Understanding Prosthetic Arms: Types, How They Work and Their Drawbacks

Prosthetics appear very often in science fiction, where they are usually colorfully described as devices far better than our biological arms. However, this is a common misconception. Prostheses that exist in the real world today are far from a true replacement for arms. Most often they are tools that serve for simple actions. 

Before figuring out what prostheses can actually do, which ones are on the market and what difficulties exist, it is necessary to understand the types of amputation: fingers, hand, forearm and shoulder. Depending on the type of amputation, a corresponding prosthesis is needed. Losing fingers is the easiest to replace, but amputation of the forearm brings serious problems. 

The reason for fitting a prosthesis can be not only accidents and other incidents that led to limb amputation, but also various congenital mutations that cause deformation of some part of the arm. Today there are three main types of prostheses:

• body-powered (mechanical)
• myoelectric (based on bioelectric sensors, the most advanced)
• cosmetic

The purpose of cosmetic prostheses is in their name: they simply make the amputation less noticeable but provide no functionality. A patient with a cosmetic prosthesis can at most support something, but nothing from normal everyday actions. 

Mechanical prostheses

Body-powered or mechanical prostheses began with the steel hand of the German Götz von Berlichingen. It had limited functionality, but it was possible to hold objects and possibly write with a pen. The device had to be controlled with a button located on the back of the palm. 

With the start of the industrial revolution, prostheses began to improve and approach the current designs. A mechanical prosthesis is a frame with many cables (tendons) that are attached to specific areas of the remaining limb. Thanks to effort, the cables tighten and the prosthesis moves or bends. Such prostheses are suitable for any type of amputation, even complete loss of the arm. 

Body-powered prostheses are better suited for shoulder amputation than myoelectric ones, due to the specifics of bioelectric sensors. The prosthesis consists not only of a frame, socket and hand, but also of a cable attached to the healthy shoulder, which enables control of the prosthesis (bending and straightening of the hand). Control at the elbow is possible only with the help of the healthy arm, or by throwing the prosthesis forward with the torso. 

Main advantages of body-powered prostheses:

• no electronics (this lowers cost and increases resistance to damage)
• can be fitted for shoulder amputation
• lower weight compared with myoelectric ones

However, such prostheses have only one type of grip, so they are designed for a narrow function, which is not always useful in daily life. 

Myoelectric prostheses

Myoelectric prostheses have a more complex working principle and design. These devices work by reading the electrical potential of the remaining muscles. To explain: you decide to move your hand; an impulse travels from the brain to the spinal cord, then enters the nerves connected to muscle fibers; the signal is amplified and spreads to certain groups of muscle fibers; one nerve can activate several muscle cells. 

In a healthy hand, the signal, for example, reaches the palm, so you clench it. However, in a hand amputated below the elbow, the signal goes "nowhere", that is, it dissipates. When special sensors are installed that register the signal before it dissipates, it can be used somehow. Myoelectric sensors do not need to be implanted into the arm; it is enough simply to put on the prosthesis. 

After the signal is registered, it goes to a processor, where it is processed. Servo drives are activated, after which the prosthesis performs some movement. Servo drives are small propulsion units that can receive a signal, compare it with program data and perform a certain movement. 

The advantage of these prostheses over mechanical ones lies in electronics. The movement of the servo drives can be programmed, and many different grip types can be set for the prosthesis. Body-powered designs can only close and open the hand, while myoelectric ones can gesture, rotate the wrist and more. They have a range of grips from 18 to 22. 

However, these grips have to be switched. It is not enough simply to want to make a certain movement for the prosthesis to perform it. The prosthesis operation depends on sensors, and it has no connection with the brain or mental processes. Switching can be done using a display or a button on the prosthesis, or a smartphone app. As with a mechanical prosthesis, one must constantly tense some muscles to perform a grip, which can be tiring during long use. 

A person will not be able to control each finger separately, but can perform movements programmed by engineers into the servo drive code. One type of prosthesis allows each finger to be controlled separately. It works on the principle of ultrasonic muscle scanning, but it is only a single prototype. 

Amputation above the forearm and the future

The difficulty with amputation above the forearm is that there is simply nothing to read the signal from. The arm is completely absent, so there is nowhere to place sensors, but engineers have found two workarounds:

1. A development of Ottobock called DynamicArm. The socket is worn where the shoulder should be and reads the signal for control. Elbow flexion is one of the grip options, and the elbow also straightens automatically under load. 
2. A development of Johns Hopkins University. The patient has both arms missing, with amputation above the forearm. Scientists placed the sensors on the chest. The prostheses are mobile and have a wide range of motion, but are limited in speed and ease of control, since it is difficult to control arms using the chest. 

Despite the technological sophistication of existing prostheses, the world still has room to grow. Patients without an arm or with bilateral amputation can rehabilitate and expand their range of activities with prostheses. However, they are still limited. These results can be improved with neuroprosthetics or neural networks, with an increased number of myoelectric sensors. 

It is worth noting that the price of prostheses remains high, from $20,000 to $100,000. Efforts are being made to make devices cheaper with 3D printing, but this is unlikely to be noticeable in the near future. Science fiction is still far away, but this branch of robotics is not standing still, so one can hope for a miracle in the future.