost his own arm in an industrial accident, patented a split hook prosthesis. Standing in the APL lab in the summer of 2007, speaking with quiet intensity, he turned his body to show the straps that linked the simple yet elegantly effective hook mechanism to his shoulder. As Kuniholm reached his prosthesis forward, the hook opened, then closed when he retracted it.

Even though the hook was unashamedly a hook, without cosmetic adornment, its simplicity and utility had made it more popular with amputees than any other design. Kuniholm, like his colleagues at APL, hoped that their latest high- tech prosthetic project would be different— but he wasn’t counting on it.

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