The $40,000 Arm

In addition to his powerful mechanical legs and zoom-lens artificial eye, television's Six Million Dollar Man has an atomic-powered arm that can knock down walls, lift cars and pull out trees by the roots. Reid Hilton's new arm is only slightly less remarkable -and considerably less expensive. Hilton, 24, a Santa Ana, Calif., karate expert who lost his right arm below the elbow in an accident, will probably not risk smashing bricks with his experimental $40,000 replacement. But the prosthesis should enable him, with practice, to function like a man with two natural arms, and in some ways outperform him.

Hilton's 8-lb. myoelectric (from the Greek myos, for muscle) arm was developed at Northwestern University and modified by engineers and researchers at the medical-products division of General Atomic and at Rancho Los Amigos, a hospital associated with the University of Southern California. The arm, which can be fitted with either a hook or a normal-looking hand, does not look much different from other powered prostheses. But the similarities are only skin deep. Most artificial arms use a system of receivers on the surface of the skin and microtransmitters under the skin to carry messages from the nerves to the arm's controls, plus belt-carried batteries for power. Hilton's arm needs neither. Its controls are directly connected to the nerves in the stump of his severed arm. The power pack is inside the prosthesis. Unlike most other artificial arms and hands, it is also equipped with feedback devices that give him a sense of touch.

Powerful Pinch. To attach the arm, Dr. Vert Mooney and his colleagues inserted three "buttons" or fasteners through the skin in the stump. (The buttons can permanently protrude through the skin without promoting infection because they are coated with pyrolytic carbon,* which Mooney says forms an antibacterial seal.) The doctors connected two of the buttons to the arm's median and ulnar nerves with stainless-steel coils, and wired the third button to another carbon plug that serves as a ground. They then connected all buttons to wires in the prosthesis itself, linking them to sensors in the hand. To operate the arm and its hand properly, Hilton moves his remaining arm muscles selectively; their contractions produce electrical impulses that, transmitted to the prosthesis, open and close his hand.

Doctors believe it will be at least five years before arms like Hilton's become generally available, and concede that further modifications may still have to be made in their prototype. They hope eventually to bring the cost of the device within reach of other amputees (Hilton's arm, without research and development expenses, would cost about $1,200). Meanwhile, Hilton is demonstrating that the prosthesis is practical; he is learning how to pick up small objects, open cigarette packages and tie his own shoes. He is also learning to be careful, especially when handling delicate objects. A normal male grip exerts a force of about 25 Ibs. Hilton's electronically assisted grip strength is a formidable 40 Ibs.

* An extremely hard and pure form produced by burning a derivative of carbon in a blend of extremely hot gases.

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