Doctors met a patient at a surgical center outside Boston to invent a new operation, a way to perform arm amputations that might allow patients to move their prosthetic hands more like real ones.

The right arm resting on a blue surgical drape before them came from a cadaver; it’s just the limb, ending at the shoulder. It came from the Anatomy Gifts Registry.

Devising a new operation is like re-engineering the anatomy. Cut open, the forearm is an intricate weave of muscles, nerves, blood vessels, and tendons. Over the course of an approximately five-hour operation, surgeons excavate the forearm down to the radius and ulna, saw those bones off midway to the elbow, build a “bone bridge” to stabilize their loose ends, and then reassemble the dissected muscles and nerves following a wiring diagram scrawled with marker on a surgical drape.

Only about a third as many people have arms amputated as legs, so it is more challenging to find suitable patients. Those who have had failed wrist or elbow replacement surgery or bone fusion operations are prime candidates.

In the new surgery, the muscle couplings are recreated, using tendons as the pulleys linking opposing muscles. When the brain thinks about bending the index finger, say, the appropriate muscle pair contracts and stretches, and that muscle activation can be picked up by an electrode and transmitted to the prosthetic hand.

But there’s limited real estate in the arm, meaning the surgeons must choose which functions of the hand are most essential to replace. Flexing the thumb and index finger, and the other fingers in unison, are a priority.

The small muscle in the hand that’s responsible for the opposable motion of the thumb is called the opponens, and it has a fragile blood supply, meaning it is risky to build a pulley with it. Surgeons removed layer after layer of tissue to expose the median nerve’s motor branch and the FCR muscle below the elbow. The nerve was snipped free from the thumb muscle and looped up to the FCR. Surgeons agreed: If they can provide proprioceptive sensation for the hand and fingers and increase thumb motion, it’s a big win.