An interesting article published by the New Yorker has explored how developments in lower limb amputation techniques have resulted in recent advances in the field of prosthetics.

Traditionally, surgeons perform below the knee amputations by sewing down the residual muscles. While one reason for this is to pad the bone, it also severely restricts how much the muscles can move, leading to atrophy. The neural signals from severed muscles are only about 3% of what they once were which is insufficient for effective communication with a neurally controllable prosthesis. 

Surgeons have now developed the agonist-antagonist myoneural interface (AMI). An AMI is made up of two muscles – an agonist and an antagonist – connected mechanically. When the agonist contracts, the antagonist is stretched, and vice versa. During an amputation procedure, a surgeon creates AMIs by linking together muscle pairs within the amputated residuum. Artificial muscle electrodes placed over each AMI muscle communicate with small computers within an advanced bionic limb that control movement of the bionic joints.

While still in the research phase, the results have been very promising in restoring function. It is the first leg design which is under a participant’s full neural control rather than having to rely on a prosthetic device which does most of the thinking via algorithms. This approach has allowed participants to achieve biomimetic gait, i.e. users can walk approximately as quickly and unthinkingly as anyone else. Even on uneven ground, the leg design allows a person to move in a truly biomimetic way. 

One patient described that literally within minutes of having the prosthesis connected, he felt like a foot was there and he could feel it through his knee. Without realising he was doing it, he was able to fidget his artificial foot by flexing and pointing it repeatedly. In a real-world experiment, he was able to return to his former hobby of rock climbing. 

Just by thinking, another patient was able to manipulate her artificial foot in all sorts of ways, e.g. pointing quickly or slowly, turning her ankle inwards and outwards, and tapping her foot as if with impatience. 

According to the researchers, M.I.T. Media Lab’s Biomechatronics Group, it will probably take five more years for neurally controlled leg prostheses to be proved safe and effective, and to become commercially available. 

In the context of catastrophic injury cases, once these prostheses are on the market and AMI surgery is more widespread in the UK, we can expect to see claims being brought for these new types of bionic limbs. They are likely to be expensive but if they are able to restore function to the extent which the early trials are suggesting, then this will increase the prospects of injured parties being able to return to work, require less care and live more independent lives. 

AMI surgery is being developed for other types of amputations, including above the knee and above and below the elbow. Similar advances in bionic limbs in these cases may therefore be on the horizon as well. 

We will keep a close eye on what the future brings.