Breakthrough in amputation narrows gap between human and machine

The writer is a science commentator

In 2005, US army sergeant Jerry Majetich was gravely injured in a bomb and gun attack by insurgents in Iraq. Two colleagues travelling in the same vehicle were killed. He underwent more than 80 operations for severe burns, spinal fractures, internal injuries and limb damage.

Last year, he volunteered for pioneering surgery: an experimental form of amputation that allows prosthetic replacements to feel as if they are a part of the body. Majetich consented to the removal of his hand, which had limited movement but gave him constant pain. A video posted by the health news website Stat shows his replacement robotic hand, and fingers, moving at will — with the pain largely banished.

The surgery appears to restore proprioception, the sense of knowing where our extremities are, even if we cannot see them. The technique could transform the way that amputation has long been viewed: not as a last-resort method that subtracts from the body but an act of rejuvenation with the potential to restore a sense of completeness.

The research into agonist-antagonist myoneural interface (AMI) surgery has been led by Hugh Herr and colleagues in the biomechatronics group at the MIT Media Lab in Cambridge, Massachusetts. While amputations traditionally involve nerves and muscles being simply cut and tidied, AMI surgery reconnects the muscle pairs that normally control joints. These pairs, such as biceps and triceps, work in a push-pull fashion: when one contracts, the other stretches. That allows elbows and knees to bend, with the movements also feeding sensory information to the brain to enable it to keep track of what limbs are doing and where they are in space.

Patients with reconnected muscle pairs after leg amputations were better able to control their prosthetic limbs — and suffered less “phantom limb” pain. The latter was unexpected but is now ascribed to the brain being less confused by incoming signals.

Majetich was the first patient to undergo the surgery in an upper limb, with his hand amputated just above the wrist. He can now feel his individual fingers; as he tries moving each one, the corresponding muscles in his arm twitch visibly under the skin. That muscle activation is picked up by electrodes in the prosthetic hand and converted into movement.

It feels like a significant development in a branch of surgery that has been practised for millennia but can still seem a little crude, especially when set against the advances made in prosthetic technology and design. The earliest confirmed proof of amputation in Europe goes back to the Neolithic period: the remains of a male dated to around 4800BC, found near Paris in 2007, included a partially healed severed leg.

Amputations later became a mainstay of military surgery. But the procedures were still highly risky, as recognised by John Woodall, a naval surgeon for the East India Company. In his 1617 manual for sea medics, The Surgions Mate, Woodall advises encouraging the luckless patient to “prepare his soule as a ready sacrifice to the Lord by earnest praiers, craving mercie and help unfainedly . . . It is no small presumption to Dismember the Image of God.”

AMI surgery joins several other refinements that promise to improve the outlook for amputees, according to Stephen Cruse, who lost both lower legs in a 2008 car accident and later founded the UK charity Amputation Foundation. Osseointegration allows replacement limbs to be surgically anchored to the bone, so the bone grows around it; targeted muscle reinnervation relocates nerve ends into the remaining muscle to enable more intuitive control of prostheses.

“AMI could be fantastic for amputees to be able to feel where they are in space, without having to stare at their limbs all the time,” says Cruse, who is fully active but suffers from intermittent phantom limb pain. The seam between human and machine is becoming smoother; it may yet become invisible.