An arm is more than a hand
An artificial arm has to do several jobs. It replaces the weight and outline of the limb, it positions a hand or hook in space, and it usually has to transfer some control from the user’s body. The history of the prosthetic arm is mostly about solving that last problem: how can someone who has lost part of an arm make the remaining device move? This article is about the arm; the grips and fingers of the hand are a separate topic.
Iron hands of the Renaissance
Some of the best-known early arms are German iron hands from around 1500. The knight Götz von Berlichingen lost his right hand in battle in 1504 and had at least one iron replacement made. The surviving examples have articulated fingers that can be set in position with a spring and release catch, which allowed the hand to hold reins or a weapon. The user had to set the fingers with his other hand, so it was not an active gripper, but it was more than a simple stump cover.
The French surgeon Ambroise Paré, in the sixteenth century, published designs for arms that included an elbow with a locking mechanism and a hand with spring-operated fingers. These designs were made by skilled metalworkers working in a tradition shared with the makers of mechanical clocks, and they were so expensive that only the wealthy could afford them. For most amputees, the realistic option was a leather or wooden cuff with a fitted hook or a table-top attachment.
Using the body as the power source
The key breakthrough in arm design was to let the user’s own movement operate the device. In the early 1800s, a German dentist named Peter Beil is often credited with an arm in which movements of the upper body, transmitted by cords, opened and closed the hand. Dates and details are disputed, but the principle caught on. Through the nineteenth century, makers refined harnesses that wrap around the shoulders and use a cable to connect the user’s movement to a terminal device at the end of the arm.
This is called body power. When the user rolls or shrugs the shoulder, or pushes the upper arm forward, the cable tightens and the hook or hand responds. Elbow joints could be locked and unlocked with a second cable or with a small movement of the other shoulder. It is a simple, direct arrangement and it gives the user a physical sense of how hard they are gripping, because the cable carries force back to the body.
War, workers and standard designs
The First World War produced very large numbers of arm amputees in Europe, and governments set up workshops and research. Surgeons and engineers developed several ideas. One, associated with the German surgeon Ferdinand Sauerbruch, involved creating tunnels in the remaining muscle so that muscle contraction could pull a lever directly. Another was the development of standard hooks and hands that could be swapped for work or social occasions. In the United States, a widely used split-hook design appeared in 1912, and it influenced hook design for decades.
After the Second World War, organized research programs studied how arms were actually used. Engineers measured the force and range of motion required for daily tasks, and made harness layouts and elbow joints lighter and more reliable. Aluminum frames and plastic laminates replaced heavy wood and steel. The same period saw the first serious attempts at powered arms.
Why the cable design has lasted
Despite decades of electronics, cable-driven arms are still in use. The cables are often braided stainless steel, and the system is mechanically simple, relatively light, and tolerant of water and dirt. Repairs are easier, and the cost is far lower than for a powered arm. For some users, a hook is more useful than a hand-shaped device because it can grip small objects, tools and handles. Disadvantages include the harness itself, which can be uncomfortable or tiring, and the fact that large movements of the shoulder are needed to move the device. Many users own more than one arm for different situations.
Comfort and fit also mattered throughout this history. Sockets for the upper arm, like those for legs, were first made from leather and wood, then plastic laminate, then custom-fitted shapes. Modern designs use soft liners and lightweight frames, and they are sometimes produced with 3D scanners and digital design.
From mechanical to electronic
Today the arm can have a mechanical elbow, a powered elbow or a powered hand, with small electric motors doing the work, and users often combine them. A common approach is to pair a body-powered elbow with a myoelectric hand, for instance. Choices depend on the level of amputation, the user’s job, cost, and personal preference. The prosthetic arm’s long history shows that a design which does one thing reliably often outlasts more complicated alternatives, and that the best device is usually the one a user will actually put on in the morning.